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

. 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

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 = Yobs Q(So - Se) Treated sludge is often referred to as Biosolids Module 9

Bulking sludge from activated sludge process . 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

Bulking sludge from physical processes . 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

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

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

. Sludge floc structure Module 9

Why must sludges be treated? . 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

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

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

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

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

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

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

Effect of pH on gas production . Effect of pH on gas production Module 9

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

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

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

Anaerobic sludge digester . Anaerobic sludge digester Module 9

Aerobic sludge digester . Aerobic sludge digester Module 9

Temperature effect on sludge digestion . Temperature effect on sludge digestion Module 9

Temperature effect on sludge digestion . Temperature effect on sludge digestion Module 9

Moisture and organic content of sludges . Moisture and organic content of sludges Module 9

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

. Septic tank Module 9

. Imhoff tank Module 9

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

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

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