Fig. 6: Relationship between the model parameters in the single-objective optimisation showed by parallel coordinate plots.

a Correlation between TN removal efficiency and key parameters under the TSA-TT optimization method; b Correlation between COD removal efficiency and key parameters under the TSA-TT optimization method; c Correlation between TN removal efficiency and key parameters under the OSA-OT optimization method; d Correlation between COD removal efficiency and key parameters under the TSA-OT optimization method.