Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 123

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
collection 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 pre-manufactured 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 concrete in 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 bio-treatment 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. However, if the soil
porosity precludes adequate percolation, the potential problems are obvious.
Limits to land application
There are, then, limits to the potential for harnessing the processes of natural
attenuation for effluent treatment. While centuries of use across the world tes-
tify to the efficacy of the approach for human sewage and animal manures, its
application to other effluents is less well indicated and the only truly 'industrial'
wastewaters routinely applied to the land in any significant proportion tend to be
those arising from food and beverage production. This industry is a consumer of
water on a major scale. Dairy production uses between 2 and 6m 3 of water per
1m 3 of milk arriving at the plant, the manufacture of preserves requires anything