State University of New York Institute of Technology
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Does treatment of wastewater produce waste ???

Sludge Treatment

Why do we treat sludge?

All methods of sewage treatment generate organic sludges as by-products and these must be managed separately from the liquid sewage. Raw (untreated) sludges have a very high oxygen demand and must not be allowed to enter the water environment.

There is, therefore, a need to deal with them in a way that permits their ultimate disposal in an environmentally acceptable and sustainable manner.  The sludge ‘route’ selected for a given sewage treatment works will depend on several factors including its location, the availability of suitable farm land, the characteristics of the sludge and the overall cost.

Sludges produced by sewage treatment are organic in nature and contain useful amounts of plant nutrients such as nitrogen, phosphorus and essential trace elements.

Therefore, the first objective should be to utilise the sludge as a fertiliser or soil conditioner on agricultural land.

The location of some sewage works is unsuitable for the sludges they produce to be used in agriculture (e.g. not enough farmland or too much manure from farm animals) and for these the most frequent ‘route’ is incineration with landfilling of the ash.

In their initial form, most raw (untreated) sludges have a high water content (96-99%), and have an offensive odour. They will also contain a variety of human and animal pathogens derived from the contributing population.

Various forms of treatment may be used to achieve volume reduction by removing some of the water content. Odour and pathogen reduction is achieved by stabilisation and disinfection processes. In recent years, the control of odour emissions to the atmosphere has become an important requirement of sludge treatment.

The following outlines the more common types of sludge treatment employed, of which various combinations are used according to the end product required. The processes of treatment, (which will not be cover in full within the scope of this blog), are listed below.

Primary consolidation

Anaerobic digestion

Mechanical dewatering


Thermal drying

Pasteurisation (disinfection)

Lime stabilisation


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