Appendix E. Mass and Energy Balances at the Gaobeidian Wastewater Treatment Plant in Beijing, China

N. Gans, S. Mobini and X. N. Zhang Water and Environmental Engineering, Department of Chemical Engineering, Lund Institute of Technology, Lund University, P.O. Box 124, SE-221 00 Lund, Sweden

Abstract Wastewater treatment is an important component in the infrastructure of every country. However, with increasingly strict outlet demands, wastewater treatment requires more and more energy. In China most of the energy is produced from coal, which leads to major problems with air pollution, especially in urban centres such as the capital Beijing. Hence, energy conservation in wastewater treatment plants does not only reduce the operational costs for the plants but can also aid in the improvement of the air quality. In this study mass and energy balances are carried out for the Gaobeidian wastewater treatment plant in Beijing, China, in order to identify the needs for process optimisation and energy conservation. The calculations of the mass balances based on measured data suggest that there might be problems with the reliability of the measurements. It was furthermore found that at the moment the removal of nutrients is not optimal and that the capacity of the sludge treatment might not be high enough. The largest electricity consumers at the treatment plant are currently the blowers for the aeration of the activated sludge tanks and the influent pumps. Up to 31% of the total electricity consumed could maximally be produced from the cogeneration of biogas. Keywords: Wastewater treatment, sludge treatment, mass balance, energy balance, energy conservation

Introduction After the opening of the Chinese economy under Deng Xiaoping in the late 1970´s, the Chinese economy developed into one of the fastest growing economies in the world. This did not only improve the living standards of the Chinese population, but also put strain on the natural resources and the environment. At present China is the 2nd biggest energy consumer after the USA (IEA, 1999) and the 2nd largest producer of greenhouse gases in the world (Wagner, 2006). Since most of the energy is generated from coal (IAE, 1999), this leads to major problems with air pollution causing severe health problems, especially in the cities. Another important issue concerning public health and the environment is the pollution and, in more than 50% of the Chinese cities, the shortage of water resources (Butler, 2005 & Horton, 2000). The Chinese capital Beijing suffers from severe water shortage as the city’s per capita water reserves are less than 300 m³. According to the United Nations a region is in a water crisis if it has 500 m³ water per person (Lu, 2004). Furthermore, the water quality in the city is said to represent one of the three most severe environmental problems in Beijing (Hou & Hunter, 1998). The Guardian’s journalist Watts (2005) calls Beijing the “air pollution capital of the world”. Although this might seem exaggerated, Chinese experts classified the air quality in Beijing as “very dangerous” causing the premature death of up to 411 000 citizens (Watts, 2005). One of the main sources of the air pollution is the generation of energy from coal (China Internet Information Center, 2003). One keystone in improving the water quality in the city is wastewater treatment. However, energy conservation should play a role in the operation of the wastewater treatment plants in order not to further aggravate the air pollution problem. Therefore, this study aims to calculate mass and energy balances for the biggest wastewater treatment plant in Beijing, the Gaobeidian wastewater treatment plant (Gaobeidian WWTP), in order to suggest possible methods for optimising the wastewater treatment processes as well as for energy conservation. Mass balances are valuable tools to assess the performance of a WWTP and its processes and to detect possible weaknesses, while energy balances can identify the major energy consumers in the WWTP which should be the focus of energy conservation plans. The Gaobeidian WWTP was constructed in two phases. The first one was completed in 1993 and the second one in 1999. The two parts of the plant are now called the first and second project. The wastewater treatment consists of bar screens and aerated grit chambers as preliminary treatment, primary sedimentation, activated sludge tanks with nitrification, denitrification and to some extent biological phosphorous removal and finally secondary 203

sedimentation. The excess sludge from the biological treatment is removed via the primary clarifiers and enters the sludge treatment together with the primary sludge as mixed sludge. The sludge treatment consists of thickening tanks, anaerobic mesophilic digestion and dewatering. Additionally there is a thickening-dewatering house which is connected to the digesters as back-up for the dewatering.

Materials and Methods The theoretical mass balances over the wastewater and the sludge treatment at the Gaobeidian WWTP are based on literature values. If no such values could be found, assumptions were made based on studies published in scientific magazines. The mass balances based on measured data were calculated based on data and information gathered during a two-months study visit at the Engineering Consultation Company located at the Gaobeidian WWTP. An investigation of the main equipment and its electrical energy consumption at Gaobeidian WWTP was carried out. The annual power consumption was calculated using the average power rating of the motors and mean running times.

Results and Discussion Mass Balance over the Wastewater Treatment Figures 1a to 1e show the results of the theoretical mass balances for the major pollutants of concern in wastewater treatment: biochemical oxygen demand (BOD5), chemical oxygen demand (COD), total nitrogen (TN), total phosphorus (TP) and suspended solids (SS). The calculations were carried out with even kg/d for masses and m3/d for flows. However, it should be recognized that the last three digits of the masses are not very reliable and there are small inconsistencies due to rounding.

100%A 2.5% Debris & grit 2 882 kg/d

Supernatant 80 044 kg/d

Influent 230 589 kg/d

BOD oxidized 53 921 kg/d Effluent 46.8% 7 258 kg/d 6.3% B C 40.5%

Influent 115 294 kg/d

Excess sludge 46 665 kg/d

A – Preliminary treatment B – Primary treatment C– Biological treatment and secondary sedimentation

A

113.9% Excess sludge 46 665 kg/d

C

B

1.2% 38.4% Debris & grit 33.5% 2 882 kg/d

69.4% 40.5%

100%

COD oxidized 83 223 kg/d

36.1% 7.4% 33.5%

93.7%

Supernatant Excess sludge 88 603 kg/d 77 142 kg/d

Effluent 17 084 kg/d

Excess sludge 77 140 kg/d

Waste

Waste sludge 131 277 kg/d

Figure 1a. Theoretical mass balance for BOD5 over the wastewater treatment in the Gaobeidian WWTP.

A – Preliminary treatment B – Primary treatment C– Biological treatment and secondary

sludge 216 006 kg/d

sedimentation

Figure 1b. Theoretical mass balance for COD over the wastewater treatment in the Gaobeidian WWTP.

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Influent 34 655 kg/d

100%

N2 gas released 5 389 kg/d

A

B

Effluent 1 630 kg/d

Influent 3 332 kg/d

Effluent 24 113 15.6% kg/d C 69.6%

100%

A

B

18.9%

39.5%

62.5%

Excess sludge 6 557 kg/d Waste sludge 13 677 kg/d

Supernatant Excess 8 522 kg/d sludge 6 558 kg/d

113.6% 46.3%

A – Preliminary treatment B – Primary treatment C– Biological treatment and secondary sedimentation

A – Preliminary treatment B – Primary treatment C – Biological treatment and secondary sedimentation

Figure 1c. Theoretical mass balance for TN over the wastewater treatment in the Gaobeidian WWTP.

100%

Effluent 13 319 kg/d

B

A 35.5%

41.9% Supernatant 65 317 kg/d

C

Excess sludge 1 543 kg/d

Supernatant 2 083 kg/d Excess sludge 1 543 kg/d

Influent 183 938 kg/d

48.9%

46.3%

18.9% 24.6%

C

Waste sludge 3 785 kg/d

Figure 1d. Theoretical mass balance for TP over the wastewater treatment in the Gaobeidian WWTP.

7.2%

41.9%

128.3% Excess sludge 77 140 kg/d

Excess sludge 77 142 kg/d

A – Preliminary treatment B – Primary treatment C – Biological treatment and secondary sedimentation

Waste sludge 235 938 kg/d

Figure 1e. Theoretical mass balance for SS over the wastewater treatment in the Gaobeidian WWTP.

As can be seen in Figures 1a to 1e, only minor fractions of organic matter are removed in the preliminary treatment while it was assumed that the preliminary treatment does not affect the other pollutants. Before primary sedimentation two flows add considerable amounts of masses to the raw wastewater. The first one is the supernatant from the sludge treatment which especially increases the load of BOD5 and TP on the following treatment steps. The second flow that is added before primary treatment is the excess sludge from the biological treatment. It was calculated by the use of iterations. Hence, there are slight inconsistencies in the masses calculated for the excess sludge flow entering before primary sedimentation and the flow leaving the secondary sedimentation tanks. The excess sludge increases the load onto the primary clarifiers, especially for TP, BOD5 and SS.

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The waste sludge is the major removal pathway for the pollutants as it does not only contain the primary, but also the excess sludge. It can be seen that the complete excess sludge that is added, is settled and removed in the primary sedimentation tanks. Only for TN the removal via the excess sludge is considerably smaller. For the mass balance calculations the activated sludge tanks and the secondary clarifiers were treated as one unit. In this unit the pollutants are either removed as excess sludge, they escape into the atmosphere in the case of BOD5, COD and TN, or they leave the treatment plant with the effluent. It can be seen that the mass of TN and TP contained in the effluent is significant and it can be concluded that there might be problems with the nutrient removal. This is confirmed by the measured effluent concentrations which exceed the Chinese effluent standards. Additionally to the theoretical mass balance calculations it was also attempted to calculate mass balances based on concentrations and flows measured at the Gaobeidian WWTP. There are four measurement points for pollutant concentrations in the WWTP. The first one is located at the influent, the second and third before and after primary sedimentation and the fourth after secondary sedimentation. However, the concentrations measured at the second point, which is situated before the addition of the excess sludge, are very high. The concentrations that had to be contained in the supernatant to add these masses to the raw wastewater seem to be out of a reasonable range. This suggests that there might be problems with the reliability of the measurements carried out at the WWTP. As the problem is less severe for TN, which is mainly present in dissolved form, it seems that there might be problems specifically with measuring particulate pollutants. Due to the high concentrations measured before primary sedimentation, the calculated masses removed with the waste sludge from the primary clarifiers are relatively high compared to the theoretical values. The removal efficiencies in the treatment unit of activated sludge tanks and secondary clarifiers are in the range of those computed in the theoretical mass balance or slightly below for BOD5 and COD respectively, while they are considerable lower for TP. Based on the measured data the removal of TN and SS is higher than computed for the theoretical case. Nevertheless, the mass balance based on the measured data also suggests that there might be problems with the nutrient removal. Mass Balance over the Sludge Treatment The mass balance over the sludge treatment was carried out both on theoretical values and based on measured values. Due to a lack of information it was only calculated for SS. Figure 2 shows the theoretical mass balance over the sludge treatment plant. From the total mass of waste sludge entering the sludge treatment plant 80% leaves the thickening tanks (A) as thickened sludge while 20% is contained in the supernatant from the second stage digester tanks (B2). In the digesters (B1) around 36% of the total mass is converted to biogas while the supernatant from the digesters contains 4%. Approximately 40% enters the dewatering (C) as digested sludge. From the dewatering unit 36% of the total mass leaves as the filter cake and 4% are contained in the supernatant. TSMg = 85.0 (ton/d)

TSM2 = 94.3 (ton/d) TSMw = 236.0 (ton/d)

A

TSM1 = 188.7 (ton/d)

B1

TSMS1 = 47.1 (ton/d)

B2

C

TSMS2 = 9.4 (ton/d)

TSMf = 85.0 (ton/d)

TSMS3= 9.4 (ton/d)

TSMS = 66.0 (ton/d)

TSMw = Mass of waste sludge TSM1 = Mass of sludge out of thickening TSMg = Mass of sludge converting to gas TSM2 = Mass of digested sludge TSMf = Mass of filter cake

TSMS = Mass of supernatant flow TSMS1 = Mass of supernatant flow from thickening TSMS2 = Mass of supernatant flow from digester TSMS3 = Mass of supernatant flow from dewatering

Figure 2. Theoretical mass balance over the sludge treatment plant in the Gaobeidian WWTP.

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Figure 3 shows the mass balance over the sludge treatment plant based on measured values. As it can be seen, 45% of the total mass goes to the digesters as thickened sludge and 55% are contained in the supernatant from the thickening tanks. In the digesters around 20% of the total mass is converted into biogas while 22% is contained in the digested sludge. The supernatant from digestion carries about 3% of the total mass. Of the 22% entering the dewatering unit about 20% leave as the filter cake and 2% are contained in the supernatant from the dewatering process. TSMg = 40.6 (ton/d)

TSMw = 202.0 (ton/d)

A

TSM1= 90.2 (ton/d)

B1

B2

TSMf = 40.6 (ton/d)

TSM2 = 45.1 (ton/d)

C

B1 TSMS1 = 111.8 (ton/d)

TSMS2 = 4.5 (ton/d)

TSMS3 =4.5 (ton/d)

TSMS = 120.8 (ton/d)

TSMw = Mass of waste sludge TSM1 = Mass of sludge out of thickening TSMg = Mass of sludge converting to gas TSM2 = Mass of digested sludge TSMf = Mass of filter cake

TSMS = Mass of supernatant flow TSMS1 = Mass of supernatant flow from thickening TSMS2 = Mass of supernatant flow from digester TSMS3 = Mass of supernatant flow from dewatering

Figure 3. Mass balance over the sludge treatment based on measured values from the Gaobeidian WWTP.

By comparing the results of the two mass balances over the sludge treatment plant in Figure 2 and 3, it can be seen that there is a significant difference in the first part (thickening), but not in the digestion and dewatering. During the calculations of the mass balance based on the measured values, it had to be realized that it was not possible to get a balance for the values measured in the digestion and dewatering units. Therefore, the theoretical values had to be used instead of the measured ones. However, the values used for the thickening process correspond to the measured ones and, as mentioned above, there is a big difference in the supernatant from thickening. This might be due to measurement errors or operational problems. From the measured waste sludge flow towards the sludge treatment (3rd Department, 2006) it can be seen that the waste sludge entering the sludge treatment exceeds its capacity. Therefore, there is a risk of overloading and hence overflow for all units in the sludge treatment (thickening, digestion and dewatering). The mass balance based on measured values in this study can only show the problem in the thickening part, but it could be assumed that there might be problems due to overflow from the digesters as well. Energy Balance Table 1 summarizes the results of the energy balance over the Gaobeidian WWTP. The minus sign (-) indicates electricity consumption whereas the plus sign (+) refers to electricity production.

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Table 1. Electricity production and consumption at the Gaobeidian WWTP. Total power (kWh/year) Preliminary treatment

% of total (%)

- 20 117 340

32.0

- 3 637 182

6.0

- 36 110 472

58.0

- 531 732

0.4

- 1 619 688

2.6

- 650 614

1.0

Total

- 62 667 028

100.0

Cogeneration

+ 19 710 000

31.0

Primary treatment Secondary treatment Thickening Digestion Dewatering

It can be seen from Table 1 that the secondary treatment accounts for 58% of the total power use followed by the preliminary treatment with 32%. In this case the secondary treatment includes both the activated sludge tanks and secondary sedimentation. In the secondary treatment the blowers, which supply the activated sludge tanks with air, are the main electricity consumers. In the Gaobeidian WWTP there is one 800 kW blower in the first project and two 900 kW blowers in the second project. Here the influent pump station is included in the preliminary treatment. It requires high power since there are three 600 kW pumps and one 410 kW pump used currently. In contrast to the other steps in the WWTP, cogeneration is the part that is used for producing electricity. If one calculates the maximum amount of electricity that could be produced, it can account for about 31% of the total power usage. In other words, 31% of the electricity could be self-supported. In the Gaobeidian WWTP, the self-produced electricity is currently provided for the aeration process.

Conclusions •

• • •

Mass balances are valuable tools for investigating the general performance of a wastewater treatment plant and an effective method to assess the reliability of the available data. In the case of the Gaobeidian WWTP, the mass balance calculations based on measured values suggest that there might be problems with the measurements. Energy balances can be calculated theoretically, based on the running time and power consumption of the equipments. From the mass balance over the Gaobeidian WWTP it could be concluded that the present nutrient removal is not optimal and the sludge treatment seems to be overloaded. Methods for the optimisation of the plant should therefore focus on these problems. The energy balance of the Gaobeidian WWTP illustrates that the aeration and the influent pump station are the biggest consumers of electricity. A maximum of 31% of the total electricity consumed could be generated from the cogeneration of biogas produced during digestion. Hence, energy conservation should focus on the blower and pumps.

Acknowledgements We would like to thank the supervisor of our Master thesis, Associate Professor Karin Jönsson, for her help and patience. We furthermore would like to express our gratitude to the staff of the Engineering Consultation Company and the Gaobeidian WWTP for the data and information provided during our study visit in Beijing.

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