A UK water supply company serves a population of approximately 675,000 consisting of homes and businesses in Surrey, Sussex and Kent and is a key part of London’s drinking water network. The largest works in Kent, which takes raw water from a reservoir that is recharged in winter by abstraction from the River Eden. The original works capacity was 35Ml/d and it is currently undergoing a phased uprate. Phase 1 in 2010 increased the capacity to 45Ml/d and Phase 2 in 2012 increased the capacity to 50Ml/d and Phase 3 will ultimately complete the uprate to 70Ml/d. As part of Phase 2, the company decided to install UV as the primary disinfection technology with residual chloramination prior to supply to protect the water in the distribution network. Replacing chlorine with a chemical-free primary disinfectant is not only more environmentally friendly and sustainable but, at an operating cost of about 25p/Ml, it is also cost effective. But even more importantly, UV is acknowledged as good practice by the Drinking Water Inspectorate, who is responsible for compliance with drinking water quality in England and Wales, as an effective treatment for Cryptosporidium.
The plan was to provide sufficient UV capacity to treat the ultimate 70Ml/d output from the Water Treatment Works, and to have sufficient spare capacity to be able to divert flow from other works though the UV system during maintenance or distribution network issues. To this end the customer specified that the UV system should be able to treat from 120Ml/d up to 240Ml/d, making it the largest municipal drinking water UV plant in the UK at the time of installation.
Evoqua’s solution was to install three ATG™ SX-1873-30 UV reactors operating as duty/assist/standby. Each 316L stainless steel reactor is fitted with 18 ultra-efficient medium pressure UV lamps and can handle up to 5,000m3/h (120Ml/d) of water. The ATG™ SX range of UV units is independently 3rd party validated and certified using bioassay testing according to the USEPA UVDGM protocols. The reactor chambers have been designed with the aid of computational fluid dynamics (CFD) to ensure that there is no short-circuiting and that the total flow is exposed to the prescribed dose. This means that, for a given micro-organism, the UV dose–response relationship can be established so that performance for UV reduction equivalent dose (RED) in the range 10 - 120 mJ/cm2 and 1 - 5 log reduction of Cryptosporidium can be guaranteed.
As is usually the case with a works upgrade or retrofit, space was at a premium. Evoqua’s large medium pressure systems are much more compact than other systems on the market. The smaller space requirement, and resulting lower building and installation costs, were key in the customer’s selection of Evoqua as their preferred supplier.
In order to comply with the project engineering standards, Evoqua provided a specially designed Form 4 Motor Control Centre with control section housing the instrumentation (flow, UVT (transmissivity at 254nm) and UV intensity) and PLC controls for the UV plant.