A new insight into highly contaminated landfill leachate treatment using Kefir grains pre-treatment combined with Ag-doped TiO2 photocatalytic process

Treatment of landfill leachate with combined biological/photocatalytic process. [Display omitted] •The first detailed study exploring Kefir grains in the economical landfill leachate pre-treatment.•Optimization of Ag-doped TiO2 photocatalytic process for the maximum removal of COD and NH4+-N using Box-Behnken design.•Kefir grains and Ag-doped TiO2 exhibit excellent recyclability.•Combined biological/photocatalytic process is a practical low-cost approach to effectively remove toxic pollutants. This study investigates the performance of the combination of biological pre-treatment with Kefir grains (KGs) and photocatalytic process using Ag-doped TiO2 nanoparticles (NPs) for the simultaneous removal of toxic pollutants from landfill leachate (LFL). After 5 days of 1% (w/v) KGs pre-treatment at 37 °C, TOC, COD, NH4+-N, and PO43- removal rates were 93, 83.33, 70 and 88.25%, respectively. The removal efficiencies were found to be 100, 94, 62.5, 53.16 and 47.52 % for Cd, Ni, Zn, Mn and Cu, respectively. The optimal conditions of Ag-doped TiO2 photocatalytic process were optimized using Box-Behnken design and response surface methodology (BBD-RSM) to enhance the quality of pre-treated LFL. Interestingly, Ag-doped TiO2 photocatalytic process increases the overall removal efficiencies to 98, 96, 85 and 93% of TOC, COD, NH4+-N, and PO43-, respectively. Furthermore, the removal efficiency of toxic heavy metals was gradually improved. In addition, KGs and Ag-doped TiO2 exhibited excellent recyclability showing the potential of combined biological/photocatalytic process to treat hazardous LFL.