Energy Consumption in Wastewater Treatment Plants in China 1
Xie Tao,1 Wang Chengwen,1 School of Environment, Tsinghua University, Beijing, China
Abstract During recent years, the number of Wastewater Treatment Plants (WWTPs) has substantially increased, making the study of energy consumption of the plants extremely important, which was related to the economic and environmental benefits of the plants. This paper presented the basic situation of energy consumption in China based on the data from 1856 plants in 2009 and discussed the impact of different factors including scales and operation rate on the energy consumption. The analysis of the result showed that average energy consumption of 1856 WWTPs in China in 2009 was 0.254 kWh/m3. The energy consumption in WWTPs decreased with the increase of scale and operation load rate. A typical WWTP in North China was selected as the case study to analyze the operation of different energy related unit, concluding that aeration was an important part and took more than a half of the total energy consumption. Therefore, a model was established based on three main treatment process, including A2O, Oxidation Ditch and SBR, to simulate the oxygen demand, which would influence the energy consumption from aeration significantly. The result could be used to guide the operation of WWTP with lower energy consumption. Keywords: Energy Consumption, Impact factors, Oxygen Demand, Wastewater Treatment Plant
1. Introduction During recent years, the number of WWTPs in China and the corresponding capacity has extremely increased. With the development of technologies and human sense of protection of environment, the amount of WWTPs in China has reached 1993 till 2009, with a total capacity of 1.056×108 m3/d. Many researches of the operation and management in WWTPs concentrated on the process efficiency and economic benefits, while the assessment of environmental benefits, especially energy consumption is rarely considered. With the development of society, limit of energy consumption of various industries in China will be gradually incorporated into management. How to evaluate the energy consumption in WWTPs is a serious problem. It’s urgent to build a method for evaluation of energy consumption in WWTPs. As a result, the
benchmarking analysis of energy consumption in WWTPs, which is affected by the differences in scale, treatment process and operation load, should be conducted to optimize the operation of the plants. The consideration of energy consumption in WWTPs should be included in both the design and the optimal management of the plants. However, the problems in China related to such fields were rarely considered. Yang S. S. estimated the consumption during primary and secondary treatment in the WWTPs based on the plant design experience, with a conclusion that the average consumption during the secondary process was 0.268 kWh/m3. Researchers in the U. S. did some more complete studies. Wesner G. made a report on almost all treatment process in the U. S. and analyzed the recovery of energy from the plants. The energy consumed during the production of the materials in the process of
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treatment, including activated carbon, aluminum salt, chlorine, ferric trichloride and lime, was even calculated in the report. Middlebrooks E. J. made a series of process combination and compared the consumption in different units and the whole process, grouped by the inflow quality and operation condition. A method to estimate the energy consumption in the WWTP was presented by Hagan R. M. and Roberts E. B., indicating that the consideration of energy reduction should be combined with the resource management and integrated balance. Owen W. F. put forward that the indirect energy consumption during the construction and transportation should also be considered since the energy consumption of the whole plants was to be evaluate. Zarnett G. and Smith R. conducted a survey on the difference of energy consumption in different units and process combination, and made the optimizing design of the system. The research methods in EU were similar to those in the U. S. Imhoff K. stressed the importance of reasonable energy consumption through the analysis of energy structure in the WWTPs. Karlsson I. put forward the conception of Oxygen Consumption Potential (OCP) to simulate the process of aeration, which took a great part of the total consumption in a plant. This paper presented the basic situation of energy consumption in WWTPs in China and analyzed some impact factors on the energy consumption. Besides, a typical WWTP in North China was selected as the case study to analyze the operation of different energy related unit. 2. Methods Analysis data in this paper were from 1856 WWTPs in different regions of China in 2009. In the database, some basic information of WWTPs, such as the scale
and operation rate, were collected. Besides, the optional situations in different months were detected so that we could get the complete data in the whole year. Energy consumption was evaluated by the units of kWh/m3 and kWh/COD removed. Data from a typical plant in North China was analyzed for better understanding of the proportion of energy consumption and the identification of the most important unit in the whole plant. As aeration took more than half the total consumption, oxygen demand during the aeration was simulated with respect to the water quality and temperature, thus providing a bench mark for energy consumption in WWTPs. 3. Results 3.1 Average energy consumption The result of this study showed that the average energy consumption of 1856 WWTPs in China in 2009 was 0.254 kWh/m3, less than the average data of 0.290 kWh/m3 of 559 WWTPs in China in 2006 by Yang L. B., but similar to the calculation data of 0.266 kWh/m3 in a typical plant by Yang S. S. If the data was compared with data in other countries, it was indicated that the average consumption in China was higher, compared to 0.26 kWh/m3 in Japan and 0.20 kWh/m3 in USA. Fig. 1 indicated that the change of energy consumption per m3 and per COD removed with the months were not so similar. From June to November, energy consumption per COD was significantly higher than that in other months, while the data of consumption per m3 showed an opposite result. This was mainly due to the fact that the pollutants concentration was lower from June to November. As a result, to select energy consumption per COD as an indicator can better reflect the change of consumption with the influence of change in the
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Amount of WWTPs
1.000
0.28
0.900
0.26 0.800 0.24 Energy Consumption per m3 Energy Consumption per COD
0.22
0.700
0.20
0.600 1
2
3
4
5
6
7
8
9
10
11
12
Month
Fig. 1. Energy consumption per m3 and per COD in WWTPs in China in 2009 3.2 Impact of different factors on energy consumption As was shown in Fig. 2 and Fig. 3, the scales of WWTPs differed from each other, and the related energy consumption decreased with the increased scale. Fig. 2 presented the basic information about the distribution of plants with different scales in China and result showed that most plants were operated with an inflow of about 5×104 m3/d and the average consumption of plants with that scale was about 2.5 kWh/m3, which was similar to the result of the average data in whole China. The scales of different plants can significantly influence the energy 3 consumption per m , as shown in Fig. 3. With the increase of scales, the consumption decreased mainly due to the scale effect during the operation of the plants. With the increase of the inflow, equipments and devices operated during the process can work with higher efficiency and the treatment environment is relatively more stable with less change in the amount of
800 600 400 200 0 ≤0.5
0.5 ~ 1
1~2
2~5
5 ~ 10
＞50
10 ~ 20 20 ~ 50
Scale of WWTPs (104 m3/d)
Fig. 2. Scale of WWTPs in 2009 Energy Consumption per m3 (kWh/m3)
0.30
water and pollutants concentration, thus providing a better condition for the growth of the microorganisms in the sludge and improving the treatment capacity.
Energy Consumption per COD (kWh/kgCOD)
Energy Consumption per m3 (kWh/m3)
concentration of pollutants in the wastewater, thus providing a relatively reasonable benchmark for evaluation of energy consumption in the process. However, in most of the plants in China, energy consumption per m3 was still a main indicator to evaluate the performance of operation in energy consumption reduction.
0.6 0.5 y = 0.5057x-0.423 R² = 0.9812
0.4 0.3 0.2 0.1 0 0.36
0.94
1.82
3.65
8.26
16.2
32.3
Scale of WWTPs (104 m3/d)
Fig. 3. Energy consumption with respect to scales The scale of plants could influence the energy consumption as the operation condition was different. While in a typical plant, the actual amount of inflow might be not match the scale and the treatment capacity quite well, the operation load rate of the plants might be different. Energy consumption increased significantly with the decrease of operation load, as presented in Fig. 4. If the plant was operated in the low load rate of less than 30%, the energy consumption might be in a quite high situation. As shown in the figure, some plants might work with an operation load rate of more than 100%, as the inflow of some plants exceeded the designed capacity. This was also not an appropriate condition for the plants as the effluent quality might not be good.
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5 kWh/m3
4
kWh/kgCOD 3 2 1 0
Energy Consumption per COD (kWh/kg COD)
Energy Consumption per m3 (kWh/m3)
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
Operation Load (%)
Fig. 4. Energy consumption with respect to operation load 3.3 Proportion of consumption in some typical plants Analysis of data in a typical plant in North China showed that pumping, aeration and material flow were the most important units if energy consumption was considered, presented in Fig. 5. The largest parts of actual energy consumption in WWTP were aeration, pumping and sludge treatment, which account for 80% of total energy consumption. Energy consumption from aeration took more than a half. The study of energy consumption in the WWTP should be focused on aeration.
(a)Energy consumption in different process
(b)Energy consumption in different units Fig. 5. Proportion of energy consumption in a typical WWTP in China 3.4 Simulation model of oxygen demand Since aeration was considered as the main part of energy consumption unit during the process, the indicator of oxygen demand should be strictly controlled. It was essential to define the basic level of oxygen demand of different process, in order to give a benchmark for the energy consumption in the plant. The impact of different factors on the energy consumption has been discussed, concluding that the months, scales, operation load rate and treatment process might have great impact on energy consumption in the WWTPs. As a result, such factors should be considered when a model was to be established to simulate the oxygen demand. The factors of scales and operation load rate could be reflected by the inflow amount. The factors of different treatment process could be reflected by the water quality indicators including COD, TN and TP. Besides, temperature should be considered as it differs in different months in a year. A model was established to simulate the relationship between oxygen demand and water quality and temperature based on the data of a large number of WWTPs, as shown
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below. Three main treatment processes in China were considered, including A2O, Oxidation Ditch and SBR. Matching parameters were different in each treatment process, as presented in Table. 1.
n 1
c -133
k 0.0078
0.9
8
0.0078
Based on the analysis of energy consumption, increased energy efficiency of WWTPs are recommended to reduce electricity needs of the plant, which should be concerned through the design and operation of WWTPs. Energy consumption in WWTP should be more and more concerned in the future and will likely develop into an quite important factor that will influence the choice of treatment processes and the overall design of WWTPs. The optimal operation of WWTPs should also be considered based on the evaluation results of energy consumption.
1.2
24
0.0078
References
Qneed=(a×CODm+b×TNn+c×TP)×exp[k×(T-20)]
Table. 1. Matching parameters for oxygen demand 2
a m b 0.4 1.4 50
AO Oxidation 0.9 1.3 56 Ditch SBR 3.8 1 28
The simulation of oxygen demand was similar to the conception of OCP by Karlsson I. However, it’s a more practical way to estimate the oxygen demand in different process in China, as the data to establish the model came from the operation of several typical plants in China. 4. Discussion and Conclusions Based on the data from different WWTPs in China in 2009, the energy consumption of the whole plants was analyzed. Average energy consumption of 1856 WWTPs in China in 2009 was 0.254 kWh/m3. Impact of different factors on energy consumption was discussed, concluding that energy consumption in WWTPs decreased with the increase of scale and operation load rate. Proportion of energy consumption in different units was analyzed in a typical plant, indicating that aeration was the most important part. A model was established to simulate the relationship between oxygen demand and water quality and temperature, so that a bench mark could be established in the process of A2O, Oxidation and SBR.
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Mats E., Berndt B., Mikal A. (2006). Control of the aeration volume in an activated sludge process using supervisory control strategies. Water Research, 40(8): 1668-1676.
Corresponding author: Xie Tao Email:
[email protected] Tel: +86-10-62797025
Middlebrooks E. J., Middlebrooks C. H., Reed S. C. (1981). Energy requirement for small wastewater treatment system. Journal WPCF, 53(7): 1172-1197.
Fax: +86-10-62788148 Address: School of Environment, Tsinghua University, Beijing 100084, China
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