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Private Sewage Systems and how they are designed to work

| Gerry Rowe

Cesspits/Cesspools

A picture of a full cesspit that needs emptying.

Cesspits can vary in different shapes and sizes and are installed entirely underground. Newer cess pits consist of a huge watertight fiberglass holding tank with an inlet, and older tanks are usually of brick construction. 

An average cesspit has a capacity of approximately 4000 gallons, designed for containment only, with no discharge. Typically, the 4000 gallons cesspit (18,000 liters per 2 users, plus another 6,800 liters per each additional user) would require emptying every 9-12 weeks, dependent on usage. There is no requirement to inform the Environment Agency or to apply for a permit if there is no discharge. However, a building notice would be required for the new installation. 

Although it is your legal right to install a cesspit, we wouldn’t usually advise it as an option and should only be considered as a last resort or when an Environmental Permit is refused or not possible. A cesspit is designed for containment only and requires emptying regularly and can vary depending on its usage. 

Furthermore, in the future when/if you decide to sell your property, a cesspit system, in our professional opinion, can devalue a property by as much as £20,000.00 as it is the least favorable sewage system to have. 

To give you an indication of the cost, our charge to install an 18,200L / 4000-gallon cesspit is in excess of £20,000.00 plus VAT. Price excludes sheet piling, de-watering, and muck-away. Price is subject to test hole, consents, and a detailed survey/estimate. 

 

Septic tanks

A picture of a septic tank inspection chamber with the covers lifted to empty.

Septic tanks work through three significant processes: initial solids separation from liquids, limited bacteriological action, and absorption. Septic Tanks should have a minimum of two chambers (or a single chamber divided by a separation wall) with dip pipes. The effluent enters the initial tank, where larger solids are separated, and fats float to the surface. Because the first chamber receives waste straight from the drains, the contents can get stirred up as new effluent enters, especially if it is a large volume all at one time, such as emptying a bath. This is why the first chamber is limited to the initial separation of larger solids and fats. 

Effluent then passes via dip pipes to the second chamber (or additional chambers), which is a calmer environment, where smaller particles of solids in suspension have the opportunity to settle to the base of the tank. While this process occurs, the effluent has a limited bacteriological breakdown. The separated effluent then passes along a further dip pipe to the soakaways/drainage field, where the separated liquid is absorbed by the surrounding soil, which provides supplementary natural filtration. 

Old septic tanks can have difficulty coping with modern living, i.e., using washing machines, dishwashers, and other kitchen appliances, along with contemporary standards of personal hygiene. Older tanks and soakaways/drainage fields can lack the capacity or capability required for modern living, with the abundant supply of mains water, even if adequate when they were originally installed. 

There must be the separation of waste required for a septic tank system to protect the soakaways/drainage fields from becoming contaminated with fats and suspended solids. Septic tanks require a large area of light, aerated, porous ground, free from tree roots, to discharge into. Otherwise, the ground will become contaminated and turn black and oily. 

Dip pipes

Broken dip pipe in an old septic tank.

Dip pipes are essential in the separation process, as without adequate separation, suspended solids and fats can pass through the septic tank, and on through to the land drains, where they can coat the surface, causing irreparable damage making it progressively waterproof. This diminishes the soakaway/drainage field's ability to dissipate water until it ceases to work altogether. 

Septic tank discharge

A septic tank comes in two parts: a chamber separating the fats and the solids and discharging separated but untreated water into light-aerated soil, where treatment, which we call secondary treatment, occurs. Discharges into clay ground are not permitted as the ground is non-porous and deprived of oxygen. 

Heavy clay soils are slowly permeable and, without drainage, can be waterlogged for long periods, particularly in areas of high rainfall. Any sewerage or separated sewerage into clay ground will turn black and oily, and during heavy rainfall, the contamination washes towards ditches, or into watercourses which lead to streams and rivers. 

Septic drainage fields, often referred to as land drains or drains that perform the secondary treatment of effluent waste from a septic tank, are a very important, and crucial part of a septic tank system, so they must be installed correctly in line with Building Regulations Guidelines. 

Septic tank land drains should only be installed in areas of light-aerated soil to remove contaminants and impurities from the effluent that emerges from a septic tank. It usually consists of trenches, often referred to as herringbone systems containing perforated pipes and gravel, sited beneath a soft landscaped area such as a lawn. 

A septic tank is only one part; the septic drain field and any other connecting pipework make up the septic system. The septic tank only performs around 45% of low treatment, (breaking down of the solids). The rest of the work is performed by the drain field, which is why it is so important to ensure it is designed correctly before installation. 

SURFACE WATER WARNING 
If surface water/rainwater can enter the tank, this can cause further problems with separation/treatment. If the water enters in a surge, such as during heavy rainfall, or a storm, the incoming water can stir up the first tank, destroying any separation that has occurred in the primary settlement chamber and or any treatment. Without adequate separation, suspended solids and fats can pass through the tank, and on through to the soakaways/drainage field, where they can coat the surface, causing irreparable damage making it progressively waterproof. This diminishes the soakaway/drainage field's ability to dissipate water until it ceases to work altogether. 

Ground conditions

Kindly note that any estimate given is subject to ground conditions and adverse weather conditions. If a job has been priced using a mechanical excavator or mole, and we encounter various services, root ingress, or poor ground that prevents the use of a mechanical excavator or mole for all or part of the works, there will be a revised estimate to excavate by hand. This will be charged per meter and is dependent on depth, surface type, etc. Similarly, should we encounter a high water table causing water infiltration during our excavation, the price will be revised to allow for shoring and dewatering to make the excavation safe. 

Don’t delay we can help you today!

Call 0800 181 684 for all your drainage needs.

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