How is Wastewater Treated? A Step-By-Step Process

How is Wastewater Treated? A Step-By-Step Process

Waste water treatment is a relatively modern practice, introduced in the 19th century. Wastewater treatment plants effectively carry out the same cleansing process that occurs naturally in oceans, lakes and rivers; but on a much quicker scale. The proper treatment of wastewater helps to protect both the environment and the people and creatures that inhabit it.

The Stages of Wastewater Treatment

Preliminary screening and treatment

First of all, wastewater has to be screened; so large objects (e.g. nappies, sanitary items, cotton buds, facial wipes, bottle lids etc.) can be removed, to prevent damage being caused to the equipment used in the following stages. This stage involves large screens that allow water to pass through, but not floating particles and other pollutants. This removes approximately 40-50% of solid materials from the wastewater, and all materials are collected and disposed of properly. Once this initial screening has been performed, the water flows on to the next stages of the treatment process.

Primary treatment

The primary treatment stage starts by filtering the wastewater through a grit chamber, which removes coarse solids, like rock sediment, metal, bone, and other larger particles of debris from the water. Once this has been completed, the sewage water moves on to a sedimentation tank, where minute particles of organic and inorganic matter are removed. The flow of the water is slowed down, causing the suspended materials to gradually sink to the bottom of the tank. This process is often aided by the addition of coagulants and flocculants. Read more about the role of flocculants and the role of coagulants in wastewater treatment, in our wastewater treatment chemicals series.

Secondary treatment

The secondary stage of treatment for wastewater generally removes 85-90% of the remaining pollutants present, and is a biological process that uses oxidation to purify the water further. This can be carried out using one of three methods:

  • Biofiltration uses sand filters, contact filters or tricking filters to remove additional sediment from wastewater. Of these three filters, trickling filters are typically the most effective for small-batch wastewater treatment.
  • Aeration involves mixing wastewater with a solution of microorganisms. The resulting mixture is then aerated for up to 30 hours at a time to ensure optimum results. A part of this mixture of solid wastes and microorganisms joined  together can often be combined with air and reused. This ‘activated sludge’ can improve the fertility of soil for growing crops, or be used as a component in the process of producing methane gas.
  • Oxidation Ponds are generally used in warmer climates, and they use natural bodies of water, like lakes and lagoons. The wastewater is allowed to pass through the body of water for a period of time, before being retained for a period of up to three weeks.

Tertiary treatment

At one time, primary and secondary treatment processes would have been sufficient to purify wastewater for reuse. However, new pollutants have created more work for wastewater treatment systems. These additional pollutants include pathogens, heavy metals, chemical compounds and other toxic substances, and they can persist in wastewater solutions after the first two treatment processes have been completed, as they are more difficult to remove from water.

Tertiary treatment tends to either consist of a type of physical or chemical process, or sometimes a combination of the two. Wastewater treatment chemicals, like chlorine and sodium hypochlorite, are added to the wastewater to remove harmful pathogens that cause cholera, polio, typhoid, and other diseases.

Phosphorus removal, as well as nitrogen removal, can be carried out using biological treatment methods; although sand or mixed media filters, cloth discs, membranes, or other treatment units, like reverse osmosis filters, can also reduce suspended solids and phosphorus to very low levels.

Air stripping or activated carbons (carbon adsorption) are also sometimes used for the removal of volatile organic chemicals (VOCs) from wastewater. You can learn more about the role of activated carbons in water treatment here.

Post treatment

Once these wastewater treatment processes, either alone or in combination, have been performed, almost any degree of pollution control can be achieved. Wastewater that has been purified by these treatments can be used for industrial, agricultural, or recreational purposes, including drinking water supplies.

Enva help customers who operate wastewater treatment plants to run more efficient plants and stay compliant. You can learn more about our wastewater treatment services here. Enva also provide a range of bespoke wastewater treatment chemicals to optimise wastewater processes. To find out more about how Enva can help with wastewater treatment, just get in touch.