Wastewater treatment in the oil and natural gas industry

• 1. Wastewater from the Oil and Natural Gas Industry: a technological challenge.
  • 1.1 Circular economy solutions for water reuse in oil&gas
  • 1.2 Oil and gas wastewater and alternatives for wastewater management
• 2. SIGMA case study: water treatment at the UPT Thermal Production Unit in Ibiza.
  • 2.1 General project data
  • 2.2 Description of the wastewater treatment plant at the Ibiza UPT
• 3. References

1. Wastewater from the Oil and Natural Gas Industry: a technological challenge.

1.1 Circular economy solutions for water reuse in oil and gas

The oil and natural gas industry generates the energy needed to carry out numerous human and industrial activities.

The high water consumption in oil and gas extraction and processing requires the development of efficient, sustainable, and cost-effective solutions for the treatment of wastewater generated in refineries, as well as sludge from extraction wells. These effluents have a direct impact on the sustainability of water resources, the environment, and other industries and sectors.

One of the main environmental objectives of the oil and gas industry is to reduce water demand , which involves implementing circular economy solutions that promote the reuse of wastewater and the conservation of resources and raw materials, thus contributing to economic and environmental sustainability.

Properly treated wastewater can be reused in a variety of applications, such as:

• Irrigation of adjacent green areas
• Fire flow network
• Refrigeration systems
• Boilers
• Cleaning water

1.2 Oil and gas wastewater and alternatives for wastewater management

The three main activities of this industry are:

• Crude oil and natural gas extraction.
• Transformation of crude oil into its fractions.
• Energy production from petroleum and natural gas derivatives.

Wastewater from the oil and gas industry can have very different characteristics depending on the plant (crude oil extraction, cracking, fuel and gas utilization, etc.), and can also fluctuate in itself. However, it has the following basic characteristics:

• High content of fats, oils and hydrocarbons.
• Dissolved organic matter.
• Petroleum residues.
• High suspended solids content.
• Considerable viscosity.
• Dissolved salts.
• Heavy metals.

Treating this wastewater requires designing an efficient, flexible, and robust process that integrates different technologies.

The most effective technologies used in the separation of oils, fats and hydrocarbons are API, CPI and dissolved air flotation (DAF) technologies.

• API: Special tanks where the heavier sediments are naturally separated. Light, low-density, and easily floating substances are collected by API scrapers and removed from the water.
• CPI: The CPI (corrugated plate interceptor) is a coalescing plate separator used for the separation of oils and hydrocarbons.

API and CPI technologies can be combined within the same tank for process optimization.

• DAF: DAF systems allow the flotation of suspended solids and colloidal substances to separate them from water. Before the water enters a DAF system, a coagulation-flocculation process is required to group these solids and colloids.

Other less common but equally effective technologies are cyclones/hydrocyclones and membrane filtration systems.

Table 1. Technologies applied in the separation of oils, greases and hydrocarbons.

Technology	Features	Performance
Separate API	It has to be designed according to the retention time.	Inefficient with emulsified oils
CPI Separator	It is designed according to density, viscosity, temperature and flow regime. Retention times are high	Inefficient with emulsified oils. Usually coupled with API separator.
DAF Clarifier	Introduction of pressurized water containing dissolved air forms micro bubbles that allow floc flotation.	Very high efficiency separation of solids, oils, greases and hydrocarbons with a well-designed coagulation-flocculation system.
Hydrocyclones	Suitable for high oil concentrations. High maintenance cost.	Efficiency is increased by arranging several hydrocyclones in series.
Membrane filtration	Pore sizes of 0.01µm	Efficiently extracts dispersed oils and aromatic compounds.

The most effective technologies for the removal of dissolved organic compounds are adsorption, extraction and advanced oxidation systems.

Technologies applied in the elimination of dissolved organic compounds.

Table 2.1. Absorption

Main component	Features	Performance
Activated carbon	Removal of benzene, toluene and traces of curdo. Applies high retention times and depends on the pore size of the AC.	A carbon activation process is necessary. Eliminations between 50 - 75%.
Zeolite	Removal of BTEX (benzene, toluene, ethylbenzene and xylene). Compact modules are manufactured.	Elimination between 70 - 80%. High regeneration costs. Depends on the hydrophobicity of the compounds.
Walnut	Removal of oils and traces of tannin.	Elimination between 60 - 80%. Low raw material cost.
Nano composites	Removal of oils and traces of tannin.	50% elimination in reduced contact times.
Polymeric	Elimination of benzene, toluene and traces of curdo. They can be PET (polyethylene terephthalate) or polystyrene.	Elimination of up to 99%.

Table 2.2. Extraction

Main component	Features	Performance
Solvent	Elimination of free and/or dissolved fats.	Effective but costly due to solvent regeneration. The solvent must be treated as waste.

Table 2.3. Oxidation

Main component	Features	Performance
Photocatalytic	Removal of TOC, phenols, BTEX and TPH (total petroleum hydrocarbons). High influence of pH. Catalyst: TiO2	Removal of >80% BTEX, >95% TOC, >60% phenols, >75% TPH.
UV / 03	Removal of naphtha acids, ammonium and aromatic hydrocarbons. Control of pH, which must not be alkaline. Unfavorable with high bicarbonate and Cl- concentrations.	Eliminations >80%.

Sludge generated during wastewater treatment must be treated as special waste. High-performance separation technologies, such as filter presses and centrifuges, are used for its dewatering and stabilization.

2. SIGMA case study: water treatment at the UPT Thermal Production Unit in Ibiza.

2.1 General project data

• PROJECT LOCATION: Ibiza Thermal Production Unit.
• OWNER: Endesa.
• OPERATOR: Endesa Generation.
• PROJECT YEAR: 2012 – 2013.

OPERATION OF THE POWER PLANT: The Ibiza thermal power plant, or UPT, is a conventional cycle thermoelectric facility located in the municipality of Ibiza. It has 13 active thermal groups with a total power of 270 MW (six engines, four gas turbines and three double gas turbines), using natural gas as the main fuel and diesel oil as auxiliary fuel:

2 x 216 MW motors

4 x 18.4 MW motors

1 x 25 MW gas turbine

1 x 14 MW gas turbine

2 x 25 MW gas turbine

3 x 25 MW double gas turbine

TREATMENT OBJECTIVE: removal of oils, hydrocarbons and suspended solids from the wastewater from the UPT. 

WATER CHARACTERISTICS: high quantity of oils and hydrocarbons, and suspended solids: 1100 mg/L.

PROCESS DESIGN: Wastewater treatment is divided into the following stages, the main objective of which is the removal of suspended solids, oils and hydrocarbons contained in the water:

• Coarse separation process of fuel oil and hydrocarbons using API + CPI technology.
• Process of separation of solids by screening and desanding
• Fine hydrocarbon separation process
• Reagent dosing process: coagulation - flocculation
• DAF flotation treatment (SIGMADAF FPAC-20-S equipment)

PERFORMANCE: The installed equipment allows concentrations of total suspended solids, oils and greases to be reached below 55 mg/L, which means a 95% removal efficiency.

2.2 Description of the wastewater treatment plant at the Ibiza UPT

COARSE SEPARATION OF FUEL OIL AND HYDROCARBONS, API + CPI TECHNOLOGY.

The system designed by SIGMA WATER GROUP uses CPI separator plate technology for the separation of hydrocarbons within the tank.

The tank is a rectangular tank with an inclined bottom, which has an extractor screw for sedimented solids and includes a skimmer system for separating hydrocarbons and oils using API technology.

It has a hot water heating circuit, built in very high quality steel.

SEPARATION OF SOLIDS BY SCREENING AND DESANDING

It includes an inclined sieve and a shaftless screw for lifting the separated solids. The sand removal section includes a horizontal screw for transporting sand.

FINE HYDROCARBON SEPARATION

It consists of a rectangular tank with trapezoidal bottom, equipped with a set of independent coalescent lamellae for optimum water distribution. It includes a skimmer system to separate hydrocarbons and oils. It has a hot water heating circuit.

This equipment is part of the screening and desanding train, completing a compact system built in high quality steel.

PHYSICAL-CHEMICAL TREATMENT AND DAF CLARIFICATION

Physical-chemical treatment consists of adding coagulants and flocculants to form flocs that adhere to the suspended solids present in the water, which cannot be naturally separated by sedimentation due to their low density. This process also facilitates the partial removal of organic matter associated with these solids and the elimination of oil and hydrocarbon residues that may have remained after the previous treatment.

The coagulant and flocculant are added in a series of agitated tanks constructed of high-quality steel. A dosing system specifically designed to treat these contaminants is supplied. These dosages are determined through prior flocculation tests performed in the laboratory, and the dosage is subsequently adjusted on an industrial scale.

The generated flocs are separated from the water using a dissolved air flotation (DAF) system . A SIGMADAF model FPAC 20 unit is manufactured and installed for the treatment plant at the University of Puerto Rico (UTP) in Ibiza.

The SIGMA DAF FPAC model is a dissolved air flotation system for wastewater clarification. These systems utilize cross-flow separation systems with a large free surface area for the accumulation of floated sludge. They operate with small to medium flow rates (between 5 and 200 m3/h) and with very high contaminant loads (total suspended solids, oils and greases, and organic matter).

DAF FPAC systems are specifically designed to treat effluents with very high solids loads (up to 40 kg of solids/m2 of free surface area of the system) that are not buoyant and require a large surface area for flotation and separation.

Flotation air is necessary to improve floc flotation when the mixing of emulsions, oils and solids affects the specific gravity.

A recirculation pump redirects part of the clarified water to a pressurization-saturation system located at the outlet of the DAF unit. The recirculated water is pressurized by the pump to approximately 6 bar and mixed with pressurized air. In this way, the pressurized water becomes saturated with pressurized air. Under these pressure conditions, the air is dissolved in the water.

Depressurization occurs inside the DAF tank, resulting in the generation of micro air bubbles. These dissolved air bubbles allow for the removal of solids, grease particles, oils, hydrocarbons, and other contaminants that lack sufficient buoyancy. The bubbles are between 30 and 50 microns in diameter, essential dimensions for efficient flotation. The bubbles quickly adhere to particles of similar or larger dimensions and rise to the surface.

This mixture of water and bubbles is evenly distributed in the DAF unit's inlet compartment. Floated particles are redirected directly to the dewatering system at the top of the unit, where they are removed by skimmers. The sludge, oils, grease, and hydrocarbons floated in the form of foam leave the unit and are stored in a sludge hopper.

The floated flocs leave the unit in the form of foam through a skimmer system and are collected in a sludge hopper.

The settleable matter descends to the sediment compartment, located at the bottom of the DAF unit, and is discharged by the auger sludge extraction system.

The clarified water leaves the unit through an adjustable supernatant system. Part of this clarified water stream will be redirected by the recirculation pump to enter the compression and saturation system described above.

3. References 

Lavariega L. 2011, Water Treatment Trains for the Oil Industry. Academic Division of Biological Sciences, Universidad Juárez Autónoma de Tabasco. 17(33), 25-28.

Mesa S.L., Orjuela J.M., Ortega A.T., Sandoval J.A. 2018. Review of the current landscape of produced water management in the Colombian oil industry. Gestión y Ambiente. 21(1), 87-98.

Petroquímica On Line Magazine. 2017. The Oil & Gas industry, among the most "critical" in the world.

Villegas J.P., Arcila N., Ortega D., Franco C.A., Cortés F.B. 2017. Removal of hydrocarbons from oil industry production waters using nanointermediates composed of SiO2 functionalized with magnetic nanoparticles. DYNA. 84(202), 6574.