Microbial technology-based oilfield sewage biological treatment process

Abstract

The present application provides a kind of oilfield sewage biological treatment process based on microbial technology, it is characterized in that, including the following steps: the oilfield sewage containing partial hydrolyzed polyacrylamide is sent into multiple alternately arranged acid-producing bacteria pool and acid-consuming bacteria pool in turn, sewage is alternately flowed between the acid-producing bacteria pool and the acid-consuming bacteria pool, the PH value of sewage is oscillated in above-mentioned range, and each oscillation period is 3-5 hours;The treated sewage enters aerobic biological treatment tank, and the aerobic biological treatment tank is added with compounded microbial inoculum, and the compounded microbial inoculum is composed of pseudomonas, bacillus subtilis and acinetobacter with mass ratio 3:2:1.The oilfield sewage biological treatment process based on microbial technology has the advantages that after physical method pretreatment, additional chemical treatment method is not needed, and the treatment effect of polyacrylamide-containing sewage can be greatly improved.

Description

Technical Field

[0001] This invention relates to the field of oilfield wastewater treatment technology, and more specifically, to a biological treatment process for oilfield wastewater based on microbial technology. Background Technology

[0002] Oilfield produced water treatment technologies are mainly classified into three categories: physical methods, chemical methods, and biological methods. Physical methods mainly remove free oil and suspended solids through gravity sedimentation, flotation, filtration, and centrifugation, but are largely ineffective against dissolved organic matter and polymers, and are usually used as pretreatment units. Chemical methods mainly include flocculation, oxidation, and acid-base adjustment, which have a high capacity for treating oily wastewater, but the cost of reagents is high and they are prone to secondary pollution. Biological methods are considered the most promising treatment technology due to their low operating costs and environmental friendliness.

[0003] In practical engineering, oilfield produced water is usually pretreated by physical methods to remove most of the free oil and suspended solids before entering a biological treatment unit to degrade dissolved organic matter. This process has achieved good results for conventional oily wastewater; however, for produced water from polymer flooding and ternary composite flooding containing large amounts of partially hydrolyzed polyacrylamide (HPAM), biological methods still face serious challenges, even after conventional physical pretreatment.

[0004] Specifically, the high viscosity of HPAM causes wastewater to easily aggregate into large bubbles during aeration, making it difficult to dissolve and sustain the dissolved oxygen required by aerobic microorganisms. Simultaneously, the diffusion rate of organic matter and nutrients in the viscous medium decreases, reducing the contact between microorganisms and the substrate and limiting the overall rate of biodegradation. Furthermore, HPAM molecules have a stable CC backbone structure, which is difficult for microbial enzymes to directly attack and cleave in the natural environment, making it a typical recalcitrant polymer. The amide groups (-CONH2) on its side chains may hydrolyze under microbial action, releasing ammonia nitrogen. High concentrations of ammonia nitrogen inhibit some aerobic bacteria, and the amide groups themselves may also interfere with normal microbial metabolic activities. The combined effect of these factors results in a BOD5 / COD ratio in polymer-containing wastewater typically below 0.2, indicating extremely poor biodegradability.

[0005] To address the challenges of treating HPAM (Polyhydric Acid-Alkali-Associated Methane) wastewater, some studies have shown that adding hydrochloric acid or sulfuric acid to lower the pH of oily wastewater to the acidic range can cause HPAM molecular chains to coil and viscosity to decrease. This is beneficial for oxygen dissolution and the contact between microorganisms and substrates. However, applying this method to the pretreatment of biological processes presents the following problems: Adding acid or alkali reduces the activity of subsequent aerobic microbial communities. Furthermore, this method only alters the ionization state of HPAM molecules, representing a reversible change at the physical conformation level. As aerobic microorganisms degrade dissolved organic matter, the pH gradually returns to normal, causing the HPAM molecular chains to re-stretch, and the viscosity to rebound significantly, again reducing dissolved oxygen levels and the contact between microorganisms and substrates. In summary, this method has limited effectiveness in improving the biodegradability of polymer-containing wastewater; the BOD5 / COD ratio of pretreated polymer-containing wastewater typically does not exceed 0.25. In addition, this method consumes a large amount of acid and alkali, resulting in high operating costs.

[0006] In order to solve the above problems, people have been seeking an ideal technological solution. Summary of the Invention

[0007] The purpose of this invention is to address the shortcomings of existing technologies by providing a biological treatment process for oilfield wastewater based on microbial technology that can significantly improve the treatment effect of polymer-containing wastewater without the need for additional chemical treatment methods after physical pretreatment.

[0008] To achieve the above objectives, the technical solution adopted by this invention is: a biological treatment process for oilfield wastewater based on microbial technology, comprising the following steps: a) Oilfield wastewater containing partially hydrolyzed polyacrylamide is sequentially fed into multiple alternating acid-producing and acid-consuming bacteria tanks, wherein: The acid-producing bacteria tank is inoculated with acid-producing bacteria, which ferment and produce acid under anaerobic conditions using fermentable organic matter, so that the pH value of the wastewater in the tank gradually decreases to 5.0-6.0. The acid-consuming bacteria tank is inoculated with acid-consuming bacteria. Under anaerobic conditions, the bacteria consume organic acids using nitrates or sulfates as electron acceptors, causing the pH value of the wastewater in the tank to gradually increase. Specifically, at the end of the first few acid consumption cycles, the pH value remains at 7.5-8.5, and at the end of the last acid consumption cycle, the pH value remains at 6.5-7.0. Wastewater flows alternately between the acid-producing bacteria tank and the acid-consuming bacteria tank, causing the pH value of the wastewater to fluctuate within the above-mentioned range, with each oscillation cycle lasting 3 to 5 hours. b) The wastewater treated in step a enters an aerobic biological treatment tank, which is infused with a compound microbial agent. The compound microbial agent is composed of Pseudomonas, Bacillus subtilis and Acinetobacter in a mass ratio of 3:2:1. In this step, Pseudomonas degrades oil hydrocarbons and secretes biosurfactants to break the emulsion, Bacillus subtilis hydrolyzes residual amide groups of polyacrylamide through amidase, and Acinetobacter oxidizes the carbon chain backbone of polyacrylamide through laccase.

[0009] Based on the above, in step a, the acid-producing bacteria are lactic acid bacteria, and the acid-consuming bacteria are denitrifying paracocci and / or desulfurizing vibrio.

[0010] Based on the above, in step a, the alternating residence time of wastewater between the acid-producing bacteria tank and the acid-consuming bacteria tank is 1.5 to 2.5 hours, the pH oscillation cycle is 3 to 5 times, and the total hydraulic residence time is 15 to 25 hours.

[0011] Based on the above, in step a, the dissolved oxygen concentration in the acid-producing bacteria tank is maintained below 0.3 mg / L, and the dissolved oxygen concentration in the acid-consuming bacteria tank is maintained below 0.5 mg / L.

[0012] Based on the above, the fermentable organic matter mentioned in step a comes from the lower alcohols and volatile acids contained in the oilfield wastewater itself, and / or one or more of the externally supplemented molasses, corn steep liquor, and starch hydrolysate, with a supplementation amount of 0.1 to 2 kg per ton of wastewater.

[0013] Based on the above, in step b, the Pseudomonas is selected from one or more of Pseudomonas putida, Pseudomonas aeruginosa, and Pseudomonas alkaliformis; the Acinetobacter is selected from Acinetobacter veneriense and / or Acinetobacter calcium acetate.

[0014] Based on the above, each of the acid-producing bacteria tanks is filled with a first packing material, on which the acid-producing bacteria are attached; each of the acid-consuming bacteria tanks is filled with a second packing material, on which the acid-consuming bacteria are attached; and the aerobic biological treatment tank is filled with a third packing material, on which the compound microbial agent is attached.

[0015] Based on the above, it also includes the following steps: c) The wastewater treated in step b) enters the nitrification tank, which is filled with a fourth packing material. Nitrifying bacteria are attached to the fourth packing material and convert the ammonia nitrogen in the wastewater into nitrate.

[0016] Based on the above, the amount of nitrifying bacteria added is 10% to 15% of the total amount of compound microbial agents added.

[0017] Based on the above, before step a, the oily wastewater undergoes physical pretreatment, which includes a screen, an oil separator, and an air flotation system.

[0018] The present invention has the following advantages: (1) Acid-producing bacteria can ferment easily degradable carbon sources (either from the wastewater itself or from external sources) in aggregate-containing wastewater into organic acids, causing the pH value of the wastewater to gradually decrease to the acidic window. Subsequently, acid-consuming bacteria can react the organic acids with H+. + Synchronous consumption gradually raises the pH value of the wastewater; the alternating action of these two processes causes the pH value of the polymer-containing wastewater to oscillate periodically. Experiments showed that after multiple oscillation treatments, the viscosity of the polymer-containing wastewater could be reduced to below 7.5 mPa·s, with a viscosity reduction rate ≥85%. After 24 hours of standing, the viscosity rebound rate was less than 10%. In the subsequent aerobic microbial degradation steps, the BOD5 / COD ratio of the wastewater could be increased from <0.2 before treatment to 0.4–0.45, and the biodegradability grade jumped from "very poor" to "good," an improvement of more than 2 times. The significant decrease in viscosity of the polymer-containing wastewater may be due to the presence of (-COO) in the side chains of HPAM. - In the acidic window, (-COO) exists in two forms: (-COO) and (-CONH2). - The side chain is protonated to (-COOH), which reduces the electrostatic repulsion between chain segments, causing the molecular chain to change from an extended state to a coiled state. When the pH rebounds to the alkaline window, (-COOH) dissociates again to (-COO). - The side chain unfolds again; during repeated stretching and coiling, the contact between the acidic substance and (-CONH2) gradually increases, and the (-CONH2) side chain will gradually hydrolyze into (-COOH) and (NH4)2. + Meanwhile, the expansion and contraction of HPAM gradually increase, which promotes the hydrolysis of (-CONH2). Moreover, due to the reduction of (-CONH2) and repeated expansion and contraction, the hydrogen bond network and entanglement between molecules gradually disintegrate, resulting in a significant decrease in viscosity and a very low rebound rate.

[0019] The reason for the significant improvement in the biodegradability of polymer-containing wastewater may be that the sustained reduction in viscosity reduces the resistance to oxygen dissolution, increases the saturated dissolved oxygen level, significantly reduces the contact resistance between microbial cells and the substrate HPAM molecular chains, and greatly improves the mass transfer efficiency. Furthermore, the final acid consumption pH remains at 6.5–7.0. This node has two advantages: first, approximately 50%–90% of (-COOH) remains protonated, allowing HPAM to maintain a strong contractile state and ensuring lower viscosity; second, this range has minimal impact on the activity of Pseudomonas, Bacillus subtilis, and Acinetobacter. During degradation, the pH slightly rises to 7.3–8.0. Within this window, laccase secreted by Acinetobacter oxidizes the α-position of the CC backbone of HPAM via a free radical mechanism, causing β-splitting of the long chain and generating oligomers. The resulting oligomers are further degraded by Pseudomonas through β-oxidation into short-chain fatty acids such as acetic acid and propionic acid, ultimately mineralized into CO2 and H2O. Bacillus subtilis hydrolyzes the residual amide group (–CONH2) of HPAM through amidase, further reducing the side chain density of HPAM and clearing steric obstacles for Acinetobacter's laccase oxidation of the CC backbone, forming a synergistic degradation pathway of clearing side chains first and then breaking the backbone. (2) In the oscillation section, acid-producing bacteria promote the hydrolysis of the amide group of the HPAM side chain under the acid window and release ammonia nitrogen; while acid-consuming bacteria (denitrifying bacteria and / or sulfate-reducing bacteria) consume organic acids and absorb some ammonia nitrogen for their own growth through assimilation, thereby reducing the concentration of ammonia nitrogen in the effluent; this helps to reduce the inhibitory effect of ammonia nitrogen on the subsequent aerobic microbial community.

[0020] (3) The oscillation section of this invention utilizes the metabolic activities of acid-producing and acid-consuming bacteria to regulate the pH on its own, without the need to add chemical agents such as hydrochloric acid and sulfuric acid. The activity of aerobic microbial communities will not be inhibited or impacted by external chemical agents, and the system operates stably.

[0021] (4) The oscillation section only requires a small amount of carbon source (such as molasses, about 0.5 to 1 yuan / ton of water) to maintain the activity of acid-producing bacteria; the acid-consuming bacteria use the sulfate naturally contained in the wastewater or supplement a small amount of nitrate as electron acceptors, resulting in low overall operating costs and making it suitable for promotion.

[0022] (5) After being degraded by aerobic microorganisms, the wastewater is finally converted into nitrate by nitrifying bacteria, so that the ammonia nitrogen in the wastewater meets the discharge standards. Detailed Implementation

[0023] The technical solution of the present invention will be further described in detail below through specific embodiments. Example 1

[0024] A biological treatment process for oilfield wastewater based on microbial technology, the process being used to treat oily wastewater containing large amounts of HPAM, includes the following steps: (1) The oily wastewater is subjected to physical pretreatment, which includes a screen, an oil separator and an air flotation system; to remove most of the solid impurities, floating oil, large particulate dispersed oil, emulsified oil and fine suspended solids.

[0025] (2) The pretreated oilfield wastewater is sequentially sent to multiple alternating acid-producing bacteria tanks and acid-consuming bacteria tanks, wherein: The acid-producing bacteria tank is filled with a first packing material on which acid-producing bacteria are attached. Under anaerobic conditions (dissolved oxygen concentration below 0.3 mg / L), fermentable organic matter is used to produce acid, gradually reducing the pH value of the wastewater in the tank to 5.0-6.0. Specifically, the acid-producing bacteria are lactic acid bacteria. The fermentable organic matter can be derived from the lower alcohols and volatile acids contained in the oilfield wastewater itself, or from one or more of the following supplemented externally: molasses, corn steep liquor, and starch hydrolysate. The supplementation amount is 0.1-2 kg per ton of wastewater. After fermentation, the lactic acid bacteria produce lactic acid, causing the pH value to fall into the acidic window.

[0026] The acid-consuming bacteria tank is filled with a second packing material on which acid-consuming bacteria, specifically denitrifying paracocci and / or desulfurizing vibrio, consume organic acids using nitrates or sulfates as electron acceptors under anaerobic conditions (dissolved oxygen concentration below 0.5 mg / L), causing the pH value of the wastewater in the tank to gradually increase. Specifically, at the end of the first few acid consumption cycles, the pH value remains at 7.5–8.5, and at the end of the last acid consumption cycle, the pH value remains at 6.5–7.0.

[0027] Oilfield wastewater generally contains sulfates naturally. During oilfield extraction, nitrates are sometimes added intentionally (to inhibit the activity of sulfate-reducing bacteria). If the content is too low, a small amount can be added. The denitrifying paracocci and desulfurizing vibrio bacteria are present in the form of (NO3) - ) or (SO4) 2- As a "substitute oxygen," it oxidizes and decomposes lactic acid, while simultaneously reducing nitrates to nitrogen (N2) or sulfates to hydrogen sulfide (H2S); this process consumes (H2S). + Therefore, the pH value will rise.

[0028] Oily wastewater flows alternately between the acid-producing bacteria tank and the acid-consuming bacteria tank, causing the pH value of the wastewater to oscillate within the aforementioned range. Each oscillation cycle lasts for 3 to 5 hours. The wastewater's residence time between the acid-producing bacteria tank and the acid-consuming bacteria tank is 1.5 to 2.5 hours, with the goal of reaching the target pH value range. There are a total of 3 to 5 pH oscillation cycles, and the total hydraulic residence time is 15 to 25 hours.

[0029] Taking a polymer-containing wastewater with an initial viscosity of 50.0 mPa·s as an example, the viscosity of the water sample after the first, second, third, fourth, and fifth shaking, as well as the viscosity rebound rate after 24 hours of standing, were tested.

[0030] The testing instrument used was a Brookfield DV-Ⅱ+ rotational viscometer equipped with a UL-type rotor.

[0031] The testing method is: After each acid consumption period, 200 ml of the water sample to be tested was taken from the acid-consuming bacteria tank and injected into a test cup. The cup was then placed in a constant temperature water bath at 30±0.5℃ for 5 minutes to equilibrate. The rotor speed was set to 30 rpm. After startup and once the reading stabilized, the apparent viscosity value was recorded. Each sample was measured in triplicate, and the average value was taken. Subsequently, the water sample was allowed to stand at 30±0.5℃ for 24 hours. The viscosity of the water sample was then tested again using the same method to calculate the viscosity rebound rate. The calculation formula is as follows: Viscosity rebound rate = (viscosity after rebound - viscosity before rebound) / (initial viscosity - viscosity before rebound); In this embodiment, the initial viscosity of the polymer-containing wastewater is 50.0 mPa·s.

[0032] Test results: The viscosity of the water sample after the first shaking was 39.5 mPa·s, and the viscosity rebound rate after 24 hours was 50.5%. The viscosity of the water sample after the second oscillation was 27.8 mPa·s, and the viscosity rebound rate after 24 hours was 34.8%. The viscosity of the water sample after the third oscillation was 13.2 mPa·s, and the viscosity rebound rate after 24 hours was 15.7%. The viscosity of the water sample after the fourth oscillation was 7.4 mPa·s, and the viscosity rebound rate after 24 hours was 9.6%. The viscosity of the water sample after the fifth oscillation was 5.1 mPa·s, and the viscosity rebound rate after 24 hours was 8.7%. The test data shows that after multiple pH oscillations, the viscosity of oily wastewater can be significantly reduced, and the viscosity rebound rate will be significantly reduced after 24 hours. Moreover, after the first three oscillations, the viscosity and viscosity rebound rate of the water sample both show an accelerated decline, but after the last two oscillations, the decline rate of the viscosity and viscosity rebound rate of the water sample slows down significantly. Based on economic considerations, the number of oscillations is more appropriate between 3 and 5. (3) The wastewater treated by pH oscillation enters the aerobic biological treatment tank, which is filled with a third packing material. The third packing material is coated with a compound microbial agent. The compound microbial agent is composed of Pseudomonas, Bacillus subtilis and Acinetobacter in a mass ratio of 3:2:1. The Pseudomonas is selected from one or more of Pseudomonas putida, Pseudomonas aeruginosa and Pseudomonas alkali-producing. The Acinetobacter is selected from Acinetobacter venereum and / or Acinetobacter calcium acetate.

[0033] In this step, Pseudomonas degrades oil hydrocarbons and secretes biosurfactants to break the emulsion, Bacillus subtilis hydrolyzes residual amide groups of polyacrylamide through amidase, and Acinetobacter oxidizes the carbon chain backbone of polyacrylamide through laccase.

[0034] (4) The wastewater treated in step b enters the nitrification tank, which is filled with a fourth packing material. Nitrifying bacteria are attached to the fourth packing material. The amount of biological addition of the nitrifying bacteria is 10% to 15% of the total biological addition of the compound microbial agent. The nitrifying bacteria can convert the residual ammonia nitrogen in the wastewater into nitrate, so that the ammonia nitrogen in the effluent meets the discharge standards.

[0035] The reason for setting up a separate nitrification tank is that Pseudomonas, Bacillus subtilis, and Acinetobacter reproduce at similar rates, but much faster than nitrifying bacteria. When mixed together, the nitrifying bacteria, which grow too slowly, are quickly eliminated from the system.

[0036] A biodegradability test was conducted to compare the raw wastewater before treatment with wastewater treated after three pH cycles, wastewater treated after four pH cycles, and wastewater treated after five pH cycles. The experimental method was as follows: Take 10ml of the wastewater to be tested, add 0.2g of mercuric sulfate to mask chloride ions, then add potassium dichromate digestion solution, and heat in a COD digester at 165℃ for 15 minutes. After cooling, read the COD value. Separately, take 50ml of the wastewater to be tested, add 950ml of dilution water (distilled water with added BOD nutrients and 50ml of the supernatant from the aerobic biological treatment tank as inoculum), mix well, and pour into a BOD culture flask. Measure the initial dissolved oxygen concentration, tighten the stopper, and incubate in a 20℃ biochemical incubator in the dark for 5 days. Measure the final dissolved oxygen concentration and calculate the BOD5 value using the following formula: BOD5 = (final dissolved oxygen concentration - initial dissolved oxygen concentration) × 20.

[0037] The experimental results are as follows: raw sewage 402 76 0.19 Three pH oscillations 398 159 0.40 Four pH fluctuations 395 170 0.43 Five pH fluctuations 393 173 0.44 As can be seen from the table above, the BOD5 / COD ratio of the wastewater after multiple pH fluctuations increased by more than 2 times compared to the original wastewater, and the biodegradability grade could jump from "very poor" to "good". Example 2

[0038] The difference between this embodiment and embodiment 1 is that in step (2), the pH was shaken 4 times, and the pH value remained at 7.5 to 8.5 when the last acid consumption ended.

[0039] For the wastewater after pH shaking in this embodiment, a biodegradability experiment was conducted according to the test method in Example 1. The final BOD5 / COD ratio was 0.37, proving that the pH value of the wastewater has a certain impact on its biodegradability after shaking.

[0040] The possible reason is that within the pH range of 6.5 to 7.0, about 50%-90% of (-COOH) remains protonated, allowing HPAM to maintain a strong contractile state and ensuring lower viscosity. At the same time, this range has little impact on the activity of Pseudomonas, Bacillus subtilis, and Acinetobacter. Example 3

[0041] The difference between this embodiment and embodiment 1 is that in step (2), the pH is shaken 4 times, and in step (3), the compound microbial agent does not contain Bacillus subtilis, but is composed of Pseudomonas and Acinetobacter in a mass ratio of 3:1.

[0042] For the wastewater after pH shaking in this embodiment, a biodegradability test was conducted according to the test method in Example 1. The final BOD5 / COD ratio was 0.40, which is a significant reduction.

[0043] The possible reason is that Acinetobacter secretes laccase, which oxidizes the α-position of the CC backbone of HPAM through a free radical mechanism, causing β-splitting of the long chain and generating oligomers. The resulting oligomers are further degraded by Pseudomonas aeruginosa through β-oxidation into short-chain fatty acids such as acetic acid and propionic acid, and finally mineralized into CO2 and H2O. Bacillus subtilis hydrolyzes the residual amide group (–CONH2) of HPAM through amidase, which on the one hand further reduces the side chain density of HPAM, and on the other hand removes steric obstacles for Acinetobacter laccase to oxidize the CC backbone, forming a synergistic degradation pathway of clearing side chains first and then breaking the backbone.

[0044] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them; although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications can still be made to the specific implementation of the present invention or equivalent substitutions can be made to some technical features without departing from the spirit of the technical solutions of the present invention, and all such modifications and substitutions should be covered within the scope of the technical solutions claimed in the present invention.

Claims

1. A biological treatment process for oilfield wastewater based on microbial technology, characterized in that, Includes the following steps: a) Oilfield wastewater containing partially hydrolyzed polyacrylamide is sequentially fed into multiple alternating acid-producing and acid-consuming bacteria tanks, wherein: The acid-producing bacteria tank is inoculated with acid-producing bacteria, which ferment and produce acid under anaerobic conditions using fermentable organic matter, so that the pH value of the wastewater in the tank gradually decreases to 5.0-6.

0. The acid-consuming bacteria tank is inoculated with acid-consuming bacteria. Under anaerobic conditions, the bacteria consume organic acids using nitrates or sulfates as electron acceptors, causing the pH value of the wastewater in the tank to gradually increase. Specifically, at the end of the first few acid consumption cycles, the pH value remains at 7.5-8.5, and at the end of the last acid consumption cycle, the pH value remains at 6.5-7.

0. Wastewater flows alternately between the acid-producing bacteria tank and the acid-consuming bacteria tank, causing the pH value of the wastewater to fluctuate within the above-mentioned range, with each oscillation cycle lasting 3 to 5 hours. b) The wastewater treated in step a enters an aerobic biological treatment tank, which is infused with a compound microbial agent. The compound microbial agent is composed of Pseudomonas, Bacillus subtilis and Acinetobacter in a mass ratio of 3:2:

1. In this step, Pseudomonas degrades oil hydrocarbons and secretes biosurfactants to break the emulsion, Bacillus subtilis hydrolyzes residual amide groups of polyacrylamide through amidase, and Acinetobacter oxidizes the carbon chain backbone of polyacrylamide through laccase.

2. The oilfield wastewater biological treatment process based on microbial technology according to claim 1, characterized in that: In step a, the acid-producing bacteria are lactic acid bacteria, and the acid-consuming bacteria are denitrifying paracocci and / or desulfurizing vibrio.

3. The oilfield wastewater biological treatment process based on microbial technology according to claim 2, characterized in that: In step a, the wastewater alternates between the acid-producing bacteria tank and the acid-consuming bacteria tank for 1.5 to 2.5 hours each, the pH oscillation cycle is 3 to 5 times, and the total hydraulic retention time is 15 to 25 hours.

4. The oilfield wastewater biological treatment process based on microbial technology according to claim 3, characterized in that: In step a, the dissolved oxygen concentration in the acid-producing bacteria tank is maintained below 0.3 mg / L, and the dissolved oxygen concentration in the acid-consuming bacteria tank is maintained below 0.5 mg / L.

5. The oilfield wastewater biological treatment process based on microbial technology according to claim 4, characterized in that: In step a, the fermentable organic matter comes from one or more of the following: lower alcohols and volatile acids contained in the oilfield wastewater itself, and / or externally supplemented molasses, corn steep liquor, and starch hydrolysate, with a supplementation amount of 0.1 to 2 kg per ton of wastewater.

6. The oilfield wastewater biological treatment process based on microbial technology according to any one of claims 1-5, characterized in that: In step b, the Pseudomonas is selected from one or more of Pseudomonas putida, Pseudomonas aeruginosa, and Pseudomonas alkaliformis; the Acinetobacter is selected from Acinetobacter veneriense and / or Acinetobacter calcium acetate.

7. The oilfield wastewater biological treatment process based on microbial technology according to claim 6, characterized in that: Each of the acid-producing bacteria tanks is filled with a first packing material, on which the acid-producing bacteria are attached; each of the acid-consuming bacteria tanks is filled with a second packing material, on which the acid-consuming bacteria are attached; the aerobic biological treatment tank is filled with a third packing material, on which the compound microbial agent is attached.

8. The oilfield wastewater biological treatment process based on microbial technology according to any one of claims 1, 2, 3, 4, 5, and 7, characterized in that, It also includes the following steps: c) The wastewater treated in step b) enters the nitrification tank, which is filled with a fourth packing material. Nitrifying bacteria are attached to the fourth packing material and convert the ammonia nitrogen in the wastewater into nitrate.

9. The oilfield wastewater biological treatment process based on microbial technology according to claim 8, characterized in that: The amount of nitrifying bacteria added is 10% to 15% of the total amount of the compound microbial agent added.

10. The oilfield wastewater biological treatment process based on microbial technology according to any one of claims 1, 2, 3, 4, 5, 7 and 9, characterized in that: Before step a, the oily wastewater undergoes physical pretreatment, which includes a screen, an oil separator, and an air flotation system.