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.