## 1 Introduction  
Biochar is a carbon produced by thermal decomposition of organic materials at elevated temperature (300-800 \({}^{\circ}\)C) in the absence of oxygen [1]. In the wastewater treatment sector, there is growing interest in using biochar as a renewable biofilm carrier and adsorbent to replace or complement sand filters. This is because utilization of biochar for wastewater purification not only increases treatment efficiency and reduces the spread of hazardous contaminants, but also helps to conserve finite natural sands and mountain landscapes and to mitigate climate change effects through carbon sequestration [2,3]. Moreover, exhausted biochar can be easily regenerated by pyrolysis or composting, or used as a soil amendment. Onsite wastewater treatment systems (OWTS) are defined as systems treating wastewater from single or multiple dwellings [4] and can be installed next to the source of generation [5]. The capacity of onsite systems varies across countries and regulating bodies, ranging from 1000 to 10,000 PE [6]. Systems with a septic tank for primary treatment, followed by a gravity-fed leaching field, infiltration bed or sand filtration bed, are widely used for OWTS, due to their low operating cost, easy maintenance and low energy consumption [7]. There had been much research on designing OWTS for enhanced removal of organic matter and phosphorus, but nitrogen removal in these systems is less well studied. Nitrate (NO\({}_{3}\)) and pathogen contamination of groundwater and lake water used as drinking water sources has been reported worldwide, with OWTS often cited as the primary source of contamination [8; 9; 10; 11; 12; 13]. The microbial quality of effluent from OWTS is important. Worldwide, microbial outbreaks have been reported in drinking water wells, bathing waters and fishing waters located close to OWTS facilities [14; 13]. For example, in Sweden, 70% of microbial outbreaks in private wells occur in wells located near an OWTS [15]. Therefore, developing OWTS which are capable of removing nitrogen and pathogens, among other pollutants, is particularly critical to protect public health and the environment. In fact, nitrogen removal in OWTS is required to ensure that effluent from such systems is in compliance with treated water quality regulations. Under Swedish regulations, OWTSs in sensitive catchments are required to remove at least 50% of nitrogen in wastewater [16]. The efficiency of single-pass and vertical flow biochar filters in removing solids, organic matter, pathogen indicators and micropollutants from OWTSs has been assessed in previous studies [2; 17; 18; 19; 20]. These unsaturated filters are designed to remove organic pollutants, with no consideration to removing nitrogen by denitrification. Utilization of biochar wetlands for removal of nitrogen in treatment of domestic wastewater effluent has been studied [21; 22]. For example, [PERSON], [PERSON] [22] investigated biological nitrogen transformation in intermittent aerated subsurface-flow constructed wetlands under various influent C/N ratios and reported 97-99% removal of ammonium nitrogen (NH\({}_{4}\)+-N) and 46-98% removal of total nitrogen (Tot-N). In a study investigating N removal from piggery wastewater using biochar wetlands [23], it was found that biochar adsorbed 60% of NH\({}_{4}\)+-N, but that absorption was sensitive to changes in particle size, pH and temperature. There is thus well-established knowledge of nitrification/denitrification, the need for nitrogen removal in OWTS and the potential of biochar for this purpose. However, OWTS incorporating biochar filters as a simple and low-cost technology to remove nitrogen and pollutants and reduce the carbon footprint need to be designed. To our knowledge, no previous study has sought to develop a compact multi-stage biochar filter for enhanced removal of nitrogen in OWTS.