# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 4 Conclusions  
The present study investigated solid external carbon sources--Oxytree chips, beech chips, and biobased polybutylene succinate derivatives (BioPBS FZ91 PB and BioPBS FD92 PB)--as biofiliter media for treating greenhouse effluent. The highest N and P removal efficiencies were noted for BioPBS FD92 PB (PBS2) at 43.11% and 42.48%, respectively. Oxytree chips performed the worst (8.80 \(\pm\) 4.88%) N removal efficiency. Beech chips and PBS1 (FZ91 PB) had similar denitrification efficiencies at 14.10 \(\pm\) 9.09% and 15.55 \(\pm\) 9.09%, respectively. The observed low efficiencies of the denitrification process were the result of high DO concentrations in biofilters. The best-performing variants for P removal were PBS2 (42.48 \(\pm\) 10.3%) and Oxytree (31.14 \(\pm\) 6.17%). Beech chips and PBS1 provided similar removal efficiencies at 25.23 \(\pm\) 6.82% and 26.42 \(\pm\) 7.45%, respectively.  
An important issue that warrants further study is the extent of organic matter (COD) recontamination in the treated effluent. For a retention time of 24 h, a high carbon overdose was observed regardless of the type of polymers used. Woodchips were found to release less COD than the biobased polybutylene succinate, but also provided lower denitrification efficiencies. Wood chips had a smaller impact on the final COD concentration of treated wastewater, Oxytree chips guaranteed a lower P concentration in the effluent than PBS1, beech chips and PBS2 provided similar values of the efficiency of removing nitrogen and phosphorus compounds.  
The research results proved that all tested materials can be a source of carbon and could be used as a filling for biofilters or constructed wetlands treating wastewater from soilless tomato cultivation. In addition, research contributes to sustainability because it has shown that wood chips, which are waste material derived from the processing of renewable raw materials, can, under certain conditions, be used instead of readily biodegradable synthetic polymers. Considering the scale of the problem related to the discharge of untreated sewage from soilless plant cultivation, it is urgent to find a simple and easy-to-use technological solution for their purification. Such a solution could be biofilters or constructed wetlands with tested fillings.  
Conceptualization, J.R. and W.J.; methodology, K.K. and K.L.B.; validation, K.J., P.B., and A.O.; formal analysis, J.R. and A.M.; investigation, K.K., A.M., and K.L.B.; resources, W.J.; data curation, K.K. and J.R.; writing--original draft preparation, K.K.; writing--review and editing, J.R.; visualization, K.K., J.R. and A.M.; supervision, W.J. All authors have read and agreed to the published version of the manuscript.  
Project financially supported by the Minister of Education and Science under the program entitled \"Regional Initiative of Excellence\" for the years 2019-2023, Project No. 010/RID/2018/19, amount of funding 12,000,000 PLN. The study was financially co-supported in the framework of a Project No. 29.610.023-300 of the University of Warmia and Mazury in Olsztyn, Poland.  
Not applicable.  
The data presented in this study are available in the article.  
Special thanks to Mitsubishi Corporation for providing the biobased polybutylene succinate (PBS) derivatives that made this study possible.