Comnletely Mixed Activated SIud2e (CMAS) Bioreactor Desi2n Eguations

Transcription

1 Comnletely Mixed Activated Sud2e (CMAS) Bioreactor Desi2n Eguations --- c;: Secondary : i Clarifier : (Q-Qw) s x. Q SO : (Q+QJ s X : XSV :... Aeration Tank : Mass Balance: Biomass: : Q S i Qw x.. S : Return Activated Sludge (RAS) : Waste Activated ~ : j Sludge(WAS) QXo + V (~~~ -kd X) = (Q -Qw) Xe + Qw Xw Ks + S Substrate: QSo -V (-~~~~;\ ) = (Q -Qw) Se + Qw Sw.L\KS + S} ( ~ where Q Qw = influent flow and waste flow respectively m3/d V = volume of aeration basin m3 ~max = maximum specific growth rate coefficient h-1 Kg = half saturation coefficient mg/l ~ = decay coefficient h-1 Xo X Xc ~ = biomass in influent bioreactor effluent and waste mg/l as ML VSS S = soluble substrate concentration in bioreactor mg/l as BOD or COD So = influent substrate concentration mg/l as BOD or COD Y = biomass yield mg biomass formed! mg substrate utilized (mg VSS/ mg BOD) Assumptions: 1. nfluent and effluent concentration is negligible 2. Aeration basin is a CSTR S = Sw = Sc Then: 3. All reactions occur in basin -~~ = ~ + k.~~~ = Q.{S -S) Ks + S VX d ' Ks + S VX~ 0 Observe: 1

2 Q -1.Q7w -1 v-e ' -vx--e: Where e = the hydraulic retention time HRT and ec = the solids residence time SRT. This results in the following design equations: S = Ks (1 + kd e c) e = Ks + S X = e c Y(So -S) e c ~max -kd) -1 ' c S (~max -kd) -Ks kd' e (1 + kd e c) The minimum soluble BOD concentration that can be achieved as ec : S. = Ks kd mm ~max -kd The minimum ec achievable as ~ -+ ~max : e = Ks + So colin So ~max -kd ) -Ks kd SteRs for Activated Slud!!e Desi!!n 1. Establish effluent soluble BODs allowable to meet BODs and SS effluent limits. 2. Determine what ec is required to meet the effluent soluble BODs allowable. 3. Solve for the mixed liquor volatile suspended solids ML YSS concentration given a particular hydraulic residence time e. Or solve for e given a particular ML YSS. 4. Calculate the return activated sludge (RAS) flow Qr' and concentration ~'. ~'Qr = X'(Qr + Q) ; ~'= 106/SY where X' = MLSS mg/l (X' typically is approximately 1.2.X) ~' = RAS concentration mg/l Qr = RAS flow rate m3/s Find ~' using the sludge volume index SY from the following figure: 2

3 ---. SUGGESTED MAXMUM MLSS ~ CONCENTRATON Z Zo (j) i=. ««300 R = Q ~ ~. ~ Z -W U U ' ~ ~ R ~ Q ~ u. ~ CONDTONS: i ~ 1000 SV AT AERATOR TEMPERATUR-E. NOT AMBENT TEMPERATURE DESGN REACTOR BASN TEMPERATURE. c FGURE Recommended maximum MLSS design versus temperature and SV. 5. Sludge production can be estimated as follows: p -y Q (s - S) kg x -OBS 0 looog where: Px = sludge production kg/d Y OBS = observed growth yield mg biomass formed VSS/ mg BODs utilized Q = influent flow m3/d So = influent BODs 's = effluent BODs Y OBS can be estimated as : y = y OBS 1 + k.8 d c 6. Oxygen requirement for carbonaceous BOD removal can be calculated as: 3

4 -c..c" ( Q(So-S) kg ) 2 req = ' -1.42'Px f 1000 g wheref= the conversion from BODs to BODL ( ) When nitrification is occurring the oxygen requirement can be calculated as: 2 req = ( Q(S 0 -S) kg ) -1.42'Px Q ( No -N ) ' kg f 1000 g 1000 g where No and N are the influent and effluent NH4-N concentrations respectively. 7 Calculate the alkalinity consumed The conversion ofnh3-n to nitrate not only requires oxygen but it also consumes considerable amount of alkalinity (7.1 mg/l as CaCO3 for every mg/l NH3-N): alk consumed (kg/d) = Q (No -N) 7.1 mg/l as CaCO3/mg NH4-N ' (kg/l000 g) 8. Settling Tank Design The design of primary and secondary settling tanks can be done on the basis of settling tests and/or established design criteria. n general the design of tanks must meet established overflow rate and weir loading criteria. 10 State Standards Criteria Criteria Primary Settling Tanks Secondary Settling Tanks Overflow Rate m3/m2'd A vg Peak Peak OR=Q/A Weir Loading m3/m'd < 1 mgd > 1 mgd < 1 mgd > 1 mgd WL = Q/L Activated Sludge Operational Considerations An operator of an activated sludge plant is concerned with three things: 1. E quality (BODs and SS) 2. S characteristics of the biomass (SV) 3. Sludge w or solids inventory (8c F/M) 4

5 These three objectives/operational parameters are interrelated. A good settling sludge will produce good effluent quality. Maintaining the proper solids inventory will produce a good settling sludge. Controlling eo will maintain the proper solids inventory. SV -Sludge v index. C().Measure of s characteristics of biomass..measured in a g cylinder after 30 minutes of settling..units of ml/g..a d SV is in the range of Sludge Bulking.Sludge bulking is the condition where the SV is h and the suspended solids.are not settling in the secondary settling tank..t is usually an indication of f organisms -long string-like organisms which outcompete the flocculent organisms because of their large surface area..filamentous organisms can be caused by a) F/M ratio b) DO c) nutrient d d) ph e) i or toxicity F/M Ratio.The f to m (F/M) ratio is an alternative control parameter to eo for the operation of an activated sludge plant. E- = ~ = mg BODs/dD M VX mg MLVSS Note: the F/M ratio is inversely proportional to ec. 5 ~

6 .Low F/M ratios are typical in c mixed activated sludge (CMAS) systems..cmas systems consequently often have - filamentous b problems. Low F 1M Single Tank CMAS High.By using a s the F/M F 1M in the first compartment of an activated Selector Low F 1M sludge system can be increased giving the f microorganisms a CMAS with Selector competitive advantage. SUMMARY OF ACRONYMS SRT S retention time also referred to as eo. The SRT is the major design variable for the activated sludge process. The longer the SRT the lower the effluent soluble BOD. MLSS Mixed suspended solids concentration. A measure of the total solids (fixed solids plus volatile solids) in the activated sludge process. ML VSS Mixed liquor v suspended solids concentration. A measure of the volatile solids in the activated sludge process. The ML VSS concentration is often used as an estimate of the biomass concentration. SV S volume index. Used as a measure of settleability of solids in the activated sludge process. SV is determined by taking a sample of mixed liquor from the aeration basin and letting it settle for 30 minutes. The volume of solids after 30 minutes divided by the mass of solids equals the SV (units of ml/g). F:M F to microorganism ratio. A low F:M ratio is desirable since it will be an indication of good substrate removal. Low F:M ratios may promote the growth of nuisance organisms such as filamentous microorganisms however. RAS R activated sludge. This is the active biomass or sludge returned to the aeration basin from the underflow of the secondary settling tank (or clarifier). 6