Download Biochemical methods of conversion

Biochemical methods of conversion
K V Rajeshwari, Fellow and Malini Balakrishnan, Fellow
Energy–Environment Technology Division, T E R I, New Delhi
Introduction 795
Basics of biochemical
processes 795
Aerobic and anaerobic systems for
treatment of liquid
wastes 815
Aerobic and anaerobic systems for
solid waste treatment 839
Nomenclature 852
Glossary 852
References 854
Bibliography 856
Introduction
Organic wastes are important resources that can
be converted into useful products by adopting
suitable bio-processes. Waste materials considered
to be bio-resources include animal residues such
as cattle dung, poultry litter, municipal solid
wastes (including food and vegetable market
wastes), industrial organic wastes such as those
from food-processing industries, sugar, and plantation industries (for example, tea and coffee
processing), leather, distilleries, pulp and paper,
and many more (Dhussa and Tiwari 2000). In addition, agricultural wastes and wetland vegetation
such as water hyacinth also form an important category of bio-resources.
Basics of biochemical processes
A biochemical process refers to a number of
complex chemical reactions occurring in the
living organisms or involving living organisms.
The basis of any biochemical reaction is
(i) hydrolysis of complex materials in organic
matter, such as carbohydrates, proteins, and fats,
into simple nutrients or (ii) the synthesis of
complex molecules from simple substances. The
process is accompanied by the release of energy for
growth and maintenance and useful by-products.
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Food + Micro-organisms → Energy + Products
For example, photosynthesis is a biochemical process in which plants
utilize solar energy for converting carbon dioxide and water into sugar and
oxygen (Figure 14.1).
In the process of respiration, organic compounds are converted into
complex molecules such as starch, proteins, and fats, which have energy stored
in bonds. This energy is released in the form of ATP (adenosine triphosphate)
during cellular respiration
Glucose + O2 → CO2 + H2O + 36ATP
H2O + ATP → ADP (adenosine diphosphate + P + Energy)
Fermentation is a biochemical process commonly used in industries for
converting organic substrates (for example, carbohydrates) into useful
products such as alcohols, acids, and so on. A common example of biochemical conversion is fermentation of milk with lactic acid bacteria. A list of starter
cultures in fermentation of milk is given in Table 14.1.
Figure 14.1 Photosynthesis process
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Table 14.1 List of starter cultures
Streptococcus
Streptococcus
Streptococcus
Streptococcus
cremoris
lactis
thermophilus
diacetylactis
Lactobacillus bulgaricus
Leuconostoc cremoris
Leuconostoc dextranicum
Lactobacillus acidophilus
Source www.techno-preneur.net/timeis/technology/MaysciTech/DairyProduct.html,
last accessed on 25 July 2005
Various food products are obtained from fermentation processes. Most
of the industrial food processes are based on fermentation by yeast, fungi, and
mould. Alcohol in beer and wine is attributed to fermentation by yeast. Most
dairy products are produced by fermentation of milk by microbes, namely
lactococcus, lactobacillus, and streptococcus. Beer, a product of alcoholic
fermentation of cereal by yeasts, can trace its origin to the Babylonian Empire (Mesopotamia) before 6000 BC. By 2000 BC, many types of beer existed
in Babylon. The Egyptians were the first to discover that dough ferment
resulted in light delicious bread. Some examples of food fermentation are
given in Table 14.2.
Classification of micro-organisms
Micro-organisms can be broadly classified into plants, animals, and higher and
lower protista. Micro-organisms belonging to the plant and animal kingdoms
are multicellular, whereas those under protista can either be unicellular or
multicellular. Microbes, which are of importance to waste treatment, are
protists such as algae, protozoa, fungi, blue-green algae, and bacteria. These
can be further classified as eukaryotic cells containing well-defined nucleus
with a membrane or prokaryotic cells without a nuclear membrane. Bacteria
and blue green algae are prokaryotic, whereas the algae, protozoa, and fungi
are eukaryotic.
Table 14.2 Examples of food fermentation
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Product
Substrate
Micro-organism
Chocolate
Cacao bean
Bread
Coffee
Sauerkraut
Soy sauce
Flour
Coffee bean
Cabbage
Soyabean
Saccharomyces cerevisiae
Candida rugosa
Kluyveromyces marxianus
S. cerevisiae
Erwinia dissolvens
Leuconostoc plantarum
Aspergillus oryzae
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Bacteria can be classified on the basis of their shape and can be cylindrical, rod-shaped, spherical, and helical. In waste utilization, various groups
of micro-organisms act on the substrate to be treated by one of the four
processes, namely, aerobic, anoxic, anaerobic, and a combination of the
aerobic/anoxic or anaerobic processes. Each process can be further subdivided into attached or suspended growth or a combination of both. The
biochemical action of the microbes by any of the above routes results in the
degradation of carbonaceous and nitrogenous matter.
Classification of micro-organisms can also be based on the use of carbon
for growth. Autotrophic organisms are those that utilize carbon dioxide as a
source of carbon, while organisms utilizing organic carbon are known as
heterotrophic. Autotrophic organisms are further classified into photosynthetic
or chemosynthetic, depending on their source of energy, that is, the sun or
inorganic oxidation–reduction reactions.
In addition to classification based on source of energy and carbon, the
oxygen requirement for growth leads to another category of classification of
micro-organisms, namely aerobic (those that require oxygen), anaerobic (those
that can survive in the absence of oxygen), and facultative (organisms that
can exist under both aerobic and anaerobic conditions). Organisms that require the presence of oxygen for their metabolism are also known as obligate
aerobes. Almost all animals, fungi, and several bacteria are obligate aerobes.
Organisms for which the presence of molecular oxygen is toxic are known as
obligate anaerobes. These are involved in acid fermentations, acetogenesis,
and methanogenesis. Facultative anaerobes can use available oxygen and also
other electron acceptors such as nitrate in absence of oxygen. Yeast and human cells are examples of facultative micro-organisms. Table 14.3 lists the
organisms that are obligate aerobes, obligate anaerobes, and facultative
anaerobes.
Table 14.3 Examples of different kinds of aerobic and anaerobic micro-organisms
Obligate aerobes
Facultative anaerobes
Obligate anaerobes
Bacillus subtilis
Bdellovibrio spp.
Bordetella pertussis
Legionella spp.
Mycobacterium leprae
Mycobacterium tuberculosis
Pseudomonas spp.
Bacillus anthracis
Corynebacterium diphtheriae
Escherichia coli
Lactobacillus spp.
Klebsiella spp.
Salmonella sp.
Staphylococcus aureus
Streptococcus spp.
Clostridium botulinum
Clostridium tetani
Source Abedon (1998)
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Enzymes in biochemical processes
Cell growth and multiplication of micro-organisms are facilitated by enzymes
that are extracellular or intracellular. These enzymes act as catalysts during
the absorption of substrate or nutrient by the cell and also during the
conversion of substrate into product. The reaction is represented by the
following equation.
[E] + [S] → [E][S] → [P] + [E]
where [E] is the enzyme, [S] the substrate, [E][S] the enzyme–substrate
complex, and [P] is the product.
The activity of enzymes depends on the substrate, its concentration,
pH, and temperature. The optimum pH for enzymatic activity is 7 and the
optimum temperature is 35–40 ºC. At higher temperatures, enzymes get
denatured. The action of enzymes can be explained in a simplified manner by
the lock-and-key mechanism (Figure 14.2). Each enzyme is specific to a
specific substrate and has an active site where the substrate binds and
reaction occurs. The conversion of a substrate into a product can be a singlestep or a multi-step reaction, catalysed by a series of enzymes. For example,
rennet, an enzyme preparation containing chymosin, is used to catalyse
changes in milk proteins for gel formation during preparation of cheese.
Metabolic pathways of various substrate conversions in a cell are very complex
due to multiple reactions. For example, formation of ethanol from pyruvate in
yeast and other micro-organisms is catalysed by two types of enzymes. In the
first step, pyruvate is converted into acetaldehyde, which is catalysed by
pyruvate decarboxylase. The second step is the reduction of acetaldehyde to
ethanol, catalysed by alcohol dehydrogenase.
Figure 14.2 Lock-and-key mechanism of enzyme and substrate
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Pyruvate + H+→Acetaldehyde + CO2
+ +
Acetaldehyde + NADH + H →
←Ethanol + NAD
The glycolytic pathway for conversion of glucose into pyruvate along with
energy production in the form of ATP is catalysed by different enzymes for each
step (Stryer 1986; Figure 14.3). For details on glycolysis, Krebs’ cycle and other
biochemical path ways, any standard book on biochemistry (e.g. Stryer 1986)
can be referred. Enzymatic activity is maximum at optimum temperature
and pH. An acidic or alkaline pH affects the shape of enzyme molecules
and inactivates the site at which reaction occurs (Figure 14.4).
Figure 14.3 Glycolytic pathway
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Figure 14.4 Effect of pH on enzymatic reaction
Figure 14.5 Optimum temperature in enzyme-catalysed reaction
At low temperatures, molecules of the substrate do not have enough energy
for the reaction to take place. At high temperatures, molecules are highly energetic and cause degeneration of enzyme shape and its activity. Most enzymes
involved in the metabolic reactions of the human body work at body temperature. Enzymes involved in the thermophilic reactions work at a higher
temperature (Figure 14.5). In addition to the optimum temperature and pH,
enzyme activity is also affected by the presence of various elements. The
enzyme systems of acetogenic and methanogenic bacteria, for example, formate dehydrogenase and hydrogenase, involved in waste treatment processes
require the presence of selenium, tungsten, and nickel (Table 14.4).
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Table 14.4 Examples of enzymes dependent on trace elements
Trace element
Enzyme
Bacteria
Selenium
Hydrogenase
Formate dehydrogenase
Glycine reductase
Nicotinic acid hydroxylase
Xanthine dehydrogenase
Formate dehydrogenase
Carbon monoxide
dehydrogenase
Hydrogenase
Methanococcus spp.
Clostridium thermoaceticum
Clostridium pasteurianum
Clostridium thermoaceticum
Methanobacterium thermoautotrophicum
Acetobacterium woodii
Tungsten
Nickel
Source Malina and Pohland (1992)
Energy requirement for growth and maintenance
The conversion of various nutrients in a cell results in the generation of
energy, which is stored in the form of ATP. The release of energy from ATP for
various purposes results in the generation of ADP, which can again absorb
energy to form ATP. In addition to the carbon source, micro-organisms,
particularly bacteria, require other nutrients like nitrogen and phosphorus
(Figure 14.3). In the process of glycolysis, glucose is converted into glucose-6phosphate, fructose-6-phosphate, and fructose 1,6-diphosphate by the
addition of phosphorus from ATP. Fructose 1,6-diphosphate produces
dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by the action
of aldolase. ATP is again released when pyruvate is formed from 1,3diphosphoglycerate.
Pyruvate formed from glycolysis is further processed through the
Kreb’s cycle in mammalian cells for additional ATP production through
oxidative phosphorylation. This is an aerobic respiration process, also known
as the citric acid cycle, in which pyruvate is converted into simpler compounds and citrate is formed as the first intermediate.
Figure 14.6 shows the Kreb’s cycle depicting the conversion of pyruvate
through TCA (tricarboxylic acid) pathways. The oxidative decarboxylation of
pyruvate to form acetyl CoA (coenzyme A) is the link between glycolysis and
the citric acid cycle.
Pyruvate + CoA + NAD+ Acetyl CoA + CO2 + NADH
In Kreb’s cycle, decarboxylation of pyruvate occurs in the presence of
CoA, resulting in the formation of acetyl CoA. This is catalysed by pyruvate dehydrogenase complex. Acetyl CoA condenses with oxaloacetate to
form citrate that isomerizes to isocitrate, which is further oxidized and
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Figure 14.6 Kreb’s cycle
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decarboxylated to α-ketoglutarate. Further, oxidative decarboxylation
reaction results in the production of succinyl CoA that is converted to
succinate, which is further oxidized to fumarate. Fumarate on hydrolysis
forms malate and on further oxidation, converts to oxaloacetate. All these
reactions are catalysed by various enzymes (see Stryer 1986 for details).
Bacterial growth and kinetics
The growth of bacteria in a batch system follows a sigmoidal curve and
includes four distinct stages, namely lag phase, log phase (or growth phase),
stationary phase, and death phase. The different phases, along with corresponding changes in substrate concentration, are shown in Figure 14.7 (a). The
lag phase is the time required for adaptation of bacteria to the growth medium. The log or growth phase is the stage in which bacteria multiply by
consuming nutrients available in media. The growth phase can be further divided into three stages: log growth phase, declining growth phase, and
endogenous phase. In the log growth phase, abundant nutrients are available for the bacteria. This is followed by the declining growth phase in
which the nutrient supply becomes limited. The endogenous phase results in
the metabolism of cell protoplasm due to the non-availability of food. During the stationary phase, there is a balance between the growth and the
decay of cells. Finally, the death phase indicates increased lysis of cells with
the cell decay rate predominating the growth rate.
Figure 14.7(a) Schematic diagram of bacterial growth curve
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As mentioned in earlier sections, in addition to the formation of various
products through biochemical conversion of resources, a major application of
biological processes is in the utilization of waste water and solid wastes. The
following section focuses on the kinetics and types of biochemical reactions
for waste degradation.
Mixed cultures
Mixed cultures are microbial cultures, involving two or more micro-organisms. The bacterial consortium used in the biological waste treatment is a
typical example of mixed cultures. This is a complex system as each microorganism has a distinct growth pattern depending on the external
parameters like pH, temperature, substrate, availability of oxygen, and
presence of other micro-organisms. Growth rates of different micro-organisms in a mixture culture are shown in Figure 14.7(b). Growth of the mixed
cultures in batch systems simultaneously follows the three steps: oxidation
of organic matter, auto-oxidation of cells for energy, and growth of cells.
These three processes are depicted using the terms COHNS for organic
matter and C5H7NO2 to represnet cell tissue as follows (Metcalf and Eddy
1993).
Figure 14.7(b) Growth curves of different microorganisms in mixed cultures
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Oxidation
COHNS + O2 + bacteria → CO2 + NH3 + other end products + energy
Synthesis
COHNS + O2 + bacteria + energy → C5H7NO2 (New cell tissue)
Endogenous rrespir
espir
ation (aut
o-o
xidation)
espiration
(auto-o
o-oxidation)
C5H7NO2 + 5O2 → 5CO2 + NH3 + 2H2O + energy
As an example, anaerobic digestion of waste involves a consortium of
bacteria, which can be broadly divided into three main groups.
Fermenting bacteria (also termed acidifying or acidogenic bacteria) These
cause hydrolysis and acidogenesis of the substrate. The exo-enzymes released
from these micro-organisms hydrolyse polymeric matter like proteins, fats,
and carbohydrates into smaller units. These, in turn, undergo an oxidation–reduction process resulting in the formation of the VFA (volatile fatty acids) as
well as some carbon dioxide and hydrogen.
Acetogenic bacteria These break down the products of the acidification
step to form acetate. In addition, hydrogen and carbon dioxide (in the case of
odd-numbered carbon compounds) are also produced.
Methanogenic bacteria These convert acetate or carbon dioxide and
hydrogen into methane. Other possible methanogenic substrates like
formate, methanol, carbon monoxide, and so on are of minor importance
in most anaerobic digestion processes (Figure 14.8).
In addition to these three main groups, hydrogen-consuming acetogenic
bacteria are always present in small numbers in an anaerobic digester.
They produce acetate from carbon dioxide and hydrogen and, therefore,
compete with the methanogenic bacteria for hydrogen. Also, the synthesis of
propionate from acetate, coupled with the production of a longer chain VFA,
occurs, to a limited extent, in anaerobic digestion. Competition for hydrogen
can also be expected from sulphate-reducing bacteria in case of sulphate-containing wastes.
The different types of reactions during anaerobic digestion are given below (Malina and Pohland 1992).
Hydrogenotrophic reactions (oxidation of hydrogen)
4H2 + CO2 → CH4 (methane) + 2H2O
4HCOOH (formic acid) → CH4 + 2H2O + 3CO2
4(CH3CHOHCH3) (2-propanol) + CO2 → CH4 + 4CH3COCH3 + 2H2O
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Figure 14.8 Steps in anaerobic digestion of waste
Source Malina and Pohland (1992)
Aceticlastic reaction (breakdown of acetate)
CH3COOH (acetic acid) → CH4 + CO2
Disproportionation reactions (oxidation of substrate)
4CH3OH (methanol) → 3CH4 + CO2 + 2H2O
4CH3NH2 (methylamine) + 3H2O → 3CH4 + CO2 + 4NH4+ (ammonium ion)
2(CH3)2S (dimethyl sulphide) + 2H2O → 3CH4 + CO2 + H2S (hydrogen
sulphide)
CH4 + 2O2 → CO2 + 2H2O
Organic wastes typically consist of proteins (40%–60%), carbohydrates
(25%–50%), and oils and fats (8%–12%). At this elemental level, the organic
matter consists of carbon, hydrogen, oxygen, and nitrogen. The aggregate organic matter in wastes and waste water can be quantified by measuring BOD
(biochemical oxygen demand), COD (chemical oxygen demand), and TOC
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(total organic carbon). BOD determination involves measurement of the oxygen used by microorganisms in the biochemical oxidation of organic matter.
The test procedure for BOD is given in standard books such as Metcalf and
Eddy, 1993.
COD measurement involves the measurement of oxygen-equivalent of
the organic matter that can be oxidized chemically using dichromate in an
acid solution, as outlined in Chapter 12. The theoretical COD can be determined if the chemical formula of the organic matter is known. This is
illustrated in the examples provided below.
Example 1
Estimate the COD (chemical oxygen demand) of (i) glucose, (ii) cell biomass,
(iii) methane, and (iv) sewage sludge with the following composition. Carbon
(31.1%), hydrogen (4.2%), oxygen (24.3%), nitrogen (3.3%), and sulphur (1.1%).
Solution
(i) Glucose
A single molecule of glucose requires 6 moles of oxygen to break down
into carbon dioxide and water. The complete oxidation of glucose is
represented by the following equation.
C6H12O6 + 6O2 → 6CO2 + 6H2O
Hence, COD of glucose is calculated as
6 × 32/180 g of O2/g of glucose = 1.07 g O2/g glucose
(ii) Cell biomass
For oxidation of the cell biomass, 5 moles of oxygen are required as represented by the following equation
C5H7NO2 (cell biomass) + 5O2 → 5CO2 + NH3 + 2H2O
Thus, COD of the cell biomass = 5 × 32/113 g of O2/g of cell biomass
= 1.42 g O2/g cells.
(iii) Methane
The complete oxidation of methane is represented as
CH4 + 2O2 → CO2 + 2H2O
The COD of methane, which is the amount of oxygen required for oxidation of each mole of methane = (2 × 32)/16 = 4 g O2/g of methane
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Biochemical methods of conversion • 809
(iv) Sewage sludge
The molecular formula is estimated by dividing the weight per cent of
each element by the atomic weight and dividing the resulting number by the
moles of carbon.
From the elemental composition given for sewage sludge, the number of
moles can be calculated as
Carbon 31.1/12
Hydrogen 4.2/1
Oxygen 24.3 /16
Nitrogen 3.3/14
Sulphur 1.1 /32
=
=
=
=
=
2.59
4.2
1.51
0.235
0.15
For molecular formulae
Carbon 2.59/2.59 = 1
Hydrogen 4.2/2.59 = 1.62
Oxygen 1.51/2.59 = 0.58
Nitrogen 0.235/2.59= 0.09
Sulphur 0.15/2.59 = 0.06
Hence, the molecular formula is
CH1.62 N0.09 O0.58 S0.06
Molecular weight = 12 × 1 + 1 × 1.62 + 14 × 0.09 + 16 × 0.581 + 32 × 0.06 = 26.096
For estimating the COD of sewage sludge, the balancing equation for the
oxidation of the sewage sludge is
CH1.62 N0.1 O0.6 S0.06 + 0.995O2 → CO2 + 0.1NH3 + 0.59H2O + 0.06H2S
Hence, COD = (0.995 × 32)/26.096 = 1.22 g of O2/g of sewage sludge.
Example 2
Estimate the theoretical methane yield for 1 g COD of organic matter.
Solution
Density of methane is 0.7167 g/litre.
Hence, 1 g of methane will occupy a volume of 1.4 litres.
As the COD of methane is 4 g [Example 1 (iii)], 1.4 litres of methane will be
equivalent to 4 g of COD.
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Thus, 1 g of COD of methane is equal to 1.4/4 litre or 0.35 litre of methane.
Theoretical methane yield is thus 0.35 litre/g COD.
Growth kinetics of micro-organisms
It is important to understand the kinetics of bacterial growth for arriving at an
optimal design of the reactor size and hence the cost. It is possible to
have an optimum growth of micro-organisms through proper control of the
environmental conditions such as pH, temperature, aerobic or anaerobic
conditions, substrate level, additional micronutrients, and so on. The digestion period and hence the retention time is dependent on the growth rate of
micro-organisms and the rate at which the substrate can be metabolized to
give stable products.
Simple kinetics in batch culture (growth phase)
The microbial growth rate can be described by the equation
...(14.1)
where rm is the rate of growth of micro-organisms (g/m3/day), µ is the specific
growth rate (day–1), and X is the microbial concentration. Microbial concentration, also called biomass concentration, is usually measured by VSS (volatile
suspended solids). See Chapter 12 for measurement of VSS.
In case of substrate limitation, where the growth rate is affected by the
limited concentration of nutrients or substrate, the cell growth is represented
by the Monod equation as
...(14.2)
where µmax is the maximum specific growth rate (g new cells/g cells/day), S is
the limiting substrate concentration (g/m3), and ks is the half-velocity constant
(g/m3).
When µ = µmax /2
Equation 14.2 becomes
Rearranging the equation, we get
...(14.3)
Biochemical methods of conversion • 811
Or
ks = S½
The graphical representation of the variation of specific growth rate
with substrate concentration is shown in Figure 14.9.
Other kinetic models proposed include
rm = µmax
(constant)
rm = µmax XS
(first order)
(mixed)
where S0 is the influent substrate concentration and k is a dimensionless kinetic parameter.
(mixed)
Substituting for µ from Equation 14.2 in Equation 14.1, the overall rate
expression can be represented as
Figure 14.9 Variation of growth rate with substrate concentration
Reproduced with permission of The McGraw-Hill Companies
Source Metcalf and Eddy (1993)
...(14.4)
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The growth of microbes is accompanied by depletion of the substrate.
The microbial growth rate and substrate depletion rate are thus related by the
equation
rm = –Y rsub
...(14.5)
where Y is the yield coefficient.
The substrate concentration is usually measured in COD or BOD. Thus
the yield coefficient is expressed in gVSS/gCOD removed.
As the concentration of substrate decreases with time, the rate is
indicated as negative.
Substituting dX/dt from Equation 14.5 in Equation 14.4 gives
...(14.6)
The term (µmax/Y) is also denoted k, the maximum specific substrate utilization rate. The decay of cells during the endogenous or death phase is given
as
...(14.7)
where rd is the rate of biomass decay (g VSS/m /day) and kd is the decay constant (g VSS/g VSS/day). The net rate of increase in biomass concentration will
thus be
3
...(14.8)
Substituting for rm from Equation 14.1, we get
...(14.9)
Net specific cell biomass growth rate (g VSS/g VSS/day) is
...(14.10)
or
Some indicative values of kinetic constants are given below.
...(14.11)
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Acid producing bacteria
Methane producing bacteria
µmax
Y
kd
> 1.33 day –1
0.54 gVSS/gCOD utilized
0.87 day –1
> 1.33 day –1
0.14 gVSS/gCOD utilized
0.02 day–1
The VSS in any bioreactor includes the cell biomass, lysed cells, or cell
debris due to the decay of the cells, and non-biodegradable VSS present in the
feed or substrate.
Effect of temperature
Temperature affects the biological reactions through its influence on
microbial metabolism, gas transfer rates, and settleability of solids. The
microbial metabolic reactions are catalysed by enzymes that are influenced
by temperature as explained earlier. The gas transfer rates are influenced
by temperature due to its effect on diffusivity and viscosity of the media.
Temperature also affects the settleability of solids by causing deflocculation
of sludge.
The effect of temperature on reaction rate can be derived from the Vant
Hoff–Arrhenius relation
...(14.12)
where k is the reaction rate constant, T is the temperature (ºC), E is the
activation energy constant in J/mole, and R is the ideal gas constant (8.314 J/
mol ºC).
Integration of Equation 14.12 gives
...(14.13)
where k2 is the rate constant at temperature T2 and k1 is the rate constant at T1.
As the waste treatment reaction occurs at ambient temperature, the
term E/RT1T2 can be considered constant C. This transforms Equation 14.13
into
...(14.14)
...(14.15)
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where η = eC
Choosing a reference temperature of 20 ºC we get
...(14.16)
...(14.17)
where kT is the reaction rate constant at a temperature T ºC, k20 is the reaction
rate constant at a temperature 20 ºC, and η is the temperature activity coefficient.
Effect of nutrients
Certain inorganic elements are required for growth, including nitrogen,
sulphur, phosphorus, magnesium, calcium, iron, sodium, and chlorine. Micronutrients such as zinc, manganese, molybdenum, selenium, cobalt, and nickel,
and growth promoters, namely amino acids and vitamins, may also be required
in certain cases.
A note on terminology
As mentioned earlier, substrate concentration is quantified as BOD and
COD in waste waters. But as waste waters contain varieties of substrates
some more terms are used to quantify, such as biodegradable COD (bCOD)
or biodegradable soluble COD (bs COD). The waste waters contain colloidal
and particulate matter, part of which may be biodegradable. Bacteria cannot
consume the particulate substrates directly. These are first hydrolysed enzymatically to convert into soluble substrates. The bCOD can be a measure of
particulate substrates also.
The bacterial concentration, also termed often as active biomass, is commonly measured as TSS (total suspended solids) and VSS (volatile suspended
solids). In waste water treatment plants in which part of the sludge produced
is recycled, the term MLVSS (mixed liquor volatile suspended solids) is used to
define biomass concentration in the reactor input. The solids consist of bacteria, nbVSS (nonbiodegradable volatile suspended solids) and iTSS (inert total
suspended solids).
The biomass yield Y can be estimated from stoichiometry considerations. For example, if we assume that the soluble substrate can be represented
as glucose (C6H12O6), and bacterial cells can be represented as C5H7NO2, a stoichiometric equation can be written as
3C6H12O6 + 8O2 + 2NH3 → 2C5H7NO2 + 8CO2 + 14H2O
Biochemical methods of conversion • 815
The yield then can be obtained as
= 0.42 g.cells/g.glucose consumed
In the above, 113 and 180 are the molecular weights of bacterial cells and
glucose respectively.
The COD of glucose can be determined by the oxidation reaction
C6H12O6 + 6O2 → 6CO2 + 6H2O
= 1.07 gO2/g. of glucose
The yield in terms of COD will then be
The actual yield in any given biological treatment process will be less
than the figures given above.
Biomass yield can also be estimated from considerations of bioenergetics
(see Metcalf and Eddy 2003).
Aerobic and anaerobic systems for treatment of liquid wastes
Different types of bioreactors for waste water treatment
Batch reactor
It is a simple vessel, with continuous mixing of contents of the reactor with
no inflow or outflow of materials. The H/D (height/diameter) ratio is
generally kept at 1:3 (Figure 14.10).
Figure 14.10 Batch reactor