Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 121

With so high a resident microbial biomass, unsurprisingly the availability of
oxygen within the soil is a critical factor in the efficiency of treatment, affecting
both the rate of degradation and the nature of the end-products thus derived.
Oxygen availability is a function of soil porosity and oxygen diffusion can con-
sequently be a rate limiting step under certain circumstances. In general, soils
which permit the fast influx of wastewater are also ideal for oxygen transfer, lead-
ing to the establishment of highly aerobic conditions, which in turn allow rapid
biodegradation to fully oxidised final products. In land that has vegetation cover,
even if its presence is incidental to the treatment process, most of the activity takes
place within the root zone. Some plants have the ability to pass oxygen derived
during photosynthesis directly into this region of the substrate. This capacity to
behave as a biological aeration pump is most widely known in relation to certain
aquatic macrophytes, notably Phragmites reeds, but a similar mechanism appears
to function in terrestrial systems also. In this respect, the plants themselves are not
directly bio-remediating the input effluent, but acting to bioenhance conditions
for the microbes which do bring about the desired treatment.
Septic tank
In many respects, the commonest rural solution to sewage treatment beyond the
reach of sewerage, namely, the septic tank, makes use of an intermediate form
of land treatment. In the so-called cesspit, a sealed underground tank, collects
and stores all the sewage arising from the household. At regular intervals, often
around once a month dependent on the capacity, it requires emptying and tanker-
ing away, typically for spreading onto, or injection into, agricultural land. By
contrast, a septic tank is a less passive system, settling and partially digesting the
input sewage, although even with a properly sized and well managed regime
the effluent produced still contains about 70% of the original nutrient input. In
most designs, this is mitigated by the slow discharge of the liquid via an offtake
pipe into a ground soakaway, introducing the residual contaminants into the soil,
where natural treatment processes can continue the amelioration of the polluting
constituents. There are various types of septic systems in use around the world,
though the most common, illustrated in Figure 6.1, is made up of an underground
tank, which is linked to some form of in-situ soil treatment system, which usually
consists of a land drainage of some kind.
Since a system, that is poorly designed, badly installed, poorly managed or
improperly sited can cause a wide range of environmental problems, most espe-
cially the pollution of both surface and ground waters, their use requires great
care. One of the most obvious considerations in this respect is the target soil's
ability to accept the effluent adequately for treatment to be a realistic possibility
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