Submerged Aerated Filter (SAF)

The WPL Submerged Aerated Filter (Fixed Film SAF) can be used to either undertake full biological treatment or to complement existing process units for wastewater treatment. The submerged aerated filter (fixed film SAF) has randomly packed high voidage plastic media arranged as segmented cells to replicate a plug flow arrangement. Air is introduced continuously from beneath utilising air diffusers, and provides both process and scouring air for oxidation of the influent settle liquor.

The SAF process designs are utilised for the following applications:

Secondary SAF

  • For settled liquor from septic tanks and primary settlement tanks
  • Carbonaceous and nitrifying applications

Tertiary SAF

  • For polishing liquor from existing biological treatment units where carbonaceous BOD has been partially reduced, but where further BOD and nitrification are required to meet consent standards

WPL SAF units above ground installation, manufacturing options include:

  • TBTM tanks – Glass reinforced plastic (GRP)
  • Modular WPL Hybrid SAF tanks, flexible footprint and reduces on-site civil installation time
  • Stainless steel or steel tanks

The WPL SAF units are suitable for permanent or temporary applications, with a number of different hire and purchase options to suit requirement.

SAF units below ground installation, manufacturing options include:

Modular WPL Hybrid SAF – Installation at Sandbach WwTW

  • 12 WPL Hybrid SAF modules have been installed in four treatment streams at Sandbach wastewater treatment works (WwTW)
  • WPL Hybrid SAF units can operate as individual treatment plants or the modular design means they can be connected in series to form larger plants.
  • The SAF plants are built off-site in a specially controlled environment at the WPL factory, which minimises the time required for on-site installation.
  • The modular nature of the SAF units allows for challenging access.
  • Read full case study | Modular treatment plants deliver flexible upgrade for UU

Transportable units – Huyton wastewater treatment works


  • Five WPL ST1000 model units were required for this project, the modular nature of the SAF equipment means it can be readily redeployed to other sites and configured in any combination – as five units or just one.
  • The units needed to meet a demand of 5Ml/day so that the nitrifying tertiary filter could be taken off line.
  • Read full case study | Transportable units keep treatment moving for United Utilities

 High Performance Results – Danesmoor wastewater treatment works


  • Challenging access | eight steel modular SAF units were transported avoiding overhead electric cables and fit down narrow country lanes.
  • The SAF tanks, when on-site were lifted, rotated and placed on a prepared concrete slab.
  • Factory ‘kit,’ build | manufactured and engineered off-site meant that less workers were required at Yorkshire Water’s site, in turn minimising the risk of health and safety and quality control.
  • High performance results | infographic 

 Features and Benefits of the WPL SAF treatment plant:

Consistent process reliability

  • Sub 5mg/l ammonia and 10mg/l BOD

Low operating costs

  • Self-scouring, no need to de-sludge units

Low maintenance

  • No internal mechanical moving parts and safe working access for maintenance
  • Retention time’s air flow rates and blower control play a major factor in the design

Simple and easy configuration

  • When on-site with existing process solutions

Quick turnaround

  • From receipt of order to providing an on-site solution


  • Final engineering solution takes in to consideration ease of transportation to difficult access sites


  • Above ground solutions easy to move off site at a later date if the process is no longer required or relocate to another site

 Can tailor air use

  • Optimise energy consumption depending on site requirements

WPL tailor our robust and reliable SAF treatment solutions to meet our clients’ demanding specifications and expectations.

WPL HiPAF sewage treatment range:

The WPL HiPAF brand (High Performance Aerated Filter) references the unique hydrodynamic profile within the filter bed.  The benefit is a stable even biomass growth, during high or low flows meaning WPL is offering higher treatment efficiencies over other similar solutions on the market.

The WPL HiPAF Range is manufactured from glass reinforced plastic (GRP) and installed below ground.  The range includes:

  • The WPL HiPAF compact HPC (1 to 60 population), a primary tank, submerged aerated filter (SAF) and final settlement as one unit. Installed above or below ground
  • The WPL HiPAF midi (up to 300 population) is an all in one unit but also has the flexibility to have a separate primary tank but integrated SAF and humus settlement depending on customer requirement. Installed below ground
  • The WPL HiPAF modular (up to 2000 population) is a WPL standard primary tank, separate SAF and humus settlement tank.  The modular design can be configured with additional options depending on site requirements.  Installed below ground
  • WPL HiPAF bespoke (up to 5000 + population) systems could use an existing primary tank on site, WPL’s SAF and combine with a WPL’s humus settlement tank, lamella or conical tank depending on customer or site requirements.  Installed below ground.

Bespoke HiPAF | above surface installation


Usually the packaged HiPAF is manufactured from glass-reinforced plastic and fitted below the ground. On this occasion the plant did not need to receive gravity flows so there was no need for a below-ground solution, allowing installation and maintenance to be more straightforward.

How the WPL Submerged aerated filter (SAF) works

WPL SAF sewage treatment plants  use a combination of fixed, fluidised bed and suspended biomass reactions. Flow from the inlet is driven through the randomly packed plastic media utilising the systems unique hydraulic design, with in the bed an ideal environment is created for carbonaceous, nitrifying and de-nitrifying (optional) processes to take place.

Air to oxidise the influent and to scour excess biomass from the filter is introduced continuously below each chamber by a series of diffusers. Each diffuser can be removed for maintenance without the necessity to shut down the sewage plant. Inert excess biomass scoured from the media pack will rise to the surface and gravitate to the outlet with treated effluent, for final settlement.

WPL’s robust biological treatment process is backed by proven WPL SAF technology based on 25 years empirical data.

 The company’s SAF processes:

  • Submerged aerated filter (fixed film SAF) – versatile modular sewage treatment systems that can be used to undertake full biological treatment or to complement existing process units in aerated fixed-film, anoxic fixed-film or aerated fluidised bed design options
  • AN-SAF (Total N treatment) – has been developed for applications that require Total N treatment. Pre and post anoxic sewage treatment is possible, in combination with the standard aerated process
  • Nitrifying-SAF Ammonia Treatment – technology for applications where the Biochemical Oxygen Demand (BOD) has been completely removed (below 20 mg/l average), but effluent is required to be fully nitrified, and further biological capacity is required for this application