Whether a home, housing development, single business or business park, if a site isn’t connected to the mains sewage system, the waste from toilets, baths, sinks, showers, washing machines and dishwashers will need to drain into an alternative system.
Sewage is wastewater which has been used for a variety of purposes at home or in work, industrial or leisure activities. The wastewater contains dissolved solids, suspended solids and bacteria and other micro-organisms living in the water. On average, we all produce 135 to 180 litres of sewage a day, consisting of 99.9% liquid and less than 0.1% solid.
Dealing with all that waste every day can sound like a daunting prospect, so it’s important to get the right system for a large site.
While septic tanks and cesspits or cesspools could be a possible solution for some smaller sites, the only solution for a site with a population of over 15 people, according to Environment Agency rules, is a sewage treatment plant.
What is a sewage treatment plant?
Sewage treatment plants are suitable for everything from single homes right up to large industrial or housing developments. Whatever the size of a site or property, they are the only option if you want to discharge your waste to a stream or ditch.
A small sewage treatment plant, sometimes called a package treatment plant, does a similar job to a septic tank, but the sewage goes through a secondary treatment using bacteria, which produces non-polluting end products. Whether it is a small-scale domestic unit or a large-scale commercial unit, they all produce clean sewage that has minimal negative impact on the environment.
The job of a sewage treatment plant is to clean all the wastewater a building (or several buildings on the same site) produces, so that it can then be discharged safely into a river or stream. With all the harmful elements removed, the water produced shouldn’t pose a risk to plants, wildlife or fish.
All sewage treatment plants work in similar ways. First, the wastewater flows into the primary chamber, where gravity separates solids from the liquid. The solids sink to the bottom to form sludge, which will be removed by a tanker – usually once a year.
The remaining liquid flows into the second chamber for treatment. This is the biological zone, where compressed air is pumped in, encouraging naturally occurring aerobic bacteria to flourish.
The treated water then goes into the final part of the tank. Here the bacteria settle at the bottom, while the clean liquid flows out to either a watercourse (subject to Environment Agency consent) or to a ground soakaway field.
Most sewage treatment plants require a power source because they work by pumping in compressed air or by rotating discs. For a site which is off grid, the power would come from the main circuit board in a house or a generator. If several households or businesses are sharing a sewage treatment plant, there needs to be a shared supply with a separate meter to ensure one household or business isn’t left to foot the entire bill.
If you’re planning to discharge your treated water to a watercourse like a river or stream, it needs to abide by certain rules laid down by the Environment Agency. If it’s not clean enough or there’s lots of it, you will need to apply for a Consent to Discharge.
How big should a sewage treatment plant be?
The size of your sewage treatment plant depends on the size of the property/ properties and number of people that will be living or working there. It is important not to go too small! Your sewage treatment plant should be big enough for the number of people it will be serving. If you need to empty it much more than once a year, it might suggest it isn’t big enough for a large site.
You must size a sewage treatment plant for the potential occupancy of a site, not how many people are there at the time of installation. It can save money in the long run to over-estimate rather than underestimate when you are at the planning stage.
If you have a three bedroom house, that is big enough for five people, even if there are only two people living there at the time of installation. If the sewage treatment system serves a number of houses, it should be sized to meet the potential occupancy of all the houses on the site.
When it comes to sizing a sewage treatment plant for commercial use, you will need to know what the population equivalent (PE) number is. The PE relates to the number of workers using a building or site. For example, an office with 30 workers is thought to be equivalent to 10 domestic residents, so its PE number is 10. If a site has a 24-hour operation, the PE number would be considerably higher than for an office working approximately 9am-5pm on weekdays only.
For small or new businesses it is important to remember that the business has a capacity to grow – whether that is more staff members on site or more customers for a hospitality business. In such cases it is necessary to plan for full capacity, for example a hotel or caravan site with an average 80% occupancy should have a sewage treatment plant able to deal with 100% occupancy.
If your site has a fluctuating population eg holiday parks, campsites or hotels, it is important to do your research about the right sewage treatment plant for your site. While it is generally better to size up, many treatment plants don’t work well when underloaded below 50% of what they were designed to deal with, so it is essential to find a treatment plant which can cope with fluctuations and low population numbers at some points of the year.
Tanks for Everything stocks sewage treatment plants for up to 50 people. These are suitable for large properties or commercial enterprises, including small hotels, care homes, campsites, factories or restaurants. Sizing the correct plant for these businesses can be difficult, so it should be carried out by a specialist drainage company, architect or building surveyor. Alternatively, contact Tanks for Everything for assistance before purchase.
If catering or meal preparation is included in the site activities, a grease separator may also be required to ensure the efficient functioning of the system.
What rules do you need to follow?
There are a number of rules you will need to be aware of when operating a sewage treatment plant on your site.
Will you need to apply for a Consent to Discharge for a sewage treatment plant on a large site? The government’s binding rules that came into effect in 2020 state that you only need to apply for one if you are:
- Discharging over 5,000 litres per day to a watercourse
- Discharging over 2,000 litres per day to the ground
- Discharging to a chalk river
- Discharging in a site of scientific special interest (see Environment Agency website for more information)
If your site can’t adhere to the general binding rules, you will need to contact your local Environment Agency office, apply for a Consent to Discharge and pay an application fee, dependent on the amount of sewage you propose to discharge.
It's fairly easy to comply with the general binding rules, even for a large site. The main things you need to remember are:
- Check with the Environment Agency before installing a new system in case you need a permit.
- Speak to the local council to check the system will meet planning requirements and building regulations.
- Have the system emptied regularly by a registered waste carrier (at least once a year unless the manufacturer says otherwise) to ensure it doesn't cause pollution.
- Maintain the system regularly, getting problems or faults fixed immediately.
- Make sure you stick with the maximum discharge limit per day. (Work out how much your system is discharging at www.gov.uk/small-sewage-rules.)
- Only install equipment which meets British Standard BS EN 12566.
- If you sell the property, inform the buyer in writing that it has a small sewage treatment plant on site.
Go to www.gov.uk/small-sewage-rules for more information, especially if you are unable to follow the rules, are concerned that your system may be causing pollution or want to check if your system needs a permit.
It is important to keep your sewage treatment plant regularly maintained, to prevent pollution (and a potential costly fine), avoid expensive repair bills and keep your energy consumption low.
The key things to remember are:
- Keep maintenance records, so you know when to service and empty your system.
- Get the sludge emptied regularly by a registered waste carrier.
- Get your sewage treatment plant serviced regularly by an accredited engineer and fix problems straightaway.
- Check for signs of pollution regularly, including sewage smells, pools of water, sludge, foam, weeds or fungus growing in the river or stream. Call a specialist for help if you spot any of these.
- Avoid flushing wipes, sanitary items, nappies or anything else down the toilet.
- Avoid putting fats, oils or chemicals down the drain or sink, as they kill the bacteria that break down the waste (if you are a business dealing with a lot of cooking fats, you may need a sewage treatment plant with a grease separator).
- Use low or no-phosphate detergents, which are kinder to the environment.
The sewage treated by a sewage treatment plant must be domestic in nature, for example from a toilet, bathroom, shower or kitchen. But domestic water doesn’t just mean it comes from a house, it could also come from a flat or business such as a pub, hotel or office. The important thing is that the water treated by a sewage treatment plant doesn’t include excessive chemicals or things like food waste – none of which are designed to be dealt with by a sewage treatment plant. If a site or business uses these things, it is important to find an alternative way of dealing with waste.
For some sites, a sewage treatment plant may need planning permission, but that is at the discretion of your local authority. If a sewage treatment plant is part of a new build scheme, it will already be part of the main planning approval.
Whatever the size of your site or your sewage treatment plant, installation definitely needs to be left to the experts. Factors to consider include the type of terrain and soil you have, the length of pipework you’ll need and how deep to excavate the hole. You will need to employ a groundwork contractor for installing the tank and an electrician to connect it up to the power supply.
Making it work efficiently
It takes about six weeks for a sewage treatment plant to start to work. So if you have the option of installing it before the site is occupied/ fully occupied, you should take advantage of this! You can speed things up a bit and get it working more quickly by seeding the unit with the sludge from an operational aerobic sewage treatment system.
Sewage treatment plants are designed to deal purely with water and human waste. Wet wipes and sanitary products are a complete no-no. Normal household cleaning products can be used, in sensible quantities, but antibacterial cleansers are a bad idea. The sewage treatment plant relies on those microorganisms, so it is important not to kill them off.
Another thing to avoid getting into your sewage treatment plant is rainwater, which can flush out helpful bacteria and flood the system.
It is also important to avoid food waste wherever possible, which is harder for the system to break down.
If the site includes a number of households or businesses, make sure everyone is made aware of what can and can’t be processed by the sewage treatment plant, so that it can continue to do its job effectively.
It is important to locate your sewage treatment plant in the best possible place to serve your site, without causing any problems for residents or businesses. There are a number of factors to take into account when deciding where to site a sewage treatment plant:
- Building regulations state it should not be closer than seven metres from the foundations of a home or business premises, and 10 metres from the watercourse.
- Can the lorry pull up close enough when it’s time to empty the tank? It needs to be less than 30 metres away from some hardstanding, or the lorry will find it hard to get close enough.
- Putting the sewage treatment plant downhill from the building(s) it serves means gravity will help the wastewater along, reducing the need for extra pumps.
- Sewage treatment plants don’t smell much at all, but when they are being emptied, you probably will be able to smell it. Even though it is only once a year, it makes sense to position it as far away from inhabited buildings as possible.
- All sewage treatment plants make some kind of buzzing or gurgling sound. So it is best to keep them as far away as possible from bedrooms or places where people need silence to work.
While there are a lot of considerations when installing a sewage treatment plant on a large site – whether residential or business – if you do your research and work with professionals for the installation and ongoing maintenance, you should have a reliable system for your site for many years to come.
