Septic Tank FAQ

Posted on 19 May 2015 by Fabian Belin

General information about septic tanks

What is a septic tank?

A septic tank is used to treat and dispose of small volumes of wastewater, usually from single houses or a number of households that are located relatively close together. Septic tanks are part of the family known as “septic systems” which includes package sewage treatment plants, also called off-mains wastewater treatment systems, private sewage treatment systems or individual sewage disposal systems. Septic tanks are usually installed where there is no access to the mains drainage network.

What is the difference between a septic tank and a Sewage Treatment Plant (often referred as Package Treatment Plants?)

Septic tanks are a cheaper option for wastewater treatment as they don’t need an electricity supply, are cheaper to buy, to run and to maintain on a short term period. Small sewage treatment plants are normally powered by electricity which is needed to aerate the wastewater. Sewage treatment plants are also more expensive, but the final effluent is of a higher quality than the effluent from a septic tank which will always require further treatment in a soakaway.

Why are these systems known as off-mains ?

These systems are considered off-mains because they are not connected to the sewerage network / mains drainage.


How do septic tanks work?

A typical septic system includes a septic tank and a soakaway system. The septic tank digests the solid organic matter and separates oils and grease (floatable matter) and larger solids from the sewage. The liquid (effluent) is then discharged into the ground from the septic tank via a soakaway system designed to slowly release the pre-treated sewage water into the soil. Alternatively, a pump can be used to help septic tank effluent trickle through the soil to remove pollutants such as disease-causing contaminants such as nitrogen or phosphorus.

How do I know if my septic tank is properly sized?

The performance of a septic tank depends on whether it has been sized correctly. The tank must be of an adequate size for the daily flow produced by the users connected to the septic tank. (number of people connected to the tank)

What do I need to do to get my septic tank started?

In a good septic tank environment, the biomass will be “self-sustaining” which means that new bacteria will replace those that die. The regular addition of so-called “bio-augmentation” products is not normally necessary but might help the development of bacteria inside the septic tank.

Ignore the historical stories about placing a ‘dead animal’ inside your septic tank to get it started, this is not the correct solution.

Why do septic tanks fail?

Most septic tank failures are related to inappropriate maintenance; examples being when soil-based disposal such as soakaways are installed in inadequate ground conditions, on excessive slopes or in areas with high ground water tables… These poor septic tank designs cause hydraulic failures leading to water resource pollution. Poor maintenance such as irregular septic tank emptying (every 3 to 5 years in general), may cause solids in the tank to block the soakaway  and clog the complete system.

What items should not be put down the toilets if I am connected to a septic system?

Avoid pouring bleach into the toilet or into sinks. Limit the amount of fat disposed of to a minimum. Your septic tank is a biological solution for treating biological waste. Avoid putting the following items into your toilets or sinks: coffee grounds, disposable diapers, animal litter, hygiene products and sanitary items, cigarette ends, fats and greases, paper towels, paints, detergents and pesticides….these products may have a negative impact on the bacteria in your septic tank.

What should I do if my septic tank backs up into my house?

If sewage from your septic tank backs up you must avoid contact with the sewage. Wastewater may contain harmful pathogens. Cleanup personnel should wear protective clothing and wash and disinfect all equipment used in the cleanup thoroughly when the cleanup is complete. The polluted area should be dried out and not be used until it has been completely dry for at least 48 hours.

I have a smell issue with my septic tank: what is going wrong and who should I contact?

If you have bad smells from your septic tank, this may be a sign that he biomass has died. First find out what may have killed the biomass, this is likely to be a product you have poured down your sink or toilet. If this is the case you may have to add bacteria to the septic tank.

For any problem related to your septic tank or if your system needs servicing or emptying please contact your local septic tank specialist. We can provide a list of local professionals upon request.

Can septic tanks cause health problems because of their impact on water quality?

Properly sized and designed septic systems can provide acceptable sewage water treatment. However, systems installed that exceed the treatment capacity of the ground or poorly designed, poorly installed, operated or maintained systems can be a source of problems. The most serious problems involve surface water pollution and ground water contamination by nitrates, excessive nitrogen discharges or phosphorus pollution.

Why do I need to empty my septic tank?

Some of the settled solids (sludge) at the bottom of your septic tank will be digested by the anaerobic action of the bacteria and will disappear. The remaining sludge will build up over time and will need to be pumped out of the tank. Your septic tank won’t be totally emptied but “desludged”: A specialised contractor will pump the sludge together with the total water volume of the tank and then put the watery effluent back in. This effluent contains the necessary bacteria for the proper functioning of the tank.

Why does my septic tank have a scum on top?

This scum consists of FOG “Fat, Oil and Grease” from daily domestic activities such as cooking washing up etc. The enzymes will break down this scum over time. The scum in your primary tank can dry out and get too thick, stopping the air reaching the liquid effluent. The bacteria need oxygen for the aerobic degradation of the pollutants in the sewage.  A hard and dry scum (crust) means that it is the right time to get in a contractor to desludge your septic tank.

How often needs my tank desudging?

Your tank should be desludged following the manufacturer's recommendations. For septic tanks, empty the tank when the level of sludge reaches 50% of the tank’s volume. For sewage treatment plants, empty the primary tank when the level of sludge reaches 30% of the primary tanks total volume.

Desludging should be carried out by a registered professional in the wastewater industry. The local farmer (unless registered!) is not an option: use a licensed waste disposal contractor for the removal of sludge.

Septic tank legislation

What is the septic tank regulation in the UK and what has been done to improve  the maintenance and operation of septic tanks?

The Environment Agency has developed permits and consents to discharge to regulate the installation and operation of septic tanks assisting  local communities in improving water quality and to protect public health.

Some water resource protected areas benefit from more detailed regulatory programs, involving site assessments to determine the soils porosity and capacity for adequate treatment, ensuring that groundwater tables will not be threatened.

For more information about the reform of the regulatory rules to control small sewage discharges from septic tanks and small sewage treatment plants in the UK, or Environmental Permitting Programme Second Phase (EPP2) please read:

The Consent to Discharge has been replaced by the Permit to Discharge under this Environmental Permitting Programme “Second Phase” (EPP2). Unfortunately very few inspections of septic tanks are conducted after they are installed.

As part of the continuing Government programme set up to improve environmental protection and health issues EPP2 affects any septic tank and sewage treatment plant installation, maintenance and operation.

EPP2 applies to any sewage treatment plant or septic tank (including septic tank replacements) and focuses on areas where better operation can be achieved by significant improvements in overall septic system performance. Criteria includes the volume of discharge, the type of septic tank or treatment plant (EN 12566-3 2005 Certified), the location of the plant, the treated water discharge point, commissioning and maintenance contracts. EPP2 also states that desludging of the septic tank or sewage treatment plant should be done by a licensed company and according to the manufacturer’s guidelines.

Who was involved in developing the EPP2 guidelines for septic tanks?

A joint initiative between the Environment Agency and the Department for Energy and Climate Change in cooperation with DEFRA and the Welsh Assembly developed the Environmental Permitting Regulations (EPR).  The Environmental Permitting Programme “Second Phase” or EPP2, which are the guidelines for the management of wastewater treatment systems in the UK, is part of the Environmental Permitting Regulation.

What do we need to do when we are installing our new tank and how much does it cost to register my Septic system with the EA if I have to?

Whether you need to register your septic tank or sewage treatment tank with the Environment Agency (EA) or not depends on many factors. Contact the Environmental Agency to find out and make sure you follow the right procedure and comply with the law. Don’t worry: the registration of your septic system is free of charge.