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. In addition, soluble organic substances are converted to bacterial
cells, and the latter is removed 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 latrines store faecal sludge, which needs to be
collected and treated. These two types of latrine are not discussed in Section
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 soilEto 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
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) 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
(Skrypsi-Mantele, et al 2000). The costs of treatment of sludge are generally of
the same order as the costs of removing the sludge from the wastewater.
Faecal sludge collected from bucket or vault latrines has a very high
biochemical oxygen demand (BOD) and is generally putrid and odorous. Primary and
secondary sludges from an activated sludge treatment plant also have a high BOD
and may be difficult to dewater. Even sludge from a septic tank, which has
undergone bacterial decomposition over at least a year, still has a high BOD.
Stabilisation is the term used to denote the process of BOD reduction. The
stabilisation process can be carried out under aerobic or anaerobic conditions.
Aerobic stabilisation of primary and secondary sludges can be carried out in
an aeration tank in the same manner as in an activated sludge process. Because
of the high oxygen requirement, this process is energy intensive and costs are
high. Aerobic stabilisation requires less energy when carried out as part of a
composting process. For composting of sludge, its solids content should be
increased to at least 15 % so that it can be handled as a solid. Thickening and
dewatering (see below) of primary and secondary sludges are required to achieve
the required solids content. Faecal sludge may contain high enough solids.
Mixing with dry materials such as dry sawdust may assist with achieving the
required solids content as well attaining the required carbon to nitrogen ratio
Composting is an aerobic bacterial decomposition process to stabilise organic
wastes and produce humus (compost). Compost contains nutrients and organic
carbon which are excellent soil conditioners. Composting takes place naturally
on a forest floor where organic materials (leaf litter, animal wastes) are
converted to more stable organic materials (humus) and the nutrients are
released and made available for plant uptake. The process is slow on a forest
floor, but can be accelerated under optimum conditions.
The optimum conditions for composting are a moisture content of about 50 %, a
carbon to nitrogen ratio of about 25 to 30, and temperature of 55 oC. Because
wastewater sludge is rich in nutrients, its carbon to nitrogen ratio is low (5
to 10). It is also high in moisture. Addition of dry sawdust, which is very high
in carbon to nitrogen ratio (500) can adjust both the moisture and carbon to
nitrogen ratio. Other waste materials that can be used for this purpose are
mulched garden wastes, forest wastes and shredded newspaper.
Composting can be carried
out in a specially built composter, such as an inclined rotating cylinder, fed
on one end with the raw materials, and the aerated product collected at the
other end. As the materials are slowly tumbled over a period of about one week,
they are mixed and aerated. Because bacterial decomposition produces heat,
temperatures in the insulated composter can easily reach 55oC. The immature
compost is then windrowed for at least 12 weeks to allow the composting process
to complete, with occasional turning of the windrow.
Composting can be more simply carried out in windrows (Figure 32). Regular
turning of the windrows assists with mixing of the materials and more
importantly supply the oxygen to the bacteria. Temperatures can reach 55 oC,
because compost has a good heat insulating property. Turning of the compost also
ensures that all parts of the windrow reach the required 55oC essential for
pathogen destruction. Turning is required every two to three days in the first
two weeks when temperature is 55oC or above. After this period frequent turning
of the compost windrow is not required as less heat is generated and less oxygen
is required while the compost undergoes maturation.
5.3 Anaerobic digestion
digestion is a bacterial decomposition process that stabilises organic wastes
and produces a mixture of methane and carbon dioxide gas (biogas). The heat
value of methane is the same as natural petroleum gas, and biogas is valuable as
an energy source. Anaerobic digestion is usually carried out in a specially
built digester, where the content is mixed and the digester maintained at 35 oC
by combusting the biogas produced. After digestion the sludge is passed to a
sedimentation tank where the sludge is thickened. Biogas is collected from the
digester (Figure 33). The thickened sludge requires further treatment prior to
reuse or disposal.
Anaerobic digestion can also be carried out at a slower rate in an unmixed
tank or pond. Covering is usually by a UV resistant plastic sheet, because
of the large area needed to be covered, and biogas is collected from the top of
the sheet. Storage of biogas can be in a cylindrical tank with a floating
roof. The cylindrical roof floats on water and its position is determined
by the volume of the gas stored under the pressure of the roof. Biogas can
also be stored in a balloon, but only under low pressure.