Water & Wastewater Treatment

20 | september 2014 | WWt | www.wwtonline.co.uk In total 68 16m beams were used to hold the treatment tank, each weighing eight tonnes O riginally constructed in the 1930s the wastewater treatment works (WwTW) at Millbrook in Southampton's Western Docks serves a population of approximately 135,000 and discharges into Hampshire's en- vironmentally sensitive Test Estuary. It has undergone two upgrades during the 1960s and 1990s and incorporates a carbonaceous activated sludge plant (ASP), however the latest upgrade is arguably the most significant to date. The project, which involved a £25M capital works scheme, was part of Southern Water's AMP5 plan and was delivered by a joint venture between Veolia Water, Costain and MWH (4Delivery) and completes this September. The physical constraints of the site itself heightened the challenge of keeping a working site operational while carrying out the upgrade work and protecting the environment says Southern Water senior project man- ager Stewart Garrett. "A key driver for this upgrade was the section of the National Envi- ronmental Programme (NEP) which specifies a new discharge permit condition of 10 mg/l of total nitrogen (TN). This was a new requirement that wasn't being measured previously. The main challenge for Southern Water was how this could be achieved using the existing assets at the WwTW while maintaining treatment capacity on a site that had space issues. It was Wastewater treatment Southern spends £25m to meet NEP conditions clear that Millbrook's existing ASP was not capable of producing effluent that met the new standards." The solution The treatment technology that South- ern Water selected to extract the TN was a 4-stage Bardenpho ASP process, which differs from a conventional car- bonaceous ASP in its ability to remove nitrogen through denitrification to produce nitrogen gas. Two alternating Project focus ● New discharge permit condition for total nitrogen drove upgrade ● process volume of 33,500m 3 ● tank is the length of 10 London buses Nick Myall FreeLaNce Water aNd eNvIroNmeNt WrIter ● The need to cut nitrogen levels in effluent being discharged into the River Test. The National Envi- ronmental Programme (NEP) specifies a new dis- charge permit condition of 10 mg/l of total nitrogen (TN) for WwTWs. ● Keeping the WwTW working at full capacity during the construction of the new Bardenpho activated sludge plant. ● The upgrade work had to be carried out in an area where the available foot- print for the new buildings was limited and where overhead cables exist. • Drivers stages of anoxic and aerated zones are employed to achieve the end result. "Nitrified liquors are returned to the upstream of the process where anoxic conditions and influent carbon lead to denitrification with a further reduction of residual nitrogen occur- ing in the secondary anoxic zone," explains Garrett. Treatment of the high-strength centrate and filtrate liquors gener- ated from the on-site Sludge Treat- ment Centre (STC), and that are then returned into the wastewater process, also had to be considered. A whole- life cost assessment showed that the most cost-effective and lowest carbon approach was to combine the liquors with the crude load and treat it in the Bardenpho process. This simplified the process on site and reduced the number of assets required, saving Southern Water money in the process. This also led to the replacement of the existing, shallow ASP tanks. How- ever, accommodating 33,500 m 3 of process volume into the limited space available at Millbrook site posed a significant challenge. Physical constraints and design Due to the limited space available and the number of existing build- ings on the site constructing the new ►

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