Long-term assessment on performance and seasonal optimal operation of a full-scale integrated multiple constructed wetland-pond system

Constructed wetlands as natural process-based water treatment technologies are popular globally. However, lack of de-tailed long-term assessment on the impact of seasonal variations on their performance with focus on optimal seasonal adjustments of controllable operating parameters significantly limits their efficient and sustainable long-term opera-tion. To address this, a full-scale integrated multiple surface flow constructed wetlands-pond system situated between slightly polluted river water and outflow-receiving waterworks in a subtropical monsoon climate area of middle -eastern China was seasonally assessed over a period of six years. During this period, the removal rate (R) and mass re-moval rate (MRR) of total nitrogen (TN), total phosphorus (TP) and chemical oxygen demand (COD) possessed strong seasonality (p < 0.05). The highest R (%) and MRR (mg/m2/d) were in summer for TN (51.53 %, 114.35), COD (16.30 %, 143.85) and TP (62.39 %, 23.89) and least in spring for TN (23.88 %, 39.36) and COD. Whereas for TP, the least R was in autumn (37.82 %) and least MRR was in winter (9.35). Applying a first-order kinetics model coupled with Spearman's rank correlation analysis, purification efficiency exhibited significant dependence on temperature as nutri-ent reaction rates constant, k generally increased with temperature and was highest in summer. Meanwhile, the R of TN, TP and COD were positively correlated with influent concentration whiles MRR of TP was negatively correlated with hydraulic retention time but positively correlated with hydraulic loading rate (HLR) (p < 0.05). Also, MRR of COD and TN were positively correlated with mass loading rates (MLR) in summer and autumn. Through linear optimi-zation, the best operating parameters according to the compliance rate were determined and a set of guidelines were proposed to determine the optimal operational change of hydrological index in each season (Spring, 0.1-0.12 m/d; Summer, 0.14-0.16 m/d; Autumn, 0.15-0.17 m/d; Winter, 0.1-0.11 m/d) for efficient and sustainable long-term op-eration.

Automated summary

Constructed wetlands in a subtropical monsoon climate area of eastern China were seasonally assessed for six years. The study found that the removal efficiency of pollutants varied significantly among different seasons. By optimizing controllable operating parameters, efficient and sustainable long-term operation of constructed wetlands can be achieved.