Sludge Treatment and Disposal

1. Sludge Treatment and Disposal On completion of this module you should be able to: • Discuss the various methods of sludge treatment • Describe the processes involved in their treatment • Have an understanding of the causes of bulking sludge • Offer options and explain factors for the disposal of treated wastewater and biosolids Module 9

2. What impact sludge treatment and disposal has in relation with wastewater treatment? . • Capital cost of sludge treatment may be one third of the total plant cost while operating costs account for about 50% but often 90% of problems are attributed to sludge treatment and disposal Module 9

3. What are sludges? . • Sludges are the solids derived from primary and secondary sedimentation • Primary sludge is largely organic containing fecal matter, food scrap etc; has a strong odour and is unstable • Secondary sludge is usually finely divided and dispersed particles. It is difficult to dewater and is generally odour free • Sludge produced per day, Px = YobsQ(So - Se) • Treated sludge is often referred to as Biosolids Module 9

4. Bulking sludge from activated sludge process . • Sludge bulking will affect settleability and result in the carry-over of floc with the effluent from the clarifier. • Factors that contribute to sludge bulking may be physical, chemical and biological Module 9

5. Bulking sludge from physical processes . • shearing of floc caused by excessive agitation • poor rate of return of sludge • excessive overflow rate or solids loading • hydraulic turbulence Module 9

6. Bulking sludge from chemical processes . • toxic wastes • low temperature • insufficient nutrients • inadequate aeration Module 9

7. Bulking sludge from biological processes . • high proportion of filamentous microorganisms • denitrification in clarifier tank • high F/M values • poor biological flocculation Module 9

8. Sludge floc structure . Module 9

9. Why must sludges be treated? . • Sludges are highly putrescible and must be disposed of safely • All sludges must be stabilised before disposal • Waste activated sludge contains 65 – 75% organic matter with energy content of about 20.5 kJ/g organic solids, which presents opportunities for reuse Module 9

10. Sludge treatment and outcomes . • Treatment may involve anaerobic digestion or aerobic stabilisation in sludge lagoons • Digestion reduces volatile solids from 40 - 80% in untreated sludge to 30 - 60% weight • Sludge treatment reduces pathogens and volume to be disposed • Processes involve concentration (thickening), treatment and dewatering (filter or mechanical presses, sludge drying beds) • Biosolids are disposed in landfill, composting, and incineration Module 9

11. Anaerobic sludge digestion . • Digestion proceeds in 2 steps using different types of bacteria • The initial step results in acid formation • In the second step, methane is produced. It is highly flammable and explosive when mixed with air and ignited • Processes are carried out in air-tight reactors Module 9

12. Anaerobic sludge digestion (cont) . Acid formation • Uses facultative and obligate anaerobic heterotrophs • Facultative heterotrophs develop quickly and are relatively insensitive to environmental conditions • pH may drop to 5; sludge becomes grey • Complex organics degrade to various simpler organic acids • C6H12O6 to 3CH3COOH Module 9

13. Anaerobic sludge digestion (cont) . Methane formation (methanogenosis) • Uses only obligate anaerobic heterotrophs • Organic acids are degraded to methane and CO2 • CH3COOH to CH4 + CO2 • pH rises to about 7; sludge changes to black • Growth of methane bacteria is slow (4 - 10 days) and highly sensitive to environment Module 9

14. Anaerobic sludge digestion (cont) . Methane formation (methanogenosis) • presence of any dissolved oxygen will stop process • temperature range of 30 - 36o C is required • pH of 6.8 - 7.2 by maintaining alkalinity > 2000 mg/L • organic loading of raw sewage should be added regularly in small amounts; large amounts may cause a pH drop • toxic substances eg. heavy metals may inhibit process Module 9

15. Gas production . • 0.5 - 0.75 m3/kg volatile suspended solids added • An energy source • Methane (65 - 69%) • Carbon dioxide (31 - 35%) • Hydrogen sulfide trace (amounts) Module 9

16. Effect of pH on gas production . Module 9

17. Types of anaerobic sludge digesters . • Low rate single-stage anaerobic digester • High rate two-stage anaerobic digester Module 9

18. Low rate single-stage sludge digester . Module 9

19. High rate two-stage sludge digester . Module 9

20. Anaerobic sludge digester . Module 9

21. Aerobic sludge digester . Module 9

22. Temperature effect on sludge digestion . Module 9

23. Temperature effect on sludge digestion . Module 9

24. Moisture and organic content of sludges . Module 9

25. Other forms of sludge digestion . • Sludge lagoons • Septic tank • Imhoff tank Module 9

26. Septic tank . Module 9

27. Imhoff tank . Module 9

28. Disposal of biosolids . • Present practice of landfill • Beneficial reuses e.g. composting, vermiculture • Other innovative reuses e.g. brick manufacture, light-weight aggregates, oil-from-sludge technology Module 9

29. Disposal of treated wastewater . • Present practice of disposal into water bodies will depend on the dilution factor of receiving waters • Increasingly treated wastewater is now considered as valuable resource for reuse • There is potential for a domestic dual system using recycled water Module 9

30. Reuse of treated wastewater . • Luggage Point WWTP now treats 10 ML/d of near-potable water for BP refinery from wastewater that flows into Moreton Bay • Similarly Caboolture WWTP treats wastewater to near-potable standard for use in golf courses, parks • Wollongong WWTP proposes to treat 20 ML/d of near-potable water for reuse at the BHP steelworks that will replace 20% of potable water from the Avon Dam Module 9