identifying opportunities to cultivate algae combined with … · 2015-05-20 · identifying...
TRANSCRIPT
Identifying opportunities to cultivate algae combined
with wastewater recycling as a source of renewable
energy in Southeast Asia
Marcus Tang
Murdoch University
School of Engineering and Information Technology
Master of Science in Renewable Energy
2014
Declaration
I declare that, apart from properly referenced quotations and citations, this
dissertation is my own work and complies with Murdoch University's academic
integrity commitments and any other conditions of submission as attached to the
dissertation. It has not been submitted previously for assessment in another unit.
Signed: Marcus Tang
iii
Abstract
Water and energy are finite resources and our demand for these resources shape the world.
The world’s population has access to only 0.007% of the total water on earth for
consumption and close to 1.3 billion people do not have access to electricity. With the vast
majority of the world’s population moving into urban areas, the need to develop the
infrastructure and protect the resources that ensure the safe and stable access to water and
energy is paramount. This is especially so for developing countries where access to these
resources are critical in the alleviation from poverty.
Cities use large amounts of water and energy to sustain its growth, while producing large
amounts of waste and wastewater. If these pollutants are not treated, it can cause serious
health problems to the population. Therefore, coupling wastewater treatment with microalgae
could be the solution. Algae, which is known as a “third generation biofuel”, offers many
benefits over other biomass resources, such as shorter cultivation time, flexibility in types of
biofuels, producing high yields and most importantly the ability to treat pollution. However,
high production cost is one of the major challenges facing the industry.
The research paper explores the feasibility of microalgae production with wastewater
treatment and the possibility of coupling wastewater treatment with microalgae production as
a solution to create a reliable stream of renewable energy production.
Contents
1 Introduction .................................................................................................................................... 7
1.1 Aims and Objectives ............................................................................................................ 9
1.2 Research Questions ............................................................................................................ 9
2 WATER & ENERGY .................................................................................................................. 10
2.1 Scarcity of freshwater resources ...................................................................................... 10
2.2 Increase in water demand ................................................................................................. 11
2.3 Access to Energy ............................................................................................................... 11
2.3.1 Renewable Energy in Asia ........................................................................................ 12
3 Wastewater ................................................................................................................................. 13
3.1 Wastewater Treatment Process ....................................................................................... 14
3.1.1 Preliminary treatment ................................................................................................. 16
3.1.2 Primary treatment ....................................................................................................... 16
3.1.3 Secondary Treatment ................................................................................................ 17
3.1.4 Tertiary Treatment ...................................................................................................... 19
3.1.5 Disinfection .................................................................................................................. 20
3.2 Advantages and disadvantages of conventional wastewater treatment .................... 21
3.2.1 High energy consumption and cost ......................................................................... 21
3.2.2 Loss of valuable nutrients and cost of sludge treatment ...................................... 21
3.2.3 Secondary pollution.................................................................................................... 22
4 Phycoremediation ....................................................................................................................... 23
4.1 Microalgae ........................................................................................................................... 23
4.2 Microalgae used in wastewater treatment ...................................................................... 25
4.3 Challenges of coupling microalgae with wastewater treatment .................................. 25
4.4 Advantages of microalgae wastewater treatment over conventional treatment ....... 26
4.4.1 Removal of nutrients .................................................................................................. 26
4.4.2 Removal of pathogens ............................................................................................... 27
4.4.3 Photosynthetic aeration ............................................................................................. 28
4.4.4 Removal of heavy metals .......................................................................................... 28
4.4.5 Reductions in sludge formation ................................................................................ 29
4.4.6 Green House Gases reduction and CO2 mitigation ............................................. 30
4.4.7 Removal of coliform bacteria .................................................................................... 30
4.4.8 Lower energy requirement and cost ........................................................................ 30
4.4.9 Production of useful biomass ................................................................................... 31
5 Cultivating Microalgae in Wastewater ..................................................................................... 32
5.1 Factors affecting growth of microalgae ........................................................................... 33
5.1.1 Climate conditions ...................................................................................................... 33
5.1.2 Carbon Dioxide ........................................................................................................... 34
5.1.3 Evaporation and salinity ............................................................................................ 35
5.2 Microalgae for biodiesel production ................................................................................. 35
5.3 Microalgae for bioethanol production .............................................................................. 37
5.4 Summary ............................................................................................................................. 38
5.5 Waste stabilisation ponds ................................................................................................. 38
5.5.1 Facultative treatment ponds ..................................................................................... 38
5.5.2 Maturation treatment ponds ...................................................................................... 40
5.5.3 High rate algae ponds................................................................................................ 40
5.6 Photobioreactors ................................................................................................................ 41
5.7 Hybrid two stage production system ............................................................................... 42
6 Southeast Asia (SEA) – Vietnam as a Case Study .............................................................. 43
6.1 Water and wastewater ....................................................................................................... 43
6.2 Energy profile and renewable energy potential ............................................................. 44
7 Economics ................................................................................................................................... 46
8 Wastewater treatment with biorefinery .................................................................................... 50
8.1 Biorefinery ........................................................................................................................... 50
8.2 Conceptual model of wastewater treatment with biorefinery ....................................... 51
8.2.1 Preliminary removal and primary treatment ........................................................... 51
8.2.2 HRAP and wastewater treatment ............................................................................ 52
8.2.3 Harvesting ................................................................................................................... 52
8.3 Recycling of nutrients and CO2 ........................................................................................ 54
8.4 Processing of microalgae .................................................................................................. 54
8.5 Extraction of microalgae oil ............................................................................................... 55
8.6 Biofuel processing .............................................................................................................. 55
8.6.1 Transesterification ...................................................................................................... 56
8.6.2 Direct transesterification ............................................................................................ 58
8.6.3 Fermentation ............................................................................................................... 58
8.6.4 Anaerobic Digestion ................................................................................................... 59
9 Conclusion and Recommendation ........................................................................................... 62
9.1 Research Limitations ......................................................................................................... 63
9.2 Follow-up Research ........................................................................................................... 63
10 Bibliography............................................................................................................................. 64
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1 Introduction
Energy is an essential driver in economic progression with access to energy playing an even
more important role in developing countries, where reliable energy can alleviate the
population from poverty (UNDP, N.D) and provide economic and social stability. As the world
continues to battle with changes to our climate caused by our demand for energy (EPA, N.D)
and depletion of our fossil fuels resources, the need to develop reliable and sustainable
renewable resources has come to the forefront in the bid to sustain our current way of life.
There is currently close to 7.2 billion people living on Earth and this number is projected to
hit 9.6 billion by 2050 (DESA, 2013). The World Health Organisation (WHO) has projected
that by 2030, 60% of the world’s population will be living in a city with the number reaching
70% by 2050 (WHO, n.d). Similarly, by 2030 close to 55% of Asia will be living in urban
areas (ADB 2011, 17). Populations in developing countries will see the greatest increase,
according to the United Nation (UN) as rural populations move to the cities in search of
better job opportunities to improve their standard of living (World Bank, n.d). The group of
people that suffers the most are the urban poor, who live in poverty and lack access to basic
necessities (UN, 2014). It is estimated worldwide that there are one billion people living in
such urban slums (UNFPA 2007).
According to the UN it is estimated that are around 1.1 billion people across the world that
do not have access to improved water supply sources and 2.5 billion people have no access
to proper sanitation facilities (UNDESA, N.D). Access to safe drinking water is a basic right
and is recognized by the UN in its Millennium Development Goals (MDGs), but the most
affected continue to be in developing countries where communities live in extreme poverty
with little access to proper sanitation and clean drinking water. The implementation of a
reliable wastewater management system in urban areas is thus a necessity, with the lack of
proper wastewater management curtailing poverty reduction and diminishing overall health.
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However, the challenge is that the cost of wastewater treatment plants can be high (UN
Habitat 2010, 27).
Whilst cities require large amounts of water and energy to sustain its growth, they also
produce large amounts of waste and wastewater. Therefore, combining the treatment of
wastewater with the production of renewable energy can address many of the challenges
facing larger scale adoption of renewable energy whilst adding value to the water treatment
process. As such, alternative sources of sustainable energy can provide energy security and
further help the development of the country. Algae, specifically microalgae, can be that
alternative source of energy for developing countries. Known as a “third generation biofuel”,
microalgae offer many benefits over other biomass resources, such as shorter cultivation
time, flexibility in types of biofuels and producing high yields. However, the current
technology and high production cost are some of the major challenges facing the industry.
For this research, Vietnam was chosen as the country to evaluate the potential of cultivating
microalgae combined with wastewater treatment. Vietnam is located in Southeast Asia
(SEA), a region comprising 11 countries with a combined population of over 600 million and
is one of the fastest growing markets in the world. A majority of these countries are in the
early stages of development except for Singapore that has a matured economy. Economic
growth can come at a cost to the environment and the community with increased industrial
and urban development. These developing economies face a larger proportion of their rural
population moving to the cities in search of better opportunities. This large urban migration
puts additional stress on their underdeveloped water and energy infrastructure. Choosing
Vietnam as a case study is based on the assumption that projects implemented in Vietnam
have similar potential for success in other developing countries of SEA due to the similarities
in their economic development. This is not taking into account the political and social aspect
in the decision process.
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1.1 Aims and Objectives
The aim of this research is to explore the feasibility of microalgae production with
wastewater treatment plant.
The objective is to evaluate the possibility of coupling wastewater treatment with microalgae
production as a solution to create a reliable stream of renewable energy production whilst
providing a public service of treating wastewater.
1.2 Research Questions
The economic viability of cultivation and harvesting of microalgae is hampered by the high
cost associated with the use of available technology and processes. There is a lack of
information available in the literature on the economic viability of a commercial algae biofuels
facility due to the lack of projects and publicly available data. As such, the research aims to
address the following research questions:
Is cultivating microalgae with wastewater a viable solution?
Can a microalgae wastewater treatment facility address the cost challenges to
increase renewable energy generation?
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2 WATER & ENERGY
Water and energy are interdependent with actions in either domain greatly affecting the
other. Without energy it would be very difficult to bring drinkable water to the population, and
without water mass energy cannot be generated. It has been estimated that close to 7% of
the total global energy generation is used in the extraction, treatment and transportation of
water (Hoffman, 2011) with the number rising to 40% in developed countries (WEF 2009, 3).
Conversely, water is critical in the generation and transmission of energy. Close to 90% of
power generation is water intensive (UN Water 2014, 33), as water is required to cool the
steam that spins the turbines (UCS, 2011).
2.1 Scarcity of freshwater resources
Water is what sustains life on earth.
Close to 70% of the earth is covered
with water, but only a small fraction of
about 3% is made up of freshwater
with the remaining comprised of salt
water in the oceans (National
Geographic, N.D; USGS, 2014).
Fig 1: Global water stress and scarcity of water (UNEP,
2008)
Within this small fraction, a large proportion of around 70% is locked in ice caps and glaciers
leaving only 0.007% of the total water on earth available for consumption (USGS, 2014;
UNEP Freshwater resources, N.D). This makes groundwater the primary source of
freshwater followed by surface water bodies such as lakes, reservoirs and rivers (UNEP
Freshwater resources, N.D). A sad fact is that much of this groundwater is non-renewable
and will be mined to exhaustion if we continue at this high rate of consumption.
As the world’s population continues to expand, our finite water resources will be placed
under further strain. The UN has projected that by 2025 there will be close to 1.8 billion
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people directly affected by water scarcity and two-thirds of the global population living in
water-stressed areas (UN-Water, N.D; UNEP, 2008).
2.2 Increase in water demand
As the world’s population increases, so will the demand for water with the largest proportion
to come from developing countries. Increased demand corresponds to a country’s expected
population growth and economic development. The four main uses of water are for
agriculture, industrial, domestic and production of energy (UNESCO, N.D). Agriculture is by
far the largest consumer of water (GWP, 2012), accounting for close to 70% of all water
withdrawn (UNESCO, N.D). The Food and Agriculture Organisation (FAO) of the United
Nations has projected that by 2050, global demand for food will increase by 70% (FAO N.D,
4). A small increase in agriculture production is expected to raise worldwide water demand
by as much as 20% (UN Water, 2014). Industrial development is the main economy activity
in developing countries with water being an important part of the process. While in urban
populations, water is required not only for consumption but also used in sanitation and
drainage (UNESCO, N.D). Increased population and industrial activity would naturally
translate to greater demand for water.
2.3 Access to Energy
Rapid urbanisation and economic growth has seen the demand for energy increase. Global
energy demand is projected to increase in 2035 with the bulk of demand coming from
emerging economies (IEA, 2013; BP, 2014). Currently, there are almost 1.3 billion people
that do not have access to electricity with the vast majority located in sub-Saharan Africa
and developing countries of Asia (IEA 2012, 51). In Southeast Asia, it is estimated that 134
million people do not have access to electricity with around 280 million people continuing to
rely on the traditional use of biomass for cooking (IEA 2013, 26).
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ADB has projected that energy demand in Asia and the Pacific region will almost double by
2030 with Asia being responsible for more than 50% of overall energy consumption by 2035
(ADB 2013, 53). This is larger driven by the economic expansion of the countries. As
developing countries of Asia continue on their path of rapid urbanization and industrial
growth, there is an urgent need to generate power in a sustainable manner. Generating a
reliable source of energy supports the growth of the country that alleviates the population
from poverty (UNDP, N.D). It creates the opportunity for the population to engage in more
productive activities and open up opportunities for education (ADB 2013, 56-59).
2.3.1 Renewable Energy in Asia
Projections have shown that by 2035, fossil fuels, coal and natural gas will be expected to
continue their dominance as the primary source of fuel (ADB 2013, 55-57). Although the
worldwide use of renewables is set to increase (IEA 2012, 53), the overall impact will be
relatively small in Asia’s energy mix (ADB 2013, 53). Coal is easily accessible in Asia due to
large deposits, but natural gas and crude oil can prove to be an issue with supplies largely
outside of Asia (ADB 2013, 56). With the rapid expansion of developing countries in Asia,
many of these countries would become heavily reliant on energy exports (ADB 2013, 56-58).
This affects energy security and ultimately the development of the countries and the region.
Therefore, it is critical to develop locally sourced alternative sources of energy to secure the
long-term energy supply for the country. Microalgae can be that alternative resource.
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3 Wastewater
For the purpose of this research, the definition from the UN on wastewater will be used:
“a combination of one or more of: domestic effluent consisting of blackwater (excreta, urine
and faecal sludge) and greywater (kitchen and bathing wastewater); water from commercial
establishments and institutions, including hospitals; industrial effluent, stormwater and other
urban run-off; agriculture, horticulture and aquaculture effluent, either dissolved or as
suspended matter” (UN-Habitat 2010, 15).
With dwindling freshwater supply, implementing good wastewater management is critical in
ensuring the quality of water and supporting an already fragile water supply system.
Untreated wastewater is generally made up of organic material, microorganisms and
nutrients (Rawat, Kumar and Bux 2011, 3412-3414) as shown in Table 1.
As the population in urban centres continue to expand, the volume of wastewater produced
will also increase (Lazarova and Bahri, 2005). With close to 75% of the total water used by
the urban population returned as wastewater, the impact of wastewater to the urban
hydrologic system is significant (Qadir et. al, 2008. In addition, wastewater has long been
used as a vital resource in agriculture (UN-Habitat 2010, 31). It has been estimated that
there is 20 million hectares of land that uses raw or diluted wastewater for irrigation in over
50 countries (FAO, 2003).
In many developing countries where the wastewater infrastructure is inadequate to manage
the growing population, it is estimated that close 90% of wastewater flows into surface water
bodies (UN Water, 2014). ). Unmanaged wastewater can curtail poverty reduction and
diminish overall health. In addition, water pollution threatens the health and environment of
the population. For example, the World Health Organisation (WHO) has estimated that there
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are about 2.2 million deaths from diarrhoeal disease with the majority caused by unsafe
water, poor sanitation and hygiene (UN-Habitat 2010, 40).
Concentration
Contaminants Unit Weak Medium Strong
Solids, total (TS) mg L-1 350 720 1200
Dissolved, total (TDS) mg L-1 250 500 850
Fixed mg L-1 145 300 525
Volatile mg L-1 105 200 325
Suspended solids (SS) mg L-1 100 220 350
Fixed mg L-1 20 55 75
Volatile mg L-1 80 165 275
Settleable solids mg L-1 5 10 20
BOD5 at 20° C mg L-1 110 220 400
Total organic carbon (TOC) mg L-1 80 160 290
Chemical oxygen demand (COD)
mg L-1 250 500 1000
Nitrogen (total as N) mg L-1 20 40 85
Organic mg L-1 8 15 35
Free ammonia mg L-1 12 25 50
Nitrites mg L-1 0 0 0
Nitrates mg L-1 0 0 0
Phosphorus (total as P) mg L-1 4 8 15
Organic mg L-1 1 3 5
Inorganic mg L-1 3 5 10
Chlorides mg L-1 30 50 100
Sulfate mg L-1 20 30 50
Alkalinity (as CaCO3) mg L-1 50 100 200
Grease mg L-1 50 100 150
Total coliform CFU 100 mg L-1
106-10
7 10
7-10
8 10
8-10
9
Volatile organic compounds (VOCs)
mg L-1 <100 100-400 >400
Table 1: Typical composition of untreated domestic wastewater (adapted from Metcalf and Eddy
(1991) cited in Rawat, Kumar and Bux (2011))
3.1 Wastewater Treatment Process
Wastewater treatment is the process that removes the toxic elements from the wastewater to
make it suitable for discharge. One of the primary aims in the treatment process is to remove
biochemical oxygen demand (BOD), which is a measurement of the levels of oxygen that is
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used by the microorganisms (EPA, 2012). The removing of nutrients, especially nitrogen and
phosphorus, is an important step in the treatment process. The inability to remove these
nutrients can lead to eutrophication in receiving water bodies, which results in reduced levels
of oxygen thus affecting the aquatic life in the water body (Abdel-Raouf, Al-Homaida and
Ibraheem 2012, 260). Microorganisms in the wastewater play an important role in the
decomposition of organic waste that consumes oxygen in the process. During the
decomposition, many of these organic compounds have at least one carbon atom that when
oxidised produce carbon dioxide (CO2) (Abdel-Raouf, Al-Homaida and Ibraheem 2012, 263).
The conventional way of treating
wastewater involves various stages
of treatment that can be
summarized into physical, chemical
and biological removal of
containments (Rawat, Kumar and
Bux 2011, 3412) as shown in Fig 2.
The various treatment stages
involve preliminary, primary,
secondary, tertiary and disinfection.
Fig 2: Wastewater Treatment Process (Rawat,
Kumar and Bux 2011, 3413)
Fig 3: Stages of wastewater treatment (Mancl, N.D)
•Comminution
• Flow equalization
• Sedmientation
• Flotation
•Granular - Medium filtration
Physical Treatment
•Adsorption
•Disinfection
•Dechlorination
•Other chemical application
Chemical Treatment
•Aerated lagoon
• Trickling filters
•Rotating biological contractors
•Anaerobic digestion
•Biological nutrient removal
Biological Treatment
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3.1.1 Preliminary treatment
During the preliminary treatment, the influent is screened to remove grit and large solid
containments that may cause issues to plant equipment before being routed to the sewerage
plant. The wastewater passes through the bar screen that catches the large solid items
before the water enters the grit settling tanks. The grit settling tanks controls the speed of the
flow such that inorganic materials such as sand and other heavy material will settle at the
bottom of the chamber, and ensure that organic solids to remain suspended in the
wastewater and move onto the next stage of treatment (FAO, n.d; World Bank Group, n.d).
Fig 4: Preliminary treatment process (RWRD, 2011)
3.1.2 Primary treatment
The next stage removes sludge and scum via sedimentation tanks also known as a clarifier.
In a typical wastewater treatment plant, there may be clarifiers located at different points of
the treatment phase. Clarifiers that are located immediately after the preliminary stage are
called the primary clarifier and following clarifiers are called secondary or final clarifier (New
Mexico Environment Department 2007, 6-1). All these clarifiers perform the same function
and the only difference is in the density of the sludge with the sludge in the primary clarifiers
normally denser (New Mexico Environment Department 2007, 6-1).
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The wastewater is held in the tanks for a few hours to allow for the solids to settle at the
bottom of the clarifers that would then be removed by scrapers (FAO, n.d). This treatment
removes close to 70% of the suspended solids, between 25-50% of BOD and 65% of oil and
grease (FAO, n.d). Often chemicals, such as coagulants and flocculants, are used to
expedite the process by encouraging aggregation of particles (Mountain Empire Community
College, n.d). Treated effluence will than flow onto the secondary treatment stage.
Fig 5: Clarifier in primary treatment (RWRD, 2011)
3.1.3 Secondary Treatment
The secondary treatment uses aerobic treatment processes to treat the effluence to remove
residual organics that reduces BOD and colloids up to 90% (Mexico Environment
Department 2007, 7-1). These aerobic treatments use microorganisms (bacteria) in the
presence of oxygen to metabolize the organic matter to produce inorganic products such as
CO2, water and ammonia (World Bank Group, N.D; FAO, N.D). There are a few different
conventional aerobic biological treatment systems that are used:
3.1.3.1 Activated Sludge
The activated sludge process puts the wastewater through three stages. The first is a
dispersed-growth reactor that contains microorganisms kept in suspension that will be
aerated vigorously with the wastewater, which also supplies oxygen to the microorganisms
(FAO, N.D). Pumping oxygen and the use of surface aerators in the reactor are some
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techniques that can be used to aerate the liquid (SSWM, N.D). After the wastewater has
gone through aeration, it is sent to the secondary clarifiers to separate the microorganisms
from the liquid through sedimentation. The last stage will have a percentage of the sludge at
the bottom of the clarifier recycled into the reactor with the remainder to be processed (FAO,
n.d).
Fig 6: Activated Sludge Process (Tilley et al. 2014, 124)
3.1.3.2 Trickling Filter
A trickling filter consists of a cylindrical tank that is filled with high specific surface material
such as stones, gravel, plastic shapes or wooden slats that creates a substantial area for the
formation of biofilm (Tilley et al. 2014, 120). A biofilm is a thin layer of microorganisms and
when the wastewater is trickled over the biofilm, the microorganisms react with the organic
containments (Tilley et al. 2014, 120). Adequate ventilation that can be either forced or
natural air ventilation must be provided so that the microorganisms can receive sufficient
oxygen to perform oxidation (New Mexico Environment Department 2007, 7-3). The
wastewater collected at the bottom is channelled into the secondary clarifier.
Fig 7: Trickling Filter Process (Tilley et al. 2014, 120)
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3.1.3.3 Rotating Biological Contactors
Rotating biological contactors (RBCs) are fixed-film reactors made up of closely spaced
series of rotating discs mounted on a horizontal shaft that is partially submerged in the
wastewater (NSFC 2004, 3). The discs are generally made of high-density plastic sheets
with ridged, corrugated or surfaces to increase the surface area (NSFC 2004, 3). RBC uses
a similar process to that of Trickling Filters with a biofilm attached to the rotating discs. The
liquid is aerated when wastewater flows through the RBC, which simultaneously supplies
oxygen to the biofilm and wastewater (FAO, N.D).
Fig 8: Rotating biological contactor (NSFC 2004, 3)
3.1.4 Tertiary Treatment
Tertiary treatment is employed when specific wastewater compounds and nutrients need to
be removed to produce effluent close to the quality of drinkable water (FAO, n.d; World Bank
Group, N.D). Although the primary and secondary treatment stages remove the majority of
BOD and suspended solids, there remain a significant percentage of Chemical Oxygen
Demand (COD) that requires to be treated to reach a high level of effluent (Mountain Empire
Community College, n.d). COD is similar to BOD and measures the total amount of
chemicals in the wastewater that is oxidised (Abdel-Raouf, Al-Homaida and Ibraheem 2012,
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263). In addition, the nutrients that remain in the secondary effluent such as nitrogen and
phosphorus if not removed can accelerate plant growth in the receiving water bodies.
The treatment process can be either biological or chemical with the latter being very costly
and the process employs advanced techniques such as chemical precipitation, ozonation,
reverse osmosis or carbon adsorption (Abdel-Raouf, Al-Homaida and Ibraheem 2012, 260).
There are instances where these advance treatment techniques are combined in the earlier
stages such as adding chemicals in the primary clarifiers to remove phosphorus (FAO, n.d).
3.1.5 Disinfection
In the final stage before being discharged, chlorine is normally injected into the effluence
to ensure that all pathogens are destroyed (World Bank Group, N.D; Abdel-Raouf, Al-
Homaida and Ibraheem 2012, 261; Rawat, Kumar and Bux 2011, 3412-3413). Chlorine
contact chambers are usually comprised of rectangular channels that provide a chlorine
contact time of about 30-120 minutes depending on the irrigation needs (FAO, N.D).
Fig 9: Clarifier in primary treatment (RWRD, 2011)
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3.2 Advantages and disadvantages of conventional wastewater
treatment
Conventional wastewater treatment technologies are proven and offer certain advantages of
being highly efficient and having low land requirements. For example, activated sludge can
operate across a range of organic and hydraulic loading rates with a high rate of reduction of
BOD and pathogens (EAWAG and Spuhler, n.d). In addition, the land area required by
conventional wastewater treatment plants is relatively small due to lower Hydraulic Retention
Time (HRT) (Muga and Mihelcic 2008, 444). This makes conventional plants suitable for
urban areas or locations where land is at a premium. However, conventional wastewater
treatment also presents certain disadvantages:
3.2.1 High energy consumption and cost
The treatment of wastewater is an energy intensive activity (ACEEE, n.d) with the
mechanical process consuming the most energy (Menendez n.d, 1-9). The treatment of
wastewater is also infrastructure intensive that requires investment into expensive
equipment and treatment systems (Barry 2007, 2). These can be barriers for many
developing countries.
3.2.2 Loss of valuable nutrients and cost of sludge treatment
Conventional wastewater treatment facilities produce large volumes of sludge from the
extraction of pollutants from the wastewater. This includes the removal of nutrients such as
nitrogen and phosphorus that could be reused as valuable additives and supplements if
treated with the right treatment process. The collected waste sludge requires treatment and
needs to be disposed of, which can be a costly procedure. For secondary treatment plants in
Europe, it is estimated that half the costs of operation is associated with the treatment and
disposal of sludge (FAO, n.db).
The composition of the sludge is dependent on the characteristics of the wastewater, which
in turn determines the treatment required. For example, sludge with high levels of heavy
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metals is not only difficult to treat but the potential for reuse is limited with loss of valuable
nutrients (UNEP, n.db). There are also instances where effluence containing nutrients is
discarded into receiving water bodies, which leads to loss of valuable resources (Phang,
1990, 418).
3.2.3 Secondary pollution
The use of chemicals in the treatment of wastewater is not only costly but can often lead to
secondary pollution (Abdel-Raouf 260). In addition, if untreated sludge is not disposed of in
the proper manner it can seep into water bodies causing secondary pollution (Akyapi and
Erdincler n.d, 1)
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4 Phycoremediation
Phycoremediation can be broadly defined as the use of algae in wastewater to eradicate
pollutants that includes nutrients and xenobiotics from wastewater (Rawat Kumar and Bux
2011, 3415). There have been many studies conducted on the feasibility of microalgae for
the treatment of municipal wastewater with extensive work conducted on the removal of
heavy metals and valuable nutrients from effluents. These nutrients and containments if not
removed would otherwise be converted in to waste or dumped into receiving water bodies
causing eutrophication (Rawat Kumar and Bux 2011, 3415; Munoz and Guieyssea 2006,
2799-2815).
4.1 Microalgae
There are thousands of different strands of algae, but they can be broadly classified as
macroalgae or microalgae. As the name suggest, macroalgae are larger in size and are
aquatic plants such as seaweed while microalgae are unicellular organisms (EUBIA, n.d)
that can thrive in both sea and fresh water environments. Although both types of algae can
be used to produce biofuels, microalgae offer greater advantages as a feedstock especially
in the case of cultivation in wastewater. The main mineral components of a typical
microalgae cell are proteins, carbohydrates, lipids and other valuable minerals (Brown et al.
1997, 320). Valuable minerals include pigments, antioxidants and vitamins (Priyadarshani
and Rath 2012, 89–100).
Fig 10: Components of microalgae (Schmid-Staiger 2009)
Microalgae
Protein
Animal Feed
Fertiliser
Lipids
Biofuel
Valuable Minerals
Pharmaceutical
Cosmetic
Food
Carbohydrates
Biofuel
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In many strains of microalgae, proteins and carbohydrates can make up close to half of a
cell’s dry weight while lipid content can reach a maximum of 40% of its dry weight (Singh
and Gu 2010, 2602-2603). The varying high content levels of these key minerals make
microalgae an ideal feedstock in biofuel production. In addition, the cellular structure of algae
enables it to readily absorb nutrients that allow it to rapidly increase its mass and multiply at
a very fast pace. This makes it very suitable for industrial applications (Li et al. 2008, 815-
816).
Table 2: Composition of dry microalgae biomass (Dragone et al. 2010, 1358)
There has been a growing interest in the use of microalgae as a “third generation biofuel”
due to the advantages it offers over traditional biomass. Advantages of microalgae include
having high lipid content, CO2 mitigation and consumption of less water compared to other
land farmed biomass (Li, et al. 2008, 816; Rawat, Kumar and Bux 2011, 3412).
Biomass Area to produce global oil
demand in [ha* ] Percentage of worlds arable land to provide global oil demand in
Soybean 11620 842
Mustard Seed 9060 656
Sunflower 5440 394
Rapeseed 4350 315
Jatropha 2740 198
Palm oil 870 63
Algae (low eff.) 430 31
Algae (mod. eff.) 50 4
Table 3: Projected area and percentage of arable land required to replace the worlds’ oil demand with
biodiesel (Andersson, Broberg and Hackl 2011, 3)
Page | 25
4.2 Microalgae used in wastewater treatment
The use of biological treatment on wastewater is considered the least expensive treatment
method while the most environmentally appropriate (Mantzavinos and Kalogerakis 2005,
290). According to Lundquist et al. (2010), there are several thousand small (< 10 hectare)
and a few large scale (>100 hectare) algae pond systems being operated in the United
States (US) with their primary function to provide dissolved oxygen for the bacterial
breakdown of the wastes. However, harvesting of the algae is only practiced on some larger
ponds due to the high cost associated with harvesting and separation of algae from the
effluence.
The focus on energy security, climate change and alternate sources of fuel has brought
about renewed interest in cultivating and harvesting of microalgae. The emphasis on the
removal of nutrients over the traditional method of oxidizing the organic material in
wastewater has also helped push microalgae to the forefront and creates the opportunity for
wastewater treatment with microalgae to be combined with the production of biofuels
(Lundquist et al. 2010, 7).
4.3 Challenges of coupling microalgae with wastewater treatment
Although research of coupling microalgae with wastewater treatment presents many
advantages that will be discussed in the following section, there are also some challenges
that need to be addressed for such a hybrid plant to be commissioned. Microalgae require
the consumption of certain primary nutrients and micronutrients to multiply, which can raise
the overall cost should these nutrients need to be added in significant amounts to promote
growth (Christenson and Sims 2011, 688). The lack of carbon in most domestic wastewater
together with photosynthesis by the microalgae can also inhibit the growth of the microalgae
affecting the treatment of the wastewater (Craggs et al. 2011, 8). In addition, microalgae
pond systems need larger areas of land compared to other sewage treatment methods
Page | 26
(Oilgae 2009, 25). This will affect the design and implementation in urban areas where large
pieces of land are limited and land prices are high.
The most pressing challenge is the cost effective harvesting and processing of the
microalgae to useful bioproducts (Christenson and Sims 2011, 688). Extracting the
unicellular microalgae, which is suspended in large volumes for water is why harvesting
accounts for a significant portion of biomass production (Griffiths et al 2011, 184). The size
of the cells and their neutral buoyancy also add to the challenge of separation (Griffiths et al
2011, 184).
4.4 Advantages of microalgae wastewater treatment over conventional
treatment
Microalgae not only address the shortcomings of conventional wastewater treatment, but
also offer many advantages that encourage the use of it in wastewater treatment.
4.4.1 Removal of nutrients
In addition to CO2 and sunlight, the main nutrients required for microalgae growth
are nitrogen and phosphorous (FAO 2009, 22) and a number of micronutrients (Knud-
Hansen 1998, 16). The principal form of nutrients in wastewater is ammonia (NH4), nitrite
(NO−2), nitrate (NO−3) and orthophosphate (PO43−) (Bhatt et al. 2014, 6). According to de la
Noue, Laliberté and Proulx (1992), the concentration of nitrogen and phosphorous in
municipal wastewater is 10–100 mg L_1 and greater than 1000 mg L_1 in agricultural
effluent. Nitrogen pollution in sewerage effluent is mainly derived from metabolic
interconversions of extra derived compounds while half or more of phosphorus comes from
synthetic detergents (Abdel-Raouf, Al-Homaida and Ibraheem 2012, 263). These hard to
remove nutrients are absorbed by microalgae as a food source, which then greatly helps the
remediation of the wastewater.
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In a study conducted by Lau et al. (1996) on Chlorella vulgaris in the removal of nutrients,
the results indicated a nutrient removal efficiency of 86% and 70% for inorganic nitrogen and
phosphorus. Colak and Kaya (1988) also reported that for industrial wastewater treatment,
removal efficiency was 50.2% and 85.7% for nitrogen and phosphorus respectively while
phosphorus removal was 97.8% in domestic wastewater treatment (cited in Abdel-Raouf, Al-
Homaida and Ibraheem 2012, 2634). These results suggest that effectiveness of microalgae
and the inability to remove these nutrients can lead to eutrophication in receiving water
bodies.
Wastewater Source Total N
removal
Total P
removal
Carbon
removal
Retention
time
Reference
Chlorella+Nitzchia Domestic WW after settling 92 % 74 % 97% (BOD)
87% (COD)
10h (McGriff Jr. &
McKinney 1972)
Chlorella
pyrenoidosa
Domestic WW after settling 94 % 80 % NA 13 days (Tam & Wong
1989)
Chlorella
pyrenoidosa
Domestic and industrial WW 60-70 % 50-60 % 80-88% (BOD)
70-82% (COD)
15 days (Aziz &
Ng 1992)
Cyanobacteria Domestic effluent after secondary
treatment + swine WW after settling
95 % 62 % NA 1 day (Pouliot et
al. 1989)
Table 4: Nutrient removal from wastewater by different microalgae strains (Andersson, Broberg and
Hackl 2011, 34)
In the case of nitrogen pollution that leads to the release of ammonium (NH4+) or nitrate
(NO3−) during biodegradation (Muñoz and Guieyssea 2006, 2800), microalgae can help in
the removal of these nutrients. Based on two studies by Muñoz et al. (2005a) and Muñoz et
al. (2005b), for every mole of acetonitrile (CH3CH) biodegraded, the net amount of NH4+
produced decreased to 0.46 mol mol−1 in photosynthetically oxygenated batch processes
from 0.74 mol mol−1 in mechanically aerated batch processes due to the assimilation of
algae. In another study conducted by Al-Balushi et al. (2012), the number of Trentepohlia
aurea cells increased considerably in wastewater while the volume of nitrate declined in
relation to growth of the cells.
4.4.2 Removal of pathogens
The simultaneous uptake of CO2, H+ ions and bicarbonate can an increase the pH in the
wastewater (Oilgae, 10). The change in the pH to 9.2 for 24 hours is known to totally
eradicate E.coli and most pathogenic bacteria (Ertas and Ponce, n.d).
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4.4.3 Photosynthetic aeration
The photosynthetic process aids in the remove of BOD and COD. Mechanical aeration is
very energy intensive (Menendez n.d, 1-9) and the use of photosynthetic aeration can help
reduce the consumption of energy and associated cost (Kiepper, 2013; Mallick, 2002). The
essential driver in the treatment of wastewater at conventional plants is the adequate supply
of oxygen (O2) to the microorganisms to support the decomposition of organic and inorganic
compounds (Kiepper, 2013). In the photosynthetic process, microalgae produce O2 to aid in
the decomposition of organic pollutants and consume CO2 released from bacterial
respiration (Muñoz and Guieyssea 2006, 2799). Colak and Kaya (1988) studied the
biological treatment of wastewater by algae and found that elimination of BOD and COD
were 68.4% and 67.2% respectively in domestic wastewater treatment (cited in Abdel-Raouf,
Al-Homaida and Ibraheem 2012, 263).
Fig 11: Principle of photosynthetic oxygenation in BOD removal process (Muñoz and Guieyssea 2006,
2799)
4.4.4 Removal of heavy metals
The effluents from the industrial sector are known to contain significant amounts of toxic
metal ions such as mercury, lead, cadmium and chromium (VI) that pose a significant health
risk to humans and animal (Ahluwalia and Goyal 2007, 2244). The discharge of these toxic
pollutants has steadily increased with the growth of the industries (Abdel-Raouf, Al-Homaida
and Ibraheem 2012, 265).
There are several strains of algae that are efficient absorbers that bind and concentrate
heavy metals acting as an ion exchanger of biological origin even from dilute aqueous
Page | 29
solutions (Ahluwalia and Goyal 2007, 2244). The efficiency of absorption is dependent on
various factors that include the physical and chemical conditions in effluents and strain of
algae used (Ahluwalia and Goyal 2007, 2245). Cañizares-Villanueva (2000) reported that
specific metal uptake of 15 mg gBiomass−1 at 99% removal efficiency makes the use of algae
competitive to other treatment avaliable methods (cited in Muñoz and Guieyssea 2006,
2801).
Metal Biomass Accumulation capacity
(mg gBiomass−1
)
Adsorption removal rate (mg l
−1 d
−1)
Experimental set-up Reference
Zn Chlorella vulgaris — 114.2 1-l column reactor with microalgae immobilized in κ-carrageenan
Travieso et al., 1999
Cr Scenedesmus acutus — 3.5
Cd Chlorella vulgaris — 2.5
Co Scenedesmus obliquus 0.82 Rotary biofilm reactor Travieso et al., 2002
Zn Euglena gracilis 7.5 — 500-ml E-flasks, free microorganisms
Fukami et al., 1988
Cd Chlorella Homosphaera 8.4 1.44 500-ml E-flasks, free microorganisms
Zn Chlorella Homosphaera 15.6 2.67 Costa and Leite (1990)
Cd Chlorella vulgaris 2.6 — 1-l E-flasks, free microorganisms
Khoshmanesh et al. (1996)
Chlorella pyrenoidosa 2.8 —
Chlamydomonas reinhardtii
2.3 —
Al Scenedesmus subspicatus
6.8 — 50-ml polyethylene-flasks, free microorganisms
Schmitt et al., 2001
Cd 7.3 —
Cu 13.2 —
Hg 9.2 —
Cd Chlorella sorokiniana 192 — Column reactor with algae immobilized on a vegetable sponge
Akhtar et al., 2003
Table 5: Reported studies on heavy metal accumulation by microalgae (Muñoz and Guieyssea 2006,
2802)
4.4.5 Reductions in sludge formation
The cost of disposing sludge produced in conventional wastewater treatment plants during
the primary, secondary and tertiary stages of treatment is very high. In addition, the sludge is
not only comprised of organic waste material but also substances that can be toxic such as
pathogenic bacteria and viruses that can pose a health risk (FAO, N.Db). In a microalgae
wastewater treatment plant, the volume of sludge is greatly reduced as the microalgae
absorb and metabolize the nutrients. The resulting sludge is energy rich that can be refined
to produce biofuel and other high valuable products (Zhou, 2014).
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4.4.6 Green House Gases reduction and CO2 mitigation
Wastewater treatment plants are major emitters of greenhouse gases (GHG) of CO2,
Methane (CH4) and Nitrous Oxide (N2O), which are released during the different treatment
processes (Gupta and Singh 2012, 131-132). CO2 is released during the anaerobic and
aerobic treatment, while CH4 is released during the degrading of sludge and N2O when
nitrogen found in the wastewater is degraded (Gupta and Singh 2012, 132-133). Conversely
CO2 is essential to the growth of microalgae. Microalgae can absorb CO2 from the
atmosphere, industrial exhaust gases and soluble carbonates such as NaHCO3 (Wang et al.
2008, 708-709). Harmelen and Oonk (2006) estimated that the global technical potential of
CO2 abatement in 2020 from wastewater is 40 million ton/year.
4.4.7 Removal of coliform bacteria
In a study conducted by Moawad (1968), it was observed that the environmental factors that
encouraged algae growth adversely affected the survival of coliforms. Coliform bacteria are
an unlikely source of illness, but their presence is an indication that pathogens could be in
the wastewater (Washington Department of Health, 2011). Pathogenic viruses, protozoa and
bacteria such as Salmonella and Shigella are of major concern (UNEP, n.d). Results from
experimental studies indicate that pathogenic bacteria have a faster die-off rate in the
environment than coliforms while viruses tend to survive longer (Abdel-Raouf, Al-Homaida
and Ibraheem 2012, 263). Therefore, the level of coliform bacteria in the water is used as an
indicator to the quality level of the water.
4.4.8 Lower energy requirement and cost
By combining the cultivation of microalgae with wastewater treatment, it leverages on the
strengths of each process to raise overall efficiency whilst lowering cost. As discussed,
microalgae plays an important role during the tertiary treatment process by enhancing the
removal of nutrients, BOD, heavy metals and pathogens that helps to lower energy
consumption and creates better efficiency in the wastewater treatment process (Abdel-Raouf,
Page | 31
Al-Homaida and Ibraheem 2012, 262; Rawat, Kumar and Bux 2011, 3414-3415; Pittman,
Dean and Olumayowa Osundeko 2011, 17-25). Oswald (2003) found that one kilogram (kg)
of BOD removed by the photosynthetic process requires no energy inputs while producing
enough algal biomass to produce one kWh of electricity from methane. In contrast to having
to use one kWh of electricity for aeration to remove about one kg of BOD, which also
produces one kg of CO2 from fossil fuel for electricity generation (cited in Nandeshwar and
Satpute 2014).
4.4.9 Production of useful biomass
The most attractive property of microalgae is the diversity of products that it can produce.
Microalgae can be used as animal feed, in medical applications and be refined for biofuels
such as biodiesel, biogas or bioethanol (Lundquist, Woertz, Quinn and Benemann 2010, 1).
This creates different channels of demand that makes cultivating microalgae financial
attractive.
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5 Cultivating Microalgae in Wastewater
One of the biggest challenges to produce biofuels from microalgae is cultivating large
volumes of the right strain of microalgae. Feedstock is a major contributor to the overall cost
of the producing biodiesel that can range from 75% – 88% of total cost (Singh et al. 2014,
218). Selecting the right strain is dependent on the desired end result. Different strains of
microalgae contain different levels of mineral components that make them better suited for
different end products.
Microalgae completes an entire growth cycle every few days. Following the growth curve as
shown in Fig 12, the microalgae goes through the growth stages of lag, exponential, linear,
stationary and the death phase (Mata, Martins, Caetano 2010, 223). Under optimal growth
conditions, microalgae can double in volume within 24 hours while growth in the exponential
phase takes around 3.5 hours (Andersson, Broberg and Hackl 2011, 18). The curve also
highlights the depletion of nutrients in relation to the growth of the microalgae.
Fig 12: Microalgae growth in batch culture against concentration of nutrients represented by dotted
line (Mata, Martins, Caetano 2010, 223).
A study conducted by Palmer, C.M., 1974, published in the Revista de Microbiologia found
that Chlorella, Ankistrodesmus, Scenedesmus, Euglena, Chlamydomonas, Oscillatoria,
Micractinium and Golenkinia were present in wastewater (citied in Abdel-Raouf, Al-Homaida
and Ibraheem 2012).
Page | 33
5.1 Factors affecting growth of microalgae
Although microalgae are a hardy species and can grow in extreme environments, there are
many factors such as availability of compounds and nutrients in the wastewater (Aravantinou
et al 2013, 1), temperature, salinity and sunlight intensity that if not managed will affect the
maximum growth potential of microalgae (Li and Wan 2011, 2). This is especially so in the
case of growing microalgae in open ponds outside of laboratory conditions.
5.1.1 Climate conditions
Understanding the climate is important in determining the best type of system that will create
optimal growth conditions for the microalgae to grow in. Climatic conditions of sunlight
intensity and ambient temperature have a direct affect on the growth of the microalgae and
vary from geographical location.
5.1.1.1 Sunlight intensity
As in all types of plants, photosynthesis is how microalgae create energy. The
photosynthesis reaction as shown below highlights the importance of sunlight and CO2 in
creating the building blocks of the microalgae cell and oxygen (Li and Wan 2011, 2).
6CO2 + 6H2O + sunlight → C6H12O6 + 6O2 (Li and Wan 2011, 2)
However, there is a limiting factor to the solar conversion efficiency of photosynthesis in
microalgae. The rate of photosynthesis as a function of light intensity shows a linear
increase at low intensity of light, but starts to levels off when the light intensity reaches the
light saturation point and slowly declines as photosynthesis is photoinhibited as shown in Fig
13 (Grobbelaar, 2013; Lundquist et al. 2010, 14-16). The simplest solution would be to
increase the surface area by placing them in vertical photobioreactors. However, this is not a
practical solution for large scale production due required land area and higher associated
cost (Lundquist et al. 2010, 14-16).
Page | 34
Fig 13: The photosynthetic irradiance response graph (Grobbelaar, 2013)
5.1.1.2 Temperature
After sunlight, temperature is the next important factor in creating the ideal environment for
cultivating microalgae. The air temperature has a direct affect on the water temperature that
should be in the range of 25–35°C for optimal growth (Li and Wan 2011, 2). Many strains of
microalgae are able to tolerate temperatures of up to 15°C lower than their optimal
temperature, but have a poor tolerance of temperatures that exceed their optimal
temperatures by 2-4°C (Mata, Martins, Caetano 2010, 223). Using strains of microalgae that
maintain high productivity at lower and high temperatures and adapt quickly to diel
temperature are a solution to overcome temperature limitations in mass cultivation
(Lundquist et al. 2010, 22).
5.1.2 Carbon Dioxide
One challenge is that domestic wastewater has low levels of carbon present, which affects
photosynthesis of CO2 during the day thus preventing efficient nitrogen removal (Craggs et
al. 2011, 8). This in turn adversely affects the growth rate of the microalgae. A solution is to
pump in CO2 to reverse this trend and double microalgae productivity (Andersson, Broberg
and Hackl 2011, 37). The optimal concentration of CO2 is 350–1000 ppm (Li and Wan 2011,
2).
Page | 35
5.1.3 Evaporation and salinity
The salinity or pH range differs from each strain of microalgae, but the preferred range to be
maintained is between 7 and 9 (Li and Wan 2011, 2). Evaporation affects the blow down
ratio (BDR) that measures the level of salinity for optimal microalgae productivity (Lundquist
et al. 2010, 39). For example, a low BDR of 0.1 results in effluent salinity to be ten times
higher that of the influent water while a high BDR of 0.9 produces effluent salinity of only 10%
higher than the influent water(Lundquist et al. 2010, 39). The easiest way to control the
levels is to add water or salt as required (Mata, Martins, Caetano 2010, 223).
5.2 Microalgae for biodiesel production
Lipid content plays a vital role in the synthesis of biodiesel as the amount extracted
determines the quality and amount of biodiesel that is produced. Lipid content in microalgae
is also different to traditional biomass as it constitutes of hydrocarbon, alcohol, wax and
alkane (Singh et al. 2013, 221). This makes the process of synthesizing biodiesel from
microalgae more complicated and increasing the cost.
Even within strains of microalgae that are suited for biodiesel production, each strain has
different levels of productivity and lipid production. There is an inverse reaction between high
lipid production and productivity. When environmental stress is placed on the microalgae to
increase lipid content, microalgae diverts energy to produce lipid thus affecting productivity and
vice versa (Singh et al. 2013, 222). Therefore, a fine balance has to be struck to ensure sufficient
productivity with sufficient lipid content. Nutrient limitation is commonly used to increase lipid
content, such as limiting the amount of nitrogen (Griffiths, Hill & Harrison 2012, 990). It is an
affordable and easily controllable method that has shown significant results in producing
higher levels of lipid content (Griffiths, Hill & Harrison 2012, 990).
Research has shown that Botryococcus Braunii (B. Braunii) has the highest oil content but
low productivity (Singh et al. 2013, 222). However, B. Braunii has been found unsuitable for
Page | 36
biodiesel production due to its chain length of more than C30 (Griffiths and Harrison 2009,
495). In another study that placed 9 strains under a nitrogen depletion scenario,
Scenedesmus Obliquus and Chlorella Vulgaris showed good promise with having over 35%
of their dry weight as Triacylglycerol and achieved the highest average productivity at 322
and 243 mg l1 day1 respectively (Breu et al. 2012, 225). An intensive study by Zhou et al.
(2011) identified 17 strains of microalgae such as Chlorella sp., Heynigia sp., Hindakia sp.,
Micractinium sp., and Scenedesmus sp. out of an initial 60 strains that were tolerant to
wastewater treatment. The results also showed that Auxenochlorella protothecoides and
Scenedesmus sp. stood out in terms of maximum growth rate and productivity (Zhou et al.
2011, 6913 -6915).
Wastewater type Microalgae species Biomass productivity (mg L_1 day_1)
Lipid content (% biomass)
Lipid productivity (mg L_1 day_1)
References
Municipal (centrate) Chlamydomonas reinhardtii (biocoil-grown)
2000 25.25 505 Kong et al. 2010
Municipal (secondary treated) Scenedesmus obliquus
26a 31.4e 8e Martinez et al. 2000
Municipal (secondary treated) Botryococcus braunii 345.6b
17.85 62 Orpez et al. 2009
Municipal (primary treated + CO2)
Mix of Chlorella sp., Micractinium sp., Actinastrum sp.
270.7c 9 24.4 Woertz et al. 2009
Agricultural (digested dairy manure, 20_ dilution)
Chlorella sp. 81.4d 13.6e 11e Wang et al. 2010
Industrial (carpet mill, untreated)
B. braunii 34 13.20 4.5 Chinnasamy et al. 2010
Industrial (carpet mill, untreated)
Chlorella saccharophila
23 18.10 4.2 Chinnasamy et al. 2010
Industrial (carpet mill, untreated)
Dunaliella tertiolecta 28 15.20 4.3 Chinnasamy et al. 2010
Industrial (carpet mill, untreated)
Pleurochrysis carterae
33 12.00 4.0 Chinnasamy et al. 2010
Artificial wastewater Scenedesmus sp. 126.54 12.8 16.2 Voltolina et al. 1999
a Estimated from biomass value of 1.1 mg L_1 h_1. b Estimated from biomass value of 14.4 mg L_1 h_1. c Estimated from biomass value of 812 mg L_1 after 3 days. d Estimated from biomass value of 1.71 g L_1 after 21 days. e Fatty acid content and productivity determined rather than total lipid.
Table 6: Biomass and lipid production of microalgae in different wastewater conditions (Adapted from
Pittman, Dean and Osundeko 2011, 21)
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5.3 Microalgae for bioethanol production
Carbohydrates are important in the process of converting microalgae into bioethanol. It has
been reported that an estimated 5,000–15,000 gal/acre of bioethanol per year can be
harvested from microalgae, which is several times larger in comparison with other feedstock
as shown in Table 7. Carbohydrates in microalgae are generally composed of starch,
glucose, cellulose and various kinds of polysaccharides (Yen et al. 2013, 167). Certain
strains of microalgae have the ability to produce higher levels of carbohydrates over lipids,
which make them better suited bioethanol production (Nguyen and Vu 2012, 26).
Source Ethanol yield (gal/acre) Ethanol yield (L/ha)
Corn stover 112–150 1,050–1,400
Wheat 277 2,590
Cassava 354 3,310
Sweet sorghum 326–435 3,050–4,070
Corn 370–430 3,460–4,020
Sugar beet 536–714 5,010–6,680
Sugarcane 662–802 6,190–7,500
Switch grass 1,150 10,760
Microalgae 5,000–15,000 46,760–140,290
Table 7: Ethanol yield from different sources (Mussatto et al. 2010, 826)
Microalgae strains like Porphyridium, Chlorella, Dunaliella, Chlamydomonas, Scenedesmus
and Spirulina contain considerable levels of starch and glycogen that are essential in the
production of bioethanol (John et al. 2011, 188). Hirano et al. (1997) found that out of 250
strains of microalgae, Chlorella vulgaris had a high starch content of 37% and when
fermented with yeast proved to be a good source for ethanol production.
Studies by Dragone et al. (2011) and Kim et al. (2014) have shown that limitation of nutrients
such as sulfur can increase the accumulation of carbohydrates while nitrogen limitation
produced the best increased yields in both studies. In Kim et al. (2014), nitrogen limiting
produced a carbohydrate increase of 16 - 22.3% of the total content for C. vulgaris. Increase
in yields could be attributed to the microalgae using the available nitrogen to synthesis
enzymes and essential cell structures with any future CO2 being converted into
carbohydrates and lipids instead of proteins (Dragone et al. 2011, 3333).
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5.4 Summary
Selecting the right algae strain is the first and most important step in the production of
microalgae on a large scale. The local environmental conditions, composition of the
wastewater and what is the intended use of the microalgae are all important factors that
need to be taken into account when selecting the most suitable strain. Ignoring these factors
will be detrimental to the design and economics of the plant.
5.5 Waste stabilisation ponds
Waste Stabilization Ponds (WSPs) are solitary or a series of man-made water bodies that
are mechanically aerated or use natural oxidation to treat wastewater (Tilley et al. 2014, 110;
Kiepper, 2013). The World Bank supports the use of waste stabilisation pond technology as
first choice for sewage treatment (Ashworth and Skinner 2011, 12). The major disadvantage
of WSP is the large amount of land that is required, which is about 3 – 5m² per person
varying on the composition of sewage and temperature (The World Bank Group, n.db Ponds
& wetlands). However, WSPs are especially suited for developing countries that have a
shortage of skilled labour and cost of land is relatively cheap (FAO, n.dc).
Natural oxidation ponds aerobically breakdown the organic matter in wastewater using
dissolved oxygen produced by microalgae (Kiepper, 2013). Four critical parameters of
temperature, net evaporation, flow and BOD affect the design parameters such as depth,
shape and layout of WSP (Kayombo et al. n.d, 8). There are 3 different types of natural
oxidation ponds of facultative ponds, maturation ponds and high rate algae ponds.
5.5.1 Facultative treatment ponds
Facultative treatment ponds (FPs) are the simplest of all WSPs and consist of an upper
aerobic zone close to the surface and anaerobic zone at a lower depth (Kiepper, 2013). The
upper zone absorbs oxygen from natural diffusion, wind mixing and photosynthesis of the
Page | 39
microalgae while the lower zone is deprived of oxygen and becomes anaerobic (Tilley et al.
2014, 110) as shown in Table 8.
Fig 14: Facultative treatment pond (Kiepper, 2013)
FPs can be either built as a primary pond to receive raw wastewater or secondary ponds
that receive treated wastewater (Kayombo et al. n.d, 7). The ponds are designed to remove
BOD within the range of 100-400 kg BOD/ha/day (Kayombo et al. n.d, 8) and are located in
open areas so as ensure that sufficient volume of surface wind is able to sweep over the
pond to support mixing (Kiepper, 2013).
Fig 15: Facultative treatment pond (Tilley et al. 2014, 110)
Surface Loading
(kg BOD ha-1d-1)
Population per ha Detention
(days)
Climate conditions
< 10 < 200 > 200 Frigid zones with seasonal ice cover, uniform low water
temperatures and variable cloud cover
10-50 200-1000 200-100 Cold seasonal climate with seasonal ice cover,
temperate
summer temperatures for short duration
50-150 1000-3000 100-33 Temperate to semi-tropical climate, rare ice cover, no
prolonged cloud cover
150-350 3000-7000 33-17 Tropical, uniformly distributed, consistent sunlight and
temperature, no seasonal cloud cover
Table 8: Generalised loading and design criteria for FPs constructed in different climate zones
(Kiepper, 2013)
Page | 40
5.5.2 Maturation treatment ponds
Maturation treatment ponds (MPs) are aerobic ponds that are mainly used as a final or
tertiary wastewater treatment to improve the effluent quality (Kiepper, 2013). The primary
function of MPs is the removal of pathogenic organisms, with only low removal of BOD due
to low organic containments (Kayombo et al. n.d, 16). MPs are very shallow with a depth of
0.5 to 1.5 m deep and a detention time of 15 to 20 days (EAWAG and Spuhler, n.d). The
microalgae population in FPs tends to be more diverse compared to FPs with non-motile
genera tending to dominate (Kayombo et al. n.d, 16). The ability of MPs to manage and
lower the effects of fluctuations and toxic loads in the effluents make them cost effective
intermediaries (Kiepper, 2013). Disadvantages of using MPs include the microalgae
becoming an added BOD load and if the microalgae is not filtered in the right manner will
flow out with the effluent that may contravene discharge regulations.
Fig 16: Maturation treatment pond (Tilley et al. 2014, 110)
5.5.3 High rate algae ponds
High rate algae ponds (HRAPs) are open ponds that have been used in the treatment of
wastewater since the 1950s at various levels (Park, Craggs and Shilton 2011, 35). While
there are other ponds systems, HRAPs are the most cost effective solution for the
management of wastewater and capture of solar (Rawat, Kumar and Bux 2011, 3416; Abdel-
Raouf, Al-Homaida and Ibraheem 2012, 267).
HRAPs are designed to encourage the growth of microalgae by being built shallow (0.2 –
0.5m) with a raceway shape and paddle wheels that are in constant operation to prevent
sedimentation (Brennan and Owende 2010, 560). This enables HRAPs to maintain the
simplicity and economic viability of conventional ponds while addressing many of their
Page | 41
challenges such as effluent quality and limited containment removal (Craggs, Sutherland
and Campbell 2012, 329). The shallow pond allows for more light saturation leading to
greater photosynthesis that in turn produces higher levels of oxygen to drive aerobic
treatment and growth of microalgae. Sunlight with the continuous fluid flow helps with the
disinfection of the wastewater (Craggs, Sutherland and Campbell 2012, 329-330).
HRAP also face certain limitations such as evaporation losses, potential for contamination
due to the ponds being open and the requirement of large amounts of land. National Institute
of Water and Atmospheric Research Ltd. (NIWA) of New Zealand launched a 5 hectare
demonstration HRAP system at the Christchurch wastewater treatment plant (NIWA, 2009).
The results of the project will help provide a clear process pathway and demonstrate the
viability of the large-scale wastewater and microalgae processing facility.
Fig 17: HRAP with CO2 addition (Park, Graggs, Shilton 2011, 36).
5.6 Photobioreactors
Photobioreactors (PBRs) are closed systems that provide a controlled environment to
support the production of microalgae. PBRs can overcome some of the issues faced by
HARPs by facilitating better control over the environmental elements such as water supply,
carbon dioxide, optimal temperature and efficient exposure to light (Oilgae, N.D). This helps
improve the level of productivity (Singh and Gu 2597). Other benefits include prevention of
pollution and the cultivation of a single-species of microalgae with reduced risk of
contamination (Brennan and Owende 2010, 562).
Page | 42
Although PBRs can produce a higher yield of biomass (Singh and Gu 2599), they are not
suitable for commercial scale phycoremediation owing to the significant volumes of
wastewater that require remediation (Rawat, Kumar and Bux 2011, 3417). In addition, the
equipment and technology behind the system translate to higher capital and operating
expenses (Singh and Gu 2597-2599).
5.7 Hybrid two stage production system
A solution to overcome the challenges of contamination and yield in open systems while
addressing the costs of PBRs is the introduction of a hybrid two-stage cultivation system.
The hybrid system will combine the growth stages of PBRs with the large scale cultivation in
HARPs. PBRs have an important role to play as key bioreactors for small scale cultivation
(Rawat, Kumar and Bux 2011, 3417) that ensures there is minimal potential for
contamination from other strains and organisms during the initial growth stages. Once the
microalgae cultures have grown to a certain volume, they will be transferred from the PBRs
to the HARPs for large scale cultivation. Huntley and Redalje (2007) deployed such a hybrid
system that used Haematococcus pluvialis to produce oil and astaxanthin.
Page | 43
6 Southeast Asia (SEA) – Vietnam as a Case Study
In the last decade, Vietnam has experienced rapid economic growth by adopting a market
economy that has led to rapid urbanization. Vietnam has a population of about 88 million
(GSO, 2012a) with a population growth rate of 1 % (CIA, 2014). Close to 31% of the
population live in urban areas with an annual urbanization rate of 3.03% (CIA, 2014).
Agriculture, forestry and fishing are corner stones of the country’s economy and together
with Industry have a combined output of 57% (GSO, 2012b). The majority of the population
continues to live in rural areas and is dependent on the land for their livelihood with
agriculture, forestry and fishing employing 48% of the workforce (CIA, 2014).
Vietnam has a tropical climate with high temperatures not varying too greatly throughout the
year and constant solar radiation. The country has high annual rainfall a single rainy season
from May to September (CIA, 2014). Vietnam’s average total solar radiation is about
4kW/h/m²/day in the northern part of the country and 5kW/h/m²/day in the rest of the country
(Dung 2009, 29). The northern provinces have lower radiation due to winter and spring,
while the central and southern provinces have the sun shining all year round (VAST, 2012).
6.1 Water and wastewater
Due to Vietnam’s geographic location, more than 50% of its total water resources are
derived from outside the country (FAO 2012, 476-478). Vietnam is estimated to have
884.1km³/year of total renewable water resources (FAO, 2012) and at 71.418km³/year the
country’s internal renewable groundwater is sufficiently abundant (FAO 2012, 476-478).
Vietnam has freshwater withdrawal rate of 82 billion m³ in 2011 with agricultural the main
consumer at 95 % (World Bank, 2014). However, Vietnam has a growing pollution problem
with increased urbanization and inadequate infrastructure.
Page | 44
It is estimated that 3,080,000m³/day of domestic
wastewater is generated of which 340,000m³/day
(Nguyen 2013, slide 4) is being treated by the 17
urban wastewater systems in operation that have a
total capacity of 530,000m³/day (World Bank 2013,
24). According to reports, only 10 % of urban
wastewater is being treated (World Bank 2013, 30;
Nguyen 2014, slide 4). Treatment of industrial
wastewater is in even a worst state. Out of the
more than 300 industrial and export zones, only
15% of wastewater are treated with the untreated
wastewater being discharged directly into the
surface water (U.S Commercial Service 2013, 4).
Contaminants Concentratio
n
(mg L-1)
Total Suspended Solids
(TSS)
400
Volatile suspended solid 180
BOD5 at 20° C 140
Total organic carbon
(TOC)
290
Chemical oxygen demand
(COD)
269
Nitrogen (total as N) -
Ammonia - N 90
Organic - N 8.3
Phosphorus (total as P) -
pH 6 – 8.5
Table 9: Untreated municipal wastewater
from cities in Southern Vietnam (Raschid-
Sally 2001, 6)
To meet this rising demand, there are more than 30 new wastewater systems in the pipeline
or being constructed across the country (World Bank 2013, 24).
6.2 Energy profile and renewable energy potential
According to statistics by the International Energy Agency (IEA), Vietnam has a Total
Primary Energy Supply (TPES) of 61.21 Mtoe in 2011 out of which 17.27 Mtoe comes from
renewable resources (IEA, 2011a). Vietnam has the second largest proved oil reserves in
the Asia Pacific according to statistics compiled by BP (BP 2014, 6) and is a net exporter of
crude oil (EIA 2013). However, Vietnam is a net importer of oil products with only one
operating refinery that cannot keep pace with the growing demand for refined products (EIA
2013). In electricity generation, natural gas is the primary fuel source followed by hydro than
coal and peat. Fig 18 shows that the industry, residential and transport sectors consume the
largest amounts of electricity.
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Fig 18: Final Consumption by Sector (Pham and Tran, 2013)
Based on the statistics on renewable energy consumption, it may seem that Vietnam is
ahead in the use of renewable energy. However, a closer look at the numbers show that
primary solid biofuels are the main source of renewable energy consumed (IEA, 2011b). IEA
defines primary solid biofuels as “any plant matter used directly as fuel or converted into
other forms before combustion” (IEA, 2014). With a large rural low income population,
traditional methods of burning wood or other organic matter for heating or cooking can
account for the large consumption of renewable resources.
The Vietnam government understands the importance of energy in the growth of the country
and views the development of renewable energy specifically biofuels as important to its
energy security and protection of the environment (Vietnamese Ministry of Industry and
Trade, N.D). The government has taken leadership to grow the industry and consumption
by enacting different legislation. Decision No. 1855/QĐ-TTg, 27/12/2007, Vietnam National
Energy Development Strategy to 2020, with 2050 vision approves Vietnam’s national energy
development strategy with an aim of raising the proportion of new and renewable energies to
5% by 2020 and 11% by 2050. Decision No. 177/2007/Qd-Ttg Of November 20, 2007,
Approving The Scheme On Development Of Biofuel Up To 2015, With A Vision To 2025 lays
the framework to increase the use of biofuels, specifically bioethanol and biodiesel to
account for 1% of total filling demand by 2015 increasing to 5% by 2025.
Residential 33%
Industry 40%
Transport 22%
Commerce & Services
4%
Agricluture 1%
Page | 46
7 Economics
Cultivating microalgae with wastewater offers many advantages over conventional
wastewater treatment facilities that reduce overall energy consumption and cost. The capital
and operating cost of HRAPs are lower than that of mechanical nutrient removal systems
and are simpler to operate (Craggs et al. 2014, 70). The added financial benefit of coupling
microalgae to wastewater treatment is the revenue from the by-products from the microalgae
biomass residue.
The greatest hurdle to commercial production of microalgae for biofuel is the high production
cost that translates to high prices, which can be higher than the cost of fossil fuel. Tax
incentives given by local government on fossil fuels further reduces the cost competiveness
of renewables.
A report by the Department of Agriculture and Food of the Government of Western
Australian (2006) on the “Economics of microalgae production and processing into biodiesel”,
which was adapted from a report by van Harmelen and Oonkthat (2006) showed that
revenue from biodiesel production alone would not be able to cover cost (Schulz, 2006). In a
report by Lundquist et al. (2010) that assessed the economics of 5 scenarios in the United
States (U.S) of America, produced similar results that showed the cost of production for
facilities that focused on the production of biofuels is not sustainable. Results showed that
when biofuel production is a by-product of wastewater treatment the economic analysis was
very favourable that could have biofuel sold significantly less than the cost of oil as shown in
Fig 19.
This is largely due to the wastewater treatment credits afforded to the project. If the revenue
from the wastewater treatment is not taken into account the final cost per barrel of
microalgae oil would be US$417, which was a similar trend for all the other scenarios. In
Page | 47
addition, the construction of a wastewater treatment facility would include the supporting
infrastructure such as roads and power lines that a standalone microalgae biorefinery would
not be able to finance or cover. There are other research cases such as Gallagher (2011)
that show the production of biofuels is not financial attractive unless there is support from the
government and the continued high price of oil.
Fig 19: Total cost of production including revenue from wastewater treatment (Lundquist et al. 2010,
129)
Comparing the 5 scenarios, the overall capital and operating cost of a wastewater treatment
facility coupled with the production of oil (Case 1) is the highest due to the primary treatment
facilities and equipment for oil production that are required as shown in Fig 20 (Lundquist et
al. 2010, 128). However, this additional cost is made up through greater electricity produced
from the treatment of primary sludge. According to the report, the closure of the facilities that
focused on biofuel production due to winter conditions account for lower production figures
and production cost.
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Fig 20: Capital cost of 5 different scenarios (Lundquist et al. 2010, vii)
Fig 21: Annual operating cost of 5 different scenarios (Lundquist et al. 2010, viii)
This case emphasis that the cost of producing biofuels from facilities solely focused on such
production is not financial sustainable. It is the revenue from the treatment of wastewater
that enables the biofuels to attain a price point that is palatable for consumers.
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Currently, the water tariff and wastewater fees in Vietnam are too low that makes the
recovery of capital expenditure, operational and management costs very difficult (Duong,
201). This is a barrier to achieve a sustainable business model and attract private
investment. However, this will change with a new legislation by the Vietnamese government,
Decree No. 80/2014/ND-CP (Decree 80), that will gradually raise the prices to achieve full
cost recovery (WMP, 2014). It should also be noted that the cost of land and labour is much
lower in Vietnam, which lowers the initial capital investment and operational cost.
The renewable energy targets set by the government ensures a demand for biofuels. It has
recently been announced that E5 RON 92, which is a blend of 5% ethanol with unleaded
gasoline, will be sold across Vietnam’s network of 13,000 petrol stations by 2015 (Viet Nam
News, 2014) and having E10 to be sold nationwide by 2017 (Viet Nam News, 2013) there
will be a steady demand. In addition, the environmental benefits afforded by the use of
microalgae in wastewater treatment could leave the facility open to attracting certified
emission reduction (CER) credits. These credits can help fund the operation of the
treatment plants that increases the profitability. There have been previous wastewater
projects that have successful applied for these credits such as “Project 1971: Anaerobic
Digestion Swine Wastewater Treatment with On-Site Power Project” (UNFCC, 2009) and
“AMS-III.H.: Methane recovery in wastewater treatment” (UNFCC, 2010).
Page | 50
8 Wastewater treatment with biorefinery
Following the results from Lundquist et al. (2010), the successful model for the cultivation of
microalgae must have wastewater treatment as its primary focus. Building on that model, it is
proposed that the wastewater facility be coupled with a biorefinery to produce different
valuable by-products.
8.1 Biorefinery
Similar to the oil refining process that produces different petrochemical products, a
biorefinery can take advantage of the different constituents of microalgae to derive different
by-products at various stages of the refining process to make the cultivation and processing
of microalgae sustainable. The structure of microalgae enables a wide variety of products to
be extracted such as biofuels, food supplements, animal feed and for pharmaceuticals
(Singh and Gu 2010, 2602). The main challenge is the separation of the different fractions
without creating damage to the other fractions (Vanthoor-Koopmans et al 2013, 143).
CO2 Recycle
Carbohydrates
Lipids
PUFA
Proteins
Fine Chemicals
Nutrients
Microalgae
cultivation in
Wastewater
Processing and
extraction
Residual biomass
Crude
microalgae
oil
Transesterification
Biogas technology
Fermentation
Separation
techniques
Cracking Distillate Fuels
Biodiesel
Bioethanol
Pigments Antioxidants
Vitamins
Biomethane
Nutrient Recycle
Page | 51
Fig 22: Proposed biorefinery flow (Singh and Gu 2010, 2607)
8.2 Conceptual model of wastewater treatment with biorefinery
The conceptual model is adapted from
Lundquist et al. (2010), Andersson,
Broberg and Hackl (2011) and Zhu
(2014) to create a closed system with all
residue to be processed to create value
added products. Fig 23 shows the layout
of the wastewater treatment facility
proposed by Lundquist et al. (2010).
Fig 23: Components of 100 hectares facility
(Lundquist et al. 2010, 77)
With Vietnam focusing on biodiesel and bioethanol, a biorefinery that produces these two
fuels would be better suited. Fig 29 shows the proposed wastewater treatment and
biorefinery process.
8.2.1 Preliminary removal and primary treatment
The initial phase of wastewater treatment will follow a conventional process with the influent
being screened for large containments before being sent to the primary clarifier. Primary
sludge collected in the clarifier contains several types of fat (Andersson, Broberg and Hackl
2011, 55) and will be sent to the anaerobic digester to be processed with residual biomass
for the production of biogas. Metcalf and Eddy (2003) proposed that clarifiers should be
designed to have a retention time of between 1.5 – 2.5 hours and an overflow rate from 30 –
50 m3/m2/d (cited in Lundquist et al. 2010, 83).
Page | 52
8.2.2 HRAP and wastewater treatment
After the wastewater has passed through the primary treatment, the effluent is pumped into
the HRAP. Designing the HRAP to have a slight elevation to the drain will encourage the use
of gravity to handle flow out and help reduce energy consumption (Lundquist et al. 2010, 85).
Using a two stage hybrid system, a selected strain of microalgae will be grown in
photobioreactor (PBR) before being mixed into the HRAP. The warm temperature and long
duration of sunlight in Vietnam offer ideal conditions for the growth of the microalgae.
8.2.3 Harvesting
To achieve the recovery of a significant volume of biomass, the most appropriate harvesting
method must be adopted to ensure optimum liquid separation at minimal cost (Mata, Martins,
Caetano 2010, 224). The harvesting process has a significant impact on the overall financial
viability of the project, contributing to 20-30% of the total production cost (Rawat et al 2011,
3418).
8.2.3.1 Bioflocculation
The conventional treatment in the wastewater process is coagulation-flocculation that
applies chemicals to the wastewater to enhance the ability of particle removal prior
to second clarifier (Mazille, n.d). Coagulation uses chemicals to neutralise the negative
charges on the dispersed non-settable solids to form a larger particles called microflocs,
which are still too small to be visible to the naked eye (MRWA 2003, 1). Flocculation uses
chemicals and gentle stirring or agitation to encourage the particles to bond together to form
visible particles called flocs that are large enough to be filtered (MRWA 2003, 2). It must be
noted that flocculation of wastewater has a different end point to that of microalgae
flocculation, which determines the type of flocculants used such as organic or inorganic
flocculants (Schlesinger et al. 2012, 1024). Flocculation of microalgae is considered costly
and not suited for large scale operations (Schlesinger et al. 2012, 1024).
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Bioflocculation is a promising alternative method that is being explored. Bioflocculation is
the process of using non-flocculating microalgae with flocculating microalgae to enable
harvesting without adding chemicals and enables recycling of the cultivation medium without
further treatment (Salim et al., 2011). Without the need of chemicals, different cultivation
conditions and not affecting downstream processing reduces cost and the chances of
contaminations (Salim et al., 2011). In a study by Salim et al. (2011) that compared 3
flocculating microalgae in different environments, results supported the use of bioflocculation
with faster sedimentation of non-flocculating microalgae while increasing the harvesting
efficiency.
8.2.3.2 Gravity thickener
There are several different methods to harvest microalgae such as gravity sedimentation,
centrifugation and filtration (Grima et al. 2003, 492). Choosing the right method is dependent
on the strain of algae used. With the use of stabilisation ponds in wastewater treatment
plants, gravity sedimentation in the form of gravity thickeners can offer a cheap and efficient
method that separate microalgae from the wastewater (Singh et al 2014, 223).
Gravity thickeners are built similar to clarifiers except for the higher torque needed to move
the sludge and the presence of pickets to stir the sludge (Daigger 1998, 162). The process
uses the natural tendency for solids of higher density to settle at the bottom of the tank by
gravity thus thickening the solids (EPA, 2003). The scrapers at the bottom of the tank will
slowly move the thickened microalgae biomass to a discharged pipe to be the next stage for
processing.
Page | 54
Fig 24: Gravity thickener (EPA, 2003)
8.3 Recycling of nutrients and CO2
To reduce cost and improve efficiency, nutrients and CO2 will be recycled. CO2 captured
during the various refining processes will be pumped into the pond to help with the
cultivation process. Effluent in the gravity thickener will also be recirculated into the HRAP to
provide nutrients for cultivation. Methanol recovered during the refining of biodiesel will be
recycled for use in the direct transesterification process.
8.4 Processing of microalgae
The conventional method to prepare the biomass for oil extraction is to first dry the biomass.
This is achieved through various methods such as solar drying and oven drying (Singh et al.
2014, 223), but this process consumes large amounts of energy. It is estimated that the
current processing and extraction methods account for more than 80% of total energy
consumption (Chiaramonti et al. 2013, 102). This high energy intensity translates to high
cost of production, which is the greatest hurdle in making microalgae economical viable. In a
bid to reduce cost, various other methods and improvements in technology are being
explored to eliminate the cost associated with drying.
Page | 55
8.5 Extraction of microalgae oil
The extraction of microalgae oil can be achieved either via mechanical or non-mechanical
methods depending on the strain of algae used. These methods use various techniques to
disrupt the cell wall to release the intercellular components (Gonclaves, Pires and Simoes
2013, 317) and can work in tandem to produce greater yields. Some of these methods, such
as microwave and ultrasonication (Kim et al. 2013, 868), are deployed in the pre-treatment
of wet biomass as an alternative solution to extract microalgae oil directly from the biomass
without drying.
Fig 25: Extraction routes (Kim et al. 2013, 867)
8.6 Biofuel processing
The conversion of processed microalgae biomass to energy can be separated into
thermochemical and biochemical conversion as shown in Fig 26. The type of conversion is
influenced the desired end energy product and economic considerations (McKendry 2002,
46).
Harvested
microalgae biomass
Pre-treatment -
Cell disruption
Drying
Extraction
Organic solvent extraction
Supercritical fluid extraction
Mechanical cell disruption
Dry Route
Wet Route
Page | 56
Fig 26: Energy conversion process (adapted from Brennan and Owende 2010, 568)
In the conceptual model shown in Fig 29, biodiesel is the primary biofuel to be processed
and supported by bioethanol and biogas production to create a sustainable refining process.
The lipids extracted from the microalgae feedstock is refined to produce biodiesel with large
amounts of residual simultaneously generated. The residue comprises of microalgae cell
constituents of carbohydrates, protein and other valuable minerals that can be refined
through fermentation to produce bioethanol. The fermentation process also leaves behind
similar residual that can be further refined through anaerobic digestion to produce biogas or
specifically methane. The remaining waste after the digestion process contains organic
nitrogen and phosphorus that can be mineralized and reused in microalgae cultivation or
processed to other products such as fertilizers.
8.6.1 Transesterification
The major biofuel converted from extracted lipid oil from microalgae is biodiesel due to its
similar physical properties to diesel (European Biofuels Technology Platform, n.d). This
enables biodiesel to be either used as a primary fuel or blended with diesel to be used in
diesel engines. Although there are several different methods to produce biodiesel
Microalgae Biomass
Thermochemical Conversion
Gasification Syngas
Thermochemical Liquefaction
Bio-oil
Pyrolysis Bio-oil, Syngas,
Charcoal
Direct Combustion Electircity
Biochemical Conversion
Fermentation Bioethanol
Transesterification Biodiesel
Page | 57
(Gonclaves, Pires and Simoes 2013, 321; Gong and Jiang 2011, 1279), the most common
process is transesterification.
Fig 27: Transesterification process (Mata, Martins and Caetano 2010, 225)
Transesterification is a chemical reaction that converts triglycerides to diglycerides to
monoglycerides resulting in the production of fatty acid methyl esters (FAME) also known as
biodiesel and glycerol that is a by-product of the reaction as shown in Fig 27. The
transesterification reaction occurs when the triglycerides react with a short-chain alcohol and
a catalyst (Mata, Martins and Caetano 2010, 225). Short-chain alcohols include methanol,
propanoal and butanol, but ethanol is widely used due to it it’s low cost, physical and
chemical properties (Gong & Jiang 2011, 1279). The transesterification reaction can be
achieved via homogenous (base, acid and enzyme) and heterogeneous catalysts (Kim et al.
2013, 872-873). The difference between the two catalysts is the phase in which they work
with the reactants (Mosali and Bobbili, 2011).
Fig 28: Physical transesterification process (DOE, N.D)
Transesterification Crude Biodiesel Refining Biodiesel
Crude Glycerol Refining Glycerol
Methanol Recovery
Methanol + Catalyst
Page | 58
8.6.2 Direct transesterification
Direct transesterification also known as in-situ transesterification combines the process of
extraction and transesterification simultaneously. This greatly reduces the amount of energy
consumed, use of environment polluting solvents, processing time that leads to results in a
reduction in overall production cost (Pragya, Pandey and Sahoo 2013, 167). A study by Li et
al. (2011) showed that in-situ transesterification on Nannochloropsis sp. produced a higher
methyl ester yield of 28.0% with a higher heating value (HHV) of 31.53 MJ kg−1 while the
convention method yielded 22.2% with a HHV of 27.1al9 MJ kg−1. Johnson and Wen (2009)
made a comparison between the two processes with the cells of Schizochytrium limacinum
and a blended mixture of methanol, sulphuric acid and chloroform, with results showing a
higher yield for in-situ transesterification.
Direct transesterification can be applied to either wet or dry biomass, but dry biomass offers
a better yield (Kim et al. 2013, 871-874). The study by Johnson and Wen (2009) also
showed that in-situ transesterification with dry biomass produced a significant higher yield
compared to wet biomass. However, the use of wet biomass is an alternative approach that
shows some promise (Tran et al. 2012) and should be further explored.
8.6.3 Fermentation
Bioethanol has the same chemical formula regardless of whether it is produced from starch
or sugar based feedstock and has a higher octane level than gasoline, which makes it an
excellent complementary product to blend with gasoline to improve vehicle emissions (DOE,
2014b). The carbohydrates and proteins found as main components in the microalgae cell
are used in the fermentation process as carbon sources (Rawat, Kumar and Bux 2011,
3421).
Fermentation is the primary method used to produce bioethanol from microalgae and the
first step in the process is releasing the carbohydrates from the cell wall through cell
Page | 59
disruption using enzymes (Suali and Sarbatly 2012, 4329-4330). Different strains of
microalgae have varying composition of carbohydrate that require different non-standard
organisms for bioethanol production, but the most used organisms in the production of
ethanol is yeast as Saccharomyces cerevisiae (Daroch, Geng and Wang 2013, 1373).
After the extraction, the complex carbohydrates are broken down into simpler carbohydrates.
Glycolysis is the first reaction of the process, where glucose (C6H12O6) is split into two
pyruvate molecules (CHCOCOO−). The coenzymes are also broken down with two
molecules of adenosine diphosphate (ADP) reduced to two molecules of adenosine
triphosphate (ATP) and two molecules of nicotinamide adenine dinucleotide (NAD⁺) broken
down to two molecules of NADH. Water (H20) and hydrogen ions (H⁺) are also produced.
Once the molecules have been broken down, CHCOCOO− is converted into acetaldehyde
(CH3CHO) that is catalysed by pyruvate decarboxylase. The process produces CO2 and
hydrogen ions (H⁺). In the third stage, the acetaldehyde with the aid of NADH is converted to
an ethanol ion (C2H5O−). In the final stage, ethanol anion is protonated by hydrogen to
produce ethanol (C2H5OH) that also results in the production of CO2 (Suali and Sarbatly
2012, 4330).
8.6.4 Anaerobic Digestion
Anaerobic digestion uses microorganisms in the absence of oxygen to break down the
microalgae to produce biogas, which is mainly comprised of methane (CH4), CO2, and
ammonia (NH3) as shown in the reaction below (Zhu 2013, 9).
CaHbOcNd + 1/4(4a-b-2c+3d) H2O → 1/8(4a+b-2c-3d) CH4 + 1/8(4a-b+2c+3d) CO2 + dNH3 (Zhu
2013, 9)
A key advantage of using microalgae as the feedstock is that the nutrients in the residue
biomass are sufficient to produce biogas. Sialve, Bernet and Bernard (2009) found the
energy conversion of methane form residue biomass after lipid extraction is higher than that
Page | 60
of lipids. Research by Chisti (2008) also found that residue microalgae with 30% of their oil
content removed can continue to provide at least 9360 MJ of energy per metric ton. Biogas
that is produced will be used in direct combustion to produce heat (Anderson, 40) for the
drying process.
Proteins
(%)
Lipids
(%)
Carbohydrates
(%)
CH4
(L CH4 g VS− 1
)
N–NH3
(mg g VS− 1
)
Euglena gracilis 39–61 14–20 14–18 0.53–0.8 54.3–84.9
Chlamydomonas
reinhardtii
48 21 17 0.69 44.7
Chlorella pyrenoidosa 57 2 26 0.8 53.1
Chlorella vulgaris 51–58 14–22 12–17 0.63–0.79 47.5–54.0
Dunaliella salina 57 6 32 0.68 53.1
Spirulina maxima 60–71 6–7 13–16 0.63–0.74 55.9–66.1
Spirulina platensis 46–63 4–9 8–14 0.47–0.69 42.8–58.7
Scenedesmus obliquus 50–56 12–14 10–17 0.59–0.69 46.6–42.2
Table 10: Gross composition of different microalgae species with theoretical methane potential and
ammonia release during anaerobic digestion (Sialve, Bernet and Bernard 2009, 411)
Page | 61
Fig 29: Proposed biorefinery with wastewater treatment plant adapted form Lundquist et al. (2010),
Zhu (2014) and Andersson, Broberg and Hackl (2011)
Lipids
Methanol Recovery
Sludge
Residue
CO2 Recovery
Value added
products
Residue
Residue
Carbohydrates
Drying
Crude
Glycerol
Methanol + Catalyst
Direct
Transesterification Biodiesel Refining
Crude
Biodiesel
Fermentation Bioethanol Distillation
Crude
Bioethanol
Anaerobic
Digestion Biogas
(Methane)
Biogas upgrade
facility
Crude
Biogas
Combustion for Heat
Recirculation
PBR
Gravity
Thickener
Primary
Clarifier
HRAP Treated
wastewater
Preliminary
treated
wastewater Bioflocculation
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9 Conclusion and Recommendation
From the literature that was reviewed in the paper, it shows that the cultivation of microalgae
in wastewater is a viable solution that not only lowers overall cost but offers environmental
benefits over conventional wastewater treatment plants. Such a facility lays the foundation
for the commercial production of microalgae for biofuels.
There are commercial algae plants such as Solazyme Inc, and Solix BioSystems Inc., which
produce products for the chemical, beauty and food industry. Recently, AlgaeTec
Limited had commissioned the first microalgae biofuel production plant in New South Wales,
Australia. These projects do not cultivate the microalgae with wastewater, but their success
affirms that a microalgae wastewater treatment with a biorefinery is a possibility.
The most critical element to the success of a combined plant is defining the right algae strain
to cultivate. In choosing the right strain, the local environmental, growth rate, ability to
accumulate lipid and chemical compounds are some of the factors that need to be
considered. Beyond the technical aspects, there are other factors that determine the
success of the plant. Support from the government through subsidies and legislation helps
create the consistent demand that determines the economic success of the plant. High
capital and production cost remain the biggest challenges, but as shown in the research
paper a combined plant can lower the production cost to the point that is financial
sustainable and not affected by the fluctuating oil prices. The price of fossil fuel is an
important factor as it is when the cost of fuel is high that consumers and investors are eager
to explore alternative forms of energy. Increased investment and advancements in
wastewater treatment and production technology will help to further bring down the overall
cost while raising efficiency.
In many developing countries, there is no lack of land to build the ponds that are required.
However, the lack of skilled manpower to build and run the facility, poor infrastructure and
Page | 63
old technology are some of the challenges face by Vietnam (Vietnamese Ministry of Industry
and Trade, N.D). Overcoming these challenges may take time, but with the projected
construction of 30 new wastewater facilities and support from the government through new
wastewater treatment and energy legislation, this would be the most ideal time to introduce
microalgae wastewater treatment facility.
9.1 Research Limitations
There are no commercial facilities with only a few test-bed projects and the majority
research is confined to laboratory experiments, which limits the literature on the subject
There is very limited economic analysis on wastewater treatment with microalgae
Data and research on wastewater in Vietnam is limited and hard to access, which is
partly due to the differences in language
9.2 Follow-up Research
Expand on the technology and process used in harvesting and processing of biofuels
Greater in-depth economic analysis on each individual process and outcome, taking into
account logistics and storage
Page | 64
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