1 ce 548 i fundamentals of biological treatment. 2 overview of biological treatment objectives of...

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CE 548 I

Fundamentals of Biological Treatment

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Overview of Biological TreatmentOverview of Biological Treatment

Objectives of Biological Treatment:

For domestic wastewater, the main objectives are:

• Transform (oxidize) dissolved and particulate biodegradable constituents into acceptable by-products

• Capture and incorporate suspended and nonsettleable colloidal solids into a biological floc or biofilm

• Transform or remove nutrients, such as nitrogen and phosphorous

• Remove specific trace organic constituents and compounds

33


Objectives of Biological Treatment:

For industrial wastewater, the main objectives are:

• Remove or reduce the concentration of organic and inorganic compounds

• Pre-treatment of industrial wastewater may be required due to presence of toxicants before being discharged to sewer line.

For agricultural wastewater, the main objective is:

• Remove nutrients, such as N and P, that stimulate the growth of aquatic life

44


Role of Microorganisms (MOs) in Wastewater Treatment:

Microorganisms (principally bacteria) oxidize dissolved and particulate carbonaceous organic matter into simple end-products:

O2, NH3, and PO43- are required as nutrients for the conversion

of organic matter to simple products

Microorganisms are required to carryout the conversion

cellsnewOHCOPONHOMatterOrganic Micro 22

3432

55


Role of Microorganisms (MOs) in Wastewater Treatment:

Ammonia can be oxidized by specific microorganisms (nitrification) to nitrite (NO2-) and nitrate (NO3-)

Other bacteria can reduce oxidized nitrogen to gaseous nitrogen

Since biomass (Bacteria flocs) has a specific gravity that is larger than that of water, It can be removed from liquid by gravity settling

productsendenergycellsHNOHONO

productsendenergycellsOHHNOONH

rNitrobactebacteriaNitrate

asNitrosomonbacteriaNitrite

2222

2232

3)(

22

22)(

23

66

Types of Biological ProcessesTypes of Biological Processes

The principle categories of biological processes are:• Suspended growth processes

• Attached growth (bio-film) processes

Successful design and operation of any process require the knowledge of the following:

Types of microorganisms involved Specific reactions they perform Environmental factor that affect their performance Nutritional needs of the microorganisms Reaction kinetics of microorganisms

77

Suspended Growth Processes

Microorganism are maintained in suspension by appropriate mixing methods

Many of the processes are operated aerobically

Anaerobic processes are also used for treatment of industrial wastewater having high organic content and organic sludge

The most common process used in domestic wastewater is the activated sludge process

88

Suspended growth

99

Suspended growth

1010

Attached Growth Processes

Microorganism are attached to an inert packing material

Packing materials include: Rock, Gravel, Sand Slag Redwood Wide range of Plastic and other synthetic materials

Operate as aerobic and anaerobic processesThe packing can be submerged completely in liquid

or not submergedThe most common process is the trickling filterThe process is followed by settling tank

1111


1212


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Introduction to Microbial MetabolismIntroduction to Microbial Metabolism

Understanding of microbial metabolism (biochemical activities) is important to design and selection of biological treatment.

Table 7-6 shows the classification of microorganisms by electron donor, electron acceptor, carbon source, and end products.

Organisms require the following for growth:

Source of energy

Carbon for cell synthesis

Nutrients

1414


Carbon source:

Microorganisms obtain their carbon for cell growth from either:

– organic matter (heterotrophs)

– or from carbon dioxide (autotrophs).

autotrophs have lower growth rate than heterotrophs

1515


Energy Source:

1616


Nutrient and growth factor requirements:

Nutrients: The principal inorganic nutrients needed:

– N, S, P, K, Mg, Ca, Fe, Na, and Cl

Growth factor: Organic nutrients required by some organisms include:

– amino acids

– purines and pyrimidines

– vitamins

1717


Nutrient and growth factor requirements:

In biological wastewater treatment process, two types of In biological wastewater treatment process, two types of organisms are important:organisms are important:

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Bacterial GrowthBacterial Growth

Bacterial reproduction;

The primary mechanism of reproduction is binary fission.

One cell becomes two new cells.

The time required for each division (generation time) can vary from days to less than 20 minutes

If generation time is 30 min, one bacterium would yield about 16 million (224)bacteria after 12 hours.

This rapid change of biomass depends on environmental conditions ; availability of substrate and nutrients.

1919


Bacterial growth pattern in batch reactor;

Figure 7-10 shows the growth pattern in batch process.

2020


Biomass Yield;

Biomass yield is defined as the ratio of the amount of biomass produced to the amount of substrate consumed:

Since wastewater contains a large number of organic compounds, the yield is expressed in terms of measurable parameters such as COD or BOD. Thus the yield would be:

consumed) (i.e., utilized substrate g

produced biomass g yieldBiomass Y

BOD g or COD g

biomass g yieldBiomass Y

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Estimating biomass yield and oxygen requirements;

A stoichiometric relationship exists between the substrate removal, the amount of oxygen consumed, and the observed biomass yield. Assuming organic matter can be represented as C6H12O6 (glucose), the following equation (7-3) can be written:

The yield based on the glucose consumed cab be obtained as follows:

O14H8CONOH2C2NH8OOH3C 2227538(32)

23(180)

6126 )113(2)17(2

used glucose cells/g g g/mole) 3(180

g/mole)

OHC

)NOH(C

6126

275 42.0113(2

)(

Y

2222



To express the yield in COD bases, the COD of glucose must be determined:

The theoretical yield expressed in terms of COD is given by:

O6H6CO6OOHC 226(32)

2(180)

6126

used COD g / cells g 0.39

glucose) 07gCOD/gg/mole)(1. 3(180

g/mole)

COD as OHC

)NOH(C

6126

275

113(2

)(Y

glucose /gO g 1.07g/mole) (180

g/mole) 6(32

)OH(C

)(OCOD 2

6126

2

2323



The amount of oxygen required can be obtained based on the stoichiometry as defined by equation (7-3) in which 8 moles of oxygen are required for 3 moles of glucose.

Study Example 7-1

used COD /gO g 0.44

glucose) COD/g g 07g/mole)(1. 3(180

/mole)O g 8(32

COD as Glucose

used Oxygen

2

2

O14H8CONOH2C2NH8OOH3C 2227538(32)

23(180)

6126 )113(2)17(2

2424

Microbial Growth KineticsMicrobial Growth Kinetics

Microbial growth kinetic terminology;

– bCOD: biodegradable COD; since wastewater contains numerous substrates, the concentration of organic compounds is defined by biodegradable COD. bCOD comprise soluble, colloidal, and particulate components.

– bsCOD: biodegradable soluble COD.

– TSS (total suspended solids) and VSS (volatile suspended solids): represents the biomass solids in the bioreactor.

– MLSS (mixed liquor suspended solids) and MLVSS (mixed liquor volatile suspended solids): the mixture of solids resulting from combining recycled sludge with influent wastewater in the bioreactor.

– nbVSS: non-biodegradable volatile suspended solids

– iTSS: inert inorganic total suspended solids

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Rate of utilization of soluble substrate;

The substrate utilization rate in biological system can be modeled with the following expression:

Where; rsu = rate of substrate change due to utilization, g/m3 d

k = max. specific substrate utilization rate, g sub/g micro d

X = biomass (microorganisms) concentration

S = growth limiting substrate concentration, g/m3

SK

kXSr

ssu

2626



2727



The maximum growth rate of bacteria is related to the maximum specific substrate utilization rate as follows:

Where; µm = max. bacteria growth rate, g new cells/g cellsd

Yk

and

kY

m

m

)( SKY

XSr

s

msu

SK

kXSr

ssu

2828


Rate of biomass growth with soluble substrate;

The relationship between cell growth rate and substrate utilization rate is given by: (not all subs. is converted to cells)

But bacteria experience loss in growth rate due to decay and predation, this is termed endogenous decay:

Therefore;

sug Yrr

Xkr dd

22)-(7 Eq XkSK

kXSY

XkYrr

ds

dsug

2929


Rate of biomass growth with soluble substrate;

If both sides of Eq. (7-22) are divided by the biomass concentration X, the specific growth rate is defined as:

Where;

µ = specific biomass growth rate, g VSS/g VSS d

ds

g kSK

kSY

X

r

3030


Kinetic Coefficients, Oxygen Uptake and Temperature

• Typical kinetic coefficients are given in T7-9.

• The rate of oxygen uptake is given by:

• Effects of temperature on reaction rate:

varies from 1.02 to 1.25 in biological systems

24)-(7 eq gsuo rrr 42.1

25)-(7 eq kk TT

)20(20

cells ggO NOHC

(O COD

O2HNH5CO5ONOHC

2275

2

2325(32)

2(113)

275

/42.1)113(

)32(5

)(

)

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