Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 120

Figure6.1 Diagrammaticseptictank
and hence the percolation and hydraulic conductivity of the ground are important
factors in the design and long-term success of this method.
Under proper operation, the untreated sewage flows into the septic tank, where
the solids separate from the liquids. Surfactants and any fat components tend to
float to the top, where they form a scum, while the faecal residues remaining
after bacterial action sink to the bottom of the tank, to form a sludge. The
biodegradation of the organic effluent in these systems is often only partially
complete and so there tends to be a steady accumulation of sediment within the
tank, necessitating its eventual emptying. This settling effect produces a liquid
phase which is permitted to flow out of the tank, along an overflow pipe situated
towards the top of the vessel and is discharged to the soil as previously described.
Internal baffles inside the tank are designed to retain the floating scum layer
and prevent undegraded faeces from leaving the system prematurely. If these
biosolids were permitted to wash out into the soil its ability to treat the septic-
tank effluent can readily become compromised, leading to a reduction in the
overall system efficiency.
The drainage arrangements associated with a septic tank system are, arguably,
perhaps the most important part of this whole approach to sewage treatment
and may be considered as effectively forming an underground microbiological
processing plant. Clearly, it is of vital importance that the soil on any given site
must be suitable for the drainage to function reliably. The only way to be certain
is, of course, by means of a percolation test, though as a general rule, clay soils are
unsuited to this purpose. In circumstances of defined clay strata, particularly when
they exist close to the surface, it is highly unlikely that straightforward drainage
arrangements will prove satisfactory. Even in the absence of a high clay content,
soils which are either too fine or very coarse can also reduce the effectiveness of
this phase of the treatment system. The former can be a problem because, like
clay, it also resists effluent infiltration, the latter because it permits it too quickly
and thus retention time becomes inadequate for the level of treatment needed.
A further consideration which must be addressed in this respect is the position
of the water table, which may cause problems for the drainage system if it