Process-based modelling of phosphorus removal in a novel constructed wetland system using dewatered alum-sludge as substrate.

A process-based model that can evaluate the transport and the fate of phosphorus (P) in agricultural wastewater was developed for a novel 4-stage dewatered alum sludge cakes (DASC) based constructed wetlands (CWs) system using STELLA software (version 9.1.4). The model considered adsorption, plant and microbial uptakes as the major forms of P involved in the transformation chains. The results were obtained by experimental procedure through laboratory measurement, from literature and/or calibration. The observed effluent P concentration in the CWs ranged from 3.62 to 8.50 mg/L (stage 1), 2.00 to 4.45 mg/L (stage 2), 1.39 to 3.76 mg/L (stage 3) and 0.52 to 2.36 mg/L (stage 4), whereas the simulated values ranged from 2.12 to 10.99 mg/L (stage 1), 1.32 to 5.65 mg/L (stage 2), 0.84 to 3.64 mg/L (stage 3) and 0.53 to 2.25 mg/L (stage 4), respectively. The simulated and observed values of P removal in the CWs system were in good agreement. A mass balance analysis was performed for all the major processes which resulted in a major pathway of P removal through adsorption (64-75%, 58-66%, 57-63% and 49-58%) followed by plant uptake (7-11%, 8-14%, 14-17% and 9-19%) and microbial uptake (3-7%, 3-5%, 9-12% and 7-12%) for stage 1, stage 2, stage 3 and stage 4, respectively. Thus the mathematical model developed in this study could be used to explain the removal processes and simulate the fate of P in the DASC-based CWS system.