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What is Sewage?


At Coastal Drains, we believe that our customers should be as informed as possible about every part of the drainage process. That is why we’re doing everything we can to expertly answer questions such as “What is sewage?”, so they’ll have all the knowledge they need before any drainage issues occur. 

We’ve also set out this information so you can learn a little bit more about the process involved with sewage treatment plants. Read through the article below to find out more, or get in touch with us right away if there is a problem with sewage on your property already.

What is the Definition of Raw Sewage?

A dictionary definition of sewage describes it as “waste matter such as faeces or dirty water from homes and factories, which flows away through sewers”. This is a basic explanation of what sewage is, but doesn’t cover aspects related to treatment, the biology of sewage or problems that you may find if your property is suddenly flooded or has a drainage issue involving sewage water. 

Sewage is normally considered “raw” if it hasn’t yet been treated.

What’s the Difference Between Sewage Water and Wastewater?

Often, the terms “sewage” and “wastewater” are used interchangeably. However, they are not completely one and the same, as sewage is a type of wastewater. It’s the wastewater which comes from domestic dwellings and other properties, which can then be split into two different types of water that make up most of its consistency. These are called “greywater” and “blackwater”.


Also known as “sullage”, greywater is all of the combined water that comes from sinks, bathtubs, showers, dishwashers, washing machines and other appliances in a home or commercial or industrial buildings. Because it doesn’t contain any fecal matter or other human waste, it’s generally considered safer to handle than blackwater.


Blackwater is the wastewater which comes from toilets, alongside the collected human waste that has been flushed into the system. It will also normally contain toilet paper, and can contain other items which should not have been flushed, such as sanitary pads or disposable wipes.

What is Sewage Water Comprised Of?

In terms of biology, raw sewage is over 99% water by weight. The remaining ~1% is the waste that’s contaminating the water. This leftover percentage is the part that causes pollution, and can have a dangerous impact on human health and the environment if it’s left unchecked and untreated.

In the biological makeup of sewage water, you will find:

  • Biodegradable organic matter
  • Inorganic matter, such as minerals, metals and compounds
  • Nutrients such as nitrogen and phosphorus
  • Living organisms, such as bacteria
  • Pathogens, such as harmful parasites and viruses

The inorganic matter can come from a number of sources, including commercial and industrial wastewater, as well as overflowing stormwater, and inflow and infiltration from cracked pipes. 

Problems with Sewage

It is vitally important that sewage is taken away and treated as efficiently as possible, both for the health of your property’s occupants, the wider general public and the surrounding environment. If this isn’t done, it poses a range of risks:

  • Land poisoning and contamination
  • Contamination of drinking water
  • Loss of resources for farming and fishing
  • The spread of diseases such as E. coli, Gastroenteritis and Hepatitis A
  • The increased presence of pests such as rats and different insects
  • Foul odours in and around your property

Industrial wastewater treatment will often take longer and require more work to remove inorganic materials, as these are often toxic to animals and humans, creating a large problem with contamination.

If sewage is leaking from a pipe or drain onto your property, you should report this immediately and have specialist repair work carried out. Get in touch with us right away and our emergency team can be with you in a short time to ensure the sewage is removed and your drains are fully restored.

How Sewage Treatment Works

The steps involved in the process of treating sewage ensures the water is clean and safe enough to be discharged before it’s released back into local water courses. We’ve written these out so that you can read about and understand the process:


This comes before the actual treatment starts. When waste is brought into wastewater treatment plants (either domestic plants for homes or an industrial plant for public sewers), it will first be screened to remove large items that won’t break down. These include bottles, cans, sticks, rags and other objects which shouldn’t be in the system. It will then be passed through to a comminutor, a device which uses cutting edges to reduce the size of solids.

The water will then pass through a grit chamber. Here, smaller items such as sand, pebbles and broken glass will be removed.

Together, this prevents larger objects and materials from clogging up the system.

Primary Treatment

At this stage, the organic and inorganic solids are still in the biology of the water. These will be removed as the water moves to a large sedimentation tank, where the waste then stays for a while so the solid particles all have time to sink to the bottom. This forms a layer known as sludge.

Secondary Treatment

Secondary treatment can be carried out two ways. This may either be through trickling filters or activated sludge. The activated sludge process involves mixing the wastewater with microbes and aerating it. The microbes feed on the organic molecules and form the activated sludge. The wastewater is then brought into an aeration tank, where more aeration takes place, before the mixture then flows to the secondary clarifiers. Here, it is referred to as “mixed liquor”, and this is where it settles out for disinfection.

Trickling filters, on the other hand, pass the water through a bed of course stone or perforated plastic. These beds have microbes with a high concentration of oxygen growing on their surfaces, which break down the organic materials in the wastewater. The water will then continue down through the filters to the secondary clarifiers to settle for disinfection.

Tertiary Treatment

Most of the bacteria and viruses will have been removed during the secondary treatment process, but components such as nitrogen and phosphorus may still be in the water by the tertiary treatment stage. These are removed through a series of processes such as nitrification, where any ammonia present is oxidised to a nitrate. This is then followed by denitrification, where the nitrate is broken down into nitrogen and released into the atmosphere.

Carbon absorption at this stage can remove any other components that have managed to pass through the previous filters. Treated water that has been processed to this point is ready for reuse as industrial water, or can be released into rivers and streams. Further cleaning can also prepare this water for other uses, such as cleaning.

Sludge Treatment

The sludge that was separated from the treated wastewater can be utilised for a variety of purposes, including fertiliser for plants and energy production for the local area.

For More About Private Sewage Treatment

If you are thinking of upgrading a private sewage system into a domestic treatment works, or even if you already have a treatment plant on your property but it needs maintenance and care right away, Coastal Drains are here to help. We have years of experience in dealing with a variety of private systems, from older cesspits to the most modern and up-to-date sewage treatment plants, and can answer any questions you may have.

We can also carry out any professional cleaning, maintenance or repair work you require to keep your system working just as it should, at a price which will suit your budget and in a way that doesn’t disrupt your day any more than it needs to. Contact us whenever you need our help and we’ll be happy to book your plant in for an appointment as soon as possible.


Is it Possible for More than One Household to Share a Sewage Treatment Plant?

Yes, it is possible for multiple households to share one sewage treatment plant. However, you should always have an agreement in place about what should be done if the plant becomes blocked up or breaks down. This ensures that the costs are split fairly, with every homeowner’s input taken into account.

If you have recently moved into a property that uses a shared sewage treatment plant, you may wish to check if there is an agreement already in place. This ensures that you’ll never be surprised by any unexpected maintenance costs further down the line. We would also recommend remaining civil with your neighbours, to reduce the chance (and avoid the inevitable frustration) of a household not paying their way towards repairs and upkeep. 

Why Use a Domestic Sewage Treatment Plant, Rather than a Septic Tank?

Domestic sewage treatment plants offer a cleaner, more environmentally friendly solution than septic tanks because they treat the water, rather than just separating it from the waste. This means any waste water that comes from your plant doesn’t have to be discharged into a drainage field. Instead, it can be more safely discharged into a drainage ditch, or a local water course such as a river.

As of the beginning of 2020, it is actually illegal to discharge wastewater from a septic tank into ditches and watercourses, because it isn’t clean enough. If you still have a septic tank that does this, you must have the system replaced as soon as possible.