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Produced water (PW) is water trapped in underground formations. When oil or gas is extracted and brought to the surface, the fluid stream also contains produced water. In fact, PW is the largest volume of waste generated in oil and gas explorations, for example, on average nearly nine barrels of PW is co-produced with each barrel of oil extracted in the US. The cost of handling PW includes lifting a large volume of water to the surface, separating it from the petroleum product, treatment for reinjection into the ground as fracturing fluid (‘frac') or for indirect potable (IP) use, and/or disposing it in evaporation ponds.

Produced water usually contains a mixture of hydrocarbons, dissolved and suspended solids, sand and silt, and chemicals injected during exploration and production (Table 1). The total dissolved solids (TDS) can be as high as 170,000 mg/L. The PW needs to be treated especially for potable use because of several factors: (a) very high salinity, (b) very small particles, (c) volatile and semi-volatile organic compounds (VOCs), (d) extractable organics (acidic, basic, neutral), (e) ammonia, and (f) hydrogen sulphide.
 

Source: Adapted from Mondal and Wickramasinghe, J. Membrane Science, 322 (2008) 162-170.
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Dissolved organics are mainly fatty acids, benzene, toluene, ethylbenzene, xylene (BTEX), and other aliphatic organics. Fatty acids are the major constituents of dissolved organics contributing to >90% of total organic carbon (TOC). Organic acids such as napthenic acids are toxic to aquatic and non-aquatic life. These compounds along with heavy metals, radioactive particles, selenium and boron must be removed for IP use.

Several treatment processes are discussed that can render produced water as a source of ‘new' water, and can be an alternate source of potable water in lieu of seawater or brackish water.
 

Background

The composition of PW is extremely site specific, and can vary appreciably for samples from the same well over time. The type of treatment required, therefore, depends not only on the constituents in PW but also on its reuse application. For example, in the case of water for reinjection, hardness is removed to reduce the risk of precipitation of sparingly soluble salts (for example, barium and strontium sulphates) that results in scaling of production pipes and/or plugging of geological formations around the production well.

Conventional treatment includes gravity separation and skimming, dissolved air flotation, de-emulsification, coagulation and flocculation. However, these treatment processes alone cannot meet the petroleum waste discharge standards. More recently, membrane separation processes – microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO) – have been investigated for treating PW. Membrane processes, however, cannot remove all the constituents in PW (Table 1). An integrated membrane pilot plant was used for treating PW from an oil well in northern California as shown in Figure 1. The produced water TDS was 10,000 ppm, contained a high level of suspended solids and was saturated with iron, silica and boron. Boron was removed to <0.75 ppm in the RO permeate in an ion exchange polisher, presumably using weak base anion resins.

In order to remove dissolved solids, RO/NF separation is required. For RO/NF membrane systems to operate efficiently, removal of foulants including fine particles, colloids, bacteria, iron and manganese is mandatory. In addition, the amount of sparingly soluble compounds such as silica, CaCO3, CaSO4, BaSO4, and SrSO4 must be controlled with chemical treatment. Otherwise, the performance drops quickly due to scaling and fouling of the membranes. This results in frequent shutdowns for cleaning the membranes resulting in higher operating and maintenance costs.
 

Treatment methods

In 1995, the API issued recommendations called Best Available Technology (BAT) for treating Produced Water. According to the BAT report, the pollutants in PW can be reduced to undetectable levels by using combinations of different technologies detailed in Table 2. Since then additional processes including thermal evaporation, ion exchange and electro-coagulation (EC) have been explored and deployed.

Source: American Petroleum Institute's Best Available Technology for Produced Water Management and Treatment, 1995

A comprehensive process for treating high TDS produced waters from inland natural gas wells (TDS = 10,000 to 40,000 mg/L) is described below. The process shown in Figure 2 includes unit operations for removing oil/grease, suspended solids, TDS, hardness, iron, silica, selenium, boron, VOCs, organic acids and other compounds.

Oil/grease removal

The first unit operation entails separation of water from the oil/water mixture followed by gravity settling. Any residual or free oil is removed by further treatment, for example, by skimming or with cross-flow tubular UF membranes.
 

Chemical oxidation

Sodium hypochlorite is injected in the pre-treated raw water line to chlorinate the raw water and provide 5-10 ppm residual chlorine. This achieves two critical objectives: (a) pre-oxidation of reduced metals, specifically iron and sulphur, and (b) destruction of bacteria. Iron reducing bacteria and sulphur reducing bacteria are typically the dominant bacteria in PW systems, and must be removed. Oxidation of iron and its removal by precipitation is a major benefit of the process since iron is a major foulant of RO membranes. Simultaneously, oxidation of sulphur prevents the formation of sulphides and odourous hydrogen sulphide. Further, hypochlorous acid reacts with ammonia to form chloramines, thereby, substantially reducing or eliminating ammonia from water. Trace amounts of ammonia in water are removed by air stripping downstream.
 

Electro-coagulation

EC is a physical-chemical process that causes destabilisation of contaminants followed by aggregation or flocculation. The process uses an electrolytic reactor with aluminium (or iron) electrodes and a separation tank. The water to be treated passes through the reactor and is subjected to coagulation/flocculation by Al or Fe ions dissolved from the electrodes by an electrolysis process. The metal anode dissolution also results in hydrogen gas evolution at the cathode. The gas bubbles capture and float the suspended solids formed, thereby, removing the contaminants. The process also generates aluminium (or iron) hydroxide precipitates. These have large specific surface areas that allow for a wide range of organic, inorganic and particulate contaminants to be physically or chemically adsorbed, for example, silica, iron, barium and manganese are substantially removed. Suspended solids are also reduced resulting in more than 96% reduction in turbidity. The precipitates are removed in an inclined plate separator. Simultaneously, EC reduces any residual oil and grease to non-detectable levels. Recently, it has been shown that EC is highly effective in removing up to 99.9% boron. This is potentially a great added benefit since undissociated boron (at pH < 9.0) is poorly rejected by RO membranes. Reduction of boron to < 0.75 ppm is required to meet IP water standards.
 

Inclined plate settler

The EC effluent flows to a Lamella-style inclined plate settler for enhanced gravity separation of solids. The flocs formed in the EC process are large and settle easily. The settled solids volume is reduced to 5%. Most importantly oil, grease and TOC get concentrated in the solids. The organics are tightly bound in the solids ensuring they do not leach out easily.
 

Media filtration

Gravity sand, dual-media, or multimedia filters (MMF) are used to remove particles and clarify water. Sand filters reduce turbidity to 5 NTU whereas MMF reduce the turbidity to 1 NTU. Multimedia filtration is capable of removing 96% of particles (> 40 μm) and 75% of particles (10 to 40 μm). Since suspended solids are collected on the media, regular cleaning (backwashing @ 25-40 m/hr) is required.
 

Membrane softening

Softening is required to remove scaling compounds CaCO3, CaSO4, BaSO4, SrSO4, MgCO3, Mg(OH)2, BaCO3 and SrCO3. Nanofiltration (NF) membranes have a high rejection for divalent ions including calcium, magnesium and sulphate. Hence, NF is being increasingly used to remove hardness compounds as shown in Figure 1. Unlike chemical softening it does not generate sludge. As compared to RO it operates at a lower feed pressure and higher recovery, and is less prone to fouling.

The NF permeate is essentially ‘soft' brine free of suspended solids, particles, oil and grease, bacteria and hardness compounds but contains 60-70% of incoming residual hydrocarbons. It can be used for reinjection into the ground as ‘frac' water. Additional treatment is required for removing VOCs, TDS and trace amounts of other compounds to render it suitable for IP use.
 

Iron removal

Any traces of iron in water can be reduced to < 0.05 ppm in Pyrolox® media filters. Iron removal is critical for protecting the RO membranes downstream from iron fouling, which is often severe and irreversible. Pyrolox®is a mineral form of manganese dioxide. It is a granular filtration media for removing/reducing hydrogen sulphide, iron and manganese. It works on the principle that hydrogen sulphide, iron and manganese are oxidised catalytically by the media, which also traps the oxidised compounds. Chlorine and other oxidants accelerate the catalytic reaction. Typically, 1 ppm of chlorine is consumed per ppm of iron oxidised. The iron level in water should be < 7-8 ppm for the process to be economical. One advantage of this media over other iron removing media such as manganese greensand is that the media does not need to be chemically regenerated. Only backwashing is required to remove entrapped particles from the media by regular cleaning (backwashing @ 50-75 m/hr).
 

Air stripping

VOCs are removed in an air stripper, a multi-staged shallow tray type that removes > 95% of VOCs as well as benzene, toluene, naphthalene, phenols and methanol. Up to 90% of semi-VOCs such as acetone are also removed along with any traces of ammonia and hydrogen sulphide. Air stripper packing material is prone to fouling by oil and scaling by iron and calcium. However, these are removed before water flows to the stripper.

Emissions from the air stripper can be treated in a regenerative thermal oxidiser heated with natural gas at 850°C. The thermal oxidiser has a destructive efficiency of > 99%.
 

Ultraviolet (UV) disinfection/dechlorination/TOC destruction

The 254-nm rated UV lamps destroy organic matter and deactivate biological organisms thus protecting the RO membranes from biological fouling. Ultraviolet light inactivates microorganisms through disruption of their DNA processes. When organisms try to replicate, they die. The UV lamps are rated for a dosage > 30,000 μwatt-seconds/cm2 @ 254 nm wavelength after 8,000 hours of operation. More powerful but expensive UV lamps [dosage > 120,000 μwatt-seconds/cm2 @ 254 nm] are also used for dechlorinating RO feed water instead of carbon filtration or sodium bisulphite. Dechlorination is required to protect polyamide RO membranes from damage by oxidants. UV lamps used for dechlorination are high intensity, broad-spectrum UV lights that dissociate both free chlorine and chloramine compounds (mono-, di- and tri-) into easily removed byproducts. Additional important benefits of using UV dechlorination are: (a) High levels of UV disinfection, and (b) TOC destruction. Since dechlorination UV units are large and expensive, TOC destruct UV (185 nm) followed by dechlorination by sodium bisulphite is often preferred.
 

Anti-scalant and sodium bisulphite in-line treatment

An anti-scalant is injected in RO feed water (UV effluent), if necessary, to inhibit scaling by any remaining sparingly soluble species and to increase product water recovery. A typical dosage is 2-5 ppm. Sodium bisulphite is injected in RO feed water to reduce the free chlorine concentration to < 0.2 ppm when carbon filtration or UV dechlorination is not used. Theoretically it takes 1.46 ppm of sodium bisulphite to remove 1 ppm of free chlorine. Typically, 1.8-2 ppm is used. A minimum contact time of 5 seconds is required.
 

RO desalination

Dechlorinated water flows through a 5.0 μm pore size cartridge filter. The microfilter removes fines and colloidal particles, thereby, protecting the RO membranes from particulate fouling. The TDS of PW typically varies between 10,000 and 50,000 ppm corresponding to the upper limit of brackish water and seawater salinity, respectively. Depending on the feed water TDS, brackish water membranes rated for salt rejection not, vert, similar98% or seawater membranes rated for salt rejection not, vert, similar99.7% are used. The RO membrane reduces the TDS to < 500 ppm. Brackish water and seawater RO systems operate at different product water recoveries, i.e. 70-80% for brackish water and 35-45% for seawater [%recovery = (product flow rate/feed flow rate) × 100].

RO product water recovery is controlled by the concentration of sparingly soluble compounds in RO reject/brine water because of scaling/fouling as discussed earlier. The pre-treated PW (RO feed water) is essentially devoid of scaling and fouling compounds and consists mostly of sodium and chloride ions. Hence, high recoveries are attainable. However, constraints due to high osmotic pressure restrict the recovery to < 70%. Higher the osmotic pressure, higher is the RO feed pressure required to overcome the osmotic pressure. Further, higher the recovery, higher is the feed pressure required to pump water through the membrane. In order to achieve higher overall recoveries, vapour compression distillation (VCD) can be used to recover purified water from the RO reject/brine stream. VCD is, however, both energy and capital intensive. The reject brine stream flows to an evaporation pond.

RO membranes typically remove 90-97% of VOCs and organic and non-organic compounds not removed by air stripping.
 

Carbon filtration

Carbon filtration is used to remove any traces of hydrocarbons and chloramines. Depending on the organic load after UV treatment, carbon filtration may be used upstream of the RO unit. Further, the filters may be used in series to maximise removal, if necessary. Carbon adsorption treatment also removes metal ions particularly cadmium, hexavalent chromium, silver and selenium as well uncharged species such as arsenic and antimony from acidic streams.
 

Ion exchange polishing

Ion exchange (IX) polishing with strong base anion resins may be required for removing trace amounts of selenium to meet the EPA's National Primary Drinking Water Standard of < 50 ppb. For the IX process to be effective, selenium must be in the oxidised form, selenate (SeO42−). The IX process can operate for 15 days until bed exhaustion provided the feed composition is as follows (approximately): SeO42– = 0.05 ppm; SO42– = 2 ppm; Cl = 160 ppm; NO3− = trace amounts. The resins are regenerated with 4.5 kg NaCl (5%), in a co-current flow mode.

In addition, IX can be deployed to remove boron. Weak base anion resins are very effective in removing boron to below detectable limits. Borate (BO3) selectivity is extremely high when the pH is > 4.0. The resin is regenerated with 0.5 to 0.6% HCl or H2SO4.
 

Calcite remineralisation

The RO permeate is usually slightly acidic. Hence, it needs to be remineralised and the pH adjusted to not, vert, similar 7.5 before it is discharged. Water is passed through a calcite (limestone) media filter. Calcite dissolution results in increasing alkalinity (HCO3−) and hardness (Ca2+). The media is replenished based on effluent pH. Calcite can be pure in composition (CaCO3) or can contain low concentrations of magnesium forming magnesium calcite. Natural limestone can contain minerals or impurities such as dolomite [CaMg(CO3)2] and quartz (SiO2). Remineralisation with calcite is simpler and cheaper than dosing with compounds such as calcium hydroxide.
 

Conclusions

Produced water handling and treatment represents an $18 billion cost to the oil and gas industry in the US. The multiple unit operation treatment process similar to the one described is typically required for reclaiming PW for indirect potable uses, i.e., groundwater recharge, discharge to rivers and lakes, irrigation, and industrial applications such as power plant boiler feed water. The treatment processes described can also improve the economic viability of oil and gas fields by providing a source of ‘new' water.