Treating wastewater from oil and gas exploration and extraction can enable the effective recovery of water for reuse andhelp manage the compliance and risks associated with overuse and discharge into the environment.

Importantly, implementing a good water management plan can substantially reduce fresh water requirements and costs in the long term, as well as improved production rates, reduced maintenance costs, and increased equipment lifespan.

Produced water is the single biggest source of wastewater in the oil and gas sector and is a complex mixture of dissolved and undissolved organic and inorganic substances. It is considered hazardous to the environment due to the presence of toxic constituents such as dissolved and dispersed oil compounds, dissolved formation minerals, production chemicals, dissolved gases, and productions solids (formation, corrosion, scale, bacteria, waxes, and asphaltenes).

A 2020 investigation by the United States EPA noted that the majority of oil and gas sector wastewater was disposed of by underground injection, where it could no longer be accessed or used.

The US EPA said: “Produced water is the largest wastewater source by volume generated during oil and gas extraction. “[This] is the fluid (often called brine) brought up from the hydrocarbon-bearing strata during the extraction of oil and gas and includes, where present, formation water, injection water, and any chemicals added downhole or during drilling, production or maintenance processes.”

Naturally occurring constituents of produced water can include bromide, calcium, chloride, magnesium, sulphate, and radioactive materials, while chemicals added downhole can include those used for hydraulic fracturing, well stimulation, and well maintenance.

When released into the environment, produced water can reduce the dissolved oxygen concentration and the formation of sludge deposition, affecting aquatic and biotic lifeforms.

Only about 20 per cent of produced water is treated, and the release of untreated produced water into the environment generates a greenhouse gas footprint three times greater than if it was treated.

Properly treated wastewater can be reused for the irrigation of adjacent green areas, fire flow network, refrigeration systems, boilers, and water for cleaning vehicles.

A study of three different process water samples from oil and gas production in Qatar found it contained harmful components including polyaromatic hydrocarbons, phenol, heavy metals, ammonia, and other hydrocarbons and non-hydrocarbons.

It also found all three had high organic-containing wastes, were enriched with zinc and iron, and contained 16 different identifiable hydrocarbon compounds, with dominant ones including acenaphthene, acenaphthylene, fluorene, anthracene, phenanthrene, benzo anthracene, and pyrene.

The authors said: “It has been estimated that the amount of process water produced is about 0.4 to 1.6 times the crude oil produced. “It is noteworthy that although many processes of the oil and gas industry use water, not each process necessarily required raw or treated water. “To aid the petroleum sector’s development while also protecting the environment, well-developed water management and wastewater treatment technologies are critically required.

“The industrial processes should be innovative, well balanced, and optimised to achieve efficient and sustainable water resources use and to eliminate the negative impact on the environment by the treatment system.”

There are many methods and technologies available to treat produced water, with varying advantages and suitability for different contaminants, including hydro-cyclones, thermal separation, adsorption, chemical treatment, membranes and biological treatment.

Hydro-cyclones are used to separate solid particles like sand and oily contents, based on density difference, and is common among oil and gas companies as they do not require chemicals or energy to operate.

There are three widely used thermal separation technologies: multistage flash distillation, vapour compression distillation, and multi-effect distillation. All three involve heating produced water and lowering its pressure to evaporate the water in a series of stages, with antiscaling agents and acids used to prevent scaling.

Thermal separation also does not require chemicals and produces no sludge, but 95 per cent of the operating cost comes from energy consumption, necessitating solar power or other renewable energy sources.

Adsorption processes filter organic materials and are often used in conjunction with other treatment methods rather than a standalone technology. Adsorbents can remove more than 80 per cent of heavy metals like iron, manganese, total organic carbon, oil, and BTEX (benzene, toluene, ethylbenzene, xylenes).

Typical materials used for adsorption treatment include zeolites, chitosan, activated carbon, copolymers, resins, activated alumina, silica gel, molecular sieves, and organoclays.

Chemical treatments involve methods such as coagulation, flocculation, precipitation, ion exchange, neutralisation, stabilisation, and advanced oxidation processes (e.g. chemical oxidation, Fenton reaction, ozone treatment, and photocatalytic oxidation.

Chemical oxidation is used to eliminate contaminants such as colour, odour, residual chemical oxygen demand, biochemical oxygen demand, organic compounds, and some inorganic compounds.

To reduce organic and inorganic materials present in produced water, membrane treatment is an effective process and consists of four different types of membrane based on pore size: microfiltration, ultrafiltration, nanofiltration, and reverse osmosis.

Membranes are thin films of synthetic organic and/or inorganic material which enable the separation of specific components within produced water.

The most widely used inorganic membrane is ceramic, which has several advantages over other membranes including high mechanical strength and chemical compatibility, long operational life, sufficient thermal stability, and potential low lifecycle cost, but comes with a high capital cost.

Most treatment methods are not able to eliminate the dissolved organic materials found in produced water, requiring the use of microbial or biological treatment methods, which are efficient because dissolved contaminants are consumed by microorganisms to form biomass.

Types of microbial/biological treatment include biological aerated filters, biofiltration, hybrid constructed wetlands, microbial fuel cells, wet air oxidation, and electrowinning.