Water and Effluent Treatment Magazine
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10 WET NEWS NOVEMBER 2016 ONSITE WatER tREatMENt the five UV reactors and flow booster pumps. all Images courtesy of trant Engineering Revised design is a ray of light for Farlington WTW • a revised design for Portsmouth Water's century-old water treatment plant is paying dividends. trant Engineering's David Martin reveals all. projEcT SpEcS • Construct a UV irradiation facility at Farlington WtW within the existing membrane filter building • Construct wastewater recirculation facility to separate supernatant flow from process solids • Refurbish wash water holding tanks and manual inlet penstocks P ortsmouth Water's Far- lington water treatment works may be more than a century old but it continues to provide potable water to more than 280,000 customers in the Portsmouth and Havant area. Farlington's supply is derived from the springs – believed to be the largest spring source in Europe – with the raw water chlorinated at the spring source pumping stations. At the treatment works, the water gravitates from the inlet splitter chamber to 13 rapid gravity filters. When the raw water quality deteriorates, aluminium sul- phate is dosed upstream of these filters. The filtered water then gravitates into a common filtered water outlet channel where it is chlorinated. The filtered and chlorinated water then gravitates to the membrane filtration plant before being dosed with phosphoric acid upstream of the contact tank. All wastewater flows from the treatment plant are currently discharged off site. Having decided to address these cryptosporidium and discharge mitigation issues, Portsmouth Water issued a tender in April last year based on an outline design for treatment and process facilities. The solution developed comprised: • An ultraviolet (UV) irradiation facility at Farlington WTW for the inactivation of cryptosporid- ium, to be installed within the existing membrane filter build- ing to replace the existing membranes • A wastewater recirculation facility to separate supernatant flow from process solids and return it to the Head of the Works The NEC3 ECC Option C (Target Cost with Activity Schedule) contract was awarded to Trant Engineering in August 2015 as an Engineering Procurement Construction (EPC) contract. Trant carried out the detailed design in-house; process, mechanical, electrical and ICA, and is responsible for the civil, mechanical, electrical installation and commissioning. Trant Systems Electrical (TSE) is responsible for the design, manufacturing, installa- tion and commissioning of the new motor control centre (MCC) and modifications to an existing MCC for the works. The membrane filtration plant (MFP) had been installed in 2003 as the only viable DWI- approved process to remove cryptosporidium oocysts from the filtered water prior to entering the service reservoirs. Elevation of the sludge balancing tank, lamella clarifiers with Integral thickeners, and centrifuge building However, the membrane plant sub modules were deemed to be nearing the end of their serviceable life. The DWI revised the standard in 2008 to permit the use of UV disinfection as a barrier to inactivate the oocysts. Potential risks The initial outline tender design to treat up to a maximum of 100Ml/d included a significant number of potential risks associ- ated with the construction of a new UV treatment plant within and around the existing opera- tional membrane treatment plant building. Following an ini- tial review by Trant it was decided to investigate an alter- native design. The alternative design is based on the provision of five (four duty/one standby) UV reactor streams instead of the three (two duty/one standby) proposed within the outline tender design. The revised design drastically reduced both the construction and operational risks, provided greater operational flexibility and security, and enabled the phased migration from the current membrane treatment process to the new UV treatment process whilst delivering considerable totex savings. Trant used 3D technology for the final UV plant design located within the site's existing membrane plant building. It utilises the existing inlet and outlet mains, membrane plant headers plus existing tank connections for 'run to waste' and drainage operations. The original MFP comprised six identical treatment streams plus provision for a future additional treatment stream. A key element of the Trant design was to minimise interference with the operation of the existing MFP, and by extending the existing inlet and outlet headers it is possible to install, test and commission the first UV treatment stream whilst leaving the existing six MF streams fully operational. Following the successful testing and commissioning of this first UV stream, to provide UV treatment of flows up to 25Ml/d, the first of the six MF streams can be taken out of ser- vice, dismantled and removed. This then allows the second UV stream to be installed, tested and commissioned to provide a further 25Ml/d of UV treatment capacity. This process is repeated until all six MF streams have been removed and the five UV treatment streams are fully operational. These carefully programmed sequential operations ensure a very gradual migration from MF treatment to UV treatment whilst providing secure and continuous treatment to supply. The five identical 25Ml/d UV reactors installed under this contract are low pressure units by atg Technology, each with a 316 stainless steel reactor chamber housing 16 800W Amalgam UV lamps with automatic wiper system, validated UV monitoring. Each UV stream is provided with a magnetic flow meter, inline axial mixed flow Bedford Surface stripping of the lamella / centrifuge building trant Engineering's site offices installed