Water treatment before use. Dissolved substances

The previous article has been dedicated to the removal of all suspended particulate substances from natural waters. These treatments, known as clarification, may be sufficient to prepare industrial waters when quality criteria are not very stringent (e.g. certain washing, transport or cooling waters; water from cardboard and packaging paper mills).

In many cases, on the other hand, treatment has to be taken a step further, removing mineral and organic micropollutants and carrying out disinfection to prepare drinking-quality water, and/or introducing or removing gases and dissolved mineral salts, until, in the most elaborate examples, a product as close as possible to the element H ₂ O ("ultrapure waters") is obtained.

This article therefore reviews all treatments that affect dissolved constituents (gases, mineral salts, organic matter, various pollutants), some of which nevertheless have a significant influence on particulate matter, and more specifically :

• strong oxidants on pathogenic organisms (disinfection), or even all micro-organisms present in the water (sterilization);

• membranes for clarification and/or disinfection/sterilization.

The treatments described here are usually applied after clarification when the resource is surface water; they can be applied directly when the water is good quality groundwater or public supply water already made potable. Under these conditions, and depending on water use, the treatment process will combine some of the following unitary procedures:

• dissolve a gas (usually oxygen), or on the contrary, expel all or part of the pre-existing dissolved gases: this is the aim of gas-liquid exchanges;

• use oxidizing reagents (chlorine, ozone, etc.) to exert a chemical and/or disinfectant or even sterilizing effect;

• fix certain compounds, such as organic micropollutants, on adsorbent products, of which activated carbon (also used in other cases for its dechlorinating power) currently represents the most common and sophisticated example; often, prior ozonation adds a biofiltration effect to the carbon bed, which complements the purely physical action of adsorption, thus increasing the removal efficiency of undesirable solutes and the interval between two successive regenerations;

• use ion exchange resins either to modify or lower water salinity, or to eliminate it altogether (demineralization); the complexity of this field is revealed both in the nature of the resins and in the technology used to implement them,...