What is sewage


Sewage is a broad term that encompasses all types of waste and wastewater produced by households and industries, It is made up of the following types of waste water:

  • Blackwater specifically refers to wastewater that is generated by toilets and contains human waste and toilet paper. It is considered the most contaminated type of sewage due to the presence of fecal matter and pathogens. As a result, blackwater requires the most extensive treatment before it can be safely released into the environment. It is important to properly treat blackwater to prevent the spread of infectious diseases and to protect the environment from contamination.
  • Greywater is wastewater that is generated by sinks, showers, and other fixtures in households and industries. It typically contains a mixture of organic and inorganic matter, including food scraps, hair, soap, and other household cleaning products. Unlike blackwater, greywater is considered less contaminated, but still requires treatment to remove harmful substances before it can be safely released into the environment. Proper treatment of greywater helps to reduce water waste and prevent the contamination of soil and water sources.

Combined wastewater refers to sewage that contains a mixture of both blackwater and greywater. Combined wastewater is typically produced by households and industries that do not have separate systems for collecting blackwater and greywater.

How it’s treated

Sewage treatment is the process of removing contaminants from wastewater to produce treated effluent that can be safely released into the environment. The process typically involves several stages, including:

  • Pre-treatment: This stage involves the removal of large solids and debris, such as rags and sticks, from the sewage before it enters the treatment plant.
  • Primary treatment: This stage involves the removal of settleable solids and floatable materials, such as grease and oil, through sedimentation and skimming.
  • Secondary treatment: This stage involves the biological breakdown of organic matter in the sewage, typically through the use of microorganisms. The treated effluent is then disinfected to remove harmful pathogens.
  • Tertiary treatment: This stage involves further treatment of the effluent to remove nutrients and other pollutants that can harm the environment. This stage may include physical, chemical, or biological treatments.
  • Sludge treatment: This stage involves the treatment of the solid waste, or sludge, generated from the previous stages. The sludge is processed to reduce its volume and improve its stability before it is disposed of or used as a soil amendment.

Each stage of treatment is designed to remove specific contaminants from the sewage and produce a treated effluent that meets regulatory standards for discharge into the environment.

The UK’s sewage problem

A combined sewer system is a type of sewer system that collects both wastewater (from toilets, sinks, showers, etc.) and stormwater runoff (from rain and snowmelt) in the same pipe. The mixture of wastewater and stormwater is then transported to a treatment plant for treatment and disposal.

The United Kingdom has a large number of combined sewer systems, particularly in older cities and towns, due to the historical development of these areas. In the past, it was common to combine wastewater and stormwater in the same system because the population was smaller and the volume of wastewater generated was relatively low. With the growth of population and industry, however, the volume of wastewater and stormwater has increased, leading to problems with combined sewer systems during heavy rainfall.

Combined sewer systems can become overwhelmed during heavy rainfall, leading to the discharge of untreated sewage and stormwater into rivers and other bodies of water as more combined wastewater is generated than the sewage works can handle. This can have serious environmental and public health consequences and becomes more likely as the weather becomes more extreme, because with more extreme weather, the chance of enormous rain downpours increases.

Fixing this issue

To address the problems, many cities and towns in the UK have been working to upgrade their combined sewer systems to separate wastewater and stormwater collection systems, or to install storage and treatment facilities to manage the excess flows during heavy rainfall.

The use of combined sewer systems in the UK is a legacy of historical development patterns and the challenges posed by older infrastructure. Converting our system to newer infrastructure, separating stormwater and sewage streams, and increasing real-time sensing within the network are all actions being undertake to fix the UK’s legacy problem and keep the outdoors clean from sewage for ourselves and future generations.

8th February 2023
Tom McNamara


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