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.