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Fracking, or hydraulic fracturing, is a water-intensive industry, and also produces immense volumes of wastewater. The treatment of this wastewater is an emerging challenge.
Where Does Fracking Wastewater Come From?
Hydraulic fracturing is performed chiefly for shale oil and gas wells within the US and elsewhere, with thousands of wellheads yielding oil and tapping natural gas reserves. However, the downside is the enormous pressure this places on groundwater and surface water reserves - up to a million gallons being required for a single wellhead.
Simultaneously, there is a concern as to how to best dispose of the huge volumes of fracking wastewater. At least 60% of the water used for fracking flows out almost immediately (“flowback”), but the water present in the well already (“produced fracking wastewater”) is discharged more slowly, at up to 100 000 gallons a day.
This wastewater potentially contains high total dissolved solids (TDS), and various chemicals used in fracking, bacteria, disinfectants, oil, gas, metals, and natural radioactive substances. For instance, it may contain benzene derivatives, toluene, and xylene (BTEX, nitrogen-containing compounds, which are known carcinogens), and dissolved metal ions such as iron and manganese.
A wellhead typically operates for over 20 years, whereas most water and wastewater handling solutions are designed for the short term. A long-term approach to this is urgently required in view of the growing recognition that water supply and disposal are both sensitive areas.
How is Fracking Wastewater Treated?
Short-term solutions include impounding ponds where the wastewater is evaporated, deep-well injection of trucked wastewater, distant treatment of wastewater, or treating and reusing the flowback at the site itself.
Reused water is typically first treated and then diluted with additional freshwater to bring down the salt concentration, if necessary. This water is used only for fracking, and is usually stored temporarily in private designated ponds not used for any other purpose. These are also often strictly regulated.
Another method is to truck the water to local sewage treatment plants for processing, either publicly owned treatment works (POTWs) - state or municipal plants - or centralized waste treatment facilities (CWTs) - privately owned plants. These plants must obtain special permits based on the technology they use and their water quality conditions. They must also meet requirements for local water quality standards, based upon the intended purpose of the water body.
Fracking wastewater treated at industrial sewage plants still contains higher than normal amounts of toxic heavy metals, for instance. This could eventually seep into the groundwater used as a source of drinking water. Three main processes are included:
- Three-phase separation in the primary treatment – removal of gas, oil, sand, suspended solids, and gel, before storage to homogenize the chemical constituents
- Secondary separation where contaminants are further removed using dissolved air or gas flotation, and bactericidal chemicals to destroy bacteria
- Precipitation of metals and salt removal by reverse osmosis
- Dewatering to convert the collected solids into sludge for further disposal
Deep well injection is regulated under Class II requirements, which prevent the wastewater from mixing with drinking water sources underground. Some US states prohibit this technology, making it necessary to truck the wastewater across thousands of miles to other states at a cost of millions of dollars over the well lifecycle. This has also sparked concern about the effect of these fleets of trucks on the road surface, community safety, and road safety.
These methods are coming under stricter legislative scrutiny on environmental counts, as well.
How Do Portable Solutions Work?
Mobile solutions for on-site recycling aim to reuse the flowback wastewater in the same or different wells. Different methods used here include mechanical, photochemical and electrochemical techniques.
One typical treatment system combines hydrodynamic cavitation, ultraviolet-ozone treatment, and an electrochemical oxidative cell, to treat flowback wastewater. Such solutions can eliminate BTEX while reducing metals and nitrogen compounds by 85% to 95%. These portable systems are cost-effective and easy to operate. They cut down the number of steps needed to treat the water. They eliminate the need for trucking, thereby cutting costs. It also prevents the need for impounding ponds, and reduces the chances of groundwater contamination.
The drawback of such systems is that they cannot deal with produced wastewater which runs for 20 years or more. Thus the long-term environmental and community aspects of fracking wastewater management are ignored.
Are Centralized Solutions the Key to Fracking Wastewater Management?
The most promising option is a centralized water management system for the whole area, designed to handle core issues like freshwater sources, wastewater management issues, and environmental impacts. This can thus provide integrated solutions for the lifetime of the well.
A single plant can handle wastewater flowback from multiple wellheads and the produced wastewater thereafter. It is also capable of reusing municipal and other sources of wastewater which would otherwise not be accessible to the mines. Alternative freshwater sources can be tapped as well, such as old mines and stormwater control basins.
One possibility is to bring flowback and produced wastewater from all the wells within a specified radius, say, 50 miles, directly via pipeline to the treatment plant. Each pipeline is specifically labeled and the use for its wastewater is defined, so that it undergoes the correct treatment and is then carried to its ultimate destination.