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<International Source Book On Environmentally Sound Technologies
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5. Sludge treatment, reuse and disposal

Sludge is produced from the treatment of wastewater in on-site (e.g. septic tank) and off-site (e.g. activated sludge) systems. This is inherently so because a primary aim of wastewater treatment is removing solids from the wastewater. Additionally soluble organic substances are converted to bacterial cells, and we remove the latter from the wastewater. Sludge is also produced from the treatment of stormwater (Section 4.3), although it is likely to be less organic in nature compared to wastewater sludge.

Bucket latrine and vault latrine store faecal sludge, which needs to be collected and treated. These two types of latrine are not discussed in Section 2 (4), because no treatment is involved at the latrines. In the former case human excreta is deposited in a bucket and the content of the bucket is emptied daily, usually at night giving the term 'night soil' to the faecal sludge. In the latter the excreta is stored in a vault for a longer period of up to two weeks before removal. The content of the vault should preferably be removed mechanically.

The characteristics of sludge vary widely from relatively fresh faecal materials generated in bucket latrines to sludge which has undergone bacterial decomposition for over a year in a double pit latrine. The treatment required is therefore dependent on the characteristics of the sludge. The former may contain large numbers of pathogens, whereas the latter will contain much less due to pathogen die-off. Sludge should, however, always be handled with care to avoid contact with pathogens.

Sludge may be contaminated with heavy metals and other pollutants, especially when industrial wastes are disposed into the sewer. Pre-treatment of industrial wastes is therefore essential before discharge to the sewer. Treatment of sludge contaminated with high concentrations of heavy metals or toxic chemicals will be more difficult and the potential for re-use of the sludge will be limited.

Faecal sludge contains essential nutrients (nitrogen and phosphorus, Section 2 (2)) and is potentially beneficial as fertilisers for plants. The organic carbon in the sludge, once stabilised, is also desirable as a soil conditioner, because it provides improved soil structure for plant roots.

Options for sludge treatment include stabilisation, thickening, dewatering, drying and incineration. The latter is most costly, because fuel is needed and air pollution control requires extensive treatment of the combustion gases. It can be used when the sludge is heavily contaminated with heavy metals or other undesirable pollutants. Prevention of contamination of the sludge by industrial wastes is preferable to incineration. A conversion process to produce oil from sludge has been developed, which can be suitable for heavily contaminated sludge (S. Skrypsi-Mantele, T.R. Bridle, P. Freeman, A. Luceks and P.D. Ye, 2000).

The costs of treatment of sludge are generally of the same order as the costs of removing the sludge from the wastewater.


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