1. Impact of pollutants in oilfield produced water and the current state of treatment
Oily industrial wastewater has a wide range of sources and is difficult to treat, and is a thorny problem faced jointly by environmental protection, chemical engineering and many other fields. If such wastewater is discharged into natural water bodies or seeps into soil, it will cause significant harm to the ecological environment and biological health. The forms and impacts of the main pollutants in oilfield produced water, together with existing treatment methods and their limitations, are described below.
Water quality indicator 1: oil content
Forms present: floating oil, dispersed oil, emulsified oil, dissolved oil, oil–solid matter, etc.
Impacts: difficult to treat, heavy impact on filtration equipment, contamination of filter media, formation plugging, etc.
Conventional treatment methods and their problems: physical methods such as heating, coalescence, ultrasound and microwave, and electrochemical, flocculation, chemical oxidation, chemical demulsification and biological treatment, etc.
Water quality indicator 2: suspended solids
Forms present: sand and mud, clay, gums, asphaltenes, corrosion and scaling products, etc.
Impacts: increased filtration load, large sludge production, formation plugging
Conventional treatment methods and their problems: flocculation settling, electrocoagulation, air flotation, cyclone, three-phase separator, filters, membrane filtration, etc.
Water quality indicator 3: pH
Forms present: characterised by the relative content of H+ and OH-
Impacts: an important cause of corrosion and scaling
Conventional treatment methods and their problems: water quality modification technology
Water quality indicator 4: mineralisation degree
Forms present: dissolved ions (anions and cations), high Ca2+ and Mg2+ content
Impacts: increases the stability of oil–water emulsions; more scale-forming ions cause severe scaling, leading to turbid water with re-precipitation and formation plugging
Conventional treatment methods and their problems: adding water quality stabilisers and scale inhibitors to reduce scaling, induced crystallisation, etc.
Water quality indicator 5: surfactants
Forms present: oilfield chemicals for polymer flooding, acidising and fracturing, wax prevention and drilling
Impacts: increased stability of emulsified oil systems and suspended particles, making treatment more difficult
Conventional treatment methods and their problems: coalescence, physical adsorption, advanced oxidation, air flotation, chemical gel breaking and viscosity reduction
Water quality indicator 6: sulphides
Forms present: S²⁻, ferrous sulphide
Impacts: severe corrosion of equipment and pipelines, easy formation of FeS precipitate, water turning black
Conventional treatment methods and their problems: advanced oxidation (H₂O₂, ClO₂, NaClO, Fenton reagent, etc.), zinc chloride chemical precipitation, etc.
Water quality indicator 7: bacteria
Forms present: sulphate-reducing bacteria (SRB), heterotrophic bacteria (TGB), iron bacteria (FB)
Impacts: aggravated corrosion; SRB readily react with sulphate to form S²⁻ and FeS, easily causing secondary pollution and formation plugging
Conventional treatment methods and their problems: chemical sterilisation with ClO₂, H₂O₂, biocide 1227, glutaraldehyde, etc.
Water quality indicator 8: iron ions
Forms present: Fe2+, Fe3+ and partly oxide forms
Impacts: corrosion of equipment and pipelines, secondary pollution, interference with scale inhibition
Conventional treatment methods and their problems: contact catalytic oxidation, flocculation settling, chlorine oxidation, potassium permanganate oxidation, ion exchange resins, etc.
Water quality indicator 9: temperature
Forms present: the temperature of the produced water coming from the formation and the upstream heating and demulsification method
Impacts: at low temperature the emulsion system is stable and flocculant performance is affected
Conventional treatment methods and their problems: coil heating to keep the wastewater between 30 and 50 °C
Water quality indicator 10: corrosion rate
Forms present: iron oxides
Impacts: difficult hydrolysis, aggravated scaling and corrosion, reduced service life of pipelines and equipment
Conventional treatment methods and their problems: dosing corrosion inhibitors, corrosion and scale inhibitors, biocides, etc.
The petroleum hydrocarbons (PHCs) contained in oily wastewater can be divided into four categories: aliphatic, aromatic, asphaltene, and compounds containing oxygen, nitrogen and sulphur. Such pollutants are often accompanied by heavy metals such as nickel, cadmium, lead and vanadium, and their molecules contain various functional groups including alcohols, carboxyl groups, phenols and amine groups. They exist in wastewater in dispersed, emulsified or dissolved states and not only inhibit the growth of plants and animals but may also induce human gene mutation and cancer risk. It is therefore essential to develop efficient, simple and low-energy oily wastewater treatment technology.
2. Development of oily wastewater treatment processes
To date, people have tried air flotation, gravity separation, oil-absorbing materials, coagulation and flocculation to treat oily wastewater. However, when treating emulsified oil–water with droplet sizes below 10 μm, these methods must apply an additional electric field or add a demulsifier, otherwise the treatment performance is very poor, and they face the problems of high energy consumption and secondary pollution.
Membrane separation technology has been widely recognised as an advanced process because of its outstanding performance in treating various emulsions, especially surfactant-stabilised emulsions. Its separation mechanism relies mainly on three effects: pore-size sieving, electrostatic interaction and adsorption. Adsorption is related to the hydrophobic interaction between solutes and the membrane material and may reduce the effective pore size of the membrane and increase rejection; pore-size sieving achieves retention of target substances based on pore size; and electrostatic interaction makes use of the charge characteristics of the membrane surface, achieving effective separation through repulsion of like charges.
3. Research and application of SINOKLE ultra-fine SiC membrane technology
Through cooperation with a renowned international membrane technology company, SINOKLE uses the internationally advanced new-generation inorganic carbon (SiC) ceramic membrane as filter media and has designed a cross-flow filter with microfiltration or ultrafiltration performance. Cross-flow filtration technology ensures that the filter can operate stably under high SS conditions, is not prone to fouling the membrane pores, and does not easily form concentration polarisation that causes membrane fouling and high operating pressure, making cleaning and maintenance easy.
The chemical properties of the membrane material surface are of great significance for efficient separation of oil–water systems. In particular, the de-oiling mechanism relies mainly on the sieving of the porous structure on the membrane surface and on hydrophilic/oleophobic characteristics.
Ultra-fine SiC membrane filter
The ultra-fine cross-flow filter of SINOKLE uses an inorganic carbon material with excellent hydrophilicity (contact angle 0.3°), which not only offers corrosion resistance, acid and alkali resistance and high temperature resistance, but also has dense and uniform pore size, easy controllability, high permeability and selectivity. It has excellent anti-fouling capability, extremely high filtration flux, super chemical stability and great mechanical strength. The membrane core adopts a honeycomb structure design, giving high mechanical strength, high packing density, good sealing performance and convenient installation and maintenance. The excellent performance and unique structural design of the inorganic carbon membrane give it broad application prospects in petroleum, refining and other wastewater treatment fields.
Advantages of ultra-fine SiC membrane technology:
1. High filtration precision, with absolute precision ≤1.0 μm (filtration pore size 40 nm);
2. Good hydrophilicity, with a water contact angle of 0.3 and strong resistance to oil fouling;
3. High water flux, 3–4 times that of conventional ceramic membranes;
4. Dynamic continuous membrane regeneration technology, with good membrane regeneration performance and long service life;
5. Good chemical stability and acid and alkali resistance (pH 0–14);
6. High structural strength and high pressure resistance;
7. Good temperature resistance, with a maximum service temperature of 800 °C.
Technical specifications of the ultra-fine SiC membrane:
Capacity per element (Q): 2 m³/h
Medium temperature (°C): ≤800
System pressure drop (MPa): ≤0.1
Filtration resistance (MPa): ≤0.12
Inlet conditions: oil content ≤50 ppm; SS ≤50 mg/L
Outlet conditions: oil content ≤5 ppm; SS ≤1 mg/L; median particle size ≤1 μm
Inlet and outlet performance of application cases
Application fields:
Oilfield produced water treatment for reinjection
Advanced treatment of petrochemical wastewater
