Power station water recycling using membrane distillation - a plant trial

This paper presents the results of work undertaken to demonstrate the potential for a membrane distillation (MD) process to exploit waste heat from heavy industry to treat saline effluent, producing high quality water for on-site reuse without an increase in greenhouse gas emissions. In partnership with City West Water (Melbourne), GWMWater and WQRA the project operated a 240 L/day capacity pilot plant for a 3 month plant trial at Ecogen Energy's Newport Power Station, a natural gas fired 500MW electricity generator. The pilot plant treated effluent from an ion exchange resin demineralisation plant of approximately 3000mg/L TDS, by employing the power station’s waste heat as the driving force for the MD process. The trial showed that permeate flux was relatively consistent up to reject concentrations of 62,300 mg/L, after which flux decline was observed. The maximum water recovery achieved in the final phase of the trial was 92.8% with a reject concentration of 71,400 mg/L and salt rejection of 99.97%. The experiment successfully tested the MD process' longevity and robustness, demonstrating a system driven by waste heat at input temperatures as low as 30°C, which achieved permeate flux of 3 4 L/hr/m 2 , of highly desalinated water. INTRODUCTION This Smart Water Fund (Victorian Government) and WQRA funded project aims to demonstrate the potential of the membrane distillation process to exploit waste heat from a natural gas fired power station to treat saline effluent, producing high quality water for on-site reuse. Membrane distillation (MD) is a thermally based desalination process that can treat water using low grade heat, i.e. solar or waste heat, for reuse in industry thus substituting precious potable water and reducing discharge volumes to sewer. Presently, desalination by available technologies is normally an electrically driven process which consumes a high value energy source while in turn adds to greenhouse gas emissions. The major focus of this work is to examine the viability of the MD water treatment solution which converts industry’s routinely discarded effluent and waste heat to valuable treated water. Membrane Distillation was first developed in the 1960’s, but is gaining recent interest due to rising water and energy scarcity [Khayet, 2010]. In an MD process for desalination, heated feedwater is passed over the surface of porous hydrophobic polymeric membranes. Water is evaporated into the pores of the membrane at the brine–membrane interface which diffuses through the membrane where it condenses as permeate. Therefore, it is the vapour pressure difference on either side of the membrane than induces distillation to occur. When very thin membranes with large pore sizes (0.1 1.0 μm) are employed, surprisingly large amounts of water are transported at relatively low vapour pressure differences [Tomaszewska, 2000]. Hence, saline waters can be distilled at relatively low temperature which renders the technique particularly applicable to utilising industrial waste heat or solar derived hot water. Furthermore, as vapour pressure does not decline significantly at high salt concentrations, MD can treat high salinity water or operate at higher per cent water recovery than pressure driven technologies such as RO. Saline effluents resulting from industrial processes are a common trade waste issue that business must manage, both internally and in negotiation with water authorities. With schemes increasingly being introduced through encouragement and regulation to reduce salt loads in sewers, the obligation to actively treat effluent is becoming more pressing. Either to recover useful water or as part of a zero liquid discharge strategy, a low cost method for that treatment is becoming more important. Hence, it is the utilisation of waste heat instead of electricity to treat water that illustrates the great potential of MD in addressing water and energy conservation simultaneously. Further, low grade heat is commonly rejected in many industrial operations so its bill has been paid and its carbon liability has already been accounted. Therefore, by utilizing this low or zero cost resource to treat effluent, the benefits of on-site water reuse substituting for precious potable water and reducing discharge volumes to sewer are a possibility. This paper is the conclusion of work initially presented at Ozwater '11 where a series of industries in Melbourne's western suburbs were examined to identify opportunities where MD could treat effluent to reduce discharge volumes to sewer and recover water of potable quality, driven by an inexpensive, low energy demand process [Dow, et. al. 2011]. After examining many factors affecting the successful demonstration of an MD operation such as; availability and temperature of waste heat, effluent quality and other site specific issues, the survey process identified Ecogen Energy’s gas fired Newport Power Station as an appropriate site to operate the demonstration phase of the project. This study follows on from work where an MD plant was operated with support from GWMWater to desalinate groundwater using only solar derived heat [Dow, et. al. 2010]. That trial identified the promise of the MD process to desalinate water at low temperatures which prompted thinking into applying the technology to treat industrial effluent with commonly available waste heat. Thus, in partnership with City West Water, GWMWater and WQRA the present project operated a 240 L/day capacity pilot plant for a 3 month trial at the Newport Power Station. The MD pilot plant was set up to operate from September to December 2011, treating the main saline effluent from the site’s demineralisation plant. This was done using the power station’s waste heat, thereby producing water for potential on-site reuse without an increase in greenhouse gas emissions. EXPERIMENTAL AND METHODS The experimental parameters of this demonstration of the MD process was to treat between 10 – 20 L/hour of industrial effluent, operate 24 hours per day for 3 months and produce potable quality water. The previous study [Dow et al., 2011] identified the industrial site through a process of examination of waste heat availability and effluent suitability to the MD process. Table 1. Effluent quality Parameter IX resin regenerant effluent