Aerobes and Effluents 119and thatPseudomonasspecies become predominant in the bacterial population
(Hardman, McEldowney and Waite 1994).
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,
leading to the establishment of highly aerobic conditions, which in turn allow
rapid biodegradation to fully oxidised final products. In land that has vegeta-
tion 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 sub-
strate. This capacity to behave as a biological aeration pump is most widely
known in relation to certain aquatic macrophytes, notablyPhragmites reeds,
but a similar mechanism appears to function in terrestrial systems also. In this
respect, the plants themselves are not directly bioremediating 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 ofin situsoil 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 groundwaters, 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