3.5 Modified washing-off techniques.
Washing-off is required to remove unreacted and hydrolysed dye from the cotton fibre at the end of the dyeing process. The washing-off steps normally recommended by dye manufacturers are a cold rinse, followed by a warm rinse, soaping at the boil, a warm rinse and finally a cold rinse. An approach of 'hot-washing', i.e. beginning with hot rinses and a final rinse at low temperature, has been reported to improve dyeing results and marginally reduce effluent chemical oxygen demand.
Bayer introduced an 'enzymatic after soaping' process for exhaust application method, later continued by BASF, for removing the unfixed dyes not only from the fibre, but also from the exhausted dyebath. The application of the patented enzymatic compounds, recommended as the best available technique by the Commission, usually takes place in the fourth or fifth rinsing step. No detergent is required and one of the hot rinsing steps can be avoided when using enzymatic after treatment. Therefore, savings in water, energy and detergent consumption are the main advantages achievable with this technique.