Only about half the world's sewage gets treated before release
Every flush starts a journey that ends in a river, the sea or the soil, and roughly half of it worldwide is cleaned along the way. The gap between countries is stark: wealthy nations treat around 74 percent of their sewage, while developing countries average just 4.2 percent.
Treatment aims to strip enough contaminants from wastewater that it can be released without polluting, or put to use again. Biological methods do most of the work, relying on microbes to break down organic matter and remove nitrogen and phosphorus, while other steps target disease-causing organisms and trace chemicals. That is why many plants now call themselves water reclamation or recycling centres. The leftover sludge can itself be treated, and in some cases spread on fields as fertiliser under the name biosolids.
Engineers size plants with a unit called the population equivalent, the daily waste of one typical person. A common definition sets it at 60 grams of biochemical oxygen demand, a measure of organic load, and 200 litres of sewage per day. The same yardstick expresses how strong a factory's effluent is compared with household waste.
There is no single best design. Low-tech options such as waste stabilisation ponds use almost no energy and cost little to run, but they need a lot of land. At the other extreme, the activated sludge process yields very clean water yet consumes heavy amounts of electricity, mostly to blow air through tanks so microbes can breathe; in American plants of this type, energy typically makes up about 30 percent of yearly operating costs. Some plants recover energy by digesting sludge without oxygen to make biogas, occasionally covering most of their own needs. Small rural plants using trickling filters on hilly ground can run on gravity alone.
Priorities shift with circumstance. Industrialised countries tend to weigh efficiency, reliability and space, while poorer regions may focus on building and running costs and simplicity. Neighbours care about smell above all. Foul odours usually signal oxygen-starved conditions, with hydrogen sulfide the gas most likely to draw complaints, so urban plants scrub their air with carbon filters, small chlorine doses or microbes that feed on the fumes.
Source: Sewage treatment
