Water Pollution and its Control

Water availability has always been a key factor controlling the distribution and density of living things in an area, and human consumption of water has risen sharply with industrialisation. Even a very small quantity of impurity -- as little as 0.1% -- can make water unfit for human use and can also harm aquatic plants and animals; while suspended solids are relatively easy to remove, dissolved substances such as nitrates, phosphates and other nutrients, toxic metal ions, and organic compounds are far harder to eliminate. Domestic sewage, one of the most common sources of water pollution, is largely biodegradable organic matter that is readily broken down by bacteria and other microorganisms, which use it as a substrate. The amount of biodegradable organic matter present in sewage can be estimated by measuring its Biochemical Oxygen Demand (BOD) -- the quantity of dissolved oxygen, expressed in milligrams of oxygen per litre, that microorganisms need to decompose the organic matter in the water; a high BOD signals intense microbial pollution, and water bodies receiving heavy organic effluent, such as wastewater from sugar mills carrying dissolved sugars, molasses and bagasse fibres, typically show sharply elevated BOD.

As microorganisms consume dissolved oxygen while breaking down this organic matter, oxygen levels in the water body fall sharply, often killing fish and other aquatic life. An abundance of nutrients in water can also trigger excessive growth of free-floating planktonic algae, particularly blue-green algae, producing what is called an algal bloom; such blooms colour the water, often release toxins that poison fish, and generally degrade water quality for both humans and animals. Another major threat to aquatic ecosystems is the water hyacinth (Eichhornia crassipes), an invasive aquatic plant native to the Amazon basin that was originally introduced to India for its attractive purple flowers, but now spreads so aggressively across water bodies -- faster than it can be physically removed -- that it is popularly called the 'Terror of Bengal'.

Natural eutrophication is the slow, natural ageing of a lake as nutrients gradually build up in its water over thousands of years, but pollutant-laden run-off from farms, industries and homes has dramatically sped up this process, a phenomenon known as cultural or accelerated eutrophication (Fig. 15.8). As nutrient levels rise, algae and other plants bloom, blocking light to submerged plants, which then die back; the decomposition of this dead plant matter, together with the bloom itself, depletes dissolved oxygen, killing fish and other aquatic organisms and reducing overall species diversity, leaving behind a stinking, turbid, discoloured lake.

Certain pollutants present in industrial wastewater, most notably persistent pesticides like DDT and heavy metals like mercury, undergo biological magnification (biomagnification) -- because they cannot be metabolised or excreted, they accumulate in body tissue and become progressively more concentrated at each successive trophic level of a food chain (Fig. 15.9 shows how DDT concentration rises at each step, from water through zooplankton, small fish and large fish, ending up highest of all in fish-eating birds of prey, which sit at the top of the food chain and have therefore accumulated the pollutant passed on from every level beneath them).

Thermal pollution of water occurs when its temperature rises abnormally, most often because thermal and nuclear power plants draw in water as a coolant and then release it back at a higher temperature; because many aquatic organisms are sensitive to temperature change, this sudden warming can cause significant loss of flora and fauna. …