Membrane Distillation Treatment of Reverse Osmosis Brine from Coal Seam Gas Water – Chemical Cleaning and Subsequent Impacts on Membrane Properties Hung C. Duong 1, Stephen Gray 2, Mike Duke 2, Long D. Nghiem 1 1 The University of Wollongong, Australia 2 The University of Victoria, Australia We’ve done several works focusing on the application of membrane separation process for the treatment of CSG water. In this presentation, we will report an experimental research on the MD treatment of the brine following a RO process of CSG water for further fresh water extraction and brine volume reduction.

Coal seam gas (CSG) water CSG essentially the methane gas trapped in underground coal seams at 200-1000 m. To release CSG, water must be co-extracted. CSG water: large in volumes, saline solution-> negative impacts on plants and ecosystem -> must be treated prior to direct discharge or beneficial reuses. (http://www.ehp.qld.gov.au/management/coal-seam-gas/)

Treatment CSG water Coal seam gas plant Reverse osmosis desalination SCG water from the gas production site is treated by RO RO can achieve only 70-75% water recovery RO brine is temporarily kept in pond for natural evaporation MD can be used to treat RO brine for more fresh water extraction and brine reduction. Membrane distillation desalination Brine pond

Membrane Distillation (MD) Workable at high salinity Use waste heat, solar energy No intensive pretreatment System simplicity MD is a phase change separation process using a hydrophobic micro-porous membrane to separate the hot saline feed and cold distillate. Only water vapor and volatile substances permeate through the membrane => Distillate of high quality is produced. Different to reverse osmosis, MD is not affected by the osmotic pressure of process solutions => MD can work at high feed salinity. In addition, MD doesn’t require intensive feed water pre-treatment. MD system is more simple and compact than RO. Most importantly, MD is operated at the feed temperature ranging from 40 – 80 oC, thus solar thermal energy, waste heat can be ideally coupled with MD.

Research questions Impacts of bicarbonate and scaling Efficiency of various cleaning agents Influences of repetitive cleaning Membrane scaling prevention The research questions are: How does membrane scaling occur in MD treatment of CSG RO brine at high water recoveries? Impacts of carbonate present in CSG RO brine to performance of MD? If scaling occur, is membrane cleaning efficient to restore to performance? What are the impacts of chemical cleaning on membrane properties?

Bench-scale MD system PTFE membrane: Pore size: 0.2 m, porosity: 70% We use a bench-scale DCMD for the experiment. The system consists of hot feed loop, membrane cell, and cold distillate loop. A PTFE membrane having pore size of 0.2 um, porosity of 70%, thickness of 175 um was used. PTFE membrane: Pore size: 0.2 m, porosity: 70% Thickness: 175 m (active layer 5 m)

Experimental protocols DCMD process of brines: Operating temperature: Tdistillate = 25 C Cross flow velocity: Vfeed = Vdistillate = 0.06 m/s Feed volume: 4 L Membrane cleaning: 1 L of cleaning agent Rinsing time: 1 hour at 25 C, velocity: 0.12 m/s In all DCMD processes of brine solutions: + 4 L of brine was used as the feed + Distillate temperature = 25 oC, three feed temperature was used: 50, 40, 35 oC + Feed and distillate cross flow velocity V = 0.06 m/s During membrane cleaning: + Feed was replaced by 1 L of cleaning agent + 2 L of distillate was used on the permeate cell + The system was rinsed for 1 hour at room temperature at cross flow velocity of 0.12 m/s

Experiment Design Impacts of bicarbonate and scaling. Feed solution: synthetic NaCl, synthetic NaCl + NaHCO3, CSG RO brine; Feed temperature = 50 C Exp. 1 Efficiency of various cleaning solutions. Feed solution: CSG RO brine; Feed temperature = 50 C Cleaning agents: tap water, 0.5% HCl, 2.5% MC3 (commercial cleaning agent) Exp. 2 Repetitive cleaning efficiency. Feed solution: CSG RO brine; Feed temperature = 50 C Cleaning agents: the most efficient one Exp. 3 In all DCMD processes of brine solutions: + 4 L of brine was used as the feed + Distillate temperature = 25 oC, three feed temperature was used: 50, 40, 35 oC + Feed and distillate cross flow velocity V = 0.06 m/s During membrane cleaning: + Feed was replaced by 1 L of cleaning agent: tap water, 0.5% HCl, 2.5% MC3 (a commercial cleaning agent) + 2 L of distillate was used on the permeate cell + The system was rinsed for 1 hour at room temperature at cross flow velocity of 0.12 m/s Scaling prevention. Feed solution: CSG RO brine; Feed temperature = 50, 40, 35 C Exp. 4

Impacts of bicarbonate and scaling Let’s talk about the experimental results. Three brine solutions having similar salinity were used: NaCl, synthetic CSG RO brine, and CSG RO brine In DCMD process of brines, as fresh water was extracted from the feed, the salinity of feed increased. DCMD process of various brines have identical salinity profile as water recoveries rose. However, different levels of flux decline were obsereved. The influence of feed salinity on MD flux was shown in DCMD of synthetic Normalised flux and feed conductivity in DCMD treatment of: NaCl solution, synthetic CSG RO brine, and CSG RO brine. Operating feed temperature: Tfeed = 50 C. The initial permeate flux ranged from 30 to 33 LMH.

Impact of bicarbonate and scaling pH and conductivity of the distillate in DCMD treatment of: NaCl solution, synthetic CSG RO brine, and CSG RO brine at the same operating conditions.

Efficiency of various cleaning solutions Normalised flux and water recovery in DCMD treatment of CSG RO brine and SEM photos of fouled membrane after membrane cleaning.

Repetitive cleaning efficiency Normalised flux and water recovery in DCMD of CSG RO brine undergone repetitive scaling-MC3 cleaning.

Repetitive cleaning efficiency Distillate conductivity and conductivity rejection in DCMD of CSG RO brine undergone repetitive scaling-MC3 cleaning .

Impacts of chemical cleaning Contact angles of membranes after soaking with 0.5 wt.% HCl and 2.5 wt.% MC3 solution in 6, 12, and 18 hours. Mili-Q water was used as reference. Cleaning agents 0.5 wt.% HCl 2.5 wt.% MC3 Soaking duration (hours) 6 12 18 Contact angle,  133.3 133.4 133.7 133.2 134.4 Contact angle of virgin membrane,  133.5

Scaling prevention Normalised flux and SEM photos of fouled membrane in DCMD operation of CSG RO brine at different feed temperature and initial flux.

Conclusions Scaling caused significant flux decline Reducing feed temperature and initial permeate flux helped preventing membrane scaling MC3 cleaning was effective; however, repeated scaling-cleaning reduced cleaning efficiency Chemical cleaning solutions had negligible impacts on membrane hydrophobicity

Acknowledgement Special thanks to: Vietnamese Government Prof Long Nghiem (UOW) Prof Stephen Gray (University of Victoria) Prof Mikel Duke (University of Victoria)