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What Is A Septic Tank?

A septic tank and drainage field or soakaway still remains the simplest and most cost effective way of treating sewage from domestic properties where the site characteristics allow. This guide explains what a septic tank is, how a septic tank works and answers questions we’re frequently asked at WCI.

Septic tanks and drainage fields are some of the most widespread and least understood of private drainage systems. Before we begin to explore them in more detail, we should clarify a couple of definitions used within the text below. A septic tank is commonly referred to as a cesspit, whereas a drainage field is referred to as a soakaway. Neither terminology is technically correct but hopefully this clarification will help you as you read on.


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.

If you are concerned that your septic tank system has a problem such as draining directly to a watercourse or is requiring more frequent emptying please call us to arrange an Engineers visit where we can inspect the existing system and provide a quotation for any required rectification or replacement works.


How does a Septic Tank work?

A scum forms on top of the tank consisting of fats and soaps, and sludge settles at the bottom of the tank. The relatively clear liquid, called septic tank effluent, which lies in between the scum and sludge, leaves the tank and then undergoes secondary treatment.

A stand alone septic tank will reduce the strength of raw incoming sewage by 30-50% under ideal conditions through anaerobic bacterial action. The bacteria action however is not fast enough to keep pace within the amount of incoming sewage and so sludges must be removed by a licensed waste carrier on average every 12 months to ensure material does not discharge into the drainage field. The removed sludge is taken to a nearby Water Company large sewage treatment works and treated further.


What types of Septic Tank are there?

Historically septic tanks were built of engineering brick or stone and pointed, of concrete block and rendered or from poured concrete in situ. Often the inlet and outlet dip pipes were salt glazed clay 4” pipe work. The tank would consist of one, two or infrequently three chambers. The chambers were often connected using dip pipes or gaps in the partition wall, as shown in the diagram below. Wooden baffles might also be found within older tanks to hold back the scum and prevent carryover into the soakaway. Often a separate pipe would supply air to the tank via a nearby vent.

The more modern septic tanks for sale today are constructed of plastic or fibreglass and are known generically as Klargesters (after the popular Kingspan brand) or onions (due to their shape).

WCI installs new and replacement septic tanks and drainage fields. Where a septic tank is not permissible due to a lack of land area for a drainage field, a lack of porosity in the soil, proximity to the mains or a high-water table, WCI can design, install and maintain a package sewage treatment plant or pumping station.


Septic Tank Regulations

In England and Wales, septic tanks are still permitted subject to the presence of an adequately sized drainage field in permitted areas. As the operator of a septic tank system you must check the system meets the Environment Agency General Binding Rules and the impending septic tank 2020 regulation. If it does not, you may require a permit to operate and discharge legally.

WCI offers septic tank system design, installation and maintenance in Somerset, Devon, Dorset and Wiltshire. Contact us on 01984 623404 or via our contact form to arrange your free site visit.

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Please complete the form and a member of the team will be in touch shortly

If you have any questions or just need some clarification on the regulations and how they apply to your property, please call us on 01984 623404

Septic Tanks FAQs


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 huge annual expense, whereas a septic tank will require emptying once a year at an average cost of £160. This is why WCI will only install a cesspool on a site when all other system possibilities have been explored and rejected.


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. 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 can enter a septic tank?

The easiest way to think about your septic tank is that it’s a pet and you are responsible for feeding it. Keep the food regular and similar in composition. Little and often excluding anything that will make it sick or will kill it. An odd comparison? Perhaps, but not if you think there is large live bacteria colony within your septic tank which will have developed dependant on the constituents of the incoming wastewater.


    1. 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.
    2. 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!
    3. Spread your clothes washing out through out the week.
        • Little and often and try to keep the same washing powder/liquid.
    4. Bleach is fine but in moderation.
        • There are septic tank friendly alternatives on the market but they are pricey.
    5. Septic tank enzymes will do little to the sludge within your septic tank.
        • Save yourself the money!
    6. DO NOT put your dog’s faeces into the septic tank.
        • It will not decompose. Bag it and bin it.
    7. 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.


Why do I need a submersible pump in my system?

If the topology at your property doesn’t allow for a gravity system then a submersible pump will be used to pump septic tank effluent to the secondary treatment system (e.g. a soakaway). The pump will activate on a demand basis. Any failure would be seen as a flashing beacon near the system.


Can I fix my soakaway by jetting it?

Probably not. The jetting may dislodge a blockage but the bottom line is that if your soakaway has failed, jetting is only likely to move the issue deeper into the soakaway and only alleviate the problem in the short term. The sign of a failing or failed soakaway includes ponding of water or nettles in the area of the soakaway or issues such as backing up of water within the drains.


I have a manhole preceding my brick-built septic tank which always blocks. It appears to have a higher level pipe above the inlet pipe which is at the bottom of the manhole. What is this and how do I solve the blockages occurring?

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 however 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.

Can I re-render the inside of my septic tank as it is taking on ground water?

The answer from WCI might differ from other contractors but based on 35 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.


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 soakaway or replace it with a package sewage treatment plant.


What is the grey cotton wool looking material in the ditch where my system discharges?

This is sewage fungus, which is a pollution indicator. It often is used as a visual sign that there is a lack of secondary treatment and that a septic tank discharges straight into a ditch or watercourse.

If you are having problems with your Septic Tank, please contact WCI on 01984 623404 or via our contact form below to arrange your free site visit.

Contact Us

Please complete the form and a member of the team will be in touch shortly

If you have any questions or just need some clarification on the regulations and how they apply to your property, please call us on 01984 623404