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Preparation and Planning to install a new sewage treatment plant.

| Gerry Rowe

Before obtaining estimates:

Drainage surveys are an essential part of the garden design – before the treatment plant estimating process. You may be able to provide visual diagrams – If you’ve had plans drawn up by an architect within 2 years, they can be very helpful to us. On most occasions, we advise a professional survey, including test holes and a porosity test. 

A six population treatment plant ready for installation.
A six population treatment plant ready for installation.

Water moves by gravity into or out of the open pore spaces in the soil, and the size of the soil particles and their spacing determines how much water can flow in or out. Wide spacing, using 50mm flintstone in a drainage field would be highly permeable. The larger the stone, the bigger the gaps around and therefore, the resistance is less, and the run-through would be fast. In other cases, a smaller flintstone, gravel or sand would allow the flow to spread wider when required. Permeable surfaces, known as porous, allow water to percolate into the soil to filter out pollutants. Impervious surfaces are solid surfaces that don't allow water to penetrate, causing ‘runoff’.

Light aerated soils can have an ideal rate of water soakage, and coarse soils have a higher rate of disbursement through drainage fields (land drains) into the permeable ground similar to limestone. In high water table areas, the water level can rise to the same level as the drains and seep into the land drainage system, causing waterlogging.

Even for smaller homes, we implore you to have a survey; it will save you money in the long run in our experience. Even if your ground is not currently suitable, we do have solutions to accommodate this. 

Planning the position:

Any installation must be “appropriate”, i.e. it must suit the ground conditions, which will sometimes vary over a very short distance. This is the responsibility of the owner of the property according to the Environment Agency Binding Rules.

The treatment plant positioning needs to abide by the rules and suit the environment, bearing in mind the servicing and emptying. 

If you choose to install your treatment plant 40 linear meters or more down a steep slope, you will certainly have issues and problems if you do not monitor and manage the emptying. There are examples where the tanker driver has emptied off only the water because of difficulty sucking up the solids over such long distances uphill. The lie of the land is significant as it affects drainage, siting and design of drainage fields, and the amount of effort needed for earthworks.

We use photographs to aid and support any surveys performed. They’re indispensable when it comes to clarifying the existing landscape. Photos are also an excellent way of capturing every step along the design process.

Surface water warning:

On regular annual emptying, our visual inspections of the chambers within a treatment plant often reveal solids, grease and fat have been pushed through into the second and the last chamber, resulting in the grease and fat covering the media. This will seriously affect the treatment process allowing un-treated greasy water to flow into the drainage fields (land drains).

In this case, I would strongly suspect surface water or roof water has been entering the sewage system via indirect or direct means; possibly an inspection chamber covering is allowing surface water between the gap of the lid and frame.

My observations

I sometimes find sewage in the treatment section and the last chamber of the treatment plant, that is a strong indicator of rainwater being in the treatment plant, which would disrupt treatment.

A big threat to the system

I can assure you if the flow of wastewater is increased significantly, because of roof water or surface water entering the system or a large increase in usage like a party, this could force the solids straight through the holding tank to the treatment area and block the outfall pump stopping any treatment. If you have a gravity-fed system; untreated raw sewage can discharge into the soakaway – without your knowledge – for several months.


Beware of roof water and Rainwater entering the sewage system

The Operator – under the General Binding Rules – the owner of the property, in this case, has the responsibility of arranging the maintenance of the surface and roof water system and is expected to remain in control, responsible for the type of water that drains to a septic tank or treatment plant. Under the general binding rules, no pollution can occur.  

As such, the owner of the house is liable and responsible for any pollution and illegal discharges. They would be advised to protect themselves and put up notices to inform all contractors; “they have not got a combined system”, or “they have a private system”. This is because no rainwater or roof water can be connected to the foul system into the public sewer, a private septic tank or treatment plant. If a contractor is not informed that they should not run surface water into the foul system and does this in error, it is still the Operator that the Environment Agency will hold accountable for any pollution. You can familiarise yourself with the Binding Rules to make sure you fully understand your position as the Operator, how you can protect yourself by avoiding these situations, saving aggravation and looking after the environment.

If rainwater, roof water, or surface water from your property enters the system in a surge, such as during heavy rainfall or a storm, the incoming water can stir up the first tank in the public treatment works, destroying separation that has occurred in the primary settlement chambers.

Without adequate separation, suspended solids and fats, especially oil, can pass through and discharge into rivers, causing pollution. 

In septic tanks or small treatment plants, the destruction will instead occur slowly to a drainage field by coating the base and walls, causing irreparable damage and making it progressively waterproof. Greece or fat covering the media will reduce the quality of clean water. Consequently, the drainage field (land drains) or soakaway‘s ability to soak and disperse will diminish until they cease to work altogether.

Due to rainwater in the system, treatment plants that are not functioning correctly will continue to contaminate and pollute (discharging into streams and rivers or a watercourse).

Are you sure you have no Rainwater Entering Sewers?

Recommendation: To make sure you have no rain or surface water draining into the pipe leading to a septic tank or a treatment plant, we advise that you have your drains checked. Carry out a survey, simulating a heavy downpour of rain, including CCTV looking for broken or cracked pipes, looking for surface water or roof water entering the system via indirect or direct means.

This should be done by a specialist drainage company conducting a survey, using equipment such as; a simple camera with a hosepipe or a state-of-the-art CCTV camera with a professionally built machine with the right flow. With a clear goal of the survey, to eliminate or confirm indirect and direct roof water or surface water entering damaged drains should be the priority. Water gullies and downpipes should also be tested to see if they discharge into the foul drainage system. This is the service you will receive if you choose ASL Limited to inspect your drains. 

We are passionate about providing proactively good service for our customers. We take the time to explain the issues that surface water can cause, even when customers are unaware that their rainwater pipes and drains run into the foul system. We are also committed to ensuring that each property we serve is not overwhelming the sewer systems or potentially causing pollution.


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