# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 1 Introduction
This appears to be a good solution in terms of sustainable use, and effectively improves nitrogen removal efficiency, comparable to that obtained with methanol [17]. Fermentation byproducts can be a source of carbon, but this is not a feasible solution in every treatment plant, and, due to different fermentation conditions, hydrolysates are not the same [15]. Alternatively, solid substances can be used: natural plant-based materials (sawdust, woodchips, straw, cotton, maize cobs) or readily biodegradable synthetic polymers (polycaprolactone (PCL), polyhydroxyalkanoate (PHA), polybutylene succinate (PBS), polylactic acid (PLA)). They are relatively cheap, readily available, and offer good results [18]. The literature data show that sewage from soilless cultivation of plants treatment was tried in constructed wetlands [19; 20; 21]. Due to their chemical composition, it was impossible to effectively remove nitrates because there was a lack of carbon. Attempts have also been made to treat greenhouse wastewater using the denitrification process by adding a liquid external carbon source (such as methanol) [22]. However, as mentioned in the previous paragraph, this substance is toxic and difficult to transport [12]. A solution that could be a step forward may be the use of biofilters with \"active\" filling, which will act as a material on which the biofilm will develop and at the same time be a source of carbon for denitrying organisms. The basic principle of biofilter activity is the biodegradation of contaminants by microorganisms attached to the filter. Biodegradable materials filling the reactor can provide a substrate for the growth of a biofilm (biofilm carrier) while being a source of organic carbon. Under the influence of extracellular enzymes secreted by biofilm-forming microorganisms, more easily digestible, small-molecule substrates are released from biodegradable biofilters, which are electron donors in the solid-phase denitrification (SPD) process [18]. There are several pathways for nitrate removal in the SPD. The most likely is the heterotrophic denitrification process, in which nitrate is ultimately reduced to gaseous nitrogen. In parallel, dissimilatory nitrate reduction to ammonium (DNRA) can occur [23]. It should be noted that, unlike biodegradable polymers, natural media are associated with slow carbon release and unstable rates of denitrification [15]. As the medium is biodegradable, dissolved organic carbon (DOC) is released and taken up by the microbes for growth and denitrification. If DOC limits are exceeded, the carbon accumulates in the treated wastewater, leading to recontamination [18]. The extensive use of external sources of organic carbon significantly increases operating costs--the cost of carbon sources and waste management accounted for up to 50% of total wastewater treatment costs [24]. Researchers have been exploring new exogenous sources of organic carbon in order to improve nitrogen/phosphorus removal from low C:N:P wastewater, as well as to reduce the cost of treatment. In addition to technological efficiency and acquisition costs, it is equally important to take into account the possibility of using renewable raw materials and waste as an external source of carbon. An example of such a cheap waste- and renewable material is wood shavings. The use of wood shavings instead of, for example, biodegradable polymers (the production of which involves the use of nonrenewable raw materials, significant energy costs, and a significant carbon footprint) will be more beneficial for the environment and contributes to sustainability. Such technological solutions will contribute to more effective water protection and meet the requirements of sustainable water management.