Looking at sewage works in the strictly literal term, the aims of treatment can be summarised as the reduction of the total biodegradable material present, the removal of any co-existing toxic substances and the removal and/or destruction of pathogens.
It is beyond the scope of this book to examine the general, non-biological processes of sewage treatment in great detail, but for the sake of establishing the broader context in which the relevant biotechnology functions, a short description of the main key events follows. It is not, nor is it intended to be, a comprehensive examination of the physical processes involved and the reader is urged to consult relevant texts if this information is authoritatively required.
The typical sewage treatment sequence normally begins with preliminary screen-ing, with mechanical grids to exclude large material which has been carried along with the flow. Paper, rags and the like are shredded by a series of rotating blades known as comminutors and any grit is removed to protect the pumps and ensure free movement of the water through the plant. Primary treatment involves the removal of fine solids by means of settlement and sedimentation, the aim being to remove as much of the suspended organic solid content as possible from the water itself and up to a 50% reduction in 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 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.