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Lecture 4: Biological Wastewater Treatment

Prepared by

Husam Al-Najar

The Islamic University of Gaza- Environmental Engineering DepartmentEnvironmental Microbiology (EENV-2321)

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Water Use Cycle

Water SourceWater Treatment

Plant

WaterDistribution

System

WaterUse

WastewaterCollection

WastewaterTreatment

Plant

Dischargeto Receiving

Water

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Wastewater: is simply that part of the water supply to the community or to the industry which has been used for different purposes and has been mixed with solids either suspended or dissolved.

Wastewater is 99.9% water and 0.1% solids. The main task in treating the wastewater is simply to remove most or all of this 0.1% of solids.

People excrete 100- 150 grams wet weight of feces

And 1- 1.3 liters of urine per person per day.

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Urine from separated toilets and urinalsYellow water

Water from flush toilet (faeces and urine with flush water)

Black water

Black water without urine or yellow waterBrown water

Washing water from the kitchen, bathroom, laundry (without faeces and urine)

Gray water

Source of wastewaterType of Wastewater

Type of wastewater from household

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The amount of organic matter in domestic wastewater determines the degree of biological treatment required.

Three tests are used to assess the amount of organic matter:

• Biochemical Oxygen Demand (BOD)

• Chemical oxygen demand (COD)

• Total Organic Carbon (TOC)

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Measurements of organic matter:-

Many parameters have been used to measure the concentration of organic matter in wastewater. The following are the most common used methods:

Biochemical oxygen demand (BOD).

BOD5 is the oxygen equivalent of organic matter. It is determined by measuring the dissolved oxygen used by microorganisms during the biochemical oxidation of organic matter in 5 days at 20oC

Chemical oxygen demand (COD)

It is the amount of oxygen necessary to oxidize all the organic carbon completely to CO2 and H2O.

Is measured by oxidation with potassium dichromate (K2Cr2O7) in the presence of sulfuric acid and silver and expressed in milligram per liter.

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Total organic carbon (TOC)

This method measures the organic carbon existing in the wastewater by injecting a sample of the WW in special device in which the carbon is oxidized to carbon dioxide then carbon dioxide is measured and used to quantify the amount of organic matter in the WW. This method is only used for small concentration of organic matter.

Theoretical oxygen (ThOD)

If the chemical formula of the organic matter existing in the WW is known the ThOD may be computed as the amount of oxygen needed to oxidize the organic carbon to carbon dioxide and a other end products.

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Biological Oxygen Demand (BOD)dissolved oxygen consumed by microorganisms during the biochemical oxidation of organic (carbonaceous BOD) and inorganic (ammonia) matter.

The 5-day BOD test (written BOD5) is a measure of the amount of oxygen consumed by a mixed population of heterotrophic bacteria in the dark at 20°C over a period of 5 days.

In this test, aliquots of wastewater are placed in a 300-ml BOD bottle and diluted in phosphate buffer (pH 7.2) containing other inorganic elements (N, Ca, Mg, Fe) and saturated with oxygen.

Sometimes acclimated microorganisms or dehydrated cultures of microorganisms, sold in capsule form, are added to municipal and industrial wastewaters, which may not have a sufficient microflora to carry out the BOD test.

In some cases a nitrification inhibitor is added to the sample to determine only the carbonaceous BOD.

Dissolved oxygen concentration is determined at time 0 and after a 5-day incubation by means of an oxygen electrode,

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Biological Oxygen Demand (BOD)

• BODt = BOD at t days (mg/L)• DOi = initial dissolved oxygen (mg/L)• DOf = final dissolved oxygen (mg/L)• Vs= sample volume (mL)• Vb = bottle volume (mL)• DF = dilution factor

( )DFDODO

VV

DODOBOD fi

b

s

fit −=

−=

10

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10 mL of a wastewater sample are placed in a 300-mL BOD bottle. The initial DO of the sample is 8.5 mg/L. The DO is 3 mg/L after 5 days. What is the 5-day BOD?

Example

165

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Periodic Table of Elements

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Theoretical Oxygen demand (ThOD): Example

Calculate the Theoretical Oxygen Demand (ThOD) for sugar C12 H22 O11dissolved in water to a concentration of 100 mg/L. Calculate "TOC".

Solution:-C12 H22 O11 + 12O2 → 12 CO2 + 11 H2O

ThOD = sugar

sugar

ggOg

gO /123.1342

32122

2 =×

ThOD = sugar

sugar

sugar

sugar

mgg

gOmgO

ggO

Lmg

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2

101

110123.1100

∗∗∗

ThOD = 112.3 mg O2 / LTOC = 144 g carbon/ 342g sugar = 0.42 gc/ gsTOC = 0.42 x 100 = 42 mg carbon/L

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Typical Wastewater Composition

1000500250mg/L Chemical oxygen demand (COD)

29016080Total organic carbon (TOC)

400220110mg/L 5-day, 20ْ C (BOD5,20ْ C)

Biochemical oxygen demand, mg/l:

20105mg/L Settle able Solids

27516580mg/L Volatile

755520mg/L Fixed

350220100mg/L Settle able solids (SS)

325200105mg/L Volatile

525300145mg/L Fixed

850500250mg/L Dissolved, total (TDS)

1200720350mg/L Solids, total (TS)

StrongMediumWeakUnitContaminants

Concentration

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> 400100 - 400<100Mg/LVolatile organic compounds (VOCs)

107 – 109107 – 108106 - 107no/100 ml Total coliformb

15010050mg/L Grease

20010050mg/L Alkalinity (as CaCO3)

503020mg/L Sulfatea

1005030mg/L Chloridesa

1053mg/L Inorganic

531mg/L Organic

1584mg/L Phosphorus (total as P)

000mg/L Nitrites

000mg/L Nitrites

502512mg/L Free ammonia

35158mg/L Organic

854020mg/L Nitrogen (total as N)

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100220350Suspended solids

3507201200Total Solids

4815Total P

204085Total N

122550NH3-N

81535Org-N

2505001000COD

110220400BOD

weakMediumStrong

Concentration (mg/l)Parameter

Typical characteristics of domestic wastewater

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540617663SS

139913061385COD

777667728BOD

RafahGazaNorth area

Wastewater characteristicsParameter(mg/l)

Wastewater characteristics in Gaza Strip

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101 - 102Enteric virus10-2 - 101Helminth ova10-1 - 101Cryptosporidium cysts10-1 - 102Giardia cysts101 - 103Clostridium perfringens100 - 102SalmonellapresentShigella102 - 103Enterococci103 - 104Fecal streptococci104 - 105Fecal coliform105 - 106Total coliform

Concentration (per ml)Organism

Types and numbers of microorganisms typically found in untreateddomestic wastewater

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• To prevent groundwater pollution

• To prevent sea shore

• To prevent soil

• To prevent marine life

• Protection of public health

• To reuse the treated effluent

For agriculture

For groundwater recharge

For industrial recycle

Why do we need to treat wastewater ?

• Solving social problems caused by the accumulation of wastewater

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• Protecting the public health:

Wastewater contains pathogenic microorganisms lead to dangerous diseases to humans and animals

Hazardous matter such as heavy metals that are toxic

Produces odorous gases and bad smell• Protecting the environment:

Raw Wastewater leads to septic conditions in the environment andconsequently leads to the deterioration of surface and groundwater quality and pollutes the soil.

Raw wastewater is rich with nitrogen and phosphorus (N, P) and leads to the phenomena of EUTROPHICATION.

EUTROPHICATION is the growth of huge amounts of algae and other aquatic plants leading to the deterioration of the water quality.

Raw wastewater is rich with organic matter which consumes oxygen in aquatic environment.

Raw wastewater may contains toxic gases and volatile organic matter

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Biological Characteristics:-

The environmental engineer must have considerable knowledge of the biological of waste water because it is a very important characteristics factor in wastewater treatment.

The Engineer should know:-

1. The principal groups of microorganisms found in wastewater. 2. The pathogenic organisms.3. Indicator organisms (indicate the – presence of pathogens). 4. The methods used to amount the microorganisms. 5. The methods to evaluate the toxicity of treated wastewater

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Treatment Process

Primary treatment

• screening

• grit removal

• removal of oil

• sedimentation

Secondary treatment

• Aerobic, anaerobic lagoons

• Trickling filter- activated sludge-oxidation ditch

• Mostly BOD removal technology

Tertiary treatment

• Nitrate removal

• Phosphorus removal

• Disinfection

O2

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Bar Screens

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Biological waste water treatmentIt is a type of waste water treatment in which microorganisms such as bacteria are

used to remove pollutants from waste water through bio-chemical reaction.

Classification of biological Waste water methods

Aerobic and anaerobicSuspended and attached treatment

Aerobic: biological treatment is a process in which the pollutants in the waste water (organic matter) are stabilized by microorganisms in the presence of molecular oxygen

Anaerobic: biological treatment is a process in which the pollutants in the waste water (organic matter) are stabilized by microorganisms in the absence of molecular oxygen

Suspended growth process is a biological w.w.t in which microorganisms are maintained in suspension while converting organic matter to gases and cell tissue (Activated sludge).

Attached growth is a biological w.w.tin which microorganisms responsible for the conversion of organic matter to gases and cell tissue are attached to some innert material such as rocks, sand, or plastic (Trickling filter).

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Design Criteria:

• Depth 2.5 to 5m

• Hydraulic retention time 1-20 days

• Design Loading for BOD removals in anaerobic lagoon (Horan, 1990)

Anaerobic lagoon

Design Temperature (oC)

Volumetric Loading (VL) (g BOD/m3/day)

BOD Removal (%)

< 10 100 40 10-20 20T-100 2T+20 > 20 300 60

Depth *V Flow *influent BOD

L=Area

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Anaerobic lagoon

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Aerobic lagoonDesign Criteria:

• Depth 2 to 5m

• The European standard is considered (>5KW/103m3)

• Hydraulic retention time R = 3-5 days

• Considering the kinetic and rate of cell synthesis equation:

Le and Li are the effluent and influent BOD respectively.

KT reaction rate where KT= K20 OT T-20 where K20=1.4/day

OT =1.056 when T ranges between 20-30oC 1.135 when T ranges between 4-20oC

R)K(1 L

T

i

+=eL

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Aerobic lagoon/ aerators

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Example:

The North Governorate (Jabalia, Beithanoun, Beit lahya) has a total population of 115,000 inhabitants. Design the treatment plant (anaerobic and aerobic ponds) to treat a wastewater of 600 mg/L to 30 mg/L.

Knowing that:

1. The wastewater production is 100 L/C/d

2. The average wastewater temperature is 23 oC

3. The treatment process is anaerobic lagoons followed by aerated lagoons.

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facultative ponds

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facultative ponds in Nature

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Influent Treated flow

Waste Sludge

Aeration tankTTTTTTTTTTTTTT

Conventional activated sludge system

The first version of activated sludge systems are called conventional activated sludge system.

This system is composed of two parts:

a. Aeration tank:

b. Final sedimentation tank

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Conventional activated sludge process

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Effluent from primary treatment is pumped into a tank and mixed with a bacteria-rich slurry known as activated sludge.

Air or pure oxygen pump through the mixture active bacterial growth and decomposition of the organic material.

The material then goes to a secondary settling tank, where water at the top of the tank and sludge is removed from the bottom.

The concentration of the pathogens is reduced in the activated sludge process by antagonistic microorganisms as well as by adsorption to or incorporation in the secondary sludge

An important characteristic of the activated sludge process is the recycling of a large proportion of the biomass. This results in a large number of microorganisms that oxidize organic matter in a relatively short time.

The content of the aeration tank is referred to as the mixed- liquor suspended solids (MLSS).

The organic part of MLSS is called Mixed- liquor volatile suspended solids (MLVSS), which is the nonmicrobial organic matter as well as dead and living microorganisms and cell debris.

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The activated sludge process must be controlled to maintain a proper ratio of substrate (organic load) to microorganisms or food to microorganisms ratio (F/M)

VMLSSBODQ

MF

**

=

Where,

Q = flow rate of sewage

BOD = biological oxygen demand

MLSS = mixed- liquor suspended solids

V= volume of the aeration tank

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PRIMARY AERATION TANK

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CRYOGENIC AIR SEPARATION FACILITY (HYPERION TREATMENT PLANT)

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The important parameters controlling the operation of the activated sludge process are organic loading rate, oxygen supply, and control and operation of the final settling tank.

For routine operation, sludge settleability is determined by use of the sludge volume index (SVI)

MLSSVSVI 1000*

=

The microbial biomass produced in the aeration tank must settle properly from suspension so that it may be wasted or returned to the aeration tank.

Good settling occurs when the sludge microorganisms are in the endogenous phase, which occurs when carbon and energy sources are limited.

A common problem in the activated sludge process is filamentous bulking, this caused when excessive growth of filamentous microorganisms. The filaments produced by these bacteria interfere with sludge settling and compaction.

Filamentous bacteria are able to predominate under conditions of low dissolved oxygen, low nutrients and high sulfide levels.

Where, V= volume of settled sludge after 30 minutes

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1000 ml 1000 ml

30 Minute

SVSludge

Volume: ml

Sludge Volume Index (SVI)

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THE BIOLOGICAL COMPONENT OF THE ACTIVATED-SLUDGE SYSTEM

The biological component of the activated sludge system is comprised of microorganisms.

The composition of these microorganisms is 70 to 90 percent organic matter and 10 to 30 percent inorganic matter.

Cell makeup depends on both the chemical composition of the wastewater and the specific characteristics of the organisms in the biological community.

Bacteria, fungi, protozoa, and rotifers constitute the biological component, or biological mass, of activated sludge.

In addition, some metazoa, such as nematode worms, may be present. However, the constant agitation in the aeration tanks and sludge recirculation are deterrents to the growth of higher organisms.

The species of microorganism that dominates a system depends on environmental conditions, process design, the mode of plant operation, and the characteristics of the secondary influent wastewater.

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The microorganisms that are of greatest numerical importance in activated sludge are bacteria, which occur as microscopic individuals from one micron in size to visible aggregations or colonies of individuals.

Some bacteria are strict aerobes (they can only live in the presence of oxygen), whereas others are anaerobes (they are active only in the absence of oxygen).

The preponderance of bacteria living in activated sludge are facultative-able to live in either the presence or absence of oxygen, an important factor in the survival of activated sludge when dissolved oxygen concentrations are low or perhaps approaching depletion.

While both heterotrophic and autotrophic bacteria reside in activated sludge, the former predominate.

Heterotrophic bacteria obtain energy from carbonaceous organic matter in influent wastewater for the synthesis of new cells. At the same time, they release energy via the conversion of organic matter into compounds such as carbon dioxide and water.

Important genera of heterotrophic bacteria include Achromobacter, Alcaligenes, Arthrobacter, Citromonas, Flavobacterium, Pseudomonas, and Zoogloea.

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Autotrophic bacteria in activated sludge reduce oxidized carbon compounds such as carbon dioxide for cell growth.

These bacteria obtain their energy by oxidizing ammonia nitrogen to nitrate nitrogen in a two-stage conversion process known as nitrification.

Due to the fact that very little energy is derived from these oxidization reactions, and because energy is required to convert carbon dioxide to cellular carbon, nitrifying bacteria represent a small percentage of the total population of microorganisms in activated sludge.

In addition, autotrophic nitrifying bacteria have a slower rate of reproduction than heterotrophic, carbon-removing bacteria.

Two genera of bacteria are responsible for the conversion of ammonia to nitrate in activated sludge, Nitrobacter and Nitrosomonas.

Nitrification generally occurs when the time that the sludge stays in the system (called the mean cell residence time, or MCRT) is increased.

A longer mean cell residence time, therefore, allows an adequate population of nitrifying bacteria to be built up. However, because the oxygen demand for complete nitrification is high, both the necessary oxygen supply and power requirements for the system will be increased.

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Moreover, optimum pH for the growth of nitrifying bacteria is in the 8 to 9 range, with pH levels below 7 causing a substantial reduction in nitrification activity.

In the process of converting ammonia to nitrate, mineral acidity is produced.

In instances when insufficient alkalinity exists, the pH in the system will drop, potentially inhibiting nitrification.

Finally, though nitrification occurs over a wide range of temperatures, a reduction in temperature produces a slower rate of reaction.

Some activated sludge systems have been designed specifically to promote the higher growth rate of bacteria that remove carbon from influent wastewater, and adding chemicals may suppress nitrification.

Other systems are operated to achieve nitrification in the second stage of a two-stage activated-sludge system due to the longer mean cell residence time (MCRT) necessary for nitrification.

Still other systems are designed to promote nitrification.

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Fungi are also a constituent of activated sludge.

These multicellular organisms metabolize organic compounds and can successfully compete with bacteria under certain environmental conditions in a mixed culture.

In addition, a small number of fungi are capable of oxidizing ammonia to nitrite, and fewer still to nitrate.

The most common sewage fungus organisms are Sphaerotilus natans and Zoogloea sp.

A number of species of protozoa have been identified in activated sludge. Protozoa are single-celled organisms that can consume food such as bacteria and particulate matter.

Ciliated protozoa are numerically the most common species in activated sludge, but flagellated protozoa and amoebae may also be present.

The species of ciliated protozoa most commonly observed in wastewater treatment processes include Aspidisca costata, Carchesium polypinum, Chilodonellauncinata, Opercularia coarcta and O. microdiscum, Trachelophyllumpusillum, Vorticella convallaria and V. microstoma. Protozoa are a useful biological indicator of the condition of the activated sludge.

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Being strict aerobes, these microorganisms prove to be excellent indicators of an aerobic environment (though some protozoa are capable of surviving up to 12 hours in the absence of oxygen).

Protozoa also act as indicators of a toxic environment, as they exhibit a greater sensitivity to toxicity than bacteria.

A Further, ciliated protozoa play the dominant role in the removal of Escherichia coli from wastewater by predation or flocculation.

The E. coli population is generally reduced by 91 to 99 percent in the activated-sludge process.

Rotifers are multicellular aquatic microorganisms that look like rapidly revolving wheels when they are in motion.

Rotifers are able to consume both microbes and particulate matter.

Like protozoa, these microorganisms are strict aerobes and are more sensitive to toxic conditions than bacteria. Rotifers are found only in a very stable activated-sludge environment.

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Finally, viruses are also found in wastewater, particularly human viruses that are excreted in large quantities in feces.

These human enteric viruses can be divided into six major subgroups: adenovirus, coxsackievirus, echovirus, infectious hepatitus, poliovirus, and reovirus.

Viruses native to animals and plants exist in lesser quantities in wastewater, and bacterial viruses may also be present.

There is a quantitative reduction of these viruses by the activated-sludge treatment process, the mechanism by which they are removed or deactivated remains to be clearly explained.

Different mechanisms indicated by the work of various researchers included inactivation of viruses by biological antagonists in the sludge, adsorption, and reduction in which suspended solids, colloidal material, aeration, and perhaps toxic substances play a role.

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PSEUDOMONAS SP NITROBACTER SP CARCHESIUM SP

OPERCULARIA SP VORTICELLA CONVALLARIA ENTAMOEBA HISTOLYTICA

ESCHERICHIA COLI LECANE SP. (ROTIFER) POLIOVIRUS

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SOLIDS SEPARATION

Microorganisms play a second important role in the activated-sludge process beyond removing carbonaceous organic material from and nitrifying ammonia in secondary influent wastewater.

This is the process of solids separation in which activated-sludge solids separate by flocculation and gravity sedimentation from treated wastewater in secondary clarifers.

The goal of this process is to create a secondary effluent low in suspended solids in the upper portion of a clarifier and a thickened activated sludge composed of flocs in the bottom portion of a clarifier that will be recycled back into the system as return activated sludge (RAS).

Activated sludge flocs, agglomerations of particles that may reach sizes of more than 1mm, are composed of the biological component discussed in the previous section and a nonbiological component.

genera of heterotrophic bacteria, including Achromobacter, Alcaligenes, Arthrobacter, Citromonas, Flavobacterium, Pseudomonas, and Zoogloea,appear to be the primary floc-forming microorganisms.

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Two levels of structure exist in activated-sludge flocs: microstructure and macrostructure.

Microbial aggregation, adhesion, and bioflocculation are the basis of the microstructure.

Although the mechanism of bioflocculation is not well understood, it is felt to be the result of bridging between extracellular microbial polymers functioning as polyelectrolytes (a substance of high molecular weight, such as a protein, that is an ionic conductor).

These extracellular microbial polymers form felt-like envelopes around cells and groups of cells.

The macrostructure of activated sludge consists of filamentous organisms that form the network within a floc onto which floc-forming bacteria cling.

This network of filamentous organisms provides activated-sludge flocs with strength and the attainment of large size.

As a consequence, their integrity is preserved in the aeration basin, where conditions of increasing shear occur in a turbulent environment.

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Water is also contained within activated-sludge flocs, and the amount varies with the size of the particles present.

The three types of water found in flocs are the water within the organisms, capillary water within the particles, and stagnant water within the intertstices formed by the collection of particles into a mass.

52Effect of filamentous الخیطیة organisms in activated sludge on morphology and

settleability

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Theory of attached growth treatment:

Liquid flow (Qin, S)

Organic matter + NH4+

O2

End products(CO2+H2O)

+ NO3-

(Qe , Se)

Solid media(rocks)(plastic)

(biomass layer)( or fixed biofilm)

The film composed of bacteria, fungi, algae, and protozoa.

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