Biological Waste Water Treatment

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EFFLUENT TREATMENT(THEORY & PRINCIPLES)

BIOLOGICAL WASTEWATER TREATMENT

1.0 INTRODUCTION

- A wide variety of different biological treatment processes are used but thesame basic concepts apply to all of them.

- Emphasis in this paper is on fundamentals of biological treatment - not on theunique features of specific treatment techniques.

- Biological treatment has been applied primarily to organic wastewateralthough there are other applications.

- In biological treatment or organic wastewater, the same transformations thatwould occur naturally in receiving waters (with associated environmentalquality degradation) are caused to occur under controlled conditions in

biological treatment facilities.

- To effectively treat wastes biologically, it is necessary to create conditions thatfavour growth of desired organisms. To do this, it is necessary to have anunderstanding of the nature of the organisms involved and their requirementsfor growth.

2.0 NATURE OF_MICROORGANISMS

- Principal Types

Bacteria Blue-green algae Algae Fungi Protozoa

- Size

o Bacteria - about 1 micron

- Uniquitous - if conditions favouring their growth are created; they will be in

plentiful supply.

- Temperature Dependance

Roughly, the rate of microbial reactions doubles with a 100Ctemperature increase.

The rate at temperature T is often expressed as

RT = R20 (T-2) (1)

Where R20 is the rate at 200

C, and is an empirical constant withtypical values in the range of 1.02 to 1.08 (METCALF and Eddy, 1979)

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- Growth

Bacteria reproduce by fission

Can do so in as short a time as 20 minutes (under extremelyfavourable conditions in an aerobic environment)

Anaerobic organisms growth slower than aerobic

Tremendous growth potential limited only by the availability ofrequired nutrients

3.0 NUTRITIONAL_REQUIREMENTS

3.1 Introduction

- If microorganisms are to function effectively in wastewater treatment plants allingredients required for their growth must bepresent.

- If required nutrients are not found in wastewater being treated, they must beprovided.

- A listing of specific nutrient requirements follows.

3.2 Energy Source

- Two types:

Photosynthetic

Algae, blue green algae, and some bacteria

Complex reactions using suns energy such as

lightH20 + C02 02 + (CH20) (2)

where (CH20) represents the synthesis new cellular material.

The oxygen produced in Equation 2 may be of use (as an electron acceptor -see section 3.3) in biological processes such as oxidation ponds. Note,however, the algae continue to use oxygen during periods of darkness (usingthe reverse of the reaction in Equation 2).

Chemosynthetic Organisms

Bacteria, fungi and protozoana

Obtain energy by oxidation of chemical compunds

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Two types:

Heterotrophs oxidize organic compounds to obtain energy

Autotrophs oxidize inorganic substances (for example Fe++,NH4, or H2S) to obtain energy.

While biological wastewater treatment ordinarily involves heterotrophicorganisms, opportunities exist for use of autotrophic organisms as well.

3.3 Carbon Source

- Carbon is a major constituent of the microbial cells synthesized in biologicalwastewater treatment processes. Thus, an abundant source of carbon mustbe available.

- This is no problem in treatment of wastewater containing organic compounds -the organic contaminants serve as the carbon source for heterotrophicorganisms.

- Autotrophic microorganisms use inorganic carbon (bicarbonate ions or carbondioxide). In some cases, carbon might have to be added.

- Nitrogen and phosphorus are significant constituents of cells. (For example,nitrogen is needed for amino acid synthesis and phosphorus is essential forthe genetic material, deoxyribonucleic acid).

- Nitrogen and phosphorus requirements depend on the net amount of

synthesis which occurs in biological wastewater treatment processes. Thus,requirements depend on design of the treatment process and the electronacceptor.

- As a crude guideline, the necessary weight ratio of BOD:N:P is about 100:5:1in aerobic processes.

- Many industrial wastewaters are deficient in nitrogen and/or phosphorus, andnutrients must be added to accomplish effective biological wastewatertreatment.

3.4 Nitrogen and Phosphorus Sources

- Nitrogen and phosphorus are significant constituents of cells. (For example,nitrogen is needed for amino acid synthesis and phosphorus is essential forthe genetic material, deoxyribonucleic acid).

- Nitrogen and phosphorus requirements depend on the net amount ofsynthesis which occurs in biological wastewater treatment processes. Thus,requirements depend on design of the treatment process and the electronacceptor.

- As a crude guideline, the necessary weight ratio of BOD:N:P is about 100:5:1

in aerobic processes.

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- Many industrial wastewaters are deficient in nitrogen and/or phosphorus, andnutrients must be added to accomplish effective biological wastewater treatment.

3.5 Trace Minerals

- In addition to C, H, O, N and P, microbes require a wide variety of inorganicmaterials for their metabolism and for synthesis of new cells.

- For example, the catalytic activity enzymes are often associated with Co, K,Mn, Zn, Cu, Fe, Mo, Zn, etc.

- Required amounts are small but, still, the trace minerals are essential.

- Orgindarily assumed to be contained in the wastewater carriage water.

- But could limit treatment effectiveness.

3.6 Growth Factors

- Some organisms can synthesize all of the organic compounds they require,but others (like humans) requires some preformed organic compounds.

- Examples are some amino acids, vitamins, etc.

- Presumably, in heterotrophic systems, lysis of organisms results in availabilityof necessary growth factors for other organisms.

3.7 Water

- Microorganisms use substances in solutions - thus water is essential.

- No problem of availability in industrial wastewater treatment systems, but themoisture content of soil can limit the rate of microbial transformations in landtreatment systems.

4.0 RATE OF SUBSTRATE UTILIZATION

- Substrate (wastewater constituents) removal in biological processes occursbecause of enzymatically catalyzed reactions. Thus, questions on the rate of

removal of wastewater constituents are answered by considering the rate atwhich enzymatically catalyzed reactions occur.

- Conceptually, it is considered that such reactions occur as follows:

E + S ES E + P (4)

where E denotes a specific enzyme, S, the substrate, P the product of thereactions, and ES a temporary enzyme-substrate complex.

- As in the nature of catalysts, the enzyme is released when the product isformed so as to combine again with new substrate.

- The maximum rate of an enzymatically catalyzed reaction occurs when allenzymes is in use (that is, it is combined as the enzyme-susbtrate complex,ES).

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- Use of the law of mass action and Equation 4 leads to the followingexpression for d(S)/dt, the rate of removal of substrate.

d (S) = Vm (S)

dt Km + (S)

- The relationship between substrate concentration and rate of substrateremoval as given in Equation 5 is shown in Figure 1.

o When the substrate concentration is high, the rate of removal becomes

independent of substrate concentration and approaches the maximum possible rate, Vrn(Zero orderkinetic).

o When the substrate concentration is low, the rate of substrate removal

becomes directly proportional to substrate concentration. (First order kinetics). Note that this

is the assumption used in developing the traditional BOD rate equation.

- Experimental data for pure substrates indicate that values of Km and Vm aresuch that zero order kinetics prevails until low substrate concentrations arereached. Note the zero order curves in Figures 2 (from ECKENFELDER(1980)). Possibly the first order BOD removal kinetics observed with realwastewaters are the results of summation of many zero order curves from thecomponents of the heterogeneous wastewater.

5.0 SLUDGE PRODUCTION IN BIOLOGICAL WASTEWATER TREATMENT

- Microorganisms in biological wastewater treatment systems oxidize substratein order get energy required to synthesize more organisms. That is, it is theirgoal to produce excess sludge.

- A certain amount of energy is required for cell maintenance and this causes adecrease in net synthesis. This reduction in cell mass is called endogenousrespiration.

- Net synthesis may be expressed as

dx = d(S) - KdX

dt dt

where x = concentration of microoganisms

dx = rate of change of x with timedt

Y = yield coefficient= mass of synthesis/ mass of substrate utilized

Kd = Endogenous decay coefficient- The magnitude of the yield coefficient, Y, depends on the nature of the

substrate and the electron acceptor. A typical value is 0.5mg cells/mg BODdestroyed. For pure substrates, it is possible to estimate Y from

5

(5)

Y (6)


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thermodynamics consideration (CHRISTENSEN and McCarty, 1975). Thevalue of Y can be estimated from experimental observations as well.

- The magnitude of the endogenous decay coefficient typically is in the order of0.05 to 0.1 day-1.

- Note from Equation 6 that if a larger population of organisms, X, is used toachieve the same rate of substrate utilization, d(S)/dt, then net synthesis willbe reduced.

6.0 OXYGEN REQUIREMENTS IN AEROBIC SYSTEMS

- The amount of oxygen required for aerobic treatment of organic wastewaterscannot be ascertained from the change in oxygen demand between theinfluent and the effluent because some of the organic matter removedremains in the form of microorganisms.

- Taking the oxygen equivalent of microbial cells as 1.42 gm 02/gm cells*, thenthe amount of oxygen required in aerobic biological waste treatment can becalculated as

O2 = BODL - 1.42 X (8)

where O2 = daily mass of oxygen required

BODL = total ultimate BOD removed /day

X = mass of excess biological sludge wasted /day

* If the approximate chemical composition of cells is taken as C5H7N02, then, theamount of oxygen required to oxidize the cells maybe calculated from the equation:

C5H7N02 + 5 O2 5 CO2 + 2 H2O + NH3 (7)

7.0 MEAN CELL RESIDENCE TIME AS A MEASURE OF PROCESS PERFORMANCE

- By writing mass balance equations on biological reactors operating at steadystate, the relationships between wastewater characteristics, process designand process performance can be explored. The reader is referred to

LAWRENCE and McCarty (1970) for details.

- The principal design and operational variable that emerges from the analysisis the mean cell residence time, c

- The mean cell residence time is the average time a cell remains in thetreatment system. The mean cell residence time may be written as:

c = VXQuXu + QeXe

Where V = Volume of the system

X = Concentration of cells in the system

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Qu = Rate of sludge wastage

Xu = Concentration of cells in waste sludge

Qe = Effluent flow rate

Xe = Concentration of cells in the effluent

- The relationship between the mean cell residence time c and the effluentsubstrate concentration is:

S = Km (1 + Kdc)c (Vm Y-Kd) 1

- The form of Equation 10 is illustrated in Figure 3. No substrate removal occursat c values below that value at which organisms that can use the substrateare washed out of the system faster than they can reproduce.

8.0 REFERENCE

CHRISTENSEN, D.R., and McCarty, P.L. (1975), Multi Process Biological TreatmentModel", Journal Water Pollution Control Federation, 47, 2652.

ECKENFELDER, W.W. Jr. (1980), Principles And Practice of Biological WastewaterTreatment, Proceedings of Summer Institute on Biological Waste Treatment,

Manhattan College, New York, N.Y.

LAWRENCE, A.W. and McCarty, P.L. (1970), Unified Basis for Biological TreatmentDesign and Operation, Journal Sanitary Engineering Division, American Society ofCivil Engineers, 96, 757.

METCALF and Eddy, Incorporated, (1979) Wastewater Engineering Treatment,Disposal, Reuse, McGraw-Hill Book Company, Singapore, 960 pp.

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Substrate Concentration, (S)

Figure 1: Rate of Substrate Removal in Biological Wastewater Treatment.

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Figure 2: Illustrations of Zero Order (Linear) Substrate Removal from Eckenfelder, (1980)

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Figure 3: Relationship between mean Cell Residence Time and Biological Wastewater TreatmentPlant Performance

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EffluentBOD

Concentration

Mean Cell Residence Time, c


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SEPTIC TANK

Wastewater Flows Through All Three Chambers

TRAVIS TANK

Wastewater Flows Through SedimentationChambers Only

IMHOFF TANK


Figure 1: Early anaerobic treatment systems which combined sedimentation and digestion ina single unit

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Figure 4: Conventional and high rate separate digestion systems, commonly used forsewage sludge digestion

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CONVENTIONAL DIGESTER

HIGH RATE DIGESTER


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Figure 5: Suspended-growth digesters designed to maintain high bacterial populations,allowing digestion at shortened hydraulic detention times

DIGESTER CONTROL

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ANAEROBIC CONTACT PROCESS

ANAEROBIC CLARIGESTER

UPFLOW ANAEROBIC SLUDGE BUCKET


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External Control

Feed Rate

a) Volume

b) Solids

Internal Control

2.1 Temperature:

a) Mesophilic (29 370C)

b) Thermophilic (40 600C)

2.2 Volatile Acids/Alkalinity Ratio (0.35)

An increase in the ratio is the first warning that trouble is starting.

Because of the alkalinity in the digester, the pH changes very slowly. In fact,the digester may be completely upset before the pH changes.

Frequent monitoring of volatile acid and alkalinity is necessary.

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Figure 1: The anaerobic degradation of organic material can be divided into three steps

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1. Relationship Of Volatile Acids To Alkalinity

2. Volatile Acids/ Alkalinity Ratio

3. Relationship Of The Change In Ratio OfCO2 To Methane (CH4) As A Result Of 1

4. Relationship Of pH Change To ChangeIn 1

Figure 4.9: Graph of change sequence in a digester

TimeThis graph shows a digester operating with agood buffering capacity (the low volatile acids200 mg/l compared to an alkalinity of 2000

mg/l. At point A, something has happened tocause the volatile acids to increase followedby a decrease in alkalinity at point D. At pointG, the digester has become sour.

This graph continues the same digesterperformance by showing the volatile acids/

alkalinity ratio. Notice that at points CD, theincrease in volatile acids produces anincrease in the ratio from 0.1 to 0.3.

By comparing this graph with Graph 2,methane production begins to drop with acorresponding increase in CO2 when the ratioin Graph 2 reaches about 0.5

pH does not change in this graph until thedigester is becoming sour at Point G

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Biological Waste Water Treatment