Advances in Water Treatment and Enviromental Management

134 WATER TREATMENTSuch deterioration has been particularly appreciated in the Netherlands whereAOC control is practised at several works; by reducing the driving force for bacterialgrowth, it has been possible to substantially reduce the need for carrying disinfectantresiduals throughout distribution, the ultimate example being the cessation of“safety” chlorination for supplies to Amsterdam (Schellart,1987). It is also worthnoting that in the Netherlands, tentative standards for Aeromonas hydrophilla havebeen promulgated due to health concerns (A Hulsmanpersonal communication).Of further relevance to disinfection practice is the requirement of the EC directivefor pathogens to be absent. Whilst this requirement is also contained in the UK’snew Regulations, neither item of legislation defines routine monitoring clearly, albeitthe EC directive does state that such monitoring, in a supplementary manner tocoliforms, may be necessary. Recognising that some pathogenic micro-organismscould be present in the absence of coliforms, it would seem appropriate for furtherclarification of minimum sampling requirements to be provided by the regulatorybodies; such a development could materially influence disinfection practices.Aesthetic and Chemical ConstraintsDespite general recognition that microbiological quality is paramount, and thereforepresumeably the means of its achievement, an antagonistic range of aesthetic andchemical standards make the fulfillment of microbiological objectives potentiallydifficult. Such constraints are:(i)(ii)the desirability of minimising chlorination by-products, various standardsfor trihalomethanes having been applied throughout Europe; theoperational implication is to reduce the use of chlorine with the potentialfor poorer microbiological qualitythe standard for nitrite of 0.1 mg/1 NO2 which, because of itsimplementation as an absolute maximum, probably precludes the useof chloramine in many supplies due to nitrite formation associated withthe ammonia used and/or the breakdown of chloramine—this constraintappears, in the UK, to be the cause of a move away from chloramine tofree chlorine as a disinfection residual in distribution, with the identifiedneed for many more secondary chlorination installations (Schroders,1989)(iii) the standard for taste of a dilution number of 3 is presently being appliedin the UK to dechlorinated samples (DoE,1989b); as such the standardshould have little constraining effect on the management of free chlorineresiduals within distribution—however, the use of dechlorinated samplesmay not be consistent with the EC directive and any future change tounaltered samples could impact on chlorine residual management; thepractical implications would then be the need to maintain effectiveresiduals by applying smaller chlorine doses at more points (ie secondarychlorination).(iv) the EC directive requires the absence of “animacules and algas” althoughthe UK’s new Regulations offer no clear standards. The UK Governmenthas stated (DoE,1984) that water undertakers should “take steps to controltheir presence”; such steps have often included the dosing of pyrethroidinsecticides but implementation of the EC directive’s pesticide parametercould preclude such dosing