epc_fa2011_lecture_8

Epc_fa2011_lecture_8 - Wastewater Treatment Processes Necessary to protect the water quality of receiving surface waterbodies Wastewater treatment

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Unformatted text preview: Wastewater Treatment Processes -- Necessary to protect the water quality of receiving surface waterbodies. -- Wastewater treatment processes can be applicable to treat various source wastewaters, however, most typically refers to the municipal wastewater. -- Types of wastewater collection system (=sewer system) 1) Sanitary Sewer System 2) Storm Sewer System 3) Combined Sewer (CS) System (in older infrastructure) 1 Wastewater Collection System (=Sewer System) 2 Combined Sewers (CS) -- A type of sewer system that collects both sanitary sewage (=normal condition) and stormwater runoff (=wet condition) in a single pipe system, constructed till 50’s in the Northeast and Great Lakes regions -- Tremendous flow variations due to irregular stormwater runoff influxes to sewers Retention time within the treatment plant decreases under high flow, i.e., wastewater is not treated sufficiently, i.e., operational problems. "Strength" of waste is diluted under high flow rate. In biological wastewater treatment systems, this is not favorable since bacteria have acclimated to feeding on a stronger waste, i.e., efficiency suffers. 3 Combined Sewer Overflow (CSO) 4 Contaminants in Municipal Wastewater -- Suspended solids -- Biodegradable organic matter -- Nutrients, especially N and P -- Pathogens -- Endocrine disruptors 5 Municipal Wastewater Treatment Processes Primary Treatment removal of mostly solid materials in the sewage to protect pumps and other mechanical equipments (physical treatment) Secondary Treatment removal of dissolved and suspended organics and inorganic particles that were not removed in primary treatment (biological treatment) 6 Primary Treatment -- Removal of mostly solid materials in the sewage to protect pumps and other mechanical equipments (physical treatment) 1) 2) 3) 4) Screen/Bar Racks & Comminutors and Barminutors Grit Chambers Equalization tank Primary Clarifier (Primary Sedimentation Basin) Primary Treatment Large solids Raw Sewage Small, settleable solids Bar Rack Screen Comminutor Barminutor Grit Chamber Equalization Basin Primary Tank/ Clarifier Regulate flow rate (=surge protector) Secondary Treatment Settling basin 7 Primary Treatment 1) Screen/Bar Racks -- Intercept large solids (2 to 3 inches screen opening) -- Consist of a series of parallel bars -- Solids removed referred to as “screenings” 8 Primary Treatment 1) Comminutors and Barminutors ------ The “Teeth” or “Shredder” or “Grinder” Genuinely grind and shred large solids into smaller pieces. Barminutor is similar to screens with additional mechanical cutter Used right after screens, Ground-up solids are removed in following grit chamber. 9 Primary Treatment 3) Grit Chambers -- Remove abrasive, largely inorganic "settleable" particles. -- Based on settling of particles due to velocity slowdown caused by helical flow pattern, and subsequent by gravitational pull-down. -- Grit removal saves wear on pumps and other mechanical equipment. -- Many grit chambers are aerated to prevent organic particles from settling (=sticky gunks!). 10 Primary Treatment 3) Equalization Tank/Basin -- Typical fluctuations/spikes in domestic water usage peak around 7 AM and 6 PM. Incoming wastewater inflow rates to a wastewater treatment plant (WWTP) would vary accordingly. Super Half Time!! -- WWTP 24/7 operations becomes not very efficient or feasible if processing capacity keeps varying with variations in incoming wastewater inflow rates. (i.e., cannot keep changing/modulating operating capacity of the primary tank corresponding to the variable influent flow rates) -- An equalization basin is to act as a surge volume protector for these fluctuations; it should have an adequate capacity to absorb these fluctuations while the WWTP processes receive a constant flow rate downstream of the equalization basin - i.e., an equalization basin is to regulate the flowrate right after the grit chamber. 11 Primary Treatment 4) Primary Clarifier (Primary Sedimentation Basin) -- Designed and operated for removal of suspended organic matter and "floatables", usually oils and grease. -- Low maintenance, typical hydraulic retention time of 2 hours. -- Treatment effectiveness is a function of the waste stream characteristics and flow rate. 12 Primary Treatment Equalization Tank/Basin Determination -- 1 -- Hourly flows and corresponding BOD5 represent a typical daily variation of inflow to a wastewater treatment plant (WWTP). -- Design target parameters are; - Size of the equalization basin required to provide for a uniform outflow equal to the average daily flow - Average BOD5 coming out of equalization basin (i.e., after equalization) and sent to the primary tank (so that subsequent process capacity after the equalization basin can be designed) 13 Primary Treatment Equalization Tank/Basin Determination -- 2 Example) Designing an equalization basin for average daily flow of 0.0978 m3/s 14 Primary Treatment Equalization Tank/Basin Determination -- 3 Step 1) - Since we are going to repeat calculations, let's use Excel to solve for equalization basin volume estimation. First, set up Excel columns for calculation. - Column B is the hourly influent stream flow that is entering the equalization basin. (and equalization basin is situated right before before the primary tank) 15 Primary Treatment Equalization Tank/Basin Determination -- 4 Step 2) -- Sort and Rearrange Time, Flow and BOD5 with time and flow that first exceeds the average daily flow rate of 0.0978 m3/s (=design flow rate). 16 Primary Treatment Equalization Tank/Basin Determination -- 5 Step 3) -- For Column C (Volume Into the equalization basin), with 1 hr interval, a simple unit conversion is done here to convert the flow rate (column B) to the volume (column C). 406.8 m3 = 0.113 (m3/sec) *1 (hr) * 3600 (sec/hr) 471.6 m3 = 0.131 (m3/sec) *1 (hr) * 3600 (sec/hr) 17 Primary Treatment Equalization Tank/Basin Determination -- 6 Step 4) -- Similar unit conversion to column C, but the same average flow rate, 0.0978 m3/s * 1 hr * 3600 (sec/hr) = 352.1 m3 was used for the entirety of the column D. This is the average stream volume coming out of the equalization basin and sent to the primary tank. 18 Primary Treatment Equalization Tank/Basin Determination -- 7 Step 5) -- Column E = column C - D; Storage, ΔS = Volume_In Volume_Out of the equalization basin. -- Column F = Cumulative storage, Sum(ΔS) (= Sum(CD)). Pay close attention to Sum(ΔS) value of 1307.5 m3 at 2300 hour, highlighted in red below. This maximum cumulative storage value is the basis for determining the size of equalization basin. -- Equalization basin size = 1307.5 m3 * 1.25 = 1634.4 m3 where 1.25 represents 25% excess or a safety factor as usual. 19 Primary Treatment Equalization Tank/Basin Determination -- 8 Step 6) -- Column G = Hourly influent BOD5 measurements (=given) that are entering the equalization basin. -- Column H = Mass of the hourly influent BOD5 (=Loading, W) that are entering the equalization basin = Q * S0(=BOD5 conc.) * time interval 50.85 kg = (0.113*125*1*3600) / 1000 66.02 kg = (0.131*140*1*3600) / 1000 = column B (m3/s) * column G (mg/L=g/m3) * 1 hr * 3600 (sec/hr) * (kg/1000 g) 20 Primary Treatment Equalization Tank/Basin Determination -- 9 Step 7) -- Column I = Average influent BOD5 concentration, S over the time interval (Use the MB!!) or Saverage = (CiGi+Fi-1Ii-1) / (Ci+Fi-1) 125 mg/L = (406.8*125)/406.8 138.44 mg/L = [(471.6*140)+(54.72*125)] / (471. 6+54.72) 146.95 mg/L = [(486.0*150)+(174.2*138.44)] / (486.0+174.2) 21 Primary Treatment Equalization Tank/Basin Determination -- 10 Step 8) -- Column J = Mass of the hourly influent BOD5 that are exiting the equalization basin = Qaverage * Saverage * Time interval = 0.0978 (m3/s) * I (mg/L=g/m3) * 1 hr * 3600 (sec/hr) * (kg/1000 g) 44.01 kg = (0.0978*125.00*1*3600)/1000 48.72 kg = (0.0978*138.44*1*3600)/1000 51.74 kg = (0.0978*146.95*1*3600)/1000 22 Primary Treatment Equalization Tank/Basin Determination -- 11 Step 9) -- Finally, average BOD5(average) after equalization value can be estimated by calculating the average from BOD5(average) values, which is 137.04 mg/L -- In summary, the size of Equalization basin should be at least 1634.4 m3 in order to be able to handle given daily inflow variations. m3/s Design average daily flow =0.0978 Design Equalization Vol. = 1307.5 * 1.25 = 1634.4 m3 -- Average BOD5 in outflow after equalization will be 137.04 mg/L. 23 Quick Recap 1) Purpose of WW treatment is to protect the water quality of receiving surface waterbodies. 2) Primary WW treatment is all Physical process. 3) Secondary WW treatment is all Biological process. 4) Equalization tank/basin acts as a surge volume protector for fluctuating inflows – it will stabilize and provide a constant flow rate downstream of the Equalization tank/basin. 24 Municipal Wastewater Treatment Processes Primary Treatment removal of mostly solid materials in the sewage to protect pumps and other mechanical equipments (physical treatment) Secondary Treatment removal of dissolved and suspended organics and inorganic particles that were not removed in primary treatment (biological treatment) 25 Secondary Treatment -- Second treatment removes dissolved and suspended organics and inorganic particles that were not removed in the primary treatment processes. -- Mainly consists of biological treatment process -- Once soluble organic matters (OMs) are converted to particulate OMs by biological treatment, they can be more easily removed from the waste stream by sedimentation method. 26 General types of Biological Secondary Treatment Systems 1) Suspended Cultures of Microorganisms (Larger system) 2) Attached Cultures of Microorganisms (Smaller system) 27 General types of Biological Secondary Treatment Systems Suspended Cultures of Microorganisms -- Numerous colonies of microorganisms (biological flocs) present in water. Water movement/turbulence keeps flocs from settling. -- Organics (= Soluble BOD) removed by aerobic respiration. -- CSTR configuration is most popular (=concentration of soluble BOD is constant throughout the reactor). -- CSTR configuration is more resistant to shock loadings (i.e., spikes) in influent BOD concentration and toxics. • 28 General types of Biological Secondary Treatment Systems Attached Cultures of Microorganisms –1 -- Microorganisms are attached to a media (= crushed stones, plastic tile or slag) as opposed to suspended in solution. -- Attached microorganisms will form a biofilm on the surface of the a media. -- Diffusion [of material concentration from higher to lower concentration] is the main mechanism for BOD removal from the water. - Elements diffused into biofilm are BOD, O2, nutrients (when anaerobic, no O2). - Elements diffused out of the biofilm are CO2, by-products of metabolism. 29 General types of Biological Secondary Treatment Systems Attached Cultures of Microorganisms –2 time = t0 Clean media time = t1 Biofilm established time = t0 Biofilm grown thick time = t3 Sloughing -- Eventually, biofilm detaches itself from media because dead microorganism no longer "holding on" to the media (referred to as "sloughing") 30 Oxygen Requirements / Diffusers and Aerators -- O2 commonly supplied by pumping or mixing air directly into the reactor water via air diffusers. Sparger 31 Oxygen Requirements / Diffusers and Aerators -- Aerator produce turbulence at the surface to enhance O2 transfer to the water. Aerator setting could be either Floating or Fixed platform. 32 Activated Sludge (Suspended Cultures ) -- 1 -- Activated sludge process is an important biological wastewater treatment technique that a mixture of wastewater and biological sludge (microorganisms) is agitated and vigorously aerated. -- As a result, microorganisms are mixed thoroughly with the organic matter under the condition that stimulate their growth through use of the organic matter as food. -- As the microorganisms grow and are mixed by the agitation of the air, the individual organisms clump together (= flocculate) to form an active mass of microbes (= biologic floc) called activated sludge. -- Activated sludge has the ability to absorb and remove suspended, colloidal and dissolved organic matter. -- Aerobic digestion by Bacteria, Fungi and Protozoa/Metazoa (=rotifiers, crustaceans, worms, larvae) 33 Activated Sludge (Suspended Cultures ) -- 2 34 Activated Sludge (Suspended Cultures ) Process Design Considerations -- 1 -- The design formulae for the activated sludge process are based on the mass balance that describes the kinetics of bacterial growth. - mass balance for the biomass (X) - mass balance for the waste (BOD or substrate, S) 35 Activated Sludge (Suspended Cultures ) Process Design Considerations -- 2 -- Most commonly used design parameters [to determine the loading criteria] include 1) Mean cell residence time (MCRT), θc 2) Food-to-microorganisms ratio (F/M ratio) 3) Sludge Production Rate 4) Oxygen Requirements and Aeration Rate 5) Nutrient Requirements 36 Activated Sludge (Suspended Cultures ) Process Design Considerations Mean cell residence time (MCRT) -- 1 -- Mean cell residence time (MCRT), θc (also expressed as Solids Retention Time, SRT, and sludge age) is defined as the average amount of time that microorganisms are kept in the system. θc = XV Q w X r + (Q − Q w )X e If X e = 0, θ c = XV QwXr Q = wastewater flow rate Qw = waste sludge flow rate (= the rate of sludge being removed) X = mixed liquor volatile suspended solids concentration (MLVSS), mg/L Xr = conc. of volatile suspended solids in clarifier underflow, mg/L Xe = volatile suspended solids concentration in the effluent, mg/L 37 Activated Sludge (Suspended Cultures ) Process Design Considerations Mean cell residence time (MCRT) -- 2 -- Conc. of soluble BOD5 in the system can be calculated once the MCRT has been established. S= k s (1 + k dθ c ) θ c ( μm - k d ) - 1 S = soluble BOD5 in aeration tank as well as in effluent water, mg/L ks = half velocity constant, substrate concentration at 1/2 the maximum growth rate, mg/L kd = decay rate of the bacteria, day-1 θc = mean cell residence time (MCRT) µm = maximum growth rate constant, day-1 38 Activated Sludge (Suspended Cultures ) Process Design Considerations Mean cell residence time (MCRT) -- 3 -- Total microbial mass in the reactor (=XV) is then calculated with S (conc. of soluble BOD5) and θc (MCRT). XV = YQ ⋅ ( S 0 - S) θ c 1 + k dθ c X= YQ ⋅ ( S 0 - S) θ c Y ⋅ ( S 0 - S) θ c = θ ⋅ (1 + k dθ c ) V (1 + k dθ c ) X = mixed liquor volatile suspended solids concentration (MLVSS), mg/L Y = cell yield coefficient, gram cell (or mg cell) Q = wastewater flow rate S0 = influent soluble BOD5 to aeration tank, mg/L S = soluble BOD5 in aeration tank as well as in effluent water, mg/L θc = mean cell residence time (MCRT) = V/Q kd = decay rate of the bacteria, day-1 39 Activated Sludge (Suspended Cultures ) Process Design Considerations Food-to-microorganisms ratio (F/M ratio) -- 1 -- F/M ratio ranges between 0.1 and 1.0 (in day-1), with typical values around 0.5. -- A very high F/M ratio (i.e., F/M ratio > 0.9) indicates that organisms in the aeration tank are saturated with food -- too much food to handle, and the efficiency of digestion/removal would be poor. -- Au contraire, a low F/M ratio (i.e., F/M ratio < 0.3) indicates that organisms are starving and ready to eat/digest! -- a condition that would result in more complete removal of the waste. -- However, an extremely low F/M ratio (i.e., F/M ratio < 0.1) indicates a very poor efficiency in digestion/removal -almost no digestion/removal would occur, and organisms will soon start dying off from starvation. 40 Activated Sludge (Suspended Cultures ) Process Design Considerations Food-to-microorganisms ratio (F/M ratio) -- 2 F/M = S0 S ⋅Q =0 θ ⋅X V⋅X F/M ratio > 0.9 – saturated with food F/M ratio < 0.3 – starving and ready to digest F/M ratio < 0.1 – poor efficiency in removal S0 = influent soluble BOD5 to aeration tank, mg/L θ = retention time, days = V/Q X = mixed liquor volatile suspended solids concentration (MLVSS), mg/L Young MLSS Aged MLSS 41 Activated Sludge (Suspended Cultures ) Operational Problems -- 1 1) Bulking Sludge -- Due to filamentous microorganisms/fungi, causes poor settling and poor compactability of removed sludge. 2) Rising Sludge -- Due to denitrification – nitrites (NO2-) and nitrates (NO3-) in the wastewater are converted to nitrogen gas. -- As nitrogen gas is formed in the sludge layer, much of it is trapped in the sludge mass. If enough gas is formed, the sludge mass becomes buoyant and rises or floats to the surface. Rising sludge can be differentiated from bulking sludge by noting the presence of small gas bubbles attached to the floating solids. 42 Activated Sludge (Suspended Cultures ) Operational Problems -- 2 3) Nocardia Foam -- The foam is associated with a particular slow-growing filamentous fungal organism of Nocardia genus. 43 Quick Recap 1) Secondary WW treatment is all Biological process. 2) Most common suspended culture process is Activated Sludge process. 3) It’s all Mass Balance! Mean cell residence time (MCRT), θc Food-to-microorganisms ratio (F/M ratio) 44 ...
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This note was uploaded on 10/24/2011 for the course CEE 350 taught by Professor Jaewanyoon during the Fall '10 term at Old Dominion.

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