# The Use of Chalcogenite as a Biosorption Bed in the Treatment of Groundwater
## 4 Discussion of Results
The lack of bed activation also contributed to an increase in the consumption of oxygen dissolved in the biofiltration process. Nitrification is a completely aerobic process, and so oxygen consumption indicates that biological processes are taking place on the filters. The total stoichiometric oxygen consumption in the nitrification reactions is 4.57 g O\({}_{2}\) per 1 g of ammonium nitrogen, whereas, in the case of iron and manganese oxidation, they are 0.14 g O\({}_{2}\)/g Fe and 0.29 g O\({}_{2}\)/g Mn, respectively [46]. In single-stage filtration with a water table above the filtration layer, in conditions of full saturation of water with oxygen (10-16 \({}^{\circ}\)C), it is possible to oxidize no more than approximately 2 g of N-NH\({}_{4}\)\({}^{+}\)/m\({}^{3}\). This limitation applies to biofiltration processes carried out in filters with a filling which does not take part in the process of removing impurities or which take part in this process to a small extent. The oxygen content below which the nitrification rate depends on the dissolved oxygen concentration is, respectively, 2 g O\({}_{2}\)/m\({}^{3}\) (for the first phase of the process) and 4 g O\({}_{2}\)/m\({}^{3}\) (for the second phase of the process). Too little a concentration of dissolved oxygen may cause the stopping of the biological oxidation reaction after the first phase of nitrification, which will result in an undesired presence of nitrite nitrogen in the water. Accumulation of nitrite ions is a negative phenomenon due to the inhibition of the growth of nitrifying bacteria. Nitric (III) acid produced from NO\({}_{2}\)\({}^{-}\) ions can be toxic to both _Nitrosomonas_ and _Nitrobacter_. The observed oxygen consumption was significantly different from the theoretical oxygen demand and amounted to approximately 2-2.4 g O\({}_{2}\) for each g of nitrogen removed in the system with chemical activation, and approximately 3.2 g O\({}_{2}\) for each g of nitrogen removed in the system without activation (Table 7). For comparison, in the biofiltration process in a sand bed with gravitational flow, the consumption of oxygen for each 1 g of removed ammonium nitrogen was between 50-150% of the theoretical oxygen demand [47]. Such high oxygen consumption was explained by the biodegradation of dead organic matter accumulated in the bed. In the discussed case, in the first stage of biofiltration, anti-gravitational flow was used which favored a natural regulation of biofilm thickness and resulted in the successive removal of dead organic matter from the biofilter, and consequently, reduced oxygen consumption. From the comparison of the biofiltration process in the analyzed filtration systems, it can be concluded that chemical activation has no effect on the efficiency of removal of iron and ammonium nitrogen but delays the start of both the first and second phase of the nitrification process. Activation of the bed positively influences the removal of manganese and results in less oxygen consumption in the treated water as compared with the bed not subjected to chemical activation (Table 7). In the research conducted thus far, it has been demonstrated that the removal of manganese and iron from water is the result not only of physicochemical processes but also of the involvement of iron and manganese oxidative bacteria [1; 48]. Other sources indicate that microorganisms may play a significant role in the formation process of the catalytic layer of manganese oxides; however, once the formation of this layer is completed, manganese oxidizing bacteria do not affect the efficiency of manganese removal from water [28]. The mechanism of the removal of manganese process with the simultaneous removal of ammonium nitrogen has been previously described in the literature [3; 19; 27; 29; 49]. Slight changes in nitrate concentration and reduced manganese removal efficiency may explain the delay of the nitrification process due to the reaction of nitrites with manganese dioxide.