(12h) Application of Advanced Oxidation Process for the Treatment of Hydrofracked Water

APPLICATION
OF ADVANCED OXIDATION PROCESS FOR THE TREATMENT OF HYDROFRACKED WATER

      
                           Shikha Sinha, Debashis
Roy, Sudarsan Neogi, Sirshendu De

                            
                     Indian Institute of Technology, Kharagpur

                                          
                        Kharagpur-721302

One
of the key technologies in the exploration of shale gas, an important
unconventional natural gas, is hydraulic fracturing. During hydraulic
fracturing, the fracturing fluid, consisting mainly of water mixed with proppant
(sand) and chemicals (such as surfactants, chelating agents, friction reducers,
scale inhibitors and biocides) is injected at high pressure (450-800 bar) into
the producing formation, creating fissures that allows natural gas to be
released from rock pores. About 70% of this fluid is returned to the surface
after the fracturing process. It is commonly known as shale gas drilling
flowback fluid [1],[2].
These fluids typically have high dissolved salt concentration (e.g., 10,000 to
300,000 mg/L TDS), and contain various metals, dissolved and suspended solids, radionuclide
and organic compounds. To minimize the adverse affect on environment, and to reduce the production cost and to conserve
fresh water, development of efficient on-site treatment technologies is
required. The primary consideration for produced water treatment to a quality
suitable for discharge or for external reuse is reducing the Total Dissolved Solids
(TDS) concentration. Significant pretreatment is required to reduce fouling and
scaling potential before produced water may be desalinated using membrane or
thermal technologies.

The
conventional primary treatment for wastewater consists of a
coagulation-flocculation process which is only able to remove suspended solids,
whereas soluble organic compounds remain unaffected. Advanced Oxidation Process
(AOP) is a secondary treatment process based on generation of highly oxidative
and non selective reactive species (free radicals) mainly hydroxyl radicals (oxidation
potential of 2.8eV, next only to fluorine) facilitating the organics removal. AOPs
have been successful in treating the non-biodegradable pollutants such as
herbicides, landfill leachate and wastewater originating from textile industries.
However, use of AOP for hydrofracked water treatment is scant in literature. AOPs
commonly include various processes like Fenton process, photo-assisted Fenton
process, photo catalysis, Ozone/UV, H₂O₂/UV process etc. Fenton
process is one of the widely used AOPs because of its efficiency, easy
operation, relatively cheap cost and rapid oxidation kinetics. The hydroxyl
radicals generated by reaction between Fenton’s reagent (ferrous sulfate and
hydrogen peroxide) is responsible for organic substrate oxidation in the Fenton
reactions. Besides, the ferric ions (Fe³⁺) produced during the
reaction can also aid in coagulation by charge neutralization making Fenton
process suitable for both coagulation and oxidation [3].
The ideal solution pH for Fenton process should be between the ranges of 3.0-4.5.
For pH˂ 3.0, the reaction is slow due to the
formation of complex ions; also, for pH˃4.5, the generation of ferric
hydroxo complexes reduces the production rate of OH^. ions, thus
reducing the oxidizing efficiency. In photo-assisted Fenton process, irradiation
with UV-VIS light accelerates the rate of degradation of organic pollutant with
Fenton reagent, which causes photolysis of Fe³⁺ to regenerate Fe²⁺.
However, the use of UV makes it an energy intensive and costly process. Photo catalysis makes use of a semiconductor
metal oxide catalyst, titanium dioxide (TiO₂) being the most
promising one. But separation of titanium dioxide nanoparticles from the sample
is difficult and not suitable for further tertiary treatment (membrane
separation) of hydrofracked water. The use of oxidation treatments based on O₃/UV
and H₂O₂/UV systems requires the use of suitable UV
sources and of appropriate photochemical reactors. Hence Fenton’s process
without UV irradiation, though a conventional process, is chosen among other AOPs.

In
this work, performance of primary and secondary water treatment processes to
treat hydrofracked water collected from Forward base, ONGC, Ankleshwar, Gujarat,
India were investigated. Coagulation with Potash alum (K₂SO₄,
Al₂(SO₄)₃, 24H₂O), an inorganic
coagulant was studied. Coagulant dose was varied between 10 to 500 mg/L. Variation
of Total Organic Carbon (TOC) with varying coagulant doses was studied. It was
found that 100 mg/l of Potash alum was giving the maximum organics removal with
52% removal efficiency. Fenton process was carried out using ferrous sulfate heptahydrate
(FeSO₄.7H₂0) and hydrogen peroxide (H₂O₂,
50 vol%). pH was maintained between 3-3.5 to avoid the formation of ferric-hydroxo
complexes which inhibits the production of hydroxyl radical. Ferrous sulfate concentration
was varied from 0.001mol/L to 0.002 mol/L while hydrogen peroxide concentration
was varied from 0.1 mol/L to 0.2 mol/L, varying the reaction time from 15 to 60
minutes. The organic removal efficiency was found to be maximum for 0.002 mol/L
ferrous sulfate concentration and 0.2 mol/L hydrogen peroxide concentration.
The TOC removal efficiency with 0.002M ferrous sulfate and 0.2M hydrogen peroxide
at pH 3-3.5 was found to be 97.7%. Graphical representation of the experiments
conducted is shown below in figures 1, 2 and 3. Figure 1 shows that organics removal
is maximum for 100 mg/L of Potash alum dose. Figure 2 shows that for a given
iron concentration, rate of dissolved organics removal increases with increase
in peroxide concentration (maximum removal for peroxide of 0.2M) due to rapid
reaction and increase in generation of hydroxyl radicals. Figure 3 shows the
variation of iron concentration for 0.2M peroxide concentration. It shows that
with increase in iron concentration organics removal rate also increases
because of increase in generation of hydroxyl radical (maximum removal for
0.002M iron sulfate). Figures show that the removal efficiency was observed to
be more for Fenton process where both ferrous and ferric ions act as coagulant
by charge neutralization. Hence, Fenton process is aiding both in coagulation
and oxidation. We found that the use of Fenton process eliminates the need of
primary treatment such as coagulation. The hydrofracked water can be directly
treated by using Fenton process (secondary treatment) for organics and
suspended solids removal eliminating the need for primary treatment.

Keywords:
Hydrofracking, Flowback water, Coagulation, Fenton process, Total Organic
Carbon

Figure
1. Variation of TOC with Potash alum dose (mg/L).

Figure
2.  Variation of TOC with Concentration ratio (Peroxide conc./ 0.002M Iron conc.)

Figure
3.  Variation of TOC with Concentration ratio (0.2M Peroxide conc. / Iron conc.)

References:

[1]      G.
Chen, Zhouwei Wang et al., “Treatment of shale gas drilling flowback
fluids (SGDFs) by forward osmosis: Membrane fouling and mitigation,” Desalination,
Vol. 366, pp. 113–120, June 2015.

[2]      R.
D. Vidic, S. L. Brantley et al., “Impact of shale gas development on
regional water quality,” Science, Vol. 340, Issue no. 6134, May 2013.

[3]      P.
Banerjee, Sunando Dasgupta et al., "Kinetic Study of Advanced Oxidation
of Eosin Dye by Fenton’s Reagent" Internal Journal of Chemical Reactor
Engineering, Vol. 6, January 2008.