Advances in Water Treatment and Enviromental Management

GROUNDWATER POLLUTION BY VOLATILE ORGANIC SOLVENTS 209Specification of Plant PerformanceAs stated in Section 2 the presence of VOCs in groundwater can be due to contaminationwhich occurred in the past and concentrations are therefore often variable. Consequently, itis usual that the concentrations of contaminants in the raw water at the time of plantcommissioning and testing are less than the maximum concentrations to be satisfactorilyhandled by the plant. Therefore if a performance specification is to be used it is essential torequest a specific removal efficiency to guarantee quality of product from a range of raw waterconcentrations.In Case B described below both outlet concentrations and removal efficiencies were specifiedto tenderers as follows:Outlet Concentration Removal Efficiencyµg/1 %TCE 12 88PCE 5 90CTC 1 80A further aspect of importance is the potential for the formation of calcium carbonate scale onthe tower packing following carbon dioxide removal. If uncontrolled the packing can becomecompletely clogged and cease to function. In order to avoid this problem a scale inhibitorsuch as Calgon must be dosed to the raw water. The dose rate found satisfactory for suchchemicals is 0.3 mg/1 as P or 1 mg/1 as Calgon chips.Lagging of pipework has been found necessary for external installations in eastern England ifproblems of freezing are to be avoided.3.3 Operating Results from Case StudiesCase studies for two works are presented. In Case A the stripping system has operated verymuch as expected but in Case B the performance has been somewhat erratic. The causes ofthis erratic performance have not yet been fully identified. Possible explanations arediscussed.Design ParametersThe design parameters for the two cases are presented in Table 1.The Performance of TowersThe performance of the towers is presented in graphical form for TCE. Similar trends wereobserved for PCE. For CTC concentrations in the raw water were so low that concentrationsin the aerated water were below detection limits.The graphical results are presented in two forms:– concentrations in the raw and aerated water against time– removal efficiency against time