Marsh - Domestic water and wastewater treatment products

Septic tanks

Horizontal septic tanks

Horizontal septic tanks

Marsh Industries' horizontal septic tanks are available in capacities ranging from 2800 - 20,000+ litres. Horizontal septic tanks provide settlement and some biological treatment to raw domestic sewage (wastewater), however, the domestic wastewater is not fully treated and must receive additional treatment[...]



Septic tank for basic treatment to off-mains wastewater The Marsh Euro Septic tank offers significant technical improvements and cost-savings over current market options. The range, available in capacities from 2800-4500 litres, is CE marked to comply with Construction Product Regulations and compliments[...]

Basic treatment to off-mains wastewater - Septic Tank

Marsh septic tanks, tested and certified to EN12566-1 for installation throughout the EU, can be supplied as either “traditional onion shape septic” or low-profile "horizontal" (cylindrical body) tanks. Both types are suitably designed for high water table or hard rock conditions.

Normally the simplest and most economical means of treating wastewater from small developments, a septic tank holds sewage and allows solids to settle into sludge at the bottom of the tank. Here it is naturally broken down by a process known as anaerobic digestion, which provides settlement and some biological treatment. The effluent is not fully treated and must receive additional treatment before discharge to the water environment – the most common method being to spread the effluent to land via an underground drainage field.

Marsh septic tanks are manufactured using virgin resin (no chalk filler) and glass and is supplied complete with cover and frame. Two Lloyds registered lifting eyes or slinging points are fitted for ease of installation.

Delivery is normally four days from ordered placement.

Septic tanks are typically installed underground in a pea-shingle surround for normal ground conditions, however, concrete must be used for high water table sites.

For new properties, it would be advisable to check that Building Control will accept a septic tank in preference to an Ensign package sewage treatment plant.

Marsh Ensign package sewage plants are extremely cost-effective and are also tested to the highest European Standards. In addition, Marsh Industries offers the option of Polylok Filters prefitted to the outlet 'TEE piece' of the septic tank. Polylok filters can reduce suspended solids up to a further 40% - they are very easy to maintain and can extend drainage filed life (soakaway).

Marsh holds EN12566 certification for all domestic Sewage Plants, Uni:Gem Septic Tank Conversion Units and three complete individual ranges of septic tanks.

Septic tanks and cesspools have been used in the UK since the middle ages. Thankfully we have come a long way in the design and functionality of sewage disposal since then. A packaged sewage treatment plant should be the preferred choice of sewage disposal if the property is not permitted to install a new glass fiber septic tank. We can offer you two different profiles. The Marsh EN septic tank is produced in the traditional spherical “onion shape” and also in the low profile shallow cylindrical style. Both designs are fully compliant with UK building regulations and are also CE marked having been fully tested to EN12566-1.

Septic tanks can only be discharged into a percolation drainage system as set out in building regulations, not into a river, watercourse or ditch (only sewage treatment plants such as the Marsh Ensign can be discharged into a watercourse). Septic tanks are not suitable for areas of the country where the soil is of a heavy clay constitution. Septic tanks need to be regularly emptied (desludged) to prevent the solids entering the drainage field. A porosity test to ensure the soil is suitable for installing a septic tank must be undertaken prior to installation. Septic tanks do not treat Ammonia levels in raw sewage and can only reduce the solids by less than 35% when compared to sewage plants which can break down solids by up to 95%.

Shallow profile or low profile two stage septic tanks have become more popular in recent years, very often they are installed in areas of the high water table or hard rock where a normal spherical tank with its deep profile is not a practical solution. Shallow tanks are easier and more economical to install.

In normal ground conditions, all septic tanks should be installed on a 150mm concrete base and concrete surround (150mm). Marsh Industries provides detailed installation guidelines in the installation handbook supplied with the tank. The handbook also covers installation in wet heavy grounds and provides details of porosity tests and drainage area design. The main failing of a septic tank generally takes place during installation, where people do not read the instructions supplied or take short cuts. We do not produce polyethene or polypropylene septic tanks, all Marsh septic tanks are manufactured using glass fibre and resin from leading UK suppliers - we do not use chalk or calcium fillers which can reduce the strength of resins.

Marsh can also produce septic tanks up to 20,000litres. As a guide to sizing a septic tank, you start with a base figure of 2000litres for a four-person dwelling then add 150 litres per additional user. The smallest septic tank available from Marsh is 2800L. We supply our septic tanks with integral lifting eyes and either flat bottoms (spherical) or with legs (cylindrical) for ease of installation and health and safety compliance. Furthermore, our unique “keying in” lip has tremendous installation benefits.

Is your septic tank 2020 compliant?

Environmental Permitting (England and Wales) (Amendment) (England) Regulations 2014 came into force on 1 January 2015 and created General Binding Rules (GBRs) for septic tanks or small sewage treatment plants for domestic use. These rules are designed to reduce the level of pollution from sewage in the nation’s watercourses. Under the new General Binding Rules, if you have a septic tank that discharges directly to surface water, ie, a water ditch, stream, river, etc, you must upgrade or replace your septic tank to a full sewage treatment plant system by 1st January 2020, or sooner if you plan to sell your property before this date.

Current methods of discharging wastewater from septic tanks

Previous to the General Binding Rules, wastewater from septic tanks were typically discharged in the following two ways:
septic tank installations
Wastewater is released through a network of pipes in surrounding sub-soils, providing an additional form of treatment for the waste from the septic tank. This helps to ensure that the wastewater being dispersed does not cause pollution.

Septic tanks discharging into a drainage field are not affected by the 2020 septic tank regulations.

Septic tanks discharging into a drainage field
Wastewater flows through a pipe directly into a stream, river or lake.

Binding rules state that direct discharge from a septic tank into a watercourse is not permitted.

So, what are your options?

If you have a septic tank and it is currently discharging to surface water, you have four options:
Septic tank option 1
Connect to the main sewer This may not be possible in remote locations, although pump chambers can be used to direct wastewater to the mains sewer.
Septic tank option 2
Install a drainage field (Designed in accordance with BS6297) This allows the septic tank to discharge wastewater into the ground instead of surface water. In many cases, the installation of a drainage field is not possible due to site constraints such as space and/or the soil not having sufficient drainage potential (ie, rock, clay, high water table).
Install a drainage field
Install a Uni:Gem septic conversion unit A Marsh Uni:Gem can be installed and connected to a pre-existing septic tank. This treats the wastewater to a sufficient quality allowing it to be discharged to surface water.
Septic tanks underground
Upgrade your septic tank to a Marsh sewage treatment plant A Marsh sewage treatment plant eliminates the need for a drainage field and ensures that the wastewater is fully treated before being discharged.
For more detailed information and guidance, please contact the Marsh technical team on or call 01933 654582.

Septic tank compliance leaflet

Download our helpful guide


A cesspool is a watertight tank with no discharge pipe. No biological treatment is involved. The smallest cesspool permitted in the UK for two people is 18000 litres plus an additional 6800litres per person. To put this in perspective, a 20000litre tank is 8m long x 2m diameter and requires approximately 20 tonne of concrete. A cesspool is an extremely costly, long-term option, with constant emptying by a registered waste carrier. Cesspools are not permitted in Scotland. For advice on installation or tank size selection please contact the Marsh Sales office on 01933 654582.