Minworth Thermal Hydrolysis (THP) Plant :

Minworth STW is Severn Trent Water’s largest sewage treatment works and serves a population equivalent of 1.75 million from Birmingham with full flow to treatment of 1,070Ml/d. The works treats sludge arising from a population equivalent of 2.3 million which is made up of indigenous sludge plus imported sludge from the nearby Coleshill STW and various works in south Staffordshire and north Warwickshire. Minworth is Severn Trent Water’s largest renewable energy facility with 8MW CHP capacity and biogas to grid which produces 30% of their green energy. The Thermal Hydrolysis Plant (THP) project will further enhance renewable energy production by increasing the biogas yield.

As part of their PR14 submission to OFWAT, Severn Trent made a commitment to cut their total carbon footprint by 10% through AMP6 and convert 25% of all of their biosolids to an enhanced status product. The THP will have capacity to process 250 tonnes of dry solids per day at peak with 3 (No.) streams and 198 tonnes of dry solids per day at average throughput. Severn Trent expects to realise significant benefits as a result of the investment required for a THP of this scale which will be one of the largest in the UK. These include:

  • TOTEX benefit from revenue increase and OPEX savings.
  • Enhanced status sludge cake (pathogen free).
  • Reduced carbon footprint (this project delivers a third of STW’s 10% reduction target for AMP6).
  • Improved dewaterability of the sludge to increase cake dry solids and reduce volume.
  • Increased capacity for sludge treatment from existing assets avoiding CAPEX.

Plant description

The new THP has been installed upstream of the existing digesters. Liquid imports, indigenous thickened surplus activated sludge (SAS) and screened primary sludges are blended in existing tanks then transferred utilising existing ram pumps to new buffer tanks for dewatering by centrifuges. Cake transfer pumps with boundary layer injection will transfer the dewatered cake (target 22% dry solids) to three reception silos then after further dilution to 16.5% into the three parallel Cambi THP streams.

Each stream consists of a pulper, 4 No reactors and a flash tank. Steam is supplied by 3 No dual fuel boilers with waste heat recovery from 5 No CHP gas engine exhausts. The hydrolysed sludge is diluted 3:1 with recirculated sludge, then cooled in heat exchangers and pumped to the existing 12 No digesters.

Each THP stream has 2 No heat exchangers each connected to a pair of digesters. Biogas will be stored in 2 No new gas holders then boosted to 33mbar to supply the CHP’s and the boiler siloxane plant.

Digested sludge will be dewatered using the 10 No existing Klampress units and the enhanced product will be stored on the cake pad. Final effluent will be filtered in two stages with 300-micron and 50-micron auto backwashing strainers then sterilised with UV to prevent contamination of the enhanced product.

Stantec were awarded the £48.4million project by Severn Trent Water and were engaged as the principal contractor to design and build the new TH plant. Stantec then contracted with Franklyn Yates Engineering to deliver various mechanical scopes of work. This scheme followed on from previous successful projects delivered on other sites working with the same management team lending itself to a very lean delivery process with a very proactive approach to managing the programming of the whole of the works whilst at the same time interfacing with other stakeholders and sub-contractors.

Franklyn Yates were selected specifically to design, supply and install various pipework systems, access platforms and support structures from an overall 3D model developed by Stantec. The new pipework systems were to be integrated into existing operational process plant and ultimately forming part of the new TH plant.

The process systems within Franklyn Yates contract were as follows;

  • Primary Screening area
  • Digester Sludge Transfer
  • Dewater feed pumping station
  • Centrifuge area
  • UV Plant

As the Severn Trent Water site is an operational site, any interface with existing systems needed to be planned with the Stantec and Severn Trent Water site management teams to minimise any down time and mitigate any knock-on effect to the overall operational systems during the critical interface work.

Due to tight time constraints, challenging logistics, difficulties interfacing with other contractors works and the complexity of the overall design and length of the pipework systems to be installed, Franklyn Yates had to site run all the pipework to mitigate programme creep and the risk of not meeting the necessary compliance dates. There was over 600-metres of DN250 & DN300 manifolds, spools, spigots, fittings, couplings, valves and flowmeters that needed to be installed alongside and above existing pipework that again threw many challenges at the designers and the site installation team throughout the project. This was further complicated with the need to design and install supporting structures that had to also act as access platforms, walkways and stairs.

As built drawings including 3D model drawings were forwarded onto the client following completion of contracted work.

If you would like to find out more information on the overall works completed on site you will find a useful case study on the link below: