North Mymms Water Treatment Works (WTW), owned and operated by Affinity Water, is situated to support the North London water supply infrastructure. It has four ground water sources which are susceptible to both historic and diffuse pollution such as pesticides, bromate and nitrate.

By Robert Reeves MSc CEng MIMechE

Senior Mechanical Design Engineer
– Trant Engineering Ltd

One of the pesticides of significant concern to a number of Water companies at present is metaldehyde as it is not readily removed by conventional pesticide removal processes. Affinity Water has an Undertaking with the Drinking Water Inspectorate (DWI) to construct and commission a new treatment process for metaldehyde removal.

Affinity Water contracted Trant Engineering to design, construct and commission a new high-rate clarification plant with powdered activated carbon (PAC) dosing which will primarily target the removal of metaldehyde. The new process treats a segregated flow from one of the raw water sources which has been identified to have consistently high metaldehyde levels.

The high rate clarification process enhances the flocculation and settlement stages by ballasting the flocs with the addition of sand and polymer. Trant Engineering’s Design Department utilised historic raw water quality data to determine the relevant process parameters for the new treatment equipment, ensuring adequate metaldehyde, and turbidity, reductions were achieved without affecting the existing treatment processes and subsequent final water quality. Additionally, integration of an existing online metaldehyde monitor into the control system has enabled optimisation of the PAC dose.

Trant has extensive experience in the development and implementation of bespoke process solutions, applying the full expertise of our civil & MEP design and delivery teams to the seamless integration of specialist technologies into our Client’s existing process streams. Paired with our supply chain management processes and procedures which ensure we engage only those organisations demonstrating the highest levels of HSE & Quality Management, we’re able to reliably deliver projects in line with the value and programme expectations of our Clients.

At North Mymms WTW, through quality communication and a collaborative approach, the Trant Engineering & Affinity Water project team are successfully delivering a bespoke solution centered around the application of high-rate clarification technology and PAC dosing. This approach has been maintained throughout the detailed design, construction and commissioning stages, ensuring a positive outcome for the end user.