Wastewater can consist of sewage, water that has been used in industrial processes or rainwater run-off from cities or agricultural land.

The waste water treatment process is aimed at getting rid of substances that are contaminating the water. To do this, the water is subjected to various biological, chemical and other processes that remove the contaminants and produce clean, environmentally safe water. This is vital for food production, drinking, recreation and marine and river life.


The raw “dirty” water arrives at the plant in its contaminated state, and the first treatment step is usually to remove any material that can easily be taken out before it clogs the equipment or damages machinery. This might involve any objects such as branches, leaves or rubbish that have entered the water. This is usually done via a screening process – highly automated in some plants but in others a more manual process.

Pre-treatment can also involve processes for getting rid of grit or similar substances. The speed at which water is entering the plant is adjusted so that these particles will drop to the bottom of the chamber containing the water. Horizontal, aerated and vortex are all types of grit chamber that operate in different ways to remove grit – a function called “sedimentation”.

Primary Treatment – Clarifying the Water
If the water tends to contain fat, it is passed through a tank where the fat globules floating on the surface are skimmed off. Sometimes air is blown across the tank to help the fat to form into froth that is easier to remove. Any sludge is pumped off.

Secondary Treatment – Biological Processes
Much wastewater has high levels of soap, detergent and human and food waste. Once the water has been through the primary clarifying processes, many plants treat it with biological methods. These are aerobic – that is, they involve aeration. The organisms and substances in the water need food and oxygen to survive. Bacteria and protozoa (such as amoeba) eat fat, sugar and other contaminants that are present in the water.

In the secondary treatment, this biomass is used to clean the water. Filters and films are used to encourage biomass growth. This is a very specialised area with a number of process types, such as aerobic granulation, rotating biological contactor, tickling filter and others.

 Monitoring Wastewater is an expert engineering discipline. A company that provides analysis of these processes has to understand the entire water life cycle and every process the water may have been subject to.

At every point in these processes, the water has to be analysed and monitored to ensure that quality tolerances are not exceeded and that variables such as flow, which may affect processes, are being monitored and analysed.

Tertiary Treatment
This gets the water ready to flow out into the sea or surrounding wetlands or rivers. The water may be filtered over activated carbon to remove sand and any toxins. The biological removal of nutrients in the water which was started in step two is sometimes continued at this stage.

Final processes at many plants are dependent on expert advice as to the particular challenges posed by the water composition and its intended outlet destination.

For example, wastewater can have large amounts of phosphorous and nitrogen, which can encourage algae and weeds if they are released into the environment. This can include the notorious blue-green algae which causes health warnings for holidaymakers and their dogs.

Expert advice on the effectiveness of the waste water treatment process is vital before the water is allowed back into the environment.