Treatment was originally by five trickling filters, three using plastic structured media and two using mineral media, followed by humus settlement and partial flow sand filtration, with final effluent being discharged into the River Derwent near wetlands of significant value for nature conservation with a wide diversity of habitats and a number of designated sites. During the winter months, lower temperatures resulted in challenges to meet ammonia consent and Yorkshire Water allocated £8 million to upgrade the works.
Process Selection
Stantec, and Aqua Consultants, carried out an optioneering exercise which concluded that the most cost-effective option was a moving bed biofilm reactor (MBBR) which would act as tertiary treatment handling the total flow from the trickling filters. MBBR is a well-established technology using a moving media on which the biomass grows. This gives a higher biomass concentration than is possible in a conventional activated sludge process and provides good contact between the wastewater and the biofilm for carbonaceous organic (BOD) removal, nitrification and denitrification.
Project Execution
Stantec and Aqua Consultants prepared an outline design for the MBBR plant. Ward & Burke was appointed as the principal contractor and worked with Stantec to confirm the commercial parameters. After technical and commercial tender adjudication ACWA Services of Skipton was selected as mechanical and electrical detailed design and build sub-contractors. ACWA Services designed, supplied and installed all the mechanical and electrical equipment including media, media retention grids, screens and air blowers together with instrumentation and control for the MBBR.
Filter Media
The media is critical to the MBBR operation and therefore it is vitally important to select media which will allow optimum treatment process performance, ACWA Services selected Warden Biomedia’s Bioflo+ 800. Although having used Warden Biomedia’s products previously, this was the first time Bioflo+ 800 had been used for an MBBR project at Yorkshire Water, so a review and comparison with the media previously used was undertaken prior to acceptance, which found Bioflo+ 800 to offer significant benefits including; large protected surface area, prepared media surface, improved durability and structural integrity.
Bioflo+ 800 is manufactured in the UK from high-density polyethylene. Its wheel-shaped structure with internal fins gives a protected specific surface area of 800 m2/m3, the highest in its class, giving enhanced nitrification and denitrification in a minimum plant footprint. A specific gravity of 0.96 gives it the buoyancy needed for the MBBR process whilst large openings allow free passage of wastewater helping to maintain a healthy and thin biofilm over the full protected surface area. The project used 380m³ of Bioflo+ 800 for the MBBR in this project.
Project Success
The project has secured the ongoing performance of Malton WwTW with the media enabling the MBBR system to achieve consistent effluent quality.
