Transcript Lecture 6

CE 548
I
Fundamentals of Biological Treatment
1
Overview 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
2
Overview of Biological Treatment
 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
3
Overview of Biological Treatment
 Role of Microorganisms (MOs) in Wastewater
Treatment:
 Microorganisms (principally bacteria) oxidize dissolved and
particulate carbonaceous organic matter into simple endproducts:
Organic Matter  O2  NH 3  PO43 Micro
 CO2  H 2O  new cells
 O2, NH3, and PO43- are required as nutrients for the conversion
of organic matter to simple products
 Microorganisms are required to carryout the conversion
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Overview of Biological Treatment
 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
Nitrite
bacteria
( Nitrosomonas )
2 NH3  3O2     2 NO2  2 H   H 2O  cells  energy  end products
Nitrate
bacteria
( Nitrobacter )
2 NO2  O2  2 H     2 NO3  2 H   cells  energy  end products
 Since biomass (Bacteria flocs) has a specific gravity that is
larger than that of water, It can be removed from liquid by
gravity settling
5
Types 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
6
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
7
Suspended growth
8
Suspended growth
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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 processes
The packing can be submerged completely in liquid
or not submerged
The most common process is the trickling filter
The process is followed by settling tank
10
Attached Growth Processes
11
Attached Growth Processes
12
Introduction 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
13
Introduction to Microbial Metabolism
 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
14
Introduction to Microbial Metabolism
 Energy Source:
15
Introduction to Microbial Metabolism
 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
16
Introduction to Microbial Metabolism
 Nutrient and growth factor requirements:
In biological wastewater treatment process, two types of
organisms are important:
17
Bacterial 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.
18
Bacterial Growth
 Bacterial growth pattern in batch reactor;
 Figure 7-10 shows the growth pattern in batch process.
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Bacterial Growth
 Biomass Yield;
 Biomass yield is defined as the ratio of the amount of biomass
produced to the amount of substrate consumed:
Biomass yield Y 
g biomass produced
g substrate utilized (i.e., 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:
Biomass yield Y 
g biomass
g COD or g BOD
20
Bacterial Growth
 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:
3C6H12O6  8O2  2NH3  2C5H7NO2  8CO2  14H2O
3(180)
8(32)
2(17)
2 (113)
The yield based on the glucose consumed cab be obtained as follows:
Y
(C 5H7NO 2 )
2(113 g/mole)

 0.42 g cells/g glucose used
(C6H12O6 ) 3(180 g/mole)
21
Bacterial Growth
 Estimating biomass yield and oxygen requirements;
To express the yield in COD bases, the COD of glucose must be
determined:
C6H12O6  6O2  6CO2  6H2O
(180)
COD 
6(32)
(O 2 )
6(32 g/mole)

 1.07 g O2 /g glucose
(C 6H12 O6 ) (180 g/mole)
The theoretical yield expressed in terms of COD is given by:
Y
(C 5H7NO 2 )
2(113 g/mole)

(C6H12O 6 as COD) 3(180 g/mole)(1. 07gCOD/g glucose)
 0.39 g cells / g COD used
22
Bacterial Growth
 Estimating biomass yield and oxygen requirements;
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.
3C6H12O6  8O2  2NH3  2C5H7NO2  8CO2  14H2O
3(180)
8(32)
2(17)
2 (113)
Oxygen used
8(32 g O 2 /mole)

Glucose as COD 3(180 g/mole)(1. 07 g COD/g glucose)
 0.44 g O2 /g COD used
Study Example 7-1
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Microbial 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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Microbial Growth Kinetics
 Rate of utilization of soluble substrate;
The substrate utilization rate in biological system can be modeled with
the following expression:
rsu  
kXS
Ks  S
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
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Microbial Growth Kinetics
 Rate of utilization of soluble substrate;
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Microbial Growth Kinetics
 Rate of utilization of soluble substrate;
The maximum growth rate of bacteria is related to the maximum
specific substrate utilization rate as follows:
 m  kY
and
k
m
Y
Where; µm = max. bacteria growth rate, g new cells/g cellsd
kXS
rsu  
Ks  S
rsu  
 m XS
Y (Ks  S )
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Microbial Growth Kinetics
 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)
rg  Yrsu
But bacteria experience loss in growth rate due to decay and
predation, this is termed endogenous decay:
rd  k d X
Therefore;
rg  Yrsu  k d X
Y
kXS
 kd X
Ks  S
Eq (7 - 22)
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Microbial Growth Kinetics
 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:

rg
X
Y
kS
 kd
Ks  S
Where;
µ = specific biomass growth rate, g VSS/g VSS d
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Microbial Growth Kinetics
 Kinetic Coefficients, Oxygen Uptake and Temperature
• Typical kinetic coefficients are given in T7-9.
• The rate of oxygen uptake is given by:
ro  rsu  1.42rg
eq (7 - 24)
C5H7NO 2  5O2  5CO2  NH3  2H2O
(113)
COD 
5(32)
(O 2 )
5(32)

 1.42 gO2 / g cells
(C5H7NO 2 ) (113)
• Effects of temperature on reaction rate:
kT  k 20 (T  20)
eq (7 - 25)
 varies from 1.02 to 1.25 in biological systems
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