Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 122

Figure6.1 Diagrammaticseptictank
bacterial action sink to the bottom of the tank, to form a sludge. The biodegrada-
tion 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 permit-
ted 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 micro-biological
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
lies within half a metre of the surface. Consequently in areas where this is a
permanent or even seasonal feature, the drains may be established much higher
than would be typical, frequently in close proximity to the soil surface. This
brings its own inevitable set of concerns, not least amongst them being that there
can be a very real possibility of the relatively untreated effluent breaking through
to above ground.
One solution to this potential problem that has been used with some success
is the sewage treatment mound. Formed using clean sand or small gravel, the