Steady state evaluation with different operating times in the direct contact membrane distillation process applied to water recovery from dyeing wastewater

In the present work, the steady state was determined and the operating time was investigated in Direct Contact Membrane Distillation (DCMD) applied to water recovery from dyeing wastewater. The determination of the steady state is noteworthy since dyeing is a batch process and DCMD will thus probably not be a continuous process in the textile industry. Applied research focused on the optimization of a wastewater reclamation associated with water sustainability may provide industrial strategies. The DCMD process was performed at 3, 12, and 24 h with dye solutions from different classes and each operating time was evaluated by reaching a steady state. The reactive Black (RB) dye solution showed a decreasing permeate flux, whereas the Disperse Black (DB) dye solution showed a stable permeate flux over all tested operating times. Different from usually reported in the literature with other dye classes, the permeate flux with disperse dye solution did not decline until 24 h. These results are due to the characteristics related to size and ionic character of each dye class: the reactive dye is large and have anionic groups when in solution, whereas the disperse dye is small and non-ionic. Disperse dye particles reduced thus the diffusion path of water vapor molecules, while reactive dye particles were repelled by the membrane interface. The process stability observed for the disperse dye is important as an industrial point of view since a decline in permeate flux would not be observed, indicating a continuous process of water recovery with posterior reuse for up to 24 h. The three state demonstration and the derivative calculation indicated a stable permeate flux i.e. steady state at 12.5 h of DCMD experiment. After 24 h of experiment, a mean permeates flux of 14.3 kg m(-2)h(-1) and 18.0 kg M(-2)h(-1) was obtained respectively for RB and DB dye solutions. Both dye classes reduced the surface contact angle over operating time up to 24.8% and increased the membrane thickness due to fouling, however, a high color rejection was observed, thus allowing studies for water reuse recovered by DCMD process.