“Ability to calculate and compare the treatment methods for particular wastes. & Ability to design and evaluate various unit operations for waste treatments.” Biological treatment / Unit operation
INTRODUCTION Objectives of Biological Treatment: a) Transform (i.e., oxidize) dissolved and particulate biodegradable constituent s by microorganisms into acceptable end products, b) Capture and incorporate suspended and non-settleable colloidal solids into a biological floc or biofilm, c) Transform and remove nutrients, such as nitrogen and phosphorus.. why??? d) In some cases, remove specific trace organic constituents and compounds.
For industrial wastewater: - To remove or reduce the concentration of organic and inorganic compounds. because some of the constituents and compounds are toxic to microorganism, pretreatment may be required before discharging to municipal collection.
Biological Processes for wastewater treatment Activated sludge process Aerated lagoons
Trickling filters Rotating biological contractors
Aerobic biological oxidation of organic matters Nutrients for microbes to Converts organic matters to CO2 and H2O Biomass produced vi = stoichiometric coefficient
Composition & Classification of Microorganisms ** Revise the cell components, compositions, structure, DNA, RNA, microbial Growth & metabolism, C & N sources … Refer Chapter 7, page 555-563
Bacteria metabolism Aerobic, autotrophic Aerobic, heterotrophic Do you understand: Aerobic? Anaerobic? Heterotrophic? Autotrophic? Phototroph? Chemotroph? Anaerobic, heterotrophic
Bacterial reproduction: In 30 min of generation time (time required bacteria to divide into 2 organisms) 1 bacterium would yield ~ 17 x 106 bacteria in 12 h and the mass ~ 8.4 µg Biomass yield Y = g (biomass produced) / g (substrate consumed) VSS- common method to measure biomass growth
Microbial Growth Kinetics Growth kinetics govern the substrate oxidation and biomass production TSS conc. in biological reactor • Organic compounds mostly defined as biodegradable COD (bCOD) or ultimate carbonaceous BOD (UBOD). bCOD and UBOD comprise of soluble (dissolved), colloidal and particulate biodegradable components. • Biomass solids in bioreactor = TSS & VSS • The mixture of solids resulting from combining recycled sludge with influent wastewater in bioreactor = mixed liquor suspended solids (MLSS) and mixed liquor volatile suspended solids (MLVSS) • Solid = biomass, nonbiodegradable volatile suspended solid (nbVSS) and inert inorganic total suspended solid (iTSS)
Rate of utilization of soluble substrates (-ve : substrate decreases with time) rsu = rate substrate conc. change due to utilization, g/m3.d k = max specific substrate utilization rate, g substrate/ g microbe . D Ks = substrate conc. at one-half max substrate utilization rate g/m3 X= biomass concentration, S= limiting substrate concentration g/m3
2) Rate of Biomass Growth with soluble substrate Specific biomass growth rate, μ= rg/X 3) Rate of oxygen uptake
Example 7-5 Determine Biomass and Solids Yields For an industrial wastewater activated sludge process, the amount of bsCOD in the influent wastewater is 300 g/m3 and the influent nbVSS concentration is 50 g/m3 . The influent flowrate is 1000 m3 /d, the biomass concentration is 2000 g/m3, the reactor bsCOD concentration is 15 g/m3 , and the reactor volume is 105 m3. If the cell debris fraction fd is 0.10, determine: • The net biomass yield • The observed solids yield • The biomass fraction in the MLVSS • Specific biomass growth rate, µ
Solution: ? ?
Aerobic Biological Oxidation Wide range of microorganisms used: Ex: Aerobic heterotrophic bacteria able to produce extracellular biopolymers that result in the formation of biological flocs, then separated by gravity settling. • Protozoa: consume free bacteria and colloidal particulates – aid effluent clarification.
Stoichiometry: Electron acceptor Electron donor
Biological Nitrification Nitrification: 2-step biological processes; • Ammonia (NH4-N) is oxidized to nitrite (NO2-N) • Nitrite is oxidized to nitrate (NO3-N) Why?? • Ammonia & nitrite– associate DO conc. & fish toxicity • Need for nitrogen removal: – control eutrophication & water-reuse application Total conc. of organic and ammonia nitrogen in municipal wastewater: 25-45 mg/L
Stoichiometry: • Two step oxidation of ammonia to nitrate Nitroso-bacteria (Nitrosococcus, Nitrosospira, etc): 2NH4+ + 3O2 2NO2- + 4H+ + 2H2O (Nitrobacter, Nitrococcus, Nitrospina, etc): Nitrate: Safer form to aquatic lives
Biological Denitrification Denitrification: The biological reduction of nitrate to (nitrite) then to nitric oxide, nitrous oxide, and nitrogen gas. Biological nitrogen removal is used in wastewater treatment : • where there are concerns for eutrophication, • and where groundwater must be protected against elevated NO3-N concentration. 2 modes of nitrate removal: • Assimilating nitrate reduction (ANR) • Dissimilating nitrate reduction (DNR)
ANR involves the reduction of nitrate to ammonia for use in cell synthesis (Fig 7-20) Assimilation occurs when NH4-N is not available and is independent of DO concentration. • DNR is coupled to the respiratory electron transport chain, and nitrate or nitrite is used as an electron acceptor for the oxidation of a variety of organic or inorganic electron donors (Fig 7-20). • Microorganism for denitrification: both heterotrophic and autotrophic ( most are facultative aerobic organisms with the ability to use oxygen as well as nitrate or nitrite). - Example: Achromobacter, Acinetobacter, Bacillus, Chromobacterium, Pseudomonas, Rhizobium, etc.
Types of denitrification process Substrate driven Endogenous driven
In the first flow, nitrate produced in the aeration tank is recycled back to the anoxic tank (anaerobic). Because the organic substrate in the influent wastewater provides the e- donor for oxidation-reduction reactions using nitrate, the process is termed substrate denitrification. Or because the anoxic process precedes the aeration tank, the process is known as a preanoxicdenitrification. In the second process, denitrification occurs after nitrification and the e- donor source is from endogenous decay. BOD removal has occurred first, and is not available to drive the nitrate reduction reaction, and called postanoxicdenitrification. It has much slower rate of reaction than preanoxicdenitrification. Often, an exogenous carbon source such as methanol or acetate is added to postanoxic processes to provide sufficient BOD for nitrate reduction and to increase rate of denitrification.
Anaerobic Fermentation & Oxidation • Used primarily for treatment of waste sludge and high-strength organic wastes. • As a pretreatment step due to low quality effluent. • Advantages: • Lower biomass yield • Energy (methane) can be recovered from biological conversion of organic substrate • Cost-effective; savings in energy, nutrient addition and reactor volume.
Three basic steps in anaerobic oxidation of wastes: • Hydrolysis: • particulate material is converted to soluble compounds that can then be hydrolyzed further to simple monomers that are used by bacteria that perform fermentation. 2) Fermentation (or acidogenesis): • Amino acids, sugars, and some fatty acids are degraded further. • The principle products are acetate, H2, CO2, and propionate and butyrate. • Acetate, H2, CO2 precursors of methane formation (Methanogenesis)
3) Methanogenesis: Carried out by 2 groups of microorganisms (or Methanogens): a) Aceticlastic methanogens – split acetate into methane and CO2 CH3COOH CH4 + CO2 b) Hydrogen-utilizing methanogens - use H2 as electron donor and CO2 as the electron acceptor to produce methane
Nuisance organisms in anaerobic fermentation - When the wastewater contains significant concentrations of sulfate - Sulfate-reducing bacteria can reduce sulfate to sulfide (toxic to methanogenic bacteria) - Then, how to solve?? How??
Environmental factors: • Anaerobic processes are sensitive to pH & inhibitory substances (ex: NH3, H2S, etc.) • pH near neutral preferred ; • pH below 6.8 methanogenic activity is inhibited • Due to about 30-35 % CO2 (high) produced in anaerobic process, high alkalinity is needed to neutralize pH • Range of alkalinity, i.e., 3000-5000 mg/L as CaCO3 is often found. In industrial wastewater applications which mainly contain carbohydrates, it is necessary to add alkalinity for pH control.
Suspended Growth Processes (SGP) • Microbes are maintained in liquid suspension by mixing methods • Most common SGP: Activated-sludge process (ASP) - ASP uses activated mass of microbes capable of stabilizing a waste under aerobic conditions - mix wastewater with microbial suspension at certain contact time, mechanically • MLSS • MLVSS • MLSS flows to clarifier (where microbial suspension is settled and thickened) “Activated sludge (AS)” AS is returned to aeration tank to continue biodegradation of organic material
Selection & Design for Activated Sludge Processes • Aeration System Aeration system must be adequate to: • Satisfy the bCOD of the wastes • Satisfy the endogenous respiration by the biomass • Satisfy the O2 demand for nitrification • Provide adequate mixing • Maintain minimum dissolved O2 conc. throughout the tank If the O2 transfer efficiency of aeration system is known, we can design /estimate the actual air requirements for diffused air aeration or installed power of mechanical surface aerators.
Aeration-achieved via diffused air (diffuser) or surfaceaerator. • Aeration equipment must be designed with enough flexibility to: • Meet minimum dan max O2 demand • Prevent excessive aeration and save energy
2) Aeration Tanks and Appurtenances (support facilities) a) Aeration Tanks - Usually constructed of reinforced concrete and left open to atmosphere - Capacity if aerated with diffused air: • capacity range of 0.22 to 0.44 m3 /s at least 2 tanks needed • capacity range of 0.44 to 2.2 m3 /s at least 4 tanks needed • capacity range over 2.2 m3 /s at least 6 tanks or more • Depth of wastewater in the tanks: between 4.5 and 7.5 m • Freeboard: 0.3 – 0.6 m above waterline • Width-to-depth ratio of the tanks (spiral-flow mixing): 1:1, 2.2:1 or 1.5:1 (most common) • Tank with diffusers on both sides, greater width are permissible. • Triangular baffles or fillet may be placed longitudinally in the channel to eliminate dead spot