Septic Tank/ Drainage Field Installation

WCI specialise in the installation of septic tanks and their associated soakaways / drainage fields.  Our professional installations include the correct percolation tests and soakaway / drainage field design giving you assurance of a system that will work and keep on working.

With over 40 years’ experience, our dedicated groundworks teams complete all our installations safely and to the highest standard of reinstatement.

Septic Tank Installation

Sometimes the old ways are best ways. WCI still recommend septic tanks over sewage treatment plants for a lot of properties including both new builds and existing properties.

While some will tell you otherwise, septic tanks are still permissible in England and Wales as long as they meet the General Binding Rules.

This means that new septic tanks and drainage fields (soakaways) have to be designed and constructed according to Building Regulations and British Standards.

With WCI you can be assured of professional design and installation of septic tanks and drainage fields (soakaways).

Soakaway Replacement

No system lasts forever and septic tanks and soakaways can fail.

We are often approached by septic tank owners who are sure their septic tank requires replacing. More often than not, it’s actually the soakaway/drainage field that is at fault.

If you are frequently emptying your septic tank, having issues in winter months, rod your drains frequently or have overflowing drainage, then talk to us. We’ll diagnose the problem and replace only what needs replacing.

Our septic tank installations generally start with a call with Naomi who will discuss the background to your enquiry.  This is followed by a free survey by our professional engineers. We will then provide a comprehensive written quotation which covers both the design and the construction of a system which meets your needs and delivers a compliant and functional system.

We aim to provide value for money but won’t cut corners and our quotes never leave hidden extras.


Septic tanks and drainage fields are some of the most widespread types of private sewage treatment system. Yet, they are also often the least understood and mismanaged treatment system. Here are some common questions and answers to help you find out more about septic tanks and drainage fields.

What is a septic tank?

A septic tank is a settlement tank in which raw incoming sewage is retained long enough for suspended solids to settle out as sludges and for liquid organic matter to undergo anaerobic decomposition.

As the sewage settles, a scum forms on top of the tank consisting of fats and soaps, and heavily organic matter settles at the bottom of the tank as sludge. The relatively clear liquid, called septic tank effluent, which lies in between the scum and sludge, leaves the tank and then undergoes secondary treatment in a drainage field.

While there is some anaerobic decomposition in the septic tank, this bacterial action is not efficient enough to fully treat the sewage to a level which is safe to disperse into the ground.  A septic tank alone will only reduce the strength of the incoming sewage by 30-50% under ideal conditions.

It is therefore the drainage field which receives the septic effluent from the septic tank which does most of the treatment and it is the septic tank’s job to separate out the solids to stop the drainage field from ‘blinding’ and becoming blocked.

To ensure efficient settlement of the suspended solids, there needs to be sufficient space for settlement to occur.  This is why sludges will typically need to be removed by a licenced waste carrier every 12 months on average.

What is a drainage field?

Septic tank drainage field design and installation is subject to Building Regulations guidelines and British Standard (reference BS6297:2007 + A1: 2008 Code of Practice for the design and installation of drainage fields for use in wastewater treatment).

A drainage field consists of interconnected parallel trenches up to 30m long, where perforated pipe is installed on a layer of clean stone. A soakaway for roof water is completely different and attenuation crates or rubble filled pits are not permitted for use with a septic tank or sewage treatment plant.

A correctly designed and installed drainage field can provide a perfectly functional and legal form of secondary treatment and way of dispersing effluent into the ground after a septic tank or sewage treatment plant.

However, WCI frequently receives calls from homeowners stating that they believe their septic tank has failed or has a problem. More often than not,  when we survey the system, the fault lies with the drainage field and not the septic tank.

Common septic tank drainage field problems include:

  • Boggy, wet areas below the septic tank often with lush green and weedy growth.
  • Backed up drains
  • Foul water coming out of the top of the septic tank or preceding manholes.
  • Manholes holding water in the bottom.
  • Toilets are slow to flush, possibly gurgling and noisy.
  • Tide marks within the septic tank.
  • When the septic tank is emptied – water rushes back from the drainage field into the septic tank.
  • Septic tank effluent breaks out to the surface of the land or into a nearby watercourse.
Foul water septic tank and drainage field diagram

What can enter a septic tank?

DO NOT pour fats, oils or grease down the drains.
Wipe out all pans with kitchen roll and dispose of the kitchen roll in the bin.

ONLY flush urine, faeces and traditional toilet paper down the toilet.
Don’t be fooled, these biodegradable moist wipes do not biodegrade in your septic tank and should be disposed of in the bin.

If you let out your property please consider installing signs on the back of toilet doors!

Spread your clothes washing out through out the week.
Little and often and try to keep the same washing powder/liquid.

Bleach is fine but in moderation.
There are septic tank friendly alternatives on the market but they are pricey.

Septic tank enzymes will do little to the sludge within your septic tank.
Save yourself the money!

DO NOT put your dog’s faeces into the septic tank.
It will not decompose. Bag it and bin it.

Use a water softener or have a swimming pool or hot tub?
Please do not plumb your backwash into the septic tank. It will kill your bacteria.

Are septic tanks illegal?

The quick answer is no. The main aim of the Environment Agency General Binding Rules is to stop pollution from private sewage treatment systems and ensure adequate treatment is taking place. If you have an existing septic tank that can meet all the appropriate General Binding Rules, then it need not be replaced. The main rule is that any septic tank must have adequate secondary treatment for example a drainage field (or reedbed under the appropriate EA permit).

I have a septic tank, do I need a service contract?

A septic tank needs to be regularly desludged but doesn’t generally need any other servicing unless the septic tank effluent is pumped to the drainage field. A pump station requires annual maintenance to ensure its correct operation.

What is the difference between and soakaway and a drainage field?

The easiest way to think about it is that a soakaway is designed for rainwater attenuation and release over time. They are often pit like structures filled with crates or rubble. A drainage field is designed to provide additional treatment after a septic tank or sewage treatment plant and release that water into the environment. Drainage fields consist of long horizontal perforated pipework laid on a very shallow gradient often within a grassed area, surrounded by gravels. You cannot use a new soakaway to discharge septic tank or sewage treatment plant effluent to ground. Some older soakaways do meet The Environment Agency General Binding Rules depending on the year and method of construction.

What is the difference between a cesspool and a septic tank?

We often get asked what the difference is between a septic tank and a cesspool. The answer is quite simple. A cesspool is a sealed underground tank where all the wastewater from a property is stored. There is no outlet. Once the tank is full, it requires emptying. A septic tank differs insofar as only the solids are retained within the tank and the septic effluent discharges from the septic tank for further treatment and dispersal into the ground.

How do you know what is installed at your property?

Well, a cesspool is a very large tank which will need emptying by a tanker every 4-6 weeks at significant annual expense, whereas a septic tank will require emptying once a year at an average cost of £180. This is why WCI will only install a cesspool on a site when all other system possibilities have been explored and rejected.

Is it illegal to sell a house with a non-compliant septic tank?

It isn’t illegal to sell a house with a non-compliant septic tank, however, it is the homeowners’ responsibility to ensure the system meets the Environment Agency General Binding Rules. There are two options of dealing with an illegal septic tank system as part of a house sale 1. The vendor installs a new system or 2. The purchaser negotiates a reduction off the sale price and carries out the installation work after they have moved in. The benefits of option number two is that the purchaser gets the system they want (taking into account noise, aesthetics and size), in the position they want (allowing for house and garden redesign) and for the price they want.

If you are selling or purchasing a house with a septic tank and need a compliance survey, please contact the office on 01984 623404 to discuss your requirements.

I have a septic tank, but it was installed before 1983, do I have to meet these new Environment Agency Regulations?

Under the Environment Agency’s General Binding Rules your septic tank must meet the relevant British Standard in place at the time of installation. If different parts of your treatment system were installed at different times, each part must meet the relevant British Standard in place at the time of installation. If your treatment system was installed before 1983 you do not need to do anything to meet the British Standard. There was no British Standard in place before then. You must still meet the other general binding rules that apply to you such as Rule No. 10 ‘Make sure your treatment system is installed and operated correctly and has enough capacity.’

The current standards for new systems are:

  • BS EN 12566 for septic tanks and small sewage treatment plants
  • BS 6297:2007 for drainage fields

My septic tank discharges directly into a ditch/watercourse. Is this illegal? What can I do about it?

If your septic tank system discharges effluent (no matter how clean) to a watercourse or ditch, the system does not meet the Environment Agency’s General Binding Rules and you are committing an offence under Section 85 of the Water Resources Act 1991, rendering you liable to conviction and a fine. If your septic tank is fit-for-purpose you may be able to add on a secondary form of treatment such as a drainage field or replace it with a package sewage treatment plant.

My septic tank drains into a clinker pit or soakaway. Is this legal?

The answer is it could be depending on the date it was constructed, its current operation and whether it meets all the required Environment Agency’s General Binding Rules.

For peace of mind, get in touch to discuss your foul drainage.

Can I re-render the inside of my septic tank if it is taking on groundwater?

The answer from WCI might differ from other contractors but based on 40 years of experience we would say no. In order to allow the render to cure, the whole water table around the tank would need to be dropped for a significant period of time and a specialist workman with confined spaces training would need to undertake the work. It’s not so much that it can’t be done but that a replacement tank would be more cost efficient and with the added insurance that it is a fiberglass or plastic vessel impenetrable to ground water! If the render has blown once, could it blow again?

Can my rainwater and septic tank discharge into the same soakaway?

No. Building Regulations require separate soakaways for foul water and rainwater.

Will the brine from my water softener damage my septic tank?

There are two schools of thought on this subject and a fair amount of research has been undertaken in the US on the topic. Researchers have found that brine wastes had no negative effects on the bacterial population living in the aerobic treatment tank, even when the system was loaded with twice the normal amount of brine. The additional amount of water discharged to a treatment tank during the regeneration process had no negative impact. It was also found that the water softener regeneration brine did not reduce the percolation rate of water in the absorption field of a normally operating septic system.

Some experts don’t agree with the findings above and would like to see more research done. Senior Engineering Scientists believe the sodium in the ‘plug flow’ of brine might cause metabolic shock to the bacteria in the septic tank. This shock could cause the bacteria to be less able to digest the cellulose fibers, which then might be carried over into a septic system’s drainage field. Other engineers have also stated that there are noticeable differences between septic tanks with, and without, water softener brine discharges in septic tank systems. This includes a reduced scum layer development and a less distinguishable ‘clear zone’ that might mean solids remain suspended instead of settling in the tank.

What is the manhole preceding my brick-built septic tank which always blocks?

What you are probably describing is an interceptor trap, disconnecting chamber or a version of a Buchan trap. Such a device uses a water seal (u-bend) to prevent sewer gases and vermin travelling back up the pipe work. The trap will collect solids, sludge and waste over time in the u-bend and can block causing wastewater to back up. It was devised in the 19th century when it was believed that cholera was an airborne infection and often denotes the end point of the domestic property’s sewer before it joins the main public sewer. They can be found on private sewage treatment systems often preceding septic tanks in clay drain runs. Today, pea traps and u-bends on sinks, baths, showers and toilets prevent smelly gases entering the property and therefore the chamber is without a use. The best thing is to replace the whole chamber with a new manhole.

Interceptor With Clay Vent Visible

Interceptor With Recent Plastic Connection

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Keith and Linda - Baghay Cottage

We were very impressed with the WCI installation team, who worked very hard and diligently in the most awful wet conditions to install our new sewage system. The team also did their very best in the pouring rain to ensure that the site was left tidy. Also, over the Xmas holidays, Brad promptly fixed a problem with the pump. Thank you for a job well done.

Emma Sherman - Greengates

We have found the service right from the very first phone call, site visits, detailed quote though to completed job exceptional. The whole team proved to be very knowledgeable in this field and we had full faith the system is the correct one for us. The site was complex and was left in a better site than it started, we can not thank WCI enough and would highly recommend.

Ms G Fraser - The Old Parsonage

I am thrilled with all you did here. The installation of the new drainage system was completed to my total satisfaction. It was a huge job but within a few weeks, there was no visible sign of the movement of heavy machinery or of the temporary disruption to the garden. Your staff were superb and the supervision and support from head office exemplary. I would recommend WCI to other people without hesitation.


Call us on 01984 623404 or click here to let us know about your current setup.