HYBACS is a patented HYBrid ACtivated Sludge process. The core benefit of HYBACS® is that it enables an existing activated sludge process to be swiftly upgraded for increased capacity or performance, by simple offline installation of SMART™ units.


  • Enables an existing activated sludge plant to treat double the flow and load
  • Construction is off-line, so no interruption to the process
  • Modular construction enables rapid deployment and minimum footprint
  • HYBACS sludge is highly settleable
  • Reduces power consumption associated with aeration by up to 30%
  • Effective for nitrogen and phosphorus removal
  • Low capex, opex and whole life costs
  • Low embodied & operational carbon
  • Intrinsically odourless


The bottleneck in the conventional activated sludge process is the hydrolysis (breakdown) of nutrients into molecules which are small enough to pass through the bacterial cell wall. HYBACS overcomes this bottleneck using a unique attached-growth reactor, called a SMART™ unit, in which the conditions cause naturally-occurring bacteria to over-produce hydrolytic enzymes. The SMART unit can be seen as a hydrolysis reactor.

Hydrolysed material is absorbed by bacteria and carried into the activated sludge process downstream, where conventional oxidation and nutrient removal takes place. However, since the hydrolysis bottleneck has been overcome, the activated sludge stage can operate at a much higher loading rate. Hence more flow and load can be treated within an existing activated sludge process, and tighter discharge standards can be achieved without the need for additional tank capacity.

A secondary benefit of pre-hydrolysis in the SMART unit is that the high floc loading helps nutrients to diffuse into the centre of sludge flocs. This promotes the growth of large, dense flocs with excellent settlement characteristics. Thus a HYBACS upgrade enables both the aeration tank and the clarifier to accept a higher load.


Activated Sludge comparison

HYBACS® – Powered by the SMART™ Unit

High intensity reactor (SMART™)

Deployed in the first stage of the HYBACS® process, is a type of biological reactor comprising plates fixed to a rotating structure. The plates are 50mm thick and manufactured from mesh with a voidage of 95%, producing a biological environment
that supports a large quantity of attached biomass, specialised for hydrolysing COD in the influent wastewater.

Aeration from rotation

The biomass partially drains as the plates rotate, enabling convection in and out of the plates both of the liquor containing pollutants and bacteria and of the surrounding air.
Thus, as the plates rotate, the biomass is aerated from the atmosphere when above the liquor, and brought into contact with pollutants when submerged. This advective flow substantially increases the transfer rate of pollutants and oxygen to the biomass, increasing the hydrolysis rate.

Off-site Manufacture

SMART™ units are pre-assembled in steel tanks, enabling the highest quality standards during manufacturing and testing, and can be installed quickly and simply onto a flat surface reducing installation time.