sign in sign up
<<
Home , Denitrification, Nitrification

water

Article Effectiveness of Nitrification and Denitrification Processes in Biofilters Treating Wastewater from De-Icing Airport Runways

Joanna Rodziewicz, Kamila Ostrowska, Wojciech Janczukowicz and Artur Mielcarek *

Department of Environment Engineering, Faculty of Environmental Sciences, University of Warmia and Mazury in Olsztyn, Warszawska 117a, 10-719 Olsztyn, Poland; [email protected] (J.R.); [email protected] (K.O.); [email protected] (W.J.) * Correspondence: [email protected]

 Received: 6 February 2019; Accepted: 21 March 2019; Published: 26 March 2019 

Abstract: The basic factors determining the efficiency of the removal of and compounds from airport wastewater containing de-icing agents are low temperature and the C/N ratio (carbon to nitrogen ratio). Biofilm reactors (biofilters) create better conditions for nitrification and denitrification than suspended reactors. The scope of the study included determination of the influence of the C/N ratio in the wastewater on nitrification, denitrification and removal in biofilm reactors depending on the temperature. The experiment was performed in 24 circular laboratory biofilters with LECA (Light Expanded Clay Aggregates) filling. The study was divided into three series differing in organic carbon loading. In each series, carried out at the same hydraulic retention time, biofilters were operated at 25, 8, 4 or 0 ◦C. The study showed the effective removal of nitrogen compounds across a very wide temperature range. The applied filling and properly selected operating parameters of the reactors resulted in effective simultaneous nitrification and denitrification. The highest efficiency of nitrogen removal at 0 ◦C (34.57 ± 4.54%) was obtained at the C/N ratio of 0.5 gC/gN. The efficiency of denitrification (the lowest at the temperature of 0 ◦C) increased as the temperature and C/N ratio increased in the wastewater.

Keywords: pavement de-icing agents; biofilters; LECA (Light Expanded Clay Aggregates); nitrogen and carbon removal; nitrification and denitrification

1. Introduction Chemical substances used for winter maintenance of airports are an important source of water and land pollution [1,2]. Every winter, a significant amount of chemicals are used at the airport for de-icing of aircraft and airport pavements to facilitate safe air travel. Even in small facilities, an average of several hundred tons of de-icing agents are utilized during the season, the majority of which penetrates with rainwater to the ground and water near the aerodrome. Chemicals commonly used for winter maintenance of airport pavements include: urea, acetate and sodium formate in solid form, and acetate and potassium formate in liquid form. The generated wastewater is contaminated primarily by nitrogen and carbon compounds, while concentrations of phosphorus compounds do not exceed the limit values [2]. The composition of wastewater produced during de-icing of airport pavements is determined by meteorological phenomena, methods used to prevent freezing and de-icing [3] and the systems used for wastewater collection and disposal. This wastewater is characterized by chemical demand 3 (COD) from several dozen to 630,000 mg O2/dm , concentration of total nitrogen (Kjeldahl) from 0 to 600 mg/dm3, concentration of total phosphorus below 1 mg/dm3 [4]. Therefore, the Environmental

Water 2019, 11, 630; doi:10.3390/w11030630 www.mdpi.com/journal/water Water 2019, 11, 630 2 of 17

Protection Agency chose COD and total nitrogen as quality indicators of wastewater containing de-icing agents [5]. The issue of wastewater treatment containing de-icing agents remains unresolved [6], which results in penetration of most of these substances into the ground and water near airports [7]. Most airports are not equipped with a wastewater treatment system. Only a few airports in the world have [8], which effectively remove pollution from wastewater. However, wetlands create favorable environmental conditions for birds [9], which may endanger airport operations. A significant environmental problem associated with runoff wastewater containing de-icing agents is and oxygen depletion in receiver water [1], resulting in the death of aquatic invertebrates and fish and causing the loss of migratory fish species [10]. The basic factor determining the efficiency of the removal of nitrogen and carbon compounds from wastewater containing de-icing agents is the temperature, because both de-icing and wastewater treatment takes place at low temperatures at the airport. Heterotrophic denitrification is also dependent on access to easily available carbon compounds. The simultaneous use of de-icing agents such as urea and easily decomposable organic compounds such as acetate or potassium formate at the airport might facilitate the effective denitrification process as well as the effective removal of organic compounds [1]. Temperature is one of the most important factors determining the of nitrifying [11,12]. The optimal temperature range of wastewater for nitrification is 25–28 ◦C, at which temperature, sp. are more sensitive to environmental conditions than sp. [13], thus a temperature below 8–10 ◦C mainly inhibits the second phase of nitrification, which results in the accumulation of in an effluent [14]. Nevertheless, nitrification was also observed at very low temperatures of 1 ◦C or 0.5 ◦C[14] and reducing the wastewater temperature resulted in a gradual decrease in the efficiency of oxidation of nitrogen. The nitrification efficiency decreases from 90–94% at 15 ◦C to about 55% at 2 ◦C. Despite the possibility of the growth of nitrifying microorganisms at low temperatures being confirmed by other studies, it is commonly accepted that nitrification is effective above 15 ◦C[15]. Hoilijoki et al. [16] observed that complete nitrification below 5 ◦C was possible only in a reactor with the immobilized microorganisms that formed biofilm. Rostron et al. [17] noted that a reduction of the temperature from 25 ◦C to 16 ◦C decreased 10% of the nitrification efficiency in the reactor with biofilm, while the nitrification efficiency decreased 58% in the reactor with suspended biomass. Literature data also shows that the expanded-clay aggregate applied in biofilm reactors (biofilters) “buffers” changes in pH and potential [18,19]. Few studies have investigated the effect of temperature on denitrification which is temperature-dependent as a biochemical reaction [20]. The rate of denitrification increases exponentially to 32–35 ◦C[21]. The highest rate of the reduction of oxidized forms of nitrogen occurred at 20 ◦C and a further increase in the temperature did not increase the denitrification rate. The literature presents little data on the influence of low temperature on denitrifying microorganisms. It is commonly accepted that the efficiency of denitrification below 10 ◦C or above 30 ◦C decreased significantly. However, the reduction of oxidized forms of nitrogen was noted from 1 ◦C to 8 ◦C[20,22]. There are no reports on the efficiency of nitrification or denitrification at 0 ◦C. The issue of wastewater treatment containing de-icing agents still remains unresolved. Most of airports in the world are not equipped with wastewater treatment systems. Urea, acetate and sodium formate in solid form and acetate and potassium formate in a liquid form are common chemicals used for the winter maintenance of airport pavements. The novelty of our work results from thermal conditions, specific wastewater and new biofilter filling. The study was carried out at very low temperatures −0.4 and 8 ◦C. The subject of research was wastewater generated during the de-icing of airport pavements. Wastewater contains urea and de-icing agents with readily biodegradable organic compounds. Wastewater characterized by a different C/N ratio was treated in biofilm reactors with light weight aggregates prepared from fly ash from sludge thermal treatment (FASST LWA). We expect that the study has proven that the denitrification and nitrification processes may take place even at 0 ◦C, due to the specificity of LECA filled bioreactors and maintaining the appropriate ratio between nitrogen and carbon compounds in the treated wastewater. Water 2019, 11, x FOR PEER REVIEW 3 of 17 Water 2019, 11, 630 3 of 17 responsible for environmental protection at airports and provide a technological solution to treat wastewaterWe hope that containing the results de-icing of our studies agents will from help airport engineers pavements responsible and forreduce environmental the pollution protection of the waterat airports and ground and provide environment. a technological solution to treat wastewater containing de-icing agents from airportThe pavements main purpose and reduceof the study the pollution was to determine of the water the and effectiveness ground environment. of removing pollutants from wastewaterThe main containing purpose agents of the studyfor de-icing was to airport determine pavements the effectiveness in biofilters. of removing The biofilter pollutants filling fromwas madewastewater of LECA containing prepared agents from fly for ash de-icing from sewage airport pavementssludge thermal in biofilters. treatment The (FASST biofilter LWA), filling which was ismade characterized of LECA prepared by a higher from specific fly ash fromdensity sewage and sludgecompressive thermal strength treatment than (FASST natural LWA), LECA. which The is specificcharacterized goals byof the a higher study specific were to density determine and compressive the effectiveness strength of than natural and LECA. The specific processesgoals of the and study carbon were and to nitrogen determine removal the effectiveness in biofilters of depending nitrification on and temperatures. denitrification The processesscope of theand study carbon also and included nitrogen determination removal in biofilters of the dependinginfluence of on the temperatures. C/N ratio in The the scope wastewater of the study on nitrification,also included denitrification determination and of theremoval influence of organic of the C/N compounds ratio in the(COD). wastewater Another on goal nitrification, was to determinedenitrification the effect and removal of temperature of organic and compounds C/N ratio on (COD). COD Anotherconcentration goal was in treated to determine wastewater. the effect In theof temperature conducted andstudy, C/N wastewater ratio on COD containing concentration pavement in treated de-icing wastewater. agents In the(urea conducted and readily study, biodegradablewastewater containing organic pavementcompounds) de-icing was treated, agents (urea assuming and readily that maintaining biodegradable a proper organic relationship compounds) betweenwas treated, the amount assuming of carbon that maintaining and nitrogen a proper in the treated relationship wastewater between can the ensure amount effective of carbon removal and ofnitrogen pollutants in the in treatedlow temperatures. wastewater can ensure effective removal of pollutants in low temperatures.

2.2. Materials Materials and and Methods Methods

2.1.2.1. Experimental Experimental Stand Stand and and Organization Organization TheThe study study was was performed performed in in 24 24 circular circular reactors reactors th thatat are are laboratory laboratory models models of of biofilters biofilters with with the the fillingfilling in the form of LECA (Figure1 1).). A A bioreactor bioreactor consisted consisted of of a a cylindrical cylindrical polyethylene polyethylene pipe pipe and and a acone-shaped cone-shaped bottom. bottom. In In the the bottom bottom of of the the reactor, reacto whichr, which was was the the outflow outflow part, part, a drain a drain valve valve was usedwas usedto collect to collect samples samples of the treatedof the wastewater.treated wastewater The technical. The technical parameters parameters of the reactor of the were: reactor surface were: area surface95 cm2, area volume 95 cm 25002, volume cm3 and 2500 active cm3 volume and active 1552 volume cm3. 1552 cm3.

FigureFigure 1. Biofilter 1. Biofilter with with LECA LECA (Light (Light Expanded Expanded Clay Clay Aggregates) filling. filling. The filling of the reactors was granulate. It had a structure of extended-clay aggregate (with The filling of the reactors was granulate. It had a structure of extended-clay aggregate (with a a diameter d60 = 8.2 mm) prepared from fly ash from sewage sludge thermal treatment in the diameter d60 = 8.2 mm) prepared from fly ash from sewage sludge thermal treatment in the “D˛ebogórze” Wastewater Treatment Plant in Gdynia (Poland). The granulate was prepared using “Dębogórze” Wastewater Treatment Plant in Gdynia (Poland). The granulate was prepared using mechanical plasticization and fragmentation of the raw material, followed by firing small balls (average mechanical plasticization and fragmentation of the◦ raw material, followed by firing small balls diameter d60 of 8.2 mm) into a rotary kiln at 1200 C[23]. The study was divided into three series

WaterWater 20192019,, 1111,, 630x FOR PEER REVIEW 44 of 1717

(average diameter d60 of 8.2 mm) into a rotary kiln at 1200 °C [23]. The study was divided into three differingseries differing in organic in organic carbon carbon dose introduced dose introduced to the reactor. to the reactor. In each series,In each four series, variants four variants with reactors with operatedreactors atoperated different at wastewater different temperatureswastewater weretemp separatederatures usingwere aseparated thermostatic using chamber a thermostatic (POL-EKO APARATURAchamber (POL-EKO ST 350/350, APARATURA EU—Poland) ST 350/350, (Figure EU—Poland)2). (Figure 2).

Figure 2. Research scheme.

2.2.2.2. Characteristics of Wastewater FedFed toto ReactorsReactors TheThe synthetic wastewater, prepared from samp samplele weight (using Sartorius Sartorius CPA225D-0CE, CPA225D-0CE, Germany),Germany), which included commonly used agents for de-icing airport airport pavements pavements and and tap tap water, water, waswas thethe substratesubstrate forfor thethe experimentexperiment (Supplementary(Supplementary TableTableS1). S1). TheThe experimentsexperiments were carriedcarried outout atat thethe samesame hydraulichydraulic retentionretention time.time. The concentration of organicorganic compoundscompounds in in the the wastewater wastewater was was increased increased to obtain to obtain higher higher organic organic loading inloading the following in the biofilters.following Thebiofilters. indicators The indicators of raw wastewater of raw wastewater are presented are inpresented Table1. in Table 1.

± Table 1.1. AverageAverage ((±SD)SD) values of raw wastewater conc concentrationentration in subsequent research series.

ParameterParameter Series Series 1 1 Series Series 22 Series Series 3 3 C/NC/N (gC/gN) (gC/gN) 0.5 0.5 2.52.5 5.0 5.0 Ntot (mg N/dm3 3) 71.56 ± 2.20 Ntot (mg N/dm ) 71.56 ± 2.20 NKjeldahl (mg/dm3 3) 70.80 ± 3.02 NKjeldahl (mg/dm ) 70.80 ± 3.02 3 TOCTOC (mg (mg C/dm C/dm3) ) 56.96 56.96± 1.98± 1.98 200.80 200.80± ± 4.79 375.71 375.71 ± 6.97± 6.97 COD (mg O2/dm3 3) 100.66 ± 1.34 386.80 ± 1.94 765.50 ± 2.90 COD (mg O2/dm ) 100.66 ± 1.34 386.80 ± 1.94 765.50 ± 2.90 2 3 3 DissolvedDissolved oxygen oxygen (mg (mg O2/dm O /dm) ) 6.64 ± 0.710.71 RedoxRedox potential potential (mV) (mV) 189.33 ± 12.2312.23 pHpH 7.75 ± 0.280.28

TheThe organic compoundscompounds loadingloading in the following seriesseries of experiments are presented in Table2 2..

Water 2019, 11, 630 5 of 17

Table 2. The average (±SD) reactor pollutant loading.

Parameter Series 1 Series 2 Series 3 C/N (gC/gN) 0.5 2.5 5.0 2 Ntot. (mg N/m ·d) 357.8 ± 11.00 2 NKjeldahl (mg N/m ·d) 354.00 ± 15.10 2 N-NH4 (mg N/m ·d) 0.00 2 COD (mg O2/m ·d) 503.30 ± 7.19 1934.00 ± 9.47 3827.50 ± 14.50

The study was carried out using biofilters with vertical flow. During the first three months, they were operated with synthetic wastewater (adaptive phase). During this period, the temperature of the biofilters dropped 1 ◦C/d until the assumed operating temperature was reached. The study was performed at a low hydraulic loading of 5 dm3/m2·d, which facilitated nitrification [22] and a hydraulic retention time of 4 d, which corresponded to the dosing frequency of de-icing agents at airports of 4 d. The adaptive phase lasted until stabilization of composition of the treated wastewater. After adaptive period, each of biofilters was operated at the temperature of 25, 8, 4 or 0 ◦C. The reactor operated at the temperature of 25 ◦C was the control reactor. Biofilters were placed in a thermostatic chamber which maintained the temperature with an accuracy of ± 0.1 ◦C. The wastewater was dispensed to biofilters once every four days in a way that ensured an even distribution of the wastewater on the surface of the filling.

2.3. Analytical Procedures Physicochemical analyses of raw and treated wastewater included: concentration (spectrophotometric method) [24], concentration (spectrophotometric method) [25], ammonium nitrogen concentration (spectrophotometric method) [26] using VWR UV-3100PC Spectrophotometer (China) Kjeldahl total nitrogen (titrimetric method) [27] using a Buchi SpeedDigester K-436 (Switzerland) and Buchi KjelFlex K-360 (Switzerland), organic compound concentration (COD) (titrimetric method) [28] using a Gerhardt KI 16 (Germany) laboratory heater, concentration of total nitrogen (TN) and total organic carbon (TOC) with a Shimadzu Corporation TOC-L CPN and TNM-L analyzer (Japan), dissolved oxygen, pH, redox potential and temperature with a HACH HQ440d multi-analyzer (USA.). The results were calculated on the basis of approx. 30 measurements of pollution indicators in the raw and treated wastewater.

2.4. Statistical Analysis To determine any statistically significant differences between variants, a one-way analysis of variance was carried out. Significant differences in the mean values of the measured parameters were estimated using the ANOVA analysis, with a post-hoc reasonable significant difference (RIR) Tukey test. If the assumptions of the ANOVA analysis were not met, a non-parametric Kruskal–Wallis test was used as well as the post-hoc test of multiple comparison test (Dunn’s test). The significance level of α = 0.05 was used. The statistical analyses were performed in the Statistica 13.1 PL package. Details of the statistical analysis are presented in Supplementary Tables S2–S10.

3. Results and Discussion The study tested oxidation reduction potential, pH and concentration of dissolved oxygen in raw and treated wastewater (Tables1 and3). Water 2019, 11, x FOR PEER REVIEW 6 of 17

Table 3. Physicochemical properties of treated wastewater. Water 2019, 11, 630 6 of 17 C/N [gC/gN] Temperature [°C] pH Oxidation-Reduction Potential [mV] 0 °C 8.37–8.55 127.30 ± 4.84 Table 3. 4Physicochemical °C 8.42–8.62 properties of treated wastewater. 118.50 ± 3.00 0.5 C/N [gC/gN] Temperature8 °C [◦C] 8.36–8.66 pH Oxidation-Reduction 114.80 ± 3.04 Potential [mV] 25 °C 8.61–8.79 104.90 ± 3.22 0 ◦C 8.37–8.55 127.30 ± 4.84 40◦ C°C 8.32–8.50 8.42–8.62 117.20 118.50 ± 3.88± 3.00 0.5 84◦ C°C 8.39–8.61 8.36–8.66 112.08 114.80 ± 4.21± 3.04 2.5 258◦ °CC 8.41–8.65 8.61–8.79 105.65 104.90 ± 3.07± 3.22 025◦C °C 8.40–8.72 8.32–8.50 93.74 117.20 ± 1.94± 3.88 ◦ 40 C°C 8.41–8.67 8.39–8.61 107.80 112.08 ± 3.21± 4.21 2.5 ◦ ± 84 C°C 8.46–8.72 8.41–8.65 96.03 105.65 ± 2.873.07 5.0 25 ◦C 8.40–8.72 93.74 ± 1.94 8 °C 8.48–8.76 92.28 ± 2.24 ◦ ± 025C °C 8.62–8.78 8.41–8.67 86.38 107.80 ± 3.313.21 4 ◦C 8.46–8.72 96.03 ± 2.87 5.0 8 ◦C 8.48–8.76 92.28 ± 2.24 The concentration of 25dissolved◦C oxygen 8.62–8.78 in the treated wastewater 86.38 ± was3.31 0.00 mg O2/dm3 independent of the series of the experiment. The redox potential in the treated wastewater was lower than in the raw wastewater (Table 1). In all series of experiments, a low temperature of reactor The concentration of dissolved oxygen in the treated wastewater was 0.00 mg O /dm3 operation increased redox potential in the treated wastewater in the same technological conditions.2 independent of the series of the experiment. The redox potential in the treated wastewater was In all series of experiments, the pH of wastewater increased during the treatment process. At low lower than in the raw wastewater (Table1). In all series of experiments, a low temperature of reactor temperatures, the pH values in the treated wastewater were lower than at higher temperatures. operation increased redox potential in the treated wastewater in the same technological conditions. The pH of wastewater increased during treatment in biofilters. The low temperatures of In all series of experiments, the pH of wastewater increased during the treatment process. At low biofilter operation decreased the values of pH in the treated wastewater. temperatures, the pH values in the treated wastewater were lower than at higher temperatures. Nitrogen compound transformations in the biofilm occurred as shown in the Figure 3 and The pH of wastewater increased during treatment in biofilters. The low temperatures of biofilter described by Equations (1)–(5). This corresponded to the removal efficiency shown in the operation decreased the values of pH in the treated wastewater. Supplementary Table S11. The mass balance of the overall wastewater treatment process is shown in Nitrogen compound transformations in the biofilm occurred as shown in the Figure3 and described Table 4. by Equations (1)–(5). This corresponded to the removal efficiency shown in the Supplementary Table S11. The mass balance of the overall wastewater treatment process is shown in Table4.

Figure 3. Transformation of nitrogen compounds in a biofilm: a—ammonification, b—immobilization,

c—first phase of nitrification, d—second phase of nitrification, e—release to atmosphere, f—denitrification,Figure 3. Transformation g—anaerobic oxidationof nitrogen of ammonium compou nitrogennds in [29a]. biofilm: a—ammonification, b—immobilization, c—first phase of nitrification, d—second phase of nitrification, e—release to atmosphere, f—denitrification, g—anaerobic oxidation of ammonium nitrogen [29].

Mass balance in nitrogen compound transformations in the biofilm: Nitrification (including biomass production—C5H7O2N) -I stage:

Water 2019, 11, 630 7 of 17

Mass balance in nitrogen compound transformations in the biofilm: Nitrification (including biomass production—C5H7O2N) -I stage:

+ − + 55NH4 + 5CO2 + 76O2 → C5H7O2N + 54NO2 + 52H2O + 109H (1)

-II stage:

− + − + 400NO2 + 5CO2 + NH4 + 195O2 + 2H2O → C5H7O2N + 400NO3 + H (2)

summary:

+ − + NH4 + 1.86O2 + 0.10CO2 → 0.021C5H7O2N + 0.941H2O + 0.98NO3 + 1.98H (3)

Denitrification (including biomass production -C5H7O2N and acetate as carbon source)

− − + − CH3COO + 1.01 NO3 + 1.01 H → 0.13 C5H7O2N + 0.36 CO2 + HCO3 + 1.06 H2O + 0.44N2 (4)

Anaerobic oxidation of ammonium nitrogen (including biomass production -CH2O0.5N0.15)

− − + − NH4++ 1.32 NO2 +0.066 HCO3 +0.13 H →1.02 N2+0.26 NO3 +0.066 CH2O0.5N0.15+2.03 H2O (5)

Table 4. Mass balance of the overall wastewater treatment process.

Influent Effluent Temperature COD Total Nitrogen COD Total Nitrogen C/N [gC/gN] ◦ 2 2 2 2 [ C] [mgO2/m ·d] [mgN/m ·d] [mgO2/m ·d] [mgN/m ·d] 0 187.98 ± 30.50 234.11 ± 16.24 4 164.83 ± 26.12 231.10 ± 12.20 0.5 503.30 ± 6.70 8 115.76 ± 13.04 216.86 ± 16.82 25 102.17 ± 8.56 194.18 ± 10.16 0 1523.80 ± 38.10 269.85 ± 8.27 4357.80 ± 11.00 1467.52 ± 38.87 231.64 ± 10.41 2.5 1934.00 ± 9.70 8 1445.08 ± 21.27 228.96 ± 10.52 25 866.63 ± 152.59 170.74 ± 12.56 0 3109.84 ± 100.66 274.75 ± 9.48 4 3018.37 ± 101.43 256.26 ± 11.34 5.0 3827.50 ± 14.50 8 2981.62± 65.07 245.31 ± 16.78 25 1446.08 ± 203.24 165.73 ± 13.99

3.1. Nitrification Efficiency The experiments determined the influence of temperature on the efficiency of ammonium nitrogen oxidation and the relationship between temperature and the efficiency of nitrification under different concentrations of organic carbon compounds in wastewater fed to biofilters. In the first series, the organic carbon to nitrogen ratio was 0.5 gC/gN and the efficiency of nitrification was 47.68 ± 2.26% in the control reactor (operated at the temperature of 25 ◦C). The low temperatures of biofilter operation significantly (p < 0.05) decreased the efficiency of the process compared with the control reactor while maintaining the same technological conditions (Figure4a). In the biofilter operated at 0 ◦C, the nitrification efficiency was 36.88 ± 6.29%. Water 2019, 11, 630 8 of 17 Water 2019, 11, x FOR PEER REVIEW 8 of 17

Figure 4.Figure Influence 4. Influence of temperature of temperature on the on the nitrification nitrification efficiency efficiency in: in: (a ()a series) series 1 (C/N 1 (C/N = 0.5 = gC/gN),0.5 gC/gN), (b) series 2 (C/N(b) series = 2.5 2 (C/N gC/gN), = 2.5 ( gC/gN),c) series (c )3 series(C/N 3 = (C/N 5.0 gC/gN). = 5.0 gC/gN). A multiple comparison test showed that in the biofilters operated at low temperatures, A multiplethe efficiencies comparison of oxidation test of showed ammonium that nitrogen in the were biofilters similar operated and did not at significantly low temperatures, differ the efficiencies(p < 0.05).of oxidation In the reactors of ammonium operated at 4nitrogen◦C and 8 ◦ Cwere the percentagesimilar and of ammonium did not significantly nitrogen that was differ (p < 0.05). Inoxidized the reactors was 39.98 operated± 3.31% andat 4 41.70 °C and± 4.02%, 8 °C respectively. the percentage of ammonium nitrogen that was oxidized wasThe 39.98 changing ± 3.31% of and the technological41.70 ± 4.02%, parameters respectively. of reactor operations in the second series, by increasing the amount of organic carbon in the influent to 2.5 gC/gN, did not significantly change Thethe changing efficiency of of nitrificationthe technological in the control parameters reactor, in of which reactor it was operations 50.08 ± 2.86%. in Thethe efficiencysecond ofseries, by increasingoxidation the amount of ammonium of organic nitrogen carbon in the in other the reactorsinfluent was to significantly2.5 gC/gN, lower did not than significantly in the control change the efficiencyreactor. of In nitrification this series of experiments,in the control the reactor, efficiency in of which nitrification it was was 50.08 from ± 24.88 2.86%.± 1.99% The inefficiency the of oxidationreactor of ammonium operated at 0 nitrogen◦C to 34.81 in± 3.72%the other in the reactors reactor operated was significantly at 8 ◦C (Figure 4lowerb). than in the control reactor. In thisThe changingseries of of experiments, the technological the parameters efficiency of reactor of nitrification operation in thewas third from series, 24.88 by increasing ± 1.99% in the the amount of organic carbon in the influent to 5.0 gC/gN, caused that in the reactor operated at the reactor operatedtemperature at of0 °C 25 ◦ toC, 34.81 the efficiency ± 3.72% of in nitrification the reactor was operated 50.93 ± 4.35%. at 8 However,°C (Figure this 4b). efficiency was Thenot changing significantly of higher the technological than in the other series.parameters In the third of series,reactor the lowoperation temperatures in the of the third biofilter series, by increasingoperation the amount significantly of organic decreased carbon the nitrification in the influent efficiency. to The5.0 lowestgC/gN, efficiency caused of that nitrification in the reactor operated(24.65 at the± 5.15%temperature) was noted of in25 the°C, biofilter the efficiency operated of at nitrification 0 ◦C. The efficiency was 50.93 of ammonium ± 4.35%. nitrogen However, this ◦ efficiencyoxidation was not was 27.46significantly± 5.58% in thehigher reactor than operated in atthe 4 Cother and 30.49series.± 7.83% In inth thee third reactor series, operated the low at 8 ◦C (Figure4c). temperatures of the biofilter operation significantly decreased the nitrification efficiency. The lowest The obtained results and the literature data indicate that the efficiency of nitrification depends efficiencyon of the nitrification temperature (24.65 conditions. ± 5.15%) Many was authors noted have in also the identified biofilter temperatureoperated at as 0 the °C. main The factor efficiency of ammoniumdetermining nitrogen the oxidation metabolism was of nitrifying27.46 ± 5.58% bacteria, in whichthe reactor inhibited operated nitrification at 4 [°C11]. and In this 30.49 study, ± 7.83% in the reactorthe operationoperated of at a biofilter8 °C (Figure at 0 ◦C significantly4c). (p < 0.05) decreased the nitrification efficiency compared Thewith obtained the control results reactor and in each the series.literature These data results indica confirmedte that the studythe efficiency carried out byof Zhangnitrification et al. [30 ]depends which showed a rapid decrease in the nitrification efficiency to 12.7% at the temperature of 2.5 ◦C and on the temperature conditions. Many authors have also identified temperature as the main factor a study carried out by Hoang et al. [31] which showed the nitrification activity of microorganisms at determininga temperature the metabolism below 1 ◦C. of In nitr theifying present bacteria, study, the influencewhich inhibited of the temperature nitrification on the [11]. nitrification In this study, the operation of a biofilter at 0 °C significantly (p < 0.05) decreased the nitrification efficiency compared with the control reactor in each series. These results confirmed the study carried out by Zhang et al. [30] which showed a rapid decrease in the nitrification efficiency to 12.7% at the temperature of 2.5 °C and a study carried out by Hoang et al. [31] which showed the nitrification activity of microorganisms at a temperature below 1 °C. In the present study, the influence of the temperature on the nitrification efficiency was the highest in the third series, in which the wastewater was characterized by the C/N ratio of 5.0 gC/gN. In this series, dropping the temperature of the reactor operation from 25 °C to 0 °C decreased the efficiency of ammonium nitrogen oxidation by more than 26%. The highest efficiency of nitrification of 36.88 ± 6.29% in the

Water 2019, 11, 630 9 of 17 efficiency was the highest in the third series, in which the wastewater was characterized by the C/N ratio of 5.0 gC/gN. In this series, dropping the temperature of the reactor operation from 25 ◦C to 0 ◦C decreased the efficiency of ammonium nitrogen oxidation by more than 26%. The highest efficiency of nitrification of 36.88 ± 6.29% in the biofilter operated at the temperature of 0 ◦C was noted in the series in which the wastewater was characterized by the lowest organic compound concentration. Young et al. [14] showed that as nitrification efficiency decreased due to the decreased temperature, the accumulation of nitrate nitrogen (III) occurred in the treated wastewater. In the present study, the accumulation of nitrate nitrogen (III) associated with a decrease in the nitrification efficiency was not observed despite the decrease in nitrogen oxidation efficiency. These results are consistent with recent literature reports, which showed that ammonium oxidizing bacteria are just as sensitive [32] to a temperature drop below 20 ◦C as nitrite oxidizing bacteria, or even more so. Many authors have noted the dependency of the nitrification efficiency on the concentration of immobilized biomass in the biofilter. It is commonly accepted that a drop in the temperature of 10 ◦C will result in a 50% decrease in microorganism activity [33]. However, numerous authors have also noted that an immobilized biomass is less sensitive to the temperature drop than suspended biomass due to limited effect of diffusion on mass transfer processes [16]. Rostron et al. [17] showed that dropping the temperature from 25 ◦C to 16 ◦C decreased the nitrification efficiency by 10% in the biofilter with immobilized biomass, while a 58% decrease in nitrification efficiency was observed in a reactor with suspended biomass. The insufficient alkalinity of wastewater results in a decrease in pH, which inhibits the nitrifying microorganisms, mainly bacteria, of the second phase of nitrification since they are more sensitive to changes in pH [34]. A literature review showed that the expanded-clay aggregate used in the BAF (Biological Aerated Filter) system “buffered” changes in pH and redox potential [18], which is consistent with the results obtained in the present study, and the pH in all experiments was maintained at a constant level of 8.41–8.70. This level facilitated nitrification [19]. Young et al. [14] observed that the unfavorable effect of high C/N ratio in the wastewater on the ammonium nitrogen removal increased at the low temperatures. The current study showed that the nitrification efficiency in the series of the study was not directly related to the C/N ratio in the wastewater. Therefore, the high nitrification efficiency observed with high organic compound concentrations in the wastewater resulted from heterotrophic nitrification. Guo et al. [35] observed that many heterotrophic microorganisms (not only bacteria, but also fungi and plants) were capable of nitrifying organic and inorganic nitrogen compounds. These microorganisms grew faster, required less oxygen and tolerated a higher C/N ratio in the wastewater than autotrophic .

3.2. Denitrification Efficiency Although the concentration of oxidized ammonium nitrogen indicates the efficiency of nitrification, the difference between the concentration of oxidized ammonium nitrogen and the concentration of and nitrates in the treated wastewater is the amount of nitrogen removed during denitrification. The study results suggest simultaneous nitrification and denitrification in the biofilm. In the first series, in which the wastewater was characterized by an organic carbon-to-nitrogen ratio of 0.5 gC/gN, the efficiency of denitrification was 89.83 ± 0.96% in the biofilter operated at 25 ◦C. The low temperatures of the biofilter operation resulted in decreased and slightly increased denitrification efficiency. However, a statistical analysis showed that only in the biofilter operated at 4 ◦C was the process efficiency significantly different (p < 0.05) than in the control biofilter. In this reactor, the efficiency of denitrification was 84.69 ± 2.72% (Figure5a). Water 2019, 11, 630 10 of 17 Water 2019, 11, x FOR PEER REVIEW 10 of 17

102 94

100 92

98 90

88 96

86 94

84 92 Denitrification efficiency [%] efficiency Denitrification Denitrification[%] efficiency

82 90

80 04825 88 04825 Temperature [°C] Temperature [°C] (a) (b) 102

100

98

96

94

92

90

88 Denitrification [%] efficiency 86

84 average 82 average±SE 04825 average±SD Temperature [°C] (c)

FigureFigure 5.5. InfluenceInfluence ofof temperaturetemperature onon thethe denitrificationdenitrification efficiencyefficiency in: in: ( a(a)) series series 1 1 (C/N (C/N == 0.50.5 gC/gN),gC/gN), ((bb)) seriesseries 22 (C/N(C/N = = 2.5 2.5 gC/gN), gC/gN), (c ()c )series series 3 3(C/N (C/N = =5.0 5.0 gC/gN). gC/gN).

◦ DecreasingDecreasing thethe temperaturetemperature to 0 °CC in the biofilterbiofilter did not significantlysignificantly change the efficiency efficiency of denitrificationdenitrification ((pp << 0.05).0.05). InIn thisthis biofilter,biofilter,− −89.88 ± 2.72%2.72% of of oxidized oxidized nitr nitrogenogen was was reduced, while in ◦ thethe biofilterbiofilter operatedoperated atat 88 °CC −−90.3290.32 ±± 2.77%2.77% of of oxidized oxidized nitrogen nitrogen was was reduced. reduced. TheThe change in in the the technological technological parameters parameters of of the the reactor reactor operation operation in inthethe second second series, series, by byincreasing increasing the theamount amount of organic of organic carbon carbon in the in theinfluent influent to 2.5 to 2.5gC/gN gC/gN significantly significantly increased increased the thedenitrification denitrification efficiency efficiency to to98.24 98.24 ± 1.56%± 1.56% (p (

Water 2019, 11, 630 11 of 17

Water 2019, 11, x FOR PEER REVIEW 11 of 17 was significantly reduced (p < 0.05) in the biofilters operated at low temperatures. In the other significantseries, relationship a significant between relationship the efficiency between theof efficiencydenitrification of denitrification and the temperature and the temperature was not was observed.not observed.Therefore, Therefore,if the temperature if the temperature was not wasthe main not the factor main determining factor determining the denitrification the denitrification efficiency,efficiency, the formation the formation of anoxic ofanoxic zones as zones a resu aslt a of result oxygen of oxygen depletion depletion in the biofilm in the biofilm was decisive. was decisive. At higherAt higher temperatures, temperatures, oxygen oxygen consumption consumption was was fast fasterer in inthe the outer, outer, more more active active layers ofof thethe biofilm, biofilm,which which resulted resulted in in the the formation formation of of anoxic anoxic zones zones and and a greatera greater share share of of denitrification denitrification in in the the removal removalof nitrogenof nitrogen from from wastewater. wastewater. The The dependency dependency of the of formationthe formation of anoxic of anoxic zones zones on the on temperature the temperaturewas confirmed was confirmed by Liu by et al.Liu [37 et]. al. The [37]. authors The authors observed observed that at lowthat temperatures, at low temperatures, the penetration the of penetrationoxygen of intooxygen the into biofilm the wasbiofilm higher. was higher. In the presentIn the present study, thestudy, porous the porous structure structure of the fillingof the could filling contributecould contribute to the formation to the formation of specific of specific oxygen-anaerobic oxygen-anaerobic conditions conditions that were that favorable were favorable to simultaneous to simultaneousnitrification nitrification and denitrification and denitrification [38]. Microscopic [38]. analysis Microscopic of extended-clay analysis of aggregates extended-clay showed the aggregatesimmobilization showed the of microorganisms,immobilization bothof microorganisms, on their surface andboth in theiron their micropores, surface intensifiedand in their biological micropores,wastewater intensified treatment biological processes. wastewater On the treatment other hand, processes. in the biofilmOn the other operated hand, with in the a high biofilm content of operatedorganic with compounds a high content in the wastewater,of organic comp the highounds number in the of heterotrophswastewater, producedthe high a number greater amount of of heterotrophsexopolymers, produced which a greater could increase amount the of thicknessexopolymers, and densitywhich could of the increase biofilm. Athe higher thickness density and biomass density(and of the lower biofilm. porosity) A higher may limit density effective biomass oxygen (and diffusion, lower porosity) even in themay case limit of effective a thin biofilm oxygen [39 ]. diffusion, even in the case of a thin biofilm [39]. 3.3. Efficiency of Nitrogen Compounds Removal 3.3. Efficiency of Nitrogen Compounds Removal In the biofilters, the nitrogen compounds were removed as a result of biomass growth and Inreduction the biofilters, of oxidized the nitrogen nitrogen compounds to nitrogen were oxides removed and nitrogen as a result gas. of biomass growth and reduction ofA oxidized statistical nitrogen study showed to nitrogen the significant oxides and influence nitrogen ofgas. the temperature of the biofilter operation Aand statistical the C/N study ratio showed in the wastewaterthe significant on theinfluenc efficiencye of the of temperature nitrogen removal of the from biofilter wastewater operation (p < 0.05) and the(Figure C/N ratio6). At in 25 the◦ C,wastewater the lowest on efficiency the efficiency of nitrogen of nitrogen removal removal of 45.73 from± 2.85% wastewater was noted (p < 0.05) at the C/N (Figureratio 6). At of 25 0.5 °C, gC/gN. the lowest Significantly efficiency higher of nitr efficiencyogen removal of nitrogen of 45.73 removal ± 2.85% of was 52.28 noted± 3.51% at the was C/N noted at ratio ofthe 0.5 C/N gC/gN. = 2.5 Significantly gC/gN and higher 53.68 ± efficiency3.91% at of the nitrogen C/N = 5.0removal gC/gN of than52.28 in ± the3.51% first was series. noted at the C/N = 2.5 gC/gN and 53.68 ± 3.91% at the C/N = 5.0 gC/gN than in the first series. 60

55

50

45

40

35

30

25 Series 1:

C/N=0.5 [gC/gN] 20 Series 2:

[%] removal compounds nitrogen of Efficiency C/N=2.5 [gC/gN] 15 Series 3: 04825 C/N=5.0 [gC/gN]

Temperature [°C]

Figure 6. Temperature influence on the efficiency of nitrogen compound removal, depending on the C/N ratio. Figure 6. Temperature influence on the efficiency of nitrogen compound removal, depending on the C/N ratio.Since the efficiency of nitrogen removal from the wastewater at the C/N = 0.5 gC/gN was the most resistant to the low temperatures, in this series the significantly highest efficiency of nitrogen Sinceremoval the efficiency from wastewater of nitrogen of 34.57removal± 4.54% from the was wastewater observed at at 0 the◦C( C/Np < = 0.05). 0.5 gC/gN The statistical was the study most resistantshowed to a significantthe low temperatures, decrease in the in efficiencythis series ofthe nitrogen significantly removal highest with efficiency an increase of in nitrogen the C/N ratio removal from wastewater of 34.57 ± 4.54% was observed at 0 °C (p < 0.05). The statistical study showed a significant decrease in the efficiency of nitrogen removal with an increase in the C/N ratio

Water 2019, 11, 630 12 of 17 at the low temperature of biofilter operation. However, in the control reactors, the reverse tendency was observed. In the first series, a multiple comparison test showed that in the reactors which were operated at low temperatures, the reduction of the nitrogen compounds was similar and did not significantly differ. The statistical study also did not show significant differences in the share of biomass growth and denitrification observed at low temperatures of the biofilter operation (p < 0.05). Only at 25 ◦C was the share of biomass growth in the removal of nitrogen compounds significantly increased to 6.36 ± 0.99%, which corresponded to a concentration of 2.08 ± 0.34 mg N/dm3. Figure7a presents the concentration of removed nitrogen (determined as the concentration of removed nitrogen compounds) resulting from biomass growth and denitrification in the first series.

Figure 7. Concentration of removed nitrogen as a result of biomass synthesis and denitrification processes in: (a) series 1 (C/N = 0.5 gC/gN), (b) series 2 (C/N = 2.5 gC/gN), (c) series 3 (C/N = 5.0 gC/gN).

A multiple comparison test showed that in the reactors operated at low temperatures, the share of biomass synthesis and denitrification in nitrogen compound removal was similar and did not significantly differ in the second series (p < 0.05). Under these conditions, denitrification predominated in nitrogen compound removal, whereas only 4.68 ± 0.19 −6.54 ± 1.12% of nitrogen was removed (due to the biomass growth). The significantly highest share of biomass growth in the nitrogen compound removal in this series was observed only in the control reactor, in which 5.80 ± 0.30% of the removed nitrogen (i.e., 2.17 ± 0.32 mg N/dm3) was used for biomass synthesis. The concentration of nitrogen compound removal resulting from biomass growth and denitrification in the second series is presented in Figure7b. A multiple comparison test showed that in the third series, independent of the variant of the experiment, denitrification predominated in the removal of nitrogen compounds (Figure7c). The share of biomass growth and denitrification was similar and did not significantly differ at the temperatures of 0 ◦C, 4 ◦C and 8 ◦C(p < 0.05). The biomass synthesis resulted in the removal of 5.91 ± 0.15 −6.98 ± 1.56% of nitrogen from the wastewater. In the control reactor, the biomass synthesis removed 5.57 ± 0.21% of nitrogen, which corresponded to concentration of nitrogen removed WaterWater 20192019,, 1111,, x 630 FOR PEER REVIEW 1313 of of 17 17 resulted in simultaneous nitrification and denitrification. The possibility of simultaneous 3 nitrificationof 2.24 ± 0.44 and mg denitrification N/dm . The in share the ofbiofilm denitrification has been inwidely the removal documented of nitrogen in the compoundsliterature [40]. from If ◦ bothwastewater nitrifying was and significantly denitrifying higher bacteria at 25 areC thanpresent at the in lowerthe biofilm temperatures. and the amount of oxygen is sufficientA low for temperature the oxidation of biofilter of organic operation compounds significantly and nitrification, decreased the yet nitrogen low enough removal to efficiency ensure denitrification,from the wastewater these processes (p < 0.05). may In all proceed biofilters, simultan nitrogeneously. was Puznava removed et as al. a result[41] determined of biomass that growth the concentrationand a reduction of dissolved of oxidized oxygen nitrogen at the forms. level The of 0.5–3.0 obtained mg results O2/dm suggest3 reduced that the the penetration technological of oxygenparameters and ofdenitrification biofilter operations was possible caused in the its formation internal layers. of anoxic The zones authors in the obtained biofilm, an which efficiency resulted of denitrificationin simultaneous of 71% nitrification with simultaneous and denitrification. nitrification The reaching possibility 96–98%. of simultaneous However, Zinatizadeh nitrification and and Ghaytoolindenitrification [42] in used the biofilm a dissolved has been oxygen widely concentration documented of in 2.5–3.0 the literature mg O2 [/dm40].3 If, depending both nitrifying on the and fillingdenitrifying of the bacteriaMBBR (moving are present bed in biofilm the biofilm reactor) and thereactor, amount and of observed oxygen is the sufficient efficiency for theof oxidationnitrogen removalof organic of compounds46–50% due and to simultaneous nitrification, yetnitrification low enough and to denitrification. ensure denitrification, Choo and these Stensel processes [43] used may aproceed dissolved simultaneously. oxygen concentration Puznava et al.of [2.841] determinedmg O2/dm3 thatand the observed concentration a 27% of dissolvedprocess efficiency. oxygen at 3 However,the level of after 0.5–3.0 a reduction mg O2/dm of thereduced oxygen the concentration penetration of below oxygen 0.6 and mg denitrification O2/dm3, the efficiency was possible of simultaneousin its internal nitrification layers. The authorsand denitrification obtained an increased efficiency to of 47%. denitrification of 71% with simultaneous nitrification reaching 96–98%. However, Zinatizadeh and Ghaytoolin [42] used a dissolved oxygen 3 3.4.concentration Efficiency of ofOrgani 2.5–3.0c Compounds mg O2/dm (COD), depending Removal on the filling of the MBBR (moving bed biofilm reactor) reactor, and observed the efficiency of nitrogen removal of 46–50% due to simultaneous The study tested the concentration of organic compounds (COD) in the raw and treated nitrification and denitrification. Choo and Stensel [43] used a dissolved oxygen concentration of wastewater. The results showed that the efficiency of organic compound removal depended on the 2.8 mg O /dm3 and observed a 27% process efficiency. However, after a reduction of the oxygen C/N ratio 2in the wastewater and the temperature of the biofilter operation. concentration below 0.6 mg O /dm3, the efficiency of simultaneous nitrification and denitrification The efficiency of organic compound2 removal was significantly influenced by the C/N ratio in increased to 47%. the wastewater (p < 0.05) (Figure 8). 3.4. Efficiency of Organic Compounds (COD) Removal The study tested the concentration of organic compounds (COD) in the raw and treated wastewater. The results showed that the efficiency of organic compound removal depended on the C/N ratio in the wastewater and the temperature of the biofilter operation. The efficiency of organic compound removal was significantly influenced by the C/N ratio in the p wastewater ( < 0.05) (Figure8).

90

80

70

60

50

40

30 20 Series 1: C/N=0.5 [gC/gN] 10 Series 2: C/N=2.5 [gC/gN] Efficiency of organic compounds (COD) removal [%] removal (COD) compounds organic of Efficiency 0 Series 3: 04825 C/N=5.0 [gC/gN] Temperature [°C]

FigureFigure 8. 8. EfficiencyEfficiency of of organic organic compound compound re removalmoval depending depending on on the the C/N C/N ratio. ratio.

TheThe significantlysignificantly highest highest efficiency efficiency of organic of organic compound compound removal, removal, independent independent of the temperature, of the temperature,was noted in was the biofiltersnoted in the operated biofilters at theoperated lowest at C/N the lowest ratioin C/N the ratio wastewater in the wastewater (p < 0.05). ( Thep < 0.05). most Theresistant most toresistant the low to temperatures the low temperatures was the efficiency was the efficiency of organic of compound organic compound removal in removal the first in series, the

WaterWater 20192019, 11, 11, x, 630FOR PEER REVIEW 14 14of of17 17 first series, i.e., at a C/N ratio of 0.5 gC/gN. For each temperature, increasing the concentration of i.e., at a C/N ratio of 0.5 gC/gN. For each temperature, increasing the concentration of organic carbon organic carbon decreased the efficiency of its removal. decreased the efficiency of its removal. A statistical analysis showed a significant effect of the C/N ratio in the wastewater on the A statistical analysis showed a significant effect of the C/N ratio in the wastewater on the concentration of organic compounds in the treated wastewater (Figure 9). The increased concentration of organic compounds in the treated wastewater (Figure9). The increased concentration concentration of organic compounds in the raw wastewater in the following series of the study of organic compounds in the raw wastewater in the following series of the study resulted in a resulted in a significant deterioration of the treated wastewater quality, which resulted from a significant deterioration of the treated wastewater quality, which resulted from a significant increase in significant increase in the concentration of organic compounds expressed in the COD of the treated the concentration of organic compounds expressed in the COD of the treated wastewater (p < 0.05). wastewater (p < 0.05).

700

600

500

] 3 400 /dm 2 300 COD [mgO 200 Series 1: 100 C/N=0.5 [gC/gN] Series 2: C/N=2.5 [gC/gN] 0 Series 3: 04825 C/N=5.0 [gC/gN] Temperature [°C]

FigureFigure 9. 9.InfluenceInfluence of of the the C/N C/N ratio ratio on on the the organic organic comp compoundound concentrations concentrations in intreated treated wastewater. wastewater.

The concentration of organic compounds expressed in the COD in the treated wastewater showed The concentration of organic compounds expressed in the COD in the treated wastewater that the application of a C/N ratio above 0.5 gC/gN in the wastewater at the low temperature resulted showed that the application of a C/N ratio above 0.5 gC/gN in the wastewater at the low in a high concentration of organic compounds in the treated wastewater. temperature resulted in a high concentration of organic compounds in the treated wastewater. The present study showed an increased denitrification efficiency which was associated with The present study showed an increased denitrification efficiency which was associated with an an increased concentration of organic compounds in the wastewater. Even in the series with the increased concentration of organic compounds in the wastewater. Even in the series with the lowest lowest concentration of organic compounds in the wastewater, the denitrification efficiency was concentration of organic compounds in the wastewater, the denitrification efficiency was 89.88 ± 89.88 ± 2.72% in the biofilter operated at 0 ◦C. The high denitrification efficiency resulted from low 2.72% in the biofilter operated at 0 °C. The high denitrification− efficiency resulted from low nitrification efficiency and a low concentration of available NOx . In the first series, the concentration nitrification efficiency and a low concentration of available NOx−. In the first series, the concentration of removed nitrogen compounds was 23.72 ± 2.17 mg N/dm3. In the second series, the efficiency of removed nitrogen compounds was 23.72 ± 2.17 mg N/dm3. In the second series, the efficiency of of denitrification was 92.36 ± 2.62%, while the concentration of removed nitrogen compounds was denitrification was 92.36 ± 2.62%, while the concentration of removed nitrogen compounds was 16.44 ± 1.52 mg N/dm3. In the third series, the denitrification efficiency was 87.59 ± 4.60%, while 16.44 ± 1.52 mg N/dm3. In the third series, the denitrification efficiency was 87.59 ± 4.60%, while the the concentration of removed nitrogen compounds was 15.45 ± 3.41 mg N/dm3. The decrease in the concentration of removed nitrogen compounds was 15.45 ± 3.41 mg N/dm3. The decrease in the efficiency of nitrogen removal at a lower concentration of organic compounds most probably resulted efficiency of nitrogen removal at a lower concentration of organic compounds most probably from the lower amount of available organic compounds and the better oxygenation of the biofilm. This resulted from the lower amount of available organic compounds and the better oxygenation of the was confirmed by the nitrogen removal in the control reactor in the first series, in which the process biofilm. This was confirmed by the nitrogen removal in the control reactor in the first series, in efficiency was 45.73 ± 2.85% and the nitrification efficiency reached almost 50%, which indicated good which the process efficiency was 45.73 ± 2.85% and the nitrification efficiency reached almost 50%, oxygen conditions inside the membrane. which indicated good oxygen conditions inside the membrane. 4. Conclusions 4. Conclusions The study showed that the effective removal of nitrogen compounds from wastewater occurred The study showed that the effective removal of nitrogen compounds from wastewater occurred across a very wide temperature range. The applied filling and properly selected operating parameters across a very wide temperature range. The applied filling and properly selected operating of the reactors resulted in effective nitrification and reduction of oxidized nitrogen forms to gaseous parameters of the reactors resulted in effective nitrification and reduction of oxidized nitrogen nitrogen. Therefore the removal of nitrogen compounds was possible due to simultaneous nitrification forms to gaseous nitrogen. Therefore the removal of nitrogen compounds was possible due to and denitrification, in which carbon sources were organic compounds contained in the wastewater.

Water 2019, 11, 630 15 of 17

Maintaining a proper relationship between the amount of carbon and nitrogen in the treated wastewater, through the simultaneous application of de-icing agents containing urea and easily biodegradable carbon compounds, can ensure the effective removal of pollutants at low temperatures. Table S7. Dependent variable: concentration of removed nitrogen as a results of biomass synthesis, qualitative variable: temperature. Table S8. Dependent variable: efficiency of organic compounds removal, qualitative variable: temperature. Table S9. Dependent variable: efficiency of organic compounds removal, qualitative variable: ratio C/N. Table S10. Dependent variable: concentration of organic compounds in treated wastewater, qualitative variable: ratio C/N.

Supplementary Materials: The following are available online at http://www.mdpi.com/2073-4441/11/3/630/s1, Table S1. Composition of model wastewater. Table S2. Dependent variable: nitrification efficiency, qualitative variable: temperature. Table S3. Dependent variable: denitrification efficiency, qualitative variable: temperature. Table S4. Dependent variable: efficiency of nitrogen compounds removal, qualitative variable: temperature. Table S5. Dependent variable: efficiency of nitrogen compounds removal, qualitative variable: ratio C/N. Table S6. Dependent variable: concentration of removed nitrogen as a result of denitrification, qualitative variable: temperature. Table S7. Dependent variable: concentration of removed nitrogen as a results of biomass synthesis, qualitative variable: temperature. Table S8. Dependent variable: efficiency of organic compounds removal, qualitative variable: temperature. Table S9. Dependent variable: efficiency of organic compounds removal, qualitative variable: ratio C/N. Table S10. Dependent variable: concentration of organic compounds in treated wastewater, qualitative variable: ratio C/N. Table S11. Nitrification and denitrification efficiencies and processes in different bioreactor configurations under the conditions tested. Author Contributions: J.R. developed the research idea and planned the research activities. K.O. carried out the research, including collecting the input data and prepared manuscript. W.J. supervised the research activities and reviewed the final draft of the manuscript. A.M. carried out a statistical analysis of the obtained results. All authors have read and approved the final manuscript. Funding: This research was funded by the National Science Centre, Poland grant number [No. DEC-2012/05/ N/ST8/02582] and the University of Warmia and Mazury in Olsztyn, Poland. grant number [18.610.008-300]. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Raspati, G.S.; Haug Lindseth, H.K.; Muthanna, T.M.; Azrague, K. Potential of biofilters for treatment of de-icing chemicals. Water 2018, 10, 620. [CrossRef] 2. Pressl, A.; Pucher, B.; Scharf, B.; Langergraber, G. Treatment of de-icing contaminated surface water runoff along an airport runway using in-situ enriched with structural filter materials. Sci. Total Environ. 2019, 660, 321–328. [CrossRef] 3. Ariyajunya, B.; Tarun, P.K.; Chen, V.C.P.; Kim, S.B. Modeling the impact of airport deicing/anti-icing activities on the dissolved oxygen levels in the receiving waterways. J. Water Manag. Model. 2018, 26. [CrossRef] 4. Shi, X.; Quilty, S.M.; Long, T.; Jayakaran, A.; Fay, L.; Xu, G. Managing airport stormwater containing deicers: Challenges and opportunities. Front. Struct. Civ. Eng. 2017, 11, 35–46. [CrossRef] 5. United States Environmental Protection Agency (USEPA). Technical Development Document for the Final Effluent Limitations Guidelines and New Source Performance Standards for the Airport Deicing Category; EPA-821-R-12-005; Engineering and Analysis Division: Washington, DC, USA, 2012. 6. Freeman, A.I.; Surridge, B.W.J.; Matthews, M.; Stewart, M.; Haygarth, P.M. New approaches to enhance pollutant removal in artificially aerated wastewater treatment systems. Sci. Total Environ. 2018, 627, 1182–1194. [CrossRef][PubMed] 7. Andersen, H.R.; Lee, H.; Yavuz, C.T.; Kim, H.; Ko, D.; Hwang, Y. Applicability of disulfide-polymer particles surface embedded on alginate beads for cadmium removal from airport derived stormwater. J. Environ. Chem. Eng. 2018, 6, 4124–4129. 8. Dou, T.; Troesch, S.; Petitjean, A.; Gábor, P.T.; Esser, D. Wastewater and rainwater management in urban areas: A role for constructed wetlands. Procedia Environ. Sci. 2017, 37, 535–541. [CrossRef] 9. Fox, B.J.; Holland, W.B.; Boyd, F.L.; Blackwell, B.F.; Armstrong, J.B. Use of stormwater impoundments near airports by birds recognized as hazardous to aviation safety. Landsc. Urban Plan. 2013, 119, 64–73. [CrossRef] Water 2019, 11, 630 16 of 17

10. Freeman, A.I.; Surridge, B.W.J.; Matthews, M.; Stewart, M.; Haygarth, P.M. Understanding and managing de-icer contamination of airport surface waters: A synthesis and future perspectives. Environ. Technol. Innov. 2015, 3, 46–62. [CrossRef] 11. Chen, M.; Chen, Y.; Dong, S.; Lan, S.; Zhou, H.; Tan, Z.; Li, X. Mixed nitrifying bacteria culture under different temperature dropping strategies: Nitrification performance, activity, and community. Chemosphere 2018, 195, 800–809. [CrossRef] 12. Yuan, J.; Dong, W.; Sun, F.; Zhao, K. Low temperature effects on nitrification and nitrifier community structure in V-ASP for decentralized wastewater treatment and its improvement by bio-augmentation. Environ. Sci. Pollut. Res. 2018, 25, 6584–6595. [CrossRef] 13. Paredes, D.; Kuschk, P.; Mbwette, T.S.A.; Stange, F.; Müler, R.A.; Köser, H. New aspects of microbial nitrogen transformations in the context of wastewater treatment—A review. Eng. Life Sci. 2007, 7, 13–25. [CrossRef] 14. Young, B.; Delatolla, R.; Kennedy, K.; Laflamme, E.; Stintzi, A. Low temperature MBBR nitrification: Microbiome analysis. Water Res. 2017, 111, 224–233. [CrossRef][PubMed] 15. Ducey, T.F.; Vanotti, M.B.; Shriner, A.D.; Szogi, A.A.; Ellison, A.Q. Characterization of a microbial community capable of nitrification at cold temperature. Bioresour. Technol. 2010, 101, 491–500. [CrossRef][PubMed] 16. Hoilijoki, T.H.; Kettunen, R.H.; Rintala, J.A. Nitrification of aerobically pretreated municipal landfill leachate at low temperature. Water Res. 2000, 34, 1435–1446. [CrossRef] 17. Rostron, W.; Stuckey, D.C.; Young, A.A. Nitrification of high strength ammonia wastewaters: Comparative study of immobilization media. Water Res. 2001, 35, 1169–1178. [CrossRef] 18. Zou, J.; Xu, G.; Pan, K.; Zhou, W.; Dai, Y.; Wang, X.; Zhang, D.; Hu, Y.-C.; Ma, M. Nitrogen removal and biofilm structure affected by COD/NH4+–N in a biofilter with porous sludge-ceramsite. Sep. Purif. Technol. 2012, 94, 9–15. [CrossRef] 19. Ruitz, G.; Jeison, D.; Chamy, R. Nitrification with nitrite accumulation for the treatment in wastewater with high ammonia concentration. Water Res. 2003, 37, 1371–1377. 20. Carrera, J.; Vicent, T.; Lafuente, F.J. Influence of temperature on denitrification of an industrial high-strength nitrogen wastewater in a two-sludge system. Water SA 2003, 29, 11–16. [CrossRef] 21. Chen, D.; Chen, X.; Huang, X.; He, S.; Huang, J.; Zhou, W. Controlling denitrification accompanied with nitrite accumulation at the sediment-water interface. Ecol. Eng. 2017, 100, 194–198. [CrossRef] 22. He, S.; Ding, L.; Pan, Y.; Hu, H.; Ye, L.; Ren, H. Nitrogen loading effects on nitrification and denitrification with functional gene quantity/transcription analysis in biochar packed reactors at 5 ◦C. Sci. Rep. 2018, 8, 9844. [CrossRef][PubMed] 23. Białowiec, A.; Janczukowicz, W.; Krzemieniewski, M. The fly ash from sewage sludge thermal treatment recovery as LECA for constructed wetlands. In Proceedings of the 24th International Conference on Solid Waste Technology and Management, Philadelphia, PA, USA, 15–18 March 2009. 24. PN-82/C-04576.08 Water and Wastewater. Determinations of Nitrogen Compounds. Determination of Nitrite Nitrogen with The Colorimetric Method Using Sodium Salicylate. (In Polish) 25. PN-EN26777:1999 Water Quality. Determination of Nitrite. Molecular Absorption Spectrometric Method. (In Polish) 26. PN-C-04576-04:1994 Water and Wastewater. Determinations of Nitrogen Compounds. Determination of Ammonium Nitrogen in Water by Direct Nesslerization. (In Polish) 27. PN-EN 25663:2001 Water Quality. Determination of Kjeldahl Nitrogen. Method after Mineralization with Selenium. (In Polish) 28. PN-ISO 6060:2006 Water and Wastewater. Research on Oxygen Demand and Organic Carbon Content. Determination of (COD) by Dichromate Method. (In Polish) 29. Wyffels, S.; Pynaert, K.; Boeckx, P.; Verstraete, W.; Van Cleemput, O. Identification and quantification of nitrogen removal in a rotating biological contactor by 15N tracer techniques. Water Res. 2003, 37, 1252–1259. [CrossRef] 30. Zhang, S.; Wang, Y.; He, W.; We, M.; Xing, M.; Yang, J.; Gao, N.; Yin, D. Responses of biofilm characteristics + to variations in temperature and NH4 -N loading in a moving-bed biofilm reactor treating micro-polluted raw water. Bioresour. Technol. 2013, 131, 365–373. [CrossRef] 31. Hoang, V.; Delatolla, R.; Laflamme, E.; Gadbois, A. An investigation of moving bed biofilm reactor nitrification during long-term exposure to cold temperatures. Water Environ. Res. 2014, 86, 36–42. [CrossRef] Water 2019, 11, 630 17 of 17

32. Zhou, H.X.; Li, X.K.; Chu, Z.R.; Zhang, J. Effect of temperature downshifts on a bench-scale hybrid A/O system: Process performance and microbial community dynamics. Chemosphere 2016, 153, 500–507. [CrossRef] 33. Rajeshwari, K.; Balakrishnan, M.; Kansal, A.; Lata, K.; Kishore, V.V. State-of-the-art of anaerobic digestion technology for industrial wastewater treatment. Renew. Sustain. Energy Rev. 2000, 4, 135–156. [CrossRef] 34. Ge, S.; Wang, S.; Yang, X.; Qiu, S.; Li, B.; Peng, Y. Detection of nitrifiers and evaluation of partial nitrification for wastewater treatment: A review. Chemosphere 2015, 140, 85–98. [CrossRef] 35. Guo, J.; Peng, Y.; Huang, H.; Wang, S.; Ge, S.; Zhang, J.; Wang, Z. Short- and long-term effects of temperature on partial nitrification in a treating domestic wastewater. J. Hazard. Mater. 2010, 179, 471–479. [CrossRef] 36. Elgood, Z.; Robertson, W.D.; Schiff, S.L. production in a stream bed bioreactor for nitrate removal. J. Ecol. Eng. 2010, 36, 1575–1580. [CrossRef] 37. Liu, T.; Quan, X.; Li, D. Evaluations of biofilm thickness and dissolved oxygen on single stage process in an up-flow biological aerated filter. Biochem. Eng. J. 2017, 119, 20–26. [CrossRef] 38. Wang, W.; Wang, Y.; Wang, X.; Zhang, Y.; Yan, Y. Dissolved oxygen microelectrode measurements to develop a more sophisticated intermittent aeration regime control strategy for biofilm-based CANON systems. Chem. Eng. J. 2019, 365, 165–174. [CrossRef] 39. Andersen, H.R.; Smets, B.F.; Bester, K.; Polesel, F.; Trapp, S.; Plósz, B.G.; Torresi, E.; Christensson, M. Diffusion and sorption of organic micropollutants in biofilms with varying thicknesses. Water Res. 2017, 123, 388–400. 40. Zheng, Z.; Huang, S.; Bian, W.; Liang, D.; Wang, X.; Zhang, K.; Ma, X.; Li, J. Enhanced nitrogen removal of the simultaneous partial nitrification, anammox and denitrification (SNAD) biofilm reactor for treating mainstream wastewater under low dissolved oxygen (DO) concentration. Bioresour. Technol. 2019.[CrossRef] [PubMed] 41. Puznava, N.; Payraudeau, M.; Thornberg, D. Simultaneous nitrification and denitrification in biofilters with real time aeration control. Water Sci. Technol. 2001, 43, 269–276. [CrossRef][PubMed] 42. Zinatizadeh, A.A.L.; Ghaytooli, E. Simultaneous nitrogen and carbon removal from wastewater at different operating conditions in a moving bed biofilm reactor (MBBR): Process modeling and optimization. J. Taiwan Inst. Chem. Eng. 2015, 53, 98–111. [CrossRef] 43. Choo, K.H.; Stensel, H.D. Sequencing batch membrane reactor treatment: Nitrogen removal and membrane fouling evaluation. Water Environ. Res. 2000, 72, 490–498. [CrossRef]

© 2019 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 (http://creativecommons.org/licenses/by/4.0/).