What do I need to know about Sewage Treatment Plants?


Do sewage treatment plants smell?

If a sewage treatment plant has a holding area, or primary settlement tank, then smells may occur as is often the case with septic tanks. However, plants which use a continuous aeration process like the WPL HiPAF Midi, or have no primary settlement and no septicity, such as the WPL Diamond and the WPL HiPAF Compact, they do not emit odours. Therefore the treatment process is odour free.

How do sewage treatment plants work?

The WPL Diamond range and WPL HiPAF range use an aerobic digestion process to naturally treat sewage and wastewater.  The sewage and wastewater enters the plant via an inlet pipe where it is then treated in the central aeration chamber.  Clarified effluent is then discharged via the outlet pipe.  For a more detailed description download our brochures.

What is the difference between a septic tank and a sewage treatment plant?

A septic tank is typically underground, receives untreated sewage and wastewater from a property and allows the heavier solids to settle at the bottom whilst the effluent rises to the top. Endemic bacteria helps the system break down the solids and sludge. The remaining liquids flow out of the tank into a drainage field. The effluent from a septic tank still contains about 70% of the polluted matter in the sewage, and further treatment is necessary, whereas a sewage treatment plant gives aerobic digestion of the wastewater contaminants, and treats to a much higher standard than a septic tank. Treatment plant effluent can then be discharged into a water course, ditch (with Environment Agency approval) or to a conventional soakaway.

Septic tank Provides partial treatment, settlement and separation of the solid waste and has an outlet pipe, normally discharging to a sub-surface soakaway or ditch. Septic tanks are discouraged because of the poor quality of what is discharged.

Sewage treatment plant is an aerated system that provides significant biological breakdown of the organic and chemical contaminants. Package sewage treatment systems “clean” the wastewater significantly, which permits discharge into less porous soils, smaller soakaways, drainage ditches and water courses.

Regulations, Planning & Permits

Is approval required to install a sewage treatment plant?

All sewage treatment plants need to be registered with the Environment Agency (EA) as they may require an environmental permit, or they will need to register for an exemption from this permit if they meet certain requirements. The environmental permit was formerly referred to as a ‘consent to discharge’ as it is an application to discharge treated water to surface water e.g. a stream or estuary, or to ground water via an infiltration system.

EPP2 (Environmental Permitting Programme): If the sewage treatment plant is eligible, such as the Diamond (CE Marked & EN12566-3) and HiPAF Compact (CE Marked & EN12566-3) it can be registered for an exemption from the need to have a permit under the Environmental Permitting (England and Wales) Regulations 2010. There are certain conditions associated with registering for this exemption concerning the design, installation and maintenance of the sewage treatment plant. If the treated water is being discharged from an approved domestic sewage treatment plant to a soakaway, the plant will usually be eligible to register for an exemption. All of the plants within the WPL domestic sewage treatment range will be eligible to apply for an exemption to the permit. The EA will inform applicants if they are not eligible for a registration and therefore that they require a permit.

Do plants have to comply with European Standard regulations?

Yes, all small treatment plants up to 50 persons must, by law, be fully type tested and certified with the CE Mark (see Diamond & HiPAF Compact certificates), accordance with EN12566-3 (see Diamond & HiPAF Compact certificate).

It is also important it complies with Environment Agency consent standards and is designed in accordance with British Water Codes of Practice Flows and Loads, to ensure correct sizing of all tanks. As per the statutory requirements, WPLs products are fully type tested and certified in accordance with all these regulations.


How are sewage treatment plants installed?

Our domestic sewage treatment plants are designed to sit below ground. A comprehensive installation manual is supplied with each plant, which you should read thoroughly prior to specification and installation. We recommend that you use an experienced and qualified installer – for details of those in your area, please contact a member of the WPL sales team.

Can I install the sewage treatment plant myself or can someone do this for me?

Full instructions are supplied with each unit WPL supplies.  We recommend you use a professional installer. If you need an installer you can find one in the installer network. They know our products well and have installed them several times.

How far from the building should the sewage treatment plant be sited?

Environment Agency Building Regulations (Part H-H2 Package Sewage Treatment Works) state that all sewage treatment plants should be sited 7m-10m from any inhabited part of the property. The building control officer for the site will make the final decision on exactly how far the plant should be sited.

If the discharge is to a soakaway a sampling chamber must be provided before the soakaway. Soakaway drains must be constructed in the aerobic soil layer, i.e. within 700mm of ground level.  Visit the Planning Portal website for further information.

Can I use the discharged water?

With further proprietary equipment the water can be used for irrigation.

What if there is a high water table?

The most suitable systems would be a plant with a pump. This means the plant can be installed above the water table to avoid problems such as overflowing or backfilling with flood water.

Can I connect surface water into a treatment plant?

Surface water cannot connect to the treatment plant.

I have been told you cannot fit a sewage plant into my driveway?

Civil installation experts can advise on whether this is possible or not due to specific site circumstances.

What if a property is on a slope and cannot use gravity to discharge the treated water coming out of the plant?

You would need a plant which has a pump that can cope with discharging up an incline. It is important to ask this question before placing an order.

Where can I discharge to?

The treated effluent can be discharged into a water course or ditch (with Environment Agency approval) or to a conventional soakaway.

The drain outlet is deep underground, can I still use a sewage treatment plant?

Yes, a range of turret extensions are available to suit site invert levels. On smaller tanks, whole body extensions are available for especially deep inverts. Always specify the required invert when ordering a system.

The WPL Diamond & HiPAF range offer one of the widest choices for invert extensions, giving it complete flexibility for different site conditions and installation requirements.

What is a percolation test?

A percolation test assesses how much water can drain away through the subsoil. This is performed by an experienced site surveyor who excavates a small area of subsoil to monitor. We advise this test is taken prior to a sewage treatment plant being installed.


How long will I have to wait for delivery?

WPL process their orders immediately and can deliver smaller standard packaged domestic plants to site between 7 to 10 working days.  For larger products such as the WPL Diamond DMC range and the HiPAF range, delivery can take 8-10 weeks.

How are deliveries arranged and what size lorry will be delivering the sewage treatment plant?

Off-loading will only be at the nearest roadway to site that is suitable for heavy goods vehicles. If there are any site restrictions these must be made aware of when ordering.  WPL’s plants are delivered direct to site, and transport can accommodate various site situations. A range of vehicles and off-loading options are available if requested.


How long does it take for a sewage treatment plant to start working?

Newly installed sewage treatment plants can be used immediately.  The biological process can take between 6-8 weeks to start working.

What quality of water is produced?

Water quality is defined by the level of three pollutants that are present in the effluent leaving the treatment plant. The three pollutants are:

  1. Biological Oxygen Demand (BOD) – a measure of organic pollution
  2. Suspended Solids (SS) – solids retained after filtration
  3. Ammonia (AMM-N) – breakdown of the organic nitrogen forms

These measurements are expressed in milligrams per litre e.g. 20mg/l (BOD), 30mg/l (SS), 20gm/l (AMM-N) – 20:30:20.

The WPL Diamond range treats wastewater to a 95% standard of BOD, SS, and AMM-N.

Care and Maintenance

What kind of guarantee does WPL provide?

All WPL Diamond and HiPAF packaged sewage treatment products are supplied with a lifetime process guarantee from date of delivery.

WPL warranty is as follows:

  • WPL Diamond – 20 years
  • WPL Diamond DMS/DMC Blowers – 2 years

These guarantees are all subject to our t&c’s.  For more information please see our warranty page.

Can I buy a Diamond or HiPAF direct from WPL?

WPL Diamond and HiPAF packaged sewage treatment systems can be purchased direct from us.

If however you are an end-user who also requires installation, it may be more cost effective to purchase our products through the installer network who can offer the complete package.

Why should I buy from WPL?

As an internationally-recognised leader in the design, manufacture and supply of both standardised and bespoke environmental solutions, WPL is dedicated to providing the highest quality products and services.

For more than 25 years, we have been dedicated to delivering superior sewage and waste water solutions that are fully supported by our qualified engineers and backed up by our quality guarantees. The result is dependable and innovative wastewater systems that consistently exceed our customers’ expectations.


What would happen in the event of a power cut?

In the event of a power cut, the tank will not cause any problems if without electricity for two to three days. If left any longer, septicity may occur.

What can be disposed into the sewage treatment plant?

To get the best from your sewage system, a comprehensive list of important information on what can be disposed into a WPL Diamond and HiPAF can be found in the manuals which can be located on the individual product pages or resource page.



How often do sewage treatment plants need to be emptied or de-sludged?

It depends on how the treatment plant works, and how much it is used. Some HiPAF plants have to be emptied every couple of months, and others like the WPL Diamond DMS can have extended emptying periods up to 3 years.  Reputable companies will know how to de-sludge our plants.

Do sewage treatment plants need service and maintenance?

The Environment Agency recommends that all treatment plants are regularly maintained to avoid polluting the environment. Most reputable manufactures will offer or be able to recommend quality service engineers to carry out this maintenance and they will also offer comprehensive warranties.

Service contracts for our larger domestic HiPAF Midi products are available through WPL.  For smaller domestic treatment plants, such as the WPL Diamond range which requires an annual service, and the WPL HiPAF Compact, service contracts are available through our installer network.