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
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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
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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 = Yobs Q(So - Se)
Treated sludge is often referred to as Biosolids
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4. 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
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5. 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
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6. Bulking sludge from chemical processes.
Bulking sludge from chemical processes
toxic wastes
low temperature
insufficient nutrients
inadequate aeration
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7. 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
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8. .
Sludge floc structure
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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
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10. 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
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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
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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
C6H12O to CH3COOH
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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 ( days) and highly sensitive to environment
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14. Anaerobic sludge digestion (cont).
Methane formation (methanogenosis)
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
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15. Gas production . 0.5 - 0.75 m3/kg volatile suspended solids added
An energy source
Methane ( %)
Carbon dioxide ( %)
Hydrogen sulfide trace (amounts)
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16. Effect of pH on gas production.
Effect of pH on gas production
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17. Types of anaerobic sludge digesters.
Types of anaerobic sludge digesters
Low rate single-stage anaerobic digester
High rate two-stage anaerobic digester
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18. Low rate single-stage sludge digester.
Low rate single-stage sludge digester
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19. High rate two-stage sludge digester.
High rate two-stage sludge digester
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20. Anaerobic sludge digester.
Anaerobic sludge digester
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21. Aerobic sludge digester.
Aerobic sludge digester
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22. Temperature effect on sludge digestion.
Temperature effect on sludge digestion
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23. Temperature effect on sludge digestion.
Temperature effect on sludge digestion
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24. Moisture and organic content of sludges.
Moisture and organic content of sludges
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25. Other forms of sludge digestion.
Sludge lagoons
Septic tank
Imhoff tank
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26. .
Septic tank
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27. .
Imhoff tank
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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
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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
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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
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