Suchergebnisse

AbstractDiffuse pollution by pesticides, particularly herbicides, has been recognized in the UK for several years. Atrazine, Simazine, Mecoprop, Dimethoate and, more recently, Isoproturon and Chlortoluron have been found frequently in surface waters at concentrations up to 0.5 μg/l, with a maximum of 11.5 μg/l. Pesticides are detected less frequently in groundwaters, at concentrations generally lower than 0.1 μg/l. Atrazine is found most frequently. Concentrations in untreated surface and ground‐waters have generally been below the maxima recommended for drinking water by the Department of the Environment (DoE). Concentrations in treated drinking water have not exceeded the DoE recommended maximum concentrations, but measures are being taken to reduce concentrations to below 0.1 μg/l.

AbstractThere has been no information available on the removal of Cryptosporidium oocysts or oocyst‐sized particles under the sand filtration conditions normally used for swimming pool water treatment. A pilot sand filtration plant has therefore been constructed at The University of Wales Swansea and operated under swimming pool conditions (25 m/h downflow, 30°C, 1.5 mg/L free chlorine, pH 7.3 to 7.5, alkalinity 120–150 mg/L as calcium carbonate). Removal of fluorescent polystyrene particles of a size range (1–7 μm) similar to the size of Cryptosporidium oocysts (about 5 μm) without added coagulant was less than 50%. Removal of the particles under the operating conditions above but with the addition of either of the coagulants polyaluminium chloride (PAC) or aluminium sulphate was capable of giving better than 99% removal of the particles.

AbstractThe paper describes some water‐treatment processes which have been investigated on a laboratory and pilot‐plant scale for their effectiveness in removing toxic algal cells and the dissolved toxins microcystin‐LR and anatoxin‐a. Oxidation with ozone or potassium permanganate, or treatment by biological activated carbon, were found to be the most effective processes for removal of the dissolved toxins. Chlorination was effective only for the removal of microcystin‐LR. Toxins, contained within algal cells, could be removed effectively by coagulation, clarification and filtration under suitable conditions. Consideration of the structure and properties of micro‐cystin variants suggests that treatments which are suitable for removal of microcystin‐LR and anatoxin‐a should be suitable for removal of other microcystins.

ABSTRACTTwo zones of a water distribution supply area receiving treated surface water were used to study the effect of booster chlorination on bacterial regrowth. In the first year of the study general patterns of bacterial regrowth were observed, and in the second year one of the areas was booster chlorinated at the outlet from a surface reservoir. In both years of the study the heterotrophic plate counts showed a seasonal pattern. In the second year there was a large increase in plate counts from mid‐July to October in both of the study areas. During this period, booster chlorination was unable to prevent bacterial regrowth. The addition of chlorine to produce an initial free chlorine residual of 0.5 mg/1 resulted in a reduction in bacterial numbers, but with further retention in the distribution system bacterial regrowth occurred. There was little increase in the trihalomethane (THM) concentration following booster chlorination, but booster chlorination appeared to increase the assimilable organic carbon concentration.

ABSTRACTA laboratory lead‐pipe rig has been used to support Anglian Water's successful orthophosphate dosing programme to reduce plumbosolvency. The hard waters in the region generally fall into low or high plumbosolvency categories according to the types of crystalline deposit formed. To improve the cost‐effectiveness of plumbosolvency control, the effects of temperature, phosphate doses, blending and alternation of these waters were investigated.Initial phosphate concentrations must be above 0.6 mg P/l (as phosphorus) to establish plumbosolvency control. Subsequently, phosphate doses may be reduced, provided that dosing is continuous and sufficient phosphate reaches the extremities of the distribution system. When high and low plumbosolvency waters are blended before distribution, both (or the mixture) must be phosphate dosed. However, where waters alternate in distribution, laboratory studies have shown that low plumbosolvency deposits are more stable, resulting in low lead concentrations. The high plumbsolvency of some hard waters may be due to the presence of low concentrations of humic substances.

AbstractA collaborative project between Anglian Water and Imperial College has been undertaken to investigate alternative disinfection regimes for the control of trihalomethane (THM) formation in lowland surface water treatment. A laboratory experimental protocol has been developed to measure THM formation as a consequence of both pre‐ and post‐disinfection stages with chlorine dioxide, ozone and potassium permanganate, evaluated as alternative pre‐disinfectants in direct comparison with breakpoint and sub‐breakpoint pre‐chlorination. The protocol has been applied to raw water samples from eight locations within the Anglian Water region under winter and summer conditions.The results have shown that THM formation can be reduced substantially, either by the application of chlorine dioxide and ozone as substitutes to breakpoint pre‐chlorination or by reducing the prechlorination dose. The application of potassium permanganate as a pre‐disinfectant had little effect on diminishing the formation of THM compounds. A further substantial reduction in THM formation can be achieved by diminishing the contact period following post‐chlorination between free‐chlorine and the treated water (e.g. by conversion of free‐chlorine to combined‐chlorine through reaction with ammonium sulphate).