Phosphate Mining Residues as Novel Substrate for Advanced Vertical Flow Constructed Wetlands: A Circular Economy Approach

Constructed wetlands offer sustainable, decentralized solution for wastewater treatment and reuse in Morocco. This study evaluates and compares four advanced vertical flow constructed wetlands (AVFCWs) incorporating locally sourced reactive media, to evaluate phosphate mining residues as a novel substrate. AVFCWs were designed with different substrate combinations, of pozzolan, phosphate mining residues (CW-B), clay (CW-C), and biochar (CW-D), alongside a control system (CW-A). The systems were operated under hydraulic retention times (HRTs) of 24, 48, and 72 hours corresponding to phase I, II and III respectively. Key water quality parameters in domestic wastewater were monitored, and evapotranspiration driven water losses were quantified. The results revealed that the use of reactive media significantly improved treatment efficiency, with CW-D achieving high removal efficiencies across most parameters. COD and TSS removal reached 80% and 88% respectively at 48h HRT, while nitrogen removal exceeded 82% in optimal configurations. CW-B achieved 76% phosphorus removal at 24h HRT, reaching 88% for CW-C at 72h. All AVFCWs exceeded 85% removal for heavy metals (Cu and Cd), with phosphate residues facilitating metal removal. On the other hand, faecal coliforms reached 3 Log reduction at an HRT of 24 h and decreased in subsequent phases, nevertheless exceeding Moroccan reuse limits. Extending hydraulic retention time improved the AVFCWs performance, but increased water loss (up to 28% at 72h) due to pronounced evapotranspiration in semi-arid to arid regions. Although all substrates demonstrated high performance, phosphate mining residues emerge as promising substrate pending further optimization, supporting circular economy objectives.