Domestic Sewage and Industrial Effluents
Even 0.1% impurities make domestic sewage unfit for human use. These impurities include:
- Suspended solids: Sand, silt, clay, etc.
- Colloidal materials: Faecal matter, bacteria, cloth, paper fibres, etc.
- Dissolved materials: Nutrients like nitrate, NH3, phosphate, Na, Ca, etc.
Removing dissolved materials, organic compounds, and toxic metal ions is the most difficult. Domestic sewage contains biodegradable organic matter, which is decomposed by microorganisms.
The amount of biodegradable organic matter in sewage water is estimated by measuring Biochemical Oxygen Demand (BOD). During biodegradation, microbes consume oxygen, resulting in a sharp decline in dissolved oxygen, which causes the death of aquatic organisms.
Effect of sewage discharge on some important characteristics of a river
The presence of excess nutrients in water causes excessive growth of planktonic algae, known as algal bloom. This imparts a distinct colour to water bodies, deteriorates water quality, and results in the death of fish. Some bloom-forming algae are extremely toxic to humans and animals.
Water hyacinth (Eichhornia crassipes), known as the Terror of Bengal, is the most problematic aquatic weed, growing abundantly in eutrophic water bodies.
Sewage from homes and hospitals contains pathogens that cause diseases such as dysentery, typhoid, jaundice, and cholera.
Industrial wastewater contains toxic substances like DDT, heavy metals (mercury, cadmium, copper, lead, etc.), and organic compounds.
Biological Magnification (Biomagnification)
Biological magnification is the accumulation of toxicants (e.g., mercury, DDT) at successive trophic levels of a food chain. Organisms cannot metabolize or excrete these toxicants, so they are passed on to the next trophic level.
Biomagnification of DDT in an aquatic food chain: Water (DDT: 0.003 ppb) → zooplankton (0.04 ppm) → small fish (0.5 ppm) → large fish (2 ppm) → birds (25 ppm).
DDT disturbs calcium metabolism in birds, causing thinning of eggshells and premature breaking, leading to a decline in bird populations.
Eutrophication
Eutrophication is the natural aging of a lake by nutrient enrichment. In a young lake, the water is cold and clear, supporting little life. Over time, streams introduce nutrients (e.g., nitrogen, phosphorus), increasing the lake’s fertility.
Plants and animals grow rapidly, and organic remains are deposited on the lake bottom, making the lake shallower and warmer, supporting warm-water organisms. Marsh plants take root in the shallows, filling in the original lake basin. Eventually, the lake becomes land.
Depending on climate, lake size, and other factors, eutrophication may span thousands of years. However, pollutants like effluents from industries and homes accelerate this process, known as Cultural or Accelerated Eutrophication.
The prime contaminants are nitrates and phosphates, which overstimulate algae growth, causing unsightly scum, unpleasant odours, and depletion of dissolved oxygen, leading to the death of other organisms.
Heated (thermal) wastewater from electricity-generating units (e.g., thermal power plants) eliminates organisms sensitive to high temperatures. It may enhance plant and fish growth in extremely cold areas but only after damaging indigenous flora and fauna.
Integrated Wastewater Treatment
Integrated wastewater treatment includes artificial and natural processes. The townspeople of Arcata (northern coast of California) and biologists from Humboldt State University created an integrated wastewater treatment process with two stages:
- Sedimentation, filtering, and chlorine treatments: After this, remaining pollutants like dissolved heavy metals were removed using an innovative approach.
- Biologists developed a series of six connected marshes over 60 hectares of marshland. Appropriate plants, algae, fungi, and bacteria were seeded into this area to neutralize, absorb, and assimilate pollutants, purifying the water naturally as it flows through the marshes.
Friends of the Arcata Marsh (FOAM) is a citizens’ group dedicated to the upkeep and safeguarding of this project.
Ecological Sanitation
Ecological sanitation is a sustainable system for handling human excreta using dry composting toilets. It is a practical, hygienic, efficient, and cost-effective solution to human waste disposal.
Human excreta can be recycled into a resource (as natural fertilizer), reducing the need for chemical fertilizers. EcoSan toilets are operational in Kerala and Sri Lanka.
The Government of India has passed the Water (Prevention & Control of Pollution) Act, 1974 to safeguard water resources.