How to Install a Sewage Treatment Plant?

Posted On: 26 March 2018

How to Install a Sewage Treatment Plant?

Posted On: 26 March 2018

The installation of a septic tank or a Sewage Treatment Plant (STP) is considered a very complicated and time-consuming process. This simple guide aims to give a step by step process of installation of a STP.

Understanding the legislation

The Environment Agency, Scottish Environment Protection Agency and the Northern Ireland Environment Agency have put together a set of guidelines called the Guidance for Pollution Prevention 4 (GPP4) provides advice to properties on choosing, installing and maintaining, sewage treatment plants and septic tanks. The local authority building control and the environment agency must be contacted before the installation. Some areas of the country may need planning permission from the local authority. The local authority will give guidance as to where a treatment plant and discharge point can be situated. The Department for Environment, Food and Rural Affairs (DEFRA) have imposed general binding rules for small sewage discharges (SSDs) effective January 2015. The binding rules vary on the volume of discharge and the type of discharge, either to ground or surface water. If you meet all of these binding rules, you do not require to register your SSD. It is mandatory in the UK for all Septic Tanks and Sewage Treatment Plants to have been tested and certified to BS EN 12566 and have the CE Mark.

Site Survey

Then a detailed site survey needs to be conducted which will give you essential information about the type of wastewater treated, input loadings, effluent output quality, proximity to properties, discharge point and site access. A Percolation Test should be carried out if the effluent is being discharged to ground. This is done to determine the suitability of the ground to receive the effluent. If the discharge is to water i.e. a river, stream or lake you will find the guidelines in the general binding rules.

Once all of this information is gathered it is then shared with the prospective manufacturer for them to correctly size and offer the correct product.

Choosing the right product

The next step is to figure out which product is best suited for your requirements. This will be determined by the results of percolation test, along with the level of treatment that is required. Consideration should be given to site access especially if it is a replacement for an existing failed/illegal system. The final discharge point will also determine your choice of product. If the discharge point is higher than the outlet of the system then you will require a treatment plant with a pumped outlet. Other factors to consider when choosing the correct treatment plant are aesthetics, running and maintenance costs. It is a legal requirement to choose a manufacturer like Harlequin whose products are CE certified.

Actual Installation of the product

The installation should be carried out by suitably trained and qualified personnel. You need to consider aspects such as depth of the bedrock, proximity of the dwelling to the source, discharge area, height of water table, slope of land, building control advice, prevailing wind and work vehicle access. Specialist advice is required when installing in an area with adverse ground conditions e.g. rock, running sand, high water table. Each manufacturer will provide you with a detailed manual for installation of your product. It’s advisable to use a reputable Sewage Treatment Plant installer when installing your product.

Following are the stages in the installation of a STP:

Conducting pre-checks – Check the depth of your incoming pipework and match this to the treatment plant. You would need to check the depth of the outfall from the treatment plant and check that you can achieve a gravity outfall to the ditch. And, find out the depth you will need to dig.

Ordering the sewage treatment plant – Once you have conducted the preliminary checks, place an order for a sewage treatment plant. When ordering the treatment plant, talk to the manufacturer about the depth of your drainage pipes so he can ensure you get the right product for your needs. When the plant is delivered, make sure that you have everything you need to install it.

Excavating a hole for the tank – Excavate a hole and choose the right base to be laid for the treatment plant.

Placing the tank in the hole – Once the hole is ready, place the tank inside it.

Backfilling the site – After the product has been placed, backfill the hole with gravel, sand or concrete. The instruction manual should be able to give you details about the material to be used for the tank.

Installing inlet and outlet – This should be fairly easy if you have the right measurements. It’s advisable to have an inspection manhole before and after the treatment plant to fix any problems arising in the future.

Setting up/wiring up – The set up and wiring of the plant should be performed by a qualified electrician.

Commissioning – It’s best to have the system commissioned by a sewage service engineer.


The manufacturer’s installation guide will have details of how you need to maintain the STP. You can even talk to your service engineer about the steps to be taken to maintain your plant.

Some of the key maintenance activities are:

Perform de-sludging once or twice a year depending on the plant size. De-sludging prevents a decrease in effluent quality and minimises potential damage to the plant.

Follow the manufacturer’s instructions for regular maintenance.

Harlequin Waste Water Treatment Solutions

At Harlequin, we provide safe and efficient domestic wastewater treatment solutions to meet the needs of our environment. They can be further customised to suit your requirements. We also work with a network of competent installers in UK & Ireland who can help you install the product and provide you with additional technical support.

To find out more about our range of wastewater treatment systems or to find a reliable installer, contact our team on 028 9261 1077 or email

If you’d like to know more about the new regulations in place for Septic Tanks, read our blog here.

If you are not sure which solution would suit your requirements, you can book a free site survey with us here. Our team of experts with 100 plus years’ experience can help you out.