Jianqiao QIN, Yongxin LI, Min DAI, Dongni YU

1. College of Environmental and Chemical Engineering, Zhaoqing University, Zhaoqing 526061, China; 2. Guangdong Provincial Key Laboratory of Environmental Health and Resource Utilization, Zhaoqing 526061, China

Abstract [Objectives] To investigate the removal effect of nitrifying bacteria on ammonia nitrogen in water. [Methods] In this experiment, the treated water (referred to as raw water hereinafter) from the Changping Town Qiaoli Water Treatment Project in Dongguan City of Guangdong Province was used as the experimental water body, and the nitrifying bacteria liquid used in the project was taken as the experimental material, to explore the removal effect of the nitrifying bacteria liquid on the ammonia nitrogen in the water body. [Results] Under the condition that other variables remain unchanged, the more the amount of nitrifying bacteria liquid added, the higher the removal efficiency of nitrifying bacteria liquid; under the same conditions, the removal effect of ammonia nitrogen in a stirred water body is better than that in an unstirred water body; the removal effect of ammonia nitrogen in a water body with bio-media/carriers is better than that without bio-media/carriers. [Conclusions] Nitrifying bacteria have a better removal effect on the ammonia nitrogen in the water body.

Key words Nitrifying bacteria, Ammonia nitrogen, Removal effect

1 Introduction

As China’s chemical industry is continuously developing and urbanization is accelerating, people’s requirements for environmental protection become higher, and China’s requirements for the limit of ammonia nitrogen in discharged sewage are becoming more and more stringent. Scholars both at home and abroad have conducted extensive studies about the removal of ammonia nitrogen in wastewater, and have found many denitrification methods. Among them, biological denitrification is an efficient biological treatment method. Due to its advantages of simple denitrification, low cost and high efficiency, the biological denitrification technology has been widely used in wastewater denitrification in recent years. Ammonia nitrogen in water can be removed by various biological denitrification processes. Compared with other physical and chemical treatment methods, biological denitrification has the advantages of lower energy consumption and lower operating cost[1]. Ammonia nitrogen in water can be removed by various biological denitrification processes.

1.1 Overview of nitrifying bacteriaNitrifying bacteria are Gram-negative bacteria and play an important role in the water purification process of nitrogen cycle. Bacillus, cocci and spiral bacteria are the main forms of nitrifying bacteria in nature, and most nitrifying bacteria are absolutely autotrophic microorganisms[2].

1.2 Brief introduction to ammonia nitrogen wastewater and its treatment method

1.2.1Sources and hazards of ammonia nitrogen wastewater. Ammonia nitrogen wastewater is characterized by a wide range of sources and large discharge. Its main sources include municipal sewage, fertilizer plant wastewater, coking wastewater, landfill leachate, gas wastewater, agricultural pollution wastewater,etc.[3]. With the continuous improvement of people’s living standards and the development of petroleum, chemical, food and pharmaceutical industries, the level of ammonia nitrogen in municipal sewage and landfills is constantly increasing.

The source industries involved in the formation of ammonia nitrogen wastewater are very wide and complex. Such wastewater contains a large amount of ammonia ions and free ammonia substances that have an impact on the environment and ecology. If these ions and substances are directly discharged into normal healthy water bodies without proper treatment by technical means, it will not only have an adverse effect on the growth of aquatic organisms, but also pose a great threat to human health[4].

1.2.2Brief introduction to treatment methods of ammonia nitrogen wastewater. (i) Chemical method. Chemical methods include chemical precipitation, folding point chlorination, ion exchange[5], and alkaline air stripping,etc.

(ii) Biological method. Biological denitrification is a process in which organic nitrogen and ammonia nitrogen in sewage undergo ammonification, nitrification, and denitrification under the combined action of microorganisms, and are finally converted into nitrogen gas. Specifically, the biological wastewater denitrification treatment process includes anaerobic-anoxic-oxic (A2O) oxidation ditch (OD) process, cyclic activated sludge technology (CAST), A2O-MBR process, A2O/A-MBR process, SBR process,etc.The principle of them: the nitrification reaction is carried out under aerobic conditions, and the denitrification reaction is carried out under anaerobic conditions; finally, the nitrogen-containing compounds in the sewage are released from the water in the form of nitrogen gas to achieve the purpose of denitrification of sewage[10].

(iii) Physical method. The physical denitrification method mainly includes the excavation of bottom sludge, increase of aeration amount, and use of zeolite and modified zeolite[11].

1.3 Significance and contents of this research

1.3.1Research significance. With the continuous development of the industrial economy and the increasing improvement of people’s living standards, the problems of water shortage and water pollution are becoming more and more serious. The large amount of nitrogen and phosphorus in the water body severely disrupts the ecological balance of the water body. China is a country with water shortages and extremely uneven temporal and spatial distribution. Various problems resulted from water pollution have seriously affected people’s daily lives and hindered the further development of society. How to remove nitrogen has become a universal issue in the field of water treatment[12].

1.3.2Research contents. In this experiment, the effect of nitrifying bacteria liquid on the removal of ammonia nitrogen in water under different conditions was studied. The method belongs to biological denitrification.

2 Materials and methods

2.1 Experimental principleNitrifying bacteria are Gram-negative bacteria and play an important role in the water purification process of nitrogen cycle. They are also a type of aerobic bacteria that can grow in oxygen-containing water or sand. Besides, nitrifying bacteria are typical chemical energy inorganic autotrophic bacteria with CO2as a carbon source. The oxidation of inorganic compounds through nitrification is the process of first oxidizing ammonia to nitrite and then oxidizing to nitrate. Nitrifying bacteria depend on the energy generated in this process for their metabolic needs[10].

In this experiment, Nessler’s reagent spectrophotometry was applied to determine the concentration of ammonia nitrogen in the water body, and the benefits of Nessler’s reagent spectrophotometry include simple and rapid operation, and high accuracy of analysis results[13].

2.2 Setting of experimental conditions(i) Stirring conditions (stirred and unstirred) and stirring method: mechanical automatic stirring, other conditions are the same: 10% nitrifying bacteria liquid, without bio-media/carriers. (ii) Bio-media/carriers (with or without bio-media/carriers), other conditions are the same: 10% nitrifying bacteria liquid, stirred; 10% nitrifying bacteria liquid, unstirred. (iii) Concentration of nitrifying bacteria liquid (5%, 10%, and 15%), other conditions are the same: stirred, without bio-media/carriers. (iv) Blank control group: the treated water (referred to as raw water hereinafter) from the Changping Town Qiaoli Water Treatment Project in Dongguan City of Guangdong Province.

2.3 Main materials, instruments and equipment

2.3.1Materials. The materials used in this experiment are mainly the treated water and nitrifying bacteria liquid (Nitrobacter) from the Changping Town Qiaoli Water Treatment Project.

2.3.2Instruments and equipment. Test tubes and multi-parameter water quality tester (model: 5B-3B (V8), Beijing Lianhua YongXing Science and Technology Development Co., Ltd., China).

2.4 Experiment process(i) In accordance with the different requirements stated in the above experimental conditions, 1 L water sample was prepared. (ii) The ammonia nitrogen concentration of the prepared water sample was measured, and then measured every 6 h measured twice, then measured every 12 h, measured twice, and then measured every 24 h, and the data were recorded. (iii) Determination of concentration of ammonia nitrogen in water: first, 10 mL of water sample was added to the test tube, 1 mL of N3reagent was added, and then 1 mL of N2reagent was added, then shook evenly and let stand for 10 min before the test. A 10 mm cuvette was used, a small amount of water sample was added to the cuvette, and then put into the multi-parameter water quality analyzer. Before putting into the analyzer, the cuvette should be cleaned to ensure that the results are not affected. (iv) The data were sorted out and analyzed.

2.5 Record of experimental data

2.5.1Record of the first group experimental data. Under different stirring conditions and the same other conditions, the measured ammonia nitrogen concentration in water is presented in Table 1.

Table 1 Ammonia nitrogen concentration in water under the first group experimental conditions

Fig.1 Ammonia nitrogen concentration in water under the first group experimental conditions

2.5.2Records of the second group experimental data. Under the same condition of 10% nitrifying bacteria liquid and different conditions of bio-media/carriers, the measured ammonia nitrogen concentration in water is shown in Table 2.

Table 2 Ammonia nitrogen concentration in water under the second group experimental conditions

2.5.3Records of the third group experimental data. Under the same conditions of stirring without bio-media/carriers and different conditions of concentration of nitrifying bacteria liquid, the ammonia nitrogen concentration in water was measured.

Due to insufficient instruments in the laboratory, the third group experiment was divided into two subgroups. The first subgroup was 5% and 10% nitrifying bacteria liquid concentration, and the second subgroup was 15% nitrifying bacteria liquid concentration, and the other experimental conditions were the same. The data of the first subgroup are shown in Table 3 and Fig.3, and the data of the second subgroup are given in Table 4 and Fig.4.

Fig 2 Ammonia nitrogen concentration in water under the second group experimental conditions

Fig.3 Ammonia nitrogen concentration in water under the first subgroup of the third group experimental conditions

Fig.4 Ammonia nitrogen concentration in water under the second subgroup of the third group experimental conditions

Table 3 Ammonia nitrogen concentration in water under the first subgroup of the third group experimental conditions

Table 4 Ammonia nitrogen concentration in water under the second subgroup of the third group experimental conditions

3 Results and analysis

3.1 Experimental resultsFrom the data obtained in the above experiment, the removal rate of ammonia nitrogen in water by nitrifying bacteria under different conditions can be obtained.

3.1.1Results of the first group experiment. Under different stirring conditions and the same other conditions, the measured removal rate of ammonia nitrogen in water by nitrifying bacteria is shown in Table 5 and Fig.5.

Table 5 Removal rate of ammonia nitrogen in water under the first group experimental conditions

Fig.5 Removal rate of ammonia nitrogen in water under the first group experimental conditions

3.1.2Results of the second group experiment. Under the same condition of 10% nitrifying bacteria liquid and different conditions of bio-media/carriers, the measured removal rate of ammonia nitrogen in water by nitrifying bacteria are presented in Table 6 and Fig.6.

Fig.6 Removal rate of ammonia nitrogen in water under the second group experimental conditions

3.1.3Results of the third group experiment. Under the same conditions of stirring without bio-media/carriers and different conditions of concentration of nitrifying bacteria liquid, the removal rate of ammonia nitrogen in water was measured. Due to insufficient instruments in the laboratory, the third group experiment was divided into two subgroups. The first subgroup was 5% and 10% nitrifying bacteria liquid concentration, and the second subgroup was 15% nitrifying bacteria liquid concentration, and the other experimental conditions were the same. The data of the first subgroup are presented in Table 7 and Fig.7, and the data of the second subgroup are shown in Table 8 and Fig.8.

Table 6 Removal rate of ammonia nitrogen in water under the second group experimental conditions

Table 7 Removal rate of ammonia nitrogen in water under the first subgroup of third group experimental conditions

Fig.7 Removal rate of ammonia nitrogen in water under the first subgroup of third group experimental conditions

Table 8 Removal rate of ammonia nitrogen in water under the second subgroup of third group experimental conditions

Fig.8 Removal rate of ammonia nitrogen in water under the second subgroup of third group experimental conditions

3.2 Analysis of experimental results(i) Removal rate of ammonia nitrogen in water by nitrifying bacteria under different stirring conditions and the same other conditions. As shown in Table 5 and Fig.5, whether it is stirred or unstirred, compared with the water sample without nitrifying bacteria liquid, the water sample with 10% nitrifying bacteria liquid has a higher removal rate of ammonia nitrogen. Besides, under the same conditions of adding 10% nitrifying bacteria liquid, the stirred water sample has a higher removal rate of ammonia nitrogen than the unstirred water sample.

(ii) Removal rate of ammonia nitrogen in water by nitrifying bacteria under the same condition of 10% nitrifying bacteria liquid and different conditions of bio-media/carriers. As shown in Table 6 and Fig.6, under the same conditions of stirring, the water sample with bio-media/carriers has a higher removal rate of ammonia nitrogen than the water sample without bio-media/carriers. In addition, under the same conditions of adding bio-media/carriers, the stirred water sample has a higher removal rate of ammonia nitrogen than the unstirred water sample.

(iii) Removal rate of ammonia nitrogen in water by nitrifying bacteria under the same condition of stirring without bio-media/carriers and different conditions of nitrifying bacteria concentration. Combining the data in Table 7 and Table 8 and charts in Fig.7 and Fig.8, it can be seen that the concentration of the added nitrifying bacteria liquid will affect the removal effect of ammonia nitrogen in the water sample. The higher the concentration of the added nitrifying bacteria liquid, the better removal effect of the ammonia nitrogen in the water sample.

3.3 Impact of COD on nitrifying bacteriaStudies have shown that if the COD value is too high, the activity of nitrifying bacteria will be inhibited. Nitrifying bacteria are autotrophic microorganisms. If the COD in the wastewater is too high, it will cause the rapid proliferation of heterotrophic bacteria, resulting in the decline of the proportion of nitrifying bacteria[14], accordingly influencing the removal effect of nitrifying bacteria liquid on ammonia nitrogen in the water body. However, the water in this experiment is the treated water, so the COD in the water is very small and close to zero, thus the influence on the experiment is very small.

4 Conclusions and discussion

4.1 ConclusionsFrom the experimental results of the first group, it can be known that if other conditions are the same, the removal rate of ammonia nitrogen in the stirred water body is significantly better than that in the unstirred water body. According to the experimental results of the second group, if other conditions are the same, the removal rate of ammonia nitrogen in the water body with bio-media/carriers is better than without bio-media/carriers.

The experimental results of the third group show that if other conditions are the same, the removal rate of ammonia nitrogen in the water body with nitrifying bacteria liquid is significantly better than that without nitrifying bacteria liquid. Also, with the increase in concentration of added nitrifying bacteria liquid, the removal rate of ammonia nitrogen also increases, but the changes are not large. After comparing the different concentrations of nitrifying bacteria liquid, it is found that the removal rate of ammonia nitrogen in the water body with 15% nitrifying bacteria liquid concentration is better than that with 10% nitrifying bacteria liquid concentration, and the removal rate of ammonia nitrogen in the water body with 10% nitrifying bacteria liquid concentration is better than that with 5% nitrifying bacteria liquid concentration. These indicate that the concentration of added nitrifying bacteria liquid will also influence the removal rate of ammonia nitrogen. In a certain range, the higher the nitrifying bacteria liquid concentration, the higher the removal rate of ammonia nitrogen.

4.2 DiscussionNitrifying bacteria have a significant effect on the removal of ammonia nitrogen in water bodies. Due to limitation of large experiment, we can only do some small experiments, but we can still see that the nitrifying bacteria have a better removal effect on the ammonia nitrogen in the water body, and the cost is low and the source is wide. At present, the eutrophication of wastewater is becoming more and more serious. How to reduce or minimize the ammonia nitrogen content in the water body is a major problem. In this context, the nitrifying bacteria are very useful in the process of treating the ammonia nitrogen in the water body.

Nevertheless, experiments on the removal effect of nitrifying bacteria on ammonia nitrogen in water bodies are limited to the simple laboratory conditions. Besides, because the water samples used are treated water, they cannot simulate the wastewater treatment in actual situations. In addition, compared with the components and properties of the water sample in this experiment, the actual wastewater is more complicated and has many influencing factors, so there is still a certain gap between practical application and theoretical application.

Biological denitrification technology has already had a certain application basis, but due to the complexity of the microbial process, some reaction mechanisms are not clear yet, and it is necessary to make further studies. What’s more, in actual operation, many treatment processes need to be improved in terms of reliability, stability, and treatment efficiency. In summary, the exploration of new denitrification treatment processes and new nitrifying bacteria will be the development direction of biological denitrification technology, and it is believed that wastewater biological denitrification treatment technology will get further improved[15].