Nitrite Accumulation during the Denitrification Process in SBR for theTreatment of Pre-treated Landfill Leachate*

SUN Hongwei (孙洪伟), YANG Qing (杨庆), PENG Yongzhen (彭永臻), SHI Xiaoning (时晓宁), WANG Shuying (王淑莹) and ZHANG Shujun (张树军)



Nitrite Accumulation during the Denitrification Process in SBR for theTreatment of Pre-treated Landfill Leachate*

SUN Hongwei (孙洪伟), YANG Qing (杨庆), PENG Yongzhen (彭永臻)**, SHI Xiaoning (时晓宁), WANG Shuying (王淑莹) and ZHANG Shujun (张树军)

Key Laboratory of Beijing Water Quality Science and Water Environment Recovery Engineering, Beijing University of Technology, Beijing 100124, China

The nitrite accumulation in the denitrification process is investigated with sequencing batch reactor (SBR) treating pre-treated landfill leachate in anoxic/anaerobic up-flow anaerobic sludge bed (UASB). Nitrite accumulates obviously at different initial nitrate concentrations (64.9,54.8,49.3 and 29.5 mg·L－1) and low temperatures, and the two break points on the oxidation-reduction potential (ORP) profile indicate the completion of nitrate and nitrite reduction. Usually, the nitrate reduction rate is used as the sole parameter to characterize the denitrification rate, and nitrite is not even measured. For accuracy, the total oxidized nitrogen (nitrate + nitrite) is used as a measure, though details characterizing the process may be overlooked. Additionally, batch tests are conducted to investigate the effects of C/N ratios and types of carbon sources on the nitrite accumulation during the denitrification. It is observed that carbon source is sufficient for the reduction of nitrate to nitrite, but for further reduction of nitrite to nitrogen gas, is deficient when C/N is below the theoretical critical level of 3.75 based on the stoichiometry of denitrification. Five carbon sources used in this work, except for glucose, may cause the nitrite accumulation. From experimental results and cited literature, it is concluded thatspecies may be contained in the SBR activated-sludge system.

landfill leachate, nitrite accumulation, denitrification, C/N ratio, carbon source, sequencing batch reactor

1 INTRODUCTION

Conventional biological denitrification involves a series of biological reduction processes, given in Eq. (1).In this process, nitrate is converted to nitrogen gas by nitrate reductase (NaR), nitrite reductase (NiR), nitric oxide reductase (NoR), and nitrous oxide reductase(N2oR), correspondingly [1].

Nitrite is one of the intermediates of nitrate reduction process. It is generally considered that reduction rate of nitrite is higher enough than that of nitrate, so that nitrite accumulation will not occur during the denitrification process [2]. However, nitrite accumulation in denitrification is frequently observed [3-9], probably due to the lower reduction rate of nitrite with respect to that of nitrate. Nitrite accumulation is caused by microbial communities with different characteristics for nitrate and nitrite reduction. Some denitrifiers with different denitrification ability are summarized in Table 1.

Table 1 Summary of characteristics of denitrifiers for nitrate and nitrite reduction

① Ref. [5];②Ref. [9].

The previous studies on nitrite accumulation in denitrification have some shortcomings. First, nitrite accumulation is mainly observed in the treatment of municipal wastewater, nitrogen fertilizer industry, and simulated wastewater, but it has not been reported on high strength landfill leachate. Second, the information on denitrification kinetics is not provide. Finally, there is not accurate and simple method to indicate the completion of denitrification to avoid the discharge of more harmful nitrite into the receiving water body.

Therefore, the objectives of this work are as follows. (1) To investigate nitrite accumulation during denitrification at different initial nitrate concentrations and low temperatures, (2) to perfect denitrification kinetics with the total oxidized nitrogen (nitrate +  nitrite), (3) to develop an accurate and simple method to indicate the complete reduction of nitrate and nitrite by using ORP as process control parameter, and (4) to investigate the effects of various carbon sources and C/N ratios on the nitrite accumulation in denitrification.

2 MATERIALS AND METHODS

2.1 Landfill leachate

2.2 Experimental setup and operation

The SBR has an effective volume of 9 L, with three ports on its sidewall for insertion of a DO probe, a pH probe and an oxidation-reduction potential (ORP) probe. Dissolved oxygen is supplied through a porous diffuser installed at the bottom of the SBR. The liquor and activated sludge are completely mixed by using a mechanical stirrer at a constant speed of 40 r·min－1.

The SBR operation is as follows. During the aerobic phase, ammonia nitrogen is oxidized completely to nitrate. Subsequently, the nitrate is reduced to nitrogen gas in the anoxic phase. Steady biological nitrogen removalnitrate is achieved for 116 days, and nitrite accumulation is not observed during the whole nitrification process.

2.3 Analytical methods

3 RESULTS AND DISCUSSION

3.1 Relation between concentrations of -N and -N and ORP in denitrification

The ORP is calculated based on Eq. (2) and the semi-reaction of nitrate and nitrite reduction, Eq. (3) and Eq. (4), for the stoichiometric calculation.

whereOis the standard oxidation-reduction potential (V),is the numbers of transferred electrons,is molar gas constant, 8.314 J·mol－1·K－1;is temperature(K);is Faraday constant, 96487 C·mol－1, [Red] is multiplication of all kinds of reduced state materials concentration, [Ox] is multiplication of all kinds of oxidized state materials concentration.

3.2 Denitrification kinetics

Generally, the denitrification rate is described by Monod equation [19]:

whereDNis denitrification rate, mg·L－1·d－1,is nitrate concentration, mg·L－1,is maximum specific utilization rate, d－1, andSis half-saturation constant, mg·L－1. The nitrate reduction rate is commonly used as the sole parameter to characterize the denitrification rate, and nitrite is not even measured. For accuracy, the total oxidized nitrogen (nitrate + nitrite) may be used as a measure, though details characterizing the process may be overlooked.

The reduction rates of nitrite and nitrate are equal at the peak point of nitrite concentration according to the Monod equation. Before the peak point, the nitrate reduction rate gradually decreases as the nitrate concentration decreases, and nitrite reduction rate increases with nitrite concentration increase. The difference between nitrate and nitrite reduction rates decrease until they are equal. After the peak point, both nitrate and nitrite reduction rates decrease as their concentrations decrease until the completion of denitrification.

3.3 Effect of C/N ratio on the rate of denitrification

Figure 2 shows the effect of C/N ratios on the denitrification rate. Significant nitrite accumulation is observed at five C/N ratios, though the C/N ratio of 2.25︰1 is below the theoretical critical value of 3.75 based on stoichiometry of denitrification. Table 2 shows the average rates of nitrate reduction, nitrite accumulation, and nitrite reduction, indicating that the nitrite accumulation is due to the lower reduction rate of nitrite with respect to that of nitrate in the denitrification process. The nitrite concentration in the final effluent is relatively high when C/N ratio is 2.25︰1, but the nitrate concentration is below 5 mg·L－1for five C/N ratios. In other words, the dosages of carbon source is sufficient for the reduction of nitrate to nitrite, but is deficient for further reduction of nitrite to nitrogen gas when C/N is below the theoretical critical level of 3.75. It is also found that both nitrate and nitrite are not reduced completely at C/N ratio of 3.75︰1, probably because some organics are assimilated by microorganism.

The nitrite reduction becomes rate-limiting step in the denitrification process with nitrite accumulation. It is concluded that if the denitrification is completed by two groups of cooperative denitrifiers, bacteria for nitrate reduction and those for nitrite reduction, the former is more competitive for trapping electron.

3.4 Effect of various carbon sources on the rate of denitrification

Batch tests were carried out using the sludge taken from the SBR reactor. In each test, 1L of mixture were used in each batch reactor. Five different carbon sources and NaNO3solution are added into the batch reactors (R1methanol; R2ethanol; R3sodium acetate; R4sodium propionate; R5glucose) at C/N ratio of 6︰1, to provide sufficient carbon sources. The temperature was around 13.9°C.

Figure 3 The effect of various carbon sources on the denitrification process at 13.9°C

△ methanol;★ ethanol; ○ sodium acetate;● sodium propionate;■ glucose

Table 3 The rates of nitrate reduction, nitrite reduction, nitrite accumulation during denitrification process

Note:Nitrite reduction rate: difference between measured rates of nitrate reduction and nitrite accumulation during nitrite accumulation phase.

Figure 3 shows the effect of five carbon sources on the nitrite accumulation in denitrification. Methanol, ethanol, sodium acetate, and sodium propionate are easily utilized by denitrifying bacteria to supply electron donor, in which nitrite accumulates obviously and reaches the peak concentrations, and then the denitrification is completed. However, when glucose is used as carbon source, both nitrate reduction and nitrite accumulation are not observed. This is different from the conclusions drawn by Blaszczyk. [4], who reported that significant nitrite accumulation in medium with glucose and nitrate. But it is agreement with the previous study, in whichcould not utilize glucose as electron donor [20] and reduction rates of nitrite is lower than nitrate [9]. Thus, it is concluded that theshould grow in the activated-sludge system. Table 3 shows that the average rates of nitrate reduction, nitrite accumulation, and nitrite reduction for different carbon sources during denitrification. The results further indicate that the main reason for nitrite accumulation is the lower reduction rate of nitrite with respect to that of nitrate.

4 CONCLUSIONS

Significant nitrite accumulation in denitrification is observed, which is caused by the lower reduction rate of nitrite than that of nitrate. The conclusions are as follows.

(1) It is feasible to use ORP as process control parameter to indicate the completion of nitrate and nitrite reduction based on the characteristic points of the “nitrate knee” and “nitrite knee” on its profile.

(2) In the denitrification with nitrite accumulation, the total oxidized nitrogen(nitrate + nitrite) should be used to perfect denitrification kinetics.

(3) The nitrite accumulation in denitrification is affected by types of carbon sources, rather than C/N ratio, andspecies should grow in the activated-sludge.

Table 2 The rates of nitrate reduction, nitrite reduction,nitrite accumulation during nitrite accumulation phase

Note:Nitrite reduction: difference between measured rates of nitrate reduction and nitrite accumulation.

NOMENCLATURE

ORP oxidation-reduction potential, mV

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the National Natural Science Foundation of China (50978003), the Natural Science Foundation of Beijing (8091001), the Funding Project for Academic Human Resources Development in Institutions of Higher Learning Under the Jurisdiction of Beijing Municipality (PHR 20090502), and the State Key Laboratory of Urban Water Resource and Environment (HIT) (QAK200802).

** To whom correspondence should be addressed. E-mail: pyz@bjut.edu.cn; shw@emails.bjut.edu.cn