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Septic tanks

Image of a septic tank.

Septic tank systems are an effective, economical way of treating domestic effluent. They consist of two main components: a watertight, underground tank (or tanks), into which raw sewage is fed, and (normally) a drainage field to which wastewater is released.  Direct discharge from the tank to a ditch or watercourse should not occur (however in Northern Ireland and Scotland, the use of a perforated pipe which drains to a waterway is allowed under certain conditions).

The tank provides suitable conditions for the settlement, storage and partial decomposition of solids, which need to be removed from time to time. The drainage field is critical for protecting the environment from pollution and removing potentially harmful impurities. Most modern tanks are constructed of glass fibre or thermoplastics. Older tanks were usually constructed with brick or concrete.

Septic tank systems require minimal maintenance, but they rely on bacteriological action and if not functioning correctly, there can be problems with odours, surface flooding, contamination and associated health risks.

What is a cesspit?
A cesspit is merely a holding tank for waste - no treatment is undertaken. These tanks need very regular emptying, dependant upon the number of occupiers and usage.

How does a septic tank work?
The primary purpose of the tank is to separate the solids from the liquids and to help break down contaminants. There are usually three layers in the tank:  a scum layer of floating solids. the liquid sewage from which solids are settled out. a bottom sludge layer which is partially treated by bacteria. The wastewater passes from the tank to the drainage field, often via a distribution/sampling chamber.

The drainage field typically consists of a system of sub-surface perforated pipes or a soakaway, which allows the liquid to drain into the surrounding soil. To function properly, it relies on adequate soil drainage and good contact being maintained between the liquid, air and bacteria the subsoil, which break down and purify the effluent.  It is better to use a network of sub-surface perforated pipes, or alternatives such as constructed reed beds, rather than soakaways. The former methods maximise contact with the soil, or plants, and aid the natural breakdown of the waste. But remember, septic tank systems will only operate properly if tanks are regularly and competently de-sludged.
Image of a septic tank scum layer. Maintenance
Septic tank systems require little maintenance, but they should be emptied once a year (samll treatment plants usually require more regular attention). This de-sludging is important to prevent solids entering the drainage field and causing blockages.  You will know the tank needs emptying too late if sludge levels are high, the drainage field floods, the drain backs-up or it smells. A regular look at your system will help you learn how it works, but take care when you do so. A well-run system will develop a surface crust, full of earthworms and will not be smelly!

The drainage field should also be checked for sogginess or flooding this indicates inadequate drainage or a clogged system.  When the tank is de-sludged, ensure the operator:  pumps out all of the contents, including the sludge (a small crust may be left for reseeding- but no more). uses the access cover, not the inspection ports. does not damage the internal pipework or breather vents and leaves the inlet pipe clear and the covers in a safe condition. When choosing a de-sludging contractor, check that the sludge will be disposed of responsibly and safely, preferably to the local sewage treatment works.

The underground drainage field is the weakest part of the system and once blocked, often the only solution is excavate and repair. Old systems may be so badly silted and infested with roots that replacement is the only option. This can be expensive and very disruptive if garden areas need to be disturbed. Typical indicators of problems with your system are slowly draining sinks, toilets, baths etc. Tank covers lifted by overflowing solids, backing up of sewage at the inlet, surface flooding downstream of the tank and nettles and vigorous plant growth downstream. Ground movement near the tank or drainage field, polluted water in ditch or watercourse and a smell of sewage in the area.

Your responsibilities
A septic tank system which does not work can pollute water or land, cause a nuisance or smell, and be a possible hazard to children and wildlife. As the owner or occupier of a property with a septic tank system, you have a legal responsibility to ensure that it functions properly. Environmental Regulators and The Local Authority have the power to prosecute offenders and can serve notice to ensure that remedial action is taken to improve unsatisfactory systems.

Purchasing a property
If you are considering buying a property with a septic tank system, ensure that the drainage field is inspected prior to signing contracts and ask the householder if a maintenance log is available. With new houses which have a warranty, make sure that it covers the system. With other properties, a site inspection which includes the inlet drainage, the tank and the drainage field should be carried out by a competent surveyor. Your building society or bank valuation report will not cover this.
Most domestic systems do not require registartion or a discharge certificate from the Environment Agency unless they are situated in a sensitive area, as defined by the general binding rules.
Image of a small sewage treatment plant. Useful advice
    ─ Drain all the wastewater from your home into the system as any of these waters may contain disease-causing germs or environmental pollutants, but not rainwater.
    ─ Use bleaches, disinfectants, sterilisers or germicides sparingly, as they could kill the bacteria which help digest the waste. Normal domestic cleaners are biodegradable and should not impair the function of the tank.
    ─ Inspect the system regularly, including water levels in the distribution chamber and the drainage field.
    ─ De-sludge the tank at least once a year.
    ─ Act immediately if you find a blockage or any sign of pollution do not allow effluent to collect on the ground surface.
    ─ Keep records of inspections (e.g. water levels), de-sludging activities and any maintenance undertaken on permanent log sheets.
    ─ Ensure that all covers can be secured, but removed when necessary.
    ─ Ensure that air vents are not blocked.
    ─ Ensure that the drainage field is protected do not dig in it, drive over it, plant anything over or near it except grass, or cover it with a hard surface such as concrete or asphalt.
    ─ Don't dispose of rags, nappies, sanitary items, plastic or other large solids in the tank they will cause blockage and backing-up.
    ─ Don't pour paints, solvents, hazardous chemicals, pesticides, fats, oils or heavy grease and engine oil down the drains.
    ─ Don't use caustic solutions to unblock drains use boiling water or drain rods instead.
    ─ Don't allow rainwater to drain into the tank or surface water to flood the drainage field. Flooding will sweep solids through the tank and inhibit treatment in the drainage field.
    ─ Don't ever enter a septic tank toxic gases are produced by the natural treatment process.

PDF Download a leaflet giving information on general binding rules.

PDF Download a table giving showing the general binding rules.