Water and Effluent Treatment Magazine
Issue link: https://read.utilityweek.co.uk/i/411335
Two plants on a small site – the Millbrook challenge • Southern Water's Millbrook wastewater treatment works upgrade, like so many in the UK, is an upgrade of an old existing works. Dean Stiles looks at the complex challenges to overcome the site's constraints. TECH KNOW • Four-stage Bardenpho activated sludge plant • Treatment of the made-ground layers using Vibro-stone columns • Design uses a propped cantilever system with tie-beams between opposing walls • 10,000m 3 of concrete and 1,500 tonnes of reinforcement placed • More than 1.5km of new pipelines up to 1.2m in diameter have been laid together with in excess of 3km of cable ducts MOANS & GROANS • 33,500m 3 of process volume squeezed into constrained site • Site had to maintain consent compliance throughout construction • 8-tonne beams craned into position T he original Millbrook wastewater treatment works was built in the 1930s when it comprised a series of rectangular storm tanks. Up- grades in the 1960s and 1990s saw the addition of a carbona- ceous activated sludge plant to provide secondary treatment. The site receives wastewater from the Southampton area with a current population equiv- alent of some 135,000. Located within the Western Docks, the site is sized for a flow to full treatment of 850l/s before dis- charging into the River Test estuary. The upgrade was triggered by the Environment Agency's (EA) National Environmental Programme that introduced a new discharge permit condition of 10mg/l of total nitrogen to satisfy the Habitats Directive. The existing activated sludge plant was not designed to pro- duce effluent anywhere near this standard. The £25M upgrade was car- ried out as part of Southern Water's AMP5 plan, and deliv- ered by 4Delivery, the joint ven- ture between Veolia Water, Cos- tain and MWH. It was completed in September 2014. "The main challenge for Southern Water was how the nitrogen limits could be achieved using the existing assets at the works while main- taining capacity on a site that had space issues," says Stewart Garrett, senior project manager, Southern Water. "With two existing plants in place there was not a great deal PROJECT SPECS • £25M capital works project • The upgrade of Millbrook WwTW is one of the largest capital non-infrastruc- ture schemes delivered on behalf of Southern Water for its AMP5 investment programme • Built by 4Delivery, a joint venture between Veolia Water, Costain and MWH • Delivery of the scheme was a chal- lenge given the scale of the new infrastructure required and the myriad of physical constraints. • Meticulous planning of the two- staged schedule with the utmost heed to health and safety, existing plant operation, and the environment. • Completed September 2014 THE VERDICT • No reportable accidents or environmental incidents during 250,000 man-hours worked over 30 months • No compliance issues throughout construction and commissioning • 90% of waste materials either reused on site or recycled off site • Collaborative approach with early input from the construction team of room and space was going to be an issue. The irregularly shaped footprint available to use, overhead high-voltage cables, shallow groundwater table, poor ground strength, and existing buried services all had to be considered," Garrett says. The process The treatment technology selected to provide the total nitrogen removal function was a four-stage Bardenpho activated sludge plant (ASP) process, which differs from conventional carbonaceous ASP in its ability to remove nitrogen through den- itrification to produce nitrogen gas, says Paul White, civil engi- neer at 4Delivery. "This is achieved through two alternating stages of anoxic and aerated zones. Nitrified liq- uors are returned to the upstream of the process where anoxic conditions and influent carbon enables denitrification to occur. Further reduction of residual nitrogen occurs in the secondary anoxic zone, but due to depletion of carbon in the mixed liquor (BOD/COD reduc- tion), an external source is pro- vided in the form of methanol dosing." An additional 80% of settle- ment tank capacity (surface area) was provided for removal of biomass from the secondary effluent to ensure clarity of the final effluent. "Sizing of the Bardenpho ASP process had to consider the projected PE figure of 155,000 (2020/21 horizon). Further NOVEMBER 2014 WET NEWS 11 ONSITE Millbrook WwTW upgrade By phasing the construction, the existing site was given a significant boost in terms of available treatment capacity during construction