It is estimated that up to 3% of properties in the UK have off-mains drainage and sewage treatment plants, which includes commercial properties. These are typically commercial properties situated in more remote, rural, locations where a connection to the main sewer is not possible. The property will have its own private pipes and treatment systems, known as off-mains drainage.
Commercial properties include but aren’t limited to:
Owners of a commercial property with off-mains drainage have three options:
A package sewage treatment plant is a small-scale wastewater processing unit, where flows from a property pass through three treatment stages. In the first chamber, gravity separates out the water from the solids, which sink to the floor of the tank.
The liquid then travels into a second chamber fitted with an air pump or rotating discs that circulate air to encourage the growth of bacteria, helping to break down contaminants in the water, effectively cleaning it prior to discharge to the environment.
Systems with a third chamber make it possible for any remaining solids to settle on the floor of this tank before the treated effluent is discharged to a drainage field or watercourse.
Modern package sewage treatment plants are designed with an aerated process to handle the increased volume of water usage in everyday life. They offer significant biological breakdown of the organic and chemical contaminants by using air to circulate and treat the contaminants to a much higher standard than a septic tank.
Package sewage treatment systems clean the wastewater significantly, which permits effluent discharge into less porous soils which can then be discharged into a water course or a drainage field.
A septic tank is an underground storage container that collects wastewater flowing from a property and uses gravity to separate the water from the solids within the tank. The solids sink to the bottom of the tank, while the liquid flows out nearer the top, and soaks into the ground.
As a commercial business, you are responsible for a package sewage treatment plant or septic tank where your property uses an off-mains system. Depending on whether you select a package sewage treatment plant, or a septic tank, different planning rules and environmental regulations apply.
In England, you must make sure they meets the Government’s general binding rules for small sewage treatment plants and septic tanks. Different rules apply in Wales, Scotland and Northern Ireland.
Planning a package sewage treatment plant
Planning a septic tank
There are other things to consider too beyond the legal rules and regulations. These include:
Land considerations
When planning to install an off mains sewage treatment plant you need to consider assessments of topography and ground conditions, water table levels, drainage and water run off ability, seasonal temperature changes, and stormwater management.
Energy considerations
For the effective running of an off main sewage treatment plant, it’s important to ensure a reliable energy source to power the system’s pumps, aerators, and control units.
Considering the availability of electricity, whether from the grid, renewable sources (solar, wind), or backup generators for remote locations will ensure a stable energy supply, as power interruptions can disrupt treatment processes.
The system’s energy consumption should be compatible with your supply capacity and it’s important to verify voltage and frequency compatibility, especially in areas with unstable grids. Installing surge protection may be necessary to protect the equipment from electrical fluctuations or lightning strikes.
Seasonal flow variations
Many commercial businesses experience significant fluctuations in occupancy, particularly during the summer months. The sewage treatment plant must be capable of handling peak demand during busy periods without compromising performance. Choosing a system based on the maximum population size and the number of facilities is essential, ensuring it can adapt to both high and low usage periods. This adaptability is key to maintaining consistent treatment quality. You should also be able to avoid system overload and protect its functioning during winter frosts and long periods of inactivity.
Future site growth and expansion
As the popularity of some commercial businesses grow, so does the strain on its sewage treatment facilities. When specifying a system, it's crucial to consider current needs and potential future growth. This foresight ensures the system won't become obsolete as visitor numbers increase, safeguarding your investment, and ensuring continuous compliance with effluent standards.
British Water’s Best Practice Guide to Flows and Loads provides a table of loadings, which allows the total daily sewage load from different kinds of commercial sites to be calculated. It is recommended that all designers use this table when sizing and designing a non-mains sewage treatment system.
Swimming pool limitations
Clear rules must be followed to ensure the biological balance of your off mains sewerage treatment system and to ensure it is not disrupted by discharge from swimming pools or spas.
The discharge of swimming pool or spa back wash waters into a treatment plant is not allowed.
Compliance for specific sectors
There are some specific industry standards for sectors such as animal shelters, healthcare, campsites/caravan parks, and hospitality. For example:
Animal shelters: All water utilities specifically exclude animal faeces going to the sewer system, especially from dogs, and regulations for Scotland and Northern Ireland under the Guidance for Pollution Prevention 24 specifically exclude dog faeces going to sewer when a treatment plant is on site.
For England, shelters should contact their local water authority for specific rules if connected to mains and what the regulations are if an off-mains treatment plant is needed on site. See our dedicated blog here
Healthcare sector: British Water’s Best Practice Guide to Flows and Loads say that nursing home wastewater treatment plants, for example, must be capable of dealing with 350 litres of fluid per resident per day, rising to 450 litres per day per patient for small hospitals. See our dedicated blog here
Campsites/caravan parks: British Water’s Best Practice Guide to Flows and Loads provides a table of loadings, which allows the total daily sewage load from different kinds of camp and caravan sites to be calculated. It is recommended that all designers use this table when sizing and designing a non-mains sewage treatment system.
You need to be aware of discharge requirements. These may include gaining permissions to discharge treated effluent from the Environment Agency and meeting the general binding rules for small sewage discharges (SSDs) that took effect on 2 October 2023. See our dedicated blog here
Hospitality: disposing of FOGs (fats, oils & grease):
Restaurants and bars operating in the UK's food industry are legally required to dispose of FOGs correctly. Non-compliance can result in fines and forces closures - as stipulated in the Food Safety Act (FSA) 1990.
Installing a grease trap can prevent a build-up of FOGs and commercial food services should be aware of regulatory requirements regarding their use and maintenance to ensure compliance and prevent environmental damage.
A grease trap, or grease interceptor, prevents blockages in off mains drainage and sewage treatment systems caused by FOGs. Grease traps efficiently collect FOGs before they enter the treatment system, storing them in a separate container to be discarded correctly and legally.
A wide selection of grease traps are available to suit different businesses for different reasons including above and below ground grease traps, grease dosing systems, and grease removal units. For more information, read out dedicated blog on FOGs here
Environmental impact
Wastewater treatment systems eliminate the harmful microorganisms and pollutants in sewage that can cause illness and disease. Both package sewage treatment plants and septic tanks separate out many contaminants in the solids that are later removed by tanker.
The additional stages of treatment in a package sewage treatment plant means a higher quality of water is achieved before the treated effluent is discharged from the tank and enters the ground or watercourse. This gives greater protection to the environment, plants and wildlife.
There are tight regulations around discharging wastewater near watercourses and these are even stricter at Sites of Specific Scientific Interest (SSSIs) or areas targeted for Nutrient Neutrality. Discharges from septic tanks directly to a watercourse are not permitted under current environmental regulations. In this scenario, septic tanks should be replaced with a package sewage treatment plant.
High effluent standards are non-negotiable in such sensitive environments, and the risk of fines and reputational damage from polluting activity is high. You need to be aware of discharge requirements. These may include gaining permissions to discharge treated effluent from the Environment Agency and meeting the general binding rules for small sewage discharges (SSDs) that took effect on 2 October 2023.
Minimising visibility, noise, and odour
Reducing a sewage treatment plant's aesthetic and sensory impact is critical in a holiday or hospitality setting. Systems should be designed to be as unobtrusive as possible, with minimal noise and odour emissions. This consideration ensures that the sewage treatment plant doesn't detract from the natural beauty and tranquillity that holidaymakers or customers seek when in a rural location.
Landscaping and gardening
Gardening over a package sewage treatment plant and drainage field with care can enhance a commercial property aesthetically, while ensuring the all-important functioning of the wastewater system. For modern systems, odours are no longer an issue, and package sewage treatment plants can be located below-ground. Additionally, some sewage packaged treatment plants, such as the WCSEE HiPAF, have smaller footprints that conventional systems, which helps to ensure the continued aesthetic appeal. Because drainage fields are moist and nutrient rich, with careful landscaping and plant selection, it is possible to create a space that is both beautiful and functional.
Maintenance considerations
One advantage of having an off-mains drainage and sewage treatment system is that you will not pay water company sewerage rates. However, you will need to have the solids in your package sewage treatment plant or septic tank collected and removed regularly.
The price of the sewage treatment system and ongoing maintenance costs should be taken into consideration before you make an investment. Consider the whole-life cost of different systems - those with fewer mechanical moving parts and electrical connections are less likely to break down and will require less maintenance over their lifespan.
Maintenance requirements for commercial sewage treatment plants vary depending on the size, type, and age of the equipment. When selecting equipment, compare the length and scope of warranties for different systems.
When undertaking an installation, costs can vary depending on the site location, quality of the ground, the model chosen, and how long installation takes. Where an existing system needs to be removed, this would also need to be factored into estimates from installation contractors.
Understanding the process guarantee and whole life costs
It’s important to consider the whole life costs of the sewage treatment system, including installation, operation, maintenance, and potential upgrades. A system that may seem cost-effective initially but requires frequent and expensive maintenance can become a financial burden. The scalability of the solution should also be considered. How easily can it be extended as your operations grow, and at what cost? A robust process guarantee will offer peace of mind, ensuring the system will perform as expected over its lifespan.
Choosing the best off-mains sewage treatment system for your site will give you peace of mind for many years to come. Here is a summary of key points:
If your commercial property has off-mains drainage, you may require a package sewage treatment plant or a septic tank.
Planning rules and environmental regulations for package treatment plants and septic tanks vary per site and per sector, so check which option is best for your site and contact your local planning authority for advice and guidance (LPA).
Make sure you understand the environmental impacts of a new off-mains sewage treatment plant as well as maintenance and whole life costs.
If the property discharges to a watercourse, a package sewage treatment plant will be necessary.
Ensure your treatment system is sized to handle peak flows and loads at your property, as well as any future expansion, or energy consumption needs.
Careful landscaping and garden planting can ensure that sewage treatment systems can meet the aesthetic expectations.
Odour is no longer an issue with modern systems.
Please get in touch if you have any questions relating to off mains drainage
WCS Group is a trading name for WCS Environmental Ltd, registered in England and Wales (Number 02184649) at 20 Grosvenor Place, London, SW1X 7HN. Head Office – 17 Wheatstone Court, Waterwells Business Park, Gloucester, GL2 2AQ. WCS Group is a Marlowe Critical Services Company owned by Marlowe plc. 2025© WCS Environmental Ltd.