Troubleshooting Activated Sludge Processes PNCWA - Southeast Idaho Operators Section Pocatello, ID February 11, 2016 Jim Goodley, P.E.
Outline Process Types & Kinetics Influent Monitoring Process Monitoring & Control Nitrification
Intensive Sampling?
Process Flow Regimes Complete Mix (CSTR) and Plug Flow Processes
Substrate Concentration Complete Mix (aka CSTR) Ideal CSTR Dilution Effects Influence on Kinetics Influent Pros: Stable Process Can Handle Shock Loads Cons: Low Rate Larger Tanks Effluent Time
Specific Growth Rate Growth Kinetics Kinetic Considerations K s Half Saturation Coefficient Low DO Low substrate µ max μ max 2 K s Limiting Nutrient Concentration
Growth Kinetics How does this affect operation/performance? Process Kinetics= Low Rate Process Monod Kinetics: (Substrate Limited Growth) μ = μ max S Ks + S Where: µ = Specific growth rate (day -1 ) µ max = Maximum Specific Growth Rate (day -1 ) K s = Half Saturation Constant (mg/l) S = Limiting Substrate Concentration (mg/l) Example Low DO on Autotroph Growth Rate µ max = 1.0 /day K s = 0.5 mg/l S = 0.25 mg/l µ = 0.33 /day 1/3 of maximum growth rate
Plug Flow Processes
Plug Flow Processes Ideal/Theoretical PFR No Longitudinal Mixing CSTR s in Series
Substrate Concentration Plug Flow Processes Concentration Gradient through tank length, High to Low Favorable Kinetics: Substrate Concentration High S >> Ks Maximum Growth Rate High Rate Process Pros: High Efficiency Low Effluent Concentrations Smaller Reactors Influent Cons: Susceptible to Shock Loads Requires Tighter Control Effluent Time
Influent Monitoring Influent changes are often the cause of operational issues.
Influent Monitoring Changes in loading Seasonal Industries Nutrient Balance Sidestreams Temperature Representative Sampling
Influent Monitoring Changes in loading Nutrient Balance Macronutrients Phosphorus Nitrogen Ratio of macronutrients required for microbial growth C:N:P 100:5:1
Influent Monitoring Nutrient Balance BOD 5 = 500 mg/l TKN = 20 mg/l TP = 5 mg/l Is this OK for normal growth?
Influent Monitoring Chemical Oxygen Demand- COD Develop Relationship with BOD 5 for your facility COD Typically 2 to 2.5 x BOD 5 COD analyses is fast compared to BOD 5 3hrs compared to 5 days min
Influent Monitoring Process Sidestreams Biosolids Processing- Typically a high strength waste stream 12 Try to avoid return during peak loading periods Ammonia at the anoxic zone 10 8 6 4 2 0 0:00 2:24 4:48 7:12 9:36 12:00 14:24 16:48 19:12 21:36 0:00 Time
Influent Monitoring Temperature Changes Microrganisms are highly sensitive to temperature Daily Influent Temperature Readings Recommended Spring & Fall Foaming Events High Runoff Events- Snowmelt Industrial Dischargers
Process Monitoring & Control Dissolved Oxygen F:M SVI
Process Monitoring & Control Dissolved Oxygen Aerobic heterotrophs (BOD 5 removal) and autotrophs (nitrification) require oxygen to respire Rule of thumb 2.0 mg/l Lower concentrations can cause low DO Bulking Competition for limited oxygen-filaments vs Rotifers Filaments will outcompete
Process Monitoring & Control Dissolved Oxygen In Plug Flow Processes monitor DO residual along length of aeration tank- create DO profile. Focus at head of tank where demand is highest Not true for CSTR process- DO should be nearly the same at all points in tank Monitor during peak loading periods
Process Monitoring & Control F/M (Food to Microorganism Ratio) Key parameter for maintaining normal operation Plug Flow Process 0.2-0.4/day CSTR Process 0.05-0.3/day
Process Monitoring & Control F/M (Food to Microorganism Ratio) Low F/M is common- Why? Engineers are conservative Facility designed for future conditions Designed to handle peak conditions Tends to be oversized for normal conditions
Process Monitoring & Control F/M (Food to Microorganism Ratio) Problems Associated with Low F/M Sludge Bulking Foaming - Nocardia
Process Monitoring & Control F/M Calculation High F/M Problems not as common (my experience) Typically found at industrial WWTF s or Industrial discharges to municipal WWTF s High F/M problems similar to low F/M Sludge Bulking (Filamentous & NonFilamentous) Viscous Bulking Foaming
Process Monitoring & Control F/M Calculation Food= lbs of BOD 5 per day into process Microorganism= lbs of microorganisms in the process F/M= lbs BOD 5 /day lbs MLVSS
Process Control F/M Calculation A WWTF has an influent flow of 5.5 MGD with a BOD 5 concentration of 200 mg/l. The aeration tank has a volume of 3.2 MG and an MLSS concentration of 2,800 mg/l. Assume the MLSS volatility is 85%. What's the F/M for the plant? Food= 5.5MGD * 200mg/L * 8.34lb/gal = 9,174 lb BOD 5 /day Microorganism = 3.2MG * 2,800 mg/l *.85 * 8.34lb/gal = 63,517 lb MLVSS F/M = 9,174/63,517 = 0.144/day
Process Monitoring & Control MCRT Mean Cell Residence Time Measure of how long microorganisms stay in the system Key operating parameter Plug Flow Process CSTR Process 5-15 days 10-30 days Longer MCRT s tend to be more susceptible to bulking & foaming
Process Monitoring & Control MCRT Mean Cell Residence Time MCRT (days) = Total Solids in System (lbs) Solids Removed From System (lbs/day) Total Solids- Can include solids in clarifier blankets Solids Removed- WAS + TSS in effluent
Process Monitoring SVI- Sludge Volume Index Measure of how well sludge settles and compacts Accounts for MLSS (compared to just SV30 test) SVI <100 Good SVI 100-150 OK SVI >150 Bulking
Process Monitoring SVI- Sludge Volume Index SVI= SV 30 (ml/l) MLSS (g/l) An SV 30 test settled to 450ml in a 1L settleometer in 30 minutes. The MLSS was 3,200 mg/l. What s the SVI? SVI = 450 ml/l = 140 ml/g 3.2g/L
Process Control Nitrification Autotrophs- Slow Growers, Primary Requirements Carbon INORGANIC- NOT BOD! Oxygen (4.6 parts/ part NH4-N oxidized) Alkalinity (7.2 parts/part NH4-N Oxidized) ph > 7 (7.5-7.8 best)
Questions?