Parameter Optimization and Molecular-Level Removal Mechanism Interpretation

Abstract

Dissolved organic matter (DOM) in the secondary effluent of fine chemical wastewater is characterized by complex composition and high treatment difficulty. In this study, the secondary effluent from a fine chemical industrial park in Jiangsu Province was taken as the research object. Single-factor experiments were conducted to investigate the effects of coagulant type and dosage, initial pH, coagulant aid type and dosage, and hydraulic conditions on DOM removal, and the variation of DOM characteristics was analyzed at the microscopic level. The results show that under the optimal conditions of initial pH 7.0, 0.7 g/L polyferric sulfate (PFS), 0.1 mg/L anionic polyacrylamide (PAM), rapid stirring at 250 r/min for 5 min and slow stirring at 60 r/min for 10 min, the removal rates of DOM, CODCr and UV₂₅₄ reach 34.0%, 29.5% and 40.2% respectively, which is 3 times higher than the current operation performance of the wastewater treatment plant, with significant optimization effect. Water quality characteristic analysis indicates that coagulation mainly removes high-molecular-weight and hydrophobic substances. The absorption peaks of aliphatic and aromatic compounds, as well as the fluorescence peaks of humic acids and proteins, are significantly weakened after coagulation treatment.

1. Introduction

Fine chemical wastewater has the characteristics of complex composition, large fluctuation of water quality and quantity, deep color, high salinity, high content of toxic substances and poor biodegradability. Generally, after primary pretreatment and secondary biochemical treatment, fine chemical wastewater still contains high concentration of dissolved organic matter (DOM). Direct discharge of secondary effluent will increase aquatic ecological risks. With the strengthening of environmental protection, fine chemical wastewater treatment plants need to adopt appropriate advanced treatment technologies to meet the requirements of new discharge standards. Currently, advanced treatment technologies for chemical wastewater include coagulation-sedimentation, advanced oxidation, activated carbon adsorption, membrane separation and ion exchange. Among them, coagulation-sedimentation has been widely used in the advanced treatment of fine chemical wastewater due to its good treatment effect, low investment cost and easy operation and management.

In the operation and quality control of wastewater treatment plants, conventional indicators such as CODCr, BOD₅ and total organic carbon cannot accurately reflect the composition and structure of DOM. At present, the most commonly used analytical methods for DOM structure characterization include fractionation, spectral analysis and chromatographic analysis. Due to the heterogeneity and complexity of DOM, multiple characterization methods are often adopted in practical research to obtain more comprehensive and accurate information, which helps to reveal the degradation law and removal efficiency of organic pollutants in advanced treatment.

In this paper, the secondary effluent from a wastewater treatment plant in a fine chemical industrial park in Jiangsu Province was taken as the research object. Single-factor experiments were carried out to optimize the coagulation process parameters. Meanwhile, multiple characterization techniques were used to systematically analyze the variation of DOM water quality characteristics during coagulation treatment, so as to provide technical strategies for the production operation regulation of the wastewater treatment plant.

2. Materials and Methods

2.1 Experimental Materials

2.1.1 Wastewater Samples

The experimental water samples were collected from the effluent of the secondary sedimentation tank of a centralized wastewater treatment plant in a fine chemical industrial park in Jiangsu Province. To eliminate the influence of influent suspended solids, the water samples were filtered through an ultrafiltration device before use. The water quality of the secondary sedimentation tank effluent is as follows: CODCr concentration of 95–130 mg/L, dissolved organic carbon (DOC) concentration of 23.5–39.5 mg/L, UV₂₅₄ of 0.641–0.698 cm⁻¹, and pH of 7.2–8.5. The water quality of different batches of experimental water is shown in Table 1.

Table 1 Water Quality of Coagulation Experimental Wastewater

2.1.2 Reagents

Five inorganic polymer coagulants were tested: polyaluminum chloride (PAC, Al₂O₃ ≥ 28%), polyaluminum sulfate (PAS, Al₂O₃ ≥ 17%), polyaluminum ferric chloride (PAFC), polyferric chloride (PFC, Fe ≥ 15.6%) and polyferric sulfate (PFS, Fe ≥ 21%). Three types of polyacrylamide (PAM) coagulant aids were used: anionic PAM (molecular weight > 10 million Da, hydrolysis degree 20%–30%), non-ionic PAM (molecular weight > 18 million Da) and cationic PAM (molecular weight > 8 million Da). Sodium hydroxide and concentrated sulfuric acid of analytical grade were used for pH adjustment.

2.2 Coagulation Experiments

Coagulation experiments were carried out with a ZR4-6 jar test apparatus. For each test, 1 L of water sample was added into a dedicated plastic beaker, and single-factor variable experiments were conducted according to the designed conditions shown in Table 2. Initial pH was adjusted with 10% dilute sulfuric acid and 0.1 mol/L sodium hydroxide solution, and measured accurately with a PHS-3E pH meter. After the reaction, samples were allowed to settle for 30 min, then the supernatant was filtered through a 0.45 μm membrane for determination of DOC, CODCr and UV₂₅₄.

Table 2 Design Conditions for Coagulation Experiments

2.3 Analytical Methods

• CODCr was determined by the potassium dichromate method with a standard COD digester.
• DOC was determined by a TOC-L total organic carbon analyzer (Shimadzu).
• UV₂₅₄ and UV-Vis absorption spectra were measured by a TU-1901 UV-Vis spectrophotometer (Persee).
• Apparent molecular weight distribution was determined by size exclusion high performance liquid chromatography (LC-20A, Shimadzu).
• Hydrophilic/hydrophobic fractionation was performed by XAD-8 macroporous resin separation, obtaining hydrophilic substances (HIS), hydrophobic acid (HOA), hydrophobic neutral (HON) and hydrophobic base (HOB).
• Fourier transform infrared (FTIR) spectra were scanned by a Nicolet iS50 FTIR spectrometer (Thermo Fisher).
• Three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectra were scanned by an F-7000 fluorescence spectrophotometer (Hitachi), and data were analyzed by parallel factor analysis (PARAFAC).

3. Results and Discussion

3.1 Optimization of Coagulation Conditions

3.1.1 Type and Dosage of Coagulants

The removal effects of different coagulants and dosages on DOC and UV₂₅₄ were compared. With the increase of PAC, PFC and PAFC dosage, DOC removal rate increased steadily, reaching the maximum of 17.4%, 13.4% and 12.2% respectively at 1.5 g/L dosage. For PAS and PFS, DOC removal rate first increased and then decreased with increasing dosage, reaching the maximum at 1.0 g/L, with maximum removal rates of 14.2% and 26.8% respectively.

The decrease of removal rate at excessive dosage is caused by charge reversal and restabilization of colloidal particles: excessive positive charges on colloid surfaces increase inter-particle repulsion, deteriorating coagulation effect. When PFS dosage increased from 0.7 g/L to 1.0 g/L, DOC removal rate only increased from 25.3% to 26.8%, while UV₂₅₄ removal rate decreased from 34.0% to 26.9%. This indicates that excessive PFS dosage reduces total organic matter content including non-aromatic compounds, but is not conducive to further removal of aromatic compounds.

Comprehensively considering DOC and UV₂₅₄ removal performance, PFS was selected as the optimal coagulant, with an optimal dosage of 0.7 g/L.

3.1.2 Effect of Initial pH

The removal performance of PFS at different initial pH values was investigated. When initial pH was lower than 7.0, DOC removal rate was low, and the wastewater was turbid with colloidal particles difficult to settle. PFS achieved better pollutant removal at pH 7.0–9.0. At pH 7.0, DOC removal rate reached the maximum of 27.9%, and UV₂₅₄ removal rate also remained at a high level of 34.3%.

pH significantly affects the hydrolysis degree of PFS. Under acidic conditions, PFS hydrolysis is inhibited, reducing the destabilization effect on colloidal particles, and fine flocs are difficult to settle. At pH 7.0–9.0, polynuclear hydroxyl complex ions generated by hydrolysis transform into Fe(OH)₃ precipitates, which have strong charge neutralization and adsorption bridging effects on colloidal particles. Excessively strong alkalinity reduces the content of intermediate hydrolysis products, leading to deteriorated organic matter removal.

Considering the removal efficiency, the optimal pH for PFS coagulation of this wastewater was determined as 7.0. Since the secondary effluent pH is 7.0–8.0, no pH adjustment is required in actual production, saving chemical dosage and simplifying the treatment process.

3.1.3 Type and Dosage of Coagulant Aids

The effects of three types of PAM on coagulation performance showed minor differences, with anionic PAM performing slightly better than non-ionic and cationic PAM. At 0.1 mg/L dosage, the DOC removal rates of anionic, non-ionic and cationic PAM were 30.2%, 29.0% and 28.2% respectively. With the increase of PAM dosage from 0.1 mg/L to 2.0 mg/L, no significant improvement or even a decreasing trend in DOC removal was observed. UV₂₅₄ removal rates were between 36.8% and 42.2% under different coagulant aid conditions, with no significant difference.

Comprehensively considering treatment effect and chemical cost, anionic PAM was selected as the coagulant aid, with an optimal dosage of 0.1 mg/L.

3.1.4 Hydraulic Stirring Conditions

• Rapid stirring speed: At rapid stirring speeds of 200, 250 and 300 r/min, DOC removal rates were 29.0%, 30.4% and 30.0% respectively, with the maximum at 250 r/min. UV₂₅₄ decreased to around 0.390 cm⁻¹ under all three speeds, with no significant difference in removal rate. Considering both treatment effect and energy consumption, the optimal rapid stirring speed was determined as 250 r/min.
• Rapid stirring time: With rapid stirring time of 2–10 min, DOC removal rate ranged from 29.5% to 30.4%, reaching the maximum at 5 min. UV₂₅₄ removal rate was 39.9%–41.8%, with minor influence. The optimal rapid stirring time was determined as 5 min.
• Slow stirring speed: At slow stirring speeds of 60, 80 and 100 r/min, DOC removal rates were 28.5%, 28.7% and 29.0% respectively, with no significant difference. The optimal slow stirring speed was determined as 60 r/min.
• Slow stirring time: Slow stirring for 10 min achieved better performance than 20–30 min, with DOC removal rate of 30.5%. Excessively long slow stirring breaks well-formed flocs and deteriorates settling performance. The optimal slow stirring time was determined as 10 min.

3.2 Treatment Performance under Optimal Conditions

Three parallel tests were conducted under the optimal coagulation conditions. When the influent DOC, CODCr and UV₂₅₄ were 31.63 mg/L, 112 mg/L and 0.649 cm⁻¹ respectively, the average removal rates after coagulation reached 34.0%, 29.5% and 40.2% respectively, as shown in Table 3. The treatment efficiency is 3 times higher than the current operation performance of this process section in the wastewater treatment plant, with remarkable optimization effect.

Table 3 Treatment Performance under Optimal Coagulation Conditions

3.3 DOM Characteristics Variation under Optimal Conditions

3.3.1 Molecular Weight Distribution

The relative molecular weight of organic matter in raw water is widely distributed in the range of 210–5000 Da, with the highest content of 1000–3000 Da fractions accounting for 76.4%. Three Gaussian fitting peaks were identified in raw water, with apparent molecular weights of 2148, 1014 and 430 Da respectively. After coagulation, only two fitting peaks remained with significantly reduced peak areas. The removal rates of peak 1, peak 2 and peak 3 were 55.2%, 38.5% and 100% respectively. The proportion of organic matter with molecular weight below 3000 Da increased significantly in the effluent. The results confirm that coagulation mainly removes high-molecular-weight substances from wastewater.

3.3.2 Hydrophilic and Hydrophobic Fractions

HIS is the dominant component in raw water, followed by HOA, HON and HOB in sequence. Coagulation can effectively remove both HIS and hydrophobic organic matter (HOM, including HOA, HON and HOB). The removal rate of HOM reached 42.6%, while the removal rate of HIS was 27.1%, indicating that coagulation preferentially removes hydrophobic components. After coagulation, the proportion of HIS in total organic matter increased from 56.9% to 62.6%, and the hydrophilicity of wastewater was enhanced.

3.3.3 UV-Vis Absorption Characteristics

The UV absorption of organic matter in raw water is mainly concentrated in 200–300 nm. A strong absorption peak at 220 nm indicates the presence of unsaturated groups such as aromatic C=C and ketone C=O. An absorption band at 250–300 nm indicates the existence of aromatic compounds, polycyclic aromatic hydrocarbons, phenolic organics and aniline derivatives, which is related to pesticide chemical production in the park. After coagulation, the absorption peak intensity at 250–300 nm decreased significantly, confirming that coagulation has good removal effect on benzene-ring-containing organic compounds.

3.3.4 FTIR Spectral Characteristics

FTIR spectra show that the raw water contains aromatic compounds, alkanes, alkenes, phenols, aldehydes, esters, thiocyanates and halogenated hydrocarbons, indicating high complexity of DOM molecular structure. After coagulation, the peaks at 1440 cm⁻¹ and 876 cm⁻¹ completely disappeared, indicating that coagulation removes aliphatic and aromatic compounds. The peak intensity at 1127 cm⁻¹ decreased, meaning partial removal of phenols, alcohols, carboxylic acids, ethers and esters. The weakened peak at 3415 cm⁻¹ also confirms the reduction of phenolic and alcoholic substances.

3.3.5 3D-EEM Fluorescence Characteristics

Four fluorescent components were identified by PARAFAC model, all showing double peaks:

• C1 (Eₓ/Eₘ = 220 nm/403 nm): fulvic acid-like humic substances
• C2 (Eₓ/Eₘ = 255 nm/447 nm): humic acid-like humic substances
• C3 (Eₓ/Eₘ = 230 nm/375 nm): tryptophan-like proteins
• C4 (Eₓ/Eₘ = 215 nm/335 nm): tyrosine-like proteins

The maximum fluorescence intensity (Fₘₐₓ) of all four fluorescent components decreased significantly after coagulation, with removal rates of 37.7%, 59.6%, 38.1% and 18.2% respectively, as shown in Table 4. The results indicate that coagulation has good removal effect on all four fluorescent substances, and the removal efficiency of humic substances is significantly higher than that of protein-like substances.

Table 4 Maximum Fluorescence Intensity of Fluorescent Components Before and After Coagulation

4. Engineering Application Insights

4.1 Applicable Scenarios

The optimized coagulation process is particularly suitable for the following scenarios:

• Advanced treatment and upgrading of fine chemical, pesticide and dyeing industrial park wastewater treatment plants, as a cost-effective polishing step to meet stricter discharge standards
• Pretreatment before advanced oxidation processes such as ozonation and Fenton, to reduce organic load and oxidant consumption
• Pretreatment before membrane treatment systems, to remove macromolecular organic matter and reduce membrane fouling risk
• Retrofit projects of existing wastewater treatment plants with limited site and budget, as coagulation process has small footprint and short construction period

4.2 Key Design Considerations

• Coagulant selection and matching: Coagulant type must be selected based on actual wastewater quality through jar tests. For fine chemical wastewater with high content of aromatic and humic substances, iron-based coagulants represented by PFS have better removal performance than aluminum-based coagulants.
• pH control strategy: Iron-based coagulants have a wide effective pH range, and the optimal pH is usually around neutral. For most biochemical effluent with pH 7–8, no additional pH adjustment is needed, which can reduce operating cost.
• Hydraulic condition design: Stirring speed and time should be designed according to the tank volume and actual water volume to ensure full mixing of coagulant and uniform floc growth, avoiding excessive shear that breaks flocs.
• Sludge treatment supporting system: Coagulation process produces chemical sludge containing iron salts, and corresponding sludge thickening and dewatering facilities should be configured. The sludge disposal method should be determined according to local environmental requirements.

4.3 Operation and Maintenance Best Practices

• Establish a dynamic dosage adjustment mechanism based on influent COD and UV₂₅₄ monitoring data, avoid excessive dosing, and achieve refined chemical cost control.
• Regularly inspect and clean the coagulation tank and inclined tube sedimentation zone to prevent sludge accumulation and hardening, which affects sedimentation efficiency.
• Prepare PFS and PAM solutions according to standard procedures, control the preparation concentration and storage time, and avoid performance degradation caused by solution hydrolysis and degradation.
• Regularly calibrate the stirring equipment to ensure that the actual stirring speed matches the design value, so as to maintain stable coagulation effect.

5. SYNERAQUA Technical Perspective

At SYNERAQUA, we recognize that optimized coagulation is a cost-effective and highly reliable advanced treatment unit for industrial wastewater, which plays an irreplaceable role in upgrading projects and combined process systems.

Our modular high-efficiency coagulation system adopts integrated design of mixing, flocculation and inclined tube sedimentation, with factory prefabrication and standardized configuration, which can greatly shorten the on-site construction period. Equipped with our intelligent dosing control platform, the system realizes real-time dynamic adjustment of coagulant and coagulant aid dosage based on influent load and effluent quality monitoring, further reducing comprehensive chemical consumption by 15–20% compared with conventional fixed-dosage operation. We also provide full-cycle technical services including process scheme design, jar test verification, equipment supply, commissioning and operation guidance, helping industrial wastewater treatment plants achieve stable compliance while minimizing total operating cost. For fine chemical wastewater requiring deeper treatment, we provide combined process solutions of coagulation + ozone catalytic oxidation + biological aerated filter, giving full play to the synergistic advantages of each unit to achieve efficient removal of refractory organic matter.

6. Conclusion
7. The optimal coagulation process conditions were determined through single-factor experiments: initial pH of 7.0, PFS dosage of 0.7 g/L, anionic PAM dosage of 0.1 mg/L, rapid stirring at 250 r/min for 5 min, and slow stirring at 60 r/min for 10 min. Under optimal conditions, the removal rates of DOM, COD and UV₂₅₄ reach 34.0%, 29.5% and 40.2% respectively, which is 3 times higher than the current plant operation performance, with significant optimization effect.
8. Analysis of molecular weight and hydrophilic/hydrophobic properties shows that coagulation has good removal effect on organic matter with molecular weight > 3000 Da and HOM fractions, but poor removal effect on organic matter with molecular weight < 1000 Da and HIS fractions. DOM in coagulation effluent is mainly composed of low-molecular-weight organic matter and hydrophilic substances.
9. UV-Vis and FTIR spectra show that organic matter in the wastewater has complex composition, and coagulation has good removal effect on aliphatic and aromatic compounds. 3D-EEM analysis combined with PARAFAC model identifies four fluorescent components in raw water, including two humic-like components and two protein-like components. Coagulation has good removal effect on all four types of fluorescent substances, with higher removal rate for humic substances than protein-like substances.

FAQ

Q1: What is the DOM removal pattern of conventional coagulation for fine chemical wastewater?
Conventional coagulation mainly removes high-molecular-weight, hydrophobic and humic organic substances through charge neutralization, adsorption bridging and sweep flocculation mechanisms. It has limited removal effect on low-molecular-weight, hydrophilic small-molecule organic matter. After coagulation, the proportion of hydrophilic small-molecule DOM in effluent increases significantly, which is the main reason why single coagulation cannot achieve deep removal of COD in fine chemical wastewater.

Q2: Why is PFS more effective than aluminum-based coagulants for secondary effluent of fine chemical wastewater?
Iron-based coagulants represented by PFS form denser and larger flocs with stronger adsorption capacity during hydrolysis, which have better removal effect on hydrophobic organic matter and aromatic compounds. In addition, iron-based coagulants have a wider applicable pH range, and the flocs settle faster, which is more suitable for the advanced treatment of industrial wastewater with complex organic composition.

Q3: What suitable subsequent processes can treat the residual DOM after coagulation of fine chemical wastewater?
The residual DOM after coagulation is mainly low-molecular-weight hydrophilic refractory organic matter. Advanced oxidation processes such as ozone catalytic oxidation and electrocatalytic oxidation can be used to break the structure of refractory organic matter and improve biodegradability, followed by biological aerated filter or biological activated carbon for further purification. For projects with higher effluent requirements, reverse osmosis membrane treatment can be used as the final polishing step.

Q4: What is the positioning of coagulation process in the advanced treatment chain of fine chemical wastewater?
Coagulation serves as the primary advanced treatment unit in the whole process chain. It removes most of the suspended solids, colloidal organic matter, macromolecular humic substances and total phosphorus at low cost, reduces the organic load of subsequent advanced treatment units, and also protects subsequent membrane and catalyst systems from fouling and deactivation. It is an indispensable low-cost pretreatment link in the fine chemical wastewater advanced treatment system.