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Article

Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters

by
Karolina Kłobukowska
1,
Joanna Rodziewicz
1,*,
Artur Mielcarek
1,
Kamil Łukasz Bryszewski
1,
Wojciech Janczukowicz
1,
Piotr Bugajski
2,
Krzysztof Jóźwiakowski
3 and
Agnieszka Operacz
2
1
Department of Environment Engineering, University of Warmia and Mazury in Olsztyn, Warszawska St. 117a, 10-719 Olsztyn, Poland
2
Department of Sanitary Engineering and Water Management, University of Agriculture in Kraków, Al. Mickiewicza St. 24, 28 30-059 Cracow, Poland
3
Department of Environmental Engineering and Geodesy, University of Life Sciences in Lublin, Leszczyńskiego St. 7, 20-069 Lublin, Poland
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(1), 176; https://doi.org/10.3390/app14010176
Submission received: 20 November 2023 / Revised: 5 December 2023 / Accepted: 23 December 2023 / Published: 24 December 2023
Browse Figures
Versions Notes

Abstract

:
The problem of wastewater discharged from facilities for soilless plant cultivation is a worldwide problem. So, there is a strong need to find a simple, maintenance-free technological solution. Such a solution may be the use of biofilters or constructed wetlands with “active” filling, which will be a source of carbon for denitrifying organisms. The aim of this study was to evaluate four different types of materials—Oxytree wood chips (Paulownia Clon in vitro 112®), biobased polybutylene succinate derivatives (BioPBS FZ91PB and BioPBS FD92PB), and beech wood chips—as biofilter media and as solid exogenous carbon sources. The highest efficiency of denitrification and dephosphatation (43.11% and 42.48%, respectively) was characterized by the biobased polybutylene succinate FD92PB. The lowest removal efficiency of phosphorus compounds was achieved for beech chips—25.23% and BioPBS FZ91PB—26.42%. The lowest denitrification efficiency—8.8%—was achieved by Oxytree wood chips. The extent of organic matter (COD) repollution in the treated wastewater requires further study. Wood chips were found to release less COD than biobased polybutylene succinate. The research results showed that all tested materials can be a source of carbon in soilless tomato cultivation wastewater treatment. This is a good starting point for further research on selecting appropriate process parameters and creating new solutions for reactor construction.

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∙ha−1 and 54 kg∙ha−1 of nitrogen and phosphorus, respectively, released to the soil or surface water monthly. During a 150 day growing season, approximately 1000 m3∙ha−1 of a nutrient solution is needed, and approximately 250 m3 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 O2/L [4]. Recent regulations introduced in the EU emphasize the modernization of soilless farming systems to 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 denitrifying 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 N2O 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. 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 denitrifying 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 biodegraded, 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.
The aim of the present study was to assess the applicability of two materials—Oxytree chips (Paulownia Clon in vitro 112®), and beech chips, which are waste material derived from the processing of renewable raw materials, and two, recommended in the literature, biobased polybutylene succinate derivatives (BioPBS FZ91PB (PBS1) and BioPBS FD92PB (PBS2)—with higher rate of biodegradability)—as biofilter media and as solid external carbon sources. The possibility of using solid carbon sources in the treatment of greenhouse wastewater has not been investigated so far. Beech wood chips have been employed as a carbon source in reactors designed to remove nitrates from biotreated municipal wastewater, aquaculture effluent or synthetic stormwater [25,26,27,28] (which had much lower nitrate concentrations in influent than greenhouse wastewater). Paulownia Clon in vitro 112® (Oxytree) woodchips’ application as a carbon source in SPD has never been examined before. Oxytree can absorb much more carbon dioxide (111 tons of CO2/hectare/year) than any other deciduous tree (e.g., oak absorbs only 9.1 tons) [29], so it seems to be a good material for solid-phase denitrification. Biodegradable polymers such as PBS are considered more effective than other applied materials [23]. The suitability of these media for biofilters was determined by the efficiency of nitrogen/phosphorus bio removal from wastewater.

2. Materials and Methods

2.1. Materials

Greenhouse wastewater from hydroponic tomato farming was used for the study (Table 1).
The effluent was treated in fixed-bed biofilters. Four types of media (carbon sources) were used: beech chips, Oxytree chips (Paulownia Clon in vitro 112®), biobased polybutylene succinate derivative BioPBS FZ91PB (51% carbon from biofuels, according to ASTM D 6866 [30]), and BioPBS FD92PB (35% carbon from biofuels, according to ASTM D 6866).

2.2. Experimental Station

Prior to the construction of the station, the woodchips were washed with water in order to rinse out any dyes that might have affected subsequent analyses of the treated effluent. Afterwards, the chips were dried at 70 °C.
Activated sludge from a municipal wastewater treatment plant in Olsztyn (population equivalent (PE) 270,000) was used as the inoculum. It was taken from denitrification chamber. This allowed the insertion of a complex community of microorganisms into the reactors at the beginning of the experiment. Due to the very high concentration of nitrates in the wastewater from soilless tomato cultivation, a single-stage system (consisting of only one reactor) would not provide us with significant nitrate removal efficiency. For this reason, the wastewater was subjected to two-step treatment (Figure 1). The first step consisted of four 1 L biofilters fully filled with either beech chips, Oxytree chips, BioPBS FZ91PB (PBS1), or BioPBS FD92PB (PBS2). The thus-treated effluent was fed into four smaller (0.5 L) reactors, filled with the respective medium, for the second step. The hydraulic retention time (HRT) was 24 h per step. The effluent was supplied into the reactor using a Masterflex peristaltic pump (Cole-Parmer GmbH, Wertheim, Germany). Samples for analysis were taken every 24 h. The experiment was conducted at 22.5 °C.

2.3. Analytical Methods

After a 3-week biofilter acclimation period, analytical process control was initiated. Samples were taken every 24 h. The samples were filtered through medium-size qualitative filter paper (PHU EUROCHEM BGD, Tarnów, Poland) prior to analysis. The influent and treated effluent were tested for the following parameters: pH and oxygen using a HQ 440d Multi-Meter (Hach Company, Loveland, CO, USA); temperature and conductivity using a CX-461 meter (Elmetron, Zabrze, Poland); total organic carbon (TOC); and total nitrogen using a Total Organic Carbon Analyzer TOC-L CPH/CPN with a TNM-L device (Shimadzu Corporation, Kyoto, Japan). A UV-VIS 5000 DR spectrophotometer (HACH Lange, Düsseldorf, Germany) was used to quantify organic matter (COD) (LCK 514 and LCK 1414), nitrites (LCK 341 and LCK 342), nitrates (LCK 340), ammonia nitrogen (LCK 304), and total phosphorus (LCK 348 and LCK 350).

2.4. Calculation Methods

Total nitrogen removal efficiency (EN) was calculated using Formula (1):
E N = C N 0 − C N e C N 0 · 100 %
where:
  • C N 0 —total nitrogen in the influent [mg N/L],
  • C N e —total nitrogen in the treated effluent [mg N/L],
Total phosphorus removal efficiency (EP) was calculated using Formula (2):
E P = C P 0 − C P e C P 0 · 100 %
where:
  • C P 0 —total phosphorus in the influent [mg P/L],
  • C P e —total phosphorus in the treated effluent [mg P/L].

2.5. Statistical Analysis

Statistical calculations (which were based on the results of physical and chemical analyses of influent and treated wastewater) were performed using MS Excel. The obtained results were subjected to general characterization by determining the minimum, maximum, mean, and standard deviation for each variable. The confidence interval (CI) was calculated at 95%.

3. Results and Discussion

3.1. Nitrogen Removal

N-NO3 levels by treatment step and nitrate removal by carbon source are shown in Figure 2 and Figure 3. Nitrate concentration in the influent wastewater was 710.0 mg/L. The highest removal efficiencies (43.11 ± 14.31% (±CI)) were noted for PBS2 (FD92PB), whereas Oxytree chips performed the worst (8.80 ± 4.88%). Beech chips and PBS1 (FZ91PB) had similar denitrification rates at 14.10 ± 9.09% and 15.55 ± 9.09%, respectively. PBS2 has a higher degree of biodegradability than PBS1. Thus, access to organic carbon from this medium was easier for microorganisms performing denitrification, resulting in higher treatment efficiency. However, these values are relatively low compared to carbon sources tested by other researchers. Polish legislation contains information on the maximum permissible concentrations of pollutants entering the aquatic environment from the production and processing of fruits and vegetables. For nitrate nitrogen, the value is 30 mg/L, while for total phosphorus, it is 3 mg/L [31]. The treatment efficiencies achieved did not guarantee a reduction in nitrate concentrations to acceptable values.
Wu et al. [32] managed to achieve high nitrate removal of 96.42–99.52% for a PHBV/PLA polymer at the same HRT (24 h). However, it should be noted that this study used synthetic waste containing much lower levels of nitrates (50 mg/L NO3-N) similar to Zhu et al. [33] who utilized low-nitrate wastewater too—50–150 mg/L NO3-N using polybutylene succinate (PBS)—71.77% and 96.73% removal.
The denitrification process is sensitive to pH change, and the enzymatic activity of denitrifiers is highly dependent on temperature [23]. Wastewater pH and temperature were controlled during each step of treatment. Table 2 shows mean parameter values by carbon source. Both factors were within the optimal range for denitrification: temperature was approx. 20–24 °C for all media, with average pH not exceeding 8. In each case, there is a slight increase in pH (the highest with the use of PBS2, the lowest—PBS1). The denitrification process produces alkalinity (OH– ions are formed), which can increase the pH of the denitrification system [9,23].
Chu and Wang [34] used PCL-filled beds and achieved 92% nitrate removal at 7.2 pH, and 20 °C. Schipper et al. [14] tested maize cobs, green waste, wheat straw, and soft/hard woodchips as potential sources of organic carbon. For almost all these feedstocks, nitrogen removal rates were higher at 23.5 °C than at 14 °C. Elefsiniotis and Li [35] investigated how temperature affects nitrate reduction, using acetic acid, propionic acid, and a mixture of both as exogenous carbon sources. Increasing the temperature from 10 °C to 20 °C had more of an impact on denitrification and carbon removal parameters than the increase from 20 °C to 30 °C. The authors found that denitrification is possible at low temperatures, and that the exact rate of denitrification and carbon consumption at a given temperature are affected by the initial nitrogen and carbon concentrations, respectively. Denitrification was found to be feasible under all tested conditions, as long as the initial pH was 6.5. Other authors have pointed to 20 °C as the optimal temperature for nitrate reduction [36]. According to Beaubien et al. [37] denitrifying microbes are the most active at neutral pH, with highest performance noted within the pH range of 6.0–8.0.
It requires 0.646 g of organic matter expressed as COD to reduce 1 g of N-NO3. For a value less than 1.13 g COD/g N-NO3, nitrogen removal efficiency is low [38]. Figure 4 shows the COD/N-NO3 ratio for the tested carbon sources at different stages of the experiment. The ratio, which was 0.05 g COD/g N-NO3 in the influent, surged after the first stage (I°) of treatment to 0.49 g COD/g N-NO3, 0.52 g COD/g N-NO3, 0.70 g COD/g N-NO3, and 1.20 g COD/g N-NO3 for beech chips, Oxytree chips, PBS1, and PBS2, respectively. After the second stage (II°) of treatment, the PBS2 variant boasted the highest ratio (5.13 g COD/g N-NO3), though still insufficient for good denitrification performance.
A solid carbon source is first hydrolyzed by extracellular enzymes (such as lipase) secreted by the microorganisms, then broken down into soluble and small-molecule substrates. Therefore, Wang and Chu [18] posit that the volume of carbon released is mainly regulated by bacteria as they react to nitrate levels in the water. However, carbon leaching can occur, as evidenced by Li et al. [39]—42.3–94.1% of the carbon was released during the initial stage (<24 h). Feng et al. [40] also noted that plant-based organic carbon is released much faster in the initial phase.
Thus, the increase in total organic carbon (TOC) and COD in the effluent is indicative of leaching and biodegradation of the solid carbon source. Figure 5 and Figure 6 show trends in TOC and COD during the first and second treatment step.
By far the highest TOC was noted for PBS2 at approx. 170.9 ± 96.6 mg/L and approx. 236.6 ± 125.5 mg/L after steps 1 and 2 of treatment, respectively. The respective values for other sources were: PBS1—61.5 ± 28.7 mg/L and 95.0 ± 45.0 mg/L; beech chips—33.2 ± 21.5 mg/L and 33.9 ± 8.5 mg/L; Oxytree chips—30.3 ± 10.5 mg/L and 44.0 ± 17.7 mg/L. By comparison, Zhu et al. [32] recorded dissolved organic carbon (DOC) levels of approx. 136.11–202.51 mg/L. Influent COD was 34.96 mg/L, but rose significantly after treatment with PBS1 and PBS2 (to 763.39 ± 219.60 mg/L and 1177.50 ± 479.32 mg/L, respectively). The COD increase for woodchips was less pronounced, but still sufficient to qualify as recontamination (approx. 300 ± 65 mg/L). The high DOC and COD may be attributable to carbon leaching or may indicate that the carbon load released by the metabolizing microorganisms exceeded the amount required by the denitrifiers to grow and remove nitrogen. Further research is needed on how to control the biodegradability of solid carbon sources to ensure optimal DOC in the wastewater (without it accumulating as a recontaminant).
According to Luo et al. [41] heterotrophic denitrification performance can be affected by the presence of dissolved oxygen (DO). This explains the observed low efficiency of the denitrification process in all biofilters (Figure 3). In our results, denitrification performance peaked at DO = 3.97 mg/L for PBS2 after the second-step treatment. The reactor filled with PBS2 had more favorable oxygen conditions for the denitrification process to occur. This is a result of the amount of organic substrate released and oxygen consumed in the process of its decomposition, as well as the size of the space between the fill granules, smaller especially than in the case of woodchip fill. The other groups had higher DO (>5 mg/L) and, accordingly, much poorer performance (Figure 7). Luo et al. [41] used PBS as an organic carbon source and biofilm carrier. Nitrate removal and TN removal were monitored for three DO concentrations: 5.2 mg/L, 1.4 mg/L, and 0.5 mg/L. The 5.2 mg/L DO group had the highest TN and N-NO3 removal rates. One potential explanation for this is that some denitrifying bacteria can use both DO and nitrate as electron acceptors [41]. Furthermore, Gutierrez-Wing et al. [42] found that, even with oxygen saturation, denitrification occurs as the depth of the bed increases, due to the irregular diffusion of oxygen and the formation of anoxic zones over polyhydroxybutyrate (PHB) pores (which were the subject of their study). The authors pointed to 4.0–5.0 mg/L as the optimal DO range for PHB. Other scientists have reported that denitrification can occur at oxygen levels below 2 mg/L, since oxygen is the preferred electron acceptor for organic carbon oxidation and competes with nitrates [43].
The presence of N-NH4 in wastewater may be indicative of dissimilative nitrate reduction—excessive levels of NH4+ ions (which can transform into ammonia) may be toxic to denitrifying microbes [36]. The wastewater in our study had low levels of ammonia nitrogen, which had little effect on the nitrate removal.

3.2. Phosphorus Removal

Regulation of the Minister of Maritime Economy and Inland Navigation of 12 July 2019 on Substances Particularly Harmful to the Aquatic Environment and Conditions to Be Met When Introducing Sewage into Waters or into the Ground, as Well as When Discharging defines the maximum permissible concentration of total phosphorus (TP)—3 mg/L (Dz.U. 2019, 1311) [31]. Mean TP concentrations in the influent (approx. 43.79 mg/L) and the treated effluent are provided in Figure 8. In each case, in the treated wastewater, the TP concentration far exceeds the value allowed by the regulation. Removal efficiencies across different organic carbon sources are given in Figure 9. The best-performing variants were PBS2 (42.48 ± 10.3%) and Oxytree (31.14 ± 6.17%). Beech chips and PBS1 provided similar removal efficiencies at 25.23 ± 6.82% and 26.42 ± 7.45%, respectively.
Feng et al. [44] noted high N and P removal in SBRs fed with food waste as the carbon source, with removal efficiencies of 92.70% and 92.38%, respectively. However, this study used influent with lower levels of phosphorus (approx. 5.36–6.19 mg PO4-P/L) and preceded the anoxic phase with an aerobic phase, which was responsible for the bulk of the P removal. A different study, by Wu et al. [45], explored the effects of acetate and propionate on nitrogen and phosphorus removal during a A2O treatment. It was found that the choice of carbon source had a considerable impact on denitrifying P removal in anoxic chambers, with 40.0% P removed in the acetate variant and 37.7% in the propionate variant. The experimental parameters were as follows: initial P = 8.75 mg/L, temperature = 21 °C, HRT = 8 h, and pH = 7.24–7.82. The lower TP removal efficiency in our study may be influenced by the much higher initial TP concentration in the influent wastewater.
Sharrer et al. [46] aimed to determine whether woodchips can release phosphorus through leaching in distilled water and aquaculture effluent. Phosphorus leaching was noted during the first 24 h (approx. 20–30 mg P per kg wood). Tests showed that woodchips can remove total phosphorus (15–54% removal efficiency; TP in the influent ≈ 4 mg/L) after initial leaching and successive drying cycles. The present study (which employed denitrifying P removal under anoxic conditions) did not detect any extra TP leaching from the woodchips.
Zhou et al. [47] found that nitrates in the anoxic zone can inhibit phosphorus release until denitrification is finished. Hu et al. [48] characterized P-removing denitrifying bacteria under anaerobic-anoxic conditions. This particular study used synthetic waste containing 20 mg P/L. Oxygen, nitrites, and nitrates served as electron acceptors. The study found that nitrites did not inhibit phosphorus removal, and instead acted as an alternative electron acceptor for oxygen and nitrates (maximal inhibitory concentration >115 mg NO2-N/L). This seems to suggest some groups of bacteria may be able to use both oxygen, nitrites, and nitrates as electron acceptors. Our study did not include a microbiological analysis, and thus we can only assume that the presence of NO2-N did not negatively impact P removal.

4. Conclusions

The present study investigated solid external carbon sources—Oxytree chips, beech chips, and biobased polybutylene succinate derivatives (BioPBS FZ91PB and BioPBS FD92PB)—as biofilter media for treating greenhouse effluent. The highest N and P removal efficiencies were noted for BioPBS FD92PB (PBS2) at 43.11% and 42.48%, respectively. Oxytree chips performed the worst (8.80 ± 4.88%) N removal efficiency. Beech chips and PBS1 (FZ91PB) had similar denitrification efficiencies at 14.10 ± 9.09% and 15.55 ± 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 ± 10.3%) and Oxytree (31.14 ± 6.17%). Beech chips and PBS1 provided similar removal efficiencies at 25.23 ± 6.82% and 26.42 ± 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.

Author Contributions

Conceptualization, J.R. and W.J.; methodology, K.K. and K.Ł.B.; validation, K.J., P.B., and A.O.; formal analysis, J.R. and A.M.; investigation, K.K., A.M., and K.Ł.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.

Funding

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.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available in the article.

Acknowledgments

Special thanks to Mitsubishi Corporation for providing the biobased polybutylene succinate (PBS) derivatives that made this study possible.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Scheme of the experimental model (V—volume of the reactor, B—filling of biofilters, VpB, VpB/0.5—volume of wastewater).
Figure 1. Scheme of the experimental model (V—volume of the reactor, B—filling of biofilters, VpB, VpB/0.5—volume of wastewater).
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Figure 2. Nitrate concentration in influent and treated wastewater.
Figure 2. Nitrate concentration in influent and treated wastewater.
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Figure 3. Nitrate removal efficiency according to the carbon source used.
Figure 3. Nitrate removal efficiency according to the carbon source used.
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Figure 4. COD/N-NO3 ratio at each treatment stage.
Figure 4. COD/N-NO3 ratio at each treatment stage.
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Figure 5. Variation in TOC concentration in different stages of treatment.
Figure 5. Variation in TOC concentration in different stages of treatment.
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Figure 6. Variation in COD concentration in different stages of treatment.
Figure 6. Variation in COD concentration in different stages of treatment.
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Figure 7. Variation in DO concentration in different stages of treatment.
Figure 7. Variation in DO concentration in different stages of treatment.
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Figure 8. Concentration of total phosphorus in influent and treated wastewater.
Figure 8. Concentration of total phosphorus in influent and treated wastewater.
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Figure 9. Total phosphorus removal efficiency according to the carbon source used.
Figure 9. Total phosphorus removal efficiency according to the carbon source used.
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Table 1. Physical and chemical properties of wastewater from soilless tomato cultivation (number of data points n = 10).
Table 1. Physical and chemical properties of wastewater from soilless tomato cultivation (number of data points n = 10).
IndicatorsArithmetic MeanMinimum ValueMaximum ValueStandard
Deviation
pH6.376.206.630.13
DO (dissolved oxygen) [mg O2/L]8.948.599.150.18
Temperature [°C]22.2021.0023.500.77
EC (electrical conductivity) [mS/cm]6.7076.4017.3910.279
TOC (total organic carbon) [mg/L]10.020.7516.624.45
COD [mg O2/L]34.9625.6049.207.91
Nitrates [mg N/L]710.0618.0920.076.3
Nitrites [mg N/L]0.0420.0200.0760.019
Ammonium nitrogen [mg N/L]0.1150.0280.4460.122
Total nitrogen
[mg N/L]		721.1643.0763.836.2
Total phosphorus
[mg P/L]		43.7934.4057.807.62
Table 2. Average temperature and pH values depending on the carbon source used.
Table 2. Average temperature and pH values depending on the carbon source used.
Temperature [°C]pH
Arithmetic MeanMinimum ValueMaximum ValueStandard DeviationArithmetic MeanMinimum ValueMaximum ValueStandard Deviation
Influent22.2021.0023.500.776.376.206.630.13
Beech wood22.5120.8024.000.957.286.567.980.37
Oxytree chips22.4620.8023.800.887.226.568.170.45
PBS 122.4920.9023.900.907.066.037.980.65
PBS 222.6520.9024.100.937.497.067.910.24
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Kłobukowska, K.; Rodziewicz, J.; Mielcarek, A.; Bryszewski, K.Ł.; Janczukowicz, W.; Bugajski, P.; Jóźwiakowski, K.; Operacz, A. Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters. Appl. Sci. 2024, 14, 176. https://doi.org/10.3390/app14010176

AMA Style

Kłobukowska K, Rodziewicz J, Mielcarek A, Bryszewski KŁ, Janczukowicz W, Bugajski P, Jóźwiakowski K, Operacz A. Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters. Applied Sciences. 2024; 14(1):176. https://doi.org/10.3390/app14010176

Chicago/Turabian Style

Kłobukowska, Karolina, Joanna Rodziewicz, Artur Mielcarek, Kamil Łukasz Bryszewski, Wojciech Janczukowicz, Piotr Bugajski, Krzysztof Jóźwiakowski, and Agnieszka Operacz. 2024. "Novel Materials as Exogenous Carbon Sources for Denitrifying Biofilters" Applied Sciences 14, no. 1: 176. https://doi.org/10.3390/app14010176

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