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
816 • Renewable energy engineering and technology
Figure 14.11 Plug flow reactor
Plug flow reactor
This type of reactor has a high length–width ratio with minimal or negligible
longitudinal dispersion. The outflow of materials follows the same sequence
as the influx, and each particle is retained in the reactor for the duration
equivalent to the retention time (Figure 14.11).
CSTR (Continuous flow stirred tank reactor)
There is a complete mixing in the reactor that results in immediate dispersion of particles entering the reactor. For completely mixed conditions, the
residence time is given as the volume of reactor divided by the volumetric flow
rate of feed. Complete mixing is often not achieved in practical reactors and
hence there is a residence time distribution, depending on the shape of reactor, degree of mixing, etc. (Figure 14.12).
Packed bed reactor
The reactor is packed with external media such as rock, plastic, and so on.
This can be operated either as an anaerobic filter in absence of oxygen by
complete loading of reactor or as a trickling filter by intermittent loading
(Figure 14.13).
Fluidized-bed bioreactor
The reactor is similar to the packed bed reactor but differs in the
characteristics of the packing medium, which is usually sand or powdered
charcoal that has fluidizing properties. The bed expands due to the flow of
liquid or air (Figure 14.14).
Figure 14.12 Continuous flow stirred tank
Figure 14.13 Packed bed
Biochemical methods of conversion • 817
Figure 14.14 Fluidized-bed reactor
The conversion of nutrients to final products and energy by complete
oxidation of the decomposition products results in a higher energy production
in the case of aerobic processes compared to anaerobic systems. However,
most of the energy production goes towards the growth of cells resulting in
a higher level of sludge production as compared to the anaerobic systems.
Another advantage with the anaerobic systems is the low energy and maintenance requirement for the process resulting in higher net energy production.
The cycle of conversion to the intermediate and final products of decomposition is shown in Figure 14.15.
Figure 14.15 Decomposition of organic compounds in aerobic and anaerobic cycles
818 • Renewable energy engineering and technology
The comparison of aerobic and anaerobic digestion is given in Table 14.5.
Biological treatment processes can be classified as aerobic, anaerobic,
and anoxic as stated earlier. Each of these processes can further be classified
as suspended growth or attached growth treatment processes (Table 14.6).
Table 14.5 Comparison of aerobic and anaerobic processes
Aerobic
Anaerobic
Electrical power required
No requirement of aeration hence less use of power
High maintenance and
Low operational and maintenance cost
operational cost
High sludge production
Less organic components converted to
necessitating further treatment
biomass
adding to the disposal cost
No useful energy production
Useful energy production in the form of methane
Table 14.6 Biological processes for waste water treatment
Type of process
Name of process
Aerobic suspended growth Activated sludge
Conventional (plug flow)
Continuous flow stirred tank
Contact stabilization
Extended aeration
Oxidation ditch
Aerobic attached growth
Anoxic
Suspended growth
Attached growth
Suspended growth nitrification
Aerated lagoons
Aerobic digestion
High-rate aerobic algal ponds
Trickling filters
Rotating biological contactors
Packed bed reactors
Suspended growth denitrification
Attached growth denitrification
Anaerobic suspended growth
Anaerobic digestion
Standard rate, single stage
High rate single stage
Two stage
UASB
Anaerobic attached growth
Anaerobic filter
Anaerobic lagoons
Source Metcalf and Eddy (2003)
Biochemical methods of conversion • 819
Aerobic processes
Some of the aerobic suspended growth and attached growth processes are
described below.
Activated sludge process
It is the most commonly used process for waste water treatment. The feed
is pumped into the reactor (known as mixed liquor tank) containing
acclimatized aerobic cultures. A mechanical stirrer along with aeration
provides an aerobic environment for the cultures. After the desired retention
time, the cells are separated from the treated waste water and partially recycled. The quantity of cells recycled and the retention time depend on the type
of waste water and treatment efficiency. The mean cell retention time in the
reactor depends on the flocculation and the settling property of micro-organisms.
A higher retention time results in better floc formation. This is because
of the formation of a slimy material due to the ageing of cells, which facilitates
the floc formation that ultimately settles down due to gravity.
Kinetics of activated sludge system in a continuous flow stirred tank
CSTR without recycle
Referring to Figure 14.16(a), the material balance equation for a CSTR can be
written as
Figure 14.16(a) CSTR without recyle 820 • Renewable energy engineering and technology
Figure 14.16(b) Schematic of the activated sludge process
Reproduced with permission from The McGraw-Hill Companies
Source Metcalf and Eddy (2003)
where V is the volume of reactor, Q is the volumetric feed rate and S and X are
the substrate and biomass concentrations. Substituting for rsub from Equation
14.6, we get
At equilibrium dS/dt = 0 and one can write
...(14.18)
A mass balance on bacterial biomass can be similarly written as
Substituting for rm,net from Equation 14.9, we get
And at equilibrium,
Equations 14.18 and 14.19 can be written as
...(14.19)
...(14.18a)
...(14.19a)
Biochemical methods of conversion • 821
where θ = V/Q
θ is called the HRT (hydraulic retention time) or simply retention time.
Assuming X0 = 0 and manipulating the above equations, one can get
...(14.20)
...(14.21)
These equations can be used to predict the performance of CSTR without recycle.
Activated sludge system using a CSTR with recycle
The system is shown in Figure 14.16(b). Mass balance on the system boundary
can be written as
V.
dX = QX –[(Q–Q )X + Q X ]+V.r
°
w
e
w r
m,net
dt
(Accumu = (input) – (output) + (net growth)
lation)
where Qw= waste sludge flow rate
Xr = biomass concentration in waste sludge
and Xe = biomass concentration in effluent stream.
...(14.22)
Assuming X0 = 0 and substituting for rm,net from Equation 14.9, we get, at
equilibrium (dX/dt = 0),
...(14.23)
The left hand term has units of (day)–1, and hence the inverse will have
the units of time. This is called the SRT (solids rentention time), which can be
written as
...(14.24)
For no recycle, Qw = 0 and Xe = X (assuming no clarification) and SRT
then equals V/Q, which is nothing but HRT described earlier. Thus, for a
complete mix CSTR with no recycle, the hydraulic and solid (or cell) retention
822 • Renewable energy engineering and technology
complete mix CSTR with no recycle, the hydraulic and solid (or cell) retention
times are the same. For increasing reaction rates (and to reduce reactor
volumes), it is thus imperative that SRT be increased, either by solids
recycling or by other processes such as suspended growth. Equation 14.23 can
be written as
The above equation can be rearranged as
which is similar to Equation 14.21 with θ replaced by (SRT).
A substrate mass balance on the reactor gives
...(14.25)
...(14.26)
...(14.27)
Accumu = Input – output + generation
lation
Substituting dX/dt = 0 for steady state and for rsub from Equation 14.6,
we get
...(14.28)
where θ is the hydraulic retention time defined earlier. Substituting for S/
(ks+S) from Equation 14.25 and rearranging, we get
...(14.29)
The above equation can be used to estimate the biomass concentration
in the reactor, which is seen to be a function of both SRT and HRT besides
the yield coefficient and decay coefficient. Further analysis of the activated
sludge system is given in Metcalf and Eddy (2003).
Analysis of a plug flow reactor with recycle
Plug flow or tubular reactors are more complicated for analysis compared to
CSTRs. These are however, industrially important. The Kompo gas system
(described later) for biomethanation of solid organic wastes relies on a plug
flow reactor. The leafy waste system of ASTRA, Bangalore and a few other
anaerobic digestion designs in India are also based on plug-flow reactors,
though it is difficult to distinguish between plug-flow and CSTR reactors
Biochemical methods of conversion • 823
when the L/D ratio is not very high, as is the case for some Indian reactor designs. For a true plug-flow reactor, all the particles entering the reactor spend
the same time in the reactor before exiting. In comparison, CSTRs have a
residence time distribution which may not be desirable, especially if mixing is
not adequate, as in family size biogas plants (Raman, Sujatha, Dasgupta, et al.,
1989).
Analysis of plug-flow designs under certain simplifying assumptions is reported by Metcalf and Eddy (2003). The resulting expression for SRT is
...(14.30)
where S0 = inlet concentration
S = outlet concentration
Sin= (S0 +αS)/(1+α)
and
α = recyle ratio
A number of CSTRs in series approximate the plug-flow reactor model.
Example 3
Waste water with an initial COD of 3 g/l is treated in a continuous flow
reactor. A COD reduction of 75% is achieved. The inlet and outlet bacterial
cell masses are 0 and 450 mg/l respectively. Calculate the yield coefficient Y.
Solution
For a COD reduction of 75%, the outlet substrate concentration would be 3 x
(1–0.75) or 0.75 g/l. The yield coefficient is obtained as
Example 4
A CSTR without recycle treats waste water with an inlet COD of 450 mg/l and
an inlet biomass concentration of 0. The outlet substrate concentration and
biomass concentration for different values of HRT are as follows.
HRT (days)
1.0
1.5
1.9
3.5
S(mg COD/l)
50
47
35
28
X(mg VSS/l)
120
125
128
132
Calculate the maximum specific substrate utilization rate k(=µmax/Y) and ks.
824 • Renewable energy engineering and technology
Solution
From Equation 14.18(a), we can write
Thus, plotting θX/(S0–S) vs (1/S) should result in a straight line with an
intercept equal to (1/k) and a slope equal to (ks/k).
From the given values of S and X for different θ, we can get the different
co-ordinates as follows.
S
(1/S)
(S0–S)
θX/(So–S)
50
47
35
28
0.02
0.021
0.028
0.035
400
403
415
422
0.3
0.465
0.58
1.09
A plot of θX/(So–S) vs (1/S) gives a straight line with an intercept of 0.1683 and
a slope of 0.14. Hence
Example 5
A CSTR has to be designed to treat waste water of COD 10 000 mg/l. The
desired COD reduction is 85%. Calculate the hydraulic retention times for reactor temperatures of 25 ºC and 35 ºC. The various rate constants are given in
the following table.
Temperature ºC
µmax, g/g.day
ks, g/l
kd, g/g.day
25
35
0.2
0.35
0.9
0.16
0.03
0.03
Biochemical methods of conversion • 825
Solution
The outlet concentration S is obtained as 10(1–0.85) = 1.5 g/l
From Equation 14.21,
θ can be calculated from the given rate parameters as
θ (25 ºC) = 10.5 days
θ (35 ºC) = 3.5 days
Aerated lagoon
This is similar to the conventional activated sludge process and has a retention time of 10 days. The reactor content is aerated through the use of surface
or diffused aerators with microbial cells in suspension. The different types
of aerated lagoons include facultative partially mixed lagoons, aerobic flowthrough partially mixed lagoons, and aerobic lagoons with solid recycling.
Attached growth aerobic systems
Attached growth systems involve the immobilization (binding of microbes,
substrate, enzymes, and extracellular polymers) on support material such as
plastic, sponge, activated carbon, and so on. This layer of micro-organisms,
substrate, and extracellular material is known as biofilm. The substrate consumption occurs in the biofilm and there is diffusion of substrates, products,
nutrients, and oxygen across a stagnant liquid layer, which separates the biofilm from the bulk liquid layer. The thickness of a biofilm varies between 100
µm and 10 mm. The substrate concentration within the biofilm is lower than
that in the bulk liquid zone. The concentration within the biofilm varies with
the depth of the film and rate of consumption. The layer of biofilm is not uniform, which makes the attached growth reactor kinetics more complex.
Anoxic processes
Although degradation of organic carbon is important in waste water, some of
the inorganic compounds such as ammonia reduce the dissolved oxygen in the
effluent by oxidizing to nitrate. The nitrification process is carried out either
in the same bioreactor as the organic carbon degradation or in a suspended
growth reactor following the activated sludge process. The design of the
settling tank and the reactor for nitrification process is similar to that of the
activated sludge process. Oxygen or air is used for nitrification.
826 • Renewable energy engineering and technology
Anaerobic reactors for waste water treatment
All modern high-rate anaerobic processes are based on the concept of
retaining a high viable biomass by some mode of bacterial sludge
immobilization. This is achieved by one of the following methods.
 Entrapment of sludge aggregates between the packing material inside the
reactor, for example, downflow/upflow AFFR (anaerobic fixed film reactor).
 Formation of highly settleable sludge aggregates combined with gas separation and sludge settling, for example, UASBR (upflow anaerobic sludge
blanket reactor) and ABR (anaerobic baffled reactor).
 Bacterial attachment to high density particulate carrier materials, for
example AFBR (anaerobic fluidized bed reactors) and AEBR (anaerobic expanded bed reactors).
Some of the reactor types in use are described in subsequent sections
(Kansal, Rajeshwari, Balakrishnan, et al. 1998).
Anaerobic fixed film reactor
This reactor employs a biofilm support structure (media) such as activated
carbon, PVC (polyvinyl chloride) supports, hard rock particles, or ceramic
rings for biomass immobilization, and can be operated in either the upflow or
downflow mode. The advantages and disadvantages of an anaerobic fixed film
reactor are given below.
Advantages
Disadvantages
 Simple configuration and easy to construct

 No mechanical mixing

 Better stability at higher loading rates
 Can withstand toxic and organic shock loads
 Quick recovery after starvation period
High reactor volume
Susceptibility to clogging due to increased
film thickness and high suspended solid concentration in waste water
In stationary fixed film reactors (Figure 14.17), cells are deliberately
attached to a large-sized solid support. The reactor has (i) a biofilm support
structure (media) for biomass immobilization, (ii) distribution system for
uniform distribution of waste water above/below the media, and (iii) effluent
draw off and recycle facilities (if required). The reactors can process different waste streams with little compromise in capacity and can adapt readily to
changes in temperature. This is important for installations where waste water
characteristics change rapidly. The reactor start-up can be very quick after a
period of starvation (one or two days to reach maximum capacity after three
weeks of starvation).
Biochemical methods of conversion • 827
Figure 14.17 Schematic of stationary fixed film reactor
The common problem associated with stationary fixed film reactors is
clogging due to non-uniform growth of biofilm thickness and/or high
suspended solid concentration in waste water. Non-uniform growth and
consequent clogging occurs especially at the influent entry. Some measures
to combat this problem include (i) recirculation of effluent and gas for
developing a relatively thin film and sloughing of biomass, (ii) provision for
a relatively thin layer of media near the inlet to accumulate the excess biofilm, and (iii) improvement in the flow distribution system to avoid very low
liquid velocity. Activated carbon, PVC supports, hard rock particles, and ceramic rings are the various types of film support that have been tried. Reactor
configuration and operation (upflow or downflow mode of operation) have a
marked effect on the performance of the reactor. With wastes containing large
amounts of hard-to-digest suspended solids, recirculation aids in degradation
as it keeps the solids in suspension (Figure 14.17).
The specific surface area of the packing averages about 100 m2/m3
and higher packing densities do not have any improvement in the process
efficiency.
Some of the common packing materials and their properties are shown
in Table 14.7. The performances of fixed bed reactors for different types of
waste water are given in Table 14.8.
Estimation of design parameters for a packed-bed reactor
The contact time of liquid with the biofilm is related to the filter depth as
follows.
828 • Renewable energy engineering and technology
Table 14.7 Packing material for attached growth reactors
Packing material
Size (mm)
Density (kg/m3)
River rock (small)
River rock (large)
Plastic (conventional)
Plastic (high specific area)
Plastic random packing (conventional)
Plastic random packing (high specific area)
2.5–7.5
10–13
61 x 61 x 122
61 x 61 x 122
Variable
1250–1450
800–1000
30–80
65–95
30–60
60
45
90
140
98
50
60
>95
>94
80
Variable
50–80
150
70
Specific surface area (m2/m3)
Void space
(%)
Source Metcalf and Eddy (2003)
Table 14.8 Performance of fixed-bed reactor for different types of waste water
Type of waste water
Packing type
Loading rate (kg COD/m3 d)
Retention time (days)
COD reduction
(%)
Guar gum
Chemical processing
Domestic
Landfill leachate
Food canning
Pall rings
Pall rings
Tubular
Cross flow
Cross flow
7.7
12–15
0.2–0.7
1.5–2.5
4–6
1.2
0.9–1.3
0.5–0.75
2.0–3.0
1.8–2.5
61
80–90
90–96
89
90
Source Metcalf and Eddy (2003)
...(14.31)
where t is the contact time of the liquid (min), c is the constant associated
with the packing material, h is the depth of packing (m), q is the hydraulic
loading rate (litre/m2 min), and n is the hydraulic constant for the packing material and is assumed to be 0.5.
where Q is the influent feed rate (litre/min) and a is the cross-section area of
the filter (m2).
Hence, the liquid contact time decreases with an increase in the flow
rate. This can be attributed to the increase in the film thickness with increase
in feed rate.
Biochemical methods of conversion • 829
Substituting for q in Equation 14.31 gives
The rate of substrate utilization in film can be represented as
...(14.32)
Integrating the above expression
Substituting for t from Equation 14.31,
Upflow anaerobic sludge blanket reactor
The main feature of a UASB (upflow anaerobic sludge blanket) reactor is
the formation of granular sludge, which is an agglomeration of microbial consortia. The system consists of a gas–solid separator (to retain the
anaerobic sludge within the reactor), a feed distribution system, and effluent
draw-off facilities (Figure 14.18). With a sophisticated feed distribution system installed in a UASB reactor, effluent recycle (to fluidize the sludge
bed) is not necessary as sufficient contact between waste water and sludge is
guaranteed even at low organic loads. The following table lists out the advantages and disadvantages of a UASB reactor.
Advantages

Disadvantages
Retention of the anaerobic sludge within
 Long start-up period
the reactor
 High mean cell retention time  Requires seed sludge for faster start-up due to formation of granular sludge
with bigger particle size and high sludge
settling and thickening property
 Highly cost-effective design
 Requirement of controlled and skilled operation to maintain efficiency of the reactor
 Low initial investment as compared to an anaerobic
filter or a fluidized bed system
830 • Renewable energy engineering and technology
Figure 14.18 Schematic of a UASB reactor
UASB processes are applied for treating high strength and
low/medium strength waste water and a variety of other substrates. The
process has been applied to the waste water generated from a wide crosssection of industries, such as distilleries, food-processing units, tanneries, and
so on, in addition to municipal waste water.
Expanded granular sludge bed reactor
The EGSB (expanded granular sludge bed) reactor is a modified form of UASB
where a slightly higher superficial liquid velocity can be applied (5–10 m/h
compared to 3 m/h for soluble waste water and 1–1.25 m/h for partially soluble
waste water in the UASB). This results in the accumulation of granular sludge,
and a part of the granular sludge bed is in an expanded or fluidized state in the
higher regions of the bed. Advantages and disadvantages of an EGSB reactor
are listed below.
Biochemical methods of conversion • 831
Advantages
Disadvantages
Sufficient contact between waste Possibility of sludge loss
water and sludge
A good flow of substrate into the sludge aggregates
Requirement of effluent recycling
High velocity
In the expanded bed design, micro-organisms are attached to an inert
support medium such as sand, gravel, or plastics as in a fluidized bed reactor.
However, the diameter of the particles is slightly bigger as compared to that
used in fluidized beds. The principle used for expansion is also similar to that
for the fluidized bed, that is, high upflow velocity and recycling.
Anaerobic fluidized bed reactor
In AFBR (anaerobic fluidized bed reactor), the media for bacterial attachment and growth – generally small-sized particles of sand, activated carbon,
and so on – is kept in a fluidized state by the drag forces exerted by the upflowing waste water. The fluidization of the media facilitates the movement of
microbial cells from the bulk to the surface and thus enhances the contact
between micro-organisms and the substrate. Increasing the recycling makes
the process similar to a completely mixed system. Design of a fluidized bed
thus consists of a feed distribution system, a media support structure, media,
head space, effluent draw off, and recycle facilities. Thickness of the biofilm
depends on the size and density of the inert media, bed regeneration, and upflow velocity. There can be a periodical removal of excess sludge from the
zones of the fluidized bed where the thickness of the biofilm is maximum. It
is possible to operate the reactor at lower retention times and/or higher loading rates. The stationary packed bed technology is adequate for the treatment
of easily biodegradable waste water or water in which high COD removal
is not required, while the fluidized bed technology is suitable for treatment of
high strength complex waste water with components that are difficult to
degrade (Figure 14.19). Advantages and disadvantages of an AFBR are given
below.
832 • Renewable energy engineering and technology
Figure 14.19 Fluidized bed reactor
Advantages
Disadvantages
 Elimination of bed clogging
 Need for recycling of effluent to achieve  Low hydraulic head loss
combined with better
hydraulic circulation
 Greater surface area per unit
of reactor volume
 Lower capital cost due to
bed expansion
reduced reactor volumes
Hybrid reactor
The hybrid reactor (Figure 14.20) is a combination of the UASB system and
fixed film reactors. By choosing a suitable highly porous packing material
with a large specific surface, the adhesion of microbes can be greatly
improved and concentration of activated sludge in the reactors can be
considerably enhanced.
Advantages of a hybrid reactor are listed below.
The reactor can withstand disturbances such as large fluctuations in the
loading rate
Biochemical methods of conversion • 833
Figure 14.20 Schematic of hybrid reactor
As microbes are held by the support media, there is no wash out even at a
very high upflow velocity of effluent
Successful phase separation (in terms of acidogenic and methanogenic
phases) can be achieved
Provides the best environment for growth of both acidogens and methanogens
Can be used for a wide variety of industrial effluents, and it is possible to
maintain the desired pH conditions for both the acidogens and methanogens
There is high rate of mass transfer because of the increased contact time
between the feed and the microbes
Minimization of sludge loss due to immobilization
Simplicity in operation and design
More economical than fixed-bed system at the industrial scale
The characteristics of different types of reactors discussed have been
summarized in Table 14.9.
Growth kinetics in anaerobic processes
The kinetic expression describing the anaerobic process is based on Monod’s
model, which is described earlier for aerobic processes in Equation 14.2.
where µ and µmax represent the specific growth rate and maximum specific
834 • Renewable energy engineering and technology
Table 14.9 Characteristics of different types of reactors
Anaerobic Start-up Channelling Effluent Gas–solid
Carrier tor period
effect
recycle
separation packing type
(days)
device
CSTR­—
Not present Not required Not required Not essential
UASB
4–16
Low
Not required Essential
Not essential
Anaerobic 3–4
High
Not required Beneficial Essential
filter
Expanded 3–4
Less
Required
Not required Essential
bed
AFB
3–4
Very less
Required
Beneficial Essential
Typical
loading
HRT reacrates (kg (days) COD/m3 day)
0.25–3
10–30
1–40
10–60
0.5–7
0.5–12
1–50
0.2–5
1–100
0.2–5
COD – chemical oxygen demand; HRT – hydraulic retention time; CSTR – continuous stirred tank; UASB –
upflow anaerobic sludge blanket; AFB – anaerobic fluidized bed;
Source Rajeshwari, Balakrishnan, Kansal, et al. (2000)
growth rate, respectively (g VSS/gVSS/d), S is the limiting substrate concentration (g/m3), and ks is the half-velocity constant (g/m3).
SRT can be derived from Equation 14.25
where Se is the substrate concentration in the effluent (g/m3). Typical values of
kinetic parameters for the anaerobic suspended growth process are: Y =
0.08 g VSS/g COD; k d = 0.03 g/g.day; µmax = 0.20 g/g.day (at 25 ºC); ks = 900
mg/L (at 25 ºC), and methane production = 0.4 m3/kg COD utilized (at 35 ºC).
Estimation of methane yields in an anaerobic reactor
If the composition of waste is known, estimation of CH4, CO2, NH3, and H2S
can be made by the method proposed by Buswell and Boruff (1932). The overall equation of biomethanation can be written as
...(14.33)
Biochemical methods of conversion • 835
The mole fractions of the evolved gases can be obtained as
...(14.34)
...(14.35)
...(14.36)
Critical design parameters for UASB process
Waste characteristics, volumetric organic loading rates, and upflow velocity
are the important parameters for designing UASB reactors. Upflow velocity is
the ratio of the feed flow rate and cross-sectional area of reactor. The reactor
volume and height are determined by organic loading, superficial velocity, and
effective liquid volume of reactor.
...(14.37)
where V is the effective liquid volume of reactor including the volume
occupied by sludge blanket and active biomass (m³), Q is the feed rate
(m 3/day), So is the influent concentration (kg/m3), and OLR is the organic loading rate (kg COD/m3 d).
The total liquid volume Ve after excluding the space for gas collector is
given as V/0.8. This is assuming 0.8 as the effectiveness factor indicating the
space occupied by the sludge blanket.
Upflow superficial velocity us (m/h) is expressed as the ratio of the flow
rate to the reactor area.
us = Q/A
or the area of the reactor A = Q/us
Height of the reactor He = Ve/A
Total height of the reactor H = He + Hg
where Hg is the additional height due to the gas collector.
The desired volumetric COD loading and upflow velocities for different
waste water strengths are given in Tables 14.10 and 14.11, respectively.
Feed distribution is an important parameter while designing a UASB
reactor. A gas–solid separator has to be designed to prevent sludge washout and
836 • Renewable energy engineering and technology
Table 14.10 Desired volumetric COD loading for 85%–95% removal at a
temperature of 30 oC for a UASB reactor with granular sludge and little loss
of TSS
Waste water COD (mg/litre)
Fraction as particulate COD
1000–2000
2000–6000
6000–9000
9000–18 000
0.1–0.3
8–12
0.3–0.6
8–14
0.6-1.0
0.1–0.3
12–18
0.3–0.6
12–24
0.6–1.0
0.1–0.3
15–20
0.3–0.6
15–24
0.6–1.0
0.1–0.3
15–24
0.3–0.6
0.6–1.0
Volumetric loading
(kg COD /m3d)
Source Metcalf and Eddy (2003)
Table 14.11 Desired upflow velocity and reactor height for different strengths of
waste water
Waste water Upflow velocity (m/h)
Reactor height (m)
COD 100% soluble COD partially soluble Domestic waste water
1.0–3.0
1.0–1.25
0.8–1.0
6–10
3–7
3–5
Note COD – chemical oxygen demand
Source Metcalf and Eddy (2003)
facilitate maximum separation of liquid and solids from the gas. The gas–solid–
liquid separator is an important feature in a UASB reactor. It has an inverted
V-shape and the slope of the settler in the gas–solid–liquid separation device
should be between 45 and 60º. The preferred height of the gas collector is 1.5–2
m at 5–7 m reactor height. Table 14.12 gives the recommended para-meters
for feed distribution. Further design details are provided in Metcalf and Eddy
(2003).
Biochemical methods of conversion • 837
Table 14.12 Recommended parameters for feed distribution
Sludge category
Organic loading
(kg COD/m3d)
Area of reactor distributed inlet (m2)
Dense flocculent sludge (>40 kg TSS/m3)
Medium flocculent
sludge, (20–40 kg TSS/m3)
Granular sludge
< 1
1–2
>2
0.5–1
1–2
2–3
<1–2
>3
1–2
2–4
>4
1–2
2–5
0.5–1
0.5–2
>2
Note TSS – total suspended solids; COD – chemical oxygen demand
Source Metcalf and Eddy (2003)
Importance of granular sludge and sludge volume index in designing
UASB reactor
Development of granular sludge is necessary for efficient operation of the
UASB reactors as well-formed granules with good settling property help
in retaining the biomass even at higher upflow velocities. A good settling
property of granules is judged based on the settling velocity. The settling
velocity or terminal velocity of a sediment particle is the rate at which the
sediment settles in a still fluid. It is dependent on granule size, shape, and density as well as on the viscosity and density of liquid. The settling velocity, ut
(m/s), can be calculated as follows (Harris 2003; Jimenez and Madsen 2003).
...(14.38)
where ρg is the density of the granules (kg/m3), ρ is the density of the liquid
(kg/m3), g is the acceleration due to gravity (m/s2), Cd is the drag coefficient dependent on size and shape of granule and viscosity of liquid, Vg is volume of
granule particle (m3), and Ag is the cross-sectional area of granule (m2).
Granules with a settling velocity of up to 20 m/h are categorized as poor
settling fraction, those between 20 and 50 m/h as moderate, and those over
50 m/h as good settling fraction (Schmidt and Ahring 1996). The shape and
composition of the granular sludge is variable. It generally has a spherical form
with diameter ranging from 0.14 mm to 5 mm. Depending on the type of waste
838 • Renewable energy engineering and technology
water being treated, the size and inorganic composition of granules (calcium,
potassium, and iron) differ. The structure and stability of granules is governed
by extracellular polymers that vary between 0.6% and 20% of the VSS and
consist of proteins and polysaccharides. An increase in the C/N ratio improves
production of extracellular polysaccharide, thus promoting attachment of
bacteria to the surface. Certain substrates, such as high concentration of fatty
acids, particularly propionate, are found to inhibit the activity of granular
sludge. Methanosaeta spp. and Methanosarcina spp. are important for initial
granulation. In addition, granules are also found to contain Methanobacterium
formicicum, Methanobacterium thermoautotrophicum, and Methanobrevibacter spp.
(Figure 14.21).
Example 6
(i) Determine the height and diameter of a UASB reactor treating 100 m3/day
of waste water with a soluble COD of 8000 mg/litre. (ii) Also estimate the
HRT and methane gas production assuming COD degradation to be 75%.
The effectiveness factor (fraction of the total liquid volume occupied by the
sludge blanket) is 0.8. The other parameters are as follows.
Upflow velocity = 1.5 m/h
Volumetric loading rate = 18 kg s (soluble) COD/m3/day
Gas storage height = 2.5 m
CH4 production = 0.35 litre/g COD (removed)
Solution
(i) From Equation 14.37
Figure 14.21 Granule formation
Source Schmidt and Ahring (1996)
Biochemical methods of conversion
•
839
V = Effective liquid volume of the reactor = 100 m3/day ÷ 8 kg COD/m3 ×
18 kg COD/m3/day = 44.4 m3
Total liquid volume of reactor, Ve = V/effectiveness factor = 44.4/0.8 = 55.5 m3
Area of the reactor = feed rate/upflow velocity
= 100 m3/day/(1.5 m/h × 24 h/day) = 100 m3/day/36 m/day = 2.77 m2
A = 3.14 × D2/4 = 2.77 m2
or D2 = 3.53 m2
D = 1.88 m
Liquid height of the reactor = Ve/A = 55.5/2.77 = 20 m
Total height = 20 + 2.5 = 22.5 m
(ii) HRT = liquid volume of reactor/Feed rate
= (55.5 m3/100 m3/day ) × 24 h/day = 13.32 h
(iii) Amount of soluble COD degraded = 8000 × 0.75 = 6000 mg/litre
= 6 kg/m³
Effluent COD concentration = 8000 – 6000 = 2000 mg/litre
COD utilized = 6 kg/m3 × 100 m3/d = 600 kg COD/day
Methane produced = 600 kg COD × 0.35 m3/kg COD (removed) = 210 m3/day
Aerobic and anaerobic systems for solid waste treatment
The treatment of the organic solid wastes can be carried out either in the
presence or absence of oxygen. Three common methods of processing
include composting, vermicomposting, and biomethanation/anaerobic
digestion.
Composting
Composting is a biological process in which the organic matter present in
waste is converted into enriched inorganic nutrients. The manure obtained
has high nitrogen, phosphorus, and potassium content. Heterotrophic microorganisms act upon the organic matter and by the action of enzymes, convert
organic compounds first into simpler intermediates like alcohol or organic acids and later into simple compound like sugars. This produces humic acid and
available plant nutrients in the form of soluble inorganic minerals like nitrates,
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