Sewage-Treatment and Health

What is Sewage?

Sewage-Treatment raw materials is mainly liquid containing some solids produced by humans which typically consists of

• washing water
• feces
• urine
• laundry waste
• bathing and showering
• toilet flushing
• washing of utensils and dishes
• paper
• sanitary items
• food residues
• wastewaters from office
• waste water from commercial properties
• waste water from industrial premises
• Rainwater from roofs and roads

Why Sewage-Treatment is required?

Sewage-Treatment is essential to ensure that the effluent which ultimately discharged is not significantly polluted when reached the receiving end.

What is the Sewage-Treatment involving?

Sewage-Treatment is the process of removing contaminants from sewage water which It's includes physical, chemical and biological processes which is involves:

• The removal of solids by physical screening or sedimentation
• The removal of soluble and fine suspended organic pollutants by a biological oxidation process.

Sewage-Treatment process

The following describes a typical Sewage-Treatment sequence which is shown in the following flow chart

In practice, there are many other processes which employed according to locality policy and applied standard of the ultimate effluent.

Step 1 - Preliminary Treatment

1A- Screening

Large solids (plastics, rag, and woody material) are removed first by mechanical screens in order to protect downstream operations. The material retained (‘screenings’) is usually washed and then compressed for disposal to landfill or to an incinerator

Step 2 - Primary Treatment

2A- Primary sedimentation

The sewage passes into large sedimentation tanks to provide a quiescent settlement period of about 8 hours. Most of the solids settle to the bottom of the tanks and form a watery sludge, known as ‘primary sludge’, which is removed for separate treatment. The sewage remaining after settlement has taken place is known as ‘settled sewage’


2B- Secondary (biological) treatment

Settled sewage then flows to an aerobic biological treatment stage where it comes into contact with micro-organisms (known as the activated sludge process) which remove and oxidize most of the remaining organic pollutants.As well as removing most of the polluting organic matter, remove much of the nitrogen and phosphorus in the sewage, thus reducing the nutrient load on the receiving waters.

In circumstances where the highest quality of effluent is required, a third (tertiary) stage of treatment can be used to remove most the remaining suspended organic matter from the effluent before it is discharged to a watercourse. Tertiary treatment is effected by sand filters, mechanical filtration or by passing the effluent through a constructed wetland such as a reed bed or grass plot

Step 3 - Sludge Treatment

3A- Why sludge treatment?

All methods of sewage treatment generate organic sludge's as by-products and these must be managed separately from the liquid sewage. Raw (untreated) sludge's have a very high oxygen demand (it will affect the bio live around by depleting the oxygen quantity) and must not be allowed to enter the water environment.

3B- What does a typical sludge contain?

Sludge’s produced by sewage treatment are organic in nature and contain the following:

Safe to use sludge

useful amounts of plant nutrients such as nitrogen, phosphorus and essential trace elements and should be to utilize the sludge as a fertilizer or soil conditioner on agricultural land.

Unsafe to use sludge

• presence of unacceptably high concentrations of heavy metals or other contaminants from industrial sources.
• most raw (untreated) sludge’s have a high water content (96-99%), are putrescent and have an offensive odor.
• They will also contain a variety of human and animal pathogens derived from the contributing population

3C- How to treat sludge

Various forms of treatment may be used to achieve volume reduction by removing some of the water content. Odor and pathogen reduction is achieved by stabilization and disinfection processes

Stage 1 - Primary consolidation

Sludge is passed through stirred tanks or subjected to centrifugation to reduce its water content and volume by up to 50 per cent

Stage 2 - Anaerobic digestion

Consolidated liquid sludge is retained in an airtight tank (digester) and maintained at 35 deg. C for 12-20 days. Under the anaerobic conditions in the tank, various bacteria break down about half of the sludge organic matter and convert it into a gas containing about 70 per cent methane.

The sludge resulting from anaerobic digestion is much less offensive in odor than the untreated raw sludge and generally suitable for use in agriculture in liquid or solid form

Stage 3 - Mechanical dewatering

Either untreated or digested sludge may be converted from a liquid to sludge ‘cake’ by treating it first with a conditioning chemical which releases much of the water initially bound to the organic matter.

Much of the free water is then removed from the sludge in a filter press, a belt filter or a centrifuge. The resultant sludge cake will have only 10-20 per cent of the volume and weight of the original sludge,

Stage 4 - Incineration

This involves the burning of sludge at 600-800 deg. C to destroy its organic content and to leave a smaller residue of mineral ash for final disposal, usually to landfill.




Other methods …….Thermal drying

Employment of thermal drying systems is to convert the sludge to palletized or granular form comprising about 90 per cent solids. The heating involved also destroys pathogens. Thermally-dried sludge's are used in agriculture or for amenity uses (for example, golf courses, parks and other amenity areas).

Other methods …….Pasteurization (disinfection)

To destroy all pathogens in liquid sludge, it is heated to about 70 deg. C for at least 30, minutes after which it is cooled and subjected to anaerobic digestion. This combination of pasteurization and digestion produces an ‘enhanced treated’ product which enables it to be used in agricultural purposes.

Other methods …….Lime stabilization

At some smaller works, lime is added to liquid sludge to raise its pH to above 12.0 for several hours. The high alkalinity improves its odor and reduces the number of pathogens.

Other methods …….Composting

A few sewage works compost sludge by the process of ‘windrowing’. The process generates heat and a rise in temperature in the composting material causes pathogen destruction. The final product may be suitable for amenity use.


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