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

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

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

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

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

6
Bulking sludge from chemical processes
.

• toxic wastes
• low temperature
• insufficient nutrients
• inadequate aeration

7
Bulking sludge from biological processes
.

• high proportion of filamentous microorganisms
• denitrification in clarifier tank
• high F/M values
• poor biological flocculation

8
Sludge floc structure
.
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

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

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

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

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

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 gt 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

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)

16
Effect of pH on gas production
.
17
Types of anaerobic sludge digesters
.

• Low rate single-stage anaerobic digester
• High rate two-stage anaerobic digester

18
Low rate single-stage sludge digester
.
19
High rate two-stage sludge digester
.
20
Anaerobic sludge digester
.
21
Aerobic sludge digester
.
22
Temperature effect on sludge digestion
.
23
Temperature effect on sludge digestion
.
24
Moisture and organic content of sludges
.
25
Other forms of sludge digestion
.

• Sludge lagoons
• Septic tank
• Imhoff tank

26
Septic tank
.
27
Imhoff tank
.
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

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

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