Process Design, Operational Performance and Techno-Economic Analysis

Abstract

Aiming at the characteristics of complex pollutant composition, poor biodegradability and large water quality/quantity fluctuation of wastewater from comprehensive industrial parks, this paper takes a wastewater treatment project of a comprehensive industrial park in Qianjiang as a case study, introduces the wastewater characteristics, treatment process, main design parameters and operational performance of the project, and analyzes the operating costs under different working conditions. After treatment by the combined process of “hydrolytic acidification + AAO + coagulation sedimentation + ozone-biological aerated filter (O₃-BAF)”, the main effluent pollutant indicators can stably meet the Class 1A standard of the Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002). The unit investment of the wastewater treatment plant is 4826 yuan per cubic meter of daily capacity. The operating cost under design conditions is 1.399 yuan/m³, and the actual operating cost under low influent load conditions is 0.755 yuan/m³. The relevant experience can provide reference for the design and upgrading of wastewater treatment plants in comprehensive industrial parks with diverse drainage enterprises and similar water quality characteristics.

1. Introduction

With the acceleration of urbanization in China and the increasingly clear functional zoning of cities, industrial parks have developed rapidly. According to incomplete statistics, there are about 90,000 various industrial parks across the country. Along with the growth of industrial parks, wastewater treatment issues in parks have become increasingly prominent.

At present, there are many reported cases of wastewater treatment projects for single-type industrial parks such as pharmaceutical, printing and dyeing, chemical, papermaking and mechanical processing parks. For these parks, the main drainage enterprises belong to the same industry, the wastewater characteristics are relatively distinct, and the treatment process can be highly targeted. However, with the cultivation of industrial clusters across the country, more and more comprehensive industrial parks have emerged. Wastewater from comprehensive industrial parks generally has poor biodegradability, complex pollutant composition and large fluctuations in water quality and quantity, which brings certain difficulties to wastewater treatment. In addition, the water quality and quantity of park wastewater are affected by the proportion of various enterprises, and may change significantly as more enterprises settle in the park. Therefore, the process selection of wastewater treatment plants in comprehensive industrial parks requires certain universality, and the operation mode requires certain flexibility. Relevant engineering cases for comprehensive industrial parks are still relatively insufficient at present.

This paper takes a comprehensive industrial park wastewater treatment project in Qianjiang as an example, proposes a combined process of “hydrolytic acidification + AAO + coagulation sedimentation + O₃-BAF” for comprehensive industrial park wastewater treatment, and verifies its technical feasibility and economic rationality through actual operation data, providing reference for similar projects.

2. Project Scale and Water Quality

The target comprehensive industrial park in Qianjiang mainly houses food manufacturing, furniture processing, mechanical manufacturing and preparation pharmaceutical enterprises. The park adopts a “pretreatment + pipeline discharge” mode: enterprise wastewater is pretreated to meet the public pipeline admission standard, then enters the park wastewater treatment plant for secondary treatment before discharge. The treated tail water flows into the Yangtze River through the Chengnan River.

The current phase design scale is 10,000 m³/d, the medium-term scale is 20,000 m³/d, and the long-term scale is 40,000 m³/d. The design influent and effluent quality are shown in Table 1. The effluent implements the Class 1A standard of Discharge Standard of Pollutants for Municipal Wastewater Treatment Plant (GB 18918-2002).

Table 1 Design Influent and Effluent Quality of the WWTP

Note: The value in brackets for ammonia nitrogen is the control index when water temperature ≤12℃, and the value outside brackets is for water temperature >12℃.

Among the current settled enterprises, food processing wastewater accounts for 77% of the total drainage volume, including lactic acid bacteria beverage processing, canned fruit processing and stewed product processing. The remaining drainage comes from mechanical/furniture processing enterprises and preparation pharmaceutical enterprises. The mixed wastewater is generally not extremely difficult to treat, but contains a certain amount of refractory organic matter, with large fluctuations in water quality and quantity.

3. Wastewater Treatment Process Scheme

3.1 Water Quality Analysis and Process Selection

According to the water quality characteristics of the park, the following key points are considered in process determination:

1. Refractory organic matter control: Most enterprises in the park have built-in pretreatment systems with physicochemical and biological treatment units. Most easily biodegradable pollutants are removed inside the enterprises, resulting in relatively low B/C ratio of the influent to the centralized WWTP. A hydrolytic acidification tank is set before the biological tank to break down macromolecular refractory organics into small-molecule biodegradable organics, improving the overall treatment efficiency of the biological system. Conventional biochemical treatment still cannot guarantee stable COD removal to below 50 mg/L, so advanced oxidation process is added in the advanced treatment stage. The O₃-BAF combined process is selected, which uses ozone to oxidize refractory organics into biodegradable fractions, and then removes residual COD and SS through biological degradation and filtration of BAF, with high cost performance.
2. Oil removal: Wastewater from stewed product, mechanical processing and furniture processing enterprises contains a certain concentration of oil substances. An aerated grit chamber is set in the pretreatment unit to control oil concentration below 15 mg/L, avoiding adverse effects on the biological system and membrane units.
3. Water quantity and quality fluctuation regulation: Influent fluctuation is significant due to enterprise production schedules and pretreatment operation status. Equalization tank and emergency tank are set up to cope with flow fluctuation and accidental discharge, and their volumes are appropriately enlarged considering future park development.

3.2 Process Flow

The complete process flow is as follows:

Multiple bypass pipelines are set throughout the process to enable flexible operation according to actual influent conditions:

• When influent fluctuation is small, effluent from the aerated grit chamber can bypass the equalization tank and directly flow into the hydrolytic acidification tank by gravity, reducing lifting times and operating costs.
• When influent quality is better than design value for a long period, ozone dosage can be reduced or suspended. According to the effluent quality of the ozone contact tank, BAF can be bypassed individually, or both BAF and fiber disc filter can be bypassed simultaneously to cut operating costs.
• When SS in BAF effluent already meets the standard, BAF effluent can bypass the fiber disc filter and directly enter the UV disinfection channel.

Excess sludge from hydrolytic acidification and AAO tanks, as well as physicochemical sludge from coagulation sedimentation tanks, are discharged to a sludge storage tank, then dewatered by a belt thickener-dewatering integrated machine to moisture content below 80%, and transported to an external sludge disposal center. Ion deodorization equipment is configured for the pretreatment unit and sludge treatment unit to collect and treat odorous gas.

4. Main Structures and Design Parameters

4.1 Pretreatment Units

The coarse screen & lift pump station, and fine screen & aerated grit chamber are all constructed with civil works for the medium-term scale of 20,000 m³/d, with equipment installed for the current 10,000 m³/d scale. The total variation coefficient of influent is 1.58. The lift pump station is equipped with 3 submersible pumps (2 operating + 1 standby), each with a flow rate of 320 m³/h, head of 13 m and power of 37 kW, all with variable frequency control. The aerated grit chamber has a hydraulic retention time (HRT) of 6.8 min, effective water depth of 2.2 m, and aeration rate of 0.2 m³ per cubic meter of wastewater, equipped with oil skimming devices.

4.2 Equalization Tank and Emergency Tank

The equalization tank and emergency tank are built together, designed for 10,000 m³/d. The equalization tank has 2 compartments with total dimensions of 25.0 m × 22.6 m × 6.5 m and HRT of 8 h, equipped with submersible mixers and lift pumps for homogeneous mixing and flow regulation. The emergency tank has 1 compartment with dimensions of 25.0 m × 11.0 m × 6.5 m and HRT of 4 h, used for temporary storage of excessive influent during shock load or accidents, with small-flow gradual return to the treatment system after recovery. For comprehensive industrial park projects, it is recommended to design the equalization tank with 8–12 h HRT when land and investment permit, to better cope with water quality fluctuations.

4.3 Hydrolytic Acidification Tank and AAO Bioreactor

The hydrolytic acidification tank and AAO tank are built together for 10,000 m³/d capacity.

• Hydrolytic acidification tank: Completely mixed type, consisting of complete mixing zone, buffer zone and inclined tube sedimentation zone, with total dimensions of 42.3 m × 19.2 m × 7.0 m. The average MLSS in the mixing zone is 4500 mg/L, effective HRT is 8.9 h. The inclined tube sedimentation zone has a surface hydraulic load of 6 m³/(m²·h), with a steep slope below for automatic sludge return to the mixing zone. A sludge supplement pipeline from the secondary sedimentation tank is reserved to maintain sludge concentration when influent organic load is low.
• AAO bioreactor: Total dimensions of 42.3 m × 38.3 m × 7.0 m, total HRT of 21.2 h, including 2.9 h anaerobic zone, 5.8 h anoxic zone and 12.5 h aerobic zone. Design MLSS is 3500 mg/L, sludge loading rate is 0.12 kg BOD₅/(kg MLSS·d), mixed liquor return ratio is 200%, sludge return ratio is 50%–100%, and maximum aeration gas-water ratio is 8.6:1.

4.4 Secondary Sedimentation Tank

Two radial flow secondary sedimentation tanks are built together with the inlet distribution well and sludge return tank. Each tank has a diameter of 18.5 m and height of 4.5 m, with a surface hydraulic load of 0.78 m³/(m²·h). Equipped with sludge return pumps and excess sludge discharge pumps, return sludge is lifted to the anaerobic zone of AAO and the hydrolytic acidification tank as needed.

4.5 Coagulation Sedimentation and Ozone Contact Tank

The coagulation sedimentation tank, ozone contact tank and intermediate lift pump station are built together, designed for 10,000 m³/d.

• Coagulation sedimentation tank: The mixing tank has 2 min HRT with mechanical stirring, maximum PAC dosage of 30 mg/L. The flocculation tank has 15 min HRT, maximum PAM dosage of 1 mg/L. The inclined tube sedimentation zone has a surface hydraulic load of 2.5 m³/(m²·h), equipped with a center-driven sludge scraper with thickening grids.
• Ozone contact tank: Total dimensions of 22.8 m × 9.0 m × 7.0 m, maximum ozone dosage to COD removal mass ratio of 2:1, effective contact time of 60 min divided into 3 chambers with volume ratio of 4:3:3. An aeration stripping zone with 33 min HRT is set after the contact zone to remove residual ozone before entering BAF. Two oxygen-source ozone generators with 7 kg/h capacity each are configured, supported by a 50 m³ liquid oxygen storage tank.

4.6 Biological Aerated Filter (BAF)

One up-flow BAF with 4 compartments is provided, designed for 10,000 m³/d. Each compartment is 9.0 m × 8.5 m × 7.5 m, with a 4 m high filter media layer. BOD₅ volumetric load is 0.18 kg BOD₅/(m³·d), surface hydraulic load is 1.53 m³/(m²·h), empty bed contact time is 2.6 h, and maximum aeration gas-water ratio is 4.5:1. The backwash cycle is 24–48 h, with a sequence of 4 min air-only washing, 6 min air-water combined washing, and 10 min water-only washing.

4.7 Advanced Treatment and Disinfection

One fiber rotary disc filter with 2 compartments is set, with a filtration rate of 8.3 m/h, equipped with bypass pipeline. The UV disinfection channel and Parshall flume are built with civil works for 20,000 m³/d and equipment for 10,000 m³/d. Reclaimed water pumps are set at the end to provide backwash water and cooling water for ozone generator auxiliary equipment.

4.8 Sludge Treatment and Deodorization System

The sludge storage tank has a 10 h sludge retention time. Two belt thickening-dewatering integrated machines are installed in the sludge dewatering workshop, reducing sludge moisture content to below 80%. Two independent ion deodorization systems are configured for the pretreatment unit (7000 m³/h air flow) and sludge system (8500 m³/h air flow) respectively.

5. Operational Performance Analysis

5.1 Design Condition Performance Prediction

Based on the design influent quality and process configuration, the predicted pollutant removal efficiency of each unit under design conditions is shown in Table 2. The final effluent can fully meet Class 1A standards, with sufficient safety margin for all indicators.

Table 2 Predicted Pollutant Removal Efficiency Along the Process (Design Conditions)

5.2 Actual Operational Performance

The project was completed and put into operation in August 2018. Due to the low settlement rate of enterprises in the park at the initial stage, the actual received industrial wastewater volume was lower than the design value, and domestic wastewater from surrounding areas was introduced to give full play to the project benefits. In 2021, the average daily treatment volume was about 8500 m³/d, of which industrial wastewater accounted for about 60%.

Since the actual influent quality was significantly lower than the maximum design value, the ozone contact tank was operated without ozone dosing, and its effluent bypassed BAF and directly entered the fiber rotary disc filter for filtration and disinfection before discharge. The actual pollutant removal performance is shown in Table 3. All effluent indicators stably met Class 1A standards, with large redundancy in treatment capacity.

Table 3 Actual Pollutant Removal Efficiency Along the Process

6. Techno-Economic Analysis

The direct construction cost of the project is 48.26 million RMB, equivalent to a unit investment of 4826 yuan per cubic meter of daily treatment capacity.

Under design water quality and quantity conditions, the calculated direct operating cost is 1.399 yuan/m³, with detailed composition shown in Table 4. Under actual low-load operation conditions, the actual direct operating cost is 0.755 yuan/m³, as shown in Table 5. The large difference is mainly due to the savings of ozone consumption and partial electricity cost under low load, as well as the reduction of chemical dosage.

Table 4 Direct Operating Cost (Design Conditions)

Table 5 Actual Direct Operating Cost

7. Engineering Application Insights

7.1 Applicable Scenarios

This combined process is particularly suitable for the following scenarios:

• Comprehensive industrial parks with mixed industries including food processing, light industry, machinery manufacturing and fine chemical industry
• Industrial park WWTPs with large influent quality and quantity fluctuations and uncertain future enterprise settlement types
• Upgrading projects requiring effluent to meet Class 1A or higher standards, with refractory organic matter in biochemical effluent
• Projects with sufficient site conditions and requirements for flexible operation mode to adapt to load changes

7.2 Key Design Considerations

• Adequate buffering capacity: Equalization tank and emergency tank with sufficient volume are essential for comprehensive industrial parks, to cope with shock loads from accidental enterprise discharge. It is recommended to reserve 8–12 h of hydraulic retention capacity.
• Multi-stage bypass design: Setting bypass pipelines in each treatment unit can realize graded operation according to actual influent load, greatly reducing unnecessary operating energy consumption and chemical cost during low-load periods, which is very important for parks in the early stage of development.
• Hydrolytic acidification optimization: The hydrolytic acidification tank with inclined tube sedimentation and sludge return function can maintain stable sludge concentration even under low organic load, ensuring stable hydrolysis effect. The sludge supplement pipeline from secondary sedimentation is a very practical design detail.
• Ozone-BAF combined advanced treatment: This combination makes full use of the oxidation effect of ozone on refractory organics and the biological degradation and filtration effect of BAF, which is more economical than complete oxidation by ozone alone, and more stable than single BAF for refractory COD removal.

7.3 Operation and Maintenance Best Practices

• Develop graded operation strategies according to seasonal and enterprise production fluctuations, dynamically adjust the operation of advanced treatment units, and maximize energy and chemical savings on the premise of ensuring effluent compliance.
• Regularly monitor the sludge concentration and activity of the hydrolytic acidification tank, and supplement sludge from the secondary sedimentation tank in time when the organic load is too low to maintain treatment efficiency.
• Optimize ozone dosage according to actual biochemical effluent COD concentration, adopt stepped dosing control, and avoid excessive ozone waste.
• Establish a standardized backwash management system for BAF, adjust backwash cycle and intensity according to actual head loss and effluent quality, and avoid excessive backwashing that leads to loss of biofilm and waste of water and electricity.

8. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that the combined process of hydrolytic acidification + biological treatment + ozone-BAF advanced treatment is an ideal solution for comprehensive industrial park wastewater treatment, with strong adaptability to variable water quality, stable effluent performance and good operation flexibility.

Our modular industrial park wastewater treatment systems adopt prefabricated standardized process units including hydrolytic acidification, AAO and O₃-BAF, which can be flexibly combined according to different park scales and water quality characteristics, greatly shortening the on-site construction period. Equipped with our intelligent operation management platform, the system realizes real-time dynamic adjustment of aeration intensity, chemical dosage and process bypass switching based on influent load monitoring, reducing comprehensive operating costs by 15–20% compared with conventional fixed-parameter operation. We also provide full-cycle technical services including park wastewater treatment planning, process design, equipment supply and operation guidance, helping industrial park operators achieve stable compliance while minimizing total life-cycle costs.

9. Conclusion
10. For comprehensive industrial parks with diverse drainage enterprises, complex pollutant composition, large water quality fluctuation and high content of refractory organic matter, the combined process of “hydrolytic acidification + AAO + coagulation sedimentation + O₃-BAF” has strong adaptability. The effluent can stably meet the Class 1A standard, and has good reference value for similar comprehensive industrial park wastewater treatment system design.
11. The multi-bypass design throughout the process enables highly flexible operation. According to actual influent load, different treatment units can be put into operation or bypassed, which greatly reduces operating costs during low-load periods and is very suitable for industrial parks with phased development.
12. The unit investment of the project is 4826 yuan/m³ of daily capacity. The direct operating cost under design conditions is about 1.399 yuan/m³, and the actual operating cost under low influent quality is 0.755 yuan/m³, showing good economic performance.
13. The hydrolytic acidification unit effectively improves the biodegradability of wastewater, and the O₃-BAF combined advanced treatment unit ensures stable removal of refractory COD. The combination of biological treatment and physicochemical advanced treatment provides double guarantee for effluent quality, which is suitable for promotion in comprehensive industrial park wastewater treatment projects.

FAQ

Q1: Why choose the O₃-BAF combined process instead of single ozone oxidation for advanced treatment?
Single ozone oxidation requires high ozone dosage to completely mineralize refractory organic matter into carbon dioxide and water, resulting in very high operating costs. The O₃-BAF combination only needs ozone to break the structure of macromolecular refractory organics and convert them into easily biodegradable small molecules, which are then removed by microbial degradation in BAF. This can achieve the same COD removal effect with only 30–50% of the ozone dosage of single ozone process, greatly reducing operating costs, and BAF can also remove SS at the same time.

Q2: What is the function of the multiple bypass pipelines in the process?
Comprehensive industrial parks usually have a phased development process. In the early stage with low enterprise settlement rate and good influent quality, the advanced treatment units can be partially or completely bypassed, which can save a lot of unnecessary energy and chemical costs. The bypass design also provides convenience for equipment maintenance without affecting the continuous operation of the whole plant, greatly improving the operation flexibility and reliability of the wastewater treatment plant.

Q3: Why is hydrolytic acidification set before the AAO bioreactor?
Most wastewater entering the park WWTP has been pretreated by enterprises, and most of the easily biodegradable organic matter has been removed, resulting in low B/C ratio and poor biodegradability. The hydrolytic acidification process uses facultative microorganisms to break macromolecular, long-chain and refractory organic matter into small-molecule easily biodegradable substances, improving the B/C ratio of wastewater, which can significantly improve the treatment efficiency of the subsequent AAO biochemical system and reduce the difficulty of advanced treatment.

Q4: How to control the operating cost for parks with large seasonal load fluctuations?
First, adopt graded operation mode: in low-load seasons, stop ozone dosing and bypass BAF and other advanced treatment units, and only operate the core biochemical system to ensure compliance. Second, optimize aeration control of the aerobic tank and BAF, and adjust aeration intensity in real time according to dissolved oxygen and influent load to avoid unnecessary aeration energy consumption. Third, dynamically adjust the dosage of coagulants according to actual effluent TP and SS indicators to avoid excessive chemical dosing.