1. Introduction
As the world’s largest developing country, there is massive sewage discharge every year in order to meet the needs of life and economic development in China. For example, 73.53 billion tons of wastewater was treated in 2015, such as 22.235 million tons of Chemical Oxygen Demand (COD) and 2.299 million tons of NH3-N in China [
1]. Total sewage treatment depends on the sewage treatment plant in China. As a manually designed system, sewage treatment plants need to consume a lot of resources and energy for sewage treatment; meanwhile, a large number of pollutants will be discharged, including exhaust gas, wastewater, and sludge waste [
2]. Therefore, a series of resource, energy, and environmental issues will be generated; meanwhile, they can bring environmentally severe load and items for China, especially since China’s economy has entered a period of rapid growth from the reform in the 1980s [
3,
4]. Under such a circumstance, it is necessary to evaluate the environmental sustainability in the sewage treatment industry.
Until now, several sustainable studies have been considered widely in view of different analysis perspectives for the sewage treatment industry. Many scholars have studied the relationship between the environment in the sewage treatment industry using biological perspective analysis [
5,
6,
7,
8], lifecycle assessment (LCA) perspective [
9], ecology models perspective [
10], energy and economy performances [
11], sewage treatment and water supply efficiency [
12], chemical and physical angles [
13,
14], construction in the wastewater treatment plants [
15], integrating wastewater treatment and incineration plants for energy-efficient study [
16], sampling strategy for the sewage sludge survey [
17], soil and the treatment of sewage treatment plant [
18], and systematic assessment framework [
19].
However, these studies have the following drawbacks: (1) for the isolated angles analysis, only one or some perspectives are chosen to demonstrate the interaction between environmental issues and the sewage treatment industry, resulting in a few unilateral conclusions; (2) ignorance of natural resources assessment leads to a negative impact on sustainability in the sewage treatment industry; (3) undifferentiated contribution degrees from different resource types could cause deviation inaccuracy to some extent; (4) there is a lack of a unified platform to evaluate sustainability in the sewage treatment industry, for example, an integrated method assessment should comprise resources, energy, labor, and others; and (5) some studies are unable to consider multiple pollutant emissions simultaneously, such as exhaust gas, wastewater, and solid waste. It will have a harmful effect on sustainability in the sewage treatment industry. In summary, the above studies cannot display the entire evaluated result in the sewage treatment industry.
Compared to the methodologies mentioned above, the emergy analysis (EMA) can improve these shortcomings and was put forward by Odum [
20]. EMA could realize a unified platform to compare different types of input through unit emergy values (UEVs), involving natural resources, energy, labor, and environmental pollutants. Therefore, it can be selected to assess the various types of natural, artificial, and complex systems effectively. Meanwhile, natural resource efficiency, environmental issue, and sustainable degree can be illustrated clearly through a series of emergy indicators to finally obtain an authoritative assessment result for the targeted system. Sustainability assessments are hot topics based on emergy analysis (EMA) and contain a large quantity of different types related to agricultural studies [
21,
22], city assessments [
23,
24], green building field [
25,
26], production systems [
27,
28], national research [
29], pollutant treatment [
30], and traffic range [
31].
Until now, several studies have been investigated to use the emergy approach in the sewage treatment industry. Yan et al. used an example of the Chinese sewage treatment industry to analyze the environmental sustainability issues based on the emergy method [
32]. Based on the municipal sewage treatment ecosystem, XiaoHong et al. evaluated the sustainable degree by making use of the emergy method [
33]. In view of four different wastewater treatment plants, sustainability and feasibility have been assessed using improved emergy indicators in the sewage treatment industry by F.L. da et al. [
34]. Taking the wastewater treatment in Happy Farmer’s Home as an example, Junsheng et al. confirmed several influencing factors on sustainability in China [
35]. Natalia et al. conducted the emergy assessment in a wastewater treatment plant, which compared two management alternatives of biosolids [
36]. The wastewater treatment system from a sugar factory in Sweden was assessed on the basis of the emergy theory [
37]. Paolo et al. proposed a preliminary evaluation of a wastewater treatment plant system by considering the environmental costs of natural fluxes [
38]. The ecological and economic effect of a decentralized sewage treatment plant was been surveyed by some authors [
39]. Through a sewage treatment system before and after implementing a cleaner production measure, XiaoHong et al. executed two emergy evaluations to assess sustainability [
40].
Several weak points can be found in these papers (shown in
Table 1), including (1) a lack of infrastructure emergy consideration, in which, through the collation of current literature, the infrastructure emergy has not been taken into account in the sewage treatment system to assess environmental sustainability (shown in
Table 1); (2) the use of an old emergy baseline, in which some articles use past emergy baselines rather than the latest emergy baseline; (3) the lack of pollutant evaluations, in which, in most items in
Table 1, pollutants are not considered for the sewage treatment industry, such as exhaust gas, wastewater, and solid waste; for environmental sustainability in the sewage treatment industry, it is necessary to assess the pollutant effect on the system; and (4) incomplete emergy indicators, in which only fundamental indicators had been computed in these papers and have no improved indexes based on infrastructure emergy, running emergy, and pollutant emergy for sewage treatment system evaluation. The above four disadvantages indicate that it is necessary to have a sustainable assessment for the sewage treatment industry based on emergy analysis (EMA) in China.
This paper aims to assess the environmental sustainability degree of the sewage treatment industry system through the EMA method. A new sewage treatment plant has been chosen for evaluation as an example, and all essential data can be obtained by investigation and calculation. Its contributions depend on several views, as follows: (1) considering the complete angles to assess the sewage treatment system, including infrastructure emergy, running emergy, and pollutant emergy; (2) establishing an improved and integral emergy analysis (EMA) and sustainable indicator group to evaluate the sewage treatment industry system; (3) calculating the unit emergy value of sewage treatment industry in China based on the environmental severity situation; and (4) proposing the corresponding strategies as reference for the factory manager.
Finally, the structure of the entire article is organized as follows: After the introduction section,
Section 2 shows the methodology, including primary data and situation of the new sewage treatment plant, emergy analysis group, and industrial pollutant emergy details.
Section 3 presents the results and discussion, containing main emergy calculated tables, emergy analysis, emergy indicators analysis, UEVs, sensitivity analysis, and industrial pollutant analysis.
Section 4 discusses preventive strategies and positive suggestions. Finally, the main conclusions are summed up in
Section 5.
3. Results and Discussions
3.1. Emergy Calculated Table of the Sewage Treatment Plant in China
Emergy Calculated Table of the Sewage Treatment Plant in China are listed as followings:
All calculated emergy baseline is 12.00 × 10
24 sej/yr [
52] and the UEVs have been corrected in
Table 9.
1—Renewable Energy calculation:
- (1)
Solar energy calculation: Area of ceramic plant = 3.18 × 10
5 m
2; insolation (Hubei Province, China) = 5.43 × 10
9 J/ m
2/y [
70]; albedo = 0.30 [
70]; energy = (insolation) × (1 − albedo) × (area) = (5.43 × 109 J/ m
2/y) × (1 − 0.30) × (3.18 × 10
5 m
2) = 1.32 × 10
8 J/y; UEV = 1.00 sej/J by definition; emergy of one year = 1.32 × 10
8 J/y × 1 y × 1.00 sej/J = 1.32 × 10
8 sej;
- (2)
Rain (chemical potential energy) calculation: Area of ceramic plant = 3.18 × 10
5 m
2; rainfall (annual average, n = 5) = 0.68 m/y [
70]; water density = 1.00 kg/ m
3; evapotranspiration rate = 60% [
69,
71]; Gibbs free energy of water = 4.94 × 10
3 J/kg; energy = (area) × (rainfall) × (evapotranspiration rate) × (water density) × (Gibbs free energy of water) = (3.18 × 10
5 m
2) × (0.68 m/y) × (60%) × (1 × 10
3 kg/m
3) × (4.94 × 10
3 J/kg) = 6.41 × 10
11 J/y; UEV = 2.35 × 10
4 sej/J [
65]; emergy of one year = 6.41 × 10
11 J/y × 1 y × 2.35 × 10
4 sej/J = 1.51 × 10
16 sej;
- (3)
Rain (geopotential energy) calculation: Area of ceramic plant = 3.18E × 10
5 m
2; rainfall (annual average, n = 5) = 0.68 m/y [
72]; average elevation = 316 m [
73]; water density = 1.00 × 10
3 kg/m
3; runoff rate = 40.00% [
69,
71]; energy = (area) × (rainfall) × (runoff rate) × (water density) × (average elevation) × (gravity) = (3.18 × 10
5 m
2) × (0.71 m/y) × (40%) × (1 × 10
3 kg/m
3) × (316 m) × (9.8 kg/m
2) = 2.80 × 10
11 J/y; UEV = 2.79 × 10
4 sej/J [
72]; emergy of one year = 2.80 × 10
11 J/y × 1 y × 2.79 × 10
4 sej/J = 7.81 × 10
15 sej;
- (4)
Wind energy calculation: Area of ceramic plant = 3.18E × 10
5 m
2; air density = 1.29 kg/m
3; wind velocity (annual average, n = 2) = 3.25 m/s [
73]; velocity of geostrophic wind = 3.25 m/s (surface winds are considered as 0.6 of geostrophic wind [
74]; drag coefficient = 1.00 × 10
−3 [
75]; energy = (area) × (air density) × (drag coefficient) × (velocity of geostrophic wind)3 = (3.18 × 10
5 m
2) × (1.29 kg/m
3) × (1.00 × 10
−3) × (3.25 m/s)3 × (3.15 × 10
7 s/y) = 4.43 × 10
11 J/y; UEV = 1.90 × 10
3 sej/J [
65]; emergy of one year = 4.43 × 10
11 J/y × 1 y × 1.90 × 10
3 sej/J = 8.42 × 10
14 sej;
- (5)
Geothermal heat calculation: Area of ceramic plant = 3.18 × 10
5 m
2; heat flow (average) = 3.50 × 10
−2 J/m
2/s. energy = (area) × (heat flow) = (3.18 × 10
5 m
2) × (3.50 × 10
-2 J/m
2/s) × (3.15 × 10
7 s/y) = 3.51 × 10
11 J/y; UEV = 3.44 × 10
4 sej/J [
72]; emergy of one year = 3.51 × 10
11 J/y × 1 y × 3.44 × 10
4 sej/J = 1.27 × 10
14 sej.
2—Nonrenewable resources:
The amount emergy: Cement = 5.44 × 107 × 1.93 × 1012 = 1.05 × 1020 sej;
steel = 2.83 × 107 × 2.75 × 1012 = 7.78 × 1019 sej; limestone = 1.31 × 105 × 1.27 × 1012 = 1.66 × 1017 sej;
gravel = 2.03 × 107 × 1.42 × 1012 = 2.28 × 1019 sej; wood = 5.79 × 105 × 2.67 × 1012 = 1.55 × 1018 sej;
brick = 1.97 × 106 × 2.82 × 1012 = 5.56 × 1018 sej; tap water = 1.45 × 106 × 2.56 × 1012 = 3.71 × 1018 sej.
3—Purchased resources:
4—Wastewater treatment chemicals
The emergy calculations:
Polyaluminium chloride = 6.52 × 109 × 3.37 × 106 = 2.20 × 1016sej;
Cl2 liquid = 3.86 × 107 × 3.37 × 106 = 1.30 × 1014sej;
polyacrylamide = 2.49 × 107 × 3.37 × 106 = 8.39 × 1013 sej;
potassium permanganate = 3.04 × 107 × 3.37 × 106 = 1.02 × 1014sej.
5—Labor and services:
(1) Auxiliary engineering labor costs:
Scaffolding project labor cost + concrete formwork and bracket labor cost + equipment installation and disassembly labor cost + night construction labor cost + special season construction (rain and winter) labor cost = $7,329,560 + $16,784,840 + $2,306,530 + $999,011 + $482,690 = $2.79E × 107.
(2) Engineering Service fee:
Construction engineering service fee + installation engineering service fee + decoration engineering service fee + municipal engineering service fee = $917,390 + $213,873 + $628,545 + $136,947 = $1.89 × 106.
(3) Management service fees for the government:
Environmental protection fee + civilization construction fee + temporary facility fee + safety construction fee + engineering sewage charges + dangerous work accident insurance = $54,547.2 + $184,536.7 + $37,435.68 + $86,615.89 + $43,999.70 = $4.18 × 105.
(4) The amount of emergy of labor and services: ($2.79 × 107 + $1.89 × 106 + $4.18 × 105) × 1.14 × 1010 = 3.44 × 1017sej.
6—Energy:
Construction electricity per kWh in China = 1 kwh/REM × 0.687 = 0.687 kwh/$.
- (1)
Infrastructure electricity of the new sewage treatment plant: (4.22 × 106$/0.687 kwh/$) × 3.6 × 106j = 2.21 × 1013j;
- (2)
annual electricity operation cost of sewage treatment plant: 16,000 kwh × 24 h × 365 d × 3.6 × 106j = 1.40 × 1014j;
- (3)
the amount of emergy of electricity = (2.21 × 1013 + 1.40 × 1014) × 4.50 × 105 = 7.29 × 1019sej.
7—Transportation:
Transportation emergy of annual sludge treatment = (55.8 tons × 10 km × 365d) × (7.61 × 1011) = 1.55 × 1017 sej.
8—Industrial pollutant emissions:
(1) Exhaust gas emergy of the new sewage treatment plant
- (1)
The economic loss emergy:
- (2)
The ecological services emergy:
- (3)
The sum of economic loss emergy and ecological services emergy:
(2) Wastewater emergy of the new Sewage Treatment Plant
(3) Sludge waste emergy of the new Sewage Treatment Plant
3.2. Emergy Analysis (EmA)
Table 10 is a comprehensive evaluated platform which integrates the infrastructure construction process emergy calculation and the sewage treatment process emergy calculation. There are eight sections for calculating emergy to assess the sustainability in the evaluated system, including renewable energy, nonrenewable resource, purchased resource, wastewater treatment chemicals, labor and service, energy, transportation, and industrial pollutant emissions.
Based on the emergy calculations of nonrenewable resources and purchased resources, the infrastructure construction process emergy (at least 92.6% of the entire emergy in the new sewage treatment plant) is more critical than the sewage treatment process emergy and has a self-evident superiority.
According to the proportion of emergy in
Table 10, nonrenewable resource is the primary contributor, which accounts for 69.1% of the total emergy amount, followed by energy (23.5%), purchased resource (7%), industrial pollutant emissions (0.15%), labor and services (0.11%), transportation (0.05%), wastewater treatment chemicals (0.01%), and renewable energy (0.01%).
From
Table 10, the nonrenewable resource has the dominant effect for the evaluated result, which is made up of cement (33.9%), steel (25.1%), gravel (7.36%), brick (1.79%), wood (0.50%), tap water (0.42%), and limestone (0.05%). Therein, the cement, steel, and gravel have dominant impacts on the nonrenewable resource emergy, and the proportions are 66.36% of the total emergy and 92.19% of the nonrenewable resource emergy. The energy holds a secondary influence for the complete result, and the main energy type is electricity. The purchased resource is the third most influential factor, and it is composed of aluminum (5.94%), tile (0.92%), and asphalt (0.16%). Among them, aluminum is the crucial part and has an 84.6% ratio for the entire purchased resource emergy.
Industrial pollutant emission consists of three factors, involving exhaust gas (0.00%), wastewater (0.15%), and solid waste (0.00%). Therein, the wastewater possesses the pivotal consequence in light of industrial pollutant emission emergy, far greater than exhaust gas and wastewater. The exhaust gas is a three-part composition, including dust, SO2, and NOX, which has hardly any impact on the emergy assessment.
Labor and service (0.11%) plays a small role in the sustainability of the new sewage treatment plant based on real productivity of China.
Finally, transportation (0.05%), wastewater treatment chemicals (0.01%), renewable energy (0.01%), and renewable energy (0.00%) have small effects on the entire evaluated system of the new sewage treatment plant.
3.3. Emergy Indicators Analysis
All the ecological indicators of the new sewage treatment plant are displayed in
Table 11.
- (1)
The renewability rate (Rr) is 0.000077, which demonstrates the weak sustainability and ecological level.
- (2)
Nonrenewability rate of local resource (Nr) reveals the ratio (0.690323), and the result illustrates the excessive local resource input and has caused considerable pressure on the local environment.
- (3)
Nonrenewability rate of purchased resource (Np) is 0.07. The result shows that the entire evaluation process requires considerable economic resource input, which hurts a sustainable level.
- (4)
Emergy personal density (Ed) is 6.20 × 1017sej/person, which represents the relatively high degree of automated production in the new sewage treatment plant.
- (5)
Emergy intensity (Ei) is 9.75 × 1017sej/m2, and it enunciates the better effect of land utilization to a certain degree.
- (6)
Purchased emergy dependence level (PEDL) is 0.101402, which explains the competitiveness of the evaluated system. Because of the lower economic input, the competitive strength is unqualified and needs to be improved.
- (7)
Pollutant environmental impact rate (PEIR) is 0.001535. It decides the poor influence on the whole system and could generate real sustainability.
- (8)
Emergy investment ratio (EIR) is 0.101391, which interprets low investment in the system. Compared to the natural input section, the proportion of the individual economic input section needs to be enhanced to improve the sustainable degree.
- (9)
The environmental loading ratio (ELR) is 9881.841, which shows the excessive pressure on the system. According to several standards of the study [
55], the high environmental load (>5) has been displayed and should carefully consider some measures to decrease the ELR. Through the integrated infrastructure construction process and sewage treatment process, the system has severe stress and should consider mitigation carefully.
- (10)
Emergy yield ratio (EYR) is 10.88479, representing the competitive ability of the evaluated system. It needs to balance the relationship between the total emergy section and purchased emergy for sustainability of the new sewage treatment plant.
- (11)
The emergy sustainability index (ESI) is 0.001101. It expresses the weak comprehensive effect of the environment on the evaluated system. Based on the literature in Reference [
55], the ESI of the new sewage treatment plant has an unsustainable status (<1) in the long term.
3.4. Unit Emergy Values (UEVs)
According to the statistics in
Table 12, some articles have been investigated for the UEVs of sewage treatment plants in China. In the last five years, there are two papers related to UEVs of sewage treatment in China, which are XiaoHong et al.; 2015 (6.79 × 10
11sej/m
3) and Yan et al., 2018 (9.03 × 10
11sej/m
3). In order to compare the UEVs, the unified emergy baseline should be adopted, and in view of 12.00 × 10
24sej/y, 6.79 × 10
11sej/m
3 can be corrected to 5.15 × 10
11sej/m
3. Based on running emergy calculations of an old sewage treatment plant, Xiaohong (2015) calculated the UEVs (5.15 × 10
11 sej/m
3), which is smaller than 9.03 × 10
11sej/m
3, on the basis of running emergy calculation and pollutant emergy calculation by Yan (2018).
In this paper, the UEVs (3.40 × 1012sej/m3) of a new sewage treatment plant have been computed based on infrastructure emergy, running emergy, and pollutant emergy. Higher UEVs illustrate the worst sustainable level, and it demonstrates the low efficiency of the new sewage treatment plant system due to the infrastructure emergy input. Several studies have been executed to enhance the environmental sustainability of a similar system by making better UEVs.
Energy-saving strategies have been considered carefully by Lemos et al., (2013) [
76] and Amores et al., (2013) [
77] and can reduce the UEVs of the sewage treatment plant. XiaoHong et al. (2018) has suggested sewage treatment process optimization [
40] in order to improve the water treatment efficiency and to lower the UEVs of the sewage treatment plant. Moreover, several comprehensive measures can also be conducted to enhance the UEVs of the sewage treatment plant, including replacement of renewable materials [
78], adjustment of industrial structure [
79], and development of renewable energy [
80,
81].
3.5. Sensitivity Analysis
As shown in
Table 10, the total emergy input is divided into eight sections, involving renewable energy, nonrenewable resource, purchased resource, wastewater treatment chemicals, labor and service, energy, transportation, and industrial pollutant emissions. Based on
Table 13, there are three main contributors to the entire emergy of the new sewage treatment plant, which are a nonrenewable resource, energy, and purchased resource. It is necessary to run a sensitivity analysis for these three parts in
Table 10. Here, a hypothesis is used to verify the sensitivity analysis: each of the three items changes by 10%, and other input items are forced to remain constant to test the changes of total emergy and indicators.
The sensitivity analysis of all details has been calculated in the paper, which has been displayed in
Table 14 and in
Figure 4 and
Figure 5.
Table 13 demonstrates sensitivity analysis situations in the hypothesis. Within −10% changes in the three sections, it can be concluded that the nonrenewable resources have the most significant fluctuation (6.903%), followed by energy (2.352%) and purchased resources (0.700%). The reason for the tendency is that nonrenewable resources play a significant consequence, more critical than others, and this can be known from the emergy contribution ratio, accounting for 69.1% of total emergy in the evaluated system. The same explanations can be for purchased resources and energy. The higher the emergy ratio, the greater the sensitivity results. This result explains that the staple emergy contributor exerts the foremost impact on sensitivity analysis.
Sensitivity analysis of all indicators has been illustrated in
Figure 4 and
Figure 5 for Rr (4.7619%), Nr (0.0850%), Np (−0.0628%), Ed (−4.7619%), ELR (−4.6816%), PEDL (−0.1477%), PEIR (4.7424%), EIR (−0.1482%), ELR (−4.6816%), EYR (0.1437%), ESI (4.8072%). It is found that ESI (4.8072%) has the biggest impact, followed by Rr (4.7619%), Ed (−4.7619%), PEIR (4.7424%), ELR (−4.6816%), and ELR (−4.6816%).
3.6. Industrial Pollutant Analysis
In the new sewage treatment plant system, there are three main types of pollutant emissions in
Table 8, including exhaust gas, wastewater, and solid waste. Specific details have been displayed in the note of
Table 10. According to
Table 10, wastewater is the major contributor and accounts for 0.15% of the entire emergy and almost 100% of the industrial pollutant emergy. Therein, exhaust gas and wastewater have virtually no impact on the system. It is clear that sludge treatment technology (
Figure 3) is feasible and has a better effect than wastewater treatment technology (
Figure 1).
5. Conclusions
Based on emergy methodology, the sustainable assessment of China’s new sewage treatment factory has been investigated, calculated, and analyzed in this paper. The main conclusions are summarized as follows.
According to the emergy calculations of nonrenewable resources and purchased resources, the infrastructure construction process emergy (at least 92.6% of the entire emergy in the new sewage treatment plant) is more critical than the sewage treatment process emergy and has a self-evident superiority.
Emergy contributor orders: nonrenewable resource accounts for 69.1% of the total emergy amount, followed by energy (23.5%), purchased resource (7%), industrial pollutant emissions (0.15%), labor and services (0.11%), transportation (0.05%), wastewater treatment chemicals (0.01%), and renewable energy (0.01%).
The cement, steel, and gravel have dominant impacts on the nonrenewable resource emergy, and the proportions are 66.36% of the total emergy and 92.19% of the nonrenewable resource emergy. The energy holds a secondary influence on the complete result, and the primary energy type is electricity. The purchased resource is the third most influential factor.
A series of indicators are 0.000077 (renewability rate); 0.690323 (nonrenewability rate of local resource); 0.101402 (purchased emergy dependence level); 0.001535 (pollutant environmental impact rate); 0.101391 (emergy investment ratio); 10.88479 (emergy yield ratio); 9881.841 (environmental loading ratio); and 0.001101 (emergy sustainability index). All indicator groups illustrate poor sustainability in the new sewage treatment plant and need to adopt some measures to improve the state.
In this paper, the UEVs (3.40 × 1012 sej/m3) of a new sewage treatment plant have been computed based on infrastructure emergy, running emergy, and pollutant emergy. Higher UEVs illustrate the worst sustainable level, and it demonstrates the low efficiency of the new sewage treatment plant system due to the infrastructure emergy input.
A hypothesis is used to verify the sensitivity analysis. The results show that the nonrenewable resources have the most significant fluctuation (6.903%), followed by energy (2.352%) and purchased resources (0.700%). For the indicators, ESI (4.8072%) has the most significant impact, followed by Rr (4.7619%), Ed (−4.7619%), PEIR (4.7424%), ELR (−4.6816%), and ELR (−4.6816%).
The wastewater is the major contributor and accounts for 0.15% of the entire emergy and almost 100% of the industrial pollutant emergy. Therein, exhaust gas and wastewater have practically no impact on the system.
To sum up, the new sewage treatment plant holds an excessive environmental loading ratio and poor emergy sustainability index. Hence, it cannot maintain sustainable growth; however, some positive measures are considered and implemented so as to ameliorate the sustainability, including enhancing the proportion of renewable energy input and recycling material replacement.