Air-gap membrane distillation as a one-step process for textile wastewater treatment

Wastewater from textile dyeing plants contains a complex mixture of dyes, salts, surfactants and other additives which make it challenging to treat and harmful to release directly into the water system. Current treatment technologies are composed of many steps, increasing the cost and complexity of water management. Air gap membrane distillation (AGMD) is a versatile water treatment process which has potential to reduce the complexity of textile dyeing wastewater but has not yet been investigated for this application. In this work, we used commercial polytetrafluoroethylene (PTFE) membranes in AGMD to recover pure water from simulated textile wastewater containing NaCl and either sunset yellow (SY) or rose bengal (RB) dyes and sodium dodecyle sulfate (SDS) surfactant. 100% salt and colour removal was achieved for binary feed solutions (NaCl+SY or RB) over 20 h of testing, whilst maintaining stable fluxes between 11.7 and 12.6 L m(-2) h(-1) (LMH) throughout. After 24 h of continuous testing of the ternary feed solution (NaCl + RB + SDS) the flux was as high as 11 LMH with permeate conductivity less than 50 mu S cm(-1). However after 70 h these had reached similar to 3 LMH and 421.8 mu S cm(-1), respectively indicating a need for cleaning or back-flushing between batches. Parallel direct contact membrane distillation (DCMD) tests achieved lower colour and total carbon removal after just 8 h of testing with the ternary feed solution. Comparisons between our tests and those found in the literature are made and indicate that AGMD may be the most suitable configuration for this application due to reduced flux decline and potentially higher thermal efficiency.