Aerobes and Effluents - Environmental Biotechnology: Theory and Application - page 117

solid loading is commonly achieved. At various times, and in many parts of
the world, discharge of primary effluent direct to the sea has been permissible,
but increasing environmental legislation means that this has 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 eutrophication of the
waterways. Primary stages have a removal efficiency of between 5 and 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
biological aspect of the regime and involves the two essentially linked steps
of initial bio-processing 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 fundamental 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 settlement in large tanks allows the fine floc
particles, principally composed of excess microbial biomass, to be removed
from the increasingly cleaned water. The effluent offtake from the biological
oxidation phase flows slowly upwards through the sedimentation vessels at a
rate of no more than 1-2m per hour, allowing residual suspended solids to
settle out as a sludge. The secondary treatment stage routinely achieve nutrient
reductions of between 30 and 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