Wastewater treatment in the oil and natural gas industry

November 15, 2021 (Reading 12 mins)
Jordi Fabregas

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

1.1 Circular economy solutions for water reuse in Oil&Gas

The oil and natural gas industry, commonly known as Oil & Gas, generates the energy necessary to carry out numerous human and industrial activities.

The high volume of water resources consumed by this industry implies the need to design efficient, sustainable and cost-effective solutions for the treatment of wastewater generated in oil refineries, sludge produced in extraction wells, etc., since these effluents have a direct impact on the sustainability of resources, on the environment and on the rest of the industries and sectors.

One of the main objectives of the oil and gas industry is the reduction of water demand, which implies implementing circular economy solutions that promote the reuse of wastewater and the saving of resources and raw materials, thus contributing to the economic and environmental sustainability of the Oil & Gas industry.

Properly treated wastewater can be reused in a wide range of activities, for example:

  • Irrigation of adjacent green areas
  • Fire flow network
  • Refrigeration systems
  • Boilers
  • Water for cleaning trucks, cars, etc.

To ensure the sustainability of water and of the Oil & Gas industry itself, these principles of the circular water economy, whereby wastewater becomes a resource, must be applied.

What is the application of DAF systems in the oil and gas sector?

Dissolved air flotation is the most efficient technology for primary wastewater treatment in the various activities of the oil and gas sector. The objective of this primary treatment is to separate the oils and suspended solids from the wastewater so that it can pass to a subsequent treatment stage.

DAF systems are widely used in numerous activities in the oil and gas sector, including oilfields, oil refineries, chemical and petrochemical plants, natural gas processing plants and similar industrial facilities.

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 & Gas industry can have very different characteristics in each plant (crude oil extraction, cracking, fuel oil and gas utilization, etc.), in addition to being fluctuating in itself, but it has basic characteristics such as the following:

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

The components to be removed most effectively are oils, greases, hydrocarbons, suspended solids and dissolved organic matter.

The treatment of this wastewater requires engineering that can provide integrated, robust, efficient and optimizable solutions.

The most effective technologies used in the separation of oils, greases and hydrocarbons are API, CPI and DAF technologies:

  • API: special tanks in which a natural separation of the heavier sediments takes place. Light, low density and floating substances are collected by API scrapers (designed according to American Petroleum Institute standards) and removed from the water.
  • CPI: The CPI (corrugated plate interceptor) is a coalescing plate separator applied in the separation of oils and hydrocarbons.

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

  • DAF: DAF equipment allows the flotation of suspended solids and colloidal substances for their separation from water, after the application of a coagulation-flocculation process for the grouping of 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.

TechnologyFeaturesPerformance
Separate APIIt has to be designed according to the retention time.Inefficient with emulsified oils
CPI SeparatorIt is designed according to density, viscosity, temperature and flow regime. Retention times are highInefficient with emulsified oils. Usually coupled with API separator.
DAF ClarifierIntroduction 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. 
HydrocyclonesSuitable for high oil concentrations. High maintenance cost.Efficiency is increased by arranging several hydrocyclones in series.
Membrane filtrationPore sizes of 0.01µmEfficiently 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 componentFeaturesPerformance
Activated carbonRemoval 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%.
ZeoliteRemoval 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.
WalnutRemoval of oils and traces of tannin.Elimination between 60 - 80%. Low raw material cost.
Nano compositesRemoval of oils and traces of tannin.50% elimination in reduced contact times.
PolymericElimination 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 componentFeaturesPerformance
SolventElimination 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 componentFeaturesPerformance
PhotocatalyticRemoval of TOC, phenols, BTEX and TPH (total petroleum hydrocarbons). High influence of pH. Catalyst: TiO2Removal of >80% BTEX, >95% TOC, >60% phenols, >75% TPH.
UV / 03Removal of naphtha acids, ammonium and aromatic hydrocarbons. Control of pH, which must not be alkaline. Unfavorable with high bicarbonate and Cl- concentrations.Eliminations >80%.

In addition to treating the water, the sludge generated must be managed as special waste. High-performance separator technologies such as filter presses and centrifuges are used for 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 Generación.

PROJECT YEAR: 2012 - 2013

Figure 1. Ibiza Thermal Production Unit UPT.

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

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

TREATMENT SUMMARY: Wastewater treatment is divided into the following stages, whose main objective 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)
Figure 2. Process designed and installed by Sigmadaf for wastewater treatment in the Oil & Gas industry. Case Study of the UPT of Ibiza.

PERFORMANCE: The equipment installed by Agua Sigma allows reaching concentrations below 55 mg/L of total suspended solids, oils and grease at the DAF equipment output, which means a removal performance of 95%.

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 uses CPI separator plate technology for hydrocarbon separation inside 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 high quality steel.

Figure 3. SIGMA CPI - API equipment for the separation of fuel oil and hydrocarbons installed at the UPT treatment plant in Ibiza.
Figure 4. Interior view of the SIGMA CPI - API equipment for the separation of fuel oil and hydrocarbons installed at the UPT treatment plant in Ibiza.

SEPARATION OF SOLIDS BY SCREENING AND DESANDING

Includes inclined screen and shaftless screw for elevation of separated solids. The desanding section includes a horizontal sand transport screw.

Figure 5. SIGMA screening and desanding equipment installed at the UPT treatment plant in Ibiza.

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.

Figure 6. SIGMA equipment for hydrocarbon separation installed at the UPT treatment plant in Ibiza.

PHYSICAL-CHEMICAL TREATMENT AND DAF CLARIFICATION

The physical-chemical treatment consists of the addition of the coagulant and flocculant that allow the formation of floccules that adhere to the suspended solids present in the water, which cannot be separated naturally by sedimentation due to their low density. This process also facilitates the partial elimination of organic matter associated with these solids and the elimination of oil and hydrocarbon residues that may have remained after the previous treatment.

Figure 7. SIGMA coagulation and flocculation reactor installed at the UPT treatment plant in Ibiza.

The addition of coagulant and flocculant is carried out in tanks in series provided with agitation and built in very high quality steel. A dosing system specially adjusted to the necessary doses of these products is supplied. These dosages are established by means of previous flocculation tests carried out in the laboratory, after which the dosage is re-adjusted on an industrial scale.

Figure 8. Coagulation and flocculation tests performed on a sample of wastewater from the UPT of Ibiza. The results of the tests are applied in the design of the coagulant and flocculant dosage on an industrial scale.

The flocs generated are separated from the water using dissolved air flotation (DAF) technology. For the treatment plant built at UPT Ibiza, a SIGMA DAF model FPAC 20 is designed and installed.

Figure 9. SIGMA DAF FPAC - 20 - S flotation equipment for wastewater clarification, installed at the UPT treatment plant in Ibiza.
Figure 10. Operation of the SIGMA DAF FPAC 20 flotation equipment for wastewater clarification, installed at the UPT treatment plant in Ibiza.
Figure 11. SIGMA DAF FPAC - 20 - S flotation equipment for wastewater clarification, installed at the UPT treatment plant in Ibiza.

The SIGMA DAF FPAC models are a dissolved air flotation system for wastewater clarification. They are cross-flow separation equipment with a large free surface to accumulate the floated sludge.

The SIGMA DAF FPAC equipment works with small to medium flow rates (between 5 and 160 m3/h) and with very high pollutant loads (total suspended solids, oils and grease, organic load).

They have applications in FBR (biological flotation reactors), sludge thickeners, rendering industry, meat and slaughterhouse industries, food industry, mining, petrochemical and paper industry, among others.

DAF FPAC systems are specially designed to treat streams with very high solids loads (up to 40 kg solids/m2 free surface area of the system) that require a large surface area for flotation and separation, and that do not have buoyancy.

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 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 will be saturated with pressurized air. Under these pressure conditions, the air is dissolved in the water.

Depressurization occurs inside the DAF unit, resulting in the generation of micro air bubbles. Dissolved air micro bubbles allow the removal of solids, grease particles, oils, hydrocarbons, etc. that do not have sufficient buoyancy. The bubbles are between 30-50 microns in diameter, essential dimensions for efficient flotation. The bubbles quickly adhere to particles of similar and larger dimensions and rise to the surface.

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

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

The settleable matter descends to the sediment compartment 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.

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