The global market for hydraulic fracturing is expected to exceed $35 billion in 2015. In comparison, in the United States alone, market for fracking fluids will likely reach $37 billion by 2018. Recycling of production water in an environmentally-acceptable and cost-effective manner is critical to sustainable oil and gas industry development. Extremely high salinity, hydrocarbons, arsenic, cadmium and barium content as well as other contaminants makes it difficult to treat or recycle (McGinnis et al., 2013).
According to Coday and Cath (2014), drilling muds, hydraulic fracturing fluid, and produced waters are the main streams that require treatment. More than 1 mil gal of freshwater is commonly used during drilling of a single well, producing fluid contaminated with dissolved solids, drilling additives, and inorganic and organic compounds leached from the formation. After drilling, these fluids receive minimal treatment and are commonly taken offsite for deep well injection (Class II deep wells) – deep enough to minimize the potential for intrusion and contamination of groundwater resources. Unfortunately, long term implications of underground injections of fluids are not adequately researched and understood as well as this method permanently removes water from the fresh water cycle therefore reducing the availability of a fresh water for the public and environment.
In many cases, the largest stream that can be treated and reused is the return flow from hydraulic fracturing. More than 4 mil gal of water-based slurry are commonly used for high-pressure fracturing of one well. Produced water can represent 70–90% of the total waste generated during the lifetime of a well and the remaining 10–30% is the drilling mud and fracturing fluid (Hickenbottom et al., 2013).
Generally, fracking fluid is treated and/or recycled through high cost and high energy demanding processes such as boiling (boiling fluid to leave solids on the bottom while distilling/evaporating the water), desalination (membrane processes to separate the salts called brine from water) and simple settling (settling solids and then decanting the fluid). Boiling or distillation is a very expensive method that could be as high as $0.25 per gallon (Altaee and Hilal, 2014). Settling allows 50% to 60% recovery of water that could be reused, unfortunately it is still contaminated. Furthermore, current stringent regulations for water and wastewater discharge recommend that the total dissolved solids should not exceed 500 mg/L, which renders the conventional treatment processes insufficient.
Scientific developments led to advanced desalination systems called reverse osmosis that moves water from a dilute solution to a concentrated one (Subramani and Jacangelo, 2014). In reverse osmosis, a thin semi-permeable membrane and high hydraulic pressure is applied that pushes the fluid through the membrane to produce the clean water. Unfortunately, high electricity expenditure is required to achieve the desirable results. Also, high salinity fracking fluid usually fouls and/or destroys the membrane thereby increasing the overall treatment costs.
Forward osmosis (FO) has been recognized as a promising solution for treatment of fracturing fluid and has been reported to achieve up to 85% water recovery of streams with salinities greater than 150,000 ppm (Zhang et al., 2014).
FO is an osmotic process that employs semi-permeable membrane to separate water from dissolved contaminants or organics. The driving force is an osmotic pressure gradient that occurs between a high and low concentration solution. The osmotic pressure gradient induce the net flow of water through the membrane into permeate, therefore concentrating the feed. Permeate usually have salts and other substances tailored for FO applications. The feed is usually the product stream, e.g. fracturing fluid containing all the organics, chemical compounds, contaminants, etc. FO can be used as standalone with minimal pre-treatment or coupled with other processes such as reversed osmosis or distillation.
First commercial and fully operational FO plant (100 m3 / day) has been built in 2010 in Al Khaluf, Oman. Some operational parameters of this plant are: shallow sea water intake (39,000 mg/L), single dual media filter (velocity 16 – 25 m/h). Recovery of 35% - 38% (design 30%). Important to mention that membranes never been chemically cleaned over the period of 24 months proving that FO system does not foul the membranes. Treatment resulted in total dissolved solids less than 200 mg/L and boron concentration in the range of 0.6 – 0.8 mg/L. Energy consumption was 4.9 kWh/m3 which was more than 50% less than during conventional reverse osmosis treatment.
Major advantage of FO is the lack of membrane fouling, due to minimal cake layer formation and lower compaction of foulants on the membrane active layer that is common for reversed osmosis processes. Other technological advantages include (Coday et al., 2014):
- High salinity and high levels of suspended solids do not pose a problem for FO or impair treatment effectiveness.
- Selectivity - during FO process, hydraulic pressure created by osmosis, passes through a semi-permeable membrane and selectively drags molecules through the membrane avoiding fouling and compaction.
- Natural process – FO mimics water filtration occurring in nature that requires little or no electricity or external power source.
- FO membranes are easily removed with osmotic backwashing or turbulent flow at the feed-membrane interface. During osmotic backwashing, permeate is replaced with deionized or fresh water. This develops an osmotic pressure gradient in the opposite direction across the FO membrane and water is pushed from permeate into the feed. The push of water back into the feed helps to dissolve and detach foulants from the membrane surface.
- Energy consumption – energy consumption is up to 30% lower than conventional reverse osmosis. FO uses ambient temperatures therefore there is no additional energy expenditure to heat the solution up. This significantly lowers capital costs associated with pumping and system design and construction.
- Treatment effectiveness – significantly reduced boron levels without post-treatment in comparison to conventional reverse osmosis.
- Ease of operation, simple cleaning and relatively low operational costs.
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Altaee, N. Hilal. 2014. Dual-stage forward osmosis/pressure retarded osmosis process for hypersaline solutions and fracking wastewater treatment. Desalination, 350: 79-85
D. Coday and T. Y. Cath. 2014. Forward osmosis: Novel desalination of produced water and fracturing flowback American WaterWorks Association, 2014: E55:E66
D. Coday, P. Xub, E. G. Beaudry, J. Herron, K. Lampi, N. T. Hancock, T. Y. Cath. 2014. The sweet spot of forward osmosis: Treatment of produced water, drilling wastewater, and other complex and difficult liquid streams. Desalination, 333: 23-35
L. Hickenbottom, N. T. Hancock, N. R. Hutchings, E. W. Appleton, E. G. Beaudry, P. Xu, T. Y. Cath. 2013. Forward osmosis treatment of drilling mud and fracturing wastewater from oil and gas operations. Desalination, 312: 60-66
L. McGinnis, N. T. Hancock, M. S. Nowosielski-Slepowron, G. D. McGurgan. 2013. Pilot demonstration of the NH3/CO2 forward osmosis desalination process on high salinity brines. Desalination, 312: 67-74
Subramani, J. G. Jacangelo. 2014. Treatment technologies for reverse osmosis concentrate volume minimization: A review. Separation and Purification Technology, 122: 472-489
Zhang, P. Wang, X. Fu, T-S. Chung. 2014. Sustainable water recovery from oily wastewater via forward osmosis-membrane distillation (FO-MD). Water Research, 52: 112-121.