## 4 Discussion
### Removal of Nitrogen and Effects of Hydraulic Loading Rate on Nitrogen Removal  
Initially, ammonium was adsorbed to the surface of the biochar in the VFF. High capacity of biochar for adsorption of NH\({}_{4}\)-N has been reported previously [22]. As the biofilm of the VFF gradually expanded, the adsorbed ammonium slowly started to be oxidized to nitrate by nitrifying bacteria under the aerobic conditions within the VFF. Nitrifying bacteria are autotrophic and use inorganic carbon sources, e.g., CO\({}_{2}\), to obtain energy, resulting in slow growth rates of nitrifying bacteria [28]. Due to this nitrification process, most of the NH\({}_{4}\)-N in the incoming wastewater (\(34.2\pm 9.1\) mg L\({}^{-1}\)) was removed (\(93\pm 2\%\)), which left only \(4.5\pm 4.3\) mg L\({}^{-1}\) in the effluent from the VFF. In parallel with NH\({}_{4}\)-N removal, the VFF effluent showed an increase in NO\({}_{3}\)-N concentration (\(29.3\pm 7.5\) mg L\({}^{-1}\)). The VFF was a critical component of the system, since without it, nitrification would have not been achieved. This was proven by the good nitrification capacity in the VFF (\(65\pm 15\%\)). The NO\({}_{3}\)-N concentration decreased in the HFF. In fact, after stabilization of the combined HFF-VFF system, most of the nitrogen removal (\(61\pm 14\%\)) occurred in the HFF (saturated section) and only a small proportion of Tot-N removal (\(10\pm 12\%\)) occurred in the VFF. Through denitrification in the HFF, NO\({}_{3}\)-N developed in the VFF was converted to nitrogen gas by facultative heterotrophic bacteria under anoxic conditions in the presence of adequate decomposable carbon. This indicates that addition of a saturated flow filter, i.e., the HFF, was crucial to achieving acceptable levels of denitrification. It appears that conditions favoring the growth of denitrifying bacteria (i.e., development of anoxic zones and anaerobic biofilm) developed over time in the biochar particles in the HFF, and therefore the denitrification efficiency increased (Figure 4). On a long-term basis, the combined VFF-HFF system achieved good removal of Tot-N (\(71\pm 12\%\)) (Figure 3). During the last 52 weeks of operation, only one measurement showed Tot-N removal \(<50\%\). Wastewater production in households varies on a daily, weekly and seasonal basis, which leads to variability in HLR and OLR to OWTS. Under peak conditions, this can lead to a temporary breakdown of the infiltration system, so-called episodic failure [29]. Thus, it is necessary for the infiltration bed to have the capacity to withstand variations in HLR and OLR and maintain resilient and consistent treatment performance. Increasing the HLR from 23 to 31 L m\({}^{-2}\) day\({}^{-1}\) increased the nitrification efficiency from 42 to 61%. The specific surface of the biochar used in the system was large (200 m\({}^{2}\) g\({}^{-1}\)). Thus, when the HLR was increased more surfaces were reached by the wastewater and hence more contact between nitrifying bacteria and NH\({}_{4}\) occurred, which led to increased nitrification. Another explanation is that HLR 23 L m\({}^{-2}\) day\({}^{-1}\) was low and not all the infiltrative surface of the VFF was used. Thus, when the HLR was increased, the infiltrative surface of the biochar was effectively utilized and nitrification was enhanced. An increase in HLR usually leads to washout of NH\({}_{4}\)-N without nitrification, owing to shorter contact time between nitrifying bacteria and NH\({}_{4}\)-N [30]. Increasing the HLR from 31 to 39 L m\({}^{-2}\) day\({}^{-1}\) decreased the denitrification efficiency, which was reflected in high concentrations of NO\({}_{3}\)-N in the effluent from the HFF (\(16.1\pm 4.0\) mg L\({}^{-1}\)) compared with in the influent (\(23.7\pm 4.8\) mg L\({}^{-1}\)). In parallel with the decrease in denitrification efficiency, the overall removal of Tot-N in the system decreased. This is because at HLR-39, more nitrate escaped from the HFF before it was reduced to nitrogen gas by the bacteria.