Greywater Management in Low and Middle-Income Countries - SSWM

treatment of greywater were
gained in Malaysia, where
operated as grease trap
and sedimentation tank
(see case study Malaysia).
Scum
velocity of wastewater of
v max
= 1.4–2 m/h must be
Sludge
maintained to avoid washout of the accumulated sludge. The ABR usually comprises
Pond systems have been successfully used as preliminary treatment units in low
and middle-income countries, though mainly for large-scale applications, as described
for example in India (Mara, 1997; Mara and Pearson, 1998). Pond systems are not
recommended as primary treatment unit for household greywater. Pond systems
look unpleasant, emit odours and offer a perfect environment for mosquitoes if not
well-operated and maintained (Ridderstolpe, 2004). The new WHO (2005) guidelines
for safe use of excreta and greywater do not promote pond systems if appropriate
mosquito control measures are not guaranteed. Septic or sedimentation tanks are
recommended as primary treatment unit.
The main objective of secondary treatment is the removal of organic matter and
reduction of pathogen and nutrient loads. After primary treatment, the organic matter
present in greywater takes the form of (von Sperling and Chernicharo, 2005):
• Dissolved organic matter that cannot be removed only by physical processes
such as in primary treatment.
• Suspended organic matter although largely removed in well-functioning primary
treatment units, possibly contains solids that settle more slowly and thus remain
in the liquid fraction.
The biological process component, where organic matter is removed by
microorganisms through biochemical reactions, is of key importance in secondary
treatment (von Sperling and Chernicharo, 2005). Microbial decomposition of organic
matter can take place under anaerobic and aerobic conditions:
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