Advances in Water Treatment and Enviromental Management

138 WATER TREATMENTMobile equipment of this complexity will not only be of value in secondary chlorinationassessment but, for example, if de-chlorination equipment is included, could alsobe satisfactorily used for mains disinfection. Similarly it could be made availablefor use in emergencies where rapid measures for disinfection are required.Panel mounted, or skid, arrangements to provide both handling, dilution anddosing of hypoehlorite, as well as the analysis and control of residuals in thenetwork, are becoming subject to more exhaustive specification by water companiesin the UK. Often specified is the requirement for instantaneous alarm should thechlorine residuals stray from the optimum. In all such systems accuracy andreliability is paramount and UK specifications are now likely to fall in line withthe Water Authorities Association Process Systems Committee requirements forprocess control and monitoring instruments. These strict requirements could befurther enhanced by triple validation of primary sensing signals at sensitive orproblematical locations.Secondary chlorination generally calls for the use of simple controllers (for cost andreliability). The use of sophisticated computer controlled systems in remote locationshas its advocates but the relative importance of re-chlorination compared to primarydisinfection must be borne in mind.On site electrolytic chlorination (OSEC) is likely to reduce the need for visits tothese secondary chlorination sites. Self-contained, easily installed systems are nowavailable at a wide range of capacities down to 2 kg per day. Modern technologyallows economic conversion of electrical energy and salt into hypochlorite with fullyintegrated control and alarm systems for local and remote monitoring. Storage ofsalt, instead of hypochlorite, provides stability of chemical and long periods betweenreplenishment. Such equipment needs to comply to BS5345 Part 2 and meet Healthand Safety Executive requirements.All of the above techniques and equipment are standard for water treatment worksapplication. The modification of such designs to suit secondary chlorination isnevertheless far from straighforward and require? a good deal of experimentationand modification before we reach the stage in the UK where optimum secondarychlorination can easily be applied.CONCLUSIONSReflecting the recently more stringent implementation of drinking water qualitystandards, a trend can be anticipated in the UK of improved disinfection control atsourceworks and the installation of many more secondary chlorination stationswithin distribution.Presently, the design of disinfection systems is related to the achievement ofmicrobiological quality objectives but this approach on its own can becompromised by the uncertainties arising from low sampling frequencies; further,various microbiological issues have tended to be over-looked or are in need ofdevelopment.A more objective basis is proposed, for managing the concentration of chlorineresiduals within distribution, which would lead to a more consistent approach toachieving microbiological standards.Optimisation of secondary chlorination will only be achieved given a more objectiveapproach and the pursuance of investigations to ensure that correct doses areapplied at correct locations.