Domestic Sewage and Industrial Effluents

What is in domestic sewage, and why it matters

Wastewater from homes carries away everything we wash down our drains, yet it takes only about 0.1 per cent impurity by mass to make domestic sewage unfit for human use — a striking illustration of how little contamination is needed to spoil a large volume of water.

• cover: 16.2 Composition of waste water — a circle representing sewage shows it is 99.9% water with a thin wedge for the remaining 0.1% impurities, which a call-out box breaks down into three categories: (1) suspended solids such as sand, silt and clay; (2) colloidal material such as faecal matter, bacteria, and cloth or paper fibres; and (3) dissolved materials such as nutrients — nitrate, ammonia, phosphate, sodium and calcium.

Of these impurities, solids are relatively easy to remove by settling and filtering. What is genuinely difficult to remove are the dissolved substances — nitrates, phosphates and other nutrients, along with toxic metal ions and dissolved organic compounds.

Domestic sewage is mostly biodegradable organic matter, meaning bacteria and other micro-organisms can break it down by using it as food. The amount of this biodegradable matter in a sample of sewage can be estimated by measuring its Biochemical Oxygen Demand (BOD) — essentially, how much dissolved oxygen the micro-organisms consume while decomposing the organic matter present.

What happens when sewage enters a river

When untreated sewage is discharged into a river, the micro-organisms that break down its organic content consume large amounts of dissolved oxygen from the water, and the dissolved-oxygen level falls sharply right at and just downstream of the discharge point. This oxygen crash is enough to kill fish and other oxygen-dependent aquatic life. Further downstream, as the organic load is diluted and consumed, dissolved oxygen gradually recovers and clean-water organisms reappear.

• cover: 16.3 Effect of sewage discharge on some important characteristics of a river — a graph plotted against distance downstream from a sewage discharge point shows dissolved oxygen dropping sharply to near zero right at the discharge point before gradually recovering further downstream, while BOD does the opposite — spiking sharply at the discharge point and then gradually falling — illustrating the "fish kill and disappearance of clean-water organisms" near the discharge and their "reappearance" as the river self-purifies downstream.

Nutrient overload: algal blooms and weed infestations

Sewage-laden water is also rich in nutrients, and this excess of nutrients triggers explosive growth of free-floating (planktonic) algae, known as an algal bloom, which visibly discolours the water. Blooms degrade water quality, kill fish, and some bloom-forming algal species are directly toxic to humans and animals.

• cover: 16.4 Pictorial view of an algal bloom — a photograph of a lake shoreline where the near-shore water is covered by a thick, bright-green mat of algae, contrasting with the clearer, greenish-blue water further from the bank.

A related problem is uncontrolled growth of water hyacinth (Eichhornia crassipes), an ornamental floating plant with attractive mauve flowers that was introduced to India but has since become the country's most troublesome aquatic weed — sometimes called the "Terror of Bengal." It thrives in nutrient-rich (eutrophic) water, grows faster than it can be manually cleared, chokes waterways, and throws the whole ecosystem of the water body out of balance.

Disease risk from untreated sewage

Sewage from homes and hospitals often carries disease-causing (pathogenic) micro-organisms, so discharging it into water without treatment can trigger outbreaks of diseases such as dysentery, typhoid, jaundice and cholera.

Industrial effluents and biomagnification

Unlike domestic sewage, wastewater from industries such as petroleum refining, paper manufacture, metal extraction and chemical manufacturing frequently contains genuinely toxic substances — notably heavy metals (dense metallic elements such as mercury, cadmium, copper and lead) along with a range of toxic organic compounds.

Some of these toxic substances undergo biomagnification as they move up an aquatic food chain — that is, their concentration increases at each successive trophic level, because an organism cannot metabolise or excrete the substance and simply passes it on, concentrated further, to whatever eats it. Mercury and the pesticide DDT are the best-known examples. Starting from a tiny concentration of about 0.003 parts per billion in water, DDT can be magnified all the way up to about 25 parts per million in fish-eating birds at the top of the chain. This build-up of DDT interferes with calcium metabolism in birds, thinning their eggshells so that eggs break prematurely — a major cause of population decline in affected bird species.

• cover: 16.5 Biomagnification of DDT in an aquatic food chain — five stacked levels connected by upward arrows that thicken as they rise show DDT concentration climbing at each step: water (0.003 ppb), zooplankton (0.04 ppb), small fish (0.5 ppm), large fish (2 ppm), and fish-eating birds (25 ppm) — a roughly 8,000-fold increase from the base of the chain to the top predator.

Eutrophication …