19 March 2025

Through this series, SCRT, on behalf of the Love Windermere Partnership Sewage Theme workstream, aims to provide essential information for owners and operators of private sewage treatment systems, including those not connected to main sewage, and what they can do to minimise environmental impact.

In the last edition, we covered the basics of what sewage is and why it needs to be treated before it is discharged into the environment. This time, we will discuss how sewage can be treated and cover three of the main types of private sewage systems – cesspits, septic tanks and package treatment plants.

While all sewage treatment aims to do essentially the same thing: remove as many harmful pollutants from our wastewater as possible before it is discharged, the degree to which this is done can vary depending on the level of technology used.

To make things easier, sewage treatment options are generally grouped into three categories based on the type of process used – Primary, Secondary or Tertiary treatment.

• Primary treatment represents the most basic level, which relies on gravity to remove suspended solids from the wastewater. Essentially, the sewage sits in a settlement tank, with heavier lumps of organic matter and other solids (e.g. toilet paper) eventually dropping to the bottom where they can be collected and removed. Surface scums of oil, grease and fats collect at the surface. Although this type of treatment can be effective at taking out particulates, it doesn’t do much to remove pollutants dissolved in the wastewater, which can still harm the environment.

• Secondary treatment aims to address this issue by making use of microorganisms like bacteria or protozoa to clean up the wastewater after it has been through primary treatment. As they grow, the bacteria break down any remaining organic matter and use up some nutrients before they die and settle out. This process uses bacteria that live in both aerobic (oxygenated) and anaerobic (no oxygen) conditions, to try and deal with chemicals like nitrates and ammonia. At this stage, the sewage has technically been cleaned up enough to be discharged. However, some pollutants like phosphates or bacteria still haven’t been fully addressed.

• Finally, tertiary treatment refers to any additional treatment steps which are used to further improve the standard of sewage effluent by removing extra nutrients, including nitrates and phosphates, or bacteria from the wastewater. This can be done using either physical processes (e.g. UV disinfection), dosing with chemicals, or using biology and nature-based solutions (e.g. treatment reedbeds), all of which can help clean up the wastewater even more before it is discharged.

There are many different types of private sewage treatment systems available to install (Fig. 1). The level of treatment provided will have a big effect on the impact your sewage has on the environment. Yet even a system with full tertiary treatment needs to be sized, operated and maintained properly to work effectively.

Subsequent newsletters will cover top tips on how to properly care for your system and your responsibilities. In the meantime, for further information, check out the Call of Nature website at https://callofnature.info/.