manoharbhai patel institute of engineering and technology
TRANSCRIPT
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1.1. GeneralThe population of glob is increasing, the problem of municipal &
industrial waste tedious day by day. The legacy of rapid urbanization,
industrialization, fertilizer & pesticide use has resulted in major pollution
problems in both terrestrial and aquatic environments. In developing
countries is major problem to treat the polluted water from above sources.
Chemical & mechanical menace are used for this purpose is expensive.
In response, conventional, remediation systems based on high physical
and chemical engineering approaches have been developed and applied to
avert or restore polluted sites. Much as these conventional remediation
systems are efficient, they are sparsely adopted because of some
economical and technical limitations. Generally, the cost of establishment
and running deter their use and meeting the demand particularly in
countries with week economy. Logical this high cost technology can
neither be applied justifiably where
1. The discharge is abruptly high for short time but the entire averageload is relatively small.
2. The discharge is very low but long term (entire load is medium).3. The discharge is continuously decreasing over a long duration.
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Thus conventional remediation approaches are best for
circumstances of high pollutants discharge like in industrial mining and
domestic waste water. Recently , it is evident that durability restoration
and long term contamination control in conventional remediation is
questionable because in the long run the pollution problem is only is
suspended or transferring from one site to another.
The efficiency of duckweed (Lemna) as an alternative cost
effective natural biological tool in wastewater treatment in general and
eliminating concentrations of both nutrients and soluble salts was
examined in an outdoor aquatic systems. Duckweed plants were
inoculated into primary treated sewage water systems (from the collector
tank) for aquatic treatment over eight days retention time period under
local outdoor natural conditions. Samples were taken below duckweed
cover after every two days to assess the plants efficiency in purifying
sewage water from different pollutants and to examine its effect on both
phytoplankton and total and fecal coli form bacteria.The Lemnaceae family consists of four genera (Lemna, Spirodela,
Wolffia & Wolffiella) and 37 species have been identified so far.
Compared to most other plants, duckweed has low fiber content (about
5%), since it does not require structural tissue to support leaves and
stems. Of these, applications ofLemna (duckweed) in wastewater
treatment was found to be very effective in the removal of nutrients,soluble salts, organic matter, heavy metals and in eliminating suspended
solids, algal abundance and total and fecal coli form densities. Duckweed
is a floating aquatic macro-phyte belonging to the botanical family
Lemnaceae, which can be found world-wide on the surface of nutrient
rich fresh and brackish waters. Outdoor experiments to evaluate the
performance of the duckweed as a purifier of domestic wastewater in
shallow mini-ponds (20 & 30 cm deep) showed that quality of resultant
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secondary effluents met irrigation reuse criteria. Wastewater ammonia
was converted into a protein rich biomass, which could be used for
animal feed or as soil fertilizer. The economic benefit of the biomass by-
product reduced wastewater expenditures to approx. US$ 0.05 per treated
m3 of wastewater, which was in the range of conventional treatment in
oxidation ponds.
The present study was concerned with decreasing pollution of
municipal waste waster up to degree Standards of Disposal as per
National pollution control board.
1.2. Pollution Problem
Municipal wastewater is producing in a huge quantity in most the
cities of the country that contain a diverse range of pollutants including,
the quality of municipal wastewater of stagnant/ slow velocity may create
problem of high epidemics of malaria & other water born diseases. Heavy
Metals ,Oil and Grease ,Phenols, Sulphide, Sulphate ,Nitrate ,Phosphate,
Dissolved Solids, Suspended Solids, COD, BOD, which its disposal and
treatment has become a challenge for the municipalities. Many of the
municipalities in growing cities neither have proper disposal system nor
have any treatment facility due to higher cost and in such a situation
municipal wastewater are discharge in to aquatic bodies like river, ponds
and lakes, where it is posing a serious threat to the water quality andbecome a big environmental problem.
1.3. Standards of Disposal
In order to protect the environmental Govt. of India established
pollution control boards. Tolerance limit for the industrial effluent as per
the environmental protection act 1986 of Govt. of India shown in table
1.1 governs the check for the pollution effect. In addition to these
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standards Maharashtra Pollution Control Board has introduced tolerance
limit for the dissolved oxygen as 5 mg/l, the minimum should be
maintained in the river course, 15 m from the discharge point of the
effluent in the river.
1.4. Treatment methodology
Primary treated sewage water were transferred to the laboratory
from the tertiary sewage water treatment plant after the preliminary
sieving step to get rid of large suspended solids. The transferred water
was immediately collected into two opaque tanks to prevent light
entering except at the top, each tank with dimensions of 150 cm long,
100 cm wide and 30 cm deep and was filled with 450 L primary treated
sewage water. Duckweed (Lemna) plants ere collected from Gadchiroli
Municipal Waste Water drain. The stock were cleaned by tap water then
washed by distilled water inocula ofLemna plants were transferred to the
water systems for aquatic treatment. The experiment was kept under
outdoor local environmental conditions for eight days retention time.
1.4.1 Water sampling: Subsurface (under duckweed mat)
water samples for physico-chemical, biological and bacteriological
parameters were collected in polyethylene bottles from all sides of tank
and then mixed. This procedure carried out every 2 days. Samples
volume taken every two days for each of phytoplankton count and
chlorophyll a determination was 100 ml.
Parameters measured. Physico-chemical analyses were carried out
according to standard methods for e examination of water and wastewater
(APHA, 1992). Field parameters (pH, conductivity & dissolved oxygen)
were measured in situ using the multi-probe system and rechecked in
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laboratory using bench-top equipment to ensure data accuracy for
biological parameters including total coli form count and fecal coli-form
count, phytoplankton identification and counting and chlorophyll a
determination.
1.4.2 Determination of duckweed growth rate: This was
determined for fresh and dry weights. Samples of 20 cm2 areas ofLemna
plants were harvested periodically at the designated time periods (every 2
days) and filtered using filter papers then fresh weights were determined.
These samples were then dried at 60oC for 48 h to a constant weight and
then dry weights were calculated.
Duckweed organic nitrogen content was estimated at the
beginning of the experiment and after 8 days retention time, then the
obtained values were multiplied by 6.25 to obtain protein content values.
1.5. Objective and scope of study
Pytoremadation has many advantages: it can clean-up a wide range
of contaminants while also being cost-effective, natural, passive, and
aesthetic. Because views of trees and green space can also provide
important physiological and social benefits, phytoremadation has the
potential to treat more than on-site contamination; it may also help to
create stronger neighborhoods and industrial/business districts.
The objective of present research is to develop new natural plants
& micro-aquatic plats to remove pollutants presenting waste water &
investigate the techno-economic feasibility. Study also aims to determine
the optimum condition operating parameters.
1. Identification of plants & micro-aquatic plant for removal ofmunicipal waste water.
2. Fabrication of experimental setup.3. Fabrication of laboratory setup.
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4. Conducting batch studies for the removal of pollutants frommunicipal waste water.
5. Conducting batch studies to find optimum opreting conditionof various parameters.
TABLE 1.1
STANDRADS FOR WASTE WATER DISPOSAL
Sr.No. Parameters Standards
Inland
water
surface
Public
sewers
Land for
irrigation
Marine & costal
area
1 Colour &
odour
All efforts should be made to remove it as fact as
possible
2 SS(mg/l) 100 500 200 i)100 for
process w.w.
ii) 10% above
for cooling
water effect.
3 pH 5.5 to 9.0
4 Temperature 40 45 -- 45 At discharge
5 Oil & grease
(mg/l)
10 20 10 20
6 Total
Nitrogen
100 -- -- 100
7 BOD 30 350 100 100
8 COD 250 -- -- 250
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2.1. General
The literature of Phytoremediation by lemna was collected from
the studies previously done by various persons. Their finding and
suggestions are listed hear. Various treatment methods are also discussed
for the treatment of municipal waste water with comparison of aerobic
and anaerobic treatments. An application of phytoremadation for waste
water done by different persons and their findings are also mentioned.
2.2. Characteristics of domestic waste water
Characteristic of waste water depend upon the raw material, process and
product made.
Oron et al. have study the waste water from ponds
Parameter Mean concentration in
waste water
Elimination
capacity %
Remark
Influent Effluent %
COD 500 320 30-40 Moderate
BOD 50 30 60 Good
Total N 40 20 50 Good
NH3 17 2 80-90 Excellent
Total P 6 3 50 Good
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2.3. Treatment Processes
The different processing waste water various authors have suggested
the methods of treatment. The methods of treatments can be broadly
classified as follows
A) Conventional methods of treatmentsi) Biological methodsii) Physiochemical methodiii) Land application method
B) Reuse of wastewater or by product recoveryC) Prevention of waste and waste strength reduction.D) Specific approach.
2.4. Process selection criteria for treatment of various
domestic waste water.
Over the years, biological treatment has established as a cost-
effective solution in a wide variety of domestic wastewater management
problems. It is therefore, desirable to consider whether the waste is
amenable to biodegradation or can be rendered biodegradable. Once the
biodegradability of the waste established. The most appropriate method
of biological treatment can be selected. The available bio treatment
alternative differ from one another in many respect such as nature of
electron acceptor (aerobic, anoxic, or anaerobic), biomass state
(suspended or fixed growth), hydraulic regime (plug flow or completely
mixed), and others. Selection of process should, however, be based
primarily on the waste water characteristics and the treatment gols
(W.W.Eckenfedr et.al 1989).
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2.4.1 Factor affecting process selection.
The factors affecting process selection for natural treatment are the
raw wastewater characteristics and the treatment objective. Additional
factors such as climatic conditions, plant location, land availability, etc.
also affect processes selection.
Wastewater Characterization: A classification of the organic
present in the domestic waste water into various fractions based on
amenability to biological treatment. The organics are relatively more
easily removed in any biological processes they are enmeshed in the
biomass and either degrader or physically separate from the liquid. The
soluble organics are generally more difficult to remove since portion of
these compounds are not readily available to the biomass. Those soluble
organic which are sorbed into biomass are also removed with relative
ease although part of such organics may degrade rather slowly. Of the
non soluble organic organic through the activity of extra cellular
enzymes, while a non degradable portion will be left in the effluent. Other
waste water characteristics of concern process selection are the organics
concentration, the presence of nutrients, toxicants or inhibitory
compounds.
Treatment Objectives :-
Treatment Objectives also play an important role in processselection. The primary treatment objective in biological system is
removal of biodegradable organic to levels specified by regulatory
agencies. Different treatment process can be tailored to achieve the desire
level of organic removal, toxicity reduction and non- degradable organic
removal.
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2.4.2 AVAILABLE BIOLOGICAL TREATMENT PROCESS :-
The essence of biological treatment is the utilization of organic
pollutants by microorganisms for growth and maintenance. This can be
represented by the following simplified equation.
Organics+Nutrients+Electron Acceptor = New Biomass +End Product
+Energy
A schematic illustration of the most common biological treatment
processes currently available. All biological treatment process can be
generally categorized as aerobic or anaerobic. In the former, molecular
oxygen systems, oxidized nitrogen serves as electron acceptor and is
reduced to nitrogen gas.
Both aerobic and anaerobic processes can further be classified as
fixed growth systems. The most common aerobic fixed growth systems
are the trickling filters and the rotating biological contactors (RBC). The
aerobic dispersed growth systems are the aerated lagoons and activated
sludge processes. The latter may assume different forms in terms of
hydraulic configuration such as plug flow, completely mixed etc. in
special cases, pure oxygen or nitrification / denitrification systems are
used.
The anaerobic treatment can also be divided into fixed and
dispersed growth processes. The dispersed growth system is also known
as anaerobic contact process and is similar to activated sludge except itdoes not use oxygen. The fixed growth anaerobic system include
fluidized beds and packed beds. A hybrid of fixed and dispersed growth
system is the up flow anaerobic sludge blanket process.
The major types of biological treatment processes that are currently
available. However wastewater characterization and establishment of
treatment objectives are necessary before screening and selection of the
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process. Some of the criteria and rationale behind this procedure are
discussed below.
2.4.3 AEROBIC VERSES ANAEROBIC TREATMENT:-
A general comparison of aerobic and anaerobic treatment process
is presented. In the aerobic process, where oxygen is the electron
accepter, the growth process is more efficient. It therefore, results in
higher sludge yields and energy requirements, but is less likely to
produces odours.
The anaerobic processes are more sensitive to environmental condition
(pH, temperature toxic shocks) and require longer start up time. One
major limitation of the anaerobic process is that it cannot economically
achieve levels, such as en effluent BOD of 20mg/L or 95% BOD
removal, as often required by regulatory agencies it can be cost effective,
however, if employed as pretreatment before aerobic polishing of high
strength industrial wastewater.
2.4.4 DISPERSED GROWTH VERSUS FIXEDBED REACTORS:
It is convenient to divide biological, reactors into dispersed growth
and fixed bed reactors. Biodegradation is carried but by biomass that is
suspended in the liquid phase of the reactor. In the fixed bed reactor, the
biomass is attached to a fixed within the reactor. Compared to thedispersed growth to a fixed within the reactor. Compared to the dispersed
growth reactors, the primary merit associated with the fixed bed reactors
stem from their simplicity and ease of operations, thus making them
ideal for remote and small industrial streams. Furthermore, because of the
relatively high concentration of the biomass attached to the surface of the
fixed media these reactors can handle higher loads per unit volume of
reactors. Therefore, they are a better choice whenever land is limited.
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sludge of relatively constant nature that can readily be removed by
sedimentation. This is particularly important whenever sludge settling
problems are expected in an alternative suspended growth systems less
affect fixed bed reactors.
The major disadvantages of the fixed be reactors compared to the
dispersed growth systems are their lesser flexibility in operation,
difficulty to achieve very high removal efficiencies, and greater
sensitivity to cold weather conditions. Another important drawback of
fixed bed system is that they are less understood, thus modeling andprocess design procedures are not as rigours and advanced as for the
dispersed growth systems. This drawback has two important
implications. First, in many cases the fixed bed reactors are improperly
designed; which leads to either over or under design. Second, it is more
difficult to estimate prototype performance based on bench scale
experiments. This kind of draw back is of particular importance in cases
where the nature of the wastewater is unknown.
Since the achievement of high removal efficiencies in fixed bed
systems is economically prohibitive these systems are often utilized as a
roughing stage preceding is dispersed growth polishing stage.
2.4.5 HIGH RATE ANAEROBIC TREATMENT
All high rate anaerobic treatment processes are based on the
achievement of a high retention of viable anaerobic sludge, combined
with a good contact between incoming wastewater with the sludge.
Although these conditions are not always sufficiently met in the available
high rate systems, the importance of high rate systems for practice is
considerable because of the following reasons.
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Very high organic loading rates can be applied. Consequently small reactor volumes suffice. Unless designed at their maximum loading potentials the
stability of high rate systems to sub optimal conditions
(lower temperature, shock loads, presence of inhibitory
compounds ) is high.
They make anaerobic treatment economically feasible at lowambient temperature and for very low strength wastes as
well.
2.5. Application of Phytoremedation to domestic waste water
The ability of duckweed to sequester nitrogen and phosphorus, and
in so doing cleanse dirty water, has been widely discussed in the
literature for nearly 30 years (Culley and Epps, 1973; Hillman and
Culley, 1978; Oran et al., 1986; Landolt and Kandeler, 1987; Leng,
1999). Systems utilising various species of duckweed, either alone, or in
combination with other plants, have been used to treat primary and
secondary effluent in the U.S.A. (Zirschky and Reed, 1988), the Middle
East (Oran et al., 1985) and the Indian subcontinent (Skillicorn et al.,
1993; van der Steen et al., 1998). Notwithstanding this reputation, some
species and isolates are apparently quite sensitive to high levels of
nitrogen and/or phosphorous (Bergman et al., 2000), and effluent with a
high biological oxygen demand (BOD), such as abattoir waste, may kill
the plants. Although duckweed has a reputation for absorbing large
amounts of dissolved nitrogen, the degree of absorption appears to vary
with concentration of nitrogen, time, species, and (at least in temperate
zones) the season. There is also strong evidence that there is a symbiotic,
or at least a synergistic relationship between duckweed and bacteria, both
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in the fixation of nitrogen (Duong and Tiedje, 1985), and the removal of
Chemical Oxygen Demand (COD) (Korner et al., 1998) from water.
Differences in methodology, scale, and the parameters, both
recorded and measured, make direct comparisons between the many trials
in published literature difficult. However most research indicates that
duckweed removes 40 to 60% of nitrogen in solution over a 12 to 24 day
period. Volatilization may account for a similar loss of nitrogen
(Vermaat and Haniff, 1998), although recent work completed in Israel
(Van der Steen et al., 1998), has suggested that direct duckweed
absorption may account for less than 20% of nitrogen loss, and
volatilization/ de-nitrification may account for over 70% In a similar
fashion, lemnacae are generally able to absorb 30 to 50% of dissolved
phosphorous, although one researcher (Alaerts et al., 1996) has claimed
over 90% removal in a working, full scale system.
Phosphorous uptake (as measured by tissue phosphorous) and
crude protein, increased linearly with increases in nutrient concentration,up to approximately 1.5 g P/l, and increased in absolute terms, up to 2.1
g P/l (Sutton and Ornes, 1975). This was recorded in conjunction with a
proportional rise in nitrogen concentration, thus the association between
nitrogen and phosphorous concentrations was unclear. COD is a measure
that quantifies water quality as determined by dissolved oxygen. All
research in the use of duckweed for improving effluent quality hasdetermined significant but variable decreases in COD (Alaerts et al.,
1996; Karpiscak et al., 1996; Bonomo et al., 1997; Vermaatand Haniff,
1998; van der Steen et al., 1999).
However, a substantial decrease in COD would be expected in
open ponds without the presence of duckweed (Al-Nozaily et al., 2000),
so this improvement may not be attributable to the actions of duckweed.
Simplistically, the duckweeds environment is somewhat two-
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Economic Cooperation and Development (OECD) have classified this plant
as a bioindicator (Kiss et al., 2003).
Symptoms of heavy metal toxicity are chlorosis, necrosis and root
damage, as well as changes in biochemicals including antioxidant enzymes.
The sensitivity ofL. minor has been tested in terms of some metabolic
indicators, in sewage ponds (Mohan and Hosetti, 1999) and under
laboratory conditions (Garnczarska and Ratajczak,2000a,b; Wang et al.,
2002). Since the data are not conclusive, duckweeds potential as a bio-
indicator for aquatic systems needs further investigation.
Duckweed commonly refers to a group of floating, flowering plants of
the family Lemnaceae. The different species (Lemna, Spirodela, Wolffia and
Wolfiella) are worldwide distributed in freshwater and wetlands, ponds and
some effluents are the most common sites to find duckweed. The plants are
fast growing and adapt easily to various aquatic conditions. They are able to
grow across a wide range of pH, from pH 3.5 to10.5 but survive best
between pH 4.5 to 8.3 (Environnement Canada, 1999; Cayuela et al., 2007).
The plants are found in temperate climates and serve as an important food
source for various water birds and fish (Drost et al., 2007). Some studies
indicate that duckweed plants are sensitive to toxicity. Other studies
however, report that duckweed plants are tolerant to environmental toxicity
(Wang, 1990).
To assess the tolerance of the speciesL. gibba to heavy metals, plants
were exposed to concentrations of copper and nickel higher than those used
in medium cultures. Toxic effect of pollutant on duckweed is generally
evaluated by phytotoxicity tests based on growth inhibition (Geoffroy et al.,
2004). Copper and nickel were chosen as the metals for this study for a
number of reasons. Their presence above trace levels in the environment is
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an indicator of industrial pollution. On the other hand, they are essential
micronutrients for plants; copper is a structural and catalytic component of
many proteins and enzymes involved in metabolic pathways (Teisseire &
Vernet, 2000) and nickel has an important role in the urease and
hydrogenase metabolism (Harish et al., 2008). However, when the
concentration reaches a threshold value, these essential metals become first
inhibitory and afterwards toxic. Copper is responsible for many alterations
of the plant cell (respiration, photosynthesis, pigment synthesis and enzyme
activity) (Teisseire & Vernet, 2000; Kanoun-Boul et al., 2009). Nickel
inhibits germination, chlorophyll production and proteins (Zhou et al.,
2009) in plants; several animal experimental studies have shown an
increased cancer incidence associated with chronic exposure to nickel.
3.2 Definition & types of Phytoremedation
3.2.1.What is phytoremadation ?
Phytoremediation is the use of living green
plants for in situ risk reduction and/or removal of
contaminants from contaminated soil, water,
sediments, and air. Specially selected or
engineered plants are used in the process. Risk
reduction can be through a process of removal, degradation of, or
containment of a contaminant or a combination of any of these factors.
Phytoremediation is an energy efficient, aesthically pleasing method of
remediating sites with low to moderate levels of contamination and it can be
used in conjunction with other more traditional remedial methods as a
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finishing step to the remedial process. One of the main advantages of
phytoremediation is that of its relatively low cost compared to other
remedial methods such as excavation. The cost of phytoremediation has
been estimated as $25 - $100 per ton of soil, and $0.60 - $6.00 per 1000
gallons of polluted water with remediation of organics being cheaoer than
remediation of metals. In many cases phytoremediation has been found to be
less than half the price of alternative methods. Phytoremediation also offers
a permanent in situ remediation rather than simply trans locating the
problem. However phytoremediation is not without its faults, it is a process
which is dependent on the depth of the roots and the tolerance of the plant to
the contaminant.
Exposure of animals to plants which act as hyper-accumulators can
also be a concern to environmentalists as herbivorous animals may
accumulate contaminates particles in their tissues which could in turn affect
a whole food web.
3.2.2 How Does It Work?
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Phytoremediation is actually a generic term for several ways in which
plants can be used to clean up contaminated soils and water. Plants may
break down or degrade organic pollutants, or remove and stabilize metal
contaminants. This may be done through one of or a combination of the
methods described in the next chapter. The methods used to phytoremediate
metal contaminants are slightly different to those used to remediate sites
polluted with organic contaminants.
Metal Organic
Phytoextraction Phytodegradation
Rhizofiltration Rhizodegradation
Phytostabilisation Phytovolatilisation
3.3 Methods of Phytoremediation
Phytoremediation of metal contaminated sites
Phytoextraction (Phytoaccumulation)
Phytoextraction is the name given to the process where plant roots
uptake metal contaminants from the soil and translocate them to their above
soil tissues. As different plant have different abilities to uptake and
withstand high levels of pollutants many different plants may be used. This
is of particular importance on sites that have been polluted with more than
one type of metal contaminant. Hyperaccumulator plant species (species
which absorb higher amounts of pollutants than most other species) are used
on may sites due to their tolerance of relatively extreme levels of pollution.
Once the plants have grown and absorbed the metal pollutants they are
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harvested and disposed of safely. This process is repeated several times to
reduce contamination to acceptable levels. In some cases it is possible to
recycle the metals through a process known as phytomining, though this is
usually reserved for use with precious metals. Metal compounds that have
been successfully phytoextracted include zinc, copper, and nickel, but there
is promising research being completed on lead and chromium absorbing
plants.
Rhizofiltration
Rhizofiltration is similar in concept to Phytoextraction but isconcerned with the remediation of contaminated groundwater rather than the
remediation of polluted soils. The contaminants are either adsorbed onto the
root surface or are absorbed by the plant roots. Plants used
for rhizoliltration are not planted directly in situ but are acclimated to the
pollutant first. Plants are hydroponically grown in clean water rather than
soil, until a large root system has developed. Once a large root system is in
place the water supply is substituted for a polluted water supply to
acclimatise the plant. After the plants become acclimatised they are planted
in the polluted area where the roots uptake the polluted water and the
contaminants along with it. As the roots become saturated they are harvested
and disposed of safely. Repeated treatments of the site can reduce pollution
to suitable levels as was exemplified in Chernobyl where sunflowers were
grown in radioactively contaminated pools.
Phytostabilisation
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Phytostabilisation is the use of certain plants to immobilise soil and
water contaminants. Contaminant are absorbed and accumulated by roots,
adsorbed onto the roots, or precipitated in the rhizosphere. This reduces or
even prevents the mobility of the contaminants preventing migration into the
groundwater or air, and also reduces the bioavailibility of the contaminant
thus preventing spread through the food chain. This technique can alos be
used to re-establish a plant community on sites that have been denuded due
to the high levels of metal contamination. Once a community of tolerant
species has been established the potential for wind erosion (and thus spread
of the pollutant) is reduced and leaching of the soil contaminants is also
reduced.
Phytoremediation of organic polluted sites
Phytodegradation (Phytotransformation)
Phytodegradation is the degradation or breakdown of organic
contaminants by internal and external metabolic processes driven by the
plant.Ex plantametabolic processes hydrolyse organic compounds into
smaller units that can be absorbed by the plant. Some contaminants can be
absorbed by the plant and are then broken down by plant enzymes. These
smaller pollutant molecules may then be used as metabolites by the plant as
it grows, thus becoming incorporated into the plant tissues. Plant enzymes
have been identified that breakdown ammunition wastes, chlorinated
solvents such as TCE (Trichloroethane), and others which degrade organic
herbicides.
Rhizodegradation:
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Rhizo-degradation (also called enhanced rhizo-sphere biodegradation,
phyto-stimulation, and plant assisted bioremediation) is the breakdown of
organic contaminants in the soil by soil dwelling microbes which is
enhanced by the rhizo-sphere's presence. Certain soil dwelling microbes
digest organic pollutants such as fuels and solvents, producing harmless
products through a process known asBioremediation. Plant root exudates
such as sugars, alcohols, and organic acids act as carbohydrate sources for
the soil micro-flora and enhance microbial growth and activity. Some of this
compound may also act as chemotactic signals for certain microbes. The
plant roots also loosen the soil and transport water to the rhizo-sphere thusadditionally enhancing microbial activity.
Phytovolatilization:
Phyto-volatilization is the process where plants uptake contaminants
which are water soluble and release them into the atmosphere as they
transpire the water. The contaminant may become modified along the way,
as the water travels along the plant's vascular system from the roots to the
leaves, whereby the contaminants evaporate or volatilize into the air
surrounding the plant. There are varying degrees of success with plants as
phyto-volatilizers with one study showing poplar trees to volatilize up to
90% of the TCE they absorb.
Hydraulic control of Pollutants :
Hydraulic control is the term given to the use of plants to control the
migration of subsurface water through the rapid upltake of large volumes of
water by the plants. The plants are effectively acting as natural hydraulic
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pumps which when a dense root network has been established near the water
table can transpire up to 300 gallons of water per day. This fact has been
utilized to decrease the migration of contaminants from surface water into
the groundwater (below the water table) and drinking water supplies. There
are two such uses for plants.
Riparian corridors :
Riparian corridors and buffer strips are the applications of many
aspects of phytoremediation along the banks of a river or the edges of
groundwater plumes. Pytodegradation, phytovolatilization, and
rhizodegradation are used to control the spread of contaminants and toremediate polluted sites. Riparian strips refer to these uses along the banks
of rivers and streams, whereas buffer strips are the use of such applications
along the perimeter of landfills.
Vegetative cover :
Vegetative cover is the name given to the use of plants as a cover orcap growing over landfill sites. The standard caps for such sites are usually
plastic or clay. Plants used in this manner are not only more aesthically
pleasing they may also help to control erosion, leaching of contaminants,
and may also help to degrade the underlying landfill.
Where has Phytoremediation Been Used?
As it is a relatively new technology phytoremediation is still mostly in it's
testing stages and as such has not been used in many places as a full scale
application. However it has bee tested successfully in many places around
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the world for many different contaminants. This table shows the extent of
testing across some sites in the USA
Location Application Pollutant Medium plant(s)
Ogden,
UT
Phytoextraction &
Rhizodegradation
Petroleum &
Hydrocarbons
Soil &
Groundwater
Alfalfa, poplar,
juniper, fescue
Anderson,
STPhytostabilisation Heavy Metals Soil
Hybrid poplar,
grasses
Ashtabula,
OHRhizofiltration Radionuclides Groundwater Sunflowers
Upton,
NYPhytoextraction Radionuclides Soil
Indian mustard,
cabbage
Milan, TN PhytodegradationExpolsives
wasteGroundwater
Duckweed,
parrotfeather
Amana,
IA
Riparian corridor,
phytodegradationNitrates Groundwater Hybrid poplar
Pro's & Con's of Phytoremediation
As with most new technologies phytoremediation has many pro's and
cons. When compared to other more traditional methods of environmental
remediation it becomes clearer what the detailed advantages and
disadvantages actually are.
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3.4 Advantages of phytoremediation
It is more economically viable using the same tools and supplies as
agriculture
It is less disruptive to the environment and does not involve waitingfor new plant communities to recolonise the site
Disposal sites are not needed It is more likely to be accepted by the public as it is more aesthetically
pleasing then traditional methods
It avoids excavation and transport of polluted media thus reducing therisk of spreading the contamination
It has the potential to treat sites polluted with more than one type ofpollutant
3.5 Disadvantages of phytoremediation
It is dependant on the growing conditions required by the plant (ieclimate, geology, altitude, temperature)
Large scale operations require access to agricultural equipment andknowledge
Success is dependant on the tolerance of the plant to the pollutant Contaminants collected in senescing tissues may be released back into
the environment in autumn
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Contaminants may be collected in woody tissues used as fuel Time taken to remediate sites far exceeds that of other technologies Contaminant solubility may be increased leading to greater
environmental damage and the possibility of leaching
The low cost of phytoremediation (up to 1000 times cheaper than
excavation and reburial) is the main advantage of phytoremediation,
however many of the pro's and cons of phytoremediation applications
depend greatly on the location of the polluted site, the contaminants in
question, and the application of phytoremediation.
3.6 Phytoremediation & Biotechnology
The first goal in phytoremediation is to find a plant species which is
resistant to or tolerates a particular contaminant with a view to maximizing
its potential for phytoremediation. Resistant plants are usually located
growing on soils with underlying metal ores or on the boundary of polluted
sites. Once a tolerant plant species has been selected traditional breeding
methods are used to optimize the tolerance of a species to a particular
contaminant. Agricultural methods such as the application of fertilisers,
chelators, and pH adjusters can be utilized to further improve the potential
forphytoremediation.
Genetic modification offers a new hope for phytoremediation as GM
approaches can be used to over express the enzymes involved in the existing
plant metabolic pathways or to introduce new pathways into plants. RichardMeagher and colleagues introduced a new pathway into Arabidopsis to
detoxify methyl-mercury, a common form of environmental pollutant to
elemental mercury which can be volatilised by the plant.
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been reported as doubling their biomass every 16 to 48 hours (Leng, 1999).
The main form of reproduction is vegetative, through the production of
daughter fronds that arise from one of two lateral pouches at the base of
the frond. Whilst vegetative growth is usual, duckweed daughter fronds do
not stay attached indefinitely, but rather break and form new colonies, only a
new generations old. This novel facility has led to the suggestion that
duckweed growth could be considered analogous to microbial growth
(Hillman, 1961). Individual fronds have a relatively short life span of 3 to 10
weeks when in the vegetative phase, depending on species, reproductive rate
and photoperiod (Landolt, 1986).
By this time, an original mother plant may have given rise to a
clonal colony of tens of thousands of personality plants over more than 50
generations. There appears to be distinctive differences in longevity and
mature size between generations (Landolt, 1986) that may be expressed as
cyclicity in the growth pattern of a colony. One of the significant attributes
of duckweed is its ability to be used as a source of proteinaceous food with a
favorable profile of important amino acids (Rusoff et al., 1980)
3.7.2GROWTH CONDITIONS FOR DUCKWEED
The growth oflemnacae may be nearly exponential, if carbon dioxide,
light and nutrient supplies are satisfactory. Discussion in this review is
limited to the three major plant macronutrients (nitrogen, phosphorus,
potassium). Calcium and sulphur are not generally considered to be limiting
to growth (Landolt, 1986), whereas nitrogen and phosphorus influence
growth strongly and have an interactive effect.
Lemnacae are able to absorb nitrogen as ammonium, nitrate, nitrite,
urea and some amino acids, however the first two represent the main
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nitrogen source for most species. Minimum, optimal, and toxic levels of
nitrogen vary greatly between species and geographic isolates and increasing
light intensity is thought to elevate optimal nitrogen requirements for
growth. Of the species studied, L. miniscula has the lowest (0.0016 mM/l)
and an unclassified species ofLemna the highest (0.08 mM/l) minimum
requirement for nitrogen (Landolt, 1986). Similarly, the maximum tolerated
level of nitrogen varies from 30 mM/l (L. miniscula) to 450 mM/l for L.
aequinoctialis (Landolt, 1986). The optimal recorded nitrogen requirement
ranges from 0.01 mM/l for W. colombia, up to 30 mM/l for S. polyrrhiza
(Landolt, 1986). Duckweeds requirement for phosphorous, is variable
(0.003-1.75 mM/l) between species as is seen for nitrogen requirement, but
appears unrelated to it (Landolt, 1986). Duckweed is reputedly able to
accumulate up to 1.5% of its weight as phosphorus in nutrient rich waters
(Leng, 1999). Between species differences are also evident for potassium,
with requirements also being influenced by light intensity.
3.7.3 FACTORS AFFECTING GROWTH AND COMPOSITION OF
DUCKWEED.
There is a great deal of literature published on actual and potential
yields of duckweed (Culley and Epps, 1973; Hillman and Culley, 1978;
Rusoff et al., 1980; Oran et al., 1987; Leng, 1999; Chowdhury et al., 2000).
Unfortunately, there is little data available that records the interactions
between genotype and environment. Many trials are based on short-term
yields in small containers, with theoretical yields extrapolated to a per
hectare per annum basis. Perhaps because of this, reported yields of
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duckweed vary widely. A summary of reported yields assembled by Leng
(1999) show yields ranging from 2 to 183 t(DM)/ha/year.
The extremely large range of recorded yields suggests that making
estimates of productivity based on results from short trials in laboratory-
scale vessels is of questionable value. Significant variances in growth have
been demonstrated between species and different geographic isolates of the
same species (Bergman et al., 2000). A composite picture of yields of l.
gubba on different media is shown in Figure 1. These published results on
actual and potential yield of duckweed indicate a general lack of agreement
on the growth of these plants. There are a number of factors that may
mediate these apparently conflicting results. Quite apart from procedural
differences (such as different tank sizes, flow rate/retention times) there are
numerous physico-chemical differences that make establishment of
equivalence, and thereby direct comparison difficult. Time of year (and
hence ambient temperature and day length), latitude, and pH of growth
media can all have a substantial influence on the physiology, and thus the
growth of the plant.
There are many factors that influence growth, and the value of
drawing comparisons between trials conducted without similar protocols and
isolates, is also of limited value. Additionally, the levels of available
nutrient, as well as species differences, can strongly influence both the
quantity and quality of material produced. These differences may be
interpreted in light of the existence of deficient, optimal and toxic levels for
nutrients. Nitrogen in particular, whilst being an essential macronutrient, is
toxic at high concentrations. Little interest has been shown in recent times in
establishing an optimum nutrient range for growth of duckweed despite
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inconsistencies in published literature. Recent work (Bergman et al., 2000;
Al-Nozaily, 2001) indicates that best growth is achieved where total nitrogen
concentrations range from 10 to 40 mg N/l.
However this conflicts with the work of Caicedo et al. (2000), who
reported that growth rates of S. polyrhiza actually declined over a range of
3.5 to 100 mg N/l. It has been demonstrated that lower (6 to 7) pH levels
ameliorate the toxic effects of nitrogen (McLay, 1976; Caicedo et al., 2000)
and Al-Nozaily (2000) has suggested that this may be because the low pH
limits ionization of ammonia species, resulting in a low proportion of
ammonia in solution. The optimal nutrient profile for growth of duckweed
doesnt necessarily produce the best quality of plant material in terms of
protein content and digestibility. Leng (1999) has suggested that optimal
protein content will be obtained where nitrogen is present at 60 mg N/l or
greater. Early field observations by Culley and Epps (1973) suggested that a
strong positive relationship existed between high levels of dissolved
nutrients and plant characteristics, especially protein and digestibility.
Subsequently, several other researchers have reported positive relationships
between nutrient concentrations and dry matter yield, crude protein and
phosphorous content (Whitehead et al., 1987; Alaerts et al., 1996). In
contrast, Bergman et al., (2000) found little difference in dry matter (DM)
yield and no difference in protein content in L. gibba grown over a wide
range of nutrient levels (52 to 176 mg N/l) In practice, the depth of water
required to grow duckweed will be determined by the purpose for which it is
being grown, as well as management considerations (Leng, 1999). Ponds of
less than 0.5 m depth may be subject to large diurnal temperature
fluctuations.
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Large scale operations require access to agricultural equipment andknowledge.
Success is dependant on the tolerance of the plant to the pollutant Contaminants collected in senescing tissues may be released back into
the environment in autumn
Contaminants may be collected in woody tissues used as fuel Time taken to remediate sites far exceeds that of other technologies Contaminant solubility may be increased leading to greater
environmental damage and the possibility of leaching.
3.10 Scope of phytoremadation by Lemna.
Now a days conventional sewage treatment plant have high
construction cost, energy and maintenance expenses and increasing labour
costs, traditional wastewater treatment systems are becoming an escalating
financial burden for the communities and industries that operate them. For
many rural communities, the availability of low-cost land has meant that
more extensive, low-energy treatment processes can be a cost-effective
alternative, especially for final treatment of effluent.
Usefulness and a cultural preference for mechanical infrastructure.
Queensland, in particular, is climatically well positioned to take advantage
of lagoon treatment systems that use aquatic plants as productive sinks for
wastewater nutrients from a wide range of sources. Of these, duckweed-
based treatment systems offer the most promise.
The result is greater discharged effluent standards in terms of reduced
total suspended solids (TSS) and nutrients. Nutrients contained in
phytoplankton are difficult to harvest and are generally released back into
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the environment, whereas duckweed is easily harvested, which results in
direct removal of nutrients from the waste stream.
In addition, evaporation from the water surface is reduced in DWT
systems (Bonomo et al. 1997), Duckweed works to purify wastewater in
collaboration with both aerobic and anaerobic bacteria. Therefore, the
duckweed plants themselves should be considered as only one scomponent
of a complete DWT system. Flow of nitrogenous nutrients within a DWT
system utilizing bacterial processing and uptake by duckweed plants.
Heterotrophic bacteria decompose organic waste matter into mineral
componentsspecifically forms of ammonia nitrogen and orthophosphates
that are readily up-taken by the duckweed plants. Bacterial decomposition
consumes oxygen and can cause the mid-water zone to become increasingly
anoxic and the bottom of the lagoon to become anaerobic, providing further
zones for specialized bacterial processing of organic matter and de-
nitrification a 10cm surface layer remains aerobic due to atmospheric
oxygen transferred by duckweed roots.
DWT has great potential for renovating effluent from a wide variety
of sources including municipal sewage treatment plants, intensive livestock
industries (including aquaculture), abattoirs and food processing plants. The
effectiveness of DWT depends on a system design that facilitates the correct
combination of organic loading rate, water depth and hydraulic retention
time. These will vary depending on the effluent source and the level of pre-
treatment.
Bacterial oxidization of organic matter and nitrification are facilitated
here, aided by the additional surface area for biofilms provided by the
duckweed roots and fronds. Most researchers, however, suggest that
efficiency gains using DWT are greater in secondary and tertiary treatment
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may need an acclimatisation period to adapt to the very high N levels in raw
agricultural wastewaters.
Most researchers, however, suggest that efficiency gains using DWT
are greater in secondary and tertiary treatment of effluent where organic
sludge has already been removed or converted into simple organic and
inorganic molecules that can be used.In the Burdekin, as with most
communities in Australia, primary sewage treatment infrastructure exists to
remove solids. The problems currently encountered with municipal
wastewater treatment include difficulties in meeting TSS and nutrient (Total
N & P, ammonia) discharge regulations. Domestic wastewater does not
contain significant concentrations of toxins or heavy metals (Skillicorn et al.
1993), polishing zones may simply be considered to be the latter reaches of a
continuous duckweed treatment process.
3.11 Design consideration for phytoremadation
DWT system design principles:
There is no single off-the-shelf DWT package that will serve all
purposes. Requirements will vary depending on: the effluent source and
volume; the level of pre-treatment; the regulated discharge quotas that need
to be met; prevailing climate and financial considerations. Large-scale
studies from both developing and western parts of the world have been
conducted using various DWT system designs and effluent sources, but
common recommended design features can be identified.
Plug-flow design
A plug-flow system is the most appropriate for secondary and tertiary
effluent treatment using DWT. A plug-flow system will ensure maximum
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contact between wastewater and duckweed, and minimise the possibility of
short-circuiting (Smith and Moelyowati 2001). This will facilitate the
incremental reduction of nutrients in the wastewater. Plug-flow systems are
also most efficient for pathogen removal (van der Steen et al. 1999).
The basic unit of plug-flow systems is a shallow rectangular lagoon.
The system can operate singly or as a series of lagoons. The length/width
ratio should be as large as possible to encourage plug-flow conditions
(Figure 2). Alaerts et al. (1996) recommend a ratio greater than 38:1
although this is often difficult to achieve due to practical reasons such as
cost. Bonomo et al. (1997) suggest a length/width ratio higher than 10:1 will
suffice.
A plug-flow lagoon design, which prevents short-circuiting of flow
between inlet and outlet, is most appropriate for DWT.
3.11.1Nutrient uptake
Since duckweed will be the major nutrient sink in these lagoons, a
greater biomass will inherently result in greater nutrient uptake. Greater
biomass growth will occur at higher nutrient concentrations (up to a
tolerance limit), but as duckweed incrementally reduces nutrients from the
water, high biomass growth cannot be maintained. Since the ultimate object
of treatment is to reduce nutrient concentration, duckweed starvation
inevitably will occur at the latter stage in the treatment process.
In a plug-flow system, nutrient concentrations will be higher at the
beginning of the effluent stream and lower towards the end. This will
facilitate a farming zone (high duckweed production/high nutrient uptake)
and a polishing zone (lower overall duckweed growth/lower nutrient
uptake). In the farming zone, where growth nutrients (N & P) are plentiful,
duckweed plants are predisposed to absorb them to the exclusion of other
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elements present in the wastewater column (Skillicorn et al. 1993). In the
polishing zone, however, duckweed plants starved of N and P nutrients will
scavenge for sustaining nutrients. In the process they can absorb toxins and
heavy metals if present in the InletEffluent flowDischarge wastewater. This
will have implications on the reuse or disposal of the harvested plants.
However, since most agricultural or domestic wastewater does not contain
significant concentrations of toxins or heavy metals (Skillicorn et al. 1993),
polishing zones may simply be considered to be the latter reaches of a
continuous duckweed treatment process.
3.11.2 Uptake efficiency :
The nutrient uptake efficiency (i.e. the percentage of influent nutrient
removed by the treatment) will be determined by the hydraulic retention
time. While a short retention time will maintain high nutrient levels (and
therefore extend the farming zone), the overall percentage of nutrients
removed from the effluent stream is lower. Conversely, a longer retention
period will result in a greater percentage of nutrients being removed, but
create a relatively less productive polishing zone when nutrients become
limiting. For example, the Burdekin pilot trial (Willett et al. 2003) tested
three effluent retention times, i.e. 3.5 days, 5.5 days and 10.4 days. The
relationship between total nitrogen (TN) uptake, uptake efficiency and
biomass production by DWT at different retention times from this trial.
Average Total Nitrogen uptake (mg/L/day), uptake efficiency
(percentage of influent TN removed by the treatment) and duckweed
biomass produced (g/m2/day) at three Effluent Retention Times (E.R.T.).
Data derived from Willett et al. (2003).
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Lemna and Spirodella the roots are believed to be adventitious, are only a
small proportion of overall plant weight and lack root hairs. The other two
genera lack roots. An important feature of the structure is the almost total
lack of woody tissue .Members of the Lemnacae family are found almost
world wide, being absent only in the Polar Regions and deserts.
Distribution of species is however, far from uniform with the
Americas having over 60% of recorded species, and Australia and Europe
each having less than 30% of the total. Species recorded in Australia
comprise Spirodella polyrrhiza; S.punctata; Lemna disperma; L. trisulca;
L. aequinoctialis; Wolffia australiana; W. angusta (Landolt, 1986). The
habitat requirements of duckweed vary between species, but all share the
need for sheltered still water. Depth of the plant mat is an important
limitation to growth. A striking feature of duckweed species is their
enormous reproductive capacity. Under favorable conditions they have
been reported as doubling their biomass every 16 to 48 hours (Leng, 1999).
The main form of reproduction is vegetative, through the production of
daughter fronds that arise from one of two lateral pouches at the base of
the frond. Whilst vegetative growth is usual, duckweed daughter fronds do
not stay attached indefinitely, but rather break and form new colonies, only a
few generations old. This novel facility has led to the suggestion that
duckweed growth could be considered analogous to microbial growth
(Hillman, 1961). Individual fronds have a relatively short life span of 3 to 10
weeks when in the vegetative phase, depending on species, reproductive rate
and photoperiod (Landolt, 1986).
By this time, an original mother plant may have given rise to a
clonal colony of tens of thousands of personality plants over more than 50
generations. There appears to be distinctive differences in longevity and
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mature size between generations (Landolt, 1986) that may be expressed as
cyclicity in the growth pattern of a colony. One of the significant attributes
of duckweed is its ability to be used as a source of proteinaceous food with a
favorable profile of important amino acids (Rusoff et al., 1980)
3.13 GROWTH CONDITIONS FOR DUCKWEED
The growth oflemnacae may be nearly exponential, if carbon dioxide,
light and nutrient supplies are satisfactory. Discussion in this review is
limited to the three major plant macronutrients (nitrogen, phosphorus,
potassium). Calcium and sulphur are not generally considered to be limiting
to growth (Landolt, 1986), whereas nitrogen and phosphorus influence
growth strongly and have an interactive effect.
Lemnacae are able to absorb nitrogen as ammonium, nitrate, nitrite,
urea and some amino acids, however the first two represent the main
nitrogen source for most species. Minimum, optimal, and toxic levels of
nitrogen vary greatly between species and geographic isolates and increasing
light intensity is thought to elevate optimal nitrogen requirements for
growth. Of the species studied, L. miniscula has the lowest (0.0016 mM/l)
and an unclassified species ofLemna the highest (0.08 mM/l) minimum
requirement for nitrogen (Landolt, 1986). Similarly, the maximum tolerated
level of nitrogen varies from 30 mM/l (L. miniscula) to 450 mM/l for L.
aequinoctialis (Landolt, 1986). The optimal recorded nitrogen requirement
ranges from 0.01 mM/l for W. colombia, up to 30 mM/l for S. polyrrhiza
(Landolt, 1986). Duckweeds requirement for phosphorous, is variable
(0.003-1.75 mM/l) between species as is seen for nitrogen requirement, but
appears unrelated to it (Landolt, 1986). Duckweed is reputedly able to
accumulate up to 1.5% of its weight as phosphorus in nutrient rich waters
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(Leng, 1999). Between species differences are also evident for potassium,
with requirements also being influenced by light intensity.
3.14 FACTORS AFFECTING GROWTH AND COMPOSITION OF
DUCKWEED.
There is a great deal of literature published on actual and potential
yields of duckweed (Culley and Epps, 1973; Hillman and Culley, 1978;
Rusoff et al., 1980; Oran et al., 1987; Leng, 1999; Chowdhury et al., 2000).
Unfortunately, there is little data available that records the interactions
between genotype and environment. Many trials are based on short-term
yields in small containers, with theoretical yields extrapolated to a per
hectare per annum basis. Perhaps because of this, reported yields of
duckweed vary widely. A summary of reported yields assembled by Leng
(1999) show yields ranging from 2 to 183 t(DM)/ha/year. The extremely
large range of recorded yields suggests that making estimates of productivity
based on results from short trials in laboratory-scale vessels is of
questionable value.
Significant variances in growth have been demonstrated between
species and different geographic isolates of the same species (Bergman et
al., 2000). A composite picture of yields of l. gubba on different media is
shown in Figure 1. These published results on actual and potential yield of
duckweed indicate a general lack of agreement on the growth of these plants.
There are a number of factors that may mediate these apparently conflicting
results. Quite apart from procedural differences (such as different tank sizes,
flow rate/retention times) there are numerous physico-chemical differences
that make establishment of equivalence, and thereby direct comparison
difficult. Time of year (and hence ambient temperature and day length),
-
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latitude, and pH of growth media can all have a substantial influence on the
physiology, and thus the growth of the plant.
There are many factors that influence growth, and the value of
drawing comparisons between trials conducted without similar protocols and
isolates, is also of limited value. Additionally, the levels of available
nutrient, as well as species differences, can strongly influence both the
quantity and quality of material produced. These differences may be
interpreted in light of the existence of deficient, optimal and toxic levels for
nutrients. Nitrogen in particular, whilst being an essential macronutrient, is
toxic at high concentrations. Little interest has been shown in recent times in
establishing an optimum nutrient range for growth of duckweed despite
inconsistencies in published literature. Recent work (Bergman et al., 2000;
Al-Nozaily, 2001) indicates that best growth is achieved where total nitrogen
concentrations range from 10 to 40 mg N/l. However this conflicts with the
work of Caicedo et al. (2000), who reported that growth rates ofS.polyrhiza
actually declined over a range of 3.5 to 100 mg N/l. It has been
demonstrated that lower (6 to 7) pH levels ameliorate the toxic effects of
nitrogen (McLay, 1976; Caicedo et al., 2000) and Al-Nozaily (2000) has
suggested that this may be because the low pH limits ionization of ammonia
species, resulting in a low proportion of ammonia in solution.
The optimal nutrient profile for growth of duckweed doesnt
necessarily produce the best quality of plant material in terms of protein
content and digestibility. Leng (1999) has suggested that optimal protein
content will be obtained where nitrogen is present at 60 mg N/l or greater.
Early field observations by Culley and Epps (1973) suggested that a strong
positive relationship existed between high levels of dissolved nutrients and
plant characteristics, especially protein and digestibility. Subsequently,
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several other researchers have reported positive relationships between
nutrient concentrations and dry matter yield, crude protein and phosphorous
content (Whitehead et al., 1987; Alaerts et al., 1996). In contrast, Bergman
et al., (2000) found little difference in dry matter (DM) yield and no
difference in protein content inL. gibba grown over a wide range of nutrient
levels (52 to 176 mg N/l) In practice, the depth of water required to grow
duckweed will be determined by the purpose for which it is being grown, as
well as management considerations (Leng, 1999). Ponds of less than 0.5 m
depth may be subject to large diurnal temperature fluctuations.
The greater the depth, the less likely it is that plants will have full
access to nutrients in the water column. Recently it has been found that
surface area, rather than depth, influences nitrogen removal in a duckweed
lagoon (Al-Nozaily et al., 2000).
3.15 APPLICATIONS DUCKWEED
The ability of duckweed to sequester nitrogen and phosphorus, and in
so doing cleanse dirty water, has been widely discussed in the literature
for nearly 30 years (Culley and Epps, 1973; Hillman and Culley, 1978;
Oran et al., 1986; Landolt and Kandeler, 1987; Leng, 1999). Systems
utilising various species of duckweed, either alone , or in combination with
other plants, have been used to treat primary and secondary effluent in the
U.S.A. (Zirschky and Reed, 1988), the Middle East (Oran et al., 1985) and
the Indian subcontinent (Skillicorn et al., 1993; van der Steen et al., 1998).
Notwithstanding this reputation, some species and isolates are apparently
quite sensitive to high levels of nitrogen and/or phosphorous (Bergman et
al., 2000), and effluent with a high biological oxygen demand (BOD), such
as abattoir waste, may kill the plants.
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Although duckweed has a reputation for absorbing large amounts of
dissolved nitrogen, the degree of absorption appears to vary with
concentration of nitrogen, time, species, and (at least in temperate zones)
the season. There is also strong evidence that there is a symbiotic, or at least
a synergistic relationship between duckweed and bacteria, both in the
fixation of nitrogen (Duong and Tiedje, 1985), and the removal of Chemical
Oxygen Demand (COD) (Korner et al., 1998) from water.
Differences in methodology, scale, and the parameters, both recorded
and measured, make direct comparisons between the many trials in
published literature difficult. However most research indicates that
duckweed removes 40 to 60% of nitrogen in solution over a 12 to 24 day
period. Volatilization may account for a similar loss of nitrogen (Vermaat
and Haniff, 1998), although recent work completed in Israel (Van der Steen
et al., 1998), has suggested that direct duckweed absorption may account for
less than 20% of nitrogen loss, and volatilization/ denitrification may
account for over 70% In a similar fashion, lemnacae are generally able to
absorb 30 to 50% of dissolved phosphorous, although one researcher
(Alaerts et al., 1996) has claimed over 90% removal in a working, full scale
system.
Phosphorous uptake (as measured by tissue phosphorous) and crude
protein, increased linearly with increases in nutrient concentration, up to
approximately 1.5 g P/l, and increased in absolute terms, up to 2.1 g P/l
(Sutton and Ornes, 1975). This was recorded in conjunction with a
proportional rise in nitrogen concentration, thus the association between
nitrogen and phosphorous concentrations was unclear. COD is a measure
that quantifies water quality as determined by dissolved oxygen. All research
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to growth, whereas nitrogen and phosphorus influence growth strongly and
have an interactive effect.
Lemnacae are able to absorb nitrogen as ammonium, nitrate, nitrite,
urea and some amino acids, however the first two represent the main
nitrogen source for most species. Minimum, optimal, and toxic levels of
nitrogen vary greatly between species and geographic isolates and increasing
light intensity is thought to elevate optimal nitrogen requirements for
growth. Of the species studied, L. miniscula has the lowest (0.0016 mM/l)
and an unclassified species ofLemna the highest (0.08 mM/l) minimum
requirement for nitrogen.. Duckweeds requirement for phosphorous, is
variable (0.003-1.75 mM/l) between species as is seen for nitrogen
requirement, but appears unrelated to it . Duckweed is reputedly able to
accumulate up to 1.5% of its weight as phosphorus in nutrient rich waters.
Between species differences are also evident for potassium, with
requirements also being influenced by light intensity.
3.17 FACTORS AFFECTING GROWTH AND COMPOSITION OF
DUCKWEED.
There is a great deal of literature published on actual and potential
yields of duckweed. Unfortunately, there is little data available that records
the interactions between genotype and environment. Many trials are based
on short-term yields in small containers, with theoretical yields extrapolated
to a per hectare per annum basis. Perhaps because of this, reported yields of
duckweed vary widely.
The extremely large range of recorded yields suggests that making
estimates of productivity based on results from short trials in laboratory-
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scale vessels is of questionable value. Significant variances in growth have
been demonstrated between species and different geographic isolates of the
same species. There are a number of factors that may mediate these
apparently conflicting results. Quite apart from procedural differences (such
as different tank sizes, flow rate/retention times) there are numerous
physico-chemical differences that make establishment of equivalence, and
thereby direct comparison difficult. Time of year (and hence ambient
temperature and day length), latitude, and pH of growth media can all have a
substantial influence on the physiology, and thus the growth of the plant.
There are many factors that influence growth, and the value of
drawing comparisons between trials conducted without similar protocols and
isolates, is also of limited value. Additionally, the levels of available
nutrient, as well as species differences, can strongly influence both the
quantity and quality of material produced. These differences may be
interpreted in light of the existence of deficient, optimal and toxic levels for
nutrients. Nitrogen in particular, whilst being an essential macronutrient, is
toxic at high concentrations. Little interest has been shown in recent times in
establishing an optimum nutrient range for growth of duckweed despite
inconsistencies in published literature. Recent work (Bergman et al., 2000;
Al-Nozaily, 2001) indicates that best growth is achieved where total nitrogen
concentrations range from 10 to 40 mg N/l.
However this conflicts ,that growth rates of duckweed actually
declined over a range of 3.5 to 100 mg N/l. It has been demonstrated that
lower (6 to 7) pH levels ameliorate the toxic effects of nitrogen that this
may be because the low pH limits ionization of ammonia species, resulting
in a low proportion of ammonia in solution. The optimal nutrient profile for
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Results obtained by evaluation of growth parameters were
represented as mean values of eight replicates. The control was represented
as 100% and the results obtained with treated plants were represented as
percentage of control. Chemicals that affected Lemna minor growth
significantly different from each other and control were marked with
different letters. Experiment for determination of chlorophyll a and
chlorophyll b contents was repeated three times. Results were calculated as
mean values and represented as percentage of control.
In this study, the growth of duckweed was assessed in laboratory
scale experiments.
They were fed with municipal wastewater at atmospheric
temperature. Temperature, DO, pH, TSS, TDS, Sulphate, Nitrate, Phosphate,
BOD5, COD, total nitrogen (TN), total phosphorus (TP) and ortho-
phosphate (OP) removal efficiencies of the reactors were monitored by
sampling influent and effluent of the system. Removal efficiency in this
study reflects optimal results: 73-84% COD removal, 83-87% TN removal,
70-85% TP removal and 83-95% OP removal. The results show that the
duckweed-based wastewater treatment is capable of treating the laboratory
wastewater. Wetland treatment process is a combination of all the unit
operations in a conventional treatment process plus other physico-chemical
processes, sedimentation, biological oxidation, nutrient incorporation,
adsorption and in precipitation. The use of duckweed in low-cost and easy-
to-operate wastewater treatment systems has been studied because of rapid
growth rates achieving high levels of nutrient removal.Whilst low fiberand high protein content make it a valuable fodder. Duckweed is a small,
free floating aquatic plant belonging to Lemnaceae family. Duckweed is
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4.1 Treatment methodology
4.1.1Materials and MethodsExperiments were performed in a non-continuous batch for
Phytoremediation potential of duckweed (Lemna ) in the removal of
pollutants in Municipal waste water was determined in Laboratory
experiment.
4.1.2 Study area and samples collection:
Samples of wastewater and Lemna (duckweed) were collected in
June2012. The assignment was to reclaim the sewage water collected fromGadchiroli Muncipal area. The used water treatment project was started in
June2012 and finalized in July2012. The total area of bio-treatment pond is
1mX1.5mX0.3m and total storage capacity is 0.45m3 (450 lit.). Laboratory
plant are aerobic. The water and plant samples were collected in every two
days.
4.1.3 Plant sampling and analysis of waste water:
Municipal wastewater characteristics were determined by analyzing of
some Physicochemical parameters like water Temperature, pH , Total
Alkalinity, Turbidity, Total Suspended Solids, Total Dissolve Solids, Sulfide
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, Sulfate, BOD5, COD, Oil and Grease , Phenols, Nitrates, Phosphates and
some Heavy Metals before and after the experiment. The value beforePhytoremediation experiment was noted as initial value, while the value
recorded after the Phytoremediation experiment was indicated by final
value. Pollutants removal were considered as the reduction (%) inconcentration according to:
(A-B)/A 100%
A= Initial Concentration (before experiment) .
B=Final Concentration (after experiment) .Ducweed (Lemna) plants were collected to study the pattern ofwaste
water from municipal area of Gadchiroli and phytoremediation process for
sample. From each samples were collected in replicates. Plant samples were
put in clean plastic bags and labeled carefully by permanent marker. All the
collected plant samples were placed in newspapers for the absorption of
excessive water. After 24 hours plant samples were digested and filtered,
and volume rose to 100mL.
4.1.4 Wastewater sampling and physio-chemical analysis:
One and half-liter of water samples were collected from all bio-
treatment ponds. For sample collection the bottles were washed with hot
water followed by distilled water. During collection bottles were filled,
rinsed with the sample water 2-3 times, tightly capped and properly labeled.
Physical parameters of collected water samples were studied immediately,
which were collected in replicates from all the 7 bio-treatment ponds. In
physio-chemical analysis different physical parameters were studied. Colour
was determined by direct comparison with standards and presented in
somewhat arbitrary terms of colour scale, which was observed by naked eye.
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It was done during the sampling of water on the spot (Peavy et al., 1985).
Temperature was measured by using a mercury thermometer of 0oC to 50oC
range and with 0.2oC least count. The temperature of water samples was
measured on the spot. The pH of water samples was determined in
laboratory with pH meter. The conductivity of water samples was
determined in laboratory with the help of conductivity meter.
First of all the instruments were washed with distilled water and
rinsed with the water sample. Bulb was also washed with distilled water
before putting in each water sample. The same procedure was repeated for
all water samples. For the measurement of total dissolved solids (TDS) clean
china dishes were put into oven at 103 to 105C for dryness, which were
then cooled and weigh.
Filtered water samples (20mL) were put in china dish and placed in
oven at 103 to 105C for evaporation, later on cooled in desiccators and
weighed. The increase in weight of china dish gave the weight of dissolved
solids. The results are shown in mg/liter using the following formula:
TDS = Final weight of china dish-initial weight of china dish x1000
mL of water sample used
Water samples were collected in triplicates and nitric acid (HNO3)
was added in water samples after it in situ pH measurement. All the
collected samples of water(100mL) were filtered with the filtration assembly
using the filter paper nitrocellulose membrane diameter of 0.45 m. For the
analysis of water and plant samples atomic absorption was powered on and
warmed up for 30 minutes.
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After the heating of hollow cathode lamp, the air acetylene flame was
ignited and instrument was calibrated or standardized with different working
standards. By atomic absorption spectrophotometer heavy metals of each
water and plant sample were noted.
Primary treated sewage water were transferred to the laboratory from
the tertiary sewage water treatment plant after the preliminary sieving step to
get rid of large suspended solids. The transferred water was immediately
collected into two opaque tanks to prevent light entering except at the top,
each tank with dimensions of 150 cm long, 100 cm wide and 30 cm deep
and was filled with 450 L primary treated sewage water. Duckweed
(Lemna) plants ere collected from Gadchiroli Municipal Waste Water drain.
The stock were cleaned by tap water then washed by distilled water inocula
of Lemna plants were transferred to the water systems for aquatic
treatment. The experiment was kept under outdoor local environmental
conditions for eight days retention time.
4.1.5 Water sampling: Subsurface (under duckweed mat) water
samples for physico-chemical, biological and bacteriological parameters
were collected in polyethylene bottles from all sides of tank and then mixed.
This procedure carried out every 2 days. Samples volume taken every two
days for each of phytoplankton count and chlorophyll a determination was
100 ml.
Parameters measured. Physico-chemical analyses (Table ) were
carried out according to standard methods for e examination of water and
wastewater (APHA, 1992). Field parameters (pH, conductivity & dissolved
oxygen) were measured in situ using the multi-probe system and rechecked
in laboratory using bench-top equipment to ensure data accuracy for
biological parameters including total coli form count and fecal coli-form
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RESULTS
Duckweed plant was inoculated into a primary treated sewage water
systems for aquatic treatment over 8 days (2nd
July to 9th
July , 2012 )retention time period to assess the plants efficiency in improving physico-
chemical, bacteriological and biological characteristics of sewage water. The
primary treated sewage water used in the experiment was taken from the
collector tank of the tertiary sewage water treatment plant.
Sr.No. Parameter Unit. Initial
concentration
2nd
Day
4th
Day
6th
Day
8th
Day
% Decrease
in
concentration
1 Temperature OC 29.4 23.4 22.5 20.6 24.2 17.69
2 pH 7.25 7.46 7.49 7.51 7.39 -1.93
3 DO mgO2/l 0.46 0.77 0.96 1.25 0.58 -26.09
4 TSS Mg/lit 379 28 20 16 14 96.31
5 EC Umhos/cm 905 852 878 899 995 -9.94
6 TDS Mg/lit 579 545 559 578 637 -10.02
5 CO3 Mg/lit 0.1 0 0 0 0 100.00
6 HCO3 Mg/lit 268.6 265.9 244.5 239.4 308.7 -14.93
7 T alkalinity Mg/lit 268.6 265.9 244.5 239.4 308.7 -14.93
8 BOD mgO2/lit 320 30 90.63
9 COD mgO2/lit 800 159 130 111 88 89.00
10 Phosphorus Mg/lit 4.91 4.68 4.13 3.35 2.56 47.86
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11 O
Phosphate
Mg/lit 1.5 1.49 1.45 1.423 0.534 64.40
12 Phosphate Mg/lit 11 10.5 9.25 8.12 6.2 43.64
13 Ammonia Mg/lit 10 6.5 4.7 2.2 2 80.00
14 Nitrate Mg/lit 8.32 1.8 0.5 0 0 100.0015 Calcium Mg/lit 120 78 80 80 120 0.00
16 Magnesium Mg/lit 124.8 72 75 76.8 115.2 7.69
17 Sodium Mg/lit 69.7 68.85 70.6 73.95 76.5 -9.76
18 Cloride Mg/lit 197.82 156.9 159.3 161.6 181.1 8.45
19 Sulfate Mg/lit 150.33 109.9 102.6 97.3 128.6 14.45
Pysico-chemical parameter.
Data recorded in Table showed that, values of pH were always
alkaline and ranged between 7.25 as a minimum value recorded at zero days
and 7.51 as maximum value obtained after six days treatment period. A 7.5
pH was found to be the most ideal for the successful establishment of a
duckweed system and optimum pond performance. Duckweed grew well at
pH 6 - 7.5 with outer limits of 4 and 8. it has observed that duckweed
growth declines as the pH becomes more alkaline. The dissolved oxygen
values increased as temperatures values decreased, revealing that the morecooler the water the more dissolved oxygen it can hold.
The sewage temperature is one of the crucial des