Mathematical Modelling and Determination of the Nickel Inhibition Constant for Nitrification

Abstract

Keywords: Nitrification; nickel; nitrification kinetics; inhibition.

1. Introduction

Nitrification and denitrification processes are widely used in nitrogen removal from wastewater. Nitrification involves conversion of nitrogenous compounds into nitrite and nitrate by Nitrosomonas and Nitrobacter respectively [1-3]. Biological nitrification is a known method to reduce high ammonium concentrations in wastewater down to acceptable levels in receiving bodies. The kinetics and performance of a nitrification system depend on the slow growth rate of nitrification bacteria and their sensitivity to metallic toxic substances [4]. Since nitrification bacteria are sensitive to chemicals, they are considered to be possible biological analysis organisms in evaluation of toxicity levels [5]. As nitrification bacteria grow slowly and are sensitive to physical, chemical and environmental conditions, this may lead to nitrification instability in purification systems [3]. Nitrification bacteria rapidly enrich even at low temperatures during a longer sludge retention time [6,7]. Ni and Zn concentrations are present on high levels in various industrial wastewaters. Nickel is found in batteries, roll coatings, copper forming, electrical and electronic components, electroplating, iron and steel waste. Nickel concentrations up to 2950 mg dm- 3 may be seen in electroplating wastewaters [8,9]. Heavy metals have a toxic effect on bacteria at certain *

Corresponding Author: e-mail: adincer@nku.edu.tr, tel: (282) 250 23 14

concentrations and a negative impact on biological treatment efficiency [10]. Discharge of industrial and urban wastewaters containing heavy metals to receiving bodies is a significant cause of pollution [11]. Sato et al. (1986) found that inhibition of the growth of Nitrosomonas europaea is highly associated with nickel amine compounds[12]. Many studies have examined the effects of heavy metals on nitrification bacteria, because these metals and their complexes can inhibit nitrification by passing through bacterial cell membranes and disrupting their protein structure [13-15]. The most common theory used to explain the effects of cations on microorganisms is that a cation damages or neutralizes the critical enzyme [16]. A study by Harper et al. (1996) showed that, in the nitrification - denitrification process in leachate treatment, chromium (Cr+ 3) and nickel (Ni+ 2) cause inhibitory effects in the system at soluble concentrations of approximately 0.30 and 0.70 mg/L, respectively[17]. At a 1.0 mg / l nickel concentration, nitrification was partially affected, and there was a decrease in the NO3-N concentration and an increase in the NH4-N concentration [18]. In a system, nitrification inhibition was investigated by emphasizing the effect of copper and nickel shock load. As a result of applying different copper and nickel concentrations, Nitrosomonas was found to be equal to or more sensitive than Nitrobacter [5]. According to Skinner et al. (1961), Ni+2 has an inhibition effect on Nitrosomonas bacteria at a concentration of 0.25 mg/L[19]. Low nickel concentrations increase growth rates, while high nickel concentrations can inhibit nitrification processes [20,21]. For ammonia oxidizing bacteria and nitrite oxidizing bacteria, based on the Ni / MLSS ratio at different temperatures, the nickel inhibition half-speed constants (KI, Ni) were found as 5.4 and

Experimental setup

The laboratory-scale experimental setup is shown Figure 1. The volumes in the aeration and settling tanks were 8.0 L and 3.0 L, respectively. The sedimentation tank was separated from the aeration tank. The reactor was continuously aerated, and the dissolved oxygen concentration was kept above 4.0 mg/L. The temperature in the reactor was 25 ± 1 °C. Synthetic Wastewater The synthetic wastewater was composed of 150 mg/L NH4-N, 800 mg/L alkalinity (CaCO3), 45 mg/L KH2PO4. 0.5 molar NaHCO3 was automatically added to the aeration tank to keep the reactor pH at 7.5 ± 0.5. The nickel concentration ranged from 0 to 0.017 g/L. NiSO 4.6H2O was used as the nickel source. Influent wastewater was prepared synthetically with pure water. The micronutrients contained 50 mg/L MgSO4.7H2O, 5 mg/L MnSO4. H2O, 10 mg/L FeSO4.7H2O,

3. Results And Discussions

The optimum sludge age and hydraulic retention time were studied in the first-phase experiments. Figure 2 shows the change in nitrate-nitrogen, nitrite-nitrogen and ammoniumnitrogen ions depending on the sludge age in a nitrification system. In this experiment, the optimum sludge age was determined in the system by using nickel-free wastewater. The hydraulic retention time was fixed for 30 hours. Each experiment was performed for a week after the system reached a steady state. The ammonium and nitrite concentrations decreased at the reactor effluent with increasing sludge age. It was determined that the nitrate concentration increased with increasing sludge age. The effluent ammonium nitrogen and nitrate nitrogen exchanges were found approximately the same in the sludge age range of 16-30 days. When the sludge age increased from 4 days to 20 days, the effluent nitrate nitrogen increased from 100.6 to 139.8 mg N/L. Contrary to the nitrate concentration, when the sludge age increased from 4 days to 20 days, the waste nitrite nitrogen decreased from 12.3 mg / L to 0.9 mg/L. When the sludge age was 4, 20 and 30 days, the effluent ammonium nitrogen concentration was found to be 32.1, 1.5 and 0.3 mg N / L, respectively. The optimum sludge age was found to be 20 days. Accumulation of nitrite in a short hydraulic retention time was due to the slow growth of Nitrobacter compared to Nitrosomonas bacteria. Hall et al. (1985) reported that more nitrate and biomass were formed during sludge retention times longer than 5 days[24]. The study conducted by Hocaoğlu et al. (2011) stated that nitrogen removal efficiency and kinetics changed significantly as a function of dissolved oxygen concentration and sludge age[25].

Sludge age (ϴc)(days) Figure 2. Variation of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen with sludge age (ϴH=30 hours) The change in NH4+-N, NO2--N and NO3--N with the hydraulic retention time for nickel-free wastewater is shown in Figure 3. The purpose of this experiment was to determine the optimum hydraulic retention time at a fixed sludge age (20 days). The effluent ammonium nitrogen concentration showed a decreasing trend with increasing hydraulic retention time. Similarly, the effluent nitrite concentration decreased with increasing hydraulic retention time. When the hydraulic retention time increased from 3 to 18 days, the effluent nitrate-nitrogen concentration increased from 55.7 to 141.2 mg N/L. Moreover, the nitrite nitrogen concentration decreased from 26.4 mg/L to 0.9 mg/L. In shorter hydraulic retention times, the nitrite buildup in the environment was due to the slow growth of Nitrobacter. The effluent ammonium nitrogen concentration decreased from 150 mg/L to 1.5 mg/L when the hydraulic retention time was 18 hours. The optimum hydraulic retention time was found to be 18 hours, resulting in almost complete nitrification (99%). Increases in the hydraulic retention time of more than 18 hours had little effect on the system’s performance. In biological treatment, nitrification is limited to several genera, often called Nitrosomonas and Nitrobacter. The slow growth of these bacteria makes the nitrification process very sensitive to inhibition [26].

Hydrolic Retention Time (ϴH)(hour) Figure 3. Variation of ammonia nitrogen, nitrate nitrogen and nitrite nitrogen with hydraulic retention time (Hours). The effects of the nickel concentrations on the ammonium removal efficiency and conversion rate of ammonium to nitrite and nitrate are given in Figure 4. The conversion rates of nitrite and nitrate at 150 mg/L influent ammonium concentration were considered as the performance criteria at different nickel concentrations. The sludge age and hydraulic retention time were 20 days and 18 hours, respectively. Dinçer et al. (2000) found that nitrification efficiency increased with increasing sludge age (>12 days) and hydraulic retention time (>15 hours) [27]. The experiments were carried out with a nickel content of 3-17 mg/L to investigate the effects of nickel on nitrification. The nitrification efficiency was not affected by nickel concentrations below 3 mg/L. When the Ni concentration increased to 9 mg/L, the nitrification efficiency decreased by more than 30.5%. In a study by Lee et al. (1997), nickel concentration did not produce a significant inhibition of up to 100 mg/L[5]. A loss of 35% yield was detected at a 150 mg/L nickel concentration [5]. As seen in Figure 4, the system did not reach a state due to the inhibitory effects of nickel. The nitrate nitrogen concentrations in the wastewater decreased, and the nitrite nitrogen slowly increased with increasing nickel concentrations. A sharp increase in the effluent ammonium nitrogen concentration was found at nickel concentrations greater than 7 mg/L. In a study similar to this study, a 1.0 mg/L nickel concentration did not adversely affect nitrification, and little inhibition occurred at 5 and 10 mg/L nickel concentrations. It resulted in significant inhibition at a concentration of 50 mg/L of nickel [3]. The effect of heavy metals on nitrifying bacteria may potentially inhibit nitrification by disrupting proteins as metals and their complexes are transported across bacterial cell membranes [14,15].

Nickel Concentrations (mg/L) Figure 4. Effect of nickel concentrations effluent NO3-Ne, NH4-Ne and NO2-Ne concentrations Figure5 shows the variation of the nitrification rate with the nickel concentration. As the nickel concentration increased from 0 to 7 mg/L and 17 mg/L, the nitrification rate decreased from 8.17 mgN/L.h to 6.93 mgN/L.h and 3.22 mgN/L.h, respectively. When the nickel concentration increased from 3mg/L to 17 mg/L, the nitrification removal rate decreased by 60.59%. The reduction in the nitrification removal rate at 3mg/L nickel concentrations was around 6%. While 2 mg / L Ni (II) showed a slight increase in microbial nitrification rates and related enzyme activities, 5–30 mg / L Ni (II) was found to decrease basic enzymatic activity [28].

Nickel Concentrations (mg/L) Figure 5. Variation of NH4+-N removal rate with nickel concentrations In the current literature, there are many studies on heavy metals that inhibit nitrification [25,8,9,12-14,17-19]. There is no mathematical model for inhibition of nickel on nitrification. Using the experimental data with different nickel concentrations, a plot of 1/NR against Ni was created as shown in Figure 6. The following rate equation was used to determine the nickel inhibition constant [13,27,29,30]. NR =

Where KNi is the nickel inhibition constant (mg/L); Ni is the nickel concentration (mg/L); NR is the nitrification rate (mg/L.h); NRo is the nickel-free nitrification rate. Equation 1 was converted to the following figure 1 NR

Figure 6 shows the 1/NR values against the Nickel concentrations. The following values were found from the slope and intersection of the line given in Figure 6. NRo=10.13 mg/L.h and KNi=8.75 mg/L. When the coefficients obtained from Figure 6 were replaced by equation 1, the following equation was obtained for nitrogen removal from nickel-containing wastewater. NR = 10.13

Nickel Concentration(mg/L) Figure 6. Variation of nitrification rate versus nickel concentration for determination of the nickel inhibition constant.

4. Conclusions

The yields of nitrification bacteria in a continuous flow reactor were investigated at different nickel concentrations. The ammonium removal efficiency decreased sharply at higher nickel concentrations in comparison to lower nickel concentrations. No sharp increase in the toxicity was observed until the nickel concentration increased to 7 mg/L. However, at 9 mg/L, there was a rapid increase in the ammonium in the effluent and a decrease in the nitrate concentration. The nitrite accumulation in the medium was limited against nickel inhibition. Increased nickel concentration had a greater impact on conversion of ammonium into nitrite. Based on the available data, the nickel inhibition constant KNi was determined to be 8.75. A mathematical model was developed in line with the available data.

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