Simulating water and pollutant transport in bark, charcoal and sand filters for greywater treatment

Sammanfattning: A septic tank combined with a sand filter is the most common onsite wastewater treatment system worldwide, since it is a simple, lowcost and reliable treatment method. Alternatives to sand in filters could be advantageous in terms of availability of material and enhanced treatment properties. In this study, flow dynamics and pollutant transport in three filter materials; sand, pine bark and activated charcoal, intermittently dosed with artificial greywater, were simulated using the HYDRUS wetland module. The simulated results were compared with observations from laboratory filters and model hydraulic and microbial parameters were calibrated. Emphasis was placed on simulating the removal of organic pollutants by each filter type. Furthermore, for the bark and charcoal filters, removal of organic matter was simulated for different hydraulic and organic loading rates (HLR = 32 and 64 l m⁻² day⁻¹ and OLR = 13-6 and 28 g BOD₅ m⁻² day⁻¹). Comparing simulated with measured cumulated effluent volume, the normalised root mean square error for all three filter materials was small (0.7-3.5%). The simulated bark filter COD removal in different loading regimes (HLR = 32 and 64 l m⁻² day⁻¹, OLR = 13-16 and 28 g BOD₅ m⁻² day⁻¹) was overestimated by 13-20 percentage points compared with the measured values. When release of organic matter from the bark material itself was accounted for, the difference was reduced to 2-10 percentage points. Simulation of the charcoal filter demonstrated 94 and 91 % removal of COD for HLR = 32 and OLR = 13 - 16 g BOD₅ m⁻² day⁻¹, which compared well with the measured values, 95 ± 2 % and 89 ± 11 %, respectively. However, simulated COD removal for Run 2 (70%) and Run 5 (72%) was low compared with the measured values (90 ± 7 and 84 ± 4 %). The measured sand filter effluent concentration of COD was 245 mg l⁻¹ and the simulated effluent concentration of COD was 134 mg l⁻¹ for HLR = 32 and OLR = 14 g BOD₅ m⁻² day⁻¹. After including an effect of water flow along the column wall in the model, the simulated effluent concentration of COD was 337 mg l⁻¹. These simulations of bark, charcoal and sand filters improved understanding of filter functions and identified possible filter design developments.