Sustainable integrated membrane contactor process for water reclamation, sodium sulfate salt and energy recovery from industrial effluent

The combined global impacts of water shortage and energy provide strong impetus to use innovative combinations of technologies to supply sustainably produced high quality water. Recent innovations have focused on the integration of membrane processes with other chemical separation unit processes and various stand-alone membrane processes. Coupling well-developed pressure driven membrane processes (such as nanofiltration (NF) and reverse osmosis (RO) with low energy demand membrane processes which allow the utilization of low-grade energy, industrial waste heat or renewable energy (such as membrane distillation (MD)) brings the promise of new sustainable technical solutions. In this work, two case studies will be reviewed and discussed — 1) integrated NF and MD-Crystallizer for resource recovery; and 2) the potential application of MD-Crystallizer for the concentration of RO brine (secondary reject from industrial wastewater treatment plant) towards a zero liquid discharge (ZLD) process. In Case I, MD flux of 15–20 Lm−2 h−1 with a water recovery of 80% was achieved (at a feed temperature (TF) of 60 °C and permeate temperature (TP) of 20 °C). In Case II, average MD flux of 4 Lm−2 h−1 with overall water recovery of 95% was achieved when operating with RO brine (TF = 45 °C; TP = 25 °C). © 2011 Elsevier B.V. All rights reserved.

Fig. 6. Photo record of lab-scale PVDF hollow fiber module with no sign of severe fouling on the surface after 8 cycles MD run. size = 0.2 μm; porosity = 80%; outside diameter (O.D.) = 1.5 mm and inside diameter (I.D.) = 1 mm). The detailed experimental description of the lab-scale MD-Crystallization apparatus can be found in our previous work [4]. The MD flux and water recovery are summarized in Fig. 4. During further concentration of RO brine, a flux of 3–5 Lm2 h−1 was achieved and the flux decline was observed when the MD feed solution concentration increased up to near saturation (TDS was increased from 19,200 mg L−1 to 212,899 mg L−1). Here, the recovery factor was evaluated from the total water lost in the feed reservoir. Fig. 4 shows the MD flux and water recovery obtained with experimental time. When the water recovery factor reached 75–80% (see Fig. 4), the MD concentrate was transferred to a crystallizer allowing the precipitation of salt crystals at an ambient temperature of 25 °C and the mother liquor was then re-circulated to the MD process for further concentration. A final water recovery factor of 90% was achieved in the integrated MDC process. Thus an RO + MDC integrated system allowed to reach overall water recovery factors as high as 95% and the remaining MD concentrate/retentate was only 5% to be discharged. At the end of the experiment, the conductivity of Case I study demonstrated that salt crystals with high purity can be recovered from sodium sulfate-rich industrial effluent using a coupled process. Since the MD feed (industrial effluent) in Case I is almost free from organic chemicals and includes a single salt only, the product water quality is higher than that achieved in Case II. Even though a trace amount of volatile organic compounds was present in the MD feed (RO brine) in Case II, no significant organic fouling was observed. A significant advantage is evident in Case I as a result of the effluent temperature being high enough to drive the MD process, but the availability of waste heat to operate the MD process may be a concern in Case II. On the other hand, in comparison with Case I, concentration of RO brine in Case II is a good starting point to operate a MD process which has more capability to handle a feed with high salt concentration. In both cases, experimental results obtained from bench scale feasibility studies on the coupling of MD with other pressure driven membrane processes suggest that integrated membrane contactor processes are promising for sustainable water supply and open a new window for zero liquid discharge processes. However, the economics of MD relies on the availability of waste heat or free energy for water reclamation and reuse. Industrialization of the concept still requires validation and demonstration plants of suitable capacity. Since the development of membrane contactor process are considered beyond academic research and laboratory scale, we believe that viable and sustainable integrated membrane processes based on membrane chiller MD distillate product 0.45 x (90% recovery) RO brine or secondary reject, 0.5 x TDS= 19, 200 mg L-1 heater (waste heat) MD circuit MD retentate TDS = 212, 899 mg L-1 Industrial wastewater, x RO Saturated salt solution RO permeate (Reclaimed water) 50% recovery, 0.5 x 0.05 x Overall water recovery = 0. 5x + 0.45 x = 0. 95 x (or) 95% Fig. 7. Schematic process flow diagram of integrated RO-MD-Crystallizer system and a mass balance based on water recovery.