AGSM 337/BAEN 465 Biological Treatment Processes Page 1 of 6 References The following are excellent sources of info on biological treatment processes nd Shuler, M. L. and F. Kargi, 2002. Bioprocess Engineering: Basic Concepts (2 edition). Prentice Hall. Metcalf and Eddy, Inc., 2003, Wastewater Engineering: Treatment and Reuse, revised by Tchobanoglous, George, and Burton, Franklin L., Fourth Ed., McGraw-Hill, New York. Viessman, Warren, Jr. and Hammer, Mark J., 2005, Water Supply and Pollution Control, 7th Ed., Prentice Hall. Treatment Goal Reduce organic material (BOD) in wastewater streams by converting to settleable solids and gaseous compounds (CO2) Microbial Characteristics Three basic types based on oxygen utilization: aerobic – air (oxygen) is required to support microbial activity anaerobic – microbial activity proceeds in the absence of oxygen facultative – microorganisms can grow either with or without oxygen using different metabolic processes Two types based on temperature preference mesophilic – organisms prefer temperatures in the range of 20 to 45 °C thermophilic – organisms prefer temperatures in the range of 45 to 60 °C Aerobic Growth Microbial biomass utilizes organic matter along with oxygen to create more biomass and oxidized metabolic byproducts: Organic matter + microbes + oxygen → More microbes + Metabolic byproducts + Energy (C, H, O, N, P, S) + biomass + O2 → more biomass + CO2, NO3, SO4, PO4, H2O + energy Anaerobic Growth Anaerobic growth takes place in the absence of oxygen. In this case, microbes must obtain oxygen from the organic molecules used as food. Other food molecules are rearranged as the microbes obtain energy from them. This leads to the formation of methane (CH4), CO2, and other metabolic byproducts. Anaerobic conversion of biomass takes place in stages with different types of organisms carrying out the processes for each stage. First anaerobic phase In the first anaerobic phase, the complex molecules in organic matter such as polysaccharides and proteins are hydrolyzed to simple sugars and amino acids. These are then converted to new biomass and organic acids by the microbes. AGSM 337/BAEN 465 Biological Treatment Processes Page 2 of 6 Hydrolysis reactions: Polysaccharides (e.g., starch) → glucose and other simple sugars Proteins → amino acids Microbial growth: Organic matter + microbes → more microbes + organic acids (glucose, amino acids, etc) + biomass → more biomass + acetic and other organic acids Second anaerobic phase In the second anaerobic phase, the organic acids, hydrogen and CO2 produced in the first phase are converted to more biomass, CO2, CH4 and other reduced metabolic byproducts. Amino acids also are broken down, releasing ammonia. Organic acids, amino acids, fats + microbes → more microbes + CO2 + CH4 + other byproducts Many of the organic acids (e.g., propionic and butyric acids) along with H2S produced during anaerobic growth have unpleasant odors. In addition, the H2S is toxic so care must be taken when working with anaerobic processes. Microbial Growth Curve In batch processes, microbial growth follows a sequence of phases as shown in the figure below: AGSM 337/BAEN 465 Biological Treatment Processes Page 3 of 6 Lag Phase There is a lag phase following inoculation of the medium during which the cells adapt to their new environment. The cells may have to make important constituents because of dilution when placed in the new medium and/or prepare to grow with different nutrients by making new enzymes. Exponential Growth Phase This is the most important phase for most microbial processes during which cells are growing and dividing on a regular time cycle. For waste treatment processes, this is the phase during which most of the organic matter is consumed. Because there are excess quantities of nutrients available during this phase, the microorganisms grow and divide on a periodic basis causing the amount of microbial biomass to double each period. This leads to exponential growth behavior which is shown in equation 1: x x0 e t (1) where x is the biomass concentration (mass basis) at time t, x0 is the biomass concentration at the start of the exponential phase, µ is specific growth rate of the organisms, and t is the time since the start of the exponential phase. The specific growth rate is a function of the environment of the microorganisms including factors such as pH, temperature and nutrients available. For given conditions with excess nutrients available, there is a maximum rate at which microorganisms can grow and divide. This is the maximum specific growth rate, µmax. As nutrients become limited, growth slows down. This behavior is modeled by the Monod equation: max s Ks s (2) where s is the substrate (nutrient) concentration and Ks is the saturation constant. Stationary Phase During the stationary phase, the supply of nutrients becomes limiting and growth slows. In addition, some cells begin to die. The growth and death processes are approximately in equilibrium so there is no net growth or death. Death Phase During the death phase, cell death exceeds cell growth so there is a decline in the population of viable cells. AGSM 337/BAEN 465 Biological Treatment Processes Page 4 of 6 CSTR Bioreactor Systems The typical bioreactor used for the activated sludge waste treatment process is a continuous stirred tank reactor or CSTR. In this type of reactor, the tank is fed continuously with a constant flow rate, Q, and effluent is removed at the same rate. The reactor is thoroughly mixed so the contents are uniform throughout, and the effluent has the same composition as the contents of the tank. The reactor operates at steady state so flow rates and compositions remain constant over time. A schematic of a CSTR is shown in the figure below: Q is volumetric flow rate VR is the liquid volume in the reactor x0 is microbial biomass concentration in the feed x is microbial biomass concentration in the reactor s0 is limiting nutrient concentration in feed s is limiting nutrient concentration in reactor A mass balance on microbial biomass gives the following equation: Qx0 Qx VR x 0 (3) The microbial biomass concentration in the feed, x0, normally is so small it can be considered negligible so the mass balance reduces to or Qx VR x (4) Q / VR D 1 (5) where D is the dilution rate and θ is the mean residence time in the reactor. If we assume Monod growth behavior, we can combine equations 2 and 5 to get D max s Ks s (6) Solving for s, we get s DKs max D (7) AGSM 337/BAEN 465 Biological Treatment Processes Page 5 of 6 Example Determine the volume of a CSTR needed to treat waste water flowing at a rate of 150 m3/h with a BOD (substrate) concentration of 250 mg/L to achieve a final concentration of 50 mg/L if µmax = 0.01 h-1 and Ks = 10 mg/L. If we assume the amount of microorganisms in the feed to the CSTR is negligible and that growth follows the Monod equation, then the dilution rate is given by equation 6. Note that the substrate concentration in the reactor is the same as the effluent concentration of 50 mg/L (because the tank is completely mixed) and that is the concentration to use in equation 6. D max s Ks s 0.01 50 mg L 0.0083 h -1 10 50 mg h L Using equation 5, the CSTR volume needed is V Q 150 m 3 h 18,000 m 3 D h 0.0083 Washout from CSTR Since there is a limit to the rate at which cells can divide under the best conditions (µmax), the dilution rate must also be limited. If the dilution rate is greater than approximately µmax, the steady state solution will be that all cells wash out of the reactor; i.e., the mean residence time in the reactor is too short for the cells to divide, so eventually (when the reactor reaches steady state) all cells will have been removed in the effluent from the reactor. CSTR with Recycle One way to avoid washout at high dilution rates is to concentrate cells from the effluent and recycle them back into the reactor creating a CSTR with recycle system: Q, x, x0, s, s0, and Vr as defined for CSTR above Qr is the volumetric flow rate of the recycle stream Qw is the volumetric flow rate of the cell waste stream xr is the concentration of biomass in the recycle stream (and also waste stream) xe is the concentration of biomass in the clarified effluent stream (often negligible) AGSM 337/BAEN 465 Biological Treatment Processes Page 6 of 6 In the CSTR with recycle system, the liquid has a hydraulic residence time, θ, given by Vr Q (8) Since cells are separated in the clarifier and recycled, they will have a residence time in the system different than the hydraulic residence. The cell residence time, θc, is c Vr x Qw xw (9) The conversion of substrate (limiting nutrient, BOD for wastewater streams) to cell biomass is generally consistent in aerobic systems and is described by the yield coefficient, Y: Y xw mass cells produced s0 s mass substrateconsumed (10) With recycle, some cells may remain in the system a very long time and eventually die. Therefore, there is a death (or decay) rate, kd, associated with cells in a recycle system. If you assume there is no biomass in the feed (x0 = 0) and there are no cells in the liquid effluent stream (xe =0), the concentration of biomass in the reactor is given by x cY s0 s 1 k d c (11) Note this is the same equation as equation 7 in the WWTP notes (Notes 5). For activated sludge systems, cell concentration (x or X) is often measured as mixed liquor volatile suspended solids (MLVSS). Mixed liquor is the mixture of wastewater and sludge in the reactor. Values of growth parameters for domestic wastewater treatment Value Parameter Units Range Typical Ks mg BOD5/L 25-100 60 kd day-1 0-0.30 0.1 -1 μm day 1-8 3 Y mg VSS/mg BOD5 0.4-0.8 0.6 Source: Davis, M.L., and Masten, S.J., 2009, Principles of Environmental Engineering and Science, 2nd Ed., McGraw-Hill, Boston.
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