Environmental Engineering Reference
In-Depth Information
chloride, barium, and strontium in fracking luid delivered to new wastewater treatment
plants [30]. Commercial treatment options include crystallization (zero liquid discharge),
thermal distillation/evaporation, electrodialysis, reverse osmosis (RO), ion exchange, and
coagulation/locculation and settling and/or iltration [30]. However, conventional meth-
ods for salt removal from seawater, such as RO, are inadequate to deal with frac luids that
are high in TDS including high chlorides (see Figure 28.10). It should be noted that initial
water lowback from the well tends to have lower TDS as the water has been in contact with
the well structure for a shorter time. Hence, as water continues to low from the well, the
salinity can increase signiicantly [29]. As shown in Figure 28.10, the TDS for frac luids can
reach >200,000 mg/l in contrast to 30,000-40,000 mg/l for seawater. Challenges associated
with membrane fouling, concentration polarization, and degradation of RO membranes
in high-chloride waters make use of most membrane technologies extremely challenging
for treating frac luids [29]. RO is generally considered uneconomical at TDS concentra-
tions >40,000 mg/l; some success has been shown in membrane vibratory shear-enhanced
processing, with lower (~18,450 mg/l) TDS waters but high (9000 mg/l) total suspended
solids (TSS) [29,31]. In addition, the presence of metallic contaminants such as barium and
strontium (which usually will exist in their ionic forms) exacerbates the scaling and for-
mation of precipitated salts, adding another layer of complexity to successful treatment.
Frac luids also contain several chemicals, notably silica and a variety of gelling agents.
Treatment of high-silica water remains a challenge for the water industry, and now with
shale energy development this aspect is directly connected to the energy industry. Silica
can form almost irreversible scale, which is a problem for water transport and piping. In
water, silica comes in two forms, colloidal and reactive. Resins can be effective at removing
reactive silica; however, colloidal silica contaminates the resins and causes them to be inef-
fective at removing the reactive silica. RO has been shown to remove approximately 80%
of reactive silica and 99.8% of colloidal silica [32]. However, due to the challenges of mem-
brane treatment, thermal distillation or crystallization has been shown to be very effective;
however, these processes are energy intensive and cannot handle large volumes of water,
necessitating holding tanks until the water can be treated [22]. The energy intensity of
these processes are likely to be similar to other mechanical desalination processes, which
require approximately 150-309 kJ/l in heat and electricity for pumping [22]. Processes such
as combined ceramic microiltration and ion exchange, membrane distillation, forward
osmosis membranes, and high-eficiency RO are all being developed for the high demands
of HF water treatment [33,34].
Nanotechnology also offers potential solutions to high-salinity water puriication, and as
the next generation of membranes continue development, treatment options for high-salinity
brines such as HF lowback are likely to expand. Advanced nanostructured membranes are
being constructed from a myriad of new materials, including carbon nanotube membranes,
graphene, graphene oxide, zeolites, and boron nitride nanotubes [35-39]. These rapid perme-
ation membranes separate by molecular sieving instead of by solution/diffusion that state-of-
the-art commercial polymeric membranes (such as RO) use [36]. Carbon nanotube membranes
have been shown to achieve water velocities approximately three times higher than pre-
dicted by Hagen-Poiseuille low, with measured low velocities ranging from 9.5 to 43.9 cm/s,
which far exceeds the predicted 0.00015-0.00057 cm/s [40] due to slip low within nanotubes.
Nanotechnology also offers membrane-less separation devices such as those that employ con-
centration polarization to separate molecules and ions from water, and have been shown to
remove 99% of salt when tested with seawater [41].
To reduce the costs associated with freshwater withdrawals and transportation costs, some
producers have developed processes to reuse fracking luid. Direct reuse, onsite treatment
