Guide to Off-Mains Drainage

Sewage treatment is the physical, chemical or biological process of removing contaminants from wastewater or sewage. This can be required for domestic, agricultural or commercial properties.  The objective is to produce an environmentally safe fluid.


Who needs a Sewage Treatment Plant?

The majority of houses in the UK are linked to the public sewerage system. You pull the plug, flush the toilet, pay the monthly bill and have little more to worry about. This is known as mains drainage.

In rural areas many homes and villages are self-contained. Either a cess pit, septic tank or sewage treatment plant will be used. This has no connection to the public sewerage system and is known as off-mains drainage.



There are 3 options if you are unable to connect to Mains Drainage, a Cess Pit, Septic Tank or Sewage Treatment Plant.  We have explained below the difference between each option:


Cess Pit

This is the most basic form for collecting wastewater. Often the cheapest of the three options initially, its high maintenance costs and its lack of sewage treatment processing making it an unlikely choice, often only being used for temporary purposes.  A cess pit is basically a holding tank for sewage until it is emptied. Cess Pits need to be emptied regularly as there is no outlet. They are banned in Scotland.



Septic Tank

A septic tanks provides a very basic level of sewage treatment by separating the solids from the liquids.  The liquid is called effluent, this effluent is then discharged from the septic tank to the surrounding ground via a soakaway. Septic Tanks can be manufactured from plastic, GRP or concrete.  A septic tank owner is then required to have the solids emptied from the tank as and when needed.


Sewage Treatment Plant

The most environmentally friendly and preferred option of the three. A Sewage/Package Treatment Plant is able to provide a higher level of treatment compared to a Septic Tank. A Sewage Treatment Plant achieves this by providing more rigorous processing of wastewater by utilising additional stages. This allows for discharge of treated water to more applications such as a watercourse.



Each sewage treatment plant manufacturer will provide you with a detailed manual for installation of your product. It is always advised that you use a reputable Sewage Treatment Plant installer when installing your product. At Harlequin, we work very closely with a number of installers. This allows us to recommend only the best and most reliable installer to our customers.

Need to find out who your nearest Harlequin approved installer is? Contact our team today on, 028 9261 1077.

British Water provide a helpful and detailed document – Code of practice for installing a Sewage Treatment Plant, which will help you understand your Sewage Treatment Plant that bit more. Click the link below to download the document:


Maintenance Involved

No matter what system you choose, there will be a certain amount of maintenance involved. This could be required monthly, quarterly, half-yearly, annually or longer. The manufacturer of your system will provide you with a basic maintenance guide when purchasing your product.

Every system will require some form of emptying/desludging and part cleaning or replacement.

To understand the importance of emptying/desludging, click and download the useful British Water guides linked below:



Throughout the UK and Ireland, there are many different regulations to adhere too. Below you will find linked each department responsible for providing and enforcing these regulations:



Reference should also be made to the following guidance notes


Harlequin Sewage Treatment Solutions

At Harlequin, we manufacture two type, of Sewage Treatment Plants to meet our customer’s needs, along with a septic tank range.

To find out more about our products, contact our team on 028 9261 1077 or email


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