A small group of committee members was given a tour of the sewage treatment plant at Walbutt’s above Strensall. Our thanks go to Yorkshire Water for allowing us this rare opportunity and to the staff at the site for welcoming us, showing us round, and answering all our questions in the process. The following are some notes that the group assembled after the visit.
- Walbutt’s receives up to 3500 cubic meters per day of raw sewage from Haxby, Strensall, Earswick and Huntington
- The first stage of treatment is a pair of screens (coarse and then 6mm) to remove debris such as wet wipes and sanitary products; these go into a skip for landfill.
- In the second stage of treatment, the flow is slowed to 0.3 m/s and grit is removed via a paddle wheel; this is also collected in a skip and can be recycled or sent to landfill.
- The third stage of treatment is a primary settlement tank. The flow is from the centre towards the middle. Sludge settles to the bottom and is removed via a rotating scraper.
- Aluminium sulphate (the only chemical addition made) is used to help the sludge settle more efficiently.
- Digesters work more efficiently on a larger scale than is possible at Walbutt’s, so sludge is removed by tanker for further processing at Naburn.
- The fourth stage of treatment is a biological filter comprising a bed of porous stones which provide a massive surface area for bacteria and various invertebrates to grow and remove both suspended solids and unwanted bacteria. There are four beds, each with several rotating spray booms.
- The fifth stage is a humus tank. There are six parallel tanks containing microporous beads (Bio-Pur) which act as a suspension medium for a final bacterial purification. This is intended to reduce phosphate, ammonia, aluminium and heavy metals. The effluent is aerated and pumped up the tanks. (Reed beds can be used instead of humus tanks for smaller throughputs, hence the name.) Sludge from this stage goes back to the primary settlement tanks.
- The output from the humus tanks is passed into a holding lagoon for final settlement to remove any further sludge created .
- At the output of the lagoon, water is tested for biological oxygen demand (BOD), chemical oxygen demand (COD), suspended solids, ammonia, iron, aluminium, and phosphorous as it is discharged into the Foss. Walbutt’s has to comply with two sets of legislation. The UK regulations require regular daily sampling both onsite and by a Yorkshire Water laboratory. To comply with EU regulations, there is a monthly check also carried out off-site.
- The site is supervised by a team leader backed up by a Technical Optimiser (who makes sure all assets are working) and a team of technicians who visit as required. Day to day operations are controlled by one operator.
- The Haxby (Cobbs Cottage) site is only a pumping station and pumps all its sewage to the Walbutt’s site. Both it and Haxby Storm Tanks STW have consent to discharge screened and diluted sewage into the Foss during storm conditions when the system is overloaded. Walbutt’s has neither the facility nor consent to do so.
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