Within the secondary treatments there are also included Filters with Decolouring Resins, which are structurally very similar to the quartz filters but which use a bed of weak anionic resin, for removing the color of the effluent. Thanks to these macroreticular resins, constituted by chemically modified synthetic copolymers, an exchange of ions between the polymer matrix and the water to be treated is obtained. 

In this specific case, the weak anionic resins in salified form (for example Cl-) retain the anions of strong or very strong free acids. During the operation, the retained ions reduce the exchange capacity of the resin, making necessary an alkaline regeneration (caustic soda solution), followed by a wash and a salification by a solution of NaCl. The use of NaOH, as regenerating, facilitates the ri-elution of organic substances, adsorbed during the exhaustion cycle. In fact, thanks to the structural porosity of the resins, there are retained even organic substances of high molecular weight present in the water to be treated.

This solution has been conceived in particular for the removal of color from the effluent of dyeing, in order to reuse them in the dye baths or washes and rinses, or for machineires cleaning or other uses. This type is based on a kinetic reaction that follows a mixed mechanism of adsorption and ion exchange: the colour molecules, in fact, are in part absorbed in a porous matrix of the exchanger and in part reteined to the ionic functional groups (free base or anion of strong acid).

The factors that determine the duration of the operating cycle and the quality of the treated water, in addition to a correct regeneration procedure, are: COD: it is partly adsorbed by the resin, thus contributes to its saturation, and exerts a competitive action with colour. An increase of COD, then, leads to a decrease of the cycle and may cause an increase of colour concentration in the treated water. After biological treatment, it is possible to obtain normal values of COD of about 100-120 mg/l; Colour: obviously also the concentration of colour and the type of dye present in the water causes a decrease of the cycle; pH: an alkaline pH adversely affects on the breacking down of the colour & Salinity: A high salinity (> 15,000 mg/l) causes the ri-elution of colour, since high concentrations of anions of strong acids, such as chlorides and sulphates, create an antagonistic action with the ion exchange reaction.

Usually the resin filter involves the use of pneumatic valves and appropriate instrumentation, in order to manage all the cycles of filtration and backwashing in completely automatic mode.