Application of Ozone in Oilfield Wastewater Treatment
Oilfield wastewater has three sources: the first source is drilling fluids and their return flow, the second source is oil well fracturing flowback, and the third source is wastewater produced during crude oil treatment. In order for the treated wastewater to meet the relevant reinjection standards, targeted selection of chemicals and formulations is required.
The difficulty in treating oilfield wastewater lies in the fact that it is a mixture of the three sources mentioned above. It has both the stability of colloids and contains insoluble carbonates and sulfates, ions such as calcium, potassium, sodium, and chloride, as well as possible heavy metal ions. Additionally, it contains crude oil, organic cyanides, unsaturated organic nitrogen, and other complex components, which cause the wastewater to have color and toxicity. Furthermore, bacteria are also present in the wastewater.
1. Selection and Function of Chemicals for Wastewater Treatment
1.1 Ozone is selected to treat substances such as unsaturated organic amines, unsaturated hydrocarbons, and organic cyanides in the wastewater, eliminating toxicity and removing color. (For example, it can reduce the harmful effects of polyacrylic compounds, carboxymethyl cellulose, and thiols.)
1.2 Saturated lime solution is selected to adjust the pH of the water to 8–12, which can precipitate carbonates and sulfates, and also accelerate ozone's precipitation and decomposition.
1.3 Destabilizing agents composed of flocculants, coagulating agents, and other chemicals are selected to break the colloidal stability of the wastewater. This causes the compounds in the wastewater to form flocs and precipitate out of the water.
1.4 Based on the electrical properties of the wastewater, ionic polyacrylamide aqueous solutions are selected. The polymer long-chain crosslinking effect of the flocs and the wrapping effect of the polymer long chains increase the floc size and improve their stability, facilitating the treatment process.
2. Chemical Reactions and Products in Wastewater Treatment
Under the catalyst's action, ozone reacts with the wastewater, producing inorganic substances mainly including calcium carbonate, calcium sulfate, magnesium carbonate, and others:
· A2+ + CO32- = ACO3
· A2+ + SO42- = ASO4
Where A can be Ca, Mg, or other precipitating metal ions. Ozone also reacts with organic substances to produce aldehydes, organic acids, and nitro compounds, with some conversion into CO2, H2O, nitrogen, and oxygen.
3. CDOF Technology Introduction
CDOF (Cyclonic Dissolved Ozone Flotation unit) is a patented technology developed by the Shenzhen Clear Science & Technology Co., Ltd (SINOKLE), integrating ozone multi-catalytic oxidation technology, hydraulic cavitation technology, and cyclonic flotation technology. It is a unique, internationally leading, highly efficient oxidation and flotation integrated device, capable of efficiently and rapidly removing various pollutants such as petroleum, suspended solids, colloids, COD, and bacteria from wastewater. This device consists of an oxygen production system, ozone generation system, ozone high-efficiency injection system, multi-catalytic reaction system, and cyclonic dissolved gas flotation system.
Structural Form:
It uses a closed, pressurized vertical tank, consisting of a main reaction tank (0.6 MPa), secondary reaction tank (0.05 MPa), jet ozone injection device, heterogeneous/homogeneous catalyst, pressure reduction hydraulic cavitation device, tail gas treatment device, ozone generator, etc.
Process Description:
After being pressurized, the wastewater enters the jet ozone injection device and mixes with ozone gas (10% ozone and 90% oxygen). The high-speed water flow shears the ozone gas into numerous microbubbles. The wastewater containing microbubbles is quickly pressurized, and ozone rapidly dissolves in the water. Undissolved oxygen microbubbles are compressed into micro-nano bubbles and enter the main reaction tank. The dissolved ozone contacts the catalyst, generating hydroxyl radicals (·OH) that rapidly react with organic pollutants (COD), breaking large molecules into smaller ones until CO2 and H2O are formed. During this process, the micro-nano oxygen bubbles clean and disturb the catalyst, enhancing contact and reaction rates, preventing catalyst clogging, and removing suspended solids, colloids, and oil from the wastewater, thus removing non-dissolved COD and reducing ozone consumption. The treated wastewater from the main reaction tank enters the secondary reaction tank after passing through the pressure reduction hydraulic cavitation device. Undissolved ozone is released as microbubbles and continues to react with the catalyst, further reducing COD, while micro-nano bubbles remove non-dissolved substances (oil, suspended solids, colloids, etc.). After meeting the effluent standards, the treated water is discharged downstream.
To prevent catalyst fouling and clogging, the CDOF skid-mounted equipment requires regular automatic backwashing (about 1-2 times per week) and acid washing regeneration (about once a month) to ensure the catalyst’s long-term stable operation.
Technical Features:
- Ozone is highly dispersed in water in dissolved and micro-nano bubble forms, accelerating reaction rates and ensuring full contact with the catalyst. The conversion efficiency of ·OH is high, and ozone consumption is only 1/4 to 1/3 of traditional methods. The operating cost is only 1/3 to 1/4 of conventional systems.
- The main reaction tank operates at a pressure of 0.3–0.6 MPa, and the secondary reaction tank at 0.03–0.1 MPa. The system runs under closed pressurized conditions, with zero ozone leakage, ensuring a safe and environmentally friendly operation.
- The floating bed catalytic reaction prevents catalyst clogging and fouling. The automatic backwashing, activation, and regeneration process ensures long-term stability and effectiveness of the catalyst, ensuring stable treatment performance.
- Fully automated DCS/PLC control with an intelligent automatic optimization and warning system, requiring no manual intervention. The equipment is easy to operate and maintain, with high stability and safety.
- The reaction time is only 15 minutes, which is 1/6 of traditional systems. The skid-mounted design occupies minimal space, has fewer supporting facilities, and a shorter construction period.
