Newsletter and Technical Publications
<International Source Book On Environmentally Sound Technologies
for Wastewater and Stormwater Management>
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.
The corresponding bacterial processes are described in Section 2 (2.3).
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 saw dust may assist with achieving the
required solids content as well attaining the required carbon to nitrogen ratio