1Module 9

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:

2Module 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

.

3Module 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

.

4Module 9

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

.

5Module 9

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

.

6Module 9

Bulking sludge from chemical processes

• toxic wastes

• low temperature

• insufficient nutrients

• inadequate aeration

.

7Module 9

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

.

9Module 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

.

10Module 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

.

11Module 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

.

12Module 9

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

• C6H12O6 to 3CH3COOH

.

Acid formation

13Module 9

Anaerobic sludge digestion (cont)

• 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

.

Methane formation (methanogenosis)

14Module 9

Anaerobic sludge digestion (cont)

• 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

.

Methane formation (methanogenosis)

15Module 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 16

Effect of pH on gas production

.

17Module 9

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

.

22Module 9

Temperature effect on sludge digestion.

Type ofdigestion

Temp of digestionoC

min - normal - max

Digestion period(days)

min - normal - max

unheated ambient 90 - 120 - 150

mesophilic 27 - 32 - 35 25 - 28 - 30

thermophilic 49 - 51 - 53 12 - 13 - 15

Module 9 23

Temperature effect on sludge digestion

.

24Module 9

Moisture and organic content of sludges.

Moisture content %wt

Organic content %dry wt

Primary sludge 93 - 97 40 - 80Activated sludge 98 - 99 65 - 75Humus sludge 94 - 99 30 - 60Digested sludge 96 - 99 65 - 75

25Module 9

Other forms of sludge digestion

• Sludge lagoons

• Septic tank

• Imhoff tank

.

Module 9 26

Septic tank

.

Module 9 27

Imhoff tank

.

28Module 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

.

29Module 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

.

30Module 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

.