Aerobes and Effluents 121lies 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
mound elevates the system so that it sits a metre or so above the level of the
seasonal highest water table. The construction of the mound needs to receive
careful consideration to produce a design which suits the local conditions, while
also guaranteeing an even distribution of the septic tank effluent throughout the
mound. Typically, these systems are intermittently fed by a pump from a collec-
tion point and the rate at which the liquid off-take flows through the soil is a
critical factor in the correct sizing of the drainage mound. In the final analysis,
the sizing of all septic tank systems, irrespective of the details of its specific
design, depends on the amount of sewage produced, the type and porosity of
soil at the site and the rate at which water flows through it. Proper dimensional
design and throughput calculations are of great importance, since the efficacy of
septic systems is readily reduced when the set-up is overloaded.
Most modern installations use premanufactured tanks, typically made of stable
polymer and formed in a spherical shape with a short shaft like the neck of
a bottle forming a ground level inspection point. They often have a series of
internal baffles moulded within them to facilitate the flow of liquids and retention
of solids and surface scum, together with the appropriate pipework inlets, outlets
and gas vents. This type of tank has become increasingly popular since they are
readily available, easier to site and can be operational much faster than the older
concrete designs.
The most common versions of these consisted of two rectangular chambers
which were originally built out of brick or stone until the advent of techniques
to cast concretein situ. Sewage digestion was incompletely divided into two
stages, with gas venting from the primary chamber and secondary also, in better
designed systems. These were sometimes associated with an alternative soil-
dosing phase, known as seepage pits and soakaways, in which the part-treated
effluent arising from the septic tank is discharged into a deep chamber, open
to, and contiguous with, the soil at its sides and base. This permitted the free
translocation of liquid from the seepage pit into the surrounding soil, the whole of
the surrounding ground becoming, in effect, a huge soakaway, allowing dilution
and dispersal of the effluent and its concomitant biotreatment within the body
of the soil. In practice, provided the character of the ground is truly suitable for
this approach, effluent infiltration and remediation can be very effective. How-
ever, if the soil porosity precludes adequate percolation, the potential problems
are obvious.