Study on treatment technology of oil production wastewater

With the increasing demand for petroleum energy, crude oil exploitation has gradually entered the middle and late stage. Many oil wells reach the late stage of high water cut, and their comprehensive water cut is almost above 80%.

The traditional primary and secondary oil recovery can not meet the original productivity. Therefore, polymer tertiary oil recovery is gradually applied.

The application of polymer flooding technology not only improves the recovery rate of crude oil, but also produces a large amount of oil production wastewater, which must be treated before being discharged.

Therefore, it is necessary to study the treatment technology of oil production wastewater. This paper mainly focuses on the oil production wastewater treatment technology. Further solve the problem of water treatment.

Oil production wastewater treatment technology

Iron carbon microelectrolysis

• Iron carbon micro electrolysis is based on corrosion electrochemistry. Iron and carbon, two kinds of materials with different electrode potentials, are directly contacted and immersed in electrolyte solution to produce battery effect, thus forming numerous micro primary cells. As anode, iron is consumed by corrosion, accompanied by electrochemical corrosion, resulting in a series of synergistic effects, such as adsorption, flocculation, bridging, sweeping, electrochemical reduction, coprecipitation, electrodeposition and so on.

Anaerobic biological process

• Anaerobic biological treatment is a process in which anaerobic microorganisms or facultative microorganisms degrade organic matter in wastewater into organic acids, alcohols, methane and carbon dioxide under anaerobic conditions. Under anaerobic conditions, microbial treatment of organic wastewater is mainly accomplished by anaerobic respiration and fermentation.

Advanced oxidation process

• One is direct oxidation, that is, oxidation of organic matter by ozone molecules, the other is indirect oxidation, that is, oxidation of organic matter by ozone decomposition products (hydroxyl radicals). Ozone / hydrogen peroxide oxidation system can not only use ozone and hydrogen peroxide to oxidize organics directly, but also use ozone decomposition products to oxidize organics indirectly. The addition of hydrogen peroxide enhances the formation of hydroxyl radicals.
• Hydroxyl radical has higher oxidizability than ozone, and its selectivity to organic compounds is low. The combination of ozone and peroxygenation can promote the decomposition reaction of ozone, so as to produce more hydroxyl radicals and improve the oxidation efficiency.

Before adopting these treatment processes, stainless steel filter element is needed for pretreatment, and wedge wire screen filter element or sintered filter element can be selected.

Influence factors of iron carbon micro electrolysis treatment effect

Effect of pH change

• The effect of pH change on COD removal rate was studied under the condition of iron carbon micro electrolysis. The ratio of iron to carbon is set at 3:1. The oil extraction wastewater is treated by micro electrolysis under the conditions of pH 2, pH 3, pH 4, pH 5 and pH 6. After flocculation and sedimentation under alkaline conditions, the supernatant is taken to determine the COD value and calculate the COD removal rate.
• Through calculation, when pH is 3, iron carbon micro electrolysis has the best degradation effect on oil production wastewater, and the COD removal rate can reach more than 50%. Overall, with the decrease of pH value, the COD removal rate increased gradually. This is because reducing the pH value is conducive to the formation of primary cells, which can produce more ferrous ions and iron ions, and enhance the reducibility of new ecological ammonia and iron ions.
• If pH < 3, COD removal rate decreased. This is because the acid is too strong to destroy the subsequent flocs. At the same time, the acid solution consumes a lot of water, and the amount of iron filings also increases, which increases the production of iron sludge, thus increasing the subsequent treatment load.

Effect of iron to carbon ratio

• However, if the mass ratio of iron to carbon is too large, the carbon particles per unit volume will be reduced, which will inhibit the electrolysis reaction and reduce the removal rate.

Influencing factors of anaerobic biological treatment effect

• Reflect the influence of temperature.
• Temperature is one of the important factors to be controlled in anaerobic biological treatment. Temperature affects biochemical reactions by affecting the growth and reproduction of microorganisms. Different microorganisms adapt to different temperatures, set the temperature of 30-37 degrees, increase the temperature, the removal rate of COD also increases; However, when the temperature is higher than 37 ℃, the removal rate of COD decreases with the increase of temperature.
• This is because the effect of temperature on microorganism is realized by enzyme activity, that is, in a certain temperature range, the increase of temperature can promote the improvement of enzyme activity, so as to accelerate the metabolism of anaerobic microorganism and increase the removal rate of COD; However, when the temperature exceeds a certain range, the increase of temperature will inhibit the enzyme activity, thus reducing the COD removal rate of anaerobic biological treatment.

Effect of water on residence time

• When HRT is in the range of 12-20 hours, the COD removal rate increases with the increase of HRT; However, when HRT exceeds 20h, the effect on COD removal rate is no longer obvious when HRT is extended.
• This is because in a certain range, extending HRT is to extend the contact time between anaerobic microorganism and organic matter in oil production wastewater, so as to improve the removal rate of COD.
• However, when the hydraulic retention time is beyond a certain range, the limiting factor of COD removal rate is no longer the contact time, but the biomass and the biodegradability of wastewater. At this time, the effect of simply extending the hydraulic retention time on microbial removal of organic matter in wastewater is no longer obvious.

Factors affecting the treatment effect of advanced oxidation process

Influence of hydrogen peroxide input

• When the dosage of ammonia peroxide is less than 0.4ml/l, increase the dosage of ammonia peroxide, and the removal rate of COD increases. When the dosage of ammonia peroxide is more than 0.4ml/l, continue to increase the dosage, and the removal rate of COD no longer increases, but decreases.
• From the advanced oxidation mechanism, it can be seen that the addition of ammonia peroxide is beneficial to the production of HO2 – and HO2 – is the inducer of hydroxyl radical. Therefore, adding ammonia peroxide to the reaction system within a certain range is beneficial to improve the COD removal rate of the reaction.
• However, the larger the dosage of hydrogen peroxide is, the better. This is because hydrogen peroxide itself can react with hydroxyl radicals quickly, which makes hydroxyl radicals inactivated by quenching. Therefore, increasing the dosage of hydrogen peroxide beyond a certain range is not conducive to the degradation of organic matter in wastewater.

Effect of reaction time

• With the extension of reaction time, the removal rate of COD is increasing. When the reaction time is 5-10min, the removal rate of COD is low, and the change range is small; When the reaction time is 10-15min, the removal rate of COD increases rapidly, and the change range is very large.
• When the reaction time was more than 15 min, the increase of COD removal rate gradually decreased.
• This is because at the initial stage of the reaction, due to the low concentration of ozone, the removal rate of COD is not high; With the extension of reaction time, ozone concentration is no longer the limiting factor of wastewater treatment effect. At this time, due to the existence of hydrogen peroxide, the concentration of hydroxyl radical in advanced oxidation system is greatly increased, thus improving the organic matter degradation rate.
• With the extension of reaction time, hydrogen peroxide is consumed up, and the removal rate of COD tends to be stable.

The influencing factors of COD removal rate of three wastewater treatment technologies, iron carbon micro electrolysis, anaerobic biological process and advanced oxidation process, were discussed. We can further understand the best treatment limit in wastewater treatment, and lay a certain foundation for the treatment of oil extraction wastewater in the future.