**About the Editors**

**Wojciech Janczukowicz** (Professor) Wojciech Janczukowicz is a Professor at the Chair of Environment Engineering, the Faculty of Geoengineering of the University of Warmia and Mazury in Olsztyn, Poland. His scientific interests are focused on wastewater treatment using reactors with attached biomass, treatment of wastewater from agriculture and the agri-food industry, the use of an external carbon source in the processes of dephosphatization and denitrification in reactors with suspended and fixed biomass, the impact of projects on the environment, environmental management systems. During his scientific career, Wojciech Janczukowicz was a Fulbright Academic Exchange Program participant. He completed a 3-month Preacademic Course at the Texas University in Austin, USA and a 9-month study of Advanced Technologies in Biological Wastewater Treatment at the New Jersey Institute of Technology in Newark, N.J., USA (1988/1989). Prof. Wojciech Janczukowicz was also a visiting scientist at University of Sunderland, Great Britain (1-month scholarship at Ecology Centre. 1993; 3-week scholarship at Ecology Centre. 1994). He is a member of the Polish Association of Sanitary Engineers and Technicians.

**Joanna Rodziewicz** (University Professor) Joanna Rodziewicz is a University Professor at the Chair of Environment Engineering, the Faculty of Geoengineering of the University of Warmia and Mazury in Olsztyn, Poland. Her scientific interests are focused on wastewater treatment technologies based on attached and suspended biomass application. Her investigations are about biological, electrochemical and electrobiological wastewater treatment in aerobic and anaerobic biofilm reactors (rotating disc contactors (RBC), bio-electrical reactors (BER and REBC) and aerobic and anaerobic reactors with biofilm and suspended biomass (sequencing batch biofilm reactor (SBBR), anaerobic rotating disc batch reactor (ARDBR), bio-elechtrochemical SBBR (BSBBR). These studies concern reactors' applications for phosphorus and nitrogen removal from specific wastewater including dairy, aquaculture, soilless plant cultivation. and de-icing airport pavements. The other area of her research relates to quantitative and qualitative characteristics of sludges formed during specific wastewater treatment in biofilm reactors. Prof. Joanna Rodziewicz works also on biofilters with innovative filling for low-temperature treatment of sewage from de-icing airport runways. Joanna Rodziewicz was a participant in the training program PHARE Partnership in AMU Center Aalborg, Denmark, 2000. She is a member of the Polish Association of Environmental and Resource Economists.

**Anna Iwaniak** (Professor) Anna Iwaniak is a Professor at the Chair of Food Biochemistry, the Faculty of Food Science of the University of Warmia and Mazury in Olsztyn. Her scientific interests are focused on the analysis of peptides derived from food proteins, mainly searching and co-developing new tools for bioinformatic analysis useful in the studies on bioactive and functional peptides. This area is related to developing BIOPEP-UWM database of protein and bioactive peptide sequences. During her scientific career, Anna Iwaniak was awarded Marie Curie scholarship for a 3-month scientific visit at the European Bioinformatics Institute in Hinxton, Cambridgeshire (Great Britain; 01 December 2001 – 01 March 2002). In 2005, she was a post-doc at the University of British Columbia, Faculty of Food and Land Systems, Vancouver (Canada). In 2009, she was a visiting scientist at the Institute of Chemistry, Faculty of Science and Technology, University of Tartu (Estonia). Prof. Anna Iwaniak was also a visiting scientist at University of Porto (Portugal) as well as Agricultural University of Athens in Greece. She is a member of: the European Peptide Society (EPS), the International Society for Nutraceuticals and Functional Foods (ISNFF), and the Polish Society of Food Technologists (in Polish: PTTZ). ˙

### *Editorial* **New Trends in Environmental Engineering, Agriculture, Food Production, and Analysis**

**Anna Iwaniak 1, Wojciech Janczukowicz <sup>2</sup> and Joanna Rodziewicz 2,\***


Modern agriculture and aquaculture, as well as related food processing, are associated with a significant use of environmental resources and a growing impact on the natural environment. Research is being carried out on the use of modern technologies of plant breeding, animal husbandry, sustainable water, energy, sewage and waste management in food production and processing, as well as new technologies for wastewater treatment and waste disposal, in order to protect the natural environment.

This Special Issue presents the latest advances in agriculture, aquaculture, food technology, and environmental engineering, discussing, among others, the following issues: New technologies in water and wastewater treatment; new sludge and waste management systems; the role of technological processes to improve food quality and safety; new trends in the analysis of food and food components including in vitro, in vivo, and in silico methods; and functional and structural aspects of bioactivities of food molecules.

This book includes a series of twenty one research studies that reveal new knowledge about environmental engineering, agriculture, and food protection. The topics covered span many diverse areas including: Environmental engineering [1–8], agriculture, food properties and protection [9–17], and aquaculture [18–21].

**Author Contributions:** All authors contributed equally to the preparation of this manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** Project financially supported by Minister of Science and Higher Education in the range of the program entitled "Regional Initiative of Excellence" for the years 2019–2022, project No. 010/RID/2018/19, amount funding 12.000.000 PLN.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This publication was only possible with the invaluable contributions from the authors, reviewers, and the editorial team of *Applied Sciences*.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Citation:** Iwaniak, A.; Janczukowicz, W.; Rodziewicz, J. New Trends in Environmental Engineering, Agriculture, Food Production, and Analysis. *Appl. Sci.* **2021**, *11*, 2745. https://doi.org/10.3390/app11062745

Received: 8 March 2021 Accepted: 15 March 2021 Published: 18 March 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

### **References**


### *Article* **Efficiency and Technological Reliability of Contaminant Removal in Household WWTPs with Activated Sludge**

**Agnieszka Micek 1, Krzysztof Józwiakowski ´ 1,\*, Michał Marzec 1, Agnieszka Listosz <sup>1</sup> and Tadeusz Grabowski 1,2**


**\*** Correspondence: krzysztof.jozwiakowski@up.lublin.pl

**Abstract:** The results of research on the efficiency and technological reliability of domestic wastewater purification in two household wastewater treatment plants (WWTPs) with activated sludge are presented in this paper. The studied facilities were located in the territory of the Roztocze National Park (Poland). The mean wastewater flow rate in the WWTPs was 1.0 and 1.6 m3/day. In 2017–2019, 20 series of analyses were done, and 40 wastewater samples were taken. On the basis of the received results, the efficiency of basic pollutant removal was determined. The technological reliability of the tested facilities was specified using the Weibull method. The average removal efficiencies for the biochemical oxygen demand in 5 days (BOD5) and chemical oxygen demand (COD) were 66–83% and 62–65%, respectively. Much lower effects were obtained for total suspended solids (TSS) and amounted to 17–48%, while the efficiency of total phosphorus (TP) and total nitrogen (TN) removal did not exceed 34%. The analyzed systems were characterized by the reliability of TSS, BOD5, and COD removal at the level of 76–96%. However, the reliability of TN and TP elimination was less than 5%. Thus, in the case of biogenic compounds, the analyzed systems did not guarantee that the quality of treated wastewater would meet the requirements of the Polish law during any period of operation. This disqualifies the discussed technological solution in terms of its wide application in protected areas and near lakes, where the requirements for nitrogen and phosphorus removal are high.

**Keywords:** efficiency of contaminant removal; technological reliability; wastewater purification; activated sludge; national park

### **1. Introduction**

In national parks and protected areas, there are usually museums, forester's lodges, hostels, or tourist trails with resting places for visitors, which should be equipped with sanitary infrastructure that ensures their proper functioning. According to the Law on Nature Protection [1] in Poland and the Council Directive 92/43/EEC [2], in the area of national parks and nature reserves, it is forbidden to build or reconstruct any buildings or technical facilities with the exception of facilities and devices that serve to achieve the goals of the given national park or nature reserve. For this reason, it is essential in protected areas to use water supply systems and wastewater treatment plants (WWTPs) that do not interfere with the environment [3,4] and meet the criteria of sustainable development and nature protection [5–7].

Domestic wastewater generated by various tourist facilities in national parks or protected areas, just as in rural areas, is most often discharged to non-return tanks (septic tanks) and is then taken to collective WWTPs or disposed of in individual wastewater treatment systems, i.e., in so-called small household WWTPs [8,9]. A similar way of dealing with domestic sewage in protected and rural areas, where sewerage systems and collective treatment plants are lacking, is also in force in other countries around the world [10–19].

**Citation:** Micek, A.; Jó ´zwiakowski, K.; Marzec, M.; Listosz, A.; Grabowski, T. Efficiency and Technological Reliability of Contaminant Removal in Household WWTPs with Activated Sludge. *Appl. Sci.* **2021**, *11*, 1889. https://doi.org/ 10.3390/app11041889

Received: 29 January 2021 Accepted: 17 February 2021 Published: 21 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

In recent years, household WWTPs have increasingly become one of the basic elements of technical infrastructure in protected areas where, due to natural and landscape values and large dispersion of tourist buildings, the installation of a sewerage network and a collective WWTP is not justified [9,13,14,17,19]. In accordance with Polish Standard PN-EN 12,566 [20], household WWTPs are defined as facilities for 50 inhabitants. However, on the basis to the Water Law [21], the maximum capacity of such systems in Poland should not exceed 5 m3/day.

Various technological solutions are currently used to treat small amounts of wastewater, such as systems with drainage pipes, systems with a sand filter, WWTPs with activated sludge, systems with a biological bed, hybrid systems (activated sludge + biological bed), and constructed wetland systems (CWs) [7,22].

A review of the literature on the evolution of wastewater management and its development over the centuries was presented by Lofrano and Brown [23]. Currently, the activated sludge method is the most commonly used for wastewater purification in the world. Activated sludge is most often used for urban wastewater treatment [24–27], as well as for industrial wastewater [28,29]. The popularity of the activated sludge method used in large WWTPs around the world has led to the development of "miniature" facilities of this type, whereby for over 30 years, attempts have been made to replicate the technological processes [24,30–35]. However, it is important for household WWTPs with activated sludge, which are used to treat small sewage amounts, to meet the appropriate criteria.

The most important aspects that should be considered during the selection of a technological solution involving small WWTPs are the efficiency of pollutant removal and the reliability of operation [4]. These criteria should be taken into account especially for WWTPs installed in protected areas [36]. In the case of WWTPs, efficiency refers to the degree of removal of particular types of pollutants, which is determined by the amount of pollutants retained in the system in relation to the amount of pollutants entering the system. Reliability, on the other hand, is defined as the ability to treat wastewater to the degree required by the wastewater receiver over the assumed operating time, including changes in the quantity and the composition of the inflow [32,33]. The reliability level corresponds to the probability of reaching a value of the indicator in the outflow from the WWTPs that is lower than the acceptable value; thus, reliability can be understood as the percentage of time during which the expected concentrations of pollutants in the treated wastewater are in accordance with the accepted standards or purification objectives [37,38].

The reliability and operational efficiency of activated sludge WWTPs have been previously studied by different authors [32,33,35,37,39–41]. However, there is still a lack of research results on the technological reliability of household WWTPs with activated sludge, which are analyzed over a longer period of time, especially in facilities operating in protected areas. A comprehensive analysis of the efficiency of operation of different types of WWTPs based on statistical inference and taking into account elements of reliability theory makes it possible to identify technological solutions which are characterized by the highest efficiency and stability of operation under changing conditions during many years of operation. This is important from the administrative, legal, and ecological point of view. It allows estimating the chances of passing possible control procedures, as well as establishing a hierarchy of particular technological solutions in terms of their influence on the environment. The results regarding the efficiency and reliability of household WWTP technologies should be an important element in planning the development of technical infrastructure, enabling the selection of optimal solutions under given conditions [32].

The purpose of this paper is to present the results of research on the technological reliability and the efficiency of domestic wastewater purification in two household WWTPs with activated sludge located in the area of the Roztocze National Park (RNP) in Poland. The paper contributes new content to science, because, in the world, there remain few studies related to the technological reliability of household WWTPs operating on a real scale.

### **2. Materials and Methods**

### *2.1. Presentation of the Studied Facilities*

The RNP is located in southeastern Poland in a temperate, transitional climate zone. Groundwater, as well as surface water, in the RNP is of a very good quality; thus, it is crucial to protect it from degradation [42]. In order to protect the quality of surface water and groundwater, in recent years, within the area of the RNP, some steps have been taken to build household WWTPs to treat wastewater outflowing from foresters' lodges.

For the analysis, two household WWTPs located in the area of the RNP were selected. The studied facilities consist of a four-chamber preliminary settling tank and a special reactor with activated sludge. They are located in Obrocz for the office building (facility no. 1) and Rybakówka for the forester's lodge (facility no. 2). The exact location of these facilities in the RNP was presented by Micek et al. [9]. The mean wastewater flow rate in the studied WWTPs was 1.0 and 1.6 m3/day, respectively. In Table 1, chosen technological parameters of the studied facilities are presented, and Figure 1 shows their technological scheme. The efficiency of pollutant removal in preliminary settling tanks, which are the first components of the selected household WWTPs in the RPN, was presented by Micek et al. [9].

**Table 1.** Technological parameters of the household WWTPs in the area of the RNP.


**Figure 1.** Technological scheme of the household wastewater treatment plants (WWTPs) in the area of the Roztocze National Park (RNP) (prepared on the basis of [43]). Scheme: 1—denitrification chamber; 2—nitrification chamber; 3—separation chamber; 4—wastewater inlet; 5—tube diffuser; 6—basket grate; 7—wastewater outlet; 8—sludge outlet; 9—air distributor; 10—sludge recirculation; 11—air supply; 12—sludge recirculation; 13—air for sludge recirculation; 14—concrete foundation.

### *2.2. Analytical and Statistical Methods*

Studies on the technological reliability and the efficiency of pollutant removal in the two chosen facilities were performed in 2017–2019. Wastewater samples for analyses were taken in different seasons (spring, summer, autumn, and winter) from (I) the four-chamber of the preliminary settling tank—after mechanical treatment, and (II) the outflow from an activated sludge chamber—after biological treatment (Figure 1).

During the study, 20 series of analyses were done and 40 wastewater samples were taken, in which parameters such as total suspended solids (TSS), biochemical oxygen demand in 5 days (BOD5), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP), pH, dissolved oxygen (DO), nitrate nitrogen, nitrite nitrogen, and ammonium nitrogen were determined. Sampling, sample transportation, processing, and analyses were completed on the basis of Polish Standards of Wastewater Examination, which are compatible with the American Public Health Association—APHA [44,45]. The laboratory apparatus used to carry out the analyses was presented in another paper published by Micek et al. [9].

The obtained measurement data enabled calculating the mean, minimum, and maximum concentration of pollutant values and their standard deviation. The mean concentrations of the analyzed pollutant parameters in the influent and effluent from the WWTPs were used to determine the efficiency of pollutant removal (Figure 2).

**Figure 2.** Average efficiency of pollutant removal in two studied household WWTPs.

The evaluation of the technological reliability of the WWTPs was carried out using elements of Weibull's reliability theory, which is a useful tool in assessing the risk of exceeding the normative values in wastewater discharged to the receiver [46].

The Weibull distribution is characterized by the following probability density function [46]:

$$f\left(\mathbf{x}\right) = \frac{c}{b} \cdot \frac{\mathbf{x} - \theta}{b}^{\left(\varepsilon - 1\right)} \cdot e^{-\left(\frac{\mathbf{x} - \theta}{b}\right)^{c}},\tag{1}$$

where *x* is a variable describing the concentration of a pollution parameter in the treated effluent, *b* is a scale parameter, *c* is a shape parameter, *θ* is a position parameter, and *e* is a constant, assuming *θ* < *x*, *b* > 0, *c* > 0, and *e* = 2.71828.

A variable specifying the values of basic pollution indicators (TSS, BOD5, COD, TN, and TP) in treated wastewater (*n* = 20) was analyzed. The analysis consisted of the estimation of the Weibull distribution parameters using the maximum-likelihood method and the verification of the null hypothesis that the analyzed variable could be

described by the Weibull distribution. The null hypothesis was verified with the Hollander– Proschan test at the significance level of 0.05 [36]. Reliability was determined from the cumulative distribution function plotted in the graphs, taking into account the normative values of the indicators specified in the Polish regulations [47] for wastewater discharged from treatment plants of up to 2000 PE (population equivalent): BOD5—40 mg O2·dm−3, COD—150 mg O2·dm<sup>−</sup>3, TSS—50 mg·dm−3, TN—30 mg·dm−3, and TP—5 mg·dm−3. In the case of TN and TP, the values defined for wastewater discharged into lakes and their tributaries, as well as directly into artificial water reservoirs located in flowing waters, were adopted as standard values [47]. The analysis was carried out using Statistica 13.

### **3. Results and Discussion**

### *3.1. The Efficiency of Pollutant Removal*

The chosen statistical values of pollutants in wastewater from the studied household WWTPs are presented in Tables 2 and 3. The concentrations of pollutants in wastewater inflowing to the chambers with activated sludge (after mechanical treatment) were relevantly lower in comparison to raw wastewater flowing into the preliminary settling tanks (the first element of the system), as described in an earlier paper [9]. The values of pollutant concentrations in wastewater inflowing to the activated sludge chambers in the studied WWTPs were close to those described in the literature for wastewater treated mechanically in the preliminary settling tanks [34,35,46,48–50].

**Table 2.** Pollutant concentrations in the inflow and outflow of facility no. 1. TSS, total suspended solids; BOD5, biochemical oxygen demand in 5 days; COD, chemical oxygen demand; TN, total nitrogen; TP, total phosphorus.


**TSS** is a measure of the floating solid content in wastewater, which indicates its clarity [51,52]. At the researched facilities no. 1 and 2, the removal efficiency of TSS was low and amounted to 17% and 48%, respectively (Figure 2). The low efficiency of TSS removal in the analyzed systems is due to the fact that a significant proportion (>60%) is removed in the preliminary settling tanks, which constitutes the first elements of the studied WWTPs [9]. Better results for TSS removal (58–94%) were found by Marzec and Józwiakowski [ ´ 31] in three facilities with activated sludge but without preliminary settling tanks. Quite high TSS removal effects (90–96%) were also obtained by Jakubaszek and Stadnik [53] in household WWTPs operating with low activated sludge technology in a sequential batch reactor (SBR) system. A study conducted on two household hybrid CW WWTPs of VF/HF type (with vertical and horizontal flow) with common reed and willow, operating in the RNP, showed that they also provide quite high efficiency (80–87%) of TSS

removal [36]. Furthermore, Marzec et al. [49], in a CW with common reed, manna grass, and Virginia mallow, obtained an efficiency of TSS removal of more than 86%.


**Table 3.** Pollutants concentrations in the inflow and outflow of the facility no. 2.

The conducted study shows that the average concentrations of TSS in outflow from the analyzed WWTPs were 23.9 mg/dm<sup>3</sup> for facility no. 1 and 18.1 mg/dm3 for facility no. 2 (Tables 2 and 3). These values are lower than the permissible value (50 mg/dm3) determined in Polish regulations [47]. However, Marzec and Józwiakowski [ ´ 31] found significantly higher concentrations of TSS (55–122 mg/dm3) in the outflow from three other facilities with activated sludge but without preliminary settling tanks.

**BOD5.** The efficiency of BOD5 removal in the studied facilities no. 1 and 2 was diverse and amounted to 83% and 66%, respectively (Figure 2). Similar effects of BOD5 removal (61–95%) were found by Marzec and Józwiakowski [ ´ 31] in three other facilities with activated sludge but without preliminary settling tanks. Quite high BOD5 removal effects (92–97%) were also obtained by Jakubaszek and Stadnik [53] in household WWTPs operating with an SBR system. Very high (98–99%) BOD5 removal efficiency was also obtained in two hybrid household CWs of VF/HF type operating in the RNP [36]. Moreover, Marzec et al. [49] in a household CW WWTP obtained an efficiency of BOD5 removal greater than 95%.

The average BOD5 in outflow from the studied facilities no. 1 and 2 was 9.0 and 29.1 mg/dm3, respectively (Tables 2 and 3). These results are lower than the permissible value (40 mg/dm3) specified in Polish regulations [47]. Significantly higher BOD5 values (24.9 to 267 mg/dm3) were found by Marzec and Józwiakowski [ ´ 31] in three WWTPs with activated sludge operating without a classical mechanical stage.

**COD.** At the studied facilities no. 1 and 2, the COD removal efficiency was similar and amounted to 65% and 62%, respectively (Figure 2). Higher effects of COD removal (59–90%) were found by Marzec and Józwiakowski [ ´ 31] in three other facilities with activated sludge without preliminary settling tanks. Similarly high COD removal effects (83–90%) were also obtained by Jakubaszek and Stadnik [53] in individual SBR systems. A study of two hybrid household CW WWTPs in the RNP showed that they also provide very high (96%) COD removal efficiency [36]. In a household hybrid CW, Marzec et al. [49] also obtained more than 95% COD removal efficiency.

The average COD values in the outflow from the analyzed facilities no. 1 and 2 were 63.0 and 95.0 mg/dm3, respectively (Tables 2 and 3). These results are lower than the permissible value (150 mg/dm3) specified in Polish regulations [47]. Marzec and Józwiakowski [ ´ 31] studied some household WWTPs of similar construction to the analyzed

facilities but lacking classical preliminary settling tanks, and they found higher COD values which amounted to 128–490 mg/dm3.

**Total Nitrogen.** In the analyzed facilities no. 1 and 2, the efficiency of TN removal was low and amounted to 21% and 34%, respectively (Figure 2). Much lower effects of TN removal (<7%) were found by Marzec and Józwiakowski [ ´ 31] in three other facilities with activated sludge without preliminary settling tanks. Significantly higher effects of TN removal (51–83%) were obtained by Jakubaszek and Stadnik [53] in household WWTPs operating with SBR systems. High TN removal effects (73–86%) were also obtained by Micek et al. [36] in hybrid household CW WWTPs of VF/HF type in the RNP. Furthermore, Marzec et al. [49], in a household hybrid CW, obtained more than 86% efficiency of TN removal.

The average values of TN concentration in the outflow from the analyzed facilities no. 1 and 2 were high and amounted to 127 and 77 mg/dm3, respectively (Tables 2 and 3). These values are several times higher than the permissible limit (30 mg/dm3) required in Poland for wastewater discharged into lakes and their tributaries [47]. Even higher concentrations of TN (124–320 mg/dm3) were previously found by Marzec and Józwiakowski [ ´ 31] in the outflow from three other facilities with activated sludge but without classical preliminary settling tanks.

The efficiency of TN removal in activated sludge systems depends primarily on the course of processes such as nitrification or denitrification, among others [54,55]. From the data presented in Tables 2 and 3, it can be seen that the nitrification process in the studied facilities proceeded properly, which is evidenced by a significant decrease in the concentration of ammonium nitrogen and an increase in the content of nitrate and nitrite nitrogen, as well as the concentration of oxygen in the treated wastewater. However, high concentrations of aerobic forms of nitrogen and TN in the outflow from the analyzed systems indicate that the household WWTPs with activated sludge are not able to create appropriate conditions for the denitrification process.

**Total Phosphorus.** In the studied facilities no. 1 and 2, the efficiency of TP removal was low and amounted to 4% and 30%, respectively (Figure 2). Marzec and Józwiakowski [ ´ 31] found higher TP removal efficiencies (3–63%) in three other activated sludge facilities without preliminary settling tanks. Higher efficiencies of TP removal (46–74%) were obtained by Jakubaszek and Stadnik [53] in household WWTPs with SBR systems. A study of two household hybrid CWs of VF/HF type operating in the RNP showed that they provide significantly higher efficiency of TP removal (90–94%) than activated sludge systems [36]. Moreover, Marzec et al. [49] achieved high TP removal efficiency (over 95%) in a household hybrid CW.

The average values of TP in the outflow from the analyzed facilities no. 1 and 2 were very high and amounted to 11.5 and 12.1 mg/dm3, respectively (Tables 2 and 3). These values are more than two times higher than the permissible value (5 mg/dm3) required in Poland for wastewater discharged into lakes and their tributaries [47]. In the outflow from three other facilities with activated sludge but without preliminary settling tanks, Marzec and Józwiakowski [ ´ 31] found even higher of TP concentrations in the outflow (23.2–50.6 mg/dm3).

In the process occurring in activated sludge, phosphorus is mainly removed from wastewater via assimilation, sorption, and chemical precipitation [55]. During wastewater treatment, phosphorus is assimilated by the growing biomass and should be removed with the excess sludge. Effective biological phosphorus removal requires alternating aerobic and anaerobic conditions to allow the selection and growth of specific microorganisms that exhibit the ability to store phosphorus compounds within cells [56,57]. However, the conducted studies and observations show that the analyzed activated sludge WWTPs lack an anaerobic zone and the sludge from the secondary settling tank is not regularly removed, which probably has a negative impact on the effective removal of phosphorus from wastewater and causes its high concentrations in the outflow.

On the basis of the obtained research results and the literature review, it is concluded that household WWTPs with activated sludge provide significantly lower effects of pollutant removal than hybrid CWs, especially in terms of nutrient removal. Therefore, the authors of this paper do not recommend the widespread use of household WWTPs with activated sludge in protected areas.

### *3.2. Technological Reliability of the Studied Systems*

The reliability of the analyzed WWTPs was determined using the Weibull method. In the first step, the distribution parameters were estimated, and the null hypothesis that empirical data can be described by a Weibull distribution was verified. The datasets were the values of the main pollutant indicators (BOD5, COD, TSS, TN, and TP) in treated wastewater.

The determined values of the distribution parameters (*θ, b, c*) were consistent with the assumptions made. The null hypothesis was positively verified. The goodness of fit of the obtained distributions at the significance level of α = 0.05 was high and was 68–98% for facility no. 1 and 51–99% for facility no. 2 (Table 4).

**Table 4.** Parameters of the Weibull distribution and the Hollander–Proschan goodness-of-fit test (*n* = 20).


Symbols: stat—value of the test statistic, *p*—significance level of the test; when *p* ≤ 0.05, the distribution of data does not obey a Weibull distribution.

The technological reliability of the studied WWTPs was determined on the basis of the distribution function (Figures 3–7), taking into account the limit values of the indicators, specified in the Regulation of the Minister of Maritime Economy and Inland Navigation [47] for WWTPs below 2000 PE.

**Total suspended solids.** The reliability of TSS removal in facility no. 1 was 86% (Figure 3A). On this basis, it can be concluded that, for a period of 51 days per year, the treatment plant malfunctioned. During this period, the concentration of TSS in treated wastewater exceeded the limit value (50 mg/dm3). According to Andraka and Dzienis [58], for WWTPs below 2000 PE, the minimum reliability level should be 97.3%. This means that a treatment plant of this size can malfunction for 9 days per year without adversely affecting the rating of the facility. Taking these assumptions into account, it can be concluded that, in facility no. 1, the concentration of TSS in the outflow from the analyzed WWTP was excessive for 42 days per year.

**Figure 3.** Weibull cumulative distribution functions and the technological reliabilities determined for TSS ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation: dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent.

**Figure 4.** Weibull cumulative distribution functions and the technological reliabilities determined for BOD5 ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation:dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent.

**Figure 5.** Weibull cumulative distribution functions and the technological reliabilities determined for COD ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation:dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent.

**Figure 6.** Weibull cumulative distribution functions and the technological reliabilities determined for TN ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation:dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent.

**Figure 7.** Weibull cumulative distribution functions and the technological reliabilities determined for TP ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation: dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent.

In facility no. 2, the reliability of TSS removal was 95% (Figure 3B), indicating faulty operation of the WWTP for 19 days per year. Taking into account the previously mentioned guidelines [58], it may be concluded that the level of TSS negatively influenced the facility assessment for 10 days per year. Lower reliability levels for the analyzed indicator (about 65%) were found in some studies concerning household WWTP with activated sludge conducted by Bugajski et al. [59] and Marzec [32]. However, 100% technological reliability of TSS removal in a small sequencing batch biofilm reactor (SBBR) with activated sludge was obtained by Jucherski et al. [35]. Research on two other household hybrid CW WWTPs of VF/HF type (with vertical and horizontal flow) operating in the RNP indicated that they also provide quite high (92–100%) reliability of TSS removal [36]. Additionally, Marzec et al. [33] obtained 100% reliability of TSS removal in a household CW. On the basis of the data obtained and the literature review, it appears that activated sludge systems have lower reliability of TSS removal than CW WWTPs.

**BOD5.** The technological reliability of BOD5 removal in facility no. 1 was 95% and that in facility no. 2 was 76% (Figure 4). In facility no. 1, the level of this indicator was higher than the permissible value for 10 days a year. Similarly, in facility no. 2, the level of BOD5 was elevated for 78 days a year.

In comparison, in a WWTP of identical design but without a preliminary settling tank, the reliability of BOD5 reduction was 70% [32]. On the other hand, Bugajski et al. [59] in a household system Biocompact BCT S-12 determined the reliability of reducing this

indicator at the level of 88%. The BOD5 removal reliability in a small SBBR was 77% [35]. On the other hand, maximum (100%) reliability of BOD5 removal was reported in two other household hybrid CW WWTPs in the RNP [36]. Moreover, Marzec et al. [33] achieved 100% reliability of BOD5 removal in a household hybrid CW.

**COD.** The reliability of COD removal was 96% in facility no. 1 and 87% in facility no. 2 (Figure 5). The obtained reliability levels are lower than the minimum required values given by Andraka and Dzienis [58]. In relation to the annual reference period, WWTP no. 1 provided the required level of organic pollutant removal expressed by COD for 359 days, while this value for WWTP no. 2 was 326 days.

The reliability indicators obtained in the analyzed activated sludge facilities in the RPN for COD are comparable to those obtained by Bugajski et al. [59] in a household WWTP working in activated sludge technology. At the same time, in the facilities in the RPN, the reliability of COD removal was higher than that found in a treatment plant based on an identical tank design but without a preliminary settling tank (60%) [32]. The reliability of COD removal in a small SBBR was 97.8% [35]. However, similarly to the case of BOD5, maximum (100%) COD removal reliability was reported in two other household hybrid CWs WWTPs operating in the RNP [36]. Moreover, Marzec et al. [33] achieved 100% COD removal reliability in a household hybrid CW. The presented results indicate that the reliability of COD removal in household WWTPs with activated sludge is lower than that in CWs.

**Total nitrogen.** Both analyzed activated sludge WWTPs were characterized by extremely low reliability of TN removal. The probability that the concentration of TN in treated wastewater will not exceed the standard value (30 mg/dm3) was 0% for facility no. 1 and 5% for facility no. 2 (Figure 6).

In the case of TN removal, facility no. 1 operated deficiently throughout the whole year and provided no assurance of passing inspection procedures. On the other hand, in WWTP no. 2, above-normative values of TN in treated wastewater occurred for a period of 338 days per year. Marzec [33], while analyzing a solution identical in terms of construction, but without a classical preliminary settling tank, obtained a reliability of TN removal at the level of 24%. The reliability of TN removal in a small SBBR was only 12.2% [35]. In contrast, TN removal reliability in two other household hybrid CW WWTPs operating in the RNP was much higher and amounted to 35% and 89% [36]. On the other hand, Marzec et al. [33] obtained 94% reliability of TN removal in a household hybrid CW. The analysis of the results of the conducted study through the prism of the results reported in the literature shows that the reliability of TN removal is lower in household WWTPs with activated sludge than in CWs.

**Total phosphorus.** The reliability of TP removal in the tested household WWTPs with activated sludge was even lower. The probability that the content of TP in the effluent from facility no. 1 will meet the permissible value at the operator risk of α = 0.05 was 1%. For facility no. 2, this value was determined to be 0% (Figure 7).

During the research concerning the effluent from the analyzed facilities, no cases were observed when the values were below the permissible level 5 mg/dm3 [47]. The obtained indicators allow concluding that household WWTPs with activated sludge are faulty in terms of TP removal throughout the year. The obtained results do not give a chance for a positive assessment of the facilities regarding the content of TP in treated sewage. In a similar facility, deprived of a classical mechanical treatment stage, a low technological reliability at the level of 5% was found [33]. On the other hand, the reliability of TN removal in a small SBBR was only 21.7% [35]. In contrast, a study conducted by Micek et al. [36] showed that TP removal reliability in two other household hybrid CW WWTPs operating in the RNP was much higher and amounted to 87% and 100%. Moreover, Marzec et al. [33] in a hybrid household CW achieved 100% reliability of TP removal. Similarly to other indicators, the reliability of TP removal in household WWTPs with activated sludge was lower than in CWs.

### **4. Conclusions**

The analyzed household WWTPs with activated sludge demonstrated relatively high levels of technological reliability and efficiency regarding the removal of pollutants in terms of TSS, BOD5, and COD. It is worth noting that the technological reliability of the analyzed systems was higher in comparison to corresponding systems with activated sludge but without a classical preliminary settling tank [32]. On this basis, it can be concluded that the modernization of the researched facilities, which included equipping them with preliminary settling tanks, brought positive effects. The use of an additional element in the form of a preliminary settling tank as the provision of a mechanical treatment stage and the extension of the retention time of wastewater in the system have improved the process of sedimentation of solids, as well as their initial biological decomposition. Unfortunately, no positive effects on the removal of biogenic compounds were observed in the studied facilities. The efficiency of nitrogen and phosphorus removal did not exceed 34%, while the levels of technological reliability oscillated below 5%. The obtained results indicate that, in the case of biogenic compounds, the researched facilities could not guarantee a quality of treated wastewater which would be consistent with the requirements specified in the Polish law during any period of their operation. This disqualifies the analyzed technological solution in terms of its wide application in protected areas and near lakes, where the requirements for nitrogen and phosphorus removal are high. On the other hand, previous research results presented in the literature indicate that hybrid constructed wetland systems are the most effective and reliable in terms of pollutant removal from domestic wastewater [36,49,50,60,61] and, therefore, should be recommended for use in protected areas.

**Author Contributions:** Conceptualization, K.J.; data curation, K.J. and M.M.; formal analysis, M.M. and K.J.; investigation, A.M. and A.L.; methodology, K.J.; resources, A.M. and K.J.; supervision, M.M. and A.L.; validation, K.J.; writing—original draft, A.M. and K.J.; writing—review and editing, K.J., M.M. and T.G. All authors read and agreed to the published version of the manuscript.

**Funding:** This paper was co-funded by the "Excellent Science" program of the Ministry of Science and Higher Education as a part of the contract no. DNK/SP/465641/2020: "The role of agricultural engineering and environmental engineering in sustainable agriculture development".

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This paper was written on the basis of research projects funded by the Polish Ministry of Science and Higher Education—project entitled "Water, wastewater, and energy management" (contracts No. TKD/DS/1, TKD/S/1, 2017–2019), the Provincial Fund for Environmental Protection and Water Management in Lublin, Poland—project entitled "Research on the optimization of the operation of household wastewater treatment plants at forest settlements in the area of Roztocza ´nski and Poleski National Park" (contract No. TKD/OS/37/2018), and the Roztocza ´ ´ nski National Park, Poland—project entitled "Performing research on the operation of six household wastewater treatment plants located at forest settlements in the Roztocza ´nski National Park" (contract No. TKD/U-1/ISGiE, 2019). ´

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


### *Article* **The Kinetics of Pollutant Removal through Biofiltration from Stormwater Containing Airport De-Icing Agents**

**Artur Mielcarek 1, Joanna Rodziewicz 1,\*, Wojciech Janczukowicz <sup>1</sup> and Kamila Ostrowska <sup>2</sup>**


**Abstract:** The present study aimed to determine the kinetics of pollutant removal in biofilters with LECA filling (used as a buffer to prevent de-icing agents from being released into the environment with stormwater runoff). It demonstrated a significant effect of temperature and a C/N ratio on the rate of nitrification, denitrification, and organic compound removal. The nitrification rate was the highest (0.32 mg N/L·h) at 25 ◦C and C/N = 0.5, whereas the lowest (0.18 mg N/L·h) at 0 ◦C and C/N = 2.5 and 5.0. Though denitrification rate is mainly affected by the available quantity of organic substrate, it actually decreased as the C/N increased and was positively correlated with the temperature levels. Its value was found to be the highest (0.31 mg N/L·h) at 25 ◦C and C/N = 0.5, and the lowest (0.18 mg N/L·h) at 0 ◦C and C/N = 5.0. As the C/N increased, so did the content of organic compounds in the treated effluent. The lowest organic removal rates were noted for C/N = 0.5, ranging between 11.20 and 18.42 mg COD/L·h at 0 and 25 ◦C, respectively. The highest rates, ranging between 27.83 and 59.43 mg COD/L·h, were recorded for C/N = 0.5 at 0 and 25 ◦C, respectively.

**Keywords:** kinetics; organic compound removal; nitrification and denitrification; low temperature; wintertime airport maintenance

### **1. Introduction**

Air travel is considered to be one of the safest and fastest means of transport. The aviation industry stimulates the economic/cultural growth and creates many jobs. However, the day-to-day airport operations generate air, water, and soil pollution [1]. One of the main issues is the water runoff containing pollutants from runways, taxiways, washing and de-/anti-icing pads, trans-shipment points, fuel storage stations and hangars. These wastewater types contain petroleum-derived substances, surfactants, pavement de-icers, aircraft de-icing/anti-icing agents, and other organic and inorganic pollutants. In climate zones at risk of icing, the pollution caused by pavement de-icing poses a severe environmental problem. Urea, acetate, and sodium formate in the solid form, and acetate and potassium formate in the liquid form are the agents most commonly used for winter maintenance of airport pavements [2]. They release organic compounds and nitrogen into the environment while also increasing the salinity of water solutions and, thereby the salinity of areas adjacent to airports. Both the temperature [3] and the C/N ratio [4] are factors that significantly influence nitrogen conversion and contaminant removal in biofilters. Consequently, they have a major impact on the capacity to treat airport de-icing wastewater and protect the environment.

The issue of the treatment of wastewater containing de-icing agents remains unresolved. Most airports are not equipped with a wastewater treatment system. Only a few airports in the world have wetlands, which effectively remove pollutants. Unfortunately wetlands create favorable environmental conditions for birds, which may endanger airport operations. Other rarely used solutions include filters with zeolite and perlite, media made of crushed clay and granular activated carbon, a mixture of granular activated alumina

**Citation:** Mielcarek, A.; Rodziewicz, J.; Janczukowicz, W.; Ostrowska, K. The Kinetics of Pollutant Removal through Biofiltration from Stormwater Containing Airport De-Icing Agents. *Appl. Sci.* **2021**, *11*, 1724. https://doi.org/10.3390/ app11041724

Academic Editor: Faisal I. Hai Received: 26 January 2021 Accepted: 11 February 2021 Published: 15 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

and porous concrete, granular activated lignite, half-burnt dolomite, and granular ferric hydroxides. Considering sustainable development principles, the best solution would be to use the filling made of waste materials, e.g., light weight aggregates prepared from fly ash from sewage sludge thermal treatment. It is characterized by a large specific surface, resistance to physicochemical factors, low heat conductivity, good phosphorus-sorption properties, and facilitates the deammonification process. These attributes provide good conditions for biofilm growth even at low temperatures [5,6].

The breakdown of urea into ammonium (NH4 +) or ammonia (NH3) is a well-known process that occurs in both aerobic and anaerobic conditions at a wide range of pH, temperature, and C/N values [7]. The resultant processes of nitrogen reduction and oxidation affect the natural environment.

The biological oxidation of NH4 <sup>+</sup> or ammonia NH3 to nitrate is known as nitrification. The nitrification of ammonium in a biofilter is a two-step process in which ammonium or ammonia is first converted to nitrite (NO2 −) and then to nitrate (NO3 −). Its conversion to nitrite is mainly conducted by a group of obligatory autotrophic bacteria. Also, a few heterotrophs have been reported to carry out nitrification in the environment, but usually at much lower rates than autotrophic bacteria [8]. Nitrification allows converting a relatively immobile ammonium-nitrogen to highly mobile nitrate, affecting environmental quality [9]. The most effective pathway for N removal from wastewater is nitrification followed by denitrification [4]. The biological denitrification is conducted by denitrifying microbes which use nitrate as a terminal electron acceptor, and organic and inorganic substances as electron donors and energy sources for sustaining the microbial growth [10]. In the course of autotrophic denitrification, microorganisms use sulfur compounds, hydrogen, and/or iron as energy sources, and carbon dioxide and hydrocarbons as carbon sources. Microorganisms that use organic carbon compounds are the most common denitrifiers in nature [11]. The heterotrophic biological denitrification is considered more economical, implementable on a large scale, and allowing for the ultimate reduction of nitrate to nitrogen gas with high selectivity [12]. The presence of organic compounds in wastewater significantly affects nitrification and denitrification, with a low organic content promoting effective nitrification [13] and a low C/N ratio significantly reducing denitrification [4]. The denitrification rate is also determined by the availability of nitrification products (nitrites and nitrates). Furthermore, the rate of biochemical conversion and the microbial activity are directly affected by the temperature of the wastewater treatment.

Some of these parameters may be regulated easily, but due to the high specific heat capacity of water, it is nearly impossible to influence temperature. Nitrification and denitrification are elements of the N-cycle critical to the removal of nitrogen from the treatment system. While the removal of organic compounds and nitrogen from municipal wastewater has been the subject of much research and is relatively well explored, there is limited knowledge on how to eliminate such pollution from airport wastewater. The previous authors' research has shown that biofilters LECA filling could be an effective method for removing nitrogen and organic compounds from wastewater containing airfield deicing fluids. The nitrogen compounds were removed as a result of the simultaneous process of nitrification and denitrification, where the organic compounds present in the treated wastewater served as a carbon source [5,14]. To that end, the present paper provides the findings of a study aimed to determine the transformation kinetics of pollutants generated from airport maintenance, providing a means to estimate the retention time for de-icing wastewater in filters located near airports or in individual, local systems for removing such pollutants. The study was conducted under four different temperature profiles (0 ◦C, 4 ◦C, 8 ◦C, and 25 ◦C) and at 3 loading rates of de-icing agents used for winter maintenance of airport pavements, with C/N ratios of 0.5, 2.5, and 5.0.

### **2. Materials and Methods**

### *2.1. Experimental Model*

The experiment was performed in laboratory-scale models of biofilters operated at the Department of Environmental Engineering at the University of Warmia and Mazury in Olsztyn (Katedra Inzynierii ˙ Srodowiska UWM w Olsztynie). The biofilters were loaded with ´ a granulate having the structure of expanded-clay aggregate (with a diameter d60 = 8.2 mm, d60/d10 = 2.27, bulk density of 0.88 g/cm3, hydraulic conductivity of Kh 0.52 cm/s and resistance to crushing of 12.6 N/mm2) prepared from fly ash from sewage sludge thermal treatment in the "D ˛ebogórze" Wastewater Treatment Plant in Gdynia (Poland). The granulate was prepared according to the method of mechanical plasticization and fragmentation of the raw material, followed by sintering of small balls in a rotary kiln at 1200 ◦C [15]. Biofilters consisted of a cylindrical polyethylene pipe and a cone-shaped bottom. In the bottom of the biofilter, which was outflow part, was a drain valve used to collect samples of wastewater. The technical parameters of the biofilter were: surface area 95 cm2, volume 2500 cm3, active volume 1552 cm3, total height 0.24 m and biofilter filling height 0.19 m [6]. Four temperature variants were adopted: 0 ◦C, 4 ◦C, 8 ◦C, and 25 ◦C (control biofilter). The experiment was conducted at C/N levels of 0.5, 2.5, and 5.0. C/N ratio affects the efficiency of the biofilters (in relation to C and N removal). The C/N ratio applied in the study, was aimed to determine the kinetics under conditions favoring nitrification (C/N = 0.5), denitrification (C/N = 5.0) and intermediate conditions (C/N = 2.5). Stable operating temperatures were maintained by a thermostatic chamber that regulated the temperature with an accuracy of ± 0.1 ◦C. Synthetic wastewater, prepared from a weighted sample including commonly used agents for de-icing airport pavements and tap water, was the substrate for the experiments. The composition and physicochemical parameters of the wastewater used in the study are given in Table 1.

**Table 1.** Composition and parameters of the wastewater tested.


To promote nitrification, a low hydraulic load of 5 dm3/m2·d and hydraulic retention time (HRT) of 4 d were adopted, equivalent to the cycle of de-icing agent application in airports. The study on the kinetics of pollutant removal started at the end of technological research lasting four months. This four month research was preceded by three months adaptive period [6]. Samples were collected after 0.5, 6, and 12 h of operation; then, at 12 h intervals for 84 h, with the final sample taken after 96 h of operation (Figure 1). The size of the final sample was 5 cm3.

**Figure 1.** Schedule of sampling for the kinetics assay during the experiment.

### *2.2. Analytical Procedures*

Determinations of nitrate concentration, nitrite concentration, ammonium nitrogen concentration, Kjeldahl total nitrogen, and organic compound concentration (COD) were carried out according to the APHA [16]. The concentration of total nitrogen (TN) was analyzed using a TNM-L analyzer (Shimadzu Corporation, Kyoto, Japan) with the "oxidative combustion-chemiluminescence" method.

### *2.3. Kinetics*

The results of the physicochemical testing were used to determine the order of reaction and rates of organic compound removal, ammonia nitrogen oxidation and oxidized nitrogen (nitrites and nitrates) reduction. The reaction rate constant was determined using the Statistica 13.1 PL package (TIBCO Software Inc., Palo Alto, CA, USA).

The ammonia nitrogen oxidation rate was described with the formula:

$$\mathbf{C}\_{\text{t.Nocx}} = \mathbf{C}\_{\text{i.Nocx}} + \mathbf{k}\_{\text{Nocx}} \cdot \mathbf{t}\_{\text{\textdegree}} \tag{1}$$

where Ct.Nox—concentration of oxidized nitrogen after time t [mgN/L]; Ci.Nox—initial concentration of oxidized nitrogen [mgN/L]; kNox—ammonia nitrogen oxidation rate constant [h<sup>−</sup>1].

The rate of nitrogen removal through denitrification was described with the formula:

$$\mathbf{C}\_{\text{t.Nre}} = \mathbf{C}\_{\text{i.Nre}} + \mathbf{k}\_{\text{Nre}} \cdot \mathbf{t}\_{\text{\textdegree}} \tag{2}$$

where Ct.Nre—concentration of nitrogen removed through denitrification after time t [mgN/L]; Ci.Nre—initial concentration of nitrogen removed through denitrification [mgN/L]; kNre—oxidized nitrogen removal rate constant [h<sup>−</sup>1].

The organic compound removal rate (expressed as COD) was calculated with the formula:

$$\mathbf{C}\_{\text{tCOD}} = \mathbf{C}\_{\text{iCOD}} \cdot \mathbf{e}^{-\text{kCOD} \cdot \text{t}},\tag{3}$$

where Ct.COD—concentration of organic compounds after time t [mg/L]; Ci.COD—initial concentration of organic compounds [mg/L]; kCOD—organic compound removal rate constant [h<sup>−</sup>1].

### **3. Results and Discussion**

The present study identified the kinetics of nitrification, denitrification, and organic pollutant removal from wastewater generated by wintertime airport maintenance. The experiment was conducted in four temperature variants and at different doses of the de-icing agents (sodium formate and potassium diacetate) characterized by C/N ratios, with the urea dose being constant. The use of biofilter filling promotes the formation of complex biofilms, providing favorable conditions for the development of both aerobic and anaerobic microorganisms [15]. A diverse range of parameters (which ensures the presence of various bacterial groups with different needs) creates favorable conditions for wastewater bio-treatment [17]. This, in the context of the present study, enabled the removal of organic pollutants and nitrogen species from airport de-icing wastewater. The reaction rates for the experimental variants and the goodness of fit for the model (R2) are given in Supplementary Table S1. Changes in nitrogen concentration due to nitrification and denitrification conformed to the zero-order reaction formula, whereas the organic removal rate followed the first-order reaction formula. A comparison between the reaction rates by temperature and the biofilter's organic load was used to identify the impact of these parameters on the biofilter performance (Figure 2).

**Figure 2.** Effect of temperature and C/N on the rates of nitrification (**A**), denitrification (**B**), and organic pollutant removal (**C**).

### *3.1. Nitrification*

The nitrifier's activity is very temperature-sensitive [3]. However, exposure to low temperatures did not impact it to the point of halting ammonia nitrogen oxidation, which followed the zero-order reaction, meaning that changes in activity levels over time were linear. Similar findings have been reported in the literature, indicating that longer HRTs may be used to improve removal performance at low temperatures [18]. The rate and efficiency of nitrification of airport de-icing wastewater increased at higher biofilter operating temperatures. In contrast, the higher organic matter content in series 2 and 3 caused a slight reduction in the ammonia nitrogen oxidation rate at lower biofilter operating temperatures (Figures 2 and 3; Supplementary Figures S1–S3). At C/N = 0.5 group, the highest nitrification rate in the entire series, i.e., 0.32 mgN/L·h, was determined in the biofilter operating at 25 ◦C. Lowering biofilter operating temperatures, with the other technical parameters being equal, reduced the process rate by 31.3% (to 0.22 mgN/L·h) at 0 ◦C. In the two remaining reactors (T = 4 and 8 ◦C), the nitrification rates were 25.0 and 15.6% lower, respectively. It is noteworthy that the effect of temperature on reaction kinetics is much harder to determine in biofilm reactors than in suspended-growth biomass, and not as drastic as predicted by the van't Hoff-Arrhenius equation [19]. Other factors, such as the reactor operating parameters, HRT or wastewater composition, may lessen the impact of temperature [20]. While lower temperatures do reduce nitrification rates, they also increase dissolved oxygen concentration in the water, meaning that the effect of temperature is mitigated by the higher oxygen availability. Thus, the resultant reduction in the biofilm nitrification rate was ultimately negligible (1.108% per 1 ◦C) [19]. The decreases in nitrification rates produced in the present experiment were similar. Autotrophic nitrification rates are, in large part, driven by high levels of readily digestible organic compounds, which promote heterotrophic bacterial growth [21]. According to De Pra et al. [22], fastgrowing heterotrophic microorganisms compete for oxygen and ammonia nitrogen with nitrifying bacteria, and their metabolic intermediates may inhibit the activity of the latter. To achieve optimal nitrification conditions, the influent organic carbon must be kept under 2 kg COD/m3·d [22]. In an experiment by Zafarzadeh et al. [23], the highest nitrification rates were produced at C/N ratios under 6. Another study, by Mosquera-Corral et al. [24], showed that C/N values as low as 0.3 led to the competition between heterotrophs and autotrophs, impairing nitrification. In turn, Young et al. [25] reported that the inhibiting effect of high C/N ratios on ammoniacal nitrogen removal from treated wastewater was more pronounced at lower temperatures. This finding is corroborated by the present study, which showed that the reductions in nitrification rate and efficiency were higher at the lowest of the test temperatures (T=0 ◦C), but minor at 25 ◦C and increased organic compound load. In series 2 (C/N = 2.5), the nitrification rate was similar to that of series 1 (0.31 mgN/L·h at 25 ◦C). As in series 1, the lowest nitrification rate, reaching 0.18 mgN/L·h (41.94% lower), was observed at the lowest treatment temperature (0 ◦C). The nitrification rates were similar in the 8 ◦C and 4 ◦C variants, reaching 0.22 and 0.21 mgN/L·h, respectively. With the higher organic carbon levels (C/N = 5) in series 3, the nitrification rate remained stable at 0.31 mgN/L·h in the control reactor (T = 25 ◦C). In the other reactors analyzed in this series, it was approx. one-third lower than in the control one and ranged from 0.18 mgN/L·h at 0 ◦C to 0.21 mg N/L·h at 8 ◦C.

The high nitrification efficiency obtained for wastewater samples with the higher organic compound levels may be explained by heterotrophic nitrification. Guo et al. [26] reported that numerous heterotrophic microorganisms (which are not limited to bacteria but extend to fungi and plants as well) were capable of nitrifying organic and inorganic nitrogen compounds. Such microorganisms grow faster, require lower oxygen concentrations, and tolerate higher C/N ratios than autotrophic nitrifying bacteria.

**Figure 3.** Nitrification efficiency in the experiment depending on temperature and C/N ratio in storm-water containing airport de-icing agents.

### *3.2. Nitrogen Removal*

The activity of heterotrophic denitrifying bacteria is less dependent on the temperature than that of nitrifying bacteria [27]. Kadlec and Reddy [28] reported that 20–35 ◦C was the optimal temperature range for denitrification, which slows down considerably at temperatures lower than 10 ◦C and greater than 30 ◦C. Champagne et al. [29] noted that providing a source of readily biodegradable organic carbon helped maintain high nitrification efficiencies even at temperatures under 10 ◦C. In the present study, the denitrification rate increased at higher temperatures and decreased at higher C/N ratios (Figure 2; Supplementary Figures S4–S6), even though the presence of a readily available source of organic carbon is one of the main drivers of effective denitrification [30,31]. In series 1 (C/N = 0.5), oxidized nitrogen was reduced in the control reactor (T = 25 ◦C) at a rate of 0.31 mgN/L·h. Nitrogen removal was impeded by lowered biofilter operating temperatures, with the other technical parameters being equal. The lowest oxidized nitrogen reduction rates, at 0.22 mgN/L·h, were recorded for the biofilter operating at 0 ◦C. The denitrification rate was 0.30 mgN/L·h at 25 ◦C in series 2 and 3 (C/N = 2.5 and 5.0). Lowering the biofilter operating temperature to 0 ◦C reduced the performance by 36.7% and 40.0% against the control, respectively. These findings indicate that the nitrogen removal rates recorded in the study across the different experimental series did not correlate directly with the processing temperature or the C/N ratio—instead, the nitrification rate and efficiency served as the main factors. This pattern was the most strikingly evident in series 3, where the biofilters were fed with wastewater having the highest C/N ratio and were operating at optimal temperature conditions (T = 25 ◦C). A decreased ammonia nitrogen concentration in the effluent is indicative of intensive nitrification in the reactors. The difference between oxidized ammonia nitrogen and the nitrate/nitrite concentration in the treated wastewater shows how much nitrogen was removed. High rates of ammonia nitrogen oxidation and a low nitrite/nitrate content in the biofilter effluent, which did not exceed 12% TN in series 1 (no available organic substrate after approx. 24 h; Supplementary Figure S7) and 8% TN in the other series, suggest that nitrification and denitrification occurred simultaneously (Figures 4 and 5). The main factor enabling simultaneous nitrification and denitrification is the gradient of dissolved oxygen levels, determined by inhibited diffusion in the film [32]. Puznava et al. [33] found that a dissolved oxygen concentration of 0.5–3.0 mgO2/dm3 reduced oxygen penetration and enabled denitrification in the internal layers. The authors obtained a denitrification efficiency of 71%, with the simultaneous nitrification reaching 96–98%. Another study, by Zinatizadeh and Ghaytoolin [34] obtained 46–50% nitrogen removal through simultaneous nitrification and denitrification at dissolved oxygen levels between 2.5 and 3.0 mgO2/dm3, depending on the filling used in the MBBR. The nitrogen removal rates recorded in the presented study ranged from 30.1 ± 1.5% (T = 0 ◦C) to 43.0 ± 2.1% (T = 25 ◦C) in series 1 (C/N = 0.5). In the series with the higher organic compound contents (higher C/N ratios), the biofilters operating between 0 and 8 ◦C exhibited lower performance. The nitrogen removal rate ranged from 26.5 ± 1.3% (T = 0 ◦C) to 35.9 ± 1.8% (T = 8 ◦C) for C/N = 2.5, and from 25.0 ± 1.3% (T = 0 ◦C) to 31.2 ± 1.6% (T=8 ◦C) for C/N = 5.0. In the case of the biofilter operating at 25 ◦C, it was higher and reached 45.9 ± 2.3% and 46.7 ± 2.3% at C/N = 2.5 and 5.0, respectively.

**Figure 4.** Effectiveness of nitrogen removal in the experiment depending on temperature and C/N ratio in storm-water containing airport de-icing agents.

**Figure 5.** TN and nitrate/nitrite in the biofilter effluent (column diagram—percentage of TN; line diagram—TN concentration).

### *3.3. Organic Removal Rate*

The temperature not only affects the rate of substrate diffusion into the cell, but also the activity of enzymes, and thus the rate of biochemical processes, which determine how quickly pollutants are biodegraded. What is more, microorganisms are able to grow and metabolize organic compounds as long as the liquid pollutant is provided, meaning that organic compounds can be efficiently removed from wastewater even at low temperatures [35]. It is believed that aerobic digestion of organic compounds proceeds at exponentially increasing rates as the temperature rises from 0 ◦C to 32 ◦C. Its rate is stable between 32 and 40 ◦C, then declines sharply, reaching zero at 45 ◦C. This correlation holds true in the systems where the process rate is not limited by substrate concentration [36]. In addition, biofilms are less susceptible to adverse variations in temperature than activated sludge systems [37]. The rates of organic compound removal obtained in the present study increased with the C/N ratios and biofilter operating temperatures (Figure 2; Supplementary Figures S7–S9). The lowest rates of organic pollutant removal were recorded for series 1 reactors (C/N = 0.5), among which the control one (T = 25 ◦C) produced the highest reaction rate at 21.33 mgCOD/L·h. Lowering biofilter operating temperatures to 8, 4, and 0 ◦C, with the other technical parameters being equal, reduced its performance by 18.4, 32.7, and 48.0%, respectively. Increasing organic carbon (C/N = 2.5) in the T = 0 ◦C biofilter decreased organic compound removal by 65.5%, compared with the removal rates produced at 25 ◦C. The reaction rates were 51.8 and 57.3% lower in the other temperature variants (8 and 4 ◦C, respectively). The highest reaction rates were noted in the series with the greatest initial organic pollutant levels (C/N = 5.0). The peak value of organic compound removal, reaching 58.03 mgCOD/L·h, was reached in the control bioreactor (T = 25 ◦C). As in the C/N = 0.5 and 2.5 series, the organic removal performance in series 3 was the poorest at 0 ◦C (26.76 mgCOD/L·h)—being 53.9% lower than at T = 25 ◦C. The biofilters operating at 8 and 4 ◦C produced 41.5 and 45.7% lower reaction rates, respectively. Efficiencies and reaction rates were the highest in biofilters operating at 25 ◦C, and the lowest values in these operating at 0 ◦C (Figure 6).

**Figure 6.** Efficiency of organic compound removal from stormwater containing airport deicing agents.

However, increased C/N ratios in the subsequent series also led to higher initial levels of organic compounds in the influent, necessitating the removal of higher pollutant loads. This caused the organic compound removal rates to decrease as the C/N increased, with the sole exception of the biofilter operating at T = 25 ◦C and C/N = 5.0, which ensured an increase in organic pollutant removal rate from 55.4 ± 2.8% (C/N = 2.5) to 61.2 ± 3.1% (C/N = 5.0). Nevertheless, the pollutant levels (as expressed by COD) observed were the highest in series 3 (289.3–614.0 mgCOD/L), and the lowest in series 1 (17.2–35.1 mgCOD/L). These findings are in line with literature data, which indicate that as the organic pollutant load increases, so does the COD of the treated effluent. Thus, one way to improve the treatment efficiency at low temperatures is to reduce the organic load rate [37].

### **4. Conclusions**

The study showed that the rates of nitrification, denitrification, and organic compound removal were determined not only by the reactor operating temperature, but also by the organic carbon content (C/N ratios) in treated wastewater. Reaction rates were found to decrease at progressively lower temperatures in all of the experimental series. In terms of nitrification, the reaction rates fell as the organic compound levels (sodium formate and potassium acetate) rose. The nitrification rate was the highest (0.32 mg N/L·h) at 25 ◦C and C/N = 0.5, whereas the lowest rate (0.18 mg N/L·h) was produced at 0 ◦C for C/N = 2.5 and 5.0. The impact of the higher C/N ratios was more evident at lower biofilter operating temperatures. The study did not indicate that increased organic substrate levels led to improved denitrification rates, which were instead linked to the nitrification efficiency. The denitrification rate was found to be the highest (0.31 mg N/L·h) at 25 ◦C and C/N = 0.5, and the lowest (0.18 mg N/L·h) at 0 ◦C and C/N = 5.0. The lowest organic removal rates were noted for C/N = 0.5, ranging between 11.20 and 18.42 mg COD/L·h at 0 and 25 ◦C, respectively. The highest rates were recorded for C/N = 0.5, ranging between 27.83 and 59.43 mg COD/L·h at 0 and 25 ◦C, respectively. These results indicate that a treated wastewater with C/N = 0.5 does not inhibit the activity of nitrifying bacteria and actually reduces organic pollutant levels in the effluent, while also creating optimal conditions for simultaneous nitrification and denitrification, as long as suitable biofilter filling is provided.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/2076-3 417/11/4/1724/s1, Figure S1: Effect of temperature on the nitrification rate—series 1 (C/N = 0.5), Figure S2: Effect of temperature on the nitrification rate—series 2 (C/N = 2.5), Figure S3: Effect of temperature on the nitrification rate—series 3 (C/N = 5.0), Figure S4: Effect of temperature on the denitrification rate—series 1 (C/N = 0.5), Figure S5: Effect of temperature on the denitrification rate—series 2 (C/N = 2.5), Figure S6: Effect of temperature on the denitrification rate—series 3 (C/N = 5.0), Figure S7: Effect of temperature on the organic removal rate—series 1 (C/N = 0.5), Figure S8: Effect of temperature on the organic removal rate—series 2 (C/N = 2.5), Figure S9: Effect of temperature on the organic removal rate—series 3 (C/N = 5.0), Table S1: Goodness of fit and the rates of nitrification, denitrification, and organic compound removal depending on the adopted operating parameters of the biofilters.

**Author Contributions:** Conceptualization, W.J. and J.R.; Methodology, J.R.; Software, A.M.; Validation, K.O., J.R. and A.M.; Formal Analysis, J.R.; Investigation, K.O.; Resources, K.O. and W.J.; Data Curation, K.O. and A.M.; Writing—Original Draft Preparation, A.M. and W.J.; Writing—Review & Editing, J.R.; Visualization, A.M.; Supervision, W.J.; Project Administration, K.O.; Funding Acquisition, K.O. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Project No. 29.610.023 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 on request from the corresponding author.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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