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Geography Essays | Eutrophication

Dissertation Proposal: Eutrophication in Freshwater Ecosystems caused by Sewage and Sewage Sludge

This proposal looks at the adverse impact on freshwater ecosystems occasioned by nutrient enrichment, specifically as a result of sewage introduction from both point and non-point, also referred to as diffuse sources. It highlights the contrasting problems in countering specific as opposed to diffuse sources and aims to examine the consequences of this pollution, especially algal blooms and other high profile symptoms. It also analyses the legislative measures introduced to combat these problems across the globe and assesses their likely impact.

Introduction to Eutrophication and the Consequences

This section briefly sets out the principles of Eutrophication and some of the potential consequences upon freshwater ecosystems.

Eutrophication is a naturally occurring process in which water bodies become enriched with inorganic plant nutrients. Human activities contribute to and accelerate the eutrophication process, which can in turn cause problems for both humans and the aquatic flora and fauna occupying the freshwater body. Anthropogenic sources of nutrients include agriculture, urban areas (especially in the form of run-off) and industry; the variety of land uses in a drainage basin partly controls the extent of nutrient enrichment of affected watercourses. The extent of the problem can be gauged from the fact that, as an example, eutrophication accounts for about half of the impaired lake area and 60% of the impaired river reaches in the U.S.A, where the problems are particularly acute.

Eutrophic water bodies are highly productive, however the nutrient rich conditions will only favour certain plant and animal species. This means that as productivity or biomass increases, often biodiversity may decrease. Opportunistic species will take advantage of the favourable conditions, rapidly reproducing with the onset of increased nutrient concentrations. Algae are particularly responsive to nutrient pulses, and under certain conditions the population can be dominated by potentially harmful species.

An algal bloom can affect the water quality in a variety ways. The blooms contribute to a wide range of water-related problems including fish deaths, foul odours, and unpleasant tastes in drinking water. Furthermore, when such water is processed in water treatment plants, the high load of organic detritus reacts with chlorine to form potential carcinogens (trihalomethanes). Water-soluble compounds dangerous to the mammalian nervous system and liver can be released when blooms die or are ingested. These can kill livestock and may pose a serious health hazard to humans.

Mobile algae species will crowd the water surface to capture light, creating over-saturated oxygen conditions and raising the potential Hydrogen value of the water as they photosynthesise. When the bloom starts to die off, the organic matter conversely accumulates at the bottom of the water body. The bacteria breaking down this matter use up considerable amounts of oxygen, and can create oxygen-starved conditions close to the sediments. This can be harmful for bottom dwelling animals like fish and macro invertebrates, some of which are particularly sensitive to low oxygen levels.Although blooms of toxic blue-green algae are so-called 'high profile' symptoms of eutrophication, the excessive growth of other algae, and other aquatic plants, can cause significant problems, particularly in waters receiving sewage effluents or nutrients from agriculture. Because of the fact that eutrophication is in essence defined by its biological symptoms, high nutrient levels alone can only indicate the potential for eutrophication. Other environmental factors, such as river flow, play a large part in determining whether elevated nutrient levels in practice result in algal growth.

Over the last two decades, nitrate levels in rivers have generally shown less of a reduction than phosphate levels. Nitrate in waters is largely a legacy of agricultural intensification during and after the Second World War, with leaching into groundwater having an especially long-term effect. In contrast, improvements in sewage treatment, particularly at larger works and the decreased use of phosphate detergents have reduced the phosphorous content of sewage outflows. Implementation of the EC Urban Waste Water Treatment Directive (91/271/EEC) should continue to ensure that this positive trend persists, but loads from smaller sewage treatment works and in particular diffuse pollution from agriculture (as will be discussed below) must also be reduced if nutrient concentrations are to fall to levels that will substantially reduce or eliminate the risks of eutrophication.

Treatment processes and development:

This section looks at the principle measures of control in existence and the ways in which they reduce the volumes of phosphorous and nitrate entering freshwater systems.

The first measure taken to deal with the disruptive effects of wastewater emission is to trap the "rubbish" in the waste water with metal grids, and allow the larger solid particles to settle on the bottom of tanks. But primary treatment of this kind still leaves large quantities of nutrient and organic material dissolved in the water, or carried in suspension.

For this reason, starting in Sweden and other areas of Scandinavia in the 1950s, secondary or biological treatment was introduced at many municipal wastewater-processing plants. In biological treatment, microorganisms are added to the water, consuming the organic material present in the water. In this way almost all of the oxygen-consuming decay of organic substances takes place inside the controlled environment of the treatment plant instead of in natural water channels. These early measures are now adopted along similar lines in many other countries and form the basis of most commercial sewage treatment in the developed world.

Biological treatment, however, is only capable of separating a small and discrete segment of the nutrients that wastewater contains. Consequently it could not prevent urban emissions of phosphorus rising exponentially when phosphorus-containing detergents became widespread in the 1960s and thereafter.

Phosphorus is in short supply in fresh water under customary conditions, and so every additional inflow of this substance furthers the growth of algae and other vegetation. Phosphorus availability is often the limiting factor on algal growth in lakes and watercourses. Algal production also perpetuated oxygen deficiency problems in many places.

In the early 1970s most municipal wastewater treatment plants installed chemical or advanced treatment, which eliminates 90 per cent or more of the phosphorus content of wastewater. Chemical treatment has led to substantial improvement in freshwater lakes and other areas that used to be highly eutrophic as a result of emissions from nearby urban areas. Chemical treatment has become an increasingly important response to the problems of sewage pollution in a European and global context as the level of regulatory burden increases as governments bid to counter the problem.

The European and UK context - regulating the utilities sector

As treatment methods have developed, and awareness of the problems of Eutrophication (and its potential causes) has increase, there has been far greater political will towards combating the problem.

Within the European Community, the Urban Waste Water Treatment Directive (91/271/EEC) was agreed in 1991 to protect the environment from the adverse effects of discharges from sewage treatment works and overflows caused by storms. This Directive was adopted by member states in May 1991 and transposed into legislation across the UK by the end of January 1995. Its objective is to protect the environment from the adverse effects of sewage discharges. It sets treatment levels on the basis of volumes of sewage discharge and the sensitivity of the waters receiving these discharges. By the end of 1998 the UK had stopped all disposal of the sewage sludge left over from treatment processes at sea or to other surface waters in accordance with the requirements of the directive.

The Directive and UK implementing legislation require that all significant discharges be treated to at least the level of secondary treatment (as outlined above). It sets standards and deadlines for the provision of sewerage systems, and treatment of sewage according to the individual community being served by sewage treatment works, and the sensitivity of receiving waters to their discharges.

Water bodies can be identified as Sensitive Areas [1] on three grounds:
(a) Where they are found to be eutrophic or where they may in the near future become eutrophic if protective action is not taken;
(b) where they exceed or could exceed a specified concentration of nitrate - to protect water supply sources,
(c) where discharges affecting them are subject to more than secondary treatment to comply with the standards of other Directives.

Where such water systems are identified authorities are required to provide additional treatment to protect these areas. This more stringent treatment involves reducing the levels of nitrogen and/or phosphorus to meet the standards set in the Directive. (i.e. the significance in this context of chemical treatment, as described earlier).
Water bodies are reviewed every four years to identify waters that meet the criteria in the Directive. Once identified sewage treatment service providers and the environmental regulators work together to assess the implications, and what improvements and funding are needed to provide more stringent treatment at particular sewage works within the next seven years.
The limitations of such an approach are briefly summarised in the following section:

Limitations - diffuse or non-point sources, a new challenge

Such measures however are most effective in tackling specific or ‘point’ sources of nutrient rich pollution, but are less effective against more widespread and long-term damage occasioned by non-specific, ‘diffuse’ sources, especially agriculture. Intensive animal production is a case in point and generally involves feeding large numbers of animals in small areas. Such large concentrations of animals create enormous amounts of waste. The disposal problems are directly comparable to those for raw human sewage, and yet the regulatory standards for disposing of animal wastes are generally far less stringent than the standards cities and towns must meet for treating human sewage. This is only recently starting to be reversed as the UK Department for the Environment, Food and Rural Affairs has launched a consultation process for a new programme aimed at tackling so called Diffuse Water Pollution in Agriculture. As more information is released into the public domain this will be a fruitful topic of further study.

Chemical inputs to rivers, lakes, and oceans originate either from specific or diffuse sources. Specific sources include effluent pipes from municipal sewage treatment plants and factories. Pollutant discharges from these sources tend to be continuous, with little variability over time, and often they can be easily monitored by measuring discharge and chemical concentrations periodically. Consequently, these specific sources are relatively simple to monitor and regulate, and can often be controlled by treatment at source. As a consequence regulatory control has tended to be more aggressive in dealing with such sources.

Diffuse inputs can also be continuous, but are more often intermittent and linked to seasonal agricultural activity such as planting and ploughing or occasional events such as heavy rains or major construction works. Diffuse sources often arise from a varied range of activities across broad stretches of the landscape, and materials enter waterways as overland flows, underground flows, or via precipitation from the atmosphere. Consequently, these diffuse sources are extremely difficult to measure and regulate. Control of diffuse pollution centres on land management practices [2] and regulation of the release of pollutants into the atmosphere. Such measures are variable in the degree of success that they may achieve and this often depends on the level of political commitment to regulation and enforcement. In Europe, the political will to regulate the farming industry in particular has historically been rather weak.

A significant amount of Phosphorous and Nitrate also enters lakes and rivers, from urban sources such as construction sites, lawn fertilizers and pet wastes, septic systems and developed areas that lack mains drainage. Urban runoff is for example the third most important cause of lake deterioration in the U. S., affecting about 28% of the lake area that does not meet water quality standards. (There are a number of studies specific to the Great Lakes that might repay further analysis in this respect). The scale of the problem, especially from diffuse sources, is only beginning to be appreciated.

Conclusions

As a result, the extent to which this is an ongoing problem ought not to be underestimated. It might appear that counteractive measures have been in place for a lengthy period of time - since the 1950s in Sweden - but the true scale of the problem is only lately becoming apparent. This proposal can do no more than scratch the surface of the extensive problems currently being combated in the USA. The greatest volume of nutrient saturated pollutants come from diffuse sources, which by their nature are the hardest both to identify and combat. Existing legislation is good at tackling specific or point sources but the hardest part of the battle remains to be fought.

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