Domestic wastewater treatment using oakwood biochar: filter column performance, influencing factors, and machine learning predictions

Abstract

Septic systems often exceed regional soil treatment capacity, leading to failures and high-load pollutant discharge into surface and groundwater. Consequently, cost-effective post-treatment solutions are needed to protect public health and environment. Biochar, a carbonaceous sorbent, effectively treats septic tank effluent. However, the few studies on this topic have focused on its role in enhancing constructed wetland performance overlooking its potential as a filter material especially for key design factors like filter clogging and particle size. Additionally, flow regime effects and effective models for contaminant removal are underexplored. To inform pilot scale biochar filter design, we assessed the effect of particle size and flow regime (intermittent, continuous) on filter performance and longevity. The biochar effectively removed nutrients and organics from septic tank effluent with fine-sized biochar achieving the highest removal (total nitrogen (TN) 50.47%, ammonium 69.25%, chemical oxygen demand (COD) 53.83%, total organic carbon 67.85%) but lowest filtration rates (0.051 ± 0.006 m h⁻¹) and higher clogging susceptibility. Large-sized biochar treated more than eight times the volume of wastewater than fine-sized biochar over the same period. The stepwise cluster inference model we developed accurately predicted COD and TN removal (R² > 0.80). Our findings suggest that effective biochar filter design should prioritize large-diameter filters to maximize surface area reducing pressure loss and use vegetation and geotextiles to extend service life. Future studies should focus on reducing clogging in fine biochar, understanding biofilm formation, and modifying biochar for better contaminant removal. This study provides valuable insights into optimizing biochar filters for septic effluent treatment.