Challenge

Severn Trent Water had been operating the Longbridge STW with rock media trickling filters but, whilst the BOD reduction was being achieved, the existing plant would not be able to meet future ammonia consent levels.

Following an assessment of options, trickling filters with BIOdek plastic structured media offered a positive solution. Consideration was given to replace some of the rock trickling filters with BIOdek plastic media.

However these filters were in good condition and so it was decided to add plastic media nitrifying filters to the plant to provide the ammonia reduction required.

Solution

2H worked with Severn Trent Water, Costain and MWH to build a tertiary nitrifying trickling filter solution.

MWH were Severn Trent Water's preferred design engineers and Costain their preferred contractor.

BIOdek plastic structured media can be suppplied in various configurations and sizes.  For this application, 2H proposed their FKC615 PVC media with a specific surface area of 190m2/m3.

From their discussions with Severn Trent, MWH and Costain agreed the design for two filters initially but, as a future proofing measure, allowed for the easy addition of a third filter when necessary to handle an expected increase in the load coming to the Works in the future.

Costain built 2 no tanks 16.2m diameter and 2H supplied and installed the timber matrix media support system and a total of 1,484m3 of BIOdek media into the tanks to a depth of 3.6m.

Timescales became short but 2H was able to deliver and install the support system and media within a 2-week period to keep the overall project on schedule.

Outcome

On completion of the installation and following a seeding period to establish the full nitrification process, the filters were deemed a remarkable success.

The trickling filters are easy to operate and manage, with the rotary distributors being the only moving part.

The initial aim was to achieve treatment to meet a 5mg/l ammonia consent from the works. Sampling of the effluent demonstrated that the BIOdek trickling filters were achieving ammonia levels down to 0.4mg/l.