WPL HiPAF® midi

60 up to 300 population equivalent (PE)

The WPL HiPAF packaged wastewater (sewage) treatment range is regularly specified by the highly regulated UK water utilities and meets stringent European environmental discharge consent standards.  This makes our products the preferred choice for sensitive locations such as Sites of Special Scientific Interest (SSSI).

The below ground WPL HiPAF midi is a compact packaged wastewater (sewage) treatment plant for properties of 60 up to 300 people equivalent, where mains drainage is unavailable. Each wastewater treatment plant includes a primary settlement, biological treatment and final settlement processes all in one compact unit.


Seasonal variations often experienced by hotels, caravan parks and cafés

Our discreet packaged plants can be design engineered to operate efficiently based on seasonal variations in loadings often experienced by hotels, caravan parks and cafés. Higher pollution loads and site expansion can be accommodated by adding WPL HiPAF modules. The unique design of the flow balancing in the primary tank means it can cope with variable flows and loads making it ideal for caravan sites and other seasonal venues.


WPL support

All WPL HiPAF solutions are backed by our continued support for the life of the project. With unrivalled specialist capabilities, we supply technical information and drawings; offering any additional assistance if required. This is to ensure a wastewater treatment solution that works effectively and efficiently, with minimal impact on the day to day running of businesses.

The WPL HiPAF midi is UK manufactured in glass reinforced plastic (GRP) which is ultra violet (UV)  stabilised with an external gel coat finish which extends the asset life to 25 years.


Why you should choose the WPL HiPAF midi system:

Ease of installation

  • Modular plants enable access and installation at space restricted sites
  • Overall compact design minimises excavation required offering small footprint and lower installation costs

Low maintenance

  • No internal mechanical or electrical moving parts
  • Optional low energy compressors with adjustable timers
  • Large easy access gas-strut assisted lids for safe, clean and accessible air diffuser system maintenance
  • Each diffuser (biological treatment) is capable of being isolated and removed individually without the need to shut down the plant or affect the air supply to the rest of the unit
  • Tankering made easier because primary sludge and humus are stored together

Energy efficiency

  • Optional duty-standby blower arrangement is designed to provide process air; additional scour is provided by the secondary blower when required, reducing energy use
  • Variable speed drives can be used to optimise power use

Minimal visual impact

  • Below ground installation means the plant does not impede views of the landscape
  • Discreet for use in tourist and leisure applications

Variable flows and loads

  • The unique design of the flow balancing in the primary tank means it can cope with variable flows and loads making it ideal for caravan sites and other seasonal venues


  • Fully compliant with UK Building Regulations and Environment Agency (EA) guidelines
  • Designed using the British Water Code of Practice Flows and Loads, to ensure correct sized tanks
  • Designed to BS EN 12255 – 1/15 and manufactured using BS 4994
  • WPL HiPAF treatment plants are CE marked where applicable


  • Proven WPL technology based on over 25 years empirical data
  • Unique internal circulation system leads to increased process performance
  • Use of high quality blowers and pumps underpins reliability of plant
  • Robust operation and stand-by facility
  • GRP is UV stabilised with an external gel coat finish which extends the asset life to 25 years
  • Quality controlled environment at the WPL factory (UK)
  • WPL tailor our robust and reliable wastewater treatment solutions to meet our clients’ demanding specifications and expectations


Standard commercial kiosk design comes with:

  • Weatherproof kiosk to house the control panel and air blowers on site
  • The kiosk is acoustically-lined, houses a control panel, which automatically manages the operation of the plant; the air distribution manifold and one or more air blowers as required
  • Thermostatic cut-off controls and air filtering monitoring which can alert the operator to loss of air pressure
  • Ten metres of high temperature resistant airline hose is supplied to connect blowers to the treatment plant – extra lengths can be supplied if the kiosk is sited further than 10m from the treatment unit

Image of WPL Midi compact sewage system installed at McDonalds  WPL HiPAF midi installed at a public house

Technical specification

For technical specification information (including indicative drawing), please download  the WPL HiPAF brochure (available on this page).

Typical commercial uses for the WPL HiPAF midi system:

  • Animal waste options such as stables and kennels
  • Agricultural growers and garden centres
  • Camping and static and touring caravan parks
  • Custodial establishments
  • Golf courses, country clubs and fishing lodges
  • Hotels, restaurants and public houses
  • Holiday Parks, lodges and resorts
  • Motorway service stations
  • Care homes and retirement villages
  • Commercial buildings and offices
  • Housing developments
  • Schools, colleges and training centres
  • Stately homes and large country estates
  • Villages and rural communities
  • Visitor centres, heritage sites, theme parks, activity centres and wildlife parks
  • Water utilities
  • Zoos, theme parks and leisure attractions and leisure centres

Design options:

  • Inverts up to 1.5m with turret extensions
  • Pumped inlet flow control
  • Control panels are available to cater for single and three phase electrical supplies
  • Alarm beacon for mechanical failure and loss of air pressure
  • Duty/standby blowers with automatic changeover to ensure an uninterrupted air supply
  • Energy-saving options such as variable speed drive blowers
  • GSM telemetry for remote monitoring of the plant
  • WPL Sand Filters/WPL Micro-screens can improve final effluent for strict consent requirements

West Gate WPL HiPAF Midi system installed below ground for off mains sewage treatment   Image of WPL HiPAF Midi sewage treatment plant at a campsite for off mains drainage

Three stage system for safer processing

Each WPL HiPAF treatment plant includes a three-stage process – primary settlement, biological treatment (biozone) and final settlement. The segmented biozone process chambers in stage two eliminate the risk of process short-cuts and mitigates risks associated with variable loads.


Stage 1 – Primary settlement

  • Incoming wastewater enters the primary settlement stage where organic and inorganic matter settles
  • The sludge is held in this section until it requires de-sludging
  • A unique forward-feed arrangement uses an airlift to deposit wastewater into the biological treatment stage (biozone), which keeps peak flows from entering the system and prevents hydraulic surges

Stage 2 – Biological treatment

  • Settled liquor enters the biological section either by displacement or via the airlift
  • High-voidage plastic media, contained between two floors within the biozone, encourages the growth of bacteria and other organisms which treat the wastewater
  • Air required for the treatment process is delivered by air-blowers housed in a weatherproof kiosk
  • The air also helps to scour the media bed, preventing the filter from blocking
  • A series of diffusers installed beneath the media bed ensure an even distribution of air

Stage 3 – Final settlement

  • The treated wastewater (final effluent) enters the final settlement section
  • Dead biomass (humus sludge) from the process settles out
  • An airlift automatically transfers settled humus sludge back to the primary settlement section for co-settlement
  • The final effluent is discharged through gravity displacement to either a watercourse or a sub-surface irrigation field