Sewage Discharge in Estuaries: The case for Trapping.

Transcription

1 Sewage Discharge in Estuaries: The case for Trapping. Group N- Sarah Wrigley, Bryony Wood, Laura Wicks, Helen Whiting, Daniel Wood, David Willock, Nicholas Wilson, Joanna Williams, Luke Warwick and Alex Wills. Introduction. Sewage is described as waste matter from domestic or industrial establishments that is carried away in sewers and drains. This then flows via rivers and waterways to eventually end up in seas and open oceans. It is primarily organic in nature and subject to bacterial decay, consisting of nitrogenous-based pollutants, and industrial waste products. Environmental legislation has banned the discharge of heavy metals and major toxins, to try to overcome some of the ecological damage. Nitrogen based compounds are the main source of eutrophication, where organic matter is bacterially decomposed removing vast quantities of oxygen from the water, thus starving aquatic organisms of their oxygen. Ammonia and nitrogen sulphide are released in this process, both of which are toxic to aquatic creatures. These issues have led to the highly debated topic of whether it is best to trap and treat the sewage or allow it to flow into the sea. Our aim is to highlight the key benefits of trapping and show that it does not cause unacceptable damage to the estuarine ecosystem. We will concentrate on the basic methods of trapping, and the varying procedures of treatment that are involved. Finally we will show the overall benefits of trapping and why this is a more viable, economically sound procedure. While allowing untreated sewage to flow into the seas and become diluted naturally may be the best ecological option short term; in the long run the cost of cleaning up the environmental damage is greater. Just because the sewage is out of sight, does not mean that is not doing any damage to our environment, the ramifications may be disastrous. We will show that although the short-term economical costs may be higher than sewage flushing, long-term savings both economically and ecologically are greater. Sewage Treatment. In most sewage treatment systems there will be some form of Preliminary Treatment. There may be several stages within the preliminary stage and these are Screening which removes all, large floating or suspended solids, rags, plastics, timber, animal carcasses. There are various types of screens from manual raked to mechanically raked and the waste which is removed by this process must be dealt with in a suitable manner for removal off site Grit Removal which usually only happens at larger works, removes as much as possible of the - silt, sand, gravel, cinders, ashes, metal, glass and even razor blades in the sewage. Flow Measurement where flow of sewage to a works is measured to control and monitor treatment and to comply with consents.

2 The next step in the process is Primary settlement of the sewage, which is necessary to remove heavy solids, remove grease and scum, balance the load, and performs some biological activity. The type of tank used will depend on many circumstances and it has to be carefully designed with all the many factors to be taken into account. From the primary settlement stage a lot of the coarser suspended solids will have been removed, and the biological content will have been reduced by between 25% and 50%. However from this stage a large amount of sludge must be removed from the tank very regularly, either by manual, automated systems, or by tank emptying. Biological Treatment of the sewage is the next part of this process as many biological pollutants are still present. The settled sewage is purified further by removing dissolved organic material, so that, after further settlement, in humus tanks, the effluent can be discharged to a watercourse. Biological filtration is the oldest form of sewage treatment dating back to The process relies upon Aerobic Bacteria and Microorganisms to break down the impurities in the settled sewage. Other forms of life will also colonise the filter bed, for example, Protozoa, Worms, Fly and insect larvae, spiders, and even birds, each species feeding off smaller members in the food chain. Final or Humus Settlement is usually carried out in tanks and is a means of removing the smaller solids from filter effluent. Filter effluent contains quantities of biological film, which has been washed from the media, dead microorganisms and worms. Again sludge must be removed from this stage almost continuously. The effluent from Humus Tanks will generally be about 90% to 95% clean, if the works are correctly designed and operated. Tertiary treatment is aimed at improving the quality of effluent produced by a conventional biological treatment plant. It is not a substitute for inadequate secondary treatment processes. Tertiary treatment can achieve a reduction in Suspended solids BOD Ammonia There are various types of tertiary treatment system, these being Grass plots Clarifiers Sand Filters Microstrainers Drum Filters Reed Beds Lagoons Nitrifying Filters The type of system to be used will depend on many factors and the designer will have to make a technical decision on which to use. Activated Sludge Systems can also be utilised to purify the settled sewage by removing dissolved organic material. Then, after further settlement, the effluent can

3 be discharged to a watercourse. The process was developed in the early 1900's and relies upon the aerobic bacteria and microorganisms to break down the impurities in the settled sewage into a culture known as activated sludge. There are many Other Sewage Treatment Processes available to the designer of a system such as: Reed Beds Contact Stabilisation Membrane Treatment Rotating Biological Contactors Oxidation Ditch's Extended Aeration Each system has its advantages and disadvantages, and a designer must take into account not only the site conditions but also the many other factors such as operation and maintenance, etc. Containment of Sewage in an estuary. Phytoremediation. Plants can be used to clean up the sewage sludge in an estuary, by a process called phytoremediation. Some plants have extensive communities of microorganisms that live close to the roots (rhizosphere), surviving on nutrients that are released by the roots. These rhizosphere microorganisms can be used to remediate toxic waste organic compounds; wetlands and reed-beds have been planted to clean up wastewater and sewage and prevent their being taken out to sea. Reed Bed Treatment Systems. Reed bed treatment systems are self-contained, artificially engineered, wetland ecosystems. They are designed to optimise the microbiological, chemical and physical processes naturally occurring in the wetland. Wetland plants, such as reeds, transfer atmospheric oxygen down through their roots in order to survive in waterlogged conditions. This creates both aerobic and anaerobic soil conditions, allowing extraordinary microbial species diversity to flourish. These bacteria and fungi can use organic pollutants as a food source, breaking down a wide range of organic chemical into harmless components. This process is often used as tertiary treatment following conventional treatment systems. The Biological and Economical Importance of Trapping. Treating sewage before it is added to an estuary is beneficial to all flora and fauna of that aquatic environment. This reduces oxygen depletion, which can lead to anoxia. The interceptor sewer installed at the Howden sewage works prevented this oxygen depletion by redirecting all the raw sewage to the treatment works before being discharged into the estuary. The reduction of debris associated with the sewage is economically very important, as the decline in tourism caused by these factors is the highest monetary cost associated with lack of sewage treatment. Also the reduction in water quality can severely affect

4 the profit margins of the fishing industry. Avoidance of these problems is economically beneficial when compared to the cost of treatment. Studies on the Tyne estuary show that sewage solids discharge into the lower water column very quickly due to its salt-wedge nature and the low levels of mixing. This has an affect on the immediate community around the river. However, effluent discharged into the upper estuary is moved up and down by the tide, causing eutrophication throughout, due to the presence of excess amounts of nutrients. This could be traced 1.7km upstream and 1.2km downstream, showing that entrapment of the sludge in the salt-wedge is preferable to stop the effects dispersing throughout the water. However, sewage sludge is an exploitable resource and should be recycled for beneficial use wherever possible. Sludge can be used to provide heat and energy from the methane produced through anaerobic digestion, as a conditioner on land and in brick manufacture. Conclusions. The release of untreated sewage into estuaries oceans has serious anthropogenic effects on the ecosystems in those areas. Research shows that entrapment and the subsequent treatment as outlined above has many advantages over the flushing of raw sewage straight into estuaries. These benefits are both biological and economic. For example, the reduction of the occurrence of anoxia in the water column leads to a healthier ecological system, as the majority of estuarine fauna and flora prefer an oxygenated environment, with low levels of contaminants. This leads to a greater species diversity particularly encouraging a greater biomass in species higher up the food web (e.g. Salmonids) which in turn provide a food resource for humans. This kind of cleaner, less contaminated environment can only be provided with the use of entrapment and the subsequent treatment that effluent must undergo. This is advantageous to the human population as reduction in odours and a more aesthetically pleasing environment leads to an increase in the tourist industry, which is therefore an economical improvement to the surrounding population. This also has health implications as the treatment of sewage prevents spreading of disease and increases water quality. Entrapment allows human control over the amount of pollution entering the water column, rather than solely relying on the affects of dilution in the water column as in the case with flushing. It is important that the pollution is dealt with as it occurs. By simply allowing the estuary to flush unmonitored amounts of unknown pollutants, society is just postponing the inevitable detrimental effects which the progressively more polluted water will result in. References

5 execsumm.html Johnson, P and Simmonds, M. Netherlands Loose papers A clean Irish Sea. A greenpeace response to the Irish Sea Study Group Hall. J,A, Frid. C,L,J and Gill. M,E. NOL 34(7) Pages The response of estuarine fish and benthos to an increasing discharge of sewage effluent