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CE 548 II Fundamentals of Biological Treatment

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## CE 548 II Fundamentals of Biological Treatment

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**CE 548**II Fundamentals of Biological Treatment**Modeling Suspended Growth Treatment Processes**• Description of treatment process: • All biological treatment reactor designs are based on using mass balances across a defined volume for each constituent of interest (i.e., biomass, substrate, etc.) • Biomass mass balance: Accumulation = inflow – outflow + net growth**Modeling Suspended Growth Treatment Processes**Assuming stead-state and Xo = 0, equation 7-32 can be simplified:**Modeling Suspended Growth Treatment Processes**Equation 7-34 can be written as: The term 1/SRT is related to µ, the specific biomass growth rate:**Modeling Suspended Growth Treatment Processes**In Eq. (7-36) the term (-rsu/X) is known as the specific substrate utilization rate U and can be calculated as the following: Substituting Eq. (7-12) into Eq. (7-36) yields: Solving Eq. (7-39) for S yields:**Modeling Suspended Growth Treatment Processes**• Substrate mass balance: Accumulation = inflow – outflow + generation Substituting for rsu and assuming steady-state, Eq. (7-41) can be written as: 0**Modeling Suspended Growth Treatment Processes**• Mixed liquor solids concentration and solids production: The solids production from a biological reactor represents the mass of material that must be removed each day to maintain the process: Eq. (7-45) can be used to calculate the amount of solids wasted for any of the mixed liquor components. For the amount of biomass wasted (PX), the biomass concentration X can be used in place of XT in Eq. (7-45).**Modeling Suspended Growth Treatment Processes**• Mixed liquor solids concentration: The total MLVSS equals the biomass concentration X plus the nbVSS concentration Xi: A mass balance is needed to determine the nbVSS conc.: Accumulation = inflow – outflow + generation**Modeling Suspended Growth Treatment Processes**• Mixed liquor solids concentration: At steady-state and substituting Eq. (7-25) for in Eq. (7-47) yields: Combining Eq. (7-43) and Eq. (7-49) for X and Xi produces the following equation that can be used to determine XT :**Modeling Suspended Growth Treatment Processes**• Solids production: The amount of VSS produced and wasted daily is as follows: Eq. (7-43) is substituted for biomass concentration (X) in Eq. (7-51) to show VSS production rate in terms of the substrate removal, influent VSS, and kinetic coefficients as follows:**Modeling Suspended Growth Treatment Processes**• Solids production: The effect of SRT on the performance of an activated sludge system for soluble substrate removal is shown in figure 7-13 The total suspended solids (TSS) production can be calculated by modifying Eq. (7-52) assuming that a typical biomass VSS/TSS ratio of 0.85 as follows:**Modeling Suspended Growth Treatment Processes**• The observed yield: The observed yield for VSS can be calculated by dividing Eq. (7-52) by the substrate removal rate Q(So-S): • Oxygen requirements: Oxygen used = bCOD removed – COD of waste sludge Study example 7-6**Modeling Suspended Growth Treatment Processes**• Design and operating parameters: Following are the design and operating parameters that are fundamentals to treatment and performance of the process: • SRT • Food to microorganisms (F/M) ratio The SRT can be related to F/M by the following equation:**Modeling Suspended Growth Treatment Processes**• Design and operating parameters: • Organic volumetric loading rate. Defined as the amount of BOD or COD applied to the aeration tank volume per day:**Modeling Suspended Growth Treatment Processes**• Modeling plug-flow reactors: Developing a kinetic model for the plug-flow reactor is mathematically difficult (X vary along the reactor). Two assumptions are made to simplify the modeling: • The concentration of microorganisms is uniform along the reactor This assumption applies only when SRT/ 5. • The rate of substrate utilization is given by:**Modeling Suspended Growth Treatment Processes**• Modeling plug-flow reactors: Integrating Eq. (7-72) over the retention time in the tank gives:**Biological Nitrification**• Nitrification is the conversion (by oxidation) of Ammonia (NH4-N) to nitrite (NO2-N) and then to nitrate (NO3-N). • The need for nitrification arises from water quality concerns: • Effect of ammonia on receiving water; DO demand, toxicity. • Need to provide nitrogen removal for eutrophication control. • Need to provide nitrogen removal for reuse applications. • The current drinking water MCL for nitrate is 45 mg/l as nitrate or 10 mg/l as nitrogen. • The total concentration of organic and ammonia nitrogen in municipal wastewater is typically in the range of 25-45 mg/l as nitrogen.**Biological Nitrification**• Process description: • Nitrification is commonly achieved with BOD removal in the same single-sludge process. • In case of the presence of toxic substances in the wastewater, a two-sludge system is considered. • Stoichiometry:**Biological Nitrification**• Process description: • The oxygen required for complete oxidation of ammonia is 4.57 g O2/g N oxidized. • The alkalinity (alk) requirement is 7.14 g alk as CaCO3 for each g of ammonia nitrogen (as N).**Biological Denitrification**• Process description: • Denitrification is the biological reduction of nitrate (NO3) to nitric oxide (NO), nitrous oxide (N2O), and nitrogen (N). • The purpose is to remove Nitrogen from wastewater. • Compared to alternatives of ammonia stripping, breakpoint chlorination, and ion exchange, biological nitrogen removal is more cost-effective and used more often. • Concerns over eutrophication and protection of groundwater against elevated NO3-N concentration.**Biological Denitrification**• Stoichiometry: • In denitrification, nitrate is used as the electron acceptor instead of oxygen and the COD or BOD as the carbon source (electron donor). • The carbon source can be the influent wastewater COD or external source (Methanol). • One equivalent of alkalinity is produced per equivalent of nitrate reduced. (3.57 g alk per g nitrate)**Biological Phosphorus Removal**• Process description: • Phosphorous removal is done to control eutrophication. • Chemical treatment using alum or iron salts is the most commonly used technology for phosphorous removal. • The principle advantages of biological phosphorous removal are reduced chemical costs and less sludge production. • In the biological removal of phosphorous, the phosphorous in the influent is incorporated into cell biomass which is removed by sludge wasting. • Phosphorous accumulating organisms (PAOs) are encouraged to grow and consume phosphorous. Therefore, the system is designed so that the reactor configuration provides advantage for PAOs to grow over other bacteria.**Anaerobic Fermentation and Oxidation**• Process description: • Used primarily for the treatment of waste sludge and high strength organic waste. • Advantages include low biomass yield and recovery of energy in the form of methane. • Conversion of organic matter occurs in three steps: • Step1 (Hydrolysis): involves the hydrolysis of higher-molecular-mass compounds into compounds suitable for use as a source of energy and carbon. • Step2 (Acidogenesis): conversion of compounds from step1 into lower-molecular-mass intermediate compounds. (nonmethanogenic bacteria) • Step3 (Methanogenesis): conversion of intermediates into simpler end products (CH4 & CO2).**Anaerobic Fermentation and Oxidation**• Process description: • For efficient anaerobic treatment, the reactor content should be: • void of O2 • free of inhibiting conc. of heavy metals and sulfides • pH ~ 6.6 – 7.6 • sufficient alkalinity to ensure pH is not <6.2 (methane bacteria will not function below 6.2). • Methanogenic bacteria has slow growth rate, therefore: • require long detention time for waste stabilization • yield is low: less sludge production and most organic matter is converted to CH4 gas. • sludge produced is stable: suitable for composting • require relatively high temp for adequate treat.