PROJECT FOCUS
ate screw PS before splitting again, with 60% of total flow going to the existing hybrid ASP and 20% to a new ASP. The new 18,500m3 activated sludge plant (ASP) at Blackburn Meadows comprises a mixed single anoxic zone that can treat 1,900m3 sludge. Flow then splits equally across four Hydrok fine-bubble diffused-air (FBDA) aeration lanes which are the heart of the system and use membrane diffusers.
Four new 35m-diameter final settlement tanks (FST) will be equipped with half-bridge scrapers complete with a dipping scum-box removal system, peripheral launders, scum boards and v- notch weirs. Scrapers on the existing two hybrid and four Simplex FSTs are being replaced and a new dipping-box scum removal system applied. The existing Simplex ASP anoxic zone mixers are being replaced, while the existing surface aerators will be retained and will be able to provide sufficient aeration for future loads. The storm tanks and storm pumping system are also being overhauled. A new automated Venturi flow-mixing system will keep solids in suspension, preventing them settling in the bottom of the tank. This negates the need for a scraper and makes the system self-cleaning, reducing odour.
The FFD plant is due to be commissioned in September 2014. According to Kevin Smith project manager for Yorkshire Water’s Asset Delivery Unit, one of the major challenges for the project was the constraint on space at the 72ha site.
passes to four new fine-perforated plate- screens, operating on a duty/duty/duty/standby basis so that three screens can treat full flow of up to 34m3/h, with a bypass channel. Three screenings handling units will discharge com- pacted and dewatered screenings to skips for periodic disposal.
When the flow exceeds the maximum flow- to-full-treatment (FFT), the overflow will be channelled by a pair of automatic storm weir penstocks receiving a signal from submerged flow meters downstream, passing temporarily to the storm tanks.
Sedimentation
Eight new 40.5m-diameter primary sedimenta- tion tanks (PST) will treat the screened sewage, while outlet flow control at the tanks’ perim- eters will be managed by v-notch weir plates. The flow is then split, with 20% flowing directly to the existing Simplex activated sludge plant (ASP) and 80% passing through an intermedi-
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Extensive modelling was required to fit in the new inlet works on a very tight footprint, further constrained by overhead cables from an electricity pylon at the site. Similarly, there was no room for a new channel to carry flow to the new PSTs, so the screened sewage channel now runs inside the existing channel.
Anaerobic digestion
In the second major project taking place at the Blackburn Meadows, anaerobic digestion (AD) is being developed through the installation of a sludge digester with a capacity of 16,000t/y dry solids (DS). The bio-energy digestion (BED) plant will allow the ‘hot stand-by’ of two incinerators, which the partners say will deliver significant opex benefits to the business, reduce sensitivity to rising energy prices and significantly reduce the whole life cost of the catchment’s sludge disposal route.
The AD plant’s throughput will be enough to deal with current sludge production as well as the additional sludge produced by the FFD scheme. It is based on a 16-day retention period with headroom for the rest of the plant for extended running hours in order to deal with any increase.
The £23.3M installation, which is expected to be commissioned in September, has a payback period of 4.8 years. The works include a new sludge-thickening plant.
Air is pumped into the digester feed tanks to get a consistent quality before being thickened by Alfa Laval thickeners. Three pumps feed sludge alternately to the two 6,575m3 digesters. Biosolids are retained for 15 days at a temperature of 35oC. Gas mixing ensures effective transfer of the bugs and efficient generation of biogas, which will be collected and stored in a two-membrane Kirk dome gas holder.
The electricity and heat generated by the gas in the new 1.9MW CHP plant will be used for heating the digesters. Boilers running on biogas and natural gas will supplement the CHP in the winter.
The digested sludge will be transferred to a centrifuge dewatering area where it is taken to 25% dry solids. It is composted with green waste or sawdust and stockpiled prior to recycling to land. The decline in Sheffield’s steel industry has seen a lowering in the level of met- als in wastewater; this means that the sludge, while not a Class A product, can now be used in agriculture.
A Plasticon biological odour control system is also being installed to help reduce complaints from neighbours of the site.
Challenges
Along with the site’s compact footprint, exceptional rainfall in 2012 impacted on the project. Mark Harrison, site batch manager for ETM told WWT: “The biggest challenge is time; programmes are getting more and more squashed. The weather last year – river levels were up – water was running through the underground strata, which is where we needed to be. It cost us three months.” Complex environmental legislation also posed issues. “Environmental matters are now as high on the agenda as health and safety,” Harrison said. “The legislation is very convoluted. Even the Environment Agency admit some of their regulations go round in circles.”
Future plans
As part of Yorkshire Water’s long-term carbon and energy plan to become energy neutral by 2030, the utility is considering the installation of a thermal hydrolysis plant (THP) at Blackburn Meadows to process 30,000t of DS. While this plant is not within the AMP5 proposal, the opportunity was highlighted in the Early Contractor Involvement investigation and the AD plant’s design ensures that a THP could be added at a later date. nnn
August 2013 Water & Wastewater Treatment 13
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