Aerobic and anaerobic systems for treatment of liquid wastes. Different types of bioreactors for waste water treatment

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1 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 C 6 H 12 O 6 + 6O 2 6CO 2 + 6H 2 O = 1.07 go 2 /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 Batch reactor

2 816 Renewable energy engineering and technology Figure 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 Continuous flow stirred tank Figure Packed bed

3 Biochemical methods of conversion 817 Figure 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 Figure Decomposition of organic compounds in aerobic and anaerobic cycles

4 818 Renewable energy engineering and technology The comparison of aerobic and anaerobic digestion is given in Table 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 Electrical power required High maintenance and operational cost High sludge production necessitating further treatment adding to the disposal cost No useful energy production Anaerobic No requirement of aeration hence less use of power Low operational and maintenance cost Less organic components converted to biomass Useful energy production in the form of methane Table 14.6 Biological processes for waste water treatment Type of process Aerobic suspended growth Aerobic attached growth Anoxic Suspended growth Attached growth Anaerobic suspended growth Anaerobic attached growth Name of process Activated sludge Conventional (plug flow) Continuous flow stirred tank Contact stabilization Extended aeration Oxidation ditch 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 digestion Standard rate, single stage High rate single stage Two stage UASB Anaerobic filter Anaerobic lagoons Source Metcalf and Eddy (2003)

5 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

6 820 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 r sub from Equation 14.6, we get At equilibrium ds/dt = 0 and one can write A mass balance on bacterial biomass can be similarly written as...(14.18) Substituting for r m,net from Equation 14.9, we get And at equilibrium, Equations and can be written as...(14.19)...(14.18a)...(14.19a)

7 Biochemical methods of conversion 821 where θ = V/Q θ is called the HRT (hydraulic retention time) or simply retention time. Assuming X 0 = 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 dx V. = QX [(Q Q w )X e + Q w X r ]+V.r dt m,net...(14.22) (Accumu = (input) (output) + (net growth) lation) where Q w = waste sludge flow rate X r = biomass concentration in waste sludge and X e = biomass concentration in effluent stream. Assuming X 0 = 0 and substituting for r m,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, Q w = 0 and X e = 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

8 822 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 can be written as The above equation can be rearranged as...(14.25) which is similar to Equation with θ replaced by (SRT). A substrate mass balance on the reactor gives...(14.26)...(14.27) Accumu = Input output + generation lation Substituting dx/dt = 0 for steady state and for r sub from Equation 14.6, we get...(14.28) where θ is the hydraulic retention time defined earlier. Substituting for S/ (k s +S) from Equation 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

9 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 S 0 = inlet concentration S = outlet concentration S in = (S 0 +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) S(mg COD/l) X(mg VSS/l) Calculate the maximum specific substrate utilization rate k(=µ max /Y) and k s.

10 824 Renewable energy engineering and technology Solution From Equation 14.18(a), we can write Thus, plotting θx/(s 0 S) vs (1/S) should result in a straight line with an intercept equal to (1/k) and a slope equal to (k s /k). From the given values of S and X for different θ, we can get the different co-ordinates as follows. S (1/S) (S 0 S) θx/(s o S) A plot of θx/(s o S) vs (1/S) gives a straight line with an intercept of and a slope of Hence Example 5 A CSTR has to be designed to treat waste water of COD 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 µ max, g/g.day k s, g/l k d, g/g.day

11 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.

12 826 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 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 Disadvantages 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).

13 Biochemical methods of conversion 827 Figure 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 m 2 /m 3 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 The performances of fixed bed reactors for different types of waste water are given in Table 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.

14 828 Renewable energy engineering and technology Table 14.7 Packing material for attached growth reactors Specific surface Void space Packing material Size (mm) Density (kg/m 3 ) area (m 2 /m 3 ) (%) River rock (small) River rock (large) Plastic (conventional) 61 x 61 x >95 Plastic (high specific area) 61 x 61 x >94 Plastic random packing Variable (conventional) Plastic random packing Variable (high specific area) Source Metcalf and Eddy (2003) Table 14.8 Performance of fixed-bed reactor for different types of waste water Loading rate Retention COD reduction Type of waste water Packing type (kg COD/m 3 d) time (days) (%) Guar gum Pall rings Chemical processing Pall rings Domestic Tubular Landfill leachate Cross flow Food canning Cross flow 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/m 2 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 (m 2 ). 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.

15 Biochemical methods of conversion 829 Substituting for q in Equation 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

16 830 Renewable energy engineering and technology Figure 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 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.

17 Biochemical methods of conversion 831 Advantages Sufficient contact between waste water and sludge A good flow of substrate into the sludge aggregates High velocity Disadvantage s Possibility of sludge loss Requirement of effluent recycling 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.

18 832 Renewable energy engineering and technology Figure Fluidized bed reactor Advantages Elimination of bed clogging Low hydraulic head loss combined with better hydraulic circulation Greater surface area per unit of reactor volume Lower capital cost due to reduced reactor volumes Disadvantages Need for recycling of effluent to achieve bed expansion 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

19 Biochemical methods of conversion 833 Figure 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 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 where µ and µ max represent the specific growth rate and maximum specific

20 834 Renewable energy engineering and technology Table 14.9 Characteristics of different types of reactors Typical Anaerobic Start-up Channelling Effluent Gas solid Carrier loading HRT reactor period effect recycle separation packing rates (kg (days) type (days) device COD/m 3 day) 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 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/m 3 ), and k s is the half-velocity constant (g/m 3 ). SRT can be derived from Equation where S e is the substrate concentration in the effluent (g/m 3 ). 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; µ max = 0.20 g/g.day (at 25 ºC); k s = 900 mg/l (at 25 ºC), and methane production = 0.4 m 3 /kg COD utilized (at 35 ºC). Estimation of methane yields in an anaerobic reactor If the composition of waste is known, estimation of CH 4, CO 2, NH 3, and H 2 S can be made by the method proposed by Buswell and Boruff (1932). The overall equation of biomethanation can be written as...(14.33)

21 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), S o is the influent concentration (kg/m 3 ), and OLR is the organic loading rate (kg COD/m 3 d). The total liquid volume V e 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 u s (m/h) is expressed as the ratio of the flow rate to the reactor area. u s = Q/A or the area of the reactor A = Q/u s Height of the reactor H e = V e /A Total height of the reactor H = H e + H g where H g 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 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

22 836 Renewable energy engineering and technology Table Desired volumetric COD loading for 85% 95% removal at a temperature of 30 o C for a UASB reactor with granular sludge and little loss of TSS Waste water COD Fraction as Volumetric loading (mg/litre) particulate COD (kg COD /m 3 d) Source Metcalf and Eddy (2003) Table 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 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 m at 5 7 m reactor height. Table gives the recommended para-meters for feed distribution. Further design details are provided in Metcalf and Eddy (2003).

23 Biochemical methods of conversion 837 Table Recommended parameters for feed distribution Organic loading Area of reactor distributed Sludge category (kg COD/m 3 d) inlet (m 2 ) Dense flocculent < sludge (>40 kg TSS/m 3 ) >2 2 3 Medium flocculent sludge, (20 40 kg TSS/m 3 ) < >3 2 5 Granular sludge >4 >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, u t (m/s), can be calculated as follows (Harris 2003; Jimenez and Madsen 2003)....(14.38) where ρ g is the density of the granules (kg/m 3 ), ρ is the density of the liquid (kg/m 3 ), g is the acceleration due to gravity (m/s 2 ), C d is the drag coefficient dependent on size and shape of granule and viscosity of liquid, V g is volume of granule particle (m 3 ), and A g is the cross-sectional area of granule (m 2 ). 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

24 838 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 m 3 /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/m 3 /day Gas storage height = 2.5 m CH 4 production = 0.35 litre/g COD (removed) Solution (i) From Equation Figure Granule formation Source Schmidt and Ahring (1996)

25 Biochemical methods of conversion 839 V = Effective liquid volume of the reactor = 100 m 3 /day 8 kg COD/m 3 18 kg COD/m 3 /day = 44.4 m 3 Total liquid volume of reactor, V e = V/effectiveness factor = 44.4/0.8 = 55.5 m 3 Area of the reactor = feed rate/upflow velocity = 100 m 3 /day/(1.5 m/h 24 h/day) = 100 m 3 /day/36 m/day = 2.77 m 2 A = 3.14 D 2 /4 = 2.77 m 2 or D 2 = 3.53 m 2 D = 1.88 m Liquid height of the reactor = V e /A = 55.5/2.77 = 20 m Total height = = 22.5 m (ii) HRT = liquid volume of reactor/feed rate = (55.5 m 3 /100 m 3 /day ) 24 h/day = h (iii) Amount of soluble COD degraded = = 6000 mg/litre = 6 kg/m³ Effluent COD concentration = = 2000 mg/litre COD utilized = 6 kg/m m 3 /d = 600 kg COD/day Methane produced = 600 kg COD 0.35 m 3 /kg COD (removed) = 210 m 3 /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,