Demonstration of membrane distillation on textile waste water: assessment of long term performance, membrane cleaning and waste heat integration

Abstract

This work reports outcomes of a pilot trial and practical assessment of direct contact membrane distillation (DCMD) towards achieving zero liquid discharge at a textile manufacturing plant. Treatment of textile wastewater is difficult due primarily to the complexity of textile processing and the wastewater produced. Combined effluent from the site, either untreated or treated with the site's existing flocculation and biological processes, were considered as the feeds to the MD testing. Initial bench scale studies found rapid membrane wetting appeared to be avoided by the novel use of foam fractionation on the untreated effluent, or by using the conventionally treated effluent. The trial was conducted on treated effluent using fractionation on a side stream within the MD process, and no wetting was observed over the entire 3 month trial duration. The flux of the 6.4 m² membrane module started at 5 L m⁻² h⁻¹ and declined to 2 L m⁻² h⁻¹ after more than 65 days. Caustic cleaning effectively restored flux to 4 L m⁻² h⁻¹. A 41-fold increase in feed concentration was verified by sulphate measurements, increasing from 567 mg L⁻¹ to 23 000 mg L⁻¹. After concentrating in the hot cycle, all ammonia entering the DCMD plant from the feedwater was found to evolve into the permeate, but non-volatile sulphate rejection was >99.9%. Water recovery at the end of the trial was 91.6%. A plant integration assessment found that zero liquid discharge would be feasible if saline waste streams were isolated and reverse osmosis processes were coupled with MD harnessing waste heat. MD application to current and future treatment scenarios with waste heat integration to textile processing appears viable.