Case Study of Surface Water Pretreatment and Advanced Treatment with Parameter Optimization

Abstract

Objective The acceleration of urbanization has led to increasing urban water supply demand. As a critical water supply source, surface water quality and purification efficiency are essential for drinking water safety. To investigate the treatment performance of the coagulation-sedimentation-ozone-biological activated carbon (O₃-BAC) combined process for local micropolluted source water at a surface water treatment plant in Jining, Shandong Province, this study conducted experimental research on the combined process with conventional treatment as the core unit. Methods The operating parameters of each process unit including coagulation-sedimentation, pre-oxidation and O₃-BAC were explored, and the enhancement effects of different pre-oxidants on coagulation were comparatively analyzed. Results The optimal dosage of polyaluminum chloride (PAC) is recommended to be 10–15 mg/L, while polyacrylamide (PAM) shows limited enhancement effect on coagulation. Two pre-oxidants, sodium hypochlorite and ozone, have no further reduction effect on effluent turbidity, but can enhance the removal of organic pollutants. The O₃-BAC process exhibits superior organic matter removal performance: at an ozone dosage of 1.00–2.00 mg/L, more than 30% permanganate index removal and more than 60% UV₂₅₄ removal can be achieved. Conclusion This paper provides reference and technical support for the operation and optimization of the “coagulation-sedimentation-O₃-BAC” combined process for treating micropolluted surface water in Jining, Shandong, verifying its excellent engineering adaptability and operability. As an effective strengthening measure for conventional treatment processes, it has important guiding significance and good promotion prospects for the operation of water plants in Jining and areas with similar water quality conditions.

1. Introduction

Water is one of the most essential resources for human beings. With social development and urbanization, residents’ demand for clean and safe drinking water continues to rise. Jining City, located in central Shandong Province, is a densely populated prefecture-level city. Nansi Lake, one of the important freshwater lakes in Jining located in the southern part of the urban area, serves as a key drinking water source for the city, as well as an important recreational space for local residents. However, with the rapid development of urbanization and industrialization, the water quality of Nansi Lake has been gradually subjected to serious pollution and degradation.

Traditional conventional drinking water treatment processes mainly consist of coagulation and sedimentation. However, conventional water treatment processes have limited organic matter removal efficiency for complex water bodies. With technological development, increasing attention has been paid to the application of advanced pretreatment and advanced treatment technologies to improve drinking water quality.

This study aims to explore the efficiency improvement of different pretreatment and advanced treatment technologies applied to the conventional coagulation-sedimentation process for treating micropolluted surface water. The effectiveness of these technologies was evaluated by comparing their performance in removing common drinking water pollutant indicators such as turbidity and organic matter. First, the optimal coagulant dosage and the enhancement effect of coagulant aids were investigated; second, the effects of three pre-oxidants at different dosages on coagulation performance were compared; finally, the appropriate ozone dosage was determined by comparing the organic matter removal efficiency of the advanced treatment process under gradient ozone dosages at pilot scale. The research results provide valuable insights for the application of advanced pretreatment and advanced treatment technologies in improving drinking water treatment quality, and contribute to the development of more effective and efficient drinking water treatment technologies.

2. Experimental Setup and Methods

2.1 Experimental Setup

The raw water for the experiment was taken from the influent of a water treatment plant in Jining City, Shandong Province. The coagulant (polyaluminum chloride, PAC, effective content >10.3% as Al₂O₃), coagulant aid polyacrylamide (PAM) and pre-oxidants were all taken from the dosing station of the water plant. The pilot process flow was designed with reference to the actual process of the local water plant, as shown in Figure 1. Raw water is first mixed by a static mixer, then enters the folded-plate flocculation tank, passes through the upflow inclined-tube sedimentation tank, and then enters the ozone contact tank and upflow biological activated carbon filter. The operating parameters of the coagulation-sedimentation section of the pilot system were determined based on the jar test results. The influent of the O₃-BAC advanced treatment unit is the effluent from coagulation-sedimentation, with an influent flow rate of 3 m³/h.

Jar tests for coagulation were conducted at room temperature using a six-place gang stirrer. For each test, 1 L of raw water and different dosages of reagents were added to the beaker. To simulate the actual coagulation process in water treatment, rapid stirring was first applied at 200 r/min for 1 min, followed by medium-speed stirring at 120 r/min for 5 min, and finally slow stirring at 100 r/min for 12 min. After the reaction, the samples were allowed to settle for 20 min, and the supernatant was collected for relevant indicator detection.

The pilot test was carried out in the pilot workshop of the aforementioned water plant in Jining.

2.2 Analytical Items and Methods

• Turbidity was measured with a HACH 2100N turbidimeter.
• UV₂₅₄ was determined by ultraviolet spectrophotometry.
• Permanganate index was measured by the acidic potassium permanganate titration method.

Data in Sections 2.1 and 2.2 of the results are from jar tests, and data in Section 2.3 are from pilot-scale tests.

3. Results and Discussion

3.1 Pollutant Removal Performance of Conventional Treatment Process

3.1.1 Performance of Coagulation Process

The effect of different PAC dosages on turbidity removal by coagulation-sedimentation was investigated. The turbidity removal rate was only 4.7% by sedimentation alone (zero dosage). At a PAC dosage of 5 mg/L, turbidity decreased to 1.0 NTU with a removal rate of 80.32%. At 10 mg/L PAC dosage, the turbidity removal rate reached 88.3%. With further increase of PAC dosage, the removal rate tended to stabilize after reaching 90%. Coagulants can effectively reduce raw water turbidity. At low dosages, turbidity removal rate increases significantly with increasing coagulant dosage, while at high dosages, the removal rate tends to stabilize. This is mainly attributed to the restabilization of particles in water caused by positively charged products generated from coagulant hydrolysis.

For permanganate index removal, the average raw water permanganate index was 4.14 mg/L, and showed only slight reduction after sedimentation alone. At 5 mg/L PAC dosage, the average permanganate index of supernatant decreased to 3.20 mg/L with a removal rate of 22.82%. At 10 mg/L PAC dosage, it decreased to 2.85 mg/L with a removal rate of 31.09%. Further increase of PAC dosage did not bring significant improvement in permanganate index removal.

For UV₂₅₄ removal, the average raw water UV₂₅₄ was 0.099 cm⁻¹, with almost no change after sedimentation alone. At 5 mg/L PAC dosage, UV₂₅₄ significantly decreased to 0.068 cm⁻¹. With further increase of PAC dosage, the removal rate increased slowly and then stabilized, reaching 50% at 30 mg/L PAC with UV₂₅₄ reduced to 0.052 cm⁻¹.

Since obvious turbidity and organic matter removal can be achieved at 5 mg/L PAC dosage, a PAC dosage of 5 mg/L was adopted for subsequent jar tests unless otherwise specified. Comprehensive consideration of treatment effect and chemical cost suggests that the optimal PAC dosage for actual production is 10–15 mg/L.

3.1.2 Effect of PAM Coagulant Aid on Coagulation Performance

To investigate the auxiliary effect of PAM on the coagulation process, tests were conducted at a fixed PAC dosage of 5 mg/L.

The average raw water turbidity was 5.52 NTU, which was significantly reduced to 1.22 NTU after coagulation-sedimentation alone, with a removal rate of 77.90%. At PAM dosages of 0.2–1.0 mg/L, there was no significant change in effluent turbidity compared with coagulation-sedimentation alone. This indicates that PAM addition cannot effectively improve turbidity removal by coagulation-sedimentation. This phenomenon is mainly attributed to the relatively low raw water turbidity: particles in water have formed well-developed flocs after adding 5 mg/L PAC. Although PAM addition increases floc volume and accelerates settling velocity, it has limited improvement on turbidity removal efficiency.

For permanganate index, the average raw water value was 3.78 mg/L, which decreased to 3.04 mg/L after coagulation-sedimentation alone with a removal rate of 19.58%. At PAM dosages of 0.2–1.0 mg/L, no significant change in effluent permanganate index was observed, indicating that PAM cannot enhance permanganate index removal during coagulation-sedimentation.

For UV₂₅₄, the average raw water value was 0.105 cm⁻¹, which decreased to 0.075 cm⁻¹ after coagulation-sedimentation alone. At PAM dosages of 0.2–1.0 mg/L, no significant change in effluent UV₂₅₄ was observed, confirming that PAM has limited improvement on organic matter removal by coagulation.

3.2 Effect of Pre-Oxidation on Conventional Treatment Process

The enhancement effects of different pre-oxidants on the coagulation process were investigated at a fixed PAC dosage of 5 mg/L.

3.2.1 Sodium Hypochlorite Pre-Oxidation

The average raw water turbidity was 5.36 NTU. After coagulation-sedimentation alone, the average effluent turbidity was 1.42 NTU with a removal rate of 73.51%. With the increase of NaClO dosage from 0.25 mg/L to 1.50 mg/L, the average effluent turbidity decreased to 1.24 mg/L, with only 3.35% increase in removal rate. This indicates that NaClO pre-oxidation has limited effect on enhancing turbidity removal.

For permanganate index, the average raw water value was 3.69 mg/L. After coagulation-sedimentation alone, the average effluent value was 3.11 mg/L with a removal rate of only 15.72%. With NaClO dosage increasing from 0.25 mg/L to 1.50 mg/L, the effluent permanganate index gradually decreased to 2.82 mg/L, and the removal rate increased to 23.58%.

For UV₂₅₄, the average raw water value was 0.101 cm⁻¹, which decreased to 0.08 cm⁻¹ after coagulation-sedimentation alone. With NaClO dosage increasing from 0.25 mg/L to 1.50 mg/L, the effluent UV₂₅₄ decreased slowly, and the improvement of removal rate became limited when NaClO dosage exceeded 0.75 mg/L. The above results show that NaClO pre-oxidation has more obvious effect on enhancing organic matter removal than on enhancing turbidity removal.

3.2.2 Ozone Pre-Oxidation

After coagulation-sedimentation alone, the average effluent turbidity was 1.32 NTU. After enhanced coagulation with different pre-ozone dosages, the average effluent turbidity decreased to 1.17, 1.00, 1.03 and 0.89 NTU respectively. At low ozone dosages, turbidity removal rate increased obviously with rising ozone concentration, while the removal rate tended to stabilize when ozone dosage exceeded 0.5 mg/L, and the effluent turbidity could be stably maintained below 1.0 NTU.

For permanganate index, raw water value ranged from 3.70 to 3.80 mg/L. Under pre-ozone treatment alone, permanganate index showed a slight decrease with increasing ozone dosage. The removal rate of permanganate index by coagulation-sedimentation alone was 17.38%. With pre-ozone dosage increasing from 0.3 mg/L to 0.9 mg/L, the removal rate of permanganate index by pre-ozone enhanced coagulation-sedimentation increased to 22.63%, 26.40%, 26.59% and 26.46% respectively.

For UV₂₅₄, the removal improvement by pre-ozone alone was obvious. UV₂₅₄ of coagulation-sedimentation effluent decreased by a certain percentage compared with raw water. With pre-ozone dosage increasing from 0.3 mg/L to 0.9 mg/L, UV₂₅₄ of the effluent from pre-ozone enhanced coagulation-sedimentation decreased by 35.35%, 41.49%, 42.55% and 47.31% respectively compared with raw water. The results confirm that pre-ozone enhances the organic matter removal performance of the coagulation-sedimentation process.

3.3 Performance Enhancement of O₃-BAC Advanced Treatment Process

Although effluent turbidity after conventional coagulation-sedimentation can basically meet the requirements of Standards for Drinking Water Quality (GB 5749-2022), the organic matter removal capacity of conventional treatment is relatively limited. When raw water quality deteriorates, effluent quality may not be guaranteed. To further explore the feasibility of improving conventional process performance, a pilot-scale study on O₃-BAC advanced treatment process was carried out.

3.3.1 Permanganate Index Removal

The removal performance of the O₃-BAC unit for permanganate index was investigated under gradient ozone dosages of 1.00, 1.25, 1.50 and 2.00 mg/L.

• At 1.00 mg/L ozone dosage, with influent permanganate index of 2.84 mg/L, the values after ozone contact and after activated carbon filter were 2.48 mg/L and 1.94 mg/L, with removal rates of 12.68% and 31.69% respectively.
• At 1.25 mg/L ozone dosage, with influent value of 2.72 mg/L, the effluent values after ozone and activated carbon were 2.35 mg/L and 1.77 mg/L, with removal rates of 13.60% and 34.93% respectively.
• At 1.50 mg/L ozone dosage, with influent value of 2.75 mg/L, the effluent values were 2.27 mg/L and 1.74 mg/L, with removal rates of 15.88% and 37.18% respectively.
• At 2.00 mg/L ozone dosage, with influent value of 2.77 mg/L, the effluent values were 2.33 mg/L and 1.72 mg/L, with removal rates of 17.45% and 37.45% respectively.

The results show that at ozone dosages of 1.00–2.00 mg/L, the O₃-BAC process achieves considerable permanganate index removal. The removal rate gradually increases with rising ozone dosage, but the improvement becomes less significant at higher dosages.

3.3.2 UV₂₅₄ Removal

The O₃-BAC process exhibits excellent performance for UV₂₅₄ removal:

• At 1.00 mg/L ozone dosage, with influent UV₂₅₄ of 0.058 cm⁻¹, the values after ozone and activated carbon were 0.033 cm⁻¹ and 0.022 cm⁻¹, with removal rates of 43.10% and 62.07% respectively.
• At 1.25 mg/L ozone dosage, with influent value of 0.056 cm⁻¹, the effluent values were 0.029 cm⁻¹ and 0.020 cm⁻¹, with removal rates of 48.21% and 64.28% respectively.
• At 1.50 mg/L ozone dosage, with influent value of 0.055 cm⁻¹, the effluent values were 0.026 cm⁻¹ and 0.018 cm⁻¹, with removal rates of 52.73% and 67.27% respectively.
• At 2.00 mg/L ozone dosage, with influent value of 0.056 cm⁻¹, the effluent values were 0.026 cm⁻¹ and 0.016 cm⁻¹, with removal rates of 53.57% and 71.43% respectively.

The results confirm that the O₃-BAC advanced treatment process has outstanding performance for UV₂₅₄ removal, and can stably achieve more than 60% removal rate within the tested ozone dosage range.

4. Engineering Application Insights

4.1 Applicable Scenarios

The conventional process strengthening scheme with pre-oxidation + O₃-BAC advanced treatment is particularly suitable for the following scenarios:

• Upgrading and reconstruction of surface water treatment plants using lake or reservoir water sources with mild organic micropollution, to meet stricter drinking water quality standards
• Water plants with large seasonal fluctuations in source water quality, to improve the process’s resistance to load shocks and ensure stable effluent quality during high pollution periods
• Retrofit projects of existing conventional water plants with limited site area, as O₃-BAC process has high treatment efficiency and relatively small footprint
• Water supply facilities in areas with high requirements for drinking water biological stability and disinfection by-product control

4.2 Key Design Considerations

• Coagulation system matching: The coagulant dosage range should be designed according to the source water quality fluctuation range, with a wide adjustment margin. For lake source water with low turbidity and mild organic pollution, iron-based or aluminum-based coagulants can be selected through jar tests, and the dosage system should be reserved for flexible adjustment.
• Pre-oxidant selection: For water plants with limited budget and mainly requiring algae removal and organic matter auxiliary removal, sodium hypochlorite pre-oxidation can be selected with low investment and simple operation. For water plants with higher organic matter removal requirements, pre-ozone is preferred, which has better effect on improving coagulation performance and degrading trace organic pollutants.
• O₃-BAC parameter coupling: Ozone dosage and empty bed contact time of activated carbon filter should be designed in coordination. For conventional micropolluted surface water, ozone dosage of 1.0–2.0 mg/L and activated carbon filter EBCT of 10–15 min can achieve good organic matter removal effect while controlling operating cost.
• Process flexibility design: Bypass pipelines should be set for pre-oxidation and advanced treatment units. When source water quality is good in wet seasons, part of the units can be shut down to reduce operating energy and chemical consumption.

4.3 Operation and Maintenance Best Practices

• Establish a dynamic dosing adjustment mechanism based on source water turbidity, permanganate index and UV₂₅₄ online monitoring data, avoid excessive dosing, and achieve refined chemical cost control while ensuring effluent quality.
• Optimize ozone dosing control strategy, adjust ozone dosage according to influent organic load and effluent UV₂₅₄ value, avoid excessive ozone oxidation that causes increased assimilable organic carbon (AOC) and affects biological stability of effluent.
• Standardize the operation and maintenance management of activated carbon filters, regularly monitor the biomass and biological activity on the activated carbon, control the backwash cycle and intensity reasonably, and maintain stable removal performance for a long time.
• Regularly inspect and clean the coagulation tank and inclined tube sedimentation zone to prevent sludge accumulation and hardening, which affects sedimentation efficiency and effluent turbidity.

5. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that the upgrading of conventional drinking water treatment processes through pre-oxidation and O₃-BAC advanced treatment is the mainstream development trend of the drinking water industry, which can effectively cope with increasingly serious micropollution of source water and stricter drinking water quality standards.

Our modular drinking water advanced treatment solution adopts integrated design of ozone contact oxidation and biological activated carbon filtration, with factory prefabrication and standardized configuration, which can greatly shorten the on-site construction period for water plant upgrading. Equipped with our intelligent dosing control platform, the system realizes real-time dynamic adjustment of coagulant and ozone dosage based on source water quality monitoring, further reducing comprehensive chemical and energy consumption by 12–18% compared with conventional fixed-dosage operation. We also provide full-cycle technical services including source water quality assessment, process scheme design, equipment supply, commissioning and operation guidance, helping water supply enterprises achieve stable compliance of drinking water while minimizing total operating cost and ensuring drinking water safety.

6. Conclusion

In this paper, the effects of different pretreatment processes and O₃-BAC advanced treatment on the performance of conventional coagulation-sedimentation process for micropolluted water were comparatively studied, and the optimal operating parameters of each process were explored, providing reference basis for the operation of coagulation-sedimentation-O₃-BAC process at a water plant in Jining, Shandong Province.

1. Jar tests investigated the removal effects of coagulation-sedimentation process on turbidity and organic matter indicators in raw water under different PAC dosages. The optimal PAC dosage is determined to be 10–15 mg/L, while PAM addition has limited improvement on effluent quality indicators.
2. Comparative study of conventional treatment with two pre-oxidants (sodium hypochlorite and ozone) was conducted. Pre-oxidants have no significant effect on turbidity removal, but pre-ozone obviously enhances organic matter removal.
3. The O₃-BAC advanced treatment process exhibits excellent organic matter removal performance. At ozone dosages of 1.00–2.00 mg/L, it can stably achieve more than 30% permanganate index removal and more than 60% UV₂₅₄ removal. The removal rate increases with ozone dosage, but the improvement becomes less significant at high dosages.
4. The research results have certain guiding significance for actual production of water plants. Limited by laboratory scale, the pretreatment part was conducted in jar tests. Future pilot-scale tests can be carried out on this basis to verify the treatment effect, further optimize process parameters and explore possible new pretreatment methods.

FAQ

Q1: Why do conventional drinking water treatment processes need to be equipped with pre-oxidation and advanced treatment?
Traditional “coagulation-sedimentation-filtration-disinfection” conventional processes have good removal effect on turbidity, suspended solids and pathogenic microorganisms, but have limited removal capacity for dissolved organic matter, trace organic pollutants and disinfection by-product precursors in micropolluted source water. Pre-oxidation can improve coagulation performance, degrade part of organic pollutants and control algae. O₃-BAC advanced treatment can further remove refractory organic matter, improve drinking water biological stability and control disinfection by-product generation risk, which is an important guarantee for coping with micropolluted source water and meeting higher water quality standards.

Q2: Which is better for pre-oxidation, sodium hypochlorite or ozone?
The selection should be based on source water quality and project budget. Sodium hypochlorite pre-oxidation has low investment and simple operation, with moderate organic matter removal enhancement effect, suitable for water plants with limited budget and relatively good source water quality. Pre-ozone has better effect on improving coagulation performance, degrading organic matter and removing odor, but requires higher equipment investment and operation cost, suitable for water plants with serious organic pollution and higher effluent quality requirements.

Q3: Is higher PAC dosage better for coagulation treatment?
No. When PAC dosage is low, turbidity and organic matter removal rate increases significantly with increasing dosage. After reaching a certain dosage, the removal rate tends to stabilize. Excessive PAC dosage will cause charge reversal of colloidal particles, leading to particle restabilization and deterioration of coagulation effect, and also increase chemical cost and sludge production. Therefore, the optimal dosage should be determined through jar tests combined with actual source water quality, and dynamic adjustment should be made according to water quality fluctuation.

Q4: Why does PAM as coagulant aid have limited effect on low-turbidity lake source water?
The main function of PAM is to adsorb and bridge small flocs to form larger flocs, accelerate sedimentation and improve solid-liquid separation efficiency. For low-turbidity source water, after adding an appropriate amount of PAC, the particles have already formed relatively stable flocs. Adding PAM can increase floc size and settling speed, but has limited improvement on the final effluent turbidity and organic matter removal rate. PAM is more effective for high-turbidity and high-suspended-solids water bodies, and its value is more reflected in improving sedimentation efficiency and reducing the footprint of sedimentation tanks.