A Guide To Off-Mains Drainage & Drainage Solutions

For the majority of properties in the UK all waste and foul water is fed to the main sewer system and transported to a central public sewage treatment works. However in many rural areas where there is no possibility of connecting to a wastewater treatment works, it is the property owners/builders/contractors requirement to look for an off-mains drainage solution.

So, whether you are building your own home, a new commercial premises such as a hotel, offices, pubs or even a campsite what are your options?

Stricter environmental standards and building control regulations place a greater responsibility on specifiers to ensure the correct wastewater treatment system is chosen and that's where Waste Water Civils can help. With years of experience in the waste water industry we can advise on things like, effluent quality, treatment process and flow rates. 

Whether you are planning for domestic or commercial use, this page is designed to answer some key questions;

What options are there for off-mains drainage?

If your property is within 30m of a mains sewer you will generally be expected by the Environment Agency to connect to it, regardless of the cost or difficulties in doing so. In most situations this will be the most practical and beneficial solution to the property owner/s.

But what if your property’s drainage sits below the level of the mains sewer connection and can't be accessed via gravity? What if your property is more than 30m from a mains sewer? In these situations, you have three options to consider:

Pump — Treat — Store


In low-lying areas where the main sewer system runs higher than the main foul connection.

A pumping chamber/station consists of a large tank designed to collect the sewage from the property and store it until it reaches a predetermined level. Once this level is reached, an internal pump is activated that pressurises the sewage and pushes it out of the pumping chamber, through a pressurised mdpe pipe until it either reaches the main sewer or a point where gravity can take over.

There are still a number of important things you need to consider when thinking about using a sewage pump chamber or pumping station:

  • Pumps need to be sized and selected specifically for your property's needs in order to reduce the risk of fat and grease build-ups, blockages, and over or under working of the pump system (both of which can severely impact the lifespan of the pump)

These are things you need to know for sizing a pumping chamber or station:

  • Overall pumping distance
  • Overall pumping lift (vertical height between the outlet and the highest point in the discharge pipe)
  • Proposed inlet depth
  • Inlet size
  • Type of property
  • Number of users
  • Power supply
  • Storage capacity required for 24 hours of use without power (domestic/residential use only)

Treatment of the waste water

Sewage Treatment Plants

Often called package treatment plants, sewage treatment plants are the preferred off-mains waste and foul water drainage systems for both property owners and environment agencies alike. Technology has evolved from just storing sewage, as septic tanks do, to using multiple treatment chambers and nurturing bacteria growth to produce cleaner (or less polluting) effluent.

A treatment plant first separates solid and liquids then introduces oxygen, via an electrical component or RBC technology, to encourage aerobic bacteria growth within the plant. The bacteria breaks down the nitrates in the liquid while digesting some of the solid waste. At the end of the cycle the effluent is considered clean and safe to discharge into the environment.

Because the effluent is treated before discharge it gives you much more flexibility in where you can discharge it to.

Subject to consent to discharge through your environment agency you may be able to discharge directly into a ditch, stream or other watercourse straight from a sewage treatment plant. If this option isn't available, or permitted, then you would still benefit from needing a smaller drainage field than if installing a septic tank (approx. 20% smaller).

Septic Tanks

Septic tanks are sometimes seen as the cheaper option but often this isn't correct when you take in to account the cost of installing a drainage field soaraway. 

The septic tank doesn't actually treat the waste; this happens in the drainage field where naturally occurring aerobic (waste-degrading) bacteria in the soil works to break down any discharged effluent before being soaked up into the surrounding ground. 

The septic tank works by utilising gravity to separate the solids from the liquids, thereby creating the effluent to be discharged, whilst storing the remaining sludge for later removal by a registered waste carrier.

This makes for a fairly simple system with low running costs and low maintenance requirements. However, as we're becoming increasingly aware of our impact on the environment, the septic tank started to fall out of favour. There are now many stringent rules and regulations that apply to the installation of new septic tanks, as well as to existing ones.

  • They should be installed no closer than 7m to any part of a habitable building
  • Installation is not permitted in Zone 1 of a Groundwater Source Protection Zone
  • They can only discharge into a soakaway/drainage field which complies with Building Regulations or BS 6297
  • They can no longer discharge directly into a watercourse under new environmental legislation


Storing waste is usually the very last resort as usually the size of system required to meet Building Regulations is much much larger than that of a sewage treatment plant or septic tank. Also on average the cesspool is designed to be emptied once a month leading to an ongoing regular cost. 

Cesspools or sometimes know as cesspits, are essentially a holding tank for waste that must be emptied on a regular basis by a registered carrier. 

These systems are becoming much less popular given the range of new modern options available, and new cesspool/cesspit systems are even illegal now in Scotland.

If you're not in Scotland, and this is your last option, you must still make sure:

  • Comply with Building Regulations, be sure you have the space to install it - a minimum of 7 metres from a building and 2 metres from a boundary is required
  • Get planning permission from your local authority and follow national environment agency guidelines
  • For domestic installations: make sure it has a minimum capacity of 18,000 litres for 2 persons (plus another 6,800 litres per additional persons)
  • For non-domestic installations: make sure it has a capacity for at least the minimum requirements for the application