4.10.1.3 - Waste water treatment

Sources and contaminants of waste water

Waste water contains substances added during use, so treatment must match both its source and its contaminants.

Urban lifestyles produce sewage from toilets, sinks and other household uses. Industrial processes use water for activities such as washing, cooling and chemical manufacture. Agriculture can produce waste water containing animal waste and runoff. Releasing these waters untreated can transfer harmful substances or microbes into rivers and other environments.

Sewage and agricultural waste water require removal of organic matter and harmful microbes. Organic matter includes material such as faeces and food waste that microorganisms can break down. If a large amount reaches a river, microorganisms breaking it down use dissolved oxygen, leaving less for aquatic organisms. Harmful microbes can also spread disease. Removing visible litter alone does not solve either problem: fine or dissolved organic material and microscopic organisms can remain.

Industrial waste water may contain organic matter and harmful chemicals, with the exact mixture depending on the process. A metal-processing plant might release dissolved metal compounds; a chemical plant might release acidic waste. Those examples illustrate why there is no single treatment suitable for every industrial discharge. Neutralisation can address an inappropriate pH, but it does not automatically remove every dissolved chemical. Some chemicals require specific separation or treatment before discharge.

The aim of waste water treatment is to reduce contaminants to a level appropriate for safe release or a stated reuse. Treated effluent is not automatically potable water. Safety depends on its remaining contamination and intended destination. Sampling and monitoring provide evidence that treatment has met the required standard.

Screening, grit removal and sedimentation

The first stages of sewage treatment physically separate materials that can be removed without a biological reaction.

Screening passes sewage through a grid or screen. Large objects, such as rags and plastic, are retained while water and smaller material pass through. This also protects pumps and other equipment from blockage. Grit removal separates heavy particles such as sand and small stones, preventing abrasion or unwanted build-up in later equipment. A screen does not remove dissolved organic compounds, so further stages are needed.

In a sedimentation tank, sewage is allowed to move slowly enough for suspended solids to settle under gravity. The settled solid-rich material is called sewage sludge. The liquid flowing onwards is called effluent. Sedimentation therefore creates two streams requiring different subsequent treatment; it is not a process that destroys all waste.

The separation depends on physical behaviour: material that settles can enter the sludge, whereas dissolved substances remain distributed through the liquid. Some small suspended particles also stay in the effluent. Clear-looking effluent can therefore still contain substances that need treatment. Explaining sedimentation should identify both outputs and not suggest that sludge is clean drinking water or that the effluent is simply discarded untreated.

Anaerobic sludge digestion and aerobic effluent treatment

Biological treatment uses microorganisms to break down organic matter, with different oxygen conditions for sludge and effluent.

Anaerobic means without oxygen. Sewage sludge is treated by anaerobic digestion in a controlled vessel. Microorganisms break down organic matter in the concentrated sludge without an oxygen supply. Digestion produces gas containing methane, which can be collected and used as a fuel, and leaves a more treated solid material requiring suitable management. Using methane can provide some energy for the treatment works; it does not mean the entire system operates with no energy input.