Characteristics of hydrogenotrophic denitrification in a combined system of gas-permeable membrane and a biofilm reactor

Journal of Hazardous Materials 168 (2009) 1581–1589

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Characteristics of hydrogenotrophic denitrification in a combined system of gas-permeable membrane and a biofilm reactor Caixia Lu a , Ping Gu a,∗ , Pan He a , Guanghui Zhang a , Chao Song b a b

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China

a r t i c l e

i n f o

Article history: Received 24 October 2008 Received in revised form 11 March 2009 Accepted 11 March 2009 Available online 21 March 2009 Keywords: Hydrogenotrophic denitrification Gas-permeable membrane Kinetics Drinking water

a b s t r a c t A double Monod form was employed to describe two-step hydrogenotrophic denitrification, and the saturation constants of nitrate, nitrite and hydrogen were determined by batch tests. A combined system of gas-permeable membrane and a biofilm reactor (GPM–BR) was employed to remove nitrate from drinking water. The gas-permeable membrane was tested to exclusively deliver hydrogen to an independent attached growth system. The denitrification performance of the GPM–BR was investigated with different nitrate loadings of 96.78, 163.16 and 342.58 mg N/(L d). The nitrate removal rate (NRR) of the reactor could achieve 471.36 mg N/(L d) with sufficient dissolved hydrogen (DH) in the batch tests. While in the continuous experiments, NRR ranged from 96.72 to 301.44 mg N/(L d) under different nitrate loadings. Although low nitrate loading of 96.78 mg N/(L d) led to better nitrate removal, the denitrification capacity of GPM–BR would be limited and sulfate reduction occurred. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Nitrate contamination of drinking water represents a significant risk to human health, as these compounds are directly responsible for methemoglobinemia in infants and may increase the probability of cancer. The Chinese Standards for Drinking Water Quality have set a maximum concentration limit of 10 mg N/L for nitrate and a referenced concentration of 0.3 mg N/L for nitrite. Hydrogenotrophic denitrification as a biological process can remove nitrate efficiently, with the added hydrogen acting as electron donor, the nitrate in raw water as electron acceptor, and inorganic carbon serving as the carbon source. Advantages of hydrogenotrophic denitrification include: (1) the clean and nontoxic nature of hydrogen; (2) easy hydrogen removal from treated water; (3) low biomass yield; (4) adaptability to the oligotrophic environment of drinking water; (5) the fact that no organic carbon is required; and (6) the low cost of hydrogen [1–3]. Therefore, hydrogenotrophic denitrification is a good option for drinking water treatment. However, high flammability, high explosivity and low solubility are the main drawbacks of hydrogen that have prevented the widespread acceptance of hydrogenotrophic denitrification [4]. In recent years, various novel approaches for hydrogen delivery have been developed and applied to hydrogenotrophic denitrification processes, including gas-permeable membranes,

∗ Corresponding author. Tel.: +86 022 27405059; fax: +86 022 27405059. E-mail addresses: [email protected] (C. Lu), [email protected] (P. Gu), [email protected] (P. He). 0304-3894/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2009.03.051

electrochemical methods and release of supersaturating hydrogen [5–7]. Gas-permeable membranes have many advantages. They can achieve better hydrogen transfer, enhance hydrogen utilization efficiency, and limit explosion risks by means of bubble-less hydrogen diffusion. Gas-permeable membranes can be used in combination with either suspended growth or attached growth systems [5,8,9]. Many investigators focus their attention on the latter because a gaspermeable membrane can act as both the hydrogen diffuser and the biofilm carrier. Hollow-fiber membranes are typically employed as gas-permeable membranes [1,5,8,10], although silicon tubes have been tested as well [11,12]. Hydrogen flows through the lumen and diffuses into the bulk liquid through membrane walls. Biofilm formation occurs on the outside membrane wall when nitrate exists as an electron acceptor in the bulk liquid. The counter-diffusion of hydrogen and nitrate has been shown to achieve high nitrate flux into the biofilm and hydrogen utilization efficiencies of up to 100% [5]. Although biofilm formation on the gas-permeable membrane surface may actually increase the utilization efficiency of hydrogen and nitrate, there are still many concerns to address. First, hydrogen diffusion and biofilm growth both depend on the permeable performance of membrane. Both processes interact with each other, leading to poor stability of denitrification system and difficulty in biomass control. Second, the transfer of hydrogen to bulk liquid will be impeded, decreasing the zone of influence around membranes [13]. Thus, in this research, a combined system of gas-permeable membrane and a biofilm reactor (GPM–BR) was employed for hydrogenotrophic denitrification. The gas-permeable membrane was used to exclusively supply hydrogen to an inde-

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Fig. 1. Apparatus for the determination of kinetic parameters.

pendent attached growth system to avoid the biomass lapse by cleaning or replacing the membrane, and the denitrifying bacteria in the biofilm reactor could take full advantage of the dissolved hydrogen (DH) in the bulk liquid. The denitrification performance of GPM–BR was investigated under continuous operation with different nitrate loadings. In addition, considering that hydrogenotrophic denitrification kinetics is expressed using a double Monod form, the denitrification performance of the reactor is concerned with the kinetic parameters of hydrogenotrophic denitrification, especially the saturation constants of substrates. Due to the mass transfer resistance, the saturation constant is usually measured higher in the suspended growth system than that in the attached growth system [14]. Therefore, batch tests in an attached growth system were conducted to determine the substrate saturation constants for characterizing denitrification. 2. Hydrogenotrophic denitrification kinetics After a series of reduction reactions, NO3 − → NO2 − → NO → N2 O → N2 , nitrate is ultimately converted to nitrogen gas and removed from the denitrification system. The sequencing denitrification can be simplified into two steps [15]: NO3 − + H2 → NO2 − + H2 O

(1)

NO2 − + (3/2)H2 + H+ → (1/2)N2 + 2H2 O

(2)

And the overall reaction is: NO3 − + (5/2)H2 + H+ → (1/2)N2 + 3H2 O

(3)

A double Monod form is employed to describe the two-step denitrification. Dual substrate limitation is assumed: nitrate and hydrogen are the limiting substrates for Reaction (1), while nitrite and hydrogen limit Reaction (2). Thus, the kinetic expressions are r1 = rmax1

CN1 CH CN1 + KN1 CH + KH1

(4)

r2 = rmax2

CN2 CH CN2 + KN2 CH + KH2

(5)

where, r1 and r2 are the reduction rates of nitrate and nitrite, respectively, in mg N/(L h). The variables rmax1 and rmax2 represent the maximum reduction rates of nitrate and nitrite, respectively, and are considered constant over a short period due to the slow growth of denitrifying bacteria, in mg N/(L h). Both r and rmax are related to biomass, but the ratio of r/rmax is not. CN1 and CN2 are nitrate and nitrite concentrations in the bulk liquid, respectively, in mg N/L. CH is the DH concentration in mg/L. KN1 and KN2 are the saturation constants of nitrate and nitrite, respectively, in mg N/L; KH1 and KH2 are the hydrogen saturation constants for Reactions (1) and (2), respectively, in mg/L.

3. Materials and methods 3.1. Experimental setups 3.1.1. Determination tests of kinetic parameters All batch tests were performed in a 2 L conical beaker that was completely sealed with a rubber stopper (Fig. 1). Hollow cylindrical media with a total area of 0.29 m2 were suspended in the upper part of the conical beaker to serve as a biofilm carrier with a density to that of water. Hydrogen was supplied by a high purity hydrogen generator (DGH-300, Lanke, Tianjin) and diffused into the bulk liquid via a porous diffuser. Samples were taken by siphoning; the residual gas was discharged to the outside atmosphere, and equal pressure was maintained inside and outside of the conical beaker. The system was inoculated with a mixed culture from a previous hydrogenotrophic denitrification system. The feed water was synthesized with tap water, phosphates, and nitrate/nitrite. 1100 mg/L of KH2 PO4 and 900 mg/L of K2 HPO4 were added to maintain a pH of 7.00 ± 0.15. Water temperature was controlled at (27 ± 1) ◦ C. A 1.5 L volume of feed water was added in each test. Four batch tests were carried out to determine KN1 , KH1 , KN2 and KH2 . 3.1.1.1. Tests of KN . Sampling was carried out 30 min after feeding to eliminate the influence of unstable factors in the beginning. Samples for KN1 test were taken every 15 min in the first 1 h and every 10 min in the following half hour, while for the KN2 test they were taken every 1 h for the first 3 h and every half hour for the following hour and a half. DH was maintained above 1.0 mg/L throughout both tests. 3.1.1.2. Tests of KH . The sampling procedure for two KH tests is illustrated in Fig. 2, wherein every pane represents a time unit of 0.5 h. The solid indicates the reaction period while the blank indicates the stabilizing period. Samples were taken at the beginning and end of each solid unit as a data pair, to calculate both the nitrate/nitrite reducing rate r and the average nitrate/nitrite concentration CN . After sampling at the end of the solid unit, 50 mL of tap water and 6–9 mg NO3 − -N (3–4 mg NO2 − -N) were added into the coni-

Fig. 2. Sampling procedure in the KN tests.

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Fig. 3. Schematics of GPM–BR in the continuous experiment.

cal beaker to compensate for water and nitrate/nitrite losses during sampling. 3.1.2. GPM–BR 3.1.2.1. Continuous denitrification experiment. The schematic diagram of the experimental system is shown in Fig. 3. The whole denitrification system comprised biological denitrification unit, hydrogen diffusion unit, and pH adjustment unit. Biological denitrification occurred in a biofilm reactor (BR) with a valid volume of 4.71 L. The carriers, which provided a total area of 1.46 m2 in BR, were identical to those in the kinetic parameter tests. Hydrogen was diffused via gas-permeable membrane (provided by Tianjin Polytechnic University) in a membrane tank (MT) with a total volume of 3.53 L. The membrane was microporous, with the maximum pore size of 0.16 ␮m to achieve the bubbleless hydrogen diffusion. The hollow fibers used in the construction of gas-permeable membrane were made from the hydrophobic polyvinylidene fluoride. The BR and MT both had double walls with 10 mm of space between them for thermal insulation. Hot water at a temperature of 30 ◦ C was circulated between the walls in the MT whenever the feed water temperature dropped below 20 ◦ C. A gas-permeable membrane with an area of 0.3 m2 was submerged in the MT for the first

32 d, and then replaced by another one with a 0.5 m2 area for the other days. A gas-permeable membrane with an area of 0.25 m2 was installed within a stainless vessel to diffuse carbon dioxide for pH adjustment. The feed water was lifted to a high-level water tank and introduced into the MT through a constant-level water tank. After being hydrogenated in the MT, the water flowed into the BR for nitrogen removal. A small portion of treated water left the denitrification system with the use of a peristaltic pump, and most was circulated and entered MT with fresh feeding water. The recycling flow rate was 60 L/h, and the effluent flow rate ranging from 0.82 to 2.75 L/h depended on the different hydraulic retention times (HRTs). The mixed culture used for inoculation also came from the previous hydrogenotrophic denitrification system. Feed water was synthesized from tap water with nitrate and phosphate additives. The feed nitrate concentration ranged from 32.09 to 51.18 mg N/L, with an average of 41.45 mg N/L. Phosphate, with an average concentration of 9.64 mg/L, served as the nutrient for denitrifying bacteria. The average influent pH was 7.71. The water temperature was maintained at (27 ± 3) ◦ C. The continuous experiment was divided into three periods with different nitrate loadings, as shown in Table 1.

Table 1 Operating conditions of the hydrogenotrophic denitrification system. Period 1 2 3 a b

Average. Standard deviation.

Time (d)

Influent nitrate (mg N/L)

HRT (h)

Nitrate loading (mg N/(L d))

0–10 11–43 44–60

40.32 ± 2.10 40.79 ± 4.00 42.82 ± 3.99

10 6 3

96.78 ± 5.05 163.16 ± 16.01 342.58 ± 31.93

a

b

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Fig. 4. Test for KN1 : (a) variation of nitrate concentration over reaction time; (b) linear relationship between 1/r1 and 1/CN1 .

3.1.2.2. Batch tests. Batch tests, called Test a and Test b, were conducted on Day 10 and Day 43 to investigate the denitrification capacity of GPM–BR in different periods of time. In the tests, feed valve and peristaltic pump were turned off. The composition of feeding water was similar to that in the continuous experiment. Water temperature was maintained at (29 ± 1) ◦ C.

by the potassium permanganate oxidation method. The pH was measured by a glass electrode (PP-15, Sartorius, Germany).

3.2. Analytical methods

A simplified approach was employed to measure the kinetic parameters. First, a DH concentration, CH , was maintained far higher than KH within the conical beaker, to ensure that CH /(CH + KH ) was approximately 1. Therefore, the reducing rate r was independent of CH according to Eqs. (4) and (5), enabling KN to be determined. KH was then measured according to the known KN . Finally, the values of KH and CH were compared to check whether the assumption that CH  KH was validated. In the tests of KN , CN was maintained within a narrow range to minimize its influence.

3.2.1. DH analysis DH concentration was measured using a headspace method. Liquid samples were taken with 21 mL auto-sampler vials. The vials were immediately sealed after sampling and shaken vigorously for a while until hydrogen reached its equilibrium between the gas and liquid phases. Hydrogen in the gas phase was measured and then converted to DH concentration according to Henry’s law and mass conservation law. The hydrogen concentration in the gas phase was determined via gas chromatography (Agilent 6890N, Agilent, USA) with a thermal conductivity detector (TCD). A 2-m long TDX-01 packed column was used for gas separation. The carrier gas (high purity nitrogen gas) flow was 10 mL/min. The injector and detector temperatures were 80 and 130 ◦ C, respectively. The initial oven of 50 ◦ C was held for 5 min, then gradually increased to 120 ◦ C at 20 ◦ C/min and maintained at 120 ◦ C for 1.5 min. The injection volume was 500 ␮L. A calibration curve for hydrogen in the gas phase was prepared following the principle that at a given temperature and atmospheric pressure, the DH concentration of hydrogen-saturated solution can be calculated according to Henry’s law [16]. Samples of different volumes can be taken from the hydrogen-saturated solution with vials. The hydrogen concentration in the gas phase in equilibrium can be calculated according to the known DH concentration and Henry’s law, and can also be measured using gas chromatography. The calculated hydrogen concentration was plotted against the detector’s output signal, and the plot was the calibration curve for hydrogen in the gas phase. The detection limit of DH concentration was below 0.001 mg/L for this method, and the coefficient of variation was 0.83%.

4. Results and discussion 4.1. Kinetic parameters

4.1.1. Kinetic parameters of the nitrate reduction reaction 4.1.1.1. Nitrate saturation constant KN1 . When CH  KH1 , Eq. (4) can be simplified as r1 = rmax

CN1 CN1 + KN1

After linearization, 1 KN1 1 1 = × + r1 rmax 1 CN1 rmax 1

(7)

The linear relationship between 1/r1 and 1/CN1 is plotted in Fig. 4. The ratios 1/rmax1 and KN1 /rmax1 were calculated using the yintercept and the slope of the linear fit graph, which are 0.0708 (L h)/mg N and 0.148 h, respectively. Thus, KN1 was found to be 2.09 mg N/L. 4.1.1.2. Hydrogen saturation constant KH1 . Since KN1 has been obtained, the value of CN1 /(CN1 + KN1 ) can be calculated. Thus, Eq. (14) becomes r1 = rmax1 R1

3.2.2. Ion, organic matter, and pH analyses Water quality analysis was conducted following the Chinese standard methods [17]. Nitrate, nitrite and sulfate were measured using an ion chromatograph (DX-600, Dionex, USA) with an ED50 conductivity detector. Dissolved organic carbon (DOC) was measured using a TOC analyzer (TOC-VCPH, Shimadzu, Japan) with an NDIR detector. Chemical oxygen demand (CODMn ) was determined

(6)

CH CH + KH1

(8)

where R1 = CN1 /(CN1 + KN1 ). After linearization, KH1 1 1 R1 = × + r1 rmax1 CH rmax1

(9)

The linear relationship between R1 /r1 and CH is plotted in Fig. 5.

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Fig. 5. Test for KH1 : (a) variations of nitrate and hydrogen concentration; (b) linear relationship between R1 /r1 and 1/CH .

According to the linear fit equation, 1/rmax1 was 0.0904 (L h)/mg N and KH1 /rmax1 was 0.0053 h. Thus, KH1 was determined to be 0.059 mg/L. Because the DH concentration in the bulk liquid was higher than 1.0 mg/L, CH /(CH + KH1 ) > 1.0/(1.0 + 0.059) ≈ 0.94, CH  KH1 was considered validated. 4.1.2. Kinetic parameters of nitrite reduction reaction Similar approaches are employed to determine kinetic parameters in the nitrite reduction reaction. 4.1.2.1. Nitrite saturation constant KN2 . After simplification and linearization, Eq. (5) becomes r2 = rmax2

CN2 CN2 + KN2

KN2 1 1 1 = × + r2 rmax2 CN2 rmax2

(10) (11)

Fig. 6 shows the linear plot of 1/r2 against 1/CN2 . The ratios 1/rmax2 and KN2 /rmax2 were determined to be 0.2058 (L h)/mg N and 0.3194 h, respectively. KN2 was calculated as 1.55 mg N/L. 4.1.2.2. Hydrogen saturation constant KH2 . Eq. (5) is converted to r2 = rmax2 R2

CH CH + KH2

where, R2 = CN2 /(CN2 + KN2 ).

(12)

After linearization, KH2 1 1 R2 = × + rmax2 CH rmax2 r2

(13)

The linear relationship between R2 /r2 and 1/CH is plotted in Fig. 7. The ratio 1/rmax2 was 0.3253 (L h)/mg N, KH2 /rmax2 was 0.002 h, KH2 was 0.006 mg/L, and CH /(CH + KH2 ) > 1.0/(1.0 + 0.006) ≈ 0.99. Thus, the assumption that CH  KH2 was verified. Because the kinetic parameters were experimentally obtained, errors were unavoidable. Four parameters were obtained from four tests without replication, respectively. In the KN1 and KH1 tests, two values were obtained for rmax1 , 14.12 and 11.06 mg N/(L h), respectively. Similarly, 4.86 and 3.07 mg N/(L h) were obtained for rmax2 in the KN2 and KH2 tests. Considering that each pair tests were conducted within a week, the biomass would not be varied too much. So each pair values of rmax are comparable to evaluate the reliability of the method. Kurt et al. [15] employed the same double Monod form to describe the two-step denitrification. The measured kinetic parameters were 0.18 mg N/L for KN1 , 0.16 mg N/L for KN2 and both KH values were lower than 0.002 mg/L, which was an order of magnitude lower than those determined in this research. This may be due to the different dominant denitrifying bacteria species in the different hydrogenotrophic denitrification reactors. In addition, it may be related with the hydraulic conditions of the reactor and the carrier of the biofilm. The turbulent diffusion in the reactor and the thickness of the biofilm layer both have an influence on mass

Fig. 6. Test for KN2 : (a) variation of nitrite concentration over reaction time; (b) linear relationship between 1/r2 and 1/CN2 .

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Fig. 7. Test for KH2 : (a) variations of nitrite and hydrogen concentration; (b) linear relationship between R2 /r2 and 1/CH .

transfer resistance, and thus on the saturation constant K. Rezania et al. [18] considered a zero-order kinetic model to describe the hydrogenotrophic denitrification based on the belief that the saturation constants of nitrate, nitrite, and hydrogen were so low that their influences on denitrification could be neglected. Vasiliadou et al. [19] proposed a kinetic model, considering the double nutrient limitation (nitrate, nitrite) with inhibition from nitrate. The DH concentration was assumed negligible. The kinetic parameters KN1 and KN2 were measured at 6.46 and 1.46 mg N/L, respectively. 4.2. Denitrification performance of GPM–BR The denitrification performance of the reactor under continuous operation was shown in Fig. 8. Overall, the effluent nitrate was lower than 1 mg N/L for most of the time, and only several higher concentration points appeared in Period 2 and Period 3. However,

Fig. 8. Nitrogen removal by GPM–BR running in continuous mode.

there was a relative high nitrite concentration in the effluent of Period 2 and Period 3, with maximum concentrations of 9.12 and 14.91 mg N/L, respectively. This indicated that further treatment of the effluent was required, which could be achieved by chlorine oxidation to convert nitrite to nitrate [20]. Therefore, achieving a total nitrogen (TN) concentration below 10 mg N/L in the effluent was considered the control target of the denitrification system. The effluent TN fluctuated due to the unstable effluent nitrite, almost undetected in Period 1, observed but less than 10 mg N/L in Period 2, and some exceeding the requirement in Period 3. The effluent DH ranged from 0.071 to 0.140 mg/L under the stable condition. 4.2.1. Denitrification performance in Period 1, Period 2 and Period 3 4.2.1.1. Period 1. Effluent concentrations of nitrate and nitrite were very low at less than 0.09 and 0.15 mg N/L. Little TN was observed in the effluent. On Day 8, a sulfate decrease of 24.87 mg/L was observed (the influent was 100.06 mg/L while the effluent was 75.19 mg/L). On Day 10, the effluent sulfate was only 40.07 mg/L. The effluent water was in a generally bad condition, with an ashy color and foul odor. Some minor flocculates suspending in the effluent were observed, inferred from the disaggregated biofilm. Due to the long HRT, the GPM–BR achieved perfect nitrogen removal, and few substrates were left in the bulk liquid for denitrifying bacteria. After this longterm dearth of substrates, the bacteria came into the decay stage, resulting in the poor effluent water quality. Moreover, superabundant hydrogen led to a DH concentration of 0.140 mg/L in the bulk liquid. High DH, little TN and 90–100 mg/L sulfate present in the original tap water created an optimal condition for sulfate-reducing bacteria growth, which would have become dominant if this condition lasted very long. It has been reported that an increase in hydrogen pressure could clearly provide suitable environment for sulfate-reducing bacteria: denitrification was almost completed over a long time, leading to a decrease in ORP. Sulfate reduction preferably occurred when ORP dropped to −300 mV and a 50% reduction of sulfate was observed [11]. In conclusion, although better nitrogen removal could be achieved under low nitrate loading, other water quality indices deteriorated. 4.2.1.2. Period 2. In the first days of Period 2, the effluent nitrate was lower than 0.33 mg N/L. The effluent nitrite experienced an increase at first, and then decreased from 5.50 to 2.35 mg N/L. This pattern was attributed to an adaptation to the shock nitrate loadings brought on by shorter HRT. On Day 20, the gas-permeable membrane was hydraulically cleaned to remove the biofilm on the

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surface of membrane. Although the biofilm was removed, the membrane pores were blocked with vapor due to the high intensity cleaning, which led to a decrease in hydrogen flux and an increase in membrane resistance. Therefore, the gas-permeable membrane was replaced with a new one with a 0.5 m2 area. The effluent DH increased abruptly from 0.082 to 0.278 mg/L, and maintained at 0.12–0.13 mg/L under stable operation. The effluent nitrate and nitrite also declined rapidly to an undetectable level. 4.2.1.3. Period 3. In the earlier days of Period 3, the effluent nitrate and nitrite reached up to 11.65 and 14.91 mg N/L, respectively, due to a higher nitrate loading up to 409.44 mg N/(L d). When the nitrate loading decreased to about 320 mg N/(L d), effluent nitrate fell below 2 mg N/L, and the effluent TN was controlled below 10 mg N/L. 4.2.2. NRR and TNRR Nitrate/TN removal rate (NRR/TNRR, mg N/(L d)) is defined as the amount of nitrate/TN removed every day by the unit-volume reactor, and it is used as a parameter to describe the denitrification performance of GPM–BR. The expressions for continuous experiment and batch tests are: Continuous experiment NRR(TNRR) =

(14)

Ci V − Ce V C − Ce × 24 = i × 24 t×V t

(15)

4.2.2.2. NRR, TNRR and DE in continuous experiment. Denitrification efficiency (DE) is defined to examine the denitrification performance of the GPM–BR,

(16)

DE(NRR) =

The differential form of Eq. (15) is NRR(TNRR) = −

dCN1(TN) dt

× 24

NRR = 24r1

(17)

where Ci is the influent nitrate/TN concentration in mg N/L; Ce is the effluent nitrate/TN concentration in mg N/L; V is the volume of the reactor in L; HRT is the hydraulic retention time for the continuous experiment in h; and t is the reaction time for the batch test in h. When nitrate is the sole nitrogenous substrate present during the denitrification process, the variation rate of nitrite concentration in the bulk liquid over time is the difference between the nitrite production rate in Reaction (1) and the nitrite consumption rate in Reaction (2). According to the stoichiometric relation between nitrite and nitrate in Reaction (1), the nitrite production rate equals r1 . Thus, dCN2 /dt = r1 – r2 , and r3 = −

correlations were obtained. The slopes of the linear fit corresponded to the reduction rates of nitrate and TN, r1 and r3 , respectively. In the tests, the feed nitrate was about 40 mg N/L. Although nitrate concentration in the reactor decreased gradually during the denitrification process, it was still much higher than its saturation constant KN1 . Moreover, the DH concentration increased due to the higher rehydrogenation rate compared to the hydrogen consumption rate. Looking at Test b for example, the DH concentration increased to 0.804 mg/L at 2 h, which was far higher than its saturation constant KH1 . Thus, r1 was approximately equal to rmax1 , and (NRR)max was calculated by Eq. (20). Furthermore, r3 /r1 was greater than 94% in both Test a and Test b, so (TNRR)max could be calculated by Eq. (21). In Test a, the initial nitrate was 42.06 mg N/L. Five hours later, the nitrate was almost completely removed with a residual concentration of 0.75 mg N/L. The accumulated nitrite concentration reached a maximum of 1.86 mg N/L. No nitrate and nitrite were observed after 6 h. The measured (NRR)max and (TNRR)max were 199.68 and 192.00 mg N/(L d), respectively. An HRT of 5 h could meet the requirements for effluent nitrate and nitrite concentrations in continuous experiment. Thus, a conservative HRT of 6 h was adopted in Period 2. In Test b, the initial nitrate concentration of 43.07 mg N/L dropped to 4.43 mg N/L after 2 h. Nitrite and TN were 2.39 and 6.82 mg N/L, respectively. No nitrate or nitrite was detected after 3 h. (NRR)max and (TNRR)max were 471.36 and 443.52 mg/(L d), respectively. Thus, a conservative HRT of 3 h was applied to Period 3.

Ci V − Ce V C − Ce × 24 = i × 24 HRT × V HRT

Batch test NRR(TNRR) =

1587

dCTN dCN1 /dt + dCN2 =− = r1 − (r1 − r2 ) dt dt

(18)

where r3 is the reduction rate of TN in mg N/(L h). TNRR = 24r3

(19)

4.2.2.1. Maximum NRR/TNRR. When CN1  KN1 and CH  KH1 , the nitrate reduction rate r1 achieves its maximum rmax1 according to Eq. (4), and it is independent of both nitrate and DH concentrations. Here, NRR achieves its maximum, (NRR)max = 24rmax1

(20)

Because r1 could not be larger than r2 when nitrate is the only nitrogenous substrate present during the denitrification process, TNRR reaches its maximum when r1 = r2 : (TNRR)max = 24rmax1

(21)

Batch tests, Test a and Test b, were carried out to determine NRR and TNRR (Fig. 9) for use in determining the appropriate HRT. Nitrate and TN removals with reaction time t were fitted linearly, and good

NRR (NRR)max

DE(TNRR) =

TNRR (TNRR)max

(22) (23)

Period 2 was divided into two stages, S1 (Days 11–32) and S2 (Days 33–43), with the membrane replacement as the borderline between them. NRR, TNRR and DE are shown in Table 2 for the different periods. The maximum NRR and TNRR were in the Test b, 471.36 and 443.52 mg N/(L d), respectively. In the continuous experiment, the average NRR in different periods ranged from 96.72 to 301.44 mg N/(L d) with the maximum 383.20 mg N/(L d). The results are comparable with that obtained by Lee and Rittmann [5], where a hollow-fiber membrane biofilm reactor was used for hydrogenotrophic denitrification. The results are also similar to that reported by Terada et al. [11], where a silicone tube membrane was used for hydrogen diffusion and fibrous slag was employed as the biofilm carrier. NRR and TNRR of GPM–BR were almost tripled as nitrate loading increased from 96.78 to 342.58 mg N/(L d). There was no obvious difference between NRR and TNRR in the first two periods. While in Period 3, a large difference appeared due to the high effluent nitrite concentration. In Period 1 and S1 of Period 2, DE values were lower than 50% and 40%, respectively. A low nitrate loading led to a low TN concentration in the bulk liquid, which was the main reason that GPM–BR did not perform at full capacity. When the nitrate loading increased from 96.78 to 163.16 mg N/(L d) and again from 163.16 to 342.58 mg N/(L d) there was approximately a 30% increase in DE each time. In S2 of Period 2 and Period 3, DE achieved 70% and 60% greater improvement over the performance in Period 1 and S2, respectively. However, 30–40% of the denitrification capacity was still not realized because of the low nitrate and DH concentrations in the bulk

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Fig. 9. Batch tests for determination of (NRR)max and (TNRR)max . Table 2 Nitrate removal rate (NRR), TN removal rate (TNRR) and denitrification efficiency (DE) in three periods, Test a and Test b. Nitrate

Period 1 Test a Period 2

S1 S2

Test b Period 3

TN

NRR (mg N/(L d))

DE (%)

TNRR (mg N/(L d))

96.72 ± 5.05 199.68 153.89 ± 18.71 167.98 ± 14.06 471.36 301.44 ± 50.06

48 ± 3 100 77 ± 9 36 ± 3 100 64 ± 11

96.65 ± 5.09 192.00 141.96 ± 16.07 166.53 ± 14.80 443.52 250.16 ± 57.76

liquid. Compared with the batch tests, there was no difference in magnitude between the nitrate concentration and its saturation constant. According to Eq. (4), the value of CN1 /(CN1 + KN1 ) depends on CN1, so the influence of nitrate concentration on denitrification cannot be neglected. Moreover, the DH concentration in the bulk liquid was generally stable at 0.07–0.08 mg/L—far lower than that in batch tests. Thus DH concentration was considered to be an important factor influencing the performance of the reactor. 4.2.3. Effluent organic matters CODMn is an aggregate indicator describing the contamination levels caused by organic matter and inorganic reductants. The

DE (%) 50 ± 3 100 71 ± 8 38 ± 3 100 56 ± 13

amount of organic matter cannot be correctly reflected by CODMn when nitrite exists in the water. Therefore CODMn was only measured in Period 1 and Period 2, when there was no nitrite in the effluent; in Period 2 and Period 3 when nitrite was present in the effluent, DOC was used instead of CODMn as the organic matter indicator. The increase in organic matter declined as nitrate loading increased (Table 3). In Period 1, the organic matter in the effluent was doubled, which might be related to the low nitrate loading. The effluent organic matter might primarily be from the decaying denitrifying bacteria that perished due to the lack of substrates. With a higher nitrate loading, nitrogenous substrates were present

Table 3 Variation of organic matter during the whole continuous experiment (mg/L). Period

1 2 3

DOC

CODMn

Influent

Effluent

– 2.45 ± 0.63 2.59 ± 0.60

– 3.32 ± 0.67 2.82 ± 0.48

Increased

Influent

Effluent

Increased

0.87 ± 0.33 0.23 ± 0.21

3.07 ± 0.33 2.44 ± 0.38 –

6.22 ± 0.12 4.00 ± 0.71 –

3.15 ± 0.51 1.56 ± 0.47

C. Lu et al. / Journal of Hazardous Materials 168 (2009) 1581–1589

in the bulk liquid and provided substrates for the denitrifying bacteria. This changed the state of the denitrifying bacteria from the decay stage to the stable stage. The organic matter discharged to the bulk liquid were mainly microbial products, and presented a lower concentration than those from decay stage denitrifying bacteria. 5. Conclusions A hydrogenotrophic denitrification system was investigated, where gas-permeable membrane was used to exclusively deliver hydrogen to an attached growth system, and thus could be operated independently, including cleaning and replacing, without biomass lapse. In the batch tests, the maximum NRR reached 471.36 mg N/(L d) with sufficient DH in the bulk liquid. In the continuous experiment, under the different nitrate loadings of 96.78, 163.16 and 342.58 mg N/(L d), the effluent nitrate and TN could meet the requirement as expected, especially under the first two loadings. The NRR ranged from 96.72 to 301.44 mg N/(L d), while TNRR from 96.65 to 250.16 mg N/(L d). Although satisfactory effluent nitrate could be achieved at the low nitrate loading of 96.78 mg N/(L d), some concerns arose from our study. First, only 40–50% of the GPM–BR denitrification capacity was achieved. Second, the presence of less effluent nitrogenous substrates resulted in a substantial increase in effluent organic matter due to the dominance of denitrifying bacteria in the decay stage. Finally, sulfate reduction occurred, and the effluent water quality was deteriorated due to sulfate reduction. It is required further investigation on the control of sulfate reduction in the hydrogenotrophic denitrification system. Acknowledgements We appreciate the Tianjin Polytechnic University for the supply of gas-permeable membranes. References [1] B.E. Rittmann, Hydrogen-based membrane-biofilm reactor solves oxidized contaminant problems, Membrane Technology 2002 (11) (2002) 6–10. [2] R.L. Smith, S.P. Buckwalter, D.A. Repert, D.N. Miller, Small-scale, hydrogenoxidizing-denitrifying bioreactor for treatment of nitrate-contaminated drinking water, Water Research 39 (10) (2005) 2014–2023.

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