Whole-Process Governance System Construction and Resource Utilization Development Trend

Abstract

The diversified industrial structure and irregular production and discharge patterns of petrochemical industrial parks lead to complex wastewater composition and large fluctuations in water quality and quantity. Coupled with increasingly stringent discharge limits and growing requirements for wastewater reuse and resource recovery, wastewater treatment has become a common environmental pressure for petrochemical parks.

Based on the understanding of wastewater characteristics and whole-process treatment technology in the petrochemical industry, this paper highlights the importance of source control for the stable operation of wastewater treatment systems. It identifies key pollutants requiring priority management, proposes recommended values for pipe-network admission indicators and corresponding wastewater pretreatment methods, and elaborates on the centralized treatment system of the park on this basis. In addition, typical treatment technologies such as wet air oxidation, catalytic ozonation and biological denitrification are analyzed and summarized with application cases. The future trend of wastewater treatment shifting from “removal” to “recovery” is further prospected, to provide reference for the resource-oriented wastewater treatment in petrochemical industrial parks.

1. Introduction

Chemical industrial parks are the core support for the development of the chemical industry. China is continuously promoting the agglomeration of chemical industries into parks and accelerating the relocation of chemical enterprises from urban areas to industrial parks. Petrochemical industry is one of the main industrial chain models in chemical parks. By the end of 2020, there were 616 key petrochemical parks or industrial parks dominated by petroleum and chemical industries in China.

While realizing efficient and intensive utilization of resources and energy, petrochemical parks generally face environmental pressure such as wastewater treatment. Wastewater generated by a large number of enterprises in raw material processing, intermediate production and deep processing links results in extremely complex composition of park wastewater, which features four major characteristics: large discharge volume, high salinity, poor biodegradability and high toxicity. Poor management or improper treatment will cause environmental impacts and restrict the development of the park. In view of this, based on the development trend and pressures of wastewater treatment in petrochemical parks, this paper analyzes the difficulties and existing technologies of wastewater treatment, and proposes targeted governance countermeasures.

2. Wastewater Characteristics and Governance Challenges in Petrochemical Parks

2.1 Development Trend and Pressure of Wastewater Governance

In recent years, with the continuous strengthening of national and local environmental supervision, petrochemical parks at all levels generally face huge pressure in wastewater governance. On the one hand, under the requirements of standard upgrading, total pollutant reduction and wastewater resource utilization, ensuring stable compliance of discharged wastewater has become a major challenge for park wastewater treatment.

As shown in Table 1, the discharge limits for conventional pollutants such as COD and ammonia nitrogen in some local standards have approached the Class IV control indicators in the Environmental Quality Standards for Surface Water (GB 3838-2002). In addition, some provinces and municipalities have added TDS discharge limits: for example, some river basins in Beijing and Shandong Province require TDS ≤ 1600 mg/L, and the chemical industry in Jiangsu Province requires TDS ≤ 5000 mg/L, which brings great pressure and difficulty to wastewater treatment plants built according to old standards.

On the other hand, frequent fluctuations of incoming water from upstream enterprises and uncertain water quality lead to irregular changes in discharge quantity and quality. Enterprises often adjust their product structure due to market changes. In addition, some newly built parks have a small number of settled enterprises in the early stage, and the nature of later settled enterprises is difficult to define, resulting in uncertain influent quality, which further increases the difficulty of wastewater treatment.

Table 1 Changes of Wastewater Discharge Standards for Petrochemical Industry

2.2 Water Quality Characteristics and Treatment Difficulties

Petrochemical parks are usually dominated by oil refining and ethylene industries, forming a complete upstream and downstream petrochemical industrial chain. Under the current transformation trend of the refining industry, the rapid development and extension of downstream chemical and new material industrial chains lead to more diversified and complex wastewater sources, which brings considerable difficulties to the process design, parameter selection and operation of wastewater treatment plants. Therefore, precise prevention and control of characteristic pollutants has become a key focus of park wastewater treatment.

Table 2 Main Composition and Properties of Typical Petrochemical Wastewater

Park wastewater treatment plants generally set pipe-network admission standards for upstream enterprises, but these standards mainly focus on conventional pollutant limits such as COD, ammonia nitrogen, total nitrogen and total phosphorus, with few provisions for characteristic pollutants with high biological toxicity, stable chemical structure and poor biodegradability. As a result, most park wastewater treatment plants are still designed with reference to municipal wastewater treatment plant models.

In recent years, with the change of understanding of wastewater treatment, national authorities have issued typical industry standards to limit the discharge concentration of industry characteristic pollutants. However, limited by the number of industry standards and the diversity of enterprise types in the park, many characteristic pollutant control indicators still cannot be included in the admission standards. At present, pretreatment of high-concentration chemical organic wastewater mostly takes COD and B/C as control indicators, without combining pollutant composition characteristics with biological effects, and environmental risk control still lacks sufficient basis.

3. Whole-Process Wastewater Governance Countermeasure System

3.1 Source Precision Control

Source control of wastewater should focus on the biodegradability of wastewater, which is usually predicted by B/C ratio combined with organic concentration. In addition, the biodegradability of organic matter can be judged according to molecular structure, functional groups and molecular weight. Generally, refractory substances include: short-chain hydrocarbons with double bonds, triple bonds, tertiary carbon or quaternary carbon structures; halogenated compounds; ethers; tertiary amines and quaternary ammonium salts; nitro, nitroso and oxidized organic nitrogen compounds; thioethers and sulfones; high molecular weight homologues; heterocyclic compounds; and bactericidal substances.

Based on the biological toxicity and degradation difficulty of organic components in petrochemical wastewater, the typical organic pollutants requiring priority control are classified in Table 3.

Table 3 Typical Organic Pollutants Requiring Priority Control in Petrochemical Wastewater

Petrochemical wastewater usually has complex composition and contains compounds that are difficult to oxidize. Using COD as the pollution indicator cannot fully reflect the organic pollution degree of some wastewater. Meanwhile, for high-chlorine wastewater, the analysis and determination error of COD index is large. Therefore, TOC should be added as a control indicator in the pipe-network admission standards.

In addition to conventional pollutants (pH, suspended solids, COD, BOD, chroma, ammonia nitrogen, total nitrogen, total phosphorus, TOC, TDS, etc.), priority control indicators are recommended to include cyanide, volatile phenol, adsorbable organic halides (AOX), anilines, nitrobenzenes, BTEX, formaldehyde, etc. The recommended limit values are shown in Table 4.

Table 4 Recommended Admission Indicators for Characteristic Pollutants in Petrochemical Park Wastewater Treatment Plants

3.2 Classified Collection and Pretreatment

To meet the pipe-network admission standards mentioned above, enterprises generally need to carry out classified collection and pretreatment of wastewater. The selection of pretreatment process routes is usually based on the idea of degrading pollutants or improving wastewater biodegradability, adopting various redox methods and chemical dosing, while often ignoring separation methods commonly used in chemical processes such as distillation and extraction. For the treatment of some high-concentration wastewater, separation methods such as distillation and extraction may achieve unique effects.

Case 1: Distillation pretreatment of high-concentration petrochemical wastewater
A petrochemical unit produces high-concentration wastewater with COD of about 20000 mg/L, containing high concentrations of biotoxic substances such as aldehydes, phenols and aromatic hydrocarbons. The original treatment scheme was to send it to a waste liquid incinerator, but the low calorific value of the wastewater led to high supplementary fuel costs.

Researchers proposed a distillation pretreatment scheme based on the material composition characteristics. Software simulation results show that the COD of the light distillate at the tower top can reach 57000 mg/L, accounting for 52% of the total COD of the influent. In particular, the aforementioned biotoxic substances are mainly enriched in this phase due to their high volatility. Sending the tower top distillate to the waste liquid incinerator reduces the treatment cost due to increased calorific value and reduced water volume. The COD of the tower bottom fraction drops below 10000 mg/L with B/C > 0.4, which can enter the subsequent biological treatment system.

Case 2: Solvent extraction for high-concentration phenol-containing wastewater
Solvent extraction can also realize the separation and recovery of valuable components in wastewater, and has been widely applied in high-concentration phenol-containing wastewater treatment. Studies on optimizing the extraction process of phenol-containing wastewater from phenol-acetone units using cumene as extractant show that when the operating pressure is 150 kPa, the extraction tower temperature is 45 ℃, and the solvent ratio is 4–5.5, the total phenol content of the wastewater after extraction can be reduced to 80 mg/L.

Other common pretreatment methods include steam stripping for ammonia removal, oxidative cyanide breaking, adsorption, incineration, etc. The applicable wastewater types of each method are listed in Table 5, which should be selected in combination with actual water quality in practical application.

Table 5 Typical Pretreatment Methods for Petrochemical Wastewater

3.3 Centralized Treatment and Zero Liquid Discharge

The centralized wastewater treatment plant of the petrochemical park undertakes the final treatment of industrial wastewater discharged by various enterprises and domestic sewage in the park. Affected by production load adjustment, product structure adjustment and discontinuous discharge of some batch processes, the quantity and quality of influent to the park wastewater treatment plant still fluctuate greatly even after respective pretreatment. Therefore, a buffer regulation capacity of 24–48 h is required.

The main treatment process generally adopts multi-stage biochemical processes, among which the combined process of hydrolytic acidification + A/O (with nitrogen removal) or A²/O (with nitrogen and phosphorus removal) is the most common.

In recent years, with the continuous upgrading of discharge standards, especially the intensive introduction of local standards approaching Class IV surface water quality, advanced treatment units for COD, TN and TP are generally added after secondary treatment. Among them, ozonation or catalytic ozonation, post-denitrification and chemical phosphorus removal are widely applied.

Pilot studies on petrochemical wastewater in chemical parks show that the “O₃/Mn²⁺” combined process for advanced treatment after biochemical treatment can meet the Class I standard of Shanghai Integrated Wastewater Discharge Standard (DB 31/199-2018). Engineering cases of petrochemical wastewater treatment plants adopting the “catalytic ozonation – biological aerated filter” combined process for advanced treatment show that the effluent ammonia nitrogen is 0.4–2.0 mg/L, total nitrogen is 8.0–15.8 mg/L, and orthophosphate is 1.1–2.3 mg/L.

For regions or river basins with salt discharge limits such as Beijing, Shandong and Jiangsu, or special industries including coal chemical industry, desalination of high-salinity wastewater is required to achieve partial or full zero liquid discharge, which is currently regarded as the “ultimate” link of wastewater treatment. High-salinity wastewater zero discharge systems usually consist of four units: pretreatment, reduction, deep concentration and salt separation crystallization. There are many application cases at present, and the process technology combinations of each section are relatively mature. However, due to high fixed investment, high operating cost, complex operation and limited value of product salt, the development of zero discharge is still obviously driven by policies.

3.4 Park-Centered Circular Economy

In July 2021, the National Development and Reform Commission issued the 14th Five-Year Plan for Circular Economy Development, which proposed the “Park Circular Development Project”, implementing the “3R Principles” of reduction, reuse and recycling to minimize waste generation and realize optimal utilization of resources and energy.

Wastewater resource utilization in industrial parks is the most direct measure to promote circular economy and sustainable development of parks, and also an effective means to address water shortage and water environment problems. The effluent from wastewater treatment plants can be reused for three purposes: industrial water in the park, urban miscellaneous water and landscape water, with corresponding water quality indicators referring to relevant national standards. Among them, reuse as industrial water for enterprises should be the main approach.

Effluent meeting advanced treatment standards can generally be reused as make-up water for circulating water plants after simple filtration and disinfection. To further increase the reuse rate or produce higher-grade reclaimed water, membrane technologies such as electrodialysis, ultrafiltration, nanofiltration and reverse osmosis are required. The most commonly used “ultrafiltration + reverse osmosis” double-membrane process can remove dissolved salts and bacteria on the basis of removing suspended solids and organic matter, and the produced water can be used as raw water for demineralized water systems.

With the proposal of the dual-carbon strategy and the future wastewater treatment concept plant concept, in addition to water regeneration and reuse, energy and resource recovery from wastewater will also become an important function of wastewater treatment plants and a key component of park circular economy. For example, the Strass Plant in Austria, a pioneer in energy-neutral wastewater treatment, maximizes the recovery of organic matter in wastewater for anaerobic fermentation to produce methane, and generates electricity and heat through cogeneration. Other typical application scenarios include carbon source utilization for nitrogen removal between different enterprises or treatment processes, utilization of waste acid and alkali, and resource utilization of salt from zero discharge systems.

4. In-Depth Analysis of Typical Core Technologies

After years of development, the overall environmental protection level of China’s petrochemical industry is in a leading position in China, and the whole-process wastewater treatment technology system of enterprises and parks is relatively complete. Based on previous research foundations, this section analyzes and discusses the key points, existing problems and development trends of several typical treatment technologies.

4.1 Wet Air Oxidation (WAO)

WAO is an advanced oxidation technology. Its oxidation process follows a quasi-first-order reaction, with reaction efficiency mainly determined by reaction temperature and reaction rate determined by oxygen transfer. When the organic matter concentration in wastewater is high (COD ≥ 20000 mg/L), the heat released by oxidation can maintain the temperature required for the reaction, thus greatly reducing energy consumption. Therefore, WAO is mainly suitable for pretreatment of high-concentration wastewater and has been widely applied in industries such as oil refining and chemical engineering.

Previously, large-scale WAO units with a capacity of 20 t/h were monopolized by foreign technologies. Through joint research and development, the first fully domestic large-scale WAO system was put into operation at Zhenhai Refining & Chemical Company in 2022, treating waste alkali liquor from the new ethylene unit. Compared with foreign technologies, the investment of this unit is reduced by 40% and the operating cost is reduced by more than 20%. The main parameters are shown in Table 6.

Table 6 Main Parameters of the First Domestic Large-Scale WAO System

When the content of benzene-ring and heterocyclic compounds in wastewater is high, the treatment effect of WAO deteriorates, and catalytic wet air oxidation (CWAO) technology should be considered. Most CWAO catalysts are heterogeneous supported catalysts. Among them, catalysts loaded with noble metals such as Pt, Pd, Ru, Rh and Ir have higher activity and longer service life than non-noble metal catalysts, and there have been industrial application cases.

To match the concept of resource recovery and realize resource or energy utilization of organic matter in high-concentration wastewater, WAO and CWAO are also upgrading from pursuing high proportion mineralization of organic matter to moderate oxidation. That is, taking small molecular organic acids as target products, constructing a controllable directional conversion process, reducing treatment energy consumption and carbon emissions, and providing high-quality carbon sources for subsequent biological denitrification or anaerobic methane production. It can be predicted that greener and low-carbon new WAO technologies will play an important role in chemical wastewater pretreatment.

4.2 Catalytic Ozonation

Heterogeneous catalytic ozonation technology using solid catalysts is widely used in advanced carbon removal, RO concentrated water treatment and refractory organic matter treatment due to its high efficiency and simple operation. However, in actual operation, some catalytic ozonation units have problems such as poor treatment effect and on-site ozone odor pollution, which have adversely affected the reputation, acceptance and promotion of this technology. The main factors affecting the efficiency of heterogeneous catalytic ozonation are as follows:

1. Catalyst performance
   Catalyst is the core of the ozonation process, which directly affects the treatment performance and long-term operation stability. At present, there are various types of catalysts on the market with uneven performance, and the price factor greatly limits the application of high-performance catalysts. Some projects also mix activated carbon in the catalyst bed, resulting in high initial efficiency but rapid subsequent efficiency attenuation.
2. Ozone generator capacity
   Sufficient ozone dosage is the basis for achieving the target treatment efficiency. Combined with experiments and industrial operation experience, the ratio of ozone dosage mass to COD reduction mass of 2–3 is appropriate. However, in some projects, ozone generators are configured at a ratio of 1 or even lower, resulting in low treatment efficiency in actual operation and inability to effectively adjust when influent COD fluctuates at high levels.
3. Ozone utilization efficiency
   The gas-liquid contact efficiency of the reaction tank or tower, aerator type, uniformity of water and gas distribution, and catalyst bed height will affect the mass transfer efficiency and effective reaction time of ozone, thus affecting the effective utilization rate of ozone and tail gas ozone concentration. Multi-stage step-by-step aeration, micro-bubble processes and higher catalyst packing height can achieve higher ozone utilization rate and are also conducive to improving reaction efficiency.
4. Other substances in water
   Suspended solids and hardness may cause blockage of the aeration system, compaction of the catalyst bed or coverage deactivation of active sites, which adversely affects mass transfer and degradation. For example, when treating sodium chloride-type high-salinity wastewater, chloride ions will be oxidized: part escapes in the form of chlorine gas leading to ineffective ozone consumption, and the generated hypochlorite has weaker oxidizing ability than hydroxyl radicals, which has a certain impact on the treatment effect. In addition, carbonate and bicarbonate are hydroxyl radical scavengers, which have obvious inhibitory effect on catalytic ozonation. Experimental data show that adding 1000 mg/L anhydrous sodium carbonate to the phenol simulated wastewater system treated by catalytic ozonation reduces the COD removal rate by 15%. Therefore, it is necessary to equip pretreatment devices such as filters for catalytic ozonation and avoid water quality conditions with high inhibition.

For common treatment scenarios, catalyst performance and ozone generation capacity have greater impacts. The “configuration reduction” of catalysts and ozone generators driven by low-price bidding is an important reason for the reduced efficiency of heterogeneous catalytic ozonation. Well-designed catalytic ozonation units with high-efficiency catalysts remain one of the preferred technologies for advanced carbon removal of wastewater. In addition, adding defoaming and liquid separation pretreatment to the tail gas destructor and strictly sealing the top of the reaction tank and stabilization tank can solve the odor pollution problem caused by ozone escape.

4.3 Biological Denitrification Technology

As an important factor causing water eutrophication, the discharge of ammonia nitrogen and total nitrogen is subject to increasingly strict restrictions. Biological denitrification is one of the core functions of petrochemical park wastewater treatment plants, and it is also a link prone to operation fluctuations.

Limited by the slow growth rate of autotrophic nitrifying bacteria and poor tolerance to water quality changes, the denitrification function cannot be established quickly during the start-up of biochemical units, often requiring a start-up period of 1–2 months. The system has poor stability and is vulnerable to impact when water quality fluctuates, with slow recovery after impact. The abundance of denitrifying microorganisms in activated sludge is low, resulting in limited removal capacity for nitrogen-containing pollutants.

In industrial practice, adding denitrifying bacterial agents is commonly used to solve these problems, but adding agents alone often cannot achieve ideal results. It is necessary to synchronously optimize and adjust operating conditions in combination with the type of denitrification process. A project adopting the combined technology of high-efficiency microorganisms and process optimization to treat nitrogen-containing wastewater achieved good denitrification effect: at a carbon-nitrogen ratio of 2:1, with influent COD of 526 mg/L and ammonia nitrogen of 164 mg/L, the effluent COD was 45 mg/L, average ammonia nitrogen was 4.7 mg/L, and average total nitrogen was 4.8 mg/L.

Due to the poor biodegradability of petrochemical wastewater or low influent carbon-nitrogen ratio, external carbon sources are often used for denitrification to remove total nitrogen, resulting in high treatment costs. Nitrification usually also requires large aeration volume and high energy consumption. In view of the current development trend of wastewater treatment, the application and promotion of low-cost, energy-neutral biological denitrification technologies such as autotrophic denitrification and anaerobic ammonia oxidation will gradually accelerate, and related technical difficulties such as the coupling regulation of short-range denitrification and anaerobic ammonia oxidation in industrial units need to be overcome urgently.

5. Engineering Application Insights

5.1 Applicable Scenarios

The whole-process wastewater governance system for petrochemical parks is particularly suitable for the following scenarios:

• Newly planned petrochemical or fine chemical parks, to build a source-pipe network-centralized treatment-reuse whole-process system from the initial stage, avoiding repeated transformation later
• Upgrading and reconstruction of existing petrochemical parks, to meet stricter discharge standards including Class IV surface water standards and TDS limits
• Zero discharge transformation projects in water-scarce areas or salt-restricted basins, to realize wastewater resource utilization and near-zero discharge
• Parks with frequent impact of toxic characteristic pollutants on biochemical systems, to strengthen source classification pretreatment and risk prevention and control

5.2 Key Design Considerations

• Classified collection and quality-based treatment: The pipe network system shall be designed according to the pollutant concentration, salinity and toxicity of wastewater from different enterprises, and multiple treatment series shall be set to avoid mixing of high-concentration toxic wastewater and low-concentration wastewater, which increases treatment difficulty and cost.
• Source pretreatment matching: The pretreatment process of enterprises shall be determined according to the pipe-network admission standards based on characteristic pollutants. For high-concentration wastewater, priority shall be given to resource recovery processes such as distillation and extraction, rather than directly adopting destructive oxidation processes.
• Sufficient buffer capacity reserved: The centralized treatment plant shall be equipped with a regulating tank with a hydraulic retention time of 24–48 h, and an emergency accident tank shall be set up to cope with the impact of unqualified discharge from enterprises on the biochemical system.
• Modular advanced treatment design: The advanced treatment unit shall adopt modular design to facilitate flexible process combination and capacity expansion according to future standard upgrading requirements, reducing the transformation cost and cycle.

5.3 Operation and Maintenance Best Practices

• Establish a multi-level source water quality monitoring system covering enterprise discharge outlets, pipe-network key nodes and the inlet of the centralized treatment plant, to realize early warning of abnormal water quality and traceability of responsibility.
• Formulate graded pretreatment management requirements for enterprises in the park, conduct regular characteristic pollutant detection and toxicity testing, and urge enterprises to implement pretreatment obligations for high-toxicity and high-concentration wastewater.
• Dynamically adjust the operating parameters of each unit according to the seasonal fluctuation of influent water quality and load, optimize the dosage of chemicals such as ozone and carbon sources, and reduce operating costs while ensuring stable effluent compliance.
• Establish a full-life-cycle management system for key equipment such as ozone generators and membrane systems, implement standardized maintenance procedures, and extend the service life of core equipment.

6. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that the whole-process systematic governance of petrochemical park wastewater is the core path to achieve stable compliance, cost reduction and efficiency improvement, and green low-carbon transformation of parks. The governance idea of “source precision control + classified quality treatment + centralized advanced treatment + resource recycling” can effectively solve the common pain points of complex water quality, large fluctuation and high treatment cost in petrochemical park wastewater treatment.

Our petrochemical park wastewater overall solution covers the whole process chain from source pretreatment, biochemical main treatment, advanced oxidation to zero discharge and reclaimed water reuse. Relying on modular high-efficiency treatment equipment and intelligent operation platform, we can provide customized whole-process technical route design and operation management schemes for different types of petrochemical parks. Combined with the characteristic pollutant control and negotiated discharge management mechanism, we help parks achieve the dual goals of stable effluent compliance and overall cost reduction, and effectively reduce the total carbon emission of the wastewater treatment system. We also provide full-cycle technical services including park water environment planning, process design, equipment supply, commissioning operation and management system construction, supporting the green, low-carbon and high-quality development of petrochemical parks.

7. Conclusion and Prospect

Due to the diversity and irregularity of wastewater generation and discharge from enterprises in petrochemical industrial parks, the top priority of wastewater governance is source control and pretreatment, which is also the key to the stable operation of park wastewater treatment plants. In addition to conventional comprehensive water quality factors, pipe-network admission standards based on characteristic pollutants or toxicity should be established and implemented to ensure that enterprises carry out effective pretreatment of high-concentration and high-toxicity wastewater. On the other hand, it is recommended that park wastewater treatment plants refer to the wastewater treatment mode of refining and chemical enterprises, set up multiple process series for quality-based treatment and reuse of wastewater according to pollutant concentration and salt concentration, which is conducive to improving the wastewater resource utilization rate.

At the technical level, the technical system of pretreatment – advanced treatment – wastewater reuse – near-zero discharge has been established, but it is necessary to find an appropriate balance between comprehensive investment, cost and performance, and treatment effect.

In the future, the concept of wastewater treatment will undergo major changes. The “nutrients + energy + reclaimed water” model of wastewater treatment concept plants will be further developed. The goal of wastewater treatment will gradually shift from “removal” to “recovery”. Technologies such as centralized sludge digestion to increase biogas production, biogas power generation, waste heat sludge drying, and dried sludge as fuel for thermal power plants will be widely applied. Wastewater treatment will transform from a gray compliance-oriented mode to a green ecological mode: recovering energy while converting pollutants, obtaining resources while removing pollutants, and realizing ecological integration in the discharge process.

FAQ

Q1: What are the core difficulties in wastewater treatment of petrochemical industrial parks?
The core difficulties are mainly reflected in three aspects. First, the water quality is complex and highly toxic, containing a large number of refractory characteristic pollutants such as aromatic hydrocarbons, halogenated hydrocarbons and nitrobenzenes, which can easily inhibit the biochemical system. Second, the water quantity and quality fluctuate greatly. Enterprise production adjustment and intermittent discharge lead to large load fluctuations, putting high requirements on the anti-shock capacity of the treatment system. Third, the discharge standards are increasingly strict. Many local standards require effluent to approach Class IV surface water quality, and some areas also add TDS control, which greatly increases the treatment process chain and operating cost.

Q2: Why is it emphasized to strengthen source pipe-network management and control of characteristic pollutants?
The conventional pipe-network admission standards focusing on COD and other conventional indicators cannot effectively control the toxic risk of petrochemical wastewater. High-toxicity characteristic pollutants discharged by enterprises will seriously inhibit the microbial activity of the centralized treatment plant’s biochemical system, leading to process collapse and effluent exceeding standards in severe cases. Strengthening the source control of characteristic pollutants can transfer the treatment responsibility of toxic and harmful substances to the generation end, reduce the treatment pressure and operation risk of the terminal centralized plant, and is also conducive to the overall cost optimization of the park’s wastewater treatment system.

Q3: What are the common reasons for poor effect of catalytic ozonation in practical engineering?
The main reasons include four aspects. First, the catalyst performance is unqualified. Low-cost catalysts with low activity and short service life are used, and some projects mix activated carbon to create the illusion of high initial efficiency. Second, the ozone generator is under-configured. The ozone dosage is far lower than the actual demand, making it impossible to achieve the expected removal effect. Third, the gas-liquid mass transfer design is unreasonable, resulting in low ozone utilization rate and large tail gas loss. Fourth, the influent water quality is poor, with high suspended solids, hardness or carbonate content, which causes catalyst bed blockage and free radical scavenging, inhibiting the catalytic reaction.

Q4: What are the main paths for wastewater resource utilization in petrochemical parks?
The main paths include three levels. The first level is water resource recovery: treated wastewater is reused as circulating cooling water make-up water, production process water, etc., through advanced treatment + double-membrane process, to reduce fresh water consumption. The second level is material resource recovery: valuable components such as phenol and organic acids are recovered from high-concentration wastewater through extraction, distillation and other methods, and salt resources are recovered through zero discharge systems. The third level is energy recovery: organic matter in wastewater and sludge is recovered through anaerobic fermentation to produce biogas for power generation and heat supply, to reduce the external energy consumption of the treatment system and realize energy neutralization of wastewater treatment.