Petrochemical wastewater has a high COD and poor biodegradability. To improve the biodegradability of subsequent treatment, anaerobic pretreatment is generally carried out first. The advantages of anaerobic treatment are low sludge production, low operating costs, high production efficiency, and simple operation; however, the disadvantages are long start-up time and unstable operation.

1.1 Upflow Anaerobic Sludge Blanket

The upflow anaerobic sludge blanket (UASB) reactor has a high sludge concentration, high organic load, short hydraulic retention time, low operating costs, and simple operation. However, the reactor start-up process is time-consuming, and the requirements for the cultivation of granular sludge are strict. It is often used for the treatment of high-concentration organic wastewater. Ling Wenhua et al. used it for the pretreatment of caprolactam production wastewater, achieving good COD removal, but the biodegradability of the effluent was not ideal. During the treatment process, the reaction conditions must be strictly controlled, the influent load fluctuation must be controlled within 15%, the influent SO42- should be less than 1000 mg/L, the influent pH should be 5.5～6.5, and the reaction temperature should be 30～38℃. To eliminate the adverse effects of S2- on anaerobic sludge, an appropriate amount of FeCl3 can be added to the influent.

1.2 Anaerobic Attached Film Expanded Bed

The anaerobic attached film expanded bed (AAFEB) reactor is a new type of high-efficiency anaerobic digestion process. The bed layer operates at a certain expansion rate (10%～20%), which improves the mass transfer conditions in the reactor. The small carrier particle size provides a large surface area for microbial attachment and growth, maintaining a high microbial concentration in the reactor. Zhuang Lining et al. investigated the operating characteristics under different temperatures and hydraulic retention times (HRTs). The results showed that the effect of treating petrochemical wastewater was good, and within a certain temperature range, increasing the temperature could improve the organic load and removal efficiency of the reactor.

1.3 Anaerobic Fixed-Film Reactor

Anaerobic fixed-film reactors are equipped with fixed fillers that can retain and attach a large number of anaerobic microorganisms. Under their action, the organic matter in the influent is converted into methane and carbon dioxide, which are then removed. The reactor has advantages such as long microbial retention time, strong resistance to shock loads, and convenient operation and management. Patel et al. used single-chamber and multi-chamber anaerobic fixed-film reactors to treat unneutralized acidic petrochemical wastewater. At an organic load of 20.4 kg/(m3·d), the COD removal rate of the multi-chamber reactor reached 95%, and the methane production was 0.38 m3/(m3·d). At a pH of 2.5, an organic load of 21.7 kg/(m3·d), and an HRT of 2.5 d, the COD removal rate of the single-chamber reactor reached 95%, and the methane production was 0.45 m3/(m3·d). In addition, they conducted similar research using an upflow anaerobic fixed-film reactor and analyzed the effects of organic load and temperature on the reaction.

In the treatment of petrochemical wastewater, there are many aerobic treatment methods, but few use aerobic biological treatment alone. It is mainly combined with anaerobic treatment. The latest aerobic treatment methods mainly include the following five types.

2.1 Sequencing Batch Reactor Activated Sludge Process

The sequencing batch reactor (SBR) activated sludge process has a simple process flow, good pollutant removal effect, small footprint, flexible operation, and easy automation. However, it is not suitable for treating large volumes of wastewater and has high control management requirements. Peng Yongzhen et al. used a two-stage SBR process system consisting of two identical SBRs in series to treat petrochemical wastewater. Stage I mainly degrades acetic acid, and Stage II mainly degrades aromatic compounds. The average wastewater volume is 1400 m3/d, COD is 400～1500 mg/L, BOD is 200～650 mg/L, HRT is 8 h, and the COD removal rate can reach 91%. This method can also overcome the glucose effect of the ordinary SBR method, shorten the reaction time, and improve the reaction efficiency. Experiments show that the two-stage SBR method integrates the advantages of the SBR method and the AB method and can eliminate sludge reflux. The reactor in Stage I can also operate under anaerobic conditions.

2.2 High-Efficiency Aerobic Bioreactor

The high-efficiency aerobic bioreactor (HCR) integrates high-speed jet aeration, phase-enhanced mass transfer, and turbulent shear technologies. It has the characteristics of deep-well aeration and sludge fluidized bed and is a third-generation bioreactor. Some scholars have used it for pilot-scale research on the treatment of petrochemical wastewater. The results show that the HCR has a fast start-up speed, high oxygen utilization rate, strong resistance to shock loads, and stable and reliable removal effect. The BOD removal rate can reach 75%～85%. However, due to the short HRT, the removal rate of ammonia nitrogen is not high, and due to the particularity of petrochemical wastewater, the sludge in the reactor is prone to non-filamentous bacterial expansion, and the sludge settling performance is poor. Compared with the ordinary activated sludge method, the HCR process has higher energy consumption, but at a shorter HRT, the BOD removal rate is higher, making it suitable as a pretreatment process.

2.3 Biological Contact Oxidation

Biological contact oxidation is a biofilm method developed based on biological filters. It combines the characteristics of biological filters and activated sludge methods, has strong adaptability to load changes, does not cause sludge expansion, has low sludge production, small footprint, flexible treatment methods, and easy operation and management. However, the load cannot be too high, and anti-clogging flushing measures are required. A large number of metazoans (such as rotifers) are produced, which can easily cause instantaneous large-scale shedding of the biofilm, affecting the effluent water quality. Huang Guangping used a biological contact oxidation tower to treat wastewater from Guangzhou Petrochemical General Plant. The main purpose was denitrification. The effluent COD was reduced from 100～200 mg/L to below 80 mg/L, and the ammonia nitrogen was reduced from 50～80 mg/L to below 10 mg/L. The denitrification effect was significant, the energy consumption was low, and the operational reliability was good.

2.4 Membrane Bioreactor

The membrane bioreactor (MBR) is a new type of wastewater treatment device developed by combining membrane separation technology with biological treatment technology. It is widely used in reclaimed water reuse and industrial wastewater treatment. Fan Yaobo et al. used an MBR device to treat petrochemical wastewater. Experiments showed that the removal rates of BOD, SS, and turbidity reached 98%, the COD removal rate reached 91%, and the treatment effects of petroleum, ammonia nitrogen, and phosphorus were also better than conventional secondary wastewater treatment. The system had good stability, a large sludge load, and a small amount of excess sludge.

2.5 Suspended Carrier Bioreactor

Suspended filler bioreactor is a new type of biofilm reactor. Its core component is a special filler that can maintain a suspended state in the reactor. The reactor is easy to operate and has good aeration and water permeability. It has the effects of collision and cutting bubbles, which can enhance the mass transfer of microorganisms, pollutants, and dissolved oxygen, improve oxygen utilization efficiency, and has no special requirements for aeration and water distribution. Xia Siqing et al. used it to treat petrochemical wastewater. The experimental results show that the suspended filler bioreactor has strong oxygenation capacity and anti-load impact capacity. When the filler addition rate is 50%, under the same conditions as an ordinary aeration tank, the oxygenation capacity of the reactor can be increased to more than twice that without filler, the pollutant removal effect is good, and the effluent water quality is stable; when the filler addition rate is 50% and HRT is 8h, the removal rates of COD, ammonia nitrogen, turbidity, and SS are 75.0%, 85.2%, 85.7%, and 86.2%, respectively. Using a multi-stage suspended filler bioreactor to treat petrochemical wastewater can further improve the removal effect of pollutants, especially ammonia nitrogen.

Petrochemical wastewater is characterized by a variety of pollutants, the presence of biological inhibitory substances, and complex water quality conditions. Using single aerobic or anaerobic treatment, it is difficult to meet the emission requirements. The combined process that effectively combines anaerobic (or anoxic) and aerobic treatment has good treatment effects and is widely used.

Wan Yurong et al. used a new combination of A/O processes, A/O1 and O2, to treat petrochemical wastewater. The system consists of membrane anoxic, sludge aerobic, and membrane aerobic processes. The influent COD is 1300 mg/L, the total HRT is 60h (20h respectively), and the effluent COD, BOD, MLSS, and oil content are below 100, 30, 70, and 10 mg/L, respectively.

Guan Weisheng et al. used a UASB reactor plus an aeration tank for anaerobic-aerobic combined treatment of petrochemical wastewater. The influent COD, BOD, emulsified oil, and volatile phenol of the system are 5200, 3160, 90, and 760 mg/L, respectively, and the effluent is 64.5, 28.0, 0.3, and 0.3 mg/L, respectively. The operation is stable, and the pollutant removal rate is high.

Zou Maorong et al. used the HOBAF process, which combines hydrolysis acidification, aerobic biological treatment, and an aeration biofilter, to treat petrochemical wastewater. The treatment efficiency is high, and the effluent water quality is good. The removal rates of COD and ammonia nitrogen are 92.8% and 73.4%, respectively, and the removal rates of oil, volatile phenol, and sulfide are all above 90%.

Chen Meirong et al. used a two-stage biological treatment process of anoxic-facultative-aerobic to treat petrochemical wastewater. The anoxic process uses hydrolysis acidification, the facultative process uses a batch-fed high-concentration activated sludge method, and the aerobic process uses a contact oxidation method. The operation is stable and reliable.

Petrochemical wastewater has complex components, high pollutant concentrations, and is difficult to degrade, causing serious environmental pollution. Single treatment processes are difficult to meet water quality discharge requirements. In practical applications, oil separation, flotation, flocculation, anaerobic, aerobic, adsorption, and membrane separation are frequently used. Their combinations are efficient and practical. Generally, physical and chemical methods are used for pretreatment, followed by anaerobic + aerobic secondary treatment. If reuse is required, further treatment such as adsorption and membrane separation is combined. Researching efficient, economical, and energy-saving treatment technologies and systematically developing effective combinations of different processes are the main content and development direction of petrochemical wastewater treatment technology research. However, end-of-pipe treatment only addresses the symptoms, not the root cause. From the overall development trend and benefits of the industry, the way out for water pollution control in the petrochemical industry lies in the following aspects:

(1) Promoting clean production. Following the concept of a circular economy, widely promoting clean production to control and reduce the generation of pollutants from the source and during the production process.

(2) Developing wastewater resource utilization. Reusing water with less pollution (such as steam condensate water, boiler wastewater, etc.) or treated reclaimed water to improve the reuse rate of water resources.

(3) Strengthening end-of-pipe treatment. After actively promoting clean production and wastewater resource utilization measures, for wastewater with no reuse value, using economical and efficient treatment technologies for effective end-of-pipe treatment to achieve compliance with discharge standards.