WPL Robust Aerobic Digestion System (RADS)

Sludge treatment process for sludge storage and reduction


The unique WPL Robust Aerobic Digestion System (RADS) is a highly effective and cost efficient sludge treatment system that significantly reduces sludge output up to 75% from an off mains drainage sewage treatment plant.

The aerobic wastewater treatment system is designed to incorporate the unique principles of WPL’s successful Diamond sewage treatment plant and offers extended de-sludge intervals. Analysis of WPL data collected from existing installations has shown that installing a RADS reduces de-sludge frequencies.

The WPL RADS can be retrofitted to existing treatment plants, septic tanks or incorporated into the design of a new packaged wastewater treatment system.


Features and Benefits of the WPL RADS:

Up to 4 times extended de-sludge intervals

  • Reduces tankering costs and frequency without placing significant biological or hydraulic load on the rest of the plant.
  • By holding sludge in suspension using WPL RADS unique airlift design further enhances bio degradation extending the periods between desludge.

Low maintenance

  • No internal mechanical moving parts therefore the plant requires minimal annual maintenance.
  • Reliable side channel blowers requiring the minimal maintenance.
  • Quick release bauer connector for de-sludging.

Reducing costs

  • Enforced by the Landfill Directive and ADAS safe sludge matrix.

Minimal footprint

  • An Integrated aerobic reactor and clarifier reduces the surface area of the plant.

Ease of installation

  • Only a small excavation is required due to the compact design saving valuable time on site.

Low visual impact

  • Below ground tanks available with only the lid showing at ground level.

Minimal impact on the treatment plant

  • From return liquors during times of high Mixed Liquor Suspended Solids (MLSS) load within the RAD reactor.

Minimal odours

  • Solids are kept in suspension using air, anaerobic conditions are avoided reducing the risk of septicity and associated odours.

Kind to the environment

  • The WPL RADS treatment process requires no chemicals.
  • Less tankering due to de-sludge intervals means fewer vehicle emissions into the environment.

Long life tank

  • WPL RADS is manufactured in glass reinforced plastic (GRP) with a life expectancy of 25 years.
  • All GRP used in WPL products is UV stabilised with an external gel coat finish which extends the asset life.

WPL’s Robust Aerobic Digestion System is a highly efficient and cost effective treatment option to consider in all sludge strategy decisions. It applies Best Available Technique values under the Integrated Pollution Prevention & Control (IPPC) directive.

Indicative Technical drawing



Specification Information


Model Range and Flow Rates


 Animal rehoming centre | UK

WPL RADS and HiPAF midi and Soma Dogs Trust

Animal waste in the UK is categorised as controlled waste.   Due to WPL’s unique robust aerobic digestion system, a significant cost saving is being made in de-sludging by extending the de-sludge tankering intervals, in turn reducing cost.  Reducing the frequency of de-sludging also means the initial cost outlay of the RADS is recuperated quickly with cost savings continuing for the whole operational life of the plant (up to 25 years based on asset life expectancy).

The WPL RADS can be part of the initial design or added to an existing treatment plants, or septic tanks for the treatment of domestic, commercial sewage or industrial wastewater.

The WPL RADS can be retrofitted to existing treatment plants, septic tanks or incorporated into the design of a new packaged wastewater treatment system.  The WPL HiPAF Midi and RADS have been successfully installed at many animal centres such as kennels, catteries and equestrian centres.

HiPAF Solution |Municipality Zgornja Kungota, Slovenia

  • WPL HiPAF, RADS, Conical Tanks and Walk-in Kiosk
  • The 3m WPL RADS used as sludge storage and sludge reduction.


Municipality Zgornja Kungota, western part of the Slovenian hills.

The project was co-funded by the European Union.

Power station solution | UK

  • WPL SAF, RADS, Lamella and Walk-in Kiosk

Image of WPL wastewater and sewage treatment specialist RADS Robust Aerobic Digestion System for aerobic wastewater treatment
The Power Station does not have the benefit of mains drainage to a water company sewage treatment works and therefore all waste water generated on site requires treatment before being discharged to the adjacent watercourse.

The RADS, unique to WPL, is part of the solution for extended sludge holding and minimising the requirement for off-site tankering.

 WPL RADS and de-sludge pipe with bauer connection

WPL RAD with quick release buaer connector for sludge treatment

How WPL RADS work

  • Sludge is fed to the unit through the inlet pipe from the primary settlement stage of the sewage treatment plant to the central reactor chamber; a circular tank with a sloped, open base
  • Located in the centre of the aeration chamber is a draught tube. This contains WPLs bespoke coarse bubble diffuser which feeds process air to the WPL RAD’s
  • As the diffused air rises in the draft tube a venturi effect is created, which circulates process fluid around the reactor, allowing suspended growth aerobes to stabilise the sludge and reduce its volume by endogenous respiration
  • A recycling loop is generated in the reactor in which aerated sludge continuously settles to the bottom of the tank and is drawn back up through the draft tube
  • Surrounding the aeration reactor is the integrated clarifier. As sludge’s continue to enter the aeration chamber, liquors are displaced from the reactor to the clarifier, in which quiescent conditions allow suspended material in the effluent to settle and be re-entrained into the aeration zone
  • The clarified (treated) effluent flows up through the clarifier and discharges via a dip pipe

Research and Development

A close partnership between WPL, the University of Portsmouth and investment by the Department of Trade and Industry (DTI), has allowed comprehensive research and development into the WPL Robust Aerobic Digestion System. This has enabled independent and in-depth analysis into effluent quality and sludge degradation, monitored to industry standards.

System development has also included clean water trials in controlled conditions. Laboratory tests have included trials using the industry recognised Sodium Sulphite test, to strip the oxygen from the water body before re-aeration. These trials have resulted in the development of a bespoke coarse bubble diffuser and the optimisation of the internal configuration of the system.

WPL RADS Robust Aerobic Digestion System  Inside a WPL robust aerobic digestion rad  above ground installation of a WPL robust aerobic digestion (RADS)