Process Design, Parameter Optimization and Full-Scale Engineering Application

Abstract

Groundwater contamination in industrial parks, especially in stainless steel manufacturing zones, typically features composite pollutants including heavy metals, petroleum hydrocarbons, fluorides, benzene series and total nitrogen, posing great challenges to remediation engineering. This paper presents a tailored pump-and-treat process train for a stainless steel industrial park, adopting a modular combination of “grid + equalization tank + dissolved air flotation / iron-carbon micro-electrolysis + coagulation-sedimentation + anoxic-oxic (AO) + membrane bioreactor (MBR)” to address different contamination profiles across four polluted blocks. Laboratory and pilot tests were conducted to optimize key operational parameters for each treatment unit. Full-scale operation results show that the process achieves stable effluent compliance with the Discharge Standard of Water Pollutants for Iron and Steel Industry (GB 13456-2012), with removal rates of 99.29% for petroleum, 99.87% for BTEX, 92.39% for nickel and 99.44% for fluoride under optimized conditions. This solution provides a reliable technical reference for groundwater remediation projects with composite industrial pollution, and supports carbon reduction targets in contaminated site management.

1. Introduction

Groundwater is a critical water resource for industrial and domestic use in China. With the acceleration of industrialization, groundwater contamination in industrial parks has become an increasingly prominent environmental issue. Stainless steel manufacturing involves complex pickling, polishing and surface treatment processes, leading to widespread groundwater pollution with diverse pollutant types, large affected areas, large water quality fluctuations and high remediation difficulty. These contaminants pose severe risks to ecological safety and human health, making effective prevention and control of great practical significance for sustainable groundwater resource utilization.

The pump-and-treat technology is currently the most widely applied ex-situ remediation method for contaminated groundwater. For composite pollution involving heavy metals, organic matter and inorganic salts, a single treatment unit cannot meet comprehensive discharge requirements. A combined process integrating physical, chemical and biological units is required to achieve step-by-step removal of different target pollutants. This study takes the contaminated groundwater from a stainless steel industrial park as the research object, develops a targeted combined process scheme through laboratory optimization tests, and verifies its feasibility and stability through full-scale engineering application.

2. Project Overview and Contaminant Characteristics

The project involves four contaminated blocks in a stainless steel industrial park in eastern China. According to site investigation and environmental risk assessment, the target pollutants vary across different blocks due to differences in historical production activities:

• Blocks 1 and 2: target pollutants include pH, nickel, total nitrogen, fluoride and petroleum hydrocarbons
• Blocks 3 and 4: target pollutants include pH, nickel, total nitrogen, fluoride and BTEX (benzene series)

The concentration ranges of key pollutants are summarized as follows:

The overall treatment objective is to treat the extracted groundwater to meet the indirect discharge requirements of relevant industrial pollutant discharge standards, and then discharge it to the park’s centralized wastewater treatment plant. The process design must adapt to large fluctuations in water quality and quantity, and achieve reliable removal of multiple types of pollutants simultaneously.

3. Overall Process Design and Technical Rationale

According to the pollutant characteristics of different blocks, two differentiated process routes are designed to achieve targeted treatment while avoiding redundant unit investment.

3.1 Process Flow for Blocks with Petroleum Contamination (Blocks 1 & 2)

Process route: Grid → Equalization Tank → Dissolved Air Flotation (DAF) → Coagulation-Sedimentation → AO Bioreactor → MBR → Clear Water Tank → Discharge

• Grid removes large suspended solids and impurities to protect subsequent equipment
• Equalization tank homogenizes water quality and quantity to mitigate impact load
• DAF unit removes petroleum hydrocarbons and partially reduces fluoride concentration
• Coagulation-sedimentation removes heavy metal nickel and fluoride, and adjusts effluent pH to 6–9
• AO + MBR combined biochemical system achieves deep removal of total nitrogen and residual organic matter
• Sludge produced by each unit is discharged to the sludge tank for unified dewatering and disposal

3.2 Process Flow for Blocks with BTEX Contamination (Blocks 3 & 4)

Process route: Grid → Equalization Tank → Iron-Carbon Micro-Electrolysis (ICME) → Coagulation-Sedimentation → AO Bioreactor → MBR → Clear Water Tank → Discharge

• The DAF unit is replaced with an iron-carbon micro-electrolysis unit, which degrades BTEX through redox reaction
• The iron ions generated by the reaction also provide coagulant aid effect for subsequent sedimentation units
• The subsequent coagulation, biochemical and membrane treatment units share the same design principle as the first route

This modular design concept enables flexible configuration according to different pollution characteristics, maximizes treatment efficiency and reduces unnecessary construction and operation costs.

4. Parameter Optimization and Performance Validation

A series of laboratory static tests were conducted to determine the optimal operating parameters for each core treatment unit, providing a reliable basis for engineering design.

4.1 Dissolved Air Flotation for Petroleum Removal

Dissolved air flotation achieves solid-liquid separation by attaching micro-bubbles to suspended oil particles. Tests were carried out to investigate the effects of pH, coagulant dosage and aeration time on petroleum removal efficiency.

The optimized parameters are as follows:

• Aeration time: 20 min
• Optimal pH: 7.0
• PAC (10% mass fraction) dosage: 1‰ of groundwater volume
• PAM (1‰ mass fraction) dosage: 2.5‰ of groundwater volume

Under these conditions, the removal rate of petroleum hydrocarbons reaches 99.29%, far exceeding the 90.25% removal rate of single air flotation without coagulant aid. The combined DAF-coagulation process effectively removes dispersed and emulsified oil in groundwater.

4.2 Iron-Carbon Micro-Electrolysis for BTEX Degradation

Iron-carbon micro-electrolysis utilizes the potential difference between iron and carbon particles to form numerous micro-galvanic cells in acidic aqueous solution. The electrochemical reaction breaks the molecular chain of organic pollutants, and the generated ferric hydroxide colloid further enhances adsorption and coagulation effects.

The optimized parameters for BTEX removal are as follows:

• Optimal initial pH: 5.0
• Iron powder dosage: 50 g/L, carbon powder dosage: 25 g/L
• Aeration rate: 1 L/min
• Hydraulic retention time (HRT): 20 min

Under these conditions, the BTEX removal rate reaches 99.87%, with effluent BTEX concentration below the detection limit. The process effectively degrades benzene series pollutants while avoiding the high investment and operating costs of advanced oxidation processes.

4.3 Coagulation-Sedimentation for Heavy Metal and Fluoride Removal

Coagulation-sedimentation is the core unit for removing nickel and fluoride. Tests investigated the effects of PAC dosage, PAM dosage and pH value on removal efficiency.

For nickel removal priority:

• Optimal pH: 10.0
• 10% PAC dosage: 1‰ of water volume
• 1‰ PAM dosage: 0.25‰ of water volume
• Nickel removal rate: 92.39%, fluoride removal rate: 59.34%

For fluoride removal priority:

• Optimal pH: 6.0
• 10% PAC dosage: 3% of water volume
• 1‰ PAM dosage: 2% of water volume
• Fluoride removal rate: 99.44%

For groundwater with both high nickel and high fluoride, a two-stage coagulation process can be adopted: first adjust pH to 10 to remove nickel, then adjust pH to 6 with high PAC dosage for deep fluoride removal.

4.4 AO-MBR Biochemical System for Nitrogen Removal

The AO-MBR process is adopted for total nitrogen and residual COD removal. The anoxic zone achieves denitrification with supplementary carbon source, and the aerobic zone completes nitrification and organic matter degradation. The MBR unit replaces secondary sedimentation to achieve efficient solid-liquid separation and maintain high sludge concentration.

Test results show:

• Anoxic zone with 2 g/L MLSS: total nitrogen removal rate reaches 48.17%
• Anoxic zone with 4 g/L MLSS: BOD₅ and COD removal rates reach 37.44% and 37.43% respectively
• Aerobic zone with 4 g/L MLSS: ammonia nitrogen, BOD₅ and COD removal rates reach 57.39%, 58.45% and 52.13% respectively

Biochemical respiration tests confirm that the pollutants in groundwater can be biodegraded, and the AO-MBR process has stable nitrogen and organic matter removal performance.

5. Full-Scale Engineering Application and Operational Performance

The optimized process was first applied to the full-scale treatment system of Contaminated Block 1. Continuous monitoring over 4 months of stable operation confirms that all effluent indicators meet the discharge standard requirements.

5.1 Effluent Quality Performance

All indicators stably meet the standard requirements, with large safety margins for heavy metal and petroleum pollutants. The system shows good adaptability to influent concentration fluctuations.

5.2 Environmental Benefits

During operation, monitoring well data shows that the water level fluctuation in dry season is less than 0.3 m, and the groundwater quality in the downstream area of the contaminated zone is continuously improved. The full-coverage extraction pipe network effectively controls the diffusion of pollution plumes, achieving the dual goals of stock pollution reduction and risk control.

6. Engineering Application Analysis

6.1 Applicable Scenarios

This combined process is suitable for the following groundwater remediation scenarios:

• Industrial parks with composite contamination of heavy metals, organic matter and inorganic salts
• Pump-and-treat projects for contaminated sites of stainless steel, electroplating and chemical industries
• Groundwater remediation projects with large water quality fluctuations and strict discharge requirements
• Sites requiring staged construction according to pollution degree and block distribution

6.2 Key Engineering Design Points

• Homogenization design: Sufficient volume of equalization tank must be reserved to cope with the large fluctuation of extracted groundwater quality and quantity
• pH regulation accuracy: Each physicochemical unit has different optimal pH requirements, so multi-stage pH regulation and online monitoring devices must be configured
• Anti-caking design for ICME unit: Regular backwashing and appropriate iron-carbon ratio design are required to avoid filler caking and channeling
• Membrane fouling control: The physicochemical pretreatment units must ensure stable effluent SS to reduce MBR membrane fouling risk and extend membrane service life
• Sludge disposal: The sludge containing heavy metals must be treated as hazardous waste in accordance with regulations, and a complete sludge dewatering and disposal system should be configured

6.3 Operation and Maintenance Best Practices

• Establish graded dosing control strategies for PAC and PAM based on influent pollutant concentration to reduce chemical consumption
• Regularly supplement iron-carbon filler and check the filler status to maintain stable treatment efficiency of the micro-electrolysis unit
• Optimize carbon source dosage in the AO unit based on real-time total nitrogen data to reduce operating costs
• Perform regular MBR membrane cleaning and maintenance to ensure stable flux and effluent quality
• Strengthen online water quality monitoring to realize early warning and rapid response to influent load shocks

6.4 Cost-Benefit Analysis

Compared with advanced oxidation processes such as Fenton and ozone catalytic oxidation, the iron-carbon micro-electrolysis process reduces investment by about 40% and operating cost by more than 30% for organic pollutant removal. The modular process configuration avoids redundant unit construction, and the targeted dosing strategy further reduces comprehensive operating costs. For medium-sized industrial groundwater treatment projects, this process has obvious economic advantages while ensuring stable compliance.

7. SYNERAQUA Technical Perspective

At SYNERAQUA, we believe that modular, targeted combined processes are the core solution for industrial groundwater remediation with complex composite pollution. The process design concept of classified treatment by pollutant type fully aligns with our technical philosophy of customized engineering solutions and efficient resource allocation.

Combined with SYNERAQUA’s skid-mounted equipment manufacturing capabilities, each treatment unit can be integrated into prefabricated modular systems, greatly shortening the on-site construction period and reducing the impact on the normal operation of the industrial park. Equipped with our SCADA automatic control platform, the system can realize automatic linkage of pH regulation, chemical dosing and biochemical operation parameters, reducing manual intervention and improving operational stability. We continue to optimize the combined process scheme for different types of industrial contaminated groundwater, providing one-stop solutions from process design, equipment manufacturing to operation and maintenance for site remediation projects.

8. Conclusion

The combined process of “grid + equalization + DAF / iron-carbon micro-electrolysis + coagulation-sedimentation + AO-MBR” provides a reliable solution for composite contaminated groundwater in stainless steel industrial parks, with the following core conclusions:

1. The modular process configuration can flexibly adapt to different pollution characteristics, achieving targeted removal of petroleum hydrocarbons, BTEX, heavy metal nickel, fluoride and total nitrogen with stable effluent compliance.
2. Under optimized operating parameters, the removal rates of petroleum, BTEX, nickel and fluoride reach 99.29%, 99.87%, 92.39% and 99.44% respectively, and the AO-MBR system achieves efficient total nitrogen removal.
3. Full-scale engineering application verifies the stability and reliability of the process, which not only achieves discharge compliance but also effectively improves regional groundwater quality and controls pollution diffusion.
4. This process has the advantages of strong impact resistance, moderate investment and low operating cost, and can provide technical reference for similar industrial park groundwater remediation projects, supporting the goals of groundwater pollution prevention and carbon reduction.

FAQ

Q1: What are the advantages of iron-carbon micro-electrolysis over advanced oxidation processes for organic contaminated groundwater?
Iron-carbon micro-electrolysis achieves organic degradation through electrochemical reaction without additional oxidant dosing, with much lower operating cost than Fenton or ozone oxidation processes. The generated iron ions also have coagulation aid effect, which can synchronously remove partial heavy metals and suspended impurities.

Q2: Why adopt differentiated process routes for different contaminated blocks?
Different polluted blocks have different main pollutant types. Configuring targeted treatment units avoids redundant equipment investment, reduces construction and operation costs, and improves treatment efficiency. The modular design also supports staged construction according to the governance priority of each block.

Q3: Can this process adapt to large fluctuations in groundwater quality?
Yes. The front-end equalization tank homogenizes water quality and quantity, and each treatment unit has a large load tolerance range. Combined with graded chemical dosing control and online monitoring system, the process can adapt to common concentration fluctuations of industrial contaminated groundwater.

Q4: What is the role of MBR in the groundwater treatment process?
MBR replaces the traditional secondary sedimentation tank, achieving efficient solid-liquid separation with high sludge concentration in the biochemical tank, which improves nitrogen and COD removal efficiency. It also ensures stable low SS effluent, avoiding the risk of residual suspended pollutants and providing reliable guarantee for discharge compliance.