THE SITUATION
Along with other wastewater companies, South West Water is under pressure to meet new discharge permit levels in place for AMP8 (Asset Planning Period starting April 2025) which have been set by the Environment Agency.
Having successfully completed a number of other projects with PWS, South West Water contacted us in November 2023 to discuss the oxidisation ditches at South Tawton, where they needed to comply with a new discharge permit phosphorous level of 2mg/l.
The ‘Oxidation Ditch’ was a popular design of activated sludge plant for a period in the second half of the twentieth century so there are several of these in the UK.
With this technology, traditional treatment processes for removal of phosphorus and nitrogen required chemical dosing, of which there are many disadvantages including cost, health, safety issues as well as sustainability concerns.
The Solution
Plantwork System’s CONVOX® is the conversion of an existing activated sludge sewage treatment works to Biological Nutrient Removal.
Biological Nutrient Removal in activated sludge plants is a recognised way to remove both phosphorus and nitrogen. In this specific case, only phosphorus levels were targeted, but nitrogen can also be addressed using our nutrient neutrality technology..
Plantwork Systems has spent many years developing a full BNR process to remove both phosphorus and nitrogen. Our system is called NUTREM®.
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.
We have been operating a fully functioning NUTREM® at Petersfield, Hampshire, for ten years; it was featured in the final report of the National Phosphorus Removal Trials.
There are many continuous flow activated sludge treatment plants in the UK and PWS is able to offer the experience and expertise gained developing and operating NUTREM®, to design upgrades for these existing activated sludge works.
The result will be removing phosphorus and nitrogen, without requiring any chemical dosing.
We call the process CONVOX®.
How Phosphorus Removal works…
The treatment process is re-arranged to favour the life cycle of Phosphorus Accumulating Organisms (PAOs).
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 RESULTS
The resulting phosphorus levels in final effluent, very comfortably exceed the new discharge permit level for phosphorus of 2mg/l.
It is to be noted that the aeration basin at South Tawton is an oxidation ditch, but in all other respects it is a normal activated sludge STW.
The principles of CONVOX® can be applied to all types of activated sludge plants to remove both phosphorus and nitrogen.
The Benefits
There is potential for reduced investment because we are using the existing assets in a new way.
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.
Article by Dave De Hoxar, PWS Chief Engineer