# Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters
## 1 Introduction  
Tomatoes are the most-grown horticultural plants in the world [1]. Tomato production in modern agriculture is dominated by soilless cultivation. Unfortunately, this type of crop generates huge amounts of wastewater with high concentrations of biogenic compounds, and therefore constitutes a significant source of pollution [2]. In most places, soilless cultivation is carried out in open irrigation systems, which means that wastewater is not recirculated [3] and is removed to the natural environment [4, 5]. Open tomato soilless cultivation may result in 230 kg\(\cdot\)ha\({}^{-1}\) and 54 kg\(\cdot\)ha\({}^{-1}\) of nitrogen and phosphorus, respectively, released to the soil or surface water monthly. During a 150 day growing season, approximately 1000 m\({}^{3}\cdot\)ha\({}^{-1}\) of a nutrient solution is needed, and approximately 250 m\({}^{3}\) of wastewater is produced [6]. The result is a high concentration of phosphorus in wastewater, ranging from 35.4 to 104.0 mg P/L, and for nitrogen pollutants from 270.0 to 614.9 mg N/L and low COD (Chemical Oxygen Demand), which is usually below 50 mg O\({}_{2}\)/L [4]. Recent regulations introduced in the EU emphasize the modernization of soilless farming systemsto recover water and fertilizer ingredients. Therefore, there is an urgent need to look for new technologies that ensure drainage purification to a degree that allows it to be discharged into the environment or recirculated [7]. In the first case, they will serve to reduce the amount of pollutants discharged into the water and soil environment, and in the second case, they will reduce the consumption of water and fertilizer ingredients. Biological processes are commonly used to treat municipal wastewater and industrial effluent by harnessing the metabolism of microbes which, under specific conditions, can remove nutrients by breaking down pollutants adsorbed to a biofilm or on activated sludge flocs [8]. One of the most important parameters for such treatment is the COD:N:P ratio, which should be 100:5:1 for aerobic processes and 250:5:1 for anaerobic ones [9]. Bacteria use organics in wastewater as electron donors, with nitrates and nitrites as the ultimate acceptors [8]. The high-nutrient, low-COD (low C:N:P) wastewater from soilless tomato cultivation in greenhouses [4] needs to be amended with external carbon prior to treatment in bioreactors. The aim is to increase feedstock availability to denitrying bacteria and to improve denitrification parameters [10]. Effluent from soilless tomato cultivation can endanger aquatic ecosystems and drive eutrophication if discharged untreated, as it contains high levels of nitrates and phosphates [11]. External carbon sources can be divided into soluble (e.g., methanol, ethanol, sodium acetate, starch, glucose, sucrose) and solids (e.g., straw, woodchips, cotton) [12]. The choice is usually dictated by the cost, rate, and effectiveness of denitrification, as well as safety during transport and storage [13]. Other important considerations include: absence of nitrogen, product availability [9], C:N, carbon loss, hydraulic efficiency, and N\({}_{2}\)O emissions [14]. Small-molecule organics (e.g., methanol, ethanol, acetic acid) provide high denitrification efficiency, as their liquid state allows them to readily react with pollutants, thus shortening acclimation time and streamlining the process [15; 16]. On the other hand, liquid sources of carbon are quickly absorbed and have to be constantly replenished for the microbes to thrive [15]. High nitrate removal can be achieved with methanol, but the chemical is highly volatile, toxic, and hazardous, making it difficult to transport [12]. In addition, inappropriate dosing of low-molecular-weight organics can easily elevate organic carbon in the effluent beyond the acceptable limits [15]. Research is also being carried out on the use of biological sewage sludge liquids (permeate) as a carbon source in the denitrification process, which are easily used by microorganisms due to their high biodegradability.