Sludge contains a high concentration of solids, but its water content is
still high. Combined primary and secondary sludge from an activated sludge
treatment plant contains about 2 % solids and hence 98 % water. One kg of dry
sludge is associated with 49 L of water. Thickening to 5 % solids means one kg
of dry solids is associated with 19 L of water, thus 30 L of water has to be
Thickening is carried out in a sedimentation tank or in a sedimentation pond
(Figure 34). The latter is advantageous if land area is available, because the
sludge can be allowed to settle over a much longer period and a higher solids
content of the thickened sludge is achieved. The water removed from thickening
needs treatment. It can be returned to the inlet of an off-site wastewater
treatment plant, or in the case of sludge from on-site units by an aerobic
treatment process such as lagooning.
5.5 Dewatering and drying
Dewatering aims to reduce the water content further so that the solids
content of the sludge is about 20 % (equivalent to 1 kg dry sludge with 4 L of
water). The sludge can then be handled like a solid. Dewatering can be done
mechanically using a filter press (employing pressure or vacuum), or a
centrifuge. It can also be done using drying beds. A drying bed consists of a 30
cm bed of sand with an under-drainage (Figure 35). Sludge is applied on the sand
bed and is allowed to dry by evaporation and drainage of excess water over a
period of several weeks depending on climatic conditions. Bacterial
decomposition of the sludge takes place during the drying process while moisture
content is sufficiently high. During the rainy season the process may take a
longer time to complete and sizing the area of the drying beds should take this
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. 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 oC 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 that has been
conditioned for reuse is called biosolidsE/p>
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.
5.7 Sludge disposal
Final or ultimate disposal of sludge, which cannot be reused, is by
landfilling or incineration. Since sludge for landfilling usually contains heavy
metals or toxic chemicals, lining of the landfill with clay or plastic liner may
be required to prevent contamination of groundwater. Incineration of sludge is
by a multiple hearth furnace or fluidised bed furnace. Energy input is required
to dry the sludge before combustion is self-sustaining. Combustion flue gases
usually need treatment to meet air pollution control standards. Investment and
operating costs are high.