The increasing demand during recent years to reduce and control water consumption has led many industries to examine the ways and opportunities in which they can re-use their processing water.  In order to re-cycle such an important resource in an efficient way, a low running-cost biological treatment plant has to be considered.

If we take water consumption in the textile industry we notice that the greatest consumption is during the rinsing and washing operations in dyeing or finishing processes. This is also pertinent to the dye preparation procedure. If the dyeing procedure also includes a printing operation, a great quantity of water is used in frames or cylinder washing.  Overall, this type of water consumption can achieve 80% of total consumed flow; a percentage that could be reduced dramatically by using recycled water.  Moreover, the required quality of water necessary for the type of operations mentioned above is slightly lower than the quality necessary for water used in dyeing baths; in such a way a great quantity of raw water can be recovered at low cost.

In the textile industry the quantity of Chlorides and Sulphates in the effluent coming out from the dyeing and finishing of wool, silk and acrylic is very low. This is due to the fact that salts are not used for either work processing or printing.  In this case the salinity could come only from head softeners due to sodium chloride that is used for their regeneration. Obviously, the greater the water hardness, the greater the consumption of sodium chloride. Since the salinity cannot be reduced either by previous biological treatment or by any following treatment, the degree of salinity will increase proportionately over time.  This means that if we attempt to carry out a theoretical 100% recovery, the water will simply be un-useable. So, some recovered water must be partly discharged or sent to an evaporator in order to achieve a balance with regards to salts content; it must be made clear that the recovery capacity must be established according to the quality of raw water and the type of work processing.

There are two ways in which we can achieve this:

• In some cases the recovered water can be stored in a tank where it is mixed with raw water and then it is used in the dyeing process.
• Alternatively, a two-tank system can be installed; one for raw water, and one for recycled water  (from which water is used for only certain processes). In this case the amount of recycled water is about 50 to 60% (sometimes 80% can be achieved). In the textile industry, this latter option is the safest, since dyeing is the most important and basic step.