Aerobes and Effluents 115has now become an increasingly rare option. Throughout the whole procedure of
sewage treatment, the effective reduction of nitrogen and phosphorus levels is a
major concern, since these nutrients may, in high concentration, lead to eutrophica-
tion of the waterways. Primary stages have a removal efficiency of between 5–15%
in respect of these nutrients, but greater reductions are typically required to meet
environmental standards for discharge, thus necessitating the supernatant effluent
produced passing to a secondary treatment phase. This contains the main biolog-
ical aspect of the regime and involves the two essentially linked steps of initial
bioprocessing and the subsequent removal of solids resulting from this enhanced
biotic activity. Oxidation is the fundamental basis of biological sewage treatment
and it is most commonly achieved in one of three systems, namely the percolating
filter, activated sludge reactor or, in the warmer regions of the globe, stabilisation
ponds. The operational details of the processing differ between these three methods
and will be described in more detail later in this section, though the fundamen-
tal underlying principle is effectively the same. Aerobic bacteria are encouraged,
thriving in the optimised conditions provided, leading to the BOD, nitrogen and
ammonia levels within the effluent being significantly reduced. Secondary settle-
ment in large tanks allows the fine floc particles, principally composed of excess
microbial biomass, to be removed from the increasingly cleaned water. The efflu-
ent offtake from the biological oxidation phase flows slowly upwards through the
sedimentation vessels at a rate of no more than 1–2 metres per hour, allowing
residual suspended solids to settle out as a sludge. The secondary treatment stage
routinely achieves nutrient reductions of between 30–50%.
In some cases, tertiary treatment is required as an advanced final polishing
stage to remove trace organics or to disinfect effluent. This is dictated by water-
course requirements, chiefly when the receiving waters are either unable to dilute
the secondary effluent sufficiently to achieve the target quality, or are themselves
particularly sensitive to some component aspect of the unmodified influx. Ter-
tiary treatment can add significantly to the cost of sewage management, not least
because it may involve the use of further sedimentation lagoons or additional
processes like filtration, microfiltration, reverse osmosis and the chemical pre-
cipitation of specific substances. It seems likely that the ever more stringent
discharge standards imposed on waterways will make this increasingly com-
monplace, particularly if today’s concerns over nitrate sensitivity and endocrine
disrupters continue to rise in the future.
Process Issues
At the end of the process, the water itself may be suitable for release but, com-
monly, there can be difficulty in finding suitable outlets for the concentrated
sewage sludge produced. Spreading this to land has been one solution which
has been successfully applied in some areas, as a useful fertiliser substitute on
agricultural or amenity land. Anaerobic digestion, which is described more fully