Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
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.