Advances in Water Treatment and Enviromental Management

200 WATER TREATMENTL m = molar liquid flow rate per unit cross sectional area of the tower (eg Ib mole/h/ft 2 kgmole/h/m 2 )x 2 = mole fraction of solute in inlet waterx 1 = mole fraction of solute in treated water m = molal density of liquid (eg lbmole/ft 3 —kgmole/m 3 )K L = liquid phase transfer coefficient (eg lbmole/h/ft 2 (lbmole/ft 3 )—kgmole/h/m 2 (kgmole/ms 3) )a = specific surface area of packing eg ft 2 /ft 3 -m 2 /m 3D = diffusivity of the VOC in water (ft 2 /h-m 2 /h)L = liquid rate (1b/h/ft 2 -kg/h/m 2 )µ = liquid viscosity (1b/ft/h—kg/m/h) = liquid density (lb/ft 3- kg/m3)1. INTRODUCTIONVolatile chlorinated solvents have been used as non-flammable cleaning and degreasing agentsin industry since the development of adequate storage and handling equipment just over 60years ago. Trichloroethene (TCE) was the most widely used solvent of this group until theintroduction of tetrachloroethene (PCE) as a lower volatility solvent after the Second WorldWar. The 1970’s and 1980’s have seen a decline in use of TCE and PCE in favour of less toxicsolvents such as 1,1,1-trichloroethane (TCA) and 1,1,2-trichlorotrifluoroethane (Freon 113),and alternative cleaning methods.The toxicity of the traditional solvents TCE and PCE in their liquid and concentrated vapourform has been recognised almost since their introduction. Hazards associated with theirpresence as trace contaminants in drinking water supplies, principally relating to their potentialcarcinogenic action, were perceived only during the 1970’s, and began to have an impact onwater supply undertakers and regulating authorities from the middle of that decade (eg, USEnvironmental Protection Agency, 1975 and European Economic Community, 1976 and 1980).There was thus a considerable period when industrial cleaning and degreasing processestook little or no account of the potential risks stemming from releases of small quantities ofsolvents into the aqueous environment. The legacies of the period, in the form of contaminatedpublic water supply (PWS) boreholes, may be revealed only after a long interval and at adistance from the sources of contamination. The discovery of contamination thus presentswater supply undertakers and regulators with urgent and unexpected problems, on the onehand relating to identification of sources of contamination, and on the other hand with treatingthe water so as to bring the source back into supply at the earliest opportunity.This paper examines two cases in which these problems were addressed, and discusses thesolutions that were implemented. Those matters of general applicability where particulardifficulties were overcome are described.2 SOURCE IDENTIFICATION2.1 Case Study AA PWS groundwater source in eastern England was found to be contaminated by volatileorganic solvents in 1987. The main contaminants detected were TCE (generally atconcentrations between 40 and 80 µg/1) and PCE (generally between 5 and 10 µg/1). TheWorld Health Organisation tentative guideline values for these substances in drinking waterare 30 µg/1 and 10 µg/I respectively (WHO, 1984) and these values have been adopted inrecent water supply regulations in the UK (Department of the Environment, 1989).