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Throughout history of existence of man, getting rid of waste man produces has always been problematic. Our Palaeolithic and Neolithic ancestors also had the same problem of waste disposal, those who lived on oysters and other shellfish simply threw the waste out forming the kitchen middens which are now of great interest to the archaeologists (Anderson, 2007). During the medieval times sewage and domestic waste was thrown everywhere and ended up flowing into nearby water bodies. Around 1900s’, the system of sewers evolved, cities did not treat sewage during that time (Ward, 2010). They simply collected the raw or untreated sewage (wastewater) using the system and discharged it into a receiving body of water either a river or a marine environment (Anderson, 2007) dictated by the location of the city.
However, it soon became apparent that raw sewage discharged in this manner was inadequate for two major reasons being; the adverse ecological impacts on receiving waters and the adverse public health impacts since some of the microorganisms in raw sewage may be pathogenic (Ward, 2010) hence the receiving water bodies cannot be used for water contact sports like swimming and water skiing.
Now municipal councils worldwide have gone to a great extent of constructing sewage treatment plants to keep up with pace of increasing human population, which tend to generate more sewage. These modern municipal sewage treatment systems utilize microbial degradation as a principal way to degrade these organic and inorganic material contained in waste water.
The ecological impacts of discharging raw sewage into rivers are dictated by the constituents of the waste. Sewage contains vast amounts of organic material, also inorganic materials such as nutrient salts consisting mainly phosphate and nitrate from partial mineralisation of the organic matter and detergents where phosphates are used as dispersion and suspension agents (Nixon, 2009). Run-off from industries and gardens also end up in the sewage. This can probably contain traces of heavy metals such as mercury and uranium(Landman and Ling, 2011). Microorganisms both living and dead also contribute to the amount of organic material present in the sewage, the latter is a pollutant.
Discharging raw sewage into rivers therefore introduces these constituents of sewage into the aquatic ecosystem. Organic material will therefore serve as a substrate for the growth of heterotrophic bacteria. The aerobic respiration activities of these microorganisms remove the dissolved oxygen in water, creating an anoxic condition. This condition is lethal to fish and other oxygen requiring living organisms in the river (Landman and Ling, 2011). Biodiversity in the aquatic system is thus hampered due to reduction of other living species population in the river. Also other terrestrial organisms depending on the river water will be affected negatively since the water is no longer in state that is conducive for them to use (Nixon, 2009). Wastes released by the microorganism tend to be toxic to other living organisms. Also animals that drink on the river that contain sewage can end up acquiring the heavy metals that concentrate up the food chain hence endangered species could be wiped out as a result of the toxicity of the heavy metals (Ward, 2010).
Since pH and temperature have effect on the solutes in the waste water, low or high will lead to cations in water complexing with the phosphate hence precipitates (Nixon, 2009). These precipitates will change the aesthetic appearance of water. Also the precipitates can be bound to aquatic organism such as fish hence blocking their pores on their bodies that are responsible for gaseous exchange.
Public health impacts due to raw sewage in rivers are that the river can longer be used as a source of drinking water and for human consumption. The river water thus tend to have a high concentration of nitrates, which are toxic to human especially infants as it can cause a disease known as blue baby syndrome (Anderson, 2007). This disease results due to high affinity of nitrates to haemoglobin hence less oxygen is carried in the blood. Water borne microorganisms such vibrio cholera from human faeces (Moubarrad,. et al 2007) also can be contained in the water hence if water is used for human consumption, this can lead to spread of diseases. Water contact sport also can be performed on such water due to the contamination by organic matter and bad odour as a result of the raw sewage.
Even though the water can be rich in plant nutrients, it cannot be used for irrigation purposes in agriculture since heavy metals that are contained in the water can end up finding their way into crops. Heavy metals such as mercury in water can be converted to methylmercury by bacteria (Ward, 2010). Methylmercury is a neurotoxin in human that affect the nervous system and brain functioning hence it can cause permanent development defects in babies (Moubarrad,. et al 2007) .
Since the severity of the impact of raw sewage discharged into rivers varies due to the volume of the raw sewage, organic and inorganic material content and the flow rate of water in the river, therefore when rivers are given sufficient time without further pollution, rivers have the natural capability to restore themselves (Ward, 2010). Oxygen can be reintroduced into the water through two activities being; oxygenic photosynthesis by cyanobacteria, algae and aquatic plants and also by the turbulence of the river, which results in the diffusion of oxygen from air. In addition, the organic material component of raw sewage is eventually degraded by the heterotrophic bacteria and when it depletes the population of these bacteria declines (Anderson, 2007).
However in the case of lakes, deposition of raw waste triggers a phenomenon known as eutrophication since water in lakes is not flowing. Eutrophication results from nitrates and phosphates contained in raw sewage being turned into cellular mass by microorganisms in presence of light ( HobÆK,. et al 2012). Eventually water is covered with green mats of algae, blocking sunlight and gases to diffuse into the water hence organisms rising in water die due to insufficient oxygen. A high population of microorganisms’ especially anaerobic ones results and this enhances other predatory microorganisms to reside in the water being attracted by the high populations of their prey. Due to eutrophication, biodiversity will be lost in terms of reduction in populations of organisms residing in water. Lakes compared to rivers are not capable of quick self-restoration since water in them is stagnant therefore diffusion of gases cannot be possibly easy.
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