Unique system for converting existing activated sewage sludge treatment works to Biological Nutrient Removal

What is CONVOX®

Plantwork Systems’ CONVOX® is the conversion of an existing activated sludge sewage treatment works to Biological Nutrient Removal (BNR).

Using CONVOX® we can remove phosphorus without any chemical dosing.

An example would be converting an existing aeration basin, such as an oxidation ditch, to comply with a new Discharge Permit Phosphorous level of 2mg/l.

The principles of CONVOX® can be applied to all types of activated sludge plants.


Why is it needed?

Discharge Permits for many Sewage Treatment Works have been tightened over recent years, and this is expected to continue in the AMP8 period (starting April 2025).

Both Phosphorus and Nitrogen are recognised as pollutants in rivers and other water bodies as they are nutrients that encourage the growth of algae, leading to depletion of oxygen in the water.

Biological Nutrient Removal in activated sludge plants is a recognised way to remove both Phosphorus and Nitrogen.

The alternative treatment processes for removal of Phosphorus and Nitrogen require chemical dosing. The problems with chemical dosing are:

  • Iron or Aluminium can be dosed to remove Phosphorus by precipitation.
  • To remove Nitrogen you can rearrange the activated sludge treatment process and dose with methanol (as a food for the biomass) near the end of the process to de-nitrify the treated effluent.
  • Chemical dosing is costly.
  • The chemicals themselves present health and safety risks, which have to be mitigated.
  • The chemical dosing increases the quantity of sludge produced, which needs to be further treated and disposed.
  • Where iron/aluminium is dosed to remove phosphorus the Environment Agency will set limits for the amount of iron/aluminium in the treated water.
  • There are sewage works that achieve the required level of phosphorus by dosing metal salts, but then fail their discharge permit by exceeding the permitted level of iron or aluminium.


The Solution

Plantwork Systems has spent many years developing a full BNR process to remove both Phosphorus and Nitrogen. NUTREM® is one of the treatment solutions we offer to achieve very high quality treated water. It is an activated sludge process in Sequencing Batch Reactors. The NUTREM® plant at Petersfield, Hampshire has been fully functioning for ten years and was featured in the final report of the National Phosphorus Removal trials.

We have used the experience we have gained developing and operating NUTREM® to design upgrades for existing activated sludge works, so that Phosphorus and Nitrogen can be removed without requiring any chemical dosing.

We call this CONVOX®.


how it works.. 

The treatment process is re-arranged to favour the life cycle of Phosphorus Accumulating Organisms (PAOs), to remove phosphorus and to achieve nitrification and de-nitrification to remove Nitrogen.

To favour the life cycle of PAOs we need to provided different stages in the activated sludge process;

  • A stage where the PAOs take in both phosphorus and excess oxygen from the sewage.
  • A stage where they take in food but release phosphorus back into the sewage.

Having set up these stages, the solids are settled in the final settlement tank when the phosphorus has been taken up by the PAOs. By now there is very little phosphorus in the clear liquid, which leaves the works as the final effluent.

The Phosphorus rich sludge is then managed to ensure that the Phosphorus stays locked in the solids, preventing it from returning to the main treatment process. This requires management of dissolved oxygen at each stage of sludge processing, to ensure the Phosphorus leaves the site in the sludge when it is finally taken off site. 

The Benefits

  • BNR is used instead of chemical dosing – less H&S restrictions, lower cost, more environmentally friendly.
  • Shorter installation time than implementing whole new plant.
  • Relatively easy and low cost to maintain.
  • Reduced investment – where existing plant is in good working condition, additional building work is minimised and the process goes through a clearly detailed upgrade installation with limited extra plant.
  • The desired result is achieved mainly by managing existing treatment works assets differently.
  • Preliminary treatment, if suitable, is retained.
  • Meets AMP 8 consent levels where low levels of Phosphorous and Nitrogen are required.