Biological Processes and Biosolids Handling

1. Biological Processes and Biosolids Handling Harris County Wastewater Symposium Wastewater Treatment Plants & Bacteria: Strategies for Compliance D. Ray Young, P.E. April 26, 2011 Biological Processes and Biosolids Handling

2. Biological Processes and Biosolids Handling • Purpose of this Section • Increase Engineer and Operator knowledge of how clarifier TSS losses occur • Reduction of E. coli bacteria in • Harris County streams Biological Processes and Biosolids Handling

3. Biological Processes and Biosolids Handling • Goal of this section: • Better understanding of how biological and biosolids handling processes typically used in Harris County affect bacteria levels in WWTP effluents Biological Processes and Biosolids Handling

4. Biological Processes and Biosolids Handling • Disclaimer: • My recent experience is related to the design, construction and operation of small to medium WWTP’s which make up the majority of plants in Harris County. Biological Processes and Biosolids Handling

5. Biological Processes and Biosolids Handling • Small WWTP’s in Harris County typically don’t have full time operation and most are only operator attended for ½ to 4 hours a day • Reliability is a big issue with these plants as there is little redundancy in process units • Typically no one is around when things go wrong Biological Processes and Biosolids Handling

6. Biological Processes and Biosolids Handling • One of the main problems for disinfection processes is biosolids loss from secondary clarifiers Biological Processes and Biosolids Handling

7. Clarifier Solids Losses • Influent toxicity to process bacteria • Rising sludge (denitrification) • Bulking sludge (filamentous organisms) • Peak flows through the plant “sweep” • high inventory solids over the weirs Biological Processes and Biosolids Handling

8. Clarifier Solids Losses • Return flows from sludge thickening & dewatering processes upset process • Inadequate scum collection • Clarifier weir cleaning Biological Processes and Biosolids Handling

9. Influent Toxicity • What operator hasn’t experienced an unexplainable upset in the activated sludge process? • “Yesterday the plant was running perfectly, but today nothing will settle in the clarifier.” Biological Processes and Biosolids Handling

10. Influent Toxicity • The best defense against the effects of toxicity is a generously sized and well mixed flow equalization basin • Equalization of influent quality is more important than keeping the rate of flow into the plant constant over 24 hours Biological Processes and Biosolids Handling

11. Influent Toxicity • Monitoring plan for frequent • toxicity occurrences • Automatic refrigerated sampler ~ discreet sample bottles • Concentrated toxicity sample analyzed with GC Mass Spec • Identify suspect compounds • Trace to source of toxicity Biological Processes and Biosolids Handling

12. Rising Sludge • Nitrification • Ammonia nitrogen (NH3-N) limit • of 2 to 3 mg/l • Influent NH3-N of 30 to 40 mg/l • Requires > 90% reduction • Creates nitrate (NO3-) • Nitrate in the mixed liquor flows into the secondary clarifier. Biological Processes and Biosolids Handling

13. Selector / Anoxic Zone • Nitrification • Alkalinity (HCO3- +CO3-2 + OH-) buffers the pH of MLSS • Nitrification destroys 7.2 mg/l of alkalinity/ 1.0 mg/l NH3-N • Nitrification of 30 mg/l NH3-N results in a loss of 210 mg/l of alkalinity • Absence of alkalinity → pH can drop to 5 Biological Processes and Biosolids Handling

14. Selector / Anoxic Zone • Nitrification • Absence of alkalinity → pH can drop to 5 • Inhibits nitrifying bacteria • Reduces or stops nitrification • TPDES permit violation (?) Biological Processes and Biosolids Handling

15. Rising Sludge • Denitrificationof Nitrate • Denitrification occurs in absence of D.O. in the secondary clarifier (2 NO3- → N2 + 3 O2) • Nitrogen gas bubbles attach to Biosolids Biological Processes and Biosolids Handling

16. Rising Sludge • Denitrification of Nitrate • Floats solids to the surface of the clarifier • Biosolids are lost in the secondary effluent • Bacteria shield E. Coli from chlorine compounds or UV light. Biological Processes and Biosolids Handling

17. Rising Sludge • Reduce denitrification in secondary clarifiers • Provide an Selector/Anoxic zone at head of aeration basin • Included in WWTP design by Engineer • Created by the Operator • Selector/Anoxic zone baffled to minimize back-mixing • Goal is to create a low D.O. Biological Processes and Biosolids Handling

18. Rising Sludge • Reduce denitrification in secondary clarifiers • Denitrifying bacteria to use • NO3- as O2 source • Reduce air flow rate into the first section of the aeration basin • Create a gentle roll or mix • D. O. in the 0.3 mg/l to 0.5 mg/l range Biological Processes and Biosolids Handling

19. Selector/Anoxic Zone in the Activated Sludge Aeration Process Biological Processes and Biosolids Handling

20. Selector / Anoxic Zone • Selecting out filamentous organisms • that thrive in high D.O. environments • Reduces filamentous organisms that cause bulking sludge Biological Processes and Biosolids Handling

21. Selector / Anoxic Zone • Subjects bacteria in mixed liquor to 1 - 2 hours of low D.O. • Selects out filamentous aerobic organisms • Selects in facultative bacteria • that settle better Biological Processes and Biosolids Handling

22. Selector / Anoxic Zone • Reduces NO3- discharged by 50% • O2 scavenged by the • denitrifying bacteria • NO3-converted to N2 gas and released to the atmosphere • “Free” O2 • Reduce Total N discharged to the receiving stream Biological Processes and Biosolids Handling

23. Selector / Anoxic Zone • Denitrification recaptures 50% of alkalinity destroyed by nitrification process • pH buffering capacity is restored • Process stability • Reduction of solids loss from secondary clarifiers Biological Processes and Biosolids Handling

24. Selector / Anoxic Zone • This is a NO BRAINER!!!! Biological Processes and Biosolids Handling

25. Clarifier Sludge Detention • Still have some denitrification in secondary clarifiers • Must minimize detention of sludge blanket • Most clarifiers are designed with a 1:12 floor slope Biological Processes and Biosolids Handling

26. Clarifier Sludge Detention • Some have flat floors and rely on hydraulic sludge removal • Effective thickening proportional to depth over withdrawal point • Clarifier floor slopes of 2:12 or 3:12 reduce sludge detention • Volume of cone = 1/3rd the volume of a similar sized cylinder Biological Processes and Biosolids Handling

27. Clarifier Sludge Detention • Reduce detention by minimizing the depth of the sludge blanket • Increasing the rate of return sludge • Reduce MLSS inventory • Sludge blanket ~ 1 to 2 feet Biological Processes and Biosolids Handling

28. High Process Solids Inventory • 10 to 15 day Mean Cell Residence Time (MCRT) (sludge age) • Ample time for reproduction of critical ammonia reducing bacteria • Longer MCRT’s result in higher MLSS concentrations • Increase amount of old, ashy sludge in the process Biological Processes and Biosolids Handling

29. High Process Solids Inventory • Higher MLSS in the clarifier results in hindered settling • Reduces effectiveness of solids removal process • BOD of 200–300 mg/l & 25 to 35 lbs BOD/ 1,000 cu ft, MLSS ~ 3,500-4,500 mg/l • Lack of MLSS, RSSS & QR data Biological Processes and Biosolids Handling

30. High Process Solids Inventory • 30 minute sludge volume (SV30) test • Run at least once or twice a week • Maintain SV30 ln the range of 25% to 35% • MCRT > 15 days and/or SV30 > 40% results in more dead cells in MLSS that inhibit settling • High MLSS can result in high solids losses from I/I peak flows Biological Processes and Biosolids Handling

31. Scum Removal • Effective clarifier operation requires good scum removal • Scum includes bacterial solids, grease balls and low density, non-biodegradable plastics • Short scum beaches are only partially effective in capturing scum • Full surface scum trough or pipe is effective in keeping the surface of the clarifier clean Biological Processes and Biosolids Handling

32. Scum Removal • Most plants return scum to the lift station or aeration basin • Scum winds up being re-circulated through the clarifier many times • Fate of scum is either removed with a net or it escapes to the effluent • Good alternative is to “strain” the skimmer flow across a static screen • Screen openings ≥ 0.1 inch to minimize fouling Biological Processes and Biosolids Handling

33. Scum Removal Screen Biological Processes and Biosolids Handling

34. Clarifier Weir Cleaning • Equipment to spray / brush weirs and launder trough available • Strategy that sloughing and discharging algae a little at a time is OK • Clarifier weir cleaning can be a huge source of TSS and bacteria if not done frequently Biological Processes and Biosolids Handling

35. Clarifier Weir Cleaning • If discharged all at once, can result in significant discharge of bacteria • Provide vacuum cleaning system to pick up algae, grease balls and TSS that are loosened in the weir cleaning process • Or provide piping and valves to recycle the clarifier launder flow Biological Processes and Biosolids Handling

36. Run-A-Way Biosolids • Biosolids settles on the underwater structure and sludge collection equipment inside the clarifier • Denitrification brings it to the surface of the clarifier • If scum floats up outside the scum baffle, it goes over the weir, into the launder, and on to the disinfection process Biological Processes and Biosolids Handling

37. Biosolids Handling • Aerobic digestion • Gravity sludge thickening • Sludge dewatering → high return flows • Sludge thickening → high ammonia Biological Processes and Biosolids Handling

38. Tertiary Solids Collection Process • Improve TSS removal reliability with tertiary solids collection process • Typically use tertiary clarifier or sand or cloth filter • When using Cl2 as disinfectant, locate after the chlorine contact chamber • Cl2 kills or shocks bacteria, making TSS removal easier • For UV disinfection, solids removal should occur first to make the UV more effective Biological Processes and Biosolids Handling

39. Monitoring Effluent Turbidity • Install a continuous monitoring turbidimeter • Test a small flow from the non-potable water system • Correlate TSS to turbidity • Connect to plant alarm dialer or SCADA system Biological Processes and Biosolids Handling

40. Monitoring Effluent Turbidity Biological Processes and Biosolids Handling

41. Summary of Tools Available • Monitor sources of toxicity • Implement a Selector/Anoxic Zone • Keep process solids a low levels • Minimize sludge blanket in clarifier Biological Processes and Biosolids Handling

42. Summary of Tools Available • Minimize effects of sludge processing return flows • Install polymer addition system to dose clarifier mixed liquor feed line • Continuous turbidity monitoring of clarifier effluent w/ alarms • Install tertiary solids collection process Biological Processes and Biosolids Handling