Treatment and reuse of unconventional oil and gas (UOG)
produced
water are important strategies that address the dual challenges of
water scarcity and pollution posed by UOG production. Considering
the high salinity and complex chemistry of UOG produced water, it
is important to comprehensively analyze the water quality and potential
ecological risk of treated produced water for reuse applications.
In this study, we evaluated and compared the efficacy of pretreatment
followed by nanofiltration (NF) and reverse osmosis (RO) using membranes
of varied permselectivity in treating produced water from the Niobrara
Shale play in Colorado. We determined the efficacy of each technology
in removing inorganic and organic constituents as well as reducing
toxicity on Daphnia magna. Our results show that
the pretreatment step resulted in a minor reduction of chemical constituents
and toxicity and that the NF permeates did not meet the water quality
criteria for irrigation and livestock drinking water. Despite high
removal rates for most contaminants in the produced water by RO, the
concentrations of chloride and boron as well as the sodium adsorption
rate (SAR) in the RO permeates exceeded irrigation guidelines. We
observed the passage of surfactants with molecular weights much higher
than the molecular weight cutoff of NF and RO membranes, suggesting
that membranes are not an absolute barrier to organic contaminants.
Our results demonstrate that thorough chemical and toxicological analyses
are needed to understand the feasibility and potential risk of treating
UOG produced water for beneficial reuse.