Greywater Management in Low and Middle-Income Countries - SSWM

double-compartment septic tank.
In a double-compartment septic
entire tank volume.
Scum
Most countries provide
national standards for domestic
septic tank design and size.
Septic tanks are designed
for gravity separation, combined
settleable solids, oil and grease.
Sludge
Substances denser than water settle at the bottom of the tank, while fats, oil and
the tank undergoes anaerobic decomposition and is converted into more stable
compounds and gases such as carbon dioxide, methane and hydrogen sulphide.
Given the anaerobic processes in such tanks, odour emissions will occur. To avoid
be sealed. Biochemical reactions occur mainly in the accumulated sludge and far
less in the liquid phase between scum and sludge layer. Dissolved and unsettleable
solids leave the tank more or less untreated. Even though the settled solids undergo
continuous anaerobic digestion, there is always a net accumulation of sludge in the
tank. This gradual build up of scum and sludge layer will progressively reduce the
effective volumetric capacity of the tank. To ensure continuous effective operation,
the accumulated material must therefore be emptied periodically. This should take
place when sludge and scum accumulation exceeds 30 percent of the tank’s liquid
volume.
which can be detrimental to subsequent treatment steps (e.g. clogging of subsurface
irrigation networks and soil porosity). Desludging of septic tanks should not be
conducted manually, as accumulated sludge is rich in pathogens. In most cities of
low and middle-income countries, septic tank desludging services are provided by
government or private enterprises.
treatment or special attention prior to its reuse in irrigation.
Septic tanks are frequently used in greywater treatment systems in low and
Jordan), ahead of a planted
Nepal
problems have seldom been observed where septic or sedimentation tanks are used
as primary treatment unit. Some minor problems were reported in Sri Lanka, where
kitchen wastewater from a hotel restaurant has led to high scum accumulation rates
Greywater Management in Low and Middle-Income Countries
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