넙치 양식장 배출수 정화 처리를 위한 고분자 응집제 및 산화처리수단의 적용과 효율 분석

Abstract

With the continuous expansion of the aquaculture industry, environmental concerns have grown regarding wastewater generated from aquaculture operations. Traditional cage and flow-through aquaculture systems widely used in Korea discharge large quantities of effluents containing suspended solids, nitrogen compounds (TN, TAN), phosphorus (TP), and dissolved organic matter from fish feces and residual feed, negatively impacting adjacent water bodies. In particular, flounder (Paralichthys olivaceus), a key species in Korean aquaculture, produces very fine fecal particles, making effective removal challenging using conventional sedimentation techniques. Thus, this study aimed to develop practical, efficient treatment technologies tailored to flounder farm wastewater by combining polymeric coagulant pretreatment with subsequent ozone and electrochemical oxidation processes to simultaneously achieve environmental sustainability and operational efficiency. Artificial wastewater samples (100 mg TSS 250 mL-1) were prepared from marine aquaculture sludge and treated with varying doses of polymeric coagulants, polyacrylamide (PAM, 0.00–1.00 µL 100 mg-1 TSS-1) and γ- polyglutamic acid (γ-PGA, 0.00–1.00 mg 100 mg-1 TSS-1), with reaction times of 5 minutes. Results showed that PAM treatment significantly reduced TSS up to a dosage of 0.8 µL 100 mg TSS-1, while slightly increasing again at the highest dosage. TN and TP also sharply decreased, with TP becoming undetectable above 0.8 µL dosage. COD decreased gradually up to 0.8 µL, but slightly increased at 1.0 µL. Particle size increased with PAM dosage, exceeding 100 µm at doses ≥ 0.8 µL, facilitating removal via geotextile filtration. Therefore, the optimal PAM dosage was determined as 0.8 µL 100 mg TSS-1. The γ-PGA coagulant showed gradual reductions in TSS and COD concentrations with increased dosage, reaching the lowest levels at the maximum dosage. TN decreased steadily, whereas TP showed an inconsistent response, with significant reduction only at the highest dosage. Particle size remained mostly below 100 µm regardless of dosage, suggesting γ-PGA effectiveness should be primarily evaluated based on improvements in water quality parameters. Hence, the optimal γ-PGA dosage was set at 1.0 mg 100 mg TSS-1. Optimal coagulation reaction times were established by evaluating water quality and particle size at mixing times of 1, 2, and 4 minutes, with coagulant dosages set at 0.8 µL 100 mg TSS-1 for PAM and 1.0 mg 100 mg TSS-1 for γ-PGA. PAM- treated wastewater showed consistent reductions in TSS and other contaminants with increased reaction time, notably achieving TSS concentrations 6-fold lower at 4 minutes compared to 1 minute, and 24-fold lower than the untreated control. γ-PGA similarly exhibited marked TSS reductions, up to 20-fold lower at 4 minutes compared to 1 minute. TP was nearly fully removed at the longest reaction time. However, particle size enlargement was temporarily reduced at the intermediate 2-minute point, then increased again due to re-coagulation. The 4- minute mixing time was thus established as optimal for both coagulants, providing the best balance of particle enlargement and contaminant removal. Subsequently, effluents treated with optimal coagulation conditions were filtered through geotextile filters and subjected to ozone (0–300 mg O₃ g TSS-1) and electrochemical oxidation (20–240 kC g TSS-1) to evaluate advanced treatment effectiveness. Ozone treatment provided limited improvement in PAM-treated effluents, even causing temporary TN concentration increases and irregular particle size changes. In contrast, γ-PGA-treated wastewater exhibited substantial TSS and COD reductions at higher ozone dosages, despite minimal improvements or temporary increases in TN and TP. Electrochemical oxidation significantly enhanced water quality for both PAM and γ-PGA-treated wastewater. For PAM-treated samples, TSS and COD reductions reached up to 80% and 39%, respectively. γ-PGA-treated wastewater showed almost complete TSS removal and substantial COD reduction from the lowest oxidation condition onwards. PAM-treated wastewater showed unusual particle enlargement at higher electrochemical oxidation doses, whereas γ-PGA treatment consistently reduced particle sizes. Electrochemical oxidation thus emerged as a stable, highly effective advanced treatment for polymer-treated aquaculture wastewater. Further evaluations were conducted on actual aquaculture wastewater samples classified as high- and low-concentration, using both PAM and γ-PGA coagulation followed by electrochemical oxidation. PAM-treated wastewater showed substantial reductions in TSS levels after coagulation-filtration (49.0±7.0 mg L-1 high-concentration, 17.8±3.6 mg L-1 low-concentration), further decreasing to 16.3±2.5 mg L-1 and 14.7±4.0 mg L-1, respectively, with electrochemical oxidation. Regression analyses demonstrated strong predictive capabilities for TSS and COD removal (high conc. TSS R²=0.9480; low conc. TSS R²=0.9861 low; COD R²>0.99 both), indicating their usefulness as predictive indicators for treatment effectiveness. Derived regression equations for PAM-treated wastewater, predicting TSS based on charge loading (x, kC g TSS-1), are as follows: TSS concentration of high-concentration wastewater treated by PAM coagulation-filtration and electrochemical oxidation = 26.60 + 23.41 × e-0.0375x - 0.0375x TSS concentration of low-concentration wastewater treated by PAM coagulation- filtration and electrochemical oxidation = 17.8014 + 34.5693 × e-0.0403x - 0.0107x TN and TP removal showed reliable regression predictability only in high- concentration wastewater (TN R²=0.9905, TP R²=0.8344), while being inconsistent in low-concentration conditions. γ-PGA treated wastewater showed irregular TSS reductions at high concentrations due to temporary particle dispersion (R²=0.5828) but stable, predictable reductions at lower concentrations (R²=0.9958). COD removal exhibited consistent, highly predictable regression relationships for both wastewater concentrations (R²>0.99). Consequently, COD emerged as the most reliable indicator for γ-PGA-treated wastewater electrochemical oxidation, with regression equations defined by COD concentrations (x, mg/L): COD concentration of high-concentration wastewater treated by γ-PGA coagulation-filtration and electrochemical oxidation = 547.3567 – 97.4317 × ln∣x+7.7373∣ (R²=0.9926) COD concentration of low-concentration wastewater treated by γ-PGA coagulation-filtration and electrochemical oxidation = 11.7661 + 221.4916 × e-0.0972x (R²=0.9922) In conclusion, this study confirmed the importance of selecting appropriate coagulants and optimizing electrochemical oxidation parameters based on wastewater characteristics and initial concentrations. For PAM-treated wastewater, TSS-based predictive models offer the most practical approach, facilitating operational simplicity and effectiveness. Electric charge required for TSS removal from high-concentration wastewater treated by PAM: 0.0375 × (𝑥 − 26.60 − 23.41 × 𝑒−0.0375𝑥) Electric charge required for TSS removal from low-concentration wastewater treated by PAM: 0.0107 × (𝑥 − 17.8014 − 34.5693 × 𝑒−0.0403𝑥) Conversely, γ-PGA-treated wastewater should utilize COD-based regression models due to structural stability concerns of coagulated particles. The developed regression models effectively predict necessary electrochemical oxidation conditions, providing a robust and practical solution for sustainable and efficient aquaculture wastewater management. Electric charge required for COD removal from high-concentration wastewater treated by γ-PGA: = exp ( 547.3567 − 𝑥 97.4317 ) − 7.7373 Electric charge required for COD removal from low-concentration wastewater treated by γ-PGA: 0.0972 𝑥 − 11.7661 221.4916 Thus, PAM and γ-PGA-based coagulants should be applied at actual active ingredient amounts of 0.8 µL and 1.0 mg per gram of TSS solids in the wastewater, respectively, followed by sufficient mixing and filtration for at least 4 minutes. Subsequently, electrochemical oxidation should be performed according to the derived equation above.