2 Biological Treatment Processes of Industrial Wastes
3 “Ability to calculate and compare the treatment methods for particular wastes.&Ability to design and evaluatevarious unit operationsfor waste treatments.”Biological treatment / Unit operation
4 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.
5 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.
6 Biological Processes for wastewater treatment Activated sludge processAerated lagoons
13 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 µgBiomass yield Y = g (biomass produced) / g (substrate consumed)
14 Microbial Growth Kinetics Growth kinetics govern the substrate oxidation and biomass production TSS conc. in biological reactorOrganic 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 & VSSThe 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)
15 Rate of utilization of soluble substrates (-ve : substrate decreases with time)rsu = rate substrate conc. change due to utilization, g/m3.dk = max specific substrate utilization rate, g substrate/ g microbe . DKs = substrate conc. at one-half max substrate utilization rate g/m3
16 2) Rate of Biomass Growth with soluble substrate 3) Rate of oxygen uptake
20 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 yieldThe observed solids yieldThe biomass fraction in the MLVSSSpecific biomass growth rate, µ
22 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.
24 Biological 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 toxicityNeed for nitrogen removal:– control eutrophication & water-reuse application
25 Stoichiometry: 2NH4+ + 3O2 2NO2- + 4H+ + 2H2O Nitroso-bacteria (Nitrosococcus, Nitrosospira, etc):2NH O NO H+ + 2H2O(Nitrobacter, Nitrococcus, Nitrospina, etc):Nitrate:Safer form toaquatic lives
27 Biological 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)
28 ANR involves the reduction of nitrate to ammonia for use in cell synthesis. 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.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.
31 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 preanoxic denitrification.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 postanoxic denitrification.It has much slower rate of reaction than preanoxic denitrification.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.
33 Anaerobic Fermentation & Oxidation 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)
34 3) Methanogenesis: – split acetate into methane and CO2 Carried out by 2 groups of microorganisms (or Methanogens):a) Aceticlastic methanogens– split acetate into methane and CO2CH3COOH CH4 + CO2b) Hydrogen-utilizing methanogens- use H2 as electron donor and CO2 as the electron acceptor to produce methane
35 How?? 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??
36 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 inhibitedDue to about % CO2 (high) produced in anaerobic process, high alkalinity is needed to neutralize pHRange of alkalinity, i.e., mg/L as CaCO3 is often found.In industrial wastewater applications which mainly contain carbohydrates, it is necessary to add alkalinity for pH control.
37 Types of Biological Process for Wastewater Treatment Suspended Growth ProcessAttached Growth Process (Biofilm)
39 Suspended Growth Processes (SGP) Microbes are maintained in liquid suspension by mixing methodsMost 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 MLSSMLVSSMLSS flows to clarifier (where microbial suspension is settled and thickened) “Activated sludge (AS)”AS is returned to aeration tank to continuebiodegradation of organic material
40 Activated Sludge Process 1Reactor- microbes are kept in suspension and aerated2Liquid-solids separation – ex: clarifier3Recycle system – returning solids from clarifier to reactor
44 Selection & Design for Activated Sludge Processes Aeration SystemAeration system must be adequate to:Satisfy the bCOD of the wastesSatisfy the endogenous respiration by the biomassSatisfy the O2 demand for nitrificationProvide adequate mixingMaintain minimum dissolved O2 conc. throughout the tank Can design /estimate the actual air requirements for diffused air aeration or installed power of mechanical surface aerators.
46 2) Aeration Tanks and Appurtenances (support facilities) a) Aeration Tanks- Usually constructed of reinforced concrete and left open to atmosphere- Capacity:capacity range of 0.22 to 0.44 m3 /s at least 2 tanks neededcapacity range of 0.44 to 2.2 m3 /s at least 4 tanks neededcapacity range over 2.2 m3 /s at least 6 tanks or moreDepth of wastewater in the tanks: between 4.5 and 7.5 mFreeboard: 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)
49 b) Flow distribution- from multiple units of primary sedimentation tanks & aeration tanks to ASP tanks- methods of splitting or controlling the flow rate, for ex: splitter boxes equipped with weirs or control valves or aeration tanks influent control gates.- Hydraulic balancing of flow by equalizing the headloss from the primary sedimentation basins.
50 c) Froth control systems - Foaming- when the aerated wastewater which contains soap, detergents & other surfactants- Foaming action produces froth that contains sludge solids, grease, and wastewater bacteria.- Wind may lift & blow the froth contaminate whatever it touches, slippery, and difficult to remove once it has dried.- solutions: remove froth by spraying clear water or screened effluent through nozzles or,adding antifoaming chemical additives in spray water
51 d) Nocardia Foam Control - Nocardia foam is a thick layer of brown biological foams that forms on the top of aeration tanks and clarifiers.- Nocardia organisms grows, tend to trap air bubbles float to the surface and accumulate as scum (dirty foam)- Controlled by:i) Spraying chlorine solution directly into foam layer in some cases, spray nozzles installed within a hood located across the width of plug flow aeration tanks may not be effective, because it can cause floc breakup & inhibit BOD removal and nitrificationii) Addition of cationic polymer
52 3) Solids Separation Facilities a) AS settling tank types- circular or rectangular- circular tank:Diameter = 2 – 60 m (or m)tank radius > 5 X sidewater depth
54 Suspended Growth Aerated Lagoons Suspended growth aerated lagoons (SGAL) are relatively shallow earthen basins varying in depth from 2 – 5 m, provided with mechanical aerators on floats or fixed platforms.Mechanical aerators:- to provide oxygen for biological treatment of wastewater ,- to keep the biological solids in suspension.SGAL are operated on a flow-through basis or with solids recycle.Types of SGAL:1. Facultative partially mixed2. Aerobic flow through with partial mixing3. Aerobic with solids recycle and nominal complete mixing
55 Process Design Considerations for flow-through lagoons: BOD RemovalEffluent characteristicsTemperature effectsOxygen requirementEnergy requirement for mixingSolids separation.
56 Example 8-14 Design of a Flow-through Aerated Lagoon Design a flow-through aerated lagoon to treat a wastewater flow of 3800 m3/d, including the number of surface aerators and their kilowatt rating. The treated liquid is to be held in a settling basin (lagoon) with a 2-d detention time before being discharged. Assume that the following conditions and requirements apply:Influent TSS = 200 g/m3 (influent TSS are not degraded biologically)Influent sBOD = 200 g/m3Effluent sBOD = 30 g/m3Effluent suspended solids after settling = 20 g/m3Kinetic coefficients: Y = 0.65 g/g, Ks = 100 g/m3, k = 6.0 g/g.d, kd = 0.07 g/g.d for T = 20 to 25 ⁰CTotal solids produced are equal to computed volatile suspended solids divided by 0.85First-order observed soluble BOD removal-rate constant k20 = 2.5 d-1 at 20 ⁰C.
57 8. Summer air temperature = 30 ⁰C 9 8. Summer air temperature = 30 ⁰C 9. Winter air temperature during coldest month = 6 ⁰C 10. Wastewater temperature during winter = 16 ⁰C 11. Wastewater temperature during summer = 22 ⁰C 12. Temperature coefficient, θ = Aeration constants: α = 0.85, β = Aerator oxygen transfer rate = 1.8 kg O2/kWh 15. Elevation = 500 m 16. Oxygen concentration to be maintained in liquid = 1.5 g/m3 17. Lagoon depth = 3.3 m 18. Design SRT = 5d 19. Power required for mixing = 8 kW/103/m3
59 Attached Growth Processes (AGP) Microorganisms are attached to an inert packing materialThe organic material and nutrients are removed from wastewater flowing past the attached growth (or Biofilm)Ex. packing material : rock, gravel, slag, sand, redwood, plastics, etc.The packing can be submerged completely in liquid or not submerged, with air or gas space above biofilm liquid layer
60 - Most common: Trickling filter – wastewater is distributed over the top area of a vessel containing non-submerged packing material
63 Process flow in Trickling Filter The wastewater is applied over the bed of supporting media (rocks, stones, ceramic pieces, slag, etc) by rotating arms.The effluent is collected in the secondary clarifier, to separate washed out biomass solids before final disposal. As the wastewater trickles through the filter media, growth of microorganism takes place on the surface of packing material known as bio-film or slime layer.When the wastewater passes over this film, contact between substrates or food (waste) and microorganism is established, thus, the waste is decomposed aerobically by the attached biomass.(Facultative bacteria –attach in trickling filters, decompose the organic material in the wastewater along with aerobic and anaerobic bacteria).Then, a stage comes when anaerobic conditions are developed nearer to the media surface and the microorganisms cannot remain attached or fixed to the media.the slime layer then eventually peels off (or washed out) and removed from the filter along with (next) flow.- the washed out of slime layer is called sloughing.
68 Design Criteria of Trickling Filter a) Dosing rateDosing rate in Trickling Filter (TF) is the depth of liquid discharged on top of packing for each pass of the distributor.For higher distributor rotational speeds, the dosing rate is lower.With 2 or 4 arms, TF is dosed every 10 – 60s.Investigations show that reducing distributor speed results in better filter performance improve BOD removal, reduce Psychoda & Anisopus fly population, biofilm thickness, and odors.
69 b) Loading criteriaQuantifying biomass / biological & hydrodynamic properties in TF are not possible. Why ??Attached growth is not uniformly distributed in TFThe biofilm thickness can varyBiofilm solids concentration may range from 40 to 100 g/LThe liquid does not uniformly flow over the entire packing surface area.Hence, use to find broader parameters such as: volumetric organic loading, unit area loadings, and hydraulic application rates used as design / operating parameters to relate treatment efficiency
70 Higher Dosing Rate Larger water volume applied / revolution Greater agitation – causes more solids to flush outGreater wetting efficiencyWash away fly eggsThinner biofilm – increase surface area & more aerobic biofilm
71 ROTATING BIOLOGICAL CONTACTORS (RBC) RBC consists of series of closely spaced circular disks (polystyrene or polyvinyl chloride) submerged (typically 40%) in wastewater and rotated through it. The dicks are attached to a horizontal shaft. The assembly of shaft, disks and rotating equipments is called one module. Normally in industrial waste treatment, more than 3 modules arranged in series or parallel.
74 Removal Mechanism of RBC As the shaft rotates, the surfaces of rotating disks alternately come in contact with the microorganisms and organic content of wastewater and atmosphere.During the rotation, the microbes get attached to the disks, and O2 from atmosphere is transferred to the wastewater to maintain aerobic condition.The microbes attached to the disks surface grow in the form of biological film and consume organic content in the wastes.After some times, as the bio-films thickens, an anaerobic condition develops nearer to the disks surface , then the slime layer gets washed out (sloughing) by incoming wastewater flow. the sloughed bio-films is ultimately removed in the secondary clarifier before final disposal of treated effluent.………>> Similar mechanism to Trickling filter <<…………
76 Example 9-7 Design of staged RBC for BOD Removal Given the following design conditions, develop a process design for a staged RBC systemParameterUnitPrimary EffluentTarget EffluentFlowrateBODsBODTSSm3/dg/m3400014090702010