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Frequently Asked Questions

Our FAQs page is here to address your most pertinent questions regarding drainage systems and sewage treatment. We recognise the challenges associated with public and private drainage, septic tanks, and treatment plants so we hope that the answers below give an initial level of guidance. For a no-obligation talk about your specific system or requirements call us on 01984 623404.

What is the difference between Public and Private drainage?

A public sewer network – an adopted sewer – is one owned and operated by the local water company, with maintenance of the infrastructure paid for through charges to customers, while private drainage is wholly the responsibility of the landowner and their delegates. Private drainage can range from a short lateral connecting a single dwelling with the public sewer, to an entire network, treatment plant and discharge points in a location not served by public sewers.

Agreements between an individual or an organisation and the local water company to make amendments or additions to the public drainage network are administered through several different legal instruments that refer to sections of the Water Industry Act 1991. The four most commonly used are Section 104, 106, 98 and 185 agreements.

Section 104

The Section 104 agreement defines how a drainage network is “adopted” by a water company, adoption being the process whereby a previously private network becomes part of the public sewer network, with the water company taking responsibility for its ongoing maintenance. The process of adoption should begin before construction commences with the preliminary designs being submitted to the water company for checking to ensure it conforms to the relevant standards, in this case a document called Sewers for Adoption, as well as checking whether the existing network being extended has sufficient capacity to accommodate the additional flows. Sewers for Adoption defines how every aspect of the drainage network should be constructed, including manholes, pipework, and pumping stations, to make the network resilient and the ongoing maintenance straightforward. Once the water company has given technical approval to the submitted designs site inspections will be coordinated between the developer and the water company at critical phases of the construction process to ensure that the work is of an appropriate standard. Once construction is completed a 12-month period elapses where the developer is responsible for the maintenance of the network before the water company takes ownership.

If a drainage network is to remain private the relevant document defining how it should be constructed is Part H of the Building Regulations which is less stringent and more limited in scope than Sewers for Adoption, and can be obtained free of charge through the Planning Portal. The bodies responsible for overseeing the design and installation of private drainage are the local authority’s building control department, and the Environment Agency (or NRW/SEPA/NIEA) if the final discharge point is not into a public sewer.

Section 106

If a single connection is being made to a public sewer with everything upstream remaining private, then a Section 106 is used. In many cases a Section 106 agreement is a more appropriate method by which to connect to a public sewer than a Section 104 agreement. The connection can be made to an existing public manhole, inspection chamber or sewer; if the sewer being connected to <300mm in diameter the pipe will need to be cut and a new T- or Y- junction installed to make the connection, if the sewer is ≥300mm a saddle connection can be used instead.

Section 98

In circumstances where connection to mains drainage is possible, but only via third party land, and an agreement cannot be arrived at between the developer and the owner of the third-party land, then a requisition can be requested from the water company. In essence, a requisition is where the water company uses their statutory powers to create a connection on behalf of a developer.

Usually it is better for both the developer and the owner of the third-party land if a private agreement is arrived at instead, and a connection is made under a 104 or 106 agreement. This a preferable approach as it reduces the cost to the developer, and allows them to offer a larger compensation payment to the third-party landowner than the one they would receive from the water company for the disruption created by the construction work, which is coming out of the developer’s pocket either way.

Section 185

If a proposed structure is to be installed over an existing sewer it will require diversion so that the foundations of the structure do not damage it, either during construction or as a result of the increased loads. A common problem encountered with diversions are a need to cross into third party land with the concomitant legal wrangling that entails, especially if the third-party land is in the public domain, for example a footway or road, or in the hands of a private third-party who is resistant to having extensive excavation work conducted in their land. If necessary, water companies can use their statutory powers to force a diversion in third party land in a similar way to a requisition, but this entails time and financial costs. Another problem encountered is complying with minimum gradients and depths for the diverted pipework, especially if the drainage run being built over is already at a shallow gradient and the diversion, by necessity, makes this even shallower.

In some cases, where there are no other options, a build-over agreement can be obtained where the structure is designed to bridge the pipework, so that the loads are transferred either side of it. Water companies are loath to issue these agreements given the risks involved with construction work around a sewer that is in service, and that the sewer cannot be maintained or replaced in the future through a straightforward excavation.

What is a cesspit?

Many of the people that contact WCI about a drainage survey, the Environment Agency’s 2020 General Binding Rules or the legal compliance of their private, off mains drainage system, often describe their foul drainage system as a ‘cesspit’.

If you research the word ‘cesspit’ in Wikipedia it states that ‘A cesspit (or cesspool or soak pit in some contexts), is a term with various meanings: it is used to describe either an underground holding tank (sealed at the bottom) or a soak pit (not sealed at the bottom).[1] It can be used for the temporary collection and storage of faeces, excreta or faecal sludge as part of an off mains private sewage treatment system and has some similarities with septic tanks or with soak pits. Traditionally, it was a deep cylindrical chamber dug into the earth, having approximate dimensions of 1 metre diameter and 2–3 metres depth. Their appearance was similar to that of a hand-dug water well’.If you ask most people in the West Country what a cesspit is, you will actually find they are referring to a septic tank.

Like the generic words ‘klargester’ or ‘digester’ that are used to describe a sewage treatment plant, the word cesspit is a generic word often used to describe a septic tank.

I’m buying a property with private drainage. What information does the Seller legally have to provide to me?

Under the Environment Agency’s General Binding Rules, when someone sells a property with private foul drainage arrangements, they must tell the new operator (the new owner or person responsible for the sewage treatment plant) in writing that a sewage discharge is in place. The information must include:

  • a description of the treatment plant and drainage system
  • the location of the main parts of the treatment plant, drainage system and discharge point
  • details of any changes made to the treatment plant and drainage system
  • details of how the treatment plant should be maintained, and the maintenance manual if you have one.
  • maintenance records if you have them

WCI would suggest asking the following questions:

1. What private foul drainage system serves the property?

This could be a septic tank and drainage field, a reed bed, a sewage treatment plant, a private pumping station connected to the public foul sewer or a cesspool. Terms such as cesspit, soakaway, clinker bed or klargester may also be used.

2. Is the system shared with other properties?

How many bedrooms does each property have?

3. Where is the system located?

Is it on land owned by you or a third party?  Is there an easement in place if the system is not on your land? Do you have rights of access, and is a management company in place which sets out maintenance and emptying arrangements and payment of such activities? Do you have a plan of the system?

4. Where does the system discharge?

To ground via a drainage field or to a surface water i.e. ditch (dry, seasonally dry or flowing), stream, rhyne, stream, river. Ask for a sketch of the system including where it discharges.

5. When was the system installed?

Is there a Building Completion Certificate covering the installation of the septic tank? Do you have a Operations and Maintenance Manual?

6. How often has the system been emptied and maintained?

Has the seller kept receipts of each empty and service event or even an annual service report?

7. Does the system’s discharge have an Environment Agency Permit?

This may be called a Discharge Consent, an Exemption or a Permit. WCI will check the Environment Agency’s Public Register as part of the Home Buyers Drainage Survey report.

8. If the system is served by a drainage field. Do you know of any wells or boreholes for potable water nearby?

Drainage fields cannot be located within 50m of any well, spring or borehole that is used to supply water for domestic or food production purposes.

9. Have you experienced any issues with the system?

These could include backing up of drains, smells, a need to empty the system more frequently, blockages or wet patches on land over the drainage field.

10. Is there any history of problems especially during wet weather?

In the winter the groundwater sits closer to the surface and can prevent a drainage field working properly. Any rainwater connected into the wastewater treatment system could also cause problems including washing out the tank.

How do I know the septic tank I am inheriting is legally compliant and fit for purpose?

Purchasing a house with a private foul drainage system should not be daunting. However, as with all things unknown, fear can creep in about the state of the septic tank you are inheriting. Lets face it – you will be able to see a couple of manholes and a vent pipe at best! You have heard about Environment Agency regulations but don’t know how this affects you and how do you ensure the septic tank you are inheriting is legally compliant and fit for purpose.

This is a prime example of when ‘Knowledge is Power’. The more information you can glean about the system from the Seller, the better. Better still, you could employ WCI to carry out a Drainage Survey at the property. Our drainage surveys will ensure that you will not be exposed to the daunting costs of rectifying or replacing an illegal system after you’ve completed your purchase.

I am worried my drainage field is failing… now what?

All too often people cut corners with the installation of a new or replacement drainage field. As we say at WCI, do it right and do it once. A drainage field should not be installed without the proper percolation tests being conducted and the drainage field being sized and designed by an expert in accordance with the British Standard BS 6297:2007+A1:2008. WCI can guide you through undertaking your own percolation tests.

The work involves you digging a minimum of three holes, carrying out the tests to a supplied specified method and submitting the data, field sketches/plan and photos for analysis. Unable or uninspired? WCI are able to undertake these works for you!

Contact WCI on 01984 623404 and talk through your options.

What are the top five reasons for drainage field failure?

1.  Connection of the roof water to the foul drains

Under storm conditions, where roof water is connected into the foul drain and enters the septic tank, settled sludges within the septic tank are mixed up and carried out into the drainage field causing blockages.

2.  Geological reasons including seasonally high-water table and poor porosity

When an engineer designs a drainage field, they should undertake porosity tests and groundwater monitoring to ascertain where the water table sits in the winter months and how good the soil is at draining water through it. Where an area suffers from clay based soils and/or a high-water table the functionality of a drainage field might not be viable or legal.

3.  Infrequent desludging of the preceding septic tank or sewage treatment plant

How many times do we hear “Oh I’ve never had my septic tank emptied!”. A septic tank acts to settle out raw sewage allowing the clear water to enter the drainage field. The more sludge within a preceding tank, the less clear water there is available to flow through into the drainage field and therefore solids start carrying over into the drainage field and can cause blockages.

4.  Damage to the preceding septic tank outlet dip pipe

Quite often we visit a property with a problematic drainage field only to find that the septic tank dip pipe is no longer present, has broken off and now resides at the bottom of the tank. The outlet dip pipe acts to stop floating material in the septic tank travelling through to the drainage field. If it is not present, solids can migrate into the drainage field causing blockages.

5.  Design issues including undersized septic tank and/or drainage field

We do just come across private drainage systems that are undersized for the property. When any part of the septic tank/drainage field system is undersized, the result is often a failed drainage field.

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.

How long should I expect my air blower/compressor to work without any maintenance?

Many sewage treatment plants that use air blowers to actively blow air into the plant are linear diaphragm models (ET, EL, JDK, XP) which require the diaphragms to be replaced every 12-18 months. Piston driven models (LA, ACO) require less frequent maintenance approximately every 3-5 years. Air blower filters should be cleaned or replaced when necessary which is typically annually.

Who will service my treatment plant?

All WCI Service Engineers are British Water Accredited Service Engineers and are fully trained.

They are polite, professional individuals that will be respectful of your plant and your property. Most first-time service visits are undertaken with the homeowner or maintenance manager present on site. They will witness first-hand what constitutes a service and often decide that they do not need to be present in the future. Some of our clients love sewage treatment as much as us and often make sure they are home to have a look and offer the Engineer tea and biscuits. The choice is yours 😊.

How often will my sewage treatment plant need emptying?

The frequency depends on the type and size of the treatment plant and what it is serving. Small domestic plants are on average serviced and emptied once a year while commercial systems can be serviced and emptied every 3 months.

For example a plant installed to serve a 4 bedroom home where only 2 people are living may only need emptying every 3 years due to underloading of the plant

Commercial plants range between every 2 months to every 6 months. When WCI services the system, we will take a sludge reading and test the discharge for the ammonia value which means we can advise when the plant needs emptying.