selecting resources

The choice of water to be treated before being distributed depends on a number of factors. The following considerations have to be evaluated for each of the available resources (groundwater, running or stored surface water):

• quantity : in any situation, the "source" must be capable of delivering the amount of water required. In countries where rainfall can be extremely variable, a dam may have to be constructed in order to hold back, during rainfall periods, those quantities that will be required during dry periods;

• quality : the quality of the available raw water must be compatible with the regulations applicable in some countries. In France, order 2001-1220 (see quality standard), based on a European directive of the 16th June 1975 (75/440/CEE) defined three categories of quality for surface raw water (A1 to A3) that match three types of treatment T1 to T3 respectively (that can be compared to direct filtration for T1, comprehensive clarification for T2, clarification and polishing for T3), all being supplemented by a disinfection stage; beyond the category A3 limits, raw water can only be used to produce food grade water subject to concessions; groundwater is subject to similar limits.

Natural water varies widely and, in each case, we have to establish the most appropriate process and treatment equipment. Processes and equipment must be appraised according to water quality fluctuations over previous years (daily, seasonal, climactic fluctuations) and to potential foreseeable fluctuations (building of a dam, additional land development, etc.). Also, we need to remember that "groundwater" is not synonymous with "pure water": in addition to the fact that groundwater frequently contains iron and manganese, many water tables have suffered bacteriological pollution involving either nitrates, plant care products, chlorine solvents or hydrocarbons;

• economics : then, we need to compare the investment and operating costs applicable to each of the available resources, raw water storage and transport, water treatment, treated water storage and distribution.