How Does A Sewage Treatment Plant Work?

Sewage treatment systems come in all shapes and sizes. Some have different ways of performing the same job but all work on the same core principle. In this article we will explain what this core principle is and help you to understand the differences in the styles of treatment process. But first we should explain what a sewage treatment plant does.

A sewage treatment plant is designed to take wastewater from a building (wastewater is defined as water from showers, baths, toilets, washing machines, dishwashers and sinks) and treat it to take out contaminants that are harmful to the environment. The final water quality will be good enough to discharge into a river or stream without harming fish or plant life.

Using bacteria to treat wastewater

A sewage treatment plant is a treatment system that uses naturally occurring bacteria to treat wastewater. The proves can be broken down into 4 key areas, all of which occur in an underground tank:

1. Settlement

When wastewater first arrives at the treatment plant it enters an initial settlement zone. This is designed to allow solids to settle down to the bottom of the tank. Often a crust will form on the top which is a build up of lighter solids that float on the water. Within this tank you will get the formation of anaerobic bacteria which survive with little oxygen. This bacteria only has a small effect when it comes to breaking down the waste. The cleanest part of the water (normally about a third from the top) is then sent to the treatment zone.

2. Biological Treatment

When the water enters the treatment zone, it has much less solids in it than the original water, but still has harmful pathogens and ammonia (which is poisonous to fish and plant life). This is where bacteria come in. Unlike in the primary chamber, the treatment zone is designed to grow Aerobic Bacteria which does a much better job at feeding on and removing contaminants.  Aerobic bacteria need high levels of oxygen to survive. Most systems add oxygen to the water using a small air blower which bubbles air up through the water in the tank. Bacteria also need food, this comes from the waste in the sewage water. If the environment is right (i.e., lots of oxygen and food) the microscopic bacteria will thrive and feed upon the contaminants.Bacteria can live freely in the water or grow on filter material; this is why you see some systems that have filter material, and some without.

3. Final Settlement

Once bacteria has done its job and broken down the waste, you will have a final settlement stage. This is designed to allow any remaining small particles (suspended solids) to drop to the bottom of the tank. There will often be a simple re-circulation from the bottom of this chamber to the primary chamber to ensure the settled solids don’t build up in the chamber.

4. Discharge

Following the settlement stage, cleaned water is then free from solids and has undergone a massive reduction in Ammonia and contaminants and can be discharged into a stream or river.

It must be noted that whilst the quality of water from a treatment plant is fine to discharge into the environment, it is still a health hazard and should not be reused without further extensive and costly treatment.

Different styles of treatment

As discussed earlier, there are 2 distinct styles of treatment that are employed in modern treatment plants. We will describe these as “fixed biology” and “active sludge.”

Fixed biology

Fixed biology treatment plants grow bacteria on some filter material. The filter material will often be lots of small plastic balls with grooves cut in to allow a large surface area. The large surface area is important as it allows lots of room for bacteria to colonise. An Example this style of treatment is the Tricel NOVA. With this style of sewage treatment, water flows through the filter material, therefore the “food” comes to the bacteria.

Active sludge

Active sludge (or activated sludge) treatment plants do not have filter material, instead, bacteria is encouraged to grow freely in the water. Waste (food) is introduced to the chamber and gets mixed with the bacteria. The bacteria bind to the food and begin feeding upon it. Examples of Active sludge treatment plants are the Premier Tech ASP and the SOLIDO

Pros and cons of each style of treatment:

Both styles work very well in the treatment of wastewater. Determining which is right for you isn’t the main consideration you should be making, strength of tank, guarantees, noise levels and aesthetics are things you should consider first. However, active sludge systems have less of an ability to block if items such as wet wipes get through the system, and there is some suggestion that bacteria can last slightly longer with no food when growing on filter material.