Presentation on theme: "Module 91 Sludge Treatment and Disposal Discuss the various methods of sludge treatment Describe the processes involved in their treatment Have an."— Presentation transcript:
Module 91 Sludge Treatment and Disposal 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 On completion of this module you should be able to:
Module 92 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 93 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, P x = Y obs Q(S o - S e ) Treated sludge is often referred to as Biosolids.
Module 94 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 95 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 96 Bulking sludge from chemical processes toxic wastes low temperature insufficient nutrients inadequate aeration.
Module 97 Bulking sludge from biological processes high proportion of filamentous microorganisms denitrification in clarifier tank high F/M values poor biological flocculation.
Module 98 Sludge floc structure.
Module 99 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 910 Sludge treatment and outcomes Treatment may involve anaerobic digestion or aerobic stabilisation in sludge lagoons Digestion reduces volatile solids from % in untreated sludge to % 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 911 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 912 Anaerobic sludge digestion (cont) 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 C 6 H 12 O 6 to 3CH 3 COOH. Acid formation
Module 913 Anaerobic sludge digestion (cont) Uses only obligate anaerobic heterotrophs Organic acids are degraded to methane and CO 2 CH 3 COOH to CH 4 + CO 2 pH rises to about 7; sludge changes to black Growth of methane bacteria is slow ( days) and highly sensitive to environment. Methane formation (methanogenosis)
Module 914 Anaerobic sludge digestion (cont) presence of any dissolved oxygen will stop process temperature range of o C is required pH of 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. Methane formation (methanogenosis)
Module 915 Gas production m 3 /kg volatile suspended solids added An energy source Methane ( %) Carbon dioxide ( %) Hydrogen sulfide trace (amounts).
Module 916 Effect of pH on gas production.
Module 917 Types of anaerobic sludge digesters Low rate single-stage anaerobic digester High rate two-stage anaerobic digester.
Module 919 High rate two-stage sludge digester.
Module 920 Anaerobic sludge digester.
Module 921 Aerobic sludge digester.
Module 922 Temperature effect on sludge digestion.
Module 923 Temperature effect on sludge digestion.
Module 924 Moisture and organic content of sludges.
Module 925 Other forms of sludge digestion Sludge lagoons Septic tank Imhoff tank.
Module 926 Septic tank.
Module 927 Imhoff tank.
Module 928 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 929 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 930 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.