## 3 Results and Discussion
### Biochemical Oxygen Demand (BOD\({}_{5}\)), Nitrogen, and Phosphorus
#### 3.2.2 Nitrogen  
The vermifiltration-hydroonic system showed the capacity to significantly eliminate ammonia by 99%. Efficient ammonia nitrogen removal by vermifiltration has been reported in the literature [13; 20; 21; 23]. Concerning ammonia content changes during the hydroponic treatment, the starting concentrations were already very low after vermifiltration. Further reduction was observed in the hydroponic unit. The results suggest activity of ammonia-oxidizing microorganisms in the hydroponic unit. In fact, since that unit was fed with vermifilter effluent and the hydroponic tray was aerated, creating the necessary aerobic conditions, the transfer of these microorganisms from the vermifilter and further nitrification activity in the hydroponic unit will likely occur. Plants may also have contributed to the decrease in ammonia concentration, as they can take up nitrogen both in the form of ammonia and nitrate [51]. Ammonia levels in the effluent fell below the LOQ after 28 days; thus, it was not possible to quantify the removal by the weighted average approach, and 100% removal was considered. Nitrite concentrations decreased after vermifiltration treatment to values below 0.1 mg NO\({}_{2}\)-N/L, the only exceptions being two days where the feed values were already particularly low. Removal efficiencies from 95 to 99% were observed, comparable to the efficiency of more sophisticated swine manure treatments such as immobilized cells' reactors [10]. Wastewater treatment by vermifiltration has been reported to favor nitrification processes [20; 21; 29]; the present results support those observations. Nitrite content showed a general decrease along the hydroponic treatment. Like with ammonia, nitrite decrease suggests possible activity of nitrite-oxidizing microorganisms transferred from the vermifilter by the liquid flow and proliferating in the hydroponic tray under the created aerobic conditions. Relative removal efficiency by this treatment stage was lower than the achieved by vermifiltration, and this could again be attributed to the lower starting values. Ecological balance of all the species involved in the nitrogen cycle requires the availability of their main nutrients on a certain level, and, for this reason, all nutrients, including nitrite, should tend towards certain sustained optimal levels. The presence and activity of nitrifying micro-organisms both in the liquid phase and in biofilms within the system should be more thoroughly investigated in the future. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Feed** & **Effluent** & **RC (\%)** & \(p\)**-Value** \\ \hline BOD\({}_{5}\) (mgO\({}_{2}\)/L) & 5.9 (\(\pm\)0.7) & 2.17 (\(\pm\)0.08) & -63 (\(\pm\)15) & \textless{}0.001 \\ NH\({}_{3}\)-N (mgN/L) & 0.075 (\(\pm\)0.004) & \textless{}LOQ & –100 & - \\ NO\({}_{2}\)-N (mgN/L) & 0.0329 (\(\pm\)0.0013) & 0.0112 (\(\pm\)0.0012) & –66 (\(\pm\)6) & \textless{}0.001 \\ NO\({}_{3}\)-N (mgN/L) & 208 (\(\pm\)13) & 153 (\(\pm\)1) & –27 (\(\pm\)3) & \textless{}0.001 \\ TP (mgP/L) & 39.2 (\(\pm\)0.9) & 20.9 (\(\pm\)0.4) & –47 (\(\pm\)3) & \textless{}0.001 \\ PO\({}_{4}\)-P (mgP/L) & 33.2 (\(\pm\)0.3) & 18.5 (\(\pm\)0.3) & –44 (\(\pm\)1) & \textless{}0.001 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Hydrogenic treatment feed, effluent, and relative change (RC) in water quality parameters. The results are presented as triplicate (\(n=3\)) mean \(\pm\) standard error. Nitrate has been shown to increase due to nitrification activity in vermifilters [21; 29]. For the hydroponic treatment, nitrate removal as nutrients for plants was hypothesized. Nitrate showed a relative increase after vermifiltration of just 11.0(\(\pm\)0.8)%, and the hydroponic treatment caused a relative removal up to 27(\(\pm\)3)%. These modest relative changes can be explained by the fact that the system was running for 28 days, and most of the volume transferred to the mixing tank corresponded to the hydroponic effluent, gathered in the final reservoir for at least seven days each time.