How does a sewage treatment plant work?

The average person doesn’t give much thought to how a sewage treatment plant works; they just flush the loo or empty waste water down the sink and then don’t worry where it goes.

In fact, keeping us all safe from water-borne diseases that used to devastate whole communities is very complex and is concentrated in sewage treatment plants. Some homes have septic tanks, biofilters or aerobic treatment systems, but most sewage from homes and businesses in the UK is dealt with centrally. What happens to treated waste depends on the area – coastal towns and cities often have what is called a long sea outfall, where treated sewage is pumped far out from the land. In inland places, it is desiccated or treated in other ways to render it safe before being used as fertilizer. The treatment is generally broken down into three stages, primary, secondary and tertiary.

Primary treatment

Primary treatment involves letting the sewage stand in a basin where heavy solids can settle out and oil, grease and lighter solids can float to the surface. This is the stage in which filtering takes place, removing very large items such as rags, cans, sticks and other flotsam. When the weather has been very wet, quite large items such as tree branches can end up in the system and they are filtered out here too. Pre-treatment at this stage may include a sand or grit channel or chamber, where particles are removed because they may damage pumps and other equipment. In the primary sedimentation stage, grease and oils rise to the surface and are skimmed off, sometimes after treatment with blowers to cause a froth which makes it easier to skim. In all but the smallest systems, the primary settling tanks are have motor driven scrapers that make sure the collected sludge moves constantly towards a hopper in the base of the tank where it is pumped to sludge treatment facilities. Again depending on the size of the system, the grease and oil from the floating material are sometimes removed and saponified.

Secondary treatment

At the secondary treatment plant (sometimes right next to the primary tanks but often some way away) the settled liquor is subjected to an aerobic biological process which breaks down the biodegradable soluble elements such as sugars, fats, short-chain carbon molecules etc and binds the less soluble elements into something called floc. Another term for these secondary treatment plants is fixed-film or suspended-growth systems. What basically is going on at this stage is that the biomass is encouraged to grow on a mesh media which is then exposed to the trickling sewage. In suspended growth systems, the biomass is actually mixed in and this can be used in a smaller space. However, the fixed film method is preferred if the biological elements are not arriving in a stable manner so each method is used depending on the particular circumstances. An area that is not residential and therefore has reduced flow at night, for example, does better on the fixed film method. The health of the biomass must be carefully watched because if it is compromised, the whole of the secondary treatment system will not be working properly. If the balance of the activated sludge biota is not right, a floating brown foam is the result – it is called ‘sewage fungus’ but isn’t really a fungus at all. The process can also be upset by too many chemicals, such as pesticides, heavy metals or a very extreme change in pH and this is why all secondary treatment systems are constantly monitored.

Tertiary treatment

This is where the final clean-up happens, enabling the final result to be used for agriculture or as returned water to various systems such as rivers, lakes or the sea. The euphemism sometimes used for this final stage is ‘effluent polishing’. Filtration is the start, and this can be over activated carbon if local conditions lead the company to suspect that there may be some toxins which need to be removed. A natural lagoon, space permitting, is a very natural way to clean the final stage effluent as natural macrophytes (both plant and animal) will remove a lot more of any remaining fine particles. It is important to make sure that the water is not too high in nitrogen or phosphorus because these can cause algal bloom which in turn decreases the oxygenation of the water, causing death of any animals living in the water and creating a very unhealthy ecosystem. Chlorination may also be used to disinfect the water and this must be carefully balanced also with some chemicals being unsuitable because of their persistence in the water table. Ozone and ultraviolet light is used in some treatment systems but they are costly and unreliable, needing high maintenance. With odour control being added throughout the system where necessary, the treatment of our sewage is something that few are aware of, but for which everyone is very grateful!