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CHEE 370 Waste Treatment Processes Last Lecture Final Exam Review 1 Wastewater Constituents Suspended solids Biodegradable organics Micro-organisms Nutrients Refractory organics Heavy metals Dissolved inorganics Physical contaminants (temperature, colour, odour, solids) 2 Measurement of Solids Evaporation and drying at 105 ºC Evaporation and drying at 105 ºC Ignition at 500 ºC TS = TVS + TFS TS = organic + inorganic 2.3 Metcalf & Eddy The BOD Test COHNS + O2 + bacteria (C5H7NO2) more C5H7NO2 + CO2 + H2O + NH3 + other An INDIRECT measure of the organic content Measures the amount of oxygen consumed as the bacteria found in the WW use the organic material as a carbon/energy source for the production of more bacteria BODt=UBOD(1 - e-kt) 4 BODt=UBOD(1 - e-kt) Chemical Oxygen Demand (COD) An INDIRECT measure of the organic content Mass of oxygen theoretically required to completely oxidize an organic compound to carbon dioxide Measured by mixing the WW with a very strong chemical oxidant CODin = CODout + O2consumed 6 Total Organic Carbon (TOC) A DIRECT measure of the organic content In theory - based on the chemical formula In practice - organic carbon is converted to carbon dioxide, which can then be measured Theoretical Oxygen Demand (ThOD) An INDIRECT theoretical measure of organic content Calculated using stoichiometric equations Considers both carbonaceous and 7 nitrogenous oxygen demand Additional Characterization Micro-organisms Total coliform, fecal coliform Toxicity Acute toxicity (LC50), Chronic toxicity Nutrients TKN, NH3, TP, ortho-phosphate Flowrates Hydraulic flowrates (peak and min), Loadings 8 WWTP for a Large Community Screening - Headloss 1 V 2 v 2 hL C 2g hL = headloss (m) C = empirical discharge coefficient to account for turbulence and eddy losses (clean screen = 0.7; clogged screen = 0.6) V = velocity through the openings (m/s) v = approach velocity in upstream channel (m/s) g = acceleration due to gravity (9.81 m/s2) 10 Types of Settling Type I: Discrete Settling Type II: Flocculent Settling Type III: Hindered or Zone Settling 11 Type I Settling Discrete Settling Settling of discrete, non-flocculating particles Found in grit removal tanks! Particles settle as individual entities at a constant velocity Minimal interaction between particles Applies only to particles in a suspension with a low solids concentration 12 Type I Settling Critical Settling Velocity vo= critical settling velocity A particle starting at the top of the inlet zone with a settling velocity of vo will just reach the bottom of the tank at the beginning of the outlet zone 13 Type I Sedimentation Analysis Use a batch settling column Withdraw samples from a fixed height “h” at time intervals and measure the solids concentration Calculate the weight fraction remaining Calculate the settling velocity for the particles at each of the time intervals (vs = h/t) Plot the weight fraction remaining versus velocity (cumulative distribution curve) 14 fo 1 F (1 f o ) vo v df s Total Removal o Type II Settling Flocculant Particle Settling Particles coalesce as they settle Rate of settling (vs) changes with time Particles change in size, shape and weight as they settle Larger particles have a higher vs 16 Type II Settling Analysis Use a batch settling column with multiple sampling ports Withdraw samples from each port at time intervals and measure the solids concentration Calculate the weight fraction removed Prepare a plot of sampling depth versus time and indicate the weight fraction removed for each of the samples Draw the “equal percent removal” lines at intervals of 10% on the plot 17 r3 r2 r1 Type I and Type II Settling Clarifier Dimensions A = L x W = surface area of the basin (m2) Default aspect geometry: L = 4W A = 4W2 19 Type I and Type II Settling Scouring Velocity Re-suspension of particles due to large horizontal velocities (u) Q u Where: HW u = horizontal velocity (m/s) Q = water flowrate (m3/s) HW = cross-sectional area (entry area) in the direction of flow (m2) To prevent scouring, u < (9 * vo) 20 Type III Settling Hindered or Zone Settling Occurs in solutions with a very high solids concentration Strong cohesive forces between the particles cause them to settle collectively as a zone Distinct interface between the settled particles and the clarified effluent 21 Type III Settling Clarifier Design Secondary clarifiers need to be designed for two purposes: Clarification Thickening 22 Type III Settling Secondary Clarifier Design 1. Calculate the area required for clarification Qe vo Ac Where: vo = initial zone settling velocity at the feed concentration (X), [m/h], (function of X) Ac = surface area for clarification [m2] Qe = overflow rate of clarified liquid [m3/h] 23 Type III Settling Secondary Clarifier Design 2. Calculate the area required for thickening • Find the gravity mass flux Gg v i X i Where: Gg = gravity flux [M/L2•T] (kg/(m2•h)) vi = settling velocity at solids concentration Xi [L/T] (m/h) Xi = local concentration of solids [M/L3] (kg/m3) 24 Type III Settling Secondary Clarifier Design 2. Calculate the area required for thickening • Find the bulk mass flux due to underflow pumping Qu Gu ub Xi ub A Where: ub = bulk downward velocity of the solids [L/T] (m/h) Qu = underflow flowrate [L3/h] (m3/h) A = surface area of settling tank [L2] (m2) 25 Type III Settling Secondary Clarifier Design 2. Calculate the area required for thickening • Find the total mass flux Plot G, Gg, Gu G Gg Gu G X i v i X i ub 26 QX o Limiting Flux AT GL 27 Type III Settling Secondary Clarifier Design Identify which area is greater: Area for clarification Area for thickening Use the larger area to size the clarifier Adesign = 1.75*Acalculated For an ideal clarifier, L = 4W 28 Designing for a Specific Underflow Solids Concentration (Not Given ub) Batch Bacterial Growth Curve 1. Lag Phase Acclimation to environment 2. Exponential Growth Phase Multiplication at max rate Rapid utilization of S 3. Stationary Phase Growth is offset by death Steady state 4. Death Phase Depletion of S Decrease in X due to cell death 30 Metcalf and Eddy; Figure 7-10 WW Treatment Bacterial Growth Rate dX X kd X dt Where: X = biomass concentration (mass/volume) = specific growth rate (time-1) kd = endogenous decay coefficient (time-1) 31 Bacterial Growth in Biological WW Treatment Monod Kinetics Specific growth rate increases as the concentration of the limiting substrate S increases = specific growth rate (time-1) max = maximum specific growth rate (time-1) m axS S = concentration of the growth limiting substrate (M/V) S KS Ks = half saturation constant (M/V) 32 Michaelis-Menten Continued Derivation carried out in class! [S ] vo vm ax S K M Now analogously, if our “product” are cells the above specific rate of cell formation is as described as: m axS S KS 33 Estimation of Kinetic Parameters It is possible to estimate the kinetic parameters (Ks, kd, max, Y) from bench-scale CSTR process data in order to design biological waste treatment facilities Perform tests starting with a known limiting substrate concentration So Measure X and S at various residence times () 34 CSTR With No Recycle Problems If the kinetic parameters (Ks, kd, max, Y) are known, and you are given So, Q, and one additional variable (i.e. S, U, V), then you can solve for the rest 35 AS Design Equations V VX Ks S c c Q X Q X c S( k ) k K Qo E E w R max d d s c Y(So S) K s(1 c kd ) So S X S U 1 kd c c ( m ax kd ) 1 X 1 S max 1 (K s So )(1 R R) kd YU kd w c Ks S c So ( max kd ) kd K s maxXSV Qr XR BiomassWasted QE X E QwX R kd XV R Ks S Qo X m axSX m axS RXR (1 R)X kd X 0 R (1 R) kd 0 Ks S Ks S F Qo So S So S o Efficiency 100% M V X X So COD Mass Balance Oxygen Consumed When working in COD units, you can always perform a mass balance COD substrate in = COD substrate out + COD biomass out + O2 consumed O2 consumed = COD substrate in - COD substrate out - COD biomass out O2 consumed = COD substrate consumed - COD biomass out 37 Conversion Factor (f) Required to determine the oxygen requirements if the substrate concentration is expressed in terms of BOD5 BOD5 converted f UBODconverted If you are given the influent substrate concentration in terms of BOD5 and UBOD, you can calculate “f” to determine the effluent substrate concentration in UBOD units 38 AS - Aeration Requirements Air supply requirements can be expressed in a variety of units kg O2/day, kmol O2/day, m3 O2/day, m3 air/day Conversion factor: 22.4 m3 gas/kmol (@ STP) Air contains ~21 % O2 If the oxygen transfer efficiency of the aeration system is known or can be estimated, the air requirements may be determined ALWAYS DESIGN AERATION SYSTEMS WITH A SAFETY FACTOR OF 2 39 SVI Determination Take a sample of MLSS from the aeration basin Settle for 30 min (usually in a 2 L container with a diameter larger than a graduated cylinder) Determine the volume and mass of the settled solids Y Y, settled volume of sludge (mL) SVI X, mass of settled solids (g) X Typical range: 50 - 150 mL/g 6 10 Xr Units of mg/L SVI 40 TF Design Equation Se n A exp z K Si Q z = depth of the packing media/bed [ft] Q = applied flow (Qo + Qr) [MG/D] A = filter bed cross-sectional area = π•r2 [Acres] n, K = constants; f(packing media); See table 6.11 41 AD Model Assumptions Design based on the rate-limiting step - breakdown of volatile fatty acids (VFAs) Non-biodegradable fractions of COD remain unchanged by the digestion process Heterotrophic bacteria only decays and the COD associated with decay will be accumulated as VFAs available to the methanogens Complete hydrolysis and fermentation of biodegradable organic matter -> fully available to methanogens Use the kinetics for the growth of the methanogens to determine the minimum SRT, then use this value with a safety factor to determine the operating conditions 43 Minimum SRT Calculation K vfa Svfa,available m in Svfa,available (m ax,m k d ,m ) K vfa k d ,m Where: umax,m = maximum specific growth rate for the methanogens Kd,m = decay rate for the methanogens 44 Factor of Safety for Growth It is necessary to provide a factor of safety for methanogen growth (prevent “stuck” digester) and headspace Use a factor of safety of at least 2.5 design 2.5 min The ministry of the environment requires at least 15 days SRT at 35 C Compare with your calculation and select the larger value 45 Heterotroph Mass Balance Assume there is no growth - only decay Perform a mass balance on the digester for the heterotrophic bacteria: X H ,o XH 1 kd , H c As the SRT increases, the amount of active heterotrophic biomass in the effluent decreases 46 Debris Mass Balance Debris (XD) can enter the digester in the influent (XDo) stream and is also generated during biomass decay Perform a debris mass balance on the digester : k X D X Do f d X H,o d ,H c kd , H c 1 Where fd = debris fraction of the degraded biomass (fd ranges from ~ 0.08 - 0.20) 47 VFAs for Methanogens Multiple Sources: Soluble biodegradable COD (Ss) Biodegradable particulate COD (Xs) Decay of heterotrophic biomass k Svfa,available Ss X S (1 f d ) X H ,o d ,H c kd , H c 1 48 Effluent VFA and Formation of Methanogenic Bacteria CSTR without recycle K vfa (1 c k d ,m ) Svfa c (m ax,m k d ,m ) 1 (Svfa,available Svfa ) X m Ym 1 c k d ,m 49 Methane Production COD balance can be performed in order to determine the amount of methane produced CODin = Q(SSo + XSo + XHo + XDo) CODout = Q(Svfa + XH + Xm + XD) CODin = CODout + CODmethane produced CH4 + 2O2 CO2 + 2H2O 64 g COD/mol CH4 50 Methane Production Use the ideal gas law to calculate the volume produced per day (V=nRT/P) Textbook example 7-9, p. 633 - Effect Of Temp! Volume of methane produced per day: mCH 4 RT FCH 4 64P Where mCH4 is mass-COD of CH4 produced/time 51 Breakpoint Chlorination Dose 52 Estimating the Kill Efficiency A correlation often used to estimate the amount of residual chlorine required to achieve a certain kill efficiency is: 1 0.23Ct Nt 3 Collins Model N0 N = number of organisms [C] = total chlorine residual, mg/L; [t] = time, minutes 53 Chlorine as a Disinfectant Advantages: Reliable Cheap Simple Provides a stable residual Limitations: Extremely toxic and corrosive Can influence water taste and odor Forms trihalomethanes by reacting with organic matter in the WW - these compounds are known carcinogens 54 Nitrification In a suspended growth (AS) process, low-rate extended aeration conditions lead to nitrification Involves two types of autotrophic bacteria: Nitrosomonas oxidizes ammonia to nitrite NH3 + 3/2 O2 HNO2 + H20 Nitrobacter oxidizes nitrite to nitrate HNO2 + 1/2 O2 HNO3 Overall conversion: NH3 + 2 O2 HNO3 + H20 55 Nitrifying Systems The AS design algorithm can be used to develop strategies to remove ammonia from the effluent through nitrification Ammonia is used as the substrate in the AS design model (in place of the biodegradable organics in the influent (BOD)) The systems are designed to facilitate longer solids retention times Accounts for the slower growth the the nitrifying bacteria In practice, typically a plug flow system is used (results in coupled PDEs) 56 One-Step Process Two-Step Process 57 Metcalf and Eddy; Figure 7-19 Summary of the BEPR Process In the anaerobic zone of the system: PAOs take up the VFAs from the liquid phase Phosphorus is released to the liquid phase due to polyphosphate cleavage to provide energy for VFA transport Glycogen is utilized and transformed into PHAs to provide enough reducing power to drive the transformation of VFAs into PHAs At the end of the anaerobic period: No VFAs left Large phosphorus concentration in the liquid phase PAOs will have low intracellular glycogen and high PHA contents 58 Summary of the BEPR Process In the aerobic zone of the system: PAOs use stored PHA as a substrate for growth and for replenishing the glycogen pool PAOs use stored PHA for phosphorus uptake and replenishment of the polyphosphate pool The amount of phosphorus taken up in the aerobic phase is higher than the amount of phosphorus released in the anaerobic phase Net P removal from the liquid phase At the end of the aerobic period: More PAOs will be present Intracellular glycogen content is high, polyphosphate content is high, PHA content is low Soluble phosphorus concentration is very low (can be zero) 59 The BEPR Process 60