Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
5.6 Sludge reuse
Raw sludge from activated sludge treatment plants has been applied directly
onto agricultural land particularly in the United Kingdom. This practice is
considered unsatisfactory because of the presence of pathogens in the sludge in
high numbers. There has been no thorough study, however, which has shown that
there is an increase in the risk of acquiring illnesses associated with
pathogens in the raw sludge when proper handling procedure and non-entry to the
land following application is observed.
Reuse of composted sludge as a soil conditioner in agriculture and
horticulture returns carbon, nitrogen, phosphorus and elements essential for
plant growth back to the soil (Section 2 (2.4)). Less chemical fertilisers are
required and the organic carbon helps to improve soil structure for soil
aeration, water percolation and root growth. The nitrogen and phosphorus are
also released gradually for plant uptake compared to the more soluble chemical
fertilisers. The potential of leaching of the nutrients to ground or surface
water by rainfall run-off is much reduced. Pathogens and heavy metals can,
however, limit the reuse of sludge.
Pathogens should be reduced to levels that do not pose health hazards to workers
handling the sludge, potential health hazards from the spreading of helminth
eggs and from horticultural produce contaminated by pathogens. Composting of
the sludge to attain a temperature of 55 °C for two weeks followed by windrow
maturation produces compost that meets these conditions. Stabilised sludge which
has been dewatered and dried on sand beds to attain a low moisture content can
meet the same conditions.
Heavy metals and toxic chemicals are difficult to remove from sludge. Preventing
these chemicals from entering the wastewater or sludge should be the aim of
wastewater management for sludge intended for reuse in agriculture or horticulture.
Reuse may still be possible for purposes such as mine site rehabilitation, highway
landscaping or for landfill cover. Sludge which has been conditioned for reuse
is also called 'biosolids'.
Conversion of sludge, which is heavily contaminated by heavy metals or toxic
chemicals, to oil is technically feasible (Enersludge process). A full scale
plant is operating in Perth, Western Australia (Bridle et al., 2000). The
conversion is by a pyrolysis process, heating dried sludge to a high temperature
in the absence of oxygen or with a controlled amount of oxygen. Capital and
running costs of an oil from sludge process are high.