Statement of Problem:

The primary goal of treating produced water is to physically remove as much free product (as hydrocarbons) as possible. Optimum removal processes should be able to reduce the free product concentration to below 100 ppm. In practice, however, we have found that produced water usually contains around 300 to 400 ppm TRPH and 200 to 300 ppm other organics (such as methanol and emulsion breakers). In addition to free product concentrations, produced water is at saturation concentrations for all the hydrocarbons present. The hydrocarbons in solution cannot be removed by physical methods.

Bioremediation of Produced Water

When evaporation ponds are used for disposal of produced water containing any hydrocarbons, a series of problems develops. Normally hydrocarbons are lighter than water and float to the surface. Lighter hydrocarbons evaporate and contribute to air pollution. Other hydrocarbons begin to be oxidized and hydrated. These modified hydrocarbons become heavier than water and sink to the bottom of the pond. At the bottom of the pond this material quickly becomes anaerobic where sulfate reducing bacteria (SRB’s) grow and produce hydrogen sulfide (rotten egg gas) and carbon dioxide.

Hydrogen sulfide first turns the sludge on the bottom of the ponds black and then as more hydrogen sulfide is produced the entire pond turns black and smells of rotten egg gas. Small bubbles of hydrogen sulfide and carbon dioxide collect oil and diffuse to the surface. This exact same process of allowing small bubbles to bring oil to the surface is used in dissolved air flotation (DAF’s) devices to speed up the separation of oil from water.

This process creates a smelly, oily scum on the surface of the pond. The scum is a hazard to water fowl and greatly reduces the rate of evaporation from the pond. It is often necessary to daily collect the oil by running a boom across the pond and then remove the collected scum with a vacuum truck. Often pond operators attempt to treat this problem by adding biocides to the water. These attempts are expensive and often ineffective. The biocides are usually inactivated by reduced sulfur compounds produced in the anaerobic ponds and the SRB’s become resistant to the biocides used (much as microbes become resistant to antibiotics used in hospitals).

Here are photographs of ponds before treatment

Here are photographs of ponds after treatment

Produced Water General Characteristics:

Produced waters from gas production have high contents of low molecular-weight aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX). The chemicals used for gas processing typically include dehydration chemicals, hydrogen sulfide-removal chemicals, and chemicals to inhibit hydrates. Well-stimulation chemicals that might be found in produced water from gas operations include mineral acids, dense brines, and additives.

Specific Produced Water Constituents and Their Significance:

In addition to “Free Product” hydrocarbons, produced water has a number of dispersed or dissolved compounds.

Dispersed Oil:

Dispersed oil consists of small droplets suspended in the aqueous phase. These dispersed oils are often 4 to 6 microns in size and physical treatments systems typically do not remove droplets smaller than 10 microns. However, these small droplets are collected by the carbon dioxide and hydrogen sulfide gas bubbles and will be brought to the surface of ponds where they will collect and create an oily pond scum.

Dissolved or Soluble Organic Compounds.

Hydrocarbons that occur naturally in produced water include organic acids, polycyclic aromatic hydrocarbons (PAHs), phenols, and volatiles. These hydrocarbons contribute to produced water toxicity, and their toxicities are additive, so that although individually the toxicities may be insignificant, when combined, create a bio-hazard.

The only way to remove soluble organics is by the bioremediation process described below. Generally, the concentration of organic compounds in produced water increases as the molecular weight of the compound decreases. The lighter weight compounds (BTEX and naphthalene) are less influenced by the efficiency of the oil/water separation process than the higher molecular weight PAHs and are not measured by the oil and grease analytical method.

In addition to volatile hydrocarbons, there are a number of very soluble organic compounds in produced water. There compounds include the low molecular weight (C2-C5) carboxylic acids (fatty acids such as acetic, and propionic acid), ketones (such as acetone), and alcohols (such as methanol). In some produced waters, the concentration of these components is greater than 5,000 ppm. Due to their high solubility, the organic solvent used in oil and grease analysis extracts virtually none of them, and therefore despite their large concentrations in produced water, they are not detected by oil and grease measurements. (See Ali, S.A., L.R. Henry, J.W. Darlington, and J. Occapinti, 1999, 9th Produced Water Seminar, Houston, TX, January 21-22).

Partially soluble components include medium to higher molecular weight hydrocarbons (C6 to C15). They are soluble in water at low concentrations. They are not easily removed from produced water and will accumulate in evaporation ponds. These compounds include aliphatic and aromatic carboxylic acids, phenols, and aliphatic and aromatic hydrocarbons.

Aromatic hydrocarbons are substances consisting of carbon and hydrogen in benzene-like cyclic systems. PAHs, are hydrocarbon molecules with several cyclic rings. Naphthalene is the most simple PAH and is present in higher concentrations than other PAHs. “Light” PAHs are more water soluble than “heavy” PAHs. PAHs increase biological oxygen demand, are highly toxic to aquatic organisms, and are carcinogenic to man and animals. All are mutagenic and harmful to reproduction. Heavy PAHs bind strongly to organic matter.

Aromatic hydrocarbons and alkylated phenols are major contributors to toxicity. Alkylated phenols are endocrine disruptors and damage reproductive effects.

The dispersed and soluble organic compounds are readily degraded by bioremediation.

Treatment Chemicals:

Treatment chemical posing the greatest concerns for aquatic toxicity include biocides, reverse emulsion breakers, and corrosion inhibitors. Some of these compounds can be lethal at as low as 0.1 parts per million. In addition, corrosion inhibitors can form more stable emulsions, thus making oil/water separation less efficient.

Produced Solids:

Scales (calcium carbonate, calcium sulfate, barium sulfate, strontium sulfate, and iron sulfate) clog flow lines, form oily sludge’s that must be removed, and form emulsions that are difficult to break.

Produced water can also contain precipitated solids, sand and silt, carbonates, clays, proppant, corrosion products and other suspended solids derived from the producing formation and from well bore operations.

Bioremediation Concept:

Select microbial consortiums are able to use hydrocarbons as food. In the presence of oxygen they convert hydrocarbons into carbon dioxide and water. This process is very similar to how animals convert sugars into carbon dioxide and water as they breathe oxygen.

In addition to oxygen, microbes require a proper pH, trace elements, inorganic nitrogen and phosphate sources.

During bioremediation, microorganisms metabolize hazardous substances found in produced water into carbon dioxide and water. The formation of scum on the surface of the pond and the accumulation of sludge on the bottom of the pond is prevented.

Procedures:

After as much hydrocarbon is removed by physical methods as possible, produced water is moved into evaporation ponds. Inorganic nutrients are added to the pond water and the water is inoculated with a consortium of microbes that have been selected and optimized to convert all of the hydrocarbons found in produced water into carbon dioxide and water.

Aerators and mixers are used to add enough oxygen to the water to oxidize the hydrocarbons and prevent anaerobic conditions from developing. The amount of oxygen and mixing needed is calculated based on pond size, volume of water entering the pond on a daily basis and the concentration of contaminates in the water.

It is also best to divide the pond into at least two sections. The first section is continually contaminated with hydrocarbons from the incoming produced water. Ninety percent of these hydrocarbons are removed before going into the second portion of the pond where the degradation process is completed.

Produced water treated by bioremediation can be reused for other purposes. We have a system in central Wyoming that was permitted to surface discharge treated produced water. We have systems in Colorado where treated produced water is reused to “Frac” new wells.

It is important to understand that this approach to treating produced water differs from the general concepts used in treating municipal waste water. In treating municipal waste water a high biomass is maintained by collecting and recalculating an old microbial population (activated sludge).

In treating produced water we maintain a small but a young and very active microbial population (similar to a continous culture bioreactor). This prevents the accumulation of a large biomass and the problems associated both with maintaining a large biomass and separating this biomass from the clean water.

Limitations:

The beneficial reuse of produced water requires that the TDS, pH, TRPH, the concentrations of chlorides and sulfates and other parameters or constituents toxic to animal or plant life as reasonably prescribed, meet strict requirements not be deemed harmful to the environment, wildlife or migratory birds. TDS, arsenic, barium, cadmium, hexavalent chromium, total chromium, lead, mercury, zinc, selenium and radioactivity contaminates as defined in 40 CRF 261 will not be removed by bioremediation.

Example:

The following is data obtained from a produced water treatment system and the percent removal of the Benzene, Ethyl Benzene, Toluene, Xylenes and Naphthalene (BETEXN).

Sample Date Analyte Percent Removed
06/15/2007 BETEXN 99 +
TRPH 99+
07/17/2007 BETEXN 57+
TRPH 99+
08/16/2007 BETEXN 99+
TRPH 99+
09/17/2007 BETEXN 99+
TRPH 99+
10/17/2007 BETEXN 99+
TRPH 99+
Typically, most of the components are at non detect levels after treatment.

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