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 in the context of waste management, has also been used as a means of sludge treatment. The use of this biotechnology has brought important benefits to the energy balance of many sewage treatment works, since sludge is readily biodegradable under this regime and generates sizeable quantities of methane gas, which can be burnt to provide onsite electricity.
At the same time, water resources are coming under increasing pressure, either through natural climatic scarcity in many of the hotter countries of the world, or through increasing industrialisation and consumer demand, or both. This clearly makes the efficient recycling of water from municipal works of considerable importance to both business and domestic users.
Though in many respects the technology base of treatment has moved on, the underlying microbiology has remained fundamentally unchanged and this has major implications, in this context. In essence, the biological players and processes involved are little modified from what would be found in nature in any aquatic system which had become effectively overloaded with biodegrad-able material. In this way, a microcosmic ecological succession is established, with each organism, or group, in turn providing separate, but integrated, steps within the overall treatment process. Hence, heterotrophic bacteria metabolise the organic inclusions within the wastewater; carbon dioxide, ammonia and water being the main byproducts of this activity. Inevitably, increased demand leads to an operational decrease in dissolved oxygen availability, which would lead to the establishment of functionally anaerobic conditions in the absence of external artificial aeration, hence the design of typical secondary treatments. Ciliate proto-zoans feed on the bacterial biomass produced in this way and nitrifying microbes convert ammonia first to nitrites and thence to nitrates, which form the nitro-gen source for algal growth. Though the role of algae in specifically engineered, plant-based monoculture systems set up to reduce the nitrogen component of wastewaters is discussed more fully in the next, it is interesting to note, in passing, their relevance to a ‘traditional’ effluent treatment system.
One of the inevitable consequences of the functional ecosystem basis underly-ing sewage treatment plants is their relative inability to cope with toxic chemicals which may often feature in certain kinds of industrial wastewaters. In particu-lar, metabolic poisons, xenobiotics and bactericidal disinfectants may arrive as components of incoming effluents and can prove of considerable challenge to the resident microbes, if arriving in sufficient concentration. This is a fact often borne out in practice. In 2001, considerable disruption was reported as a result of large quantities of agricultural disinfectant entering certain sewage works as a consequence of the UK’s foot and mouth disease outbreak. A number of poten-tial consequences arise from such events. The most obvious is that they kill off all or part of the biological systems in the treatment facility. However, depen-dent on the nature of the substances, in microbially sublethal concentrations, they may either become chemically bound to either the biomass or the substrate, or be subject to incomplete biodegradation. The effective outcome of this is that the degree of contaminant removal achievable becomes uncertain and less easily controlled. Partial mineralisation of toxic substances is a particular con-cern, often leading to the accumulation of intermediate metabolites in the treated wastewater, which may represent the production of a greater biological threat. The incomplete metabolism of these chemicals under aerobic conditions typi-cally results in oxidised intermediary forms which, though less intrinsically toxic than their parent molecules, are often more mobile within the environment. In addition, when the treatment efficiency is subject to monitoring, as intermedi-ate metabolites, these substances may not be picked up by standard analytical techniques, which may result in an unfairly high measure of pollutant removal being obtained.
Moreover, the extension of sewage treatment facilities to ameliorate trade efflu-ents also has implications for the management of true sewage sludge. It is not economically viable to develop processing regimes which do not lead to the concentration of toxic contaminants within the derived sludge. This was shown to be a particular problem for plants using the activated sludge process, which relies on a high aeration rate for pollutant removal, which proceeds by making use of biotransformation, air stripping and adsorption onto the biomass. Adsorp-tion of toxic inorganic substances like heavy metals, or structurally complex organic ones, onto the resident biomass, poses a problem when the microbial excess is removed from the bioreactor, particularly since dewatering activities applied to the extracted sludge can, in addition, catalyse a variety of chemical transformations. Accordingly, sewage sludge disposal will always require careful consideration if the significant levels of these chemicals are not subsequently to cause environmental pollution themselves.