A low-cost graphitized sand filter to deliver MC-LR-free potable water: Water treatment plants and household perspective

Abstract

Scale-up feasibility of the graphitized sand filter (GS1) for Microcystin-LR (MC-LR) removal and its impact on other water pollutants (WPs) was assessed through a mass-balance study, using a laboratory-based drinking water treatment plant (DWTP) micromodel named: SAP-1©. The treatment system comprised: raw water tank, pre-oxidation tank (oxidant: potassium permanganate), followed by a coagulation/flocculation tank (alum supplemented), sedimentation tank, filtration module and finally disinfection tank (dosed with hypochlorite solution). Two filter modules (FMs) were studied: a) FM1: graphitized-sand media + sand media = ½ GS1 + ½ sand and b) FM2: ½ sand + ½ sand. The MC-LR removal study (initial concentration: 50 μg/L) was per- formed for two varieties of MC-LR source: a) commercial MC-LR, and b) algal-biomass released MC-LR. Along with MC-LR, other WPs were also evaluated including metal ions (Fe2+ and Cu2+), total coliform, turbidity, ammonia-N and dissolved organic carbon. The removal efficiency of these WPs was determined for each treat- ment unit (as it passed). FM1 was able to reduce the inflow residual of MC-LR (coming from the preceding unit: sedimentation unit)from 12.1μg/L and 25.4 μg/L (for commercial and algal-cell MC-LR source, respectively) to <0.61 μg/L and hence successfully complying the WHO guidelines (<1 μg/L). The protein phosphatase 1A (PP1A) toxicity assay confirmed a much safer and more toxic-free filtrate (by 40%–50%) for FM1 as compared to the filtrate obtained from FM2. The techno-economic evaluation showed that for an annual household filter application, 160 CAD needs to be spent on one GS1-based filter unit as compared to over 6000 CAD (equivalent price) for the conventional sand-based filter to provide MC-LR-free water. The present study demonstrates the feasibility of the utilization of these units in household filtration systems.