evaluation of treatment technologies for leachate
EVALUATION OF TREATMENT TECHNOLOGIES FOR LEACHATE CONCENTRATE FROM NF/RO USING ANALYTIC HIERARCHY PROCESS L. ZHANG*, D. YUE** * Key Laboratory of Solid Waste Management and Environment Safety (Tsinghua University), Ministry of Education of China, 100084 Beijing, P.R. China ** School of Environment, Tsinghua University, Beijing 100084, China
SUMMARY: Municipal solid waste (MSW) landfill leachate varies widely in composition, representing one of the most challenging tasks for landfill management, and needs to be treated effectively before being discharged into the environment. With further progress of environment protection, both national and local leachate discharge standards become more and more stringent in China. Therefore, having advantages in high removal efficiency, low cost, and easy operation, membrane separation processes, e.g. Nano Filtration (NF) and Reverse Osmosis (RO), are becoming quite popular for leachate treatment at most Chinese MSW landfills. However, the need for further treatment of the concentrate from NF/RO has been identified as a major hamper for implementation of membrane processes. This study aims to evaluate current treatment technologies of membrane concentrate, including advanced oxidation, evaporation, etc. An assessment indicator system for leachate concentrate treatment was raised, taking into consideration treatment efficiency, operation cost, environmental benefits, and operating level. An analytic hierarchy process (AHP) was applied to the evaluation of leachate concentrate treatment. The weight of each index was determined with expert evaluation. Typical treatment technologies were evaluated and ranked based on data collected from full scale treatment plants. The result is helpful to concentrate treatment technology determination and better environmental protection with reasonable investment.
1. INTRODUCTION Since the implementation of new national standards on municipal solid waste (MSW) landfill sites and incineration plants in 2008 and 2014, respectively, requirements for leachate quailty and discharge became distinctly strict in China. In order to accord the demand, a Membrane Bio-Reactor (MBR) coupled with membrane-based treatment system has been spreadly applied (He et al., 2015), which effectively guaranteed leachate effluent to reach the standards. Nevertheless, the interception of pollutants generates a large volume of membrane concentrate (MC) (Hou et al., 2017). The MC enriches large quantities of refractory organics and salinity from leachate, and proves to be a particularly problematic issue in waste management. If the MC is directly discharged or recirculated to the MSW landfill, the accumulation of persistent pollutants and salts will lead to an decrease in biological activity and increase in electrical conductivity respectively (Talalaj and Biedka, 2015). High conductivity depresses the RO Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
process effectiveness and even invalidates the entire filtration system. Therefore, it is significant to explore a scientific and rational route for MC and refrain it from causing severe environmental contamination. A series of technology have been adopted in this field, including Recirculation, Advanced oxidation process (AOP), Mechanical Vapor Recompression or Mechanical Vapor Compression (MVR/MVC), Submerged Combustion Evaporation (SCE), etc. Recirculating the MC to MSW landfill used to be considered as the most convenient and highest cost-effective technology, similar to leachate recirculation at a bioreactor landfill to adjust water content and degrade organic pollutants as well (He et al., 2015). However, in the later stage of recirculation and aged landfill the degradation rate declines remarkably (Morello et al., 2016). AOP possessing high untargeted oxidation capacity represents feasible measure to dispose of refractory organics. Researchers testified that AOP reduces the chemical oxygen demand (COD) and potentiates the increase in the biodegradability of the treated effluent (Labiadh et al., 2016; Li et al., 2016; Lu et al., 2013), but cannot meet the national limitation. Both MVR and SCE intent to evaporate water and volatile components from wastewater and further concentrate the MC. Diferent from MVR with indirect heat transfer mode, SCE is based on direct heat transfer by emerging hot gas into the liquid (Yue et al., 2007). Although laboratory researches for MC treatment have been extensively carried out, investigations on technical performance, treatment effects, economic cost and other aspects of practical projects have rarely been conducted. And almost no evaluation was adopted due to the limited data resource and the variety of technical forms. AHP is the method solving multicriteria decision and calculating the index weight by comparing relative factors with each other, which accords with the characteristics of current processes (Shabbir and Ahmad, 2016). In this study, according to data from six concentrate treatment projects on pragmatic operation condition, control effects on pollutants, expenses of investment and running, etc., comprehensive evaluation of all alternatives is developed based on AHP, combining qualitative weights with quantitative survey data. The designed rational hierarchy model and the assessment conclusion provides instructions for selecting membrane concentrate technology with different demand.
2. MATERIALS AND METHODS 2.1 Date survey A total of 6 leachate concentrate treatment projects were involved (Table 1), including AOP, MVR and SCE processes and their improved processes. Meanwhile, pretreatment facilities (leachate treatment) had a vital influence on the water quality of concentrate, the types of preprocessing were also under investigation.Therefore, instead of appraising diverse alternatives, the same measure with different membrane concentrates were also compared in this paper. In order to obtain more detailed data and lay a foundation for the future research, when promoting research, more specific research catalogues were listed for some evaluation indicators, such as: (i) The cost of investment is subdivided into two items: construction costs and equipment purchase and installation costs. (ii) The cost of operation is subdivided into four items: electricity costs, medicament costs, comprehensive energy costs and equipment maintenance and depreciation costs. Only two Environmental indicators (COD and NH3-N) were adopted, due to the following reasons:(i) COD and ammonia nitrogen are two key standards in the water environment protection in China, and they are also important control indicators of leachate pollution. (ii)
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Taking into account the specific requirements of China's water environment protection work and the restriction on test condition, COD and NH3-N have been essential monitoring parameters. Table 1. Leachate concentrate treatment projects (Alternatives) under investigation
*
No.
Leachate treatment
Concentrate
Alternatives
D1
A²O+MBR+NF+RO
RO concentrate
AOP
D2
MBR+NF+RO
RO concentrate
C&S +AOP
D3
UASB+MBR+RO
RO concentrate
SCE1
D4
RO concentrate
SCE2
D5
UBF+MBR+NF+RO UASB+MBR+NF
NF concentrate
MVR (NF)
D6
MVR
Raw Leachate
MVR (RL)
*
Coagulation and Sedimentation.
2.2 The hierarchical structure The analytic hierarchy process (AHP) has been widely applied and extensively studied in a number of fields since developed by Thomas L. Saaty in the 1970s(Saaty, 1977). Especially for the multi-criteria decision making (MCDM) problems(Naziris et al., 2016), it decomposed the decision into a hierarchy of relevant criteria which are the main influencing factors. Criteria could also be split into many layers and analyzed independently, which were more easily to comprehend. Usually, when AHP was applied to evaluation, it consisted of three levels: (i) Goal level which was the target of the assessment, (ii) Criteria level which was components of the aspects impact the goal, (iii) Alternatives level which denoted measures or plans participating in the analysis and will be sorted eventually. Before creating a systematic model for the assessment of leachate concentrate treatment, some factors should be taken into account. Leachate emission concentration was strictly limited in China’s new standards for solid waste landfill site and incineration plant, which clearly defined the limit concentration of pollutants. These control indexes were instructive for establishing the index system of pollutant emission. What’s more, processes had different pollution control ideas and methods, and the analytic hierarchy index system should fully demonstrate its advantages and disadvantages for a rational evaluation. The demands in production practices, such as economic costs and stable operation, also need to be considered. Integrating the above reflections and the AHP basic model, the hierarchy index system (represented in Figure 1.) for evaluating leachate concentrate treatment, which engaged at the highest level (Goal level), was constituted. On criteria level, the objectives were general aspects appraising pollution control technology: B1 - Environmental benefit, characterizing pollutant treatment efficiency of the techniques in alternative level, B2 - Economic benefit, presenting fund occupancy and cost expenditure indicators B3 - Management benefit, conversing stability in operating and convenience in management. On sub-criteria level under B1 (Environmental benefit) the seven parameters which intented to convey the removal rate of contamination are chosen: C1 - Chemical Oxygen Demand (COD), C2 - Biochemical Oxygen Demand (BOD), C3 - Chromaticity, C4 - suspended solids (SS), C5 Total Nitrogen (TN), C6 - ammonia nitrogen(NH3-N), C7 - Total Phosphorus (TP). The selection of these seven indexes is in accord with the limitation of national standard in China and the characteristics of effluent after disposed.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. AHP tree for evaluating the leachate concentrate treatment
On sub-criteria level under B2 (Economic benefit) three factors were opted:C8 - Investment cost per ton of concentrate, including construction costs, equipment costs and installation costs, C9 - Operation cost per ton of concentrate contain medicament costs added to the treatment process to promote treatment and removal of rust and clogging, comprehensive energy consumption for electricity, coal (or steam), natural gas, diesel and other energy during runtime and equipment maintenance and depreciation expenses, C10 - Floor space per ton of concentrate. On sub-criteria level under B3 (Management benefit) two factors were selected: C11 Convenience of operation, measuring automation, stability and complexity of manual management, C12 – Technology Readiness Level(TPL), estimating technology maturity and popularity.
2.3 Evaluation process of index weight 2.3.1 Group decision making The method to determining the weights of each layer was one of the decisive indexes to determine whether the assessment is scientific and accurate. If the traditional single expert evaluation approach was applied simply, the pairwise comparison judgment matrices of different levels would be obtained, which would be of great subjectivity. In addition, by referring to the weight of the previous study, the inconsistent objects and experts would lead that the settled weights are not in conformity with the actual situation. Group decision making was adopted to determine the index weight, and the group was composed of experts engaged in leachate treatment from universities, enterprises, authorities and so on. Proficients participating in this paper were not limited to a small institution and were involved in scientific research or practical work related to leachate for many years. They had a comprehensive understanding of the characteristics of leachate concentrate and its main process and the conclusion had certain reference value. In order to facilitate data collection and comprehension of respondents, the expert evaluation table was established,and distributed by network. In the web-based questionnaire, experts could make pairwise comparisons using a 1–9 preference scale to determine the relative weights of two criteria, shown in Table 3.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 3. Comparative scale of 9 points used in the paired comparison(Saaty, 1977) Scale
Definition
aij=1
I is equally important with j
aij=3
I is moderately more important than j
aij=5
I is strongly more important than j
aij=7
I is very strongly more important than j
aij=9
I is extremely more important than j
aij=2,4,6,8,
Intermediate values
Reciprocal
aij= 1/aij
2.3.2 Evaluation process Each index weight(Wi) took the average value evaluated by all experts(Wik) that was calculated by the pairwise comparison judgment matrix. Nine questionnaires were selected by Internet background ascertaining relations among factors and there will be nine groups of matrices. The normalized weight vector w=(w1,w2,…wn) was obtained by solving the following matrix equation based on pairwise comparison data Eq.(1): Aw=λmaxw (1) λmax is maximum eigenvalue of the matrix A. However, the above vectors could be considered as weight after checking the consistency.The value of Consistency Index(CI) was defined as an index measuring the uniformity of matrix and calculated by Eq.(2): CI =
!!"# !! !!!
(2)
n is the number of level. To estimate the consistency of pairwise comparisons, Consistency ratio (CR) was given by Eq.(3): !" CR = ......(3) !"
Random Index(RI) was the random consistency index and shown in Table 4.If CR was less than 0.1, the inconsistency are acceptable, otherwise the judgment matrix will need adjustment. Table 4. Consistency random index RI(Saaty, 1977) n RI
3 0.58
4 0.90
5 1.12
6 1.24
7 1.32
8 1.41
9 1.45
10 1.49
3. RESULTS AND DISCUSSION 3.1 Survey results analysis Results of data survey were processed by benchmark ratio method, D1 was set as standard and all of its indicators as 1. The ratio of others to D1 took place the original data and were substituted in the AHP model(Table 5). Due to some data missing, simplified processing is done.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 5. Datum ratio table of sub-criteria No.
C1
C6
C8
C9
C10
C11
C12
1
1
1
1
1
1
1
1
2
2.256
1.333
0.291
0.573
0.323
1
1
3
2.326
7.724
0.152
5.867
0.593
1.244
1.104
4
2.332
7.968
0.16
5.867
0.635
1.244
1.104
5
2.31
5.333
0.827
0.49
0.622
0.905
0.941
6
2.294
7.973
0.286
0.497
0.489
0.905
0.941
In terms of the above research, actually, it was arduous to seek continuous cases of AOP or MVR processes for concentrate and the data were obtained in the previous run-time. SCE was the only technique which made it possible to stable operation in current China. Compared with other influents of Reverse Osmosis(RO) concentrate, MVR were more inclined to dispose leachate and Nanofiltration(NF) concentrate which had low salt content and might apparently decreased the impact of scaling and corrosion. 3.2 Index weight Following the AHP process, nine groups of comparison judgment matrices were constructed and owing to consistency ratio ( NH3-N removal rate> Investment cost> Operation cost> Convenience of operation> Technology Readiness Level. Although RO concentrate is hard-pressed to deal with, SCE manifests the optimum benefit in all three aspects, and has distinct advantages in Economic perspective. MVR is more suitable to evaporate raw leachate, which gets higher total score than to treat NF concentrate with the same process. Coupled with coagulation and sedimentation, AOP could have higher efficiency of controlling pollutants than its own. There is not much difference between three techniques for management benifits as they are still in developing period in this field. The sequence of the comprehensive evaluation was obtained: SCE>MVR>AOP. AKNOWLEDGEMENTS The authors are grateful for the financial support from the Special Fund of Environmental Protection Research for Public Welfare of China (No. 201509055).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
REFERENCES He, R., Wei, X.M., Tian, B.H., Su, Y., Lu, Y.L., (2015). Characterization of a joint recirculation of concentrated leachate and leachate to landfills with a microaerobic bioreactor for leachate treatment. Waste management, vol. 46, 380-388. Hou, C., Lu, G., Zhao, L., Yin, P., Zhu, L., (2017). Estrogenicity assessment of membrane concentrates from landfill leachate treated by the UV-Fenton process using a human breast carcinoma cell line. Chemosphere, vol. 180, 192-200. Labiadh, L., Fernandes, A., Ciriaco, L., Pacheco, M.J., Gadri, A., Ammar, S., Lopes, A., (2016). Electrochemical treatment of concentrate from reverse osmosis of sanitary landfill leachate. Journal of environmental management, vol. 181, 515-521. Li, J., Zhao, L., Qin, L., Tian, X., Wang, A., Zhou, Y., Meng, L., Chen, Y., (2016). Removal of refractory organics in nanofiltration concentrates of municipal solid waste leachate treatment plants by combined Fenton oxidative-coagulation with photo--Fenton processes. Chemosphere, vol. 146, 442-449. Lu, J., Fan, L., Roddick, F.A., (2013). Potential of BAC combined with UVC/H2O2 for reducing organic matter from highly saline reverse osmosis concentrate produced from municipal wastewater reclamation. Chemosphere, vol. 93, 683-688. Morello, L., Cossu, R., Raga, R., Pivato, A., Lavagnolo, M.C., (2016). Recirculation of reverse osmosis concentrate in lab-scale anaerobic and aerobic landfill simulation reactors. Waste management, vol. 56, 262-270. Naziris, I.A., Lagaros, N.D., Papaioannou, K., (2016). Optimized fire protection of cultural heritage structures based on the analytic hierarchy process. Journal of Building Engineering, vol. 8, 292-304. Saaty, T.L., (1977). A scaling method for priorities in hierarchical structures. Journal of Mathematical Psychology, vol. 15, 234-281. Shabbir, R., Ahmad, S.S., (2016). Water resource vulnerability assessment in Rawalpindi and Islamabad, Pakistan using Analytic Hierarchy Process (AHP). Journal of King Saud University Science, vol. 28, 293-299. Talalaj, I.A., Biedka, P., (2015). Impact of concentrated leachate recirculation on effectiveness of leachate treatment by reverse osmosis. Ecological Engineering, vol. 85, 185-192. Yue, D., Xu, Y., Mahar, R.B., Liu, F., Nie, Y., (2007). Laboratory-scale experiments applied to the design of a two-stage submerged combustion evaporation system. Waste management, vol. 27, 704-710.
SUMMARY: Municipal solid waste (MSW) landfill leachate varies widely in composition, representing one of the most challenging tasks for landfill management, and needs to be treated effectively before being discharged into the environment. With further progress of environment protection, both national and local leachate discharge standards become more and more stringent in China. Therefore, having advantages in high removal efficiency, low cost, and easy operation, membrane separation processes, e.g. Nano Filtration (NF) and Reverse Osmosis (RO), are becoming quite popular for leachate treatment at most Chinese MSW landfills. However, the need for further treatment of the concentrate from NF/RO has been identified as a major hamper for implementation of membrane processes. This study aims to evaluate current treatment technologies of membrane concentrate, including advanced oxidation, evaporation, etc. An assessment indicator system for leachate concentrate treatment was raised, taking into consideration treatment efficiency, operation cost, environmental benefits, and operating level. An analytic hierarchy process (AHP) was applied to the evaluation of leachate concentrate treatment. The weight of each index was determined with expert evaluation. Typical treatment technologies were evaluated and ranked based on data collected from full scale treatment plants. The result is helpful to concentrate treatment technology determination and better environmental protection with reasonable investment.
1. INTRODUCTION Since the implementation of new national standards on municipal solid waste (MSW) landfill sites and incineration plants in 2008 and 2014, respectively, requirements for leachate quailty and discharge became distinctly strict in China. In order to accord the demand, a Membrane Bio-Reactor (MBR) coupled with membrane-based treatment system has been spreadly applied (He et al., 2015), which effectively guaranteed leachate effluent to reach the standards. Nevertheless, the interception of pollutants generates a large volume of membrane concentrate (MC) (Hou et al., 2017). The MC enriches large quantities of refractory organics and salinity from leachate, and proves to be a particularly problematic issue in waste management. If the MC is directly discharged or recirculated to the MSW landfill, the accumulation of persistent pollutants and salts will lead to an decrease in biological activity and increase in electrical conductivity respectively (Talalaj and Biedka, 2015). High conductivity depresses the RO Proceedings Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium/ 2 - 6 October 2017 S. Margherita di Pula, Cagliari, Italy / © 2017 by CISA Publisher, Italy
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
process effectiveness and even invalidates the entire filtration system. Therefore, it is significant to explore a scientific and rational route for MC and refrain it from causing severe environmental contamination. A series of technology have been adopted in this field, including Recirculation, Advanced oxidation process (AOP), Mechanical Vapor Recompression or Mechanical Vapor Compression (MVR/MVC), Submerged Combustion Evaporation (SCE), etc. Recirculating the MC to MSW landfill used to be considered as the most convenient and highest cost-effective technology, similar to leachate recirculation at a bioreactor landfill to adjust water content and degrade organic pollutants as well (He et al., 2015). However, in the later stage of recirculation and aged landfill the degradation rate declines remarkably (Morello et al., 2016). AOP possessing high untargeted oxidation capacity represents feasible measure to dispose of refractory organics. Researchers testified that AOP reduces the chemical oxygen demand (COD) and potentiates the increase in the biodegradability of the treated effluent (Labiadh et al., 2016; Li et al., 2016; Lu et al., 2013), but cannot meet the national limitation. Both MVR and SCE intent to evaporate water and volatile components from wastewater and further concentrate the MC. Diferent from MVR with indirect heat transfer mode, SCE is based on direct heat transfer by emerging hot gas into the liquid (Yue et al., 2007). Although laboratory researches for MC treatment have been extensively carried out, investigations on technical performance, treatment effects, economic cost and other aspects of practical projects have rarely been conducted. And almost no evaluation was adopted due to the limited data resource and the variety of technical forms. AHP is the method solving multicriteria decision and calculating the index weight by comparing relative factors with each other, which accords with the characteristics of current processes (Shabbir and Ahmad, 2016). In this study, according to data from six concentrate treatment projects on pragmatic operation condition, control effects on pollutants, expenses of investment and running, etc., comprehensive evaluation of all alternatives is developed based on AHP, combining qualitative weights with quantitative survey data. The designed rational hierarchy model and the assessment conclusion provides instructions for selecting membrane concentrate technology with different demand.
2. MATERIALS AND METHODS 2.1 Date survey A total of 6 leachate concentrate treatment projects were involved (Table 1), including AOP, MVR and SCE processes and their improved processes. Meanwhile, pretreatment facilities (leachate treatment) had a vital influence on the water quality of concentrate, the types of preprocessing were also under investigation.Therefore, instead of appraising diverse alternatives, the same measure with different membrane concentrates were also compared in this paper. In order to obtain more detailed data and lay a foundation for the future research, when promoting research, more specific research catalogues were listed for some evaluation indicators, such as: (i) The cost of investment is subdivided into two items: construction costs and equipment purchase and installation costs. (ii) The cost of operation is subdivided into four items: electricity costs, medicament costs, comprehensive energy costs and equipment maintenance and depreciation costs. Only two Environmental indicators (COD and NH3-N) were adopted, due to the following reasons:(i) COD and ammonia nitrogen are two key standards in the water environment protection in China, and they are also important control indicators of leachate pollution. (ii)
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Taking into account the specific requirements of China's water environment protection work and the restriction on test condition, COD and NH3-N have been essential monitoring parameters. Table 1. Leachate concentrate treatment projects (Alternatives) under investigation
*
No.
Leachate treatment
Concentrate
Alternatives
D1
A²O+MBR+NF+RO
RO concentrate
AOP
D2
MBR+NF+RO
RO concentrate
C&S +AOP
D3
UASB+MBR+RO
RO concentrate
SCE1
D4
RO concentrate
SCE2
D5
UBF+MBR+NF+RO UASB+MBR+NF
NF concentrate
MVR (NF)
D6
MVR
Raw Leachate
MVR (RL)
*
Coagulation and Sedimentation.
2.2 The hierarchical structure The analytic hierarchy process (AHP) has been widely applied and extensively studied in a number of fields since developed by Thomas L. Saaty in the 1970s(Saaty, 1977). Especially for the multi-criteria decision making (MCDM) problems(Naziris et al., 2016), it decomposed the decision into a hierarchy of relevant criteria which are the main influencing factors. Criteria could also be split into many layers and analyzed independently, which were more easily to comprehend. Usually, when AHP was applied to evaluation, it consisted of three levels: (i) Goal level which was the target of the assessment, (ii) Criteria level which was components of the aspects impact the goal, (iii) Alternatives level which denoted measures or plans participating in the analysis and will be sorted eventually. Before creating a systematic model for the assessment of leachate concentrate treatment, some factors should be taken into account. Leachate emission concentration was strictly limited in China’s new standards for solid waste landfill site and incineration plant, which clearly defined the limit concentration of pollutants. These control indexes were instructive for establishing the index system of pollutant emission. What’s more, processes had different pollution control ideas and methods, and the analytic hierarchy index system should fully demonstrate its advantages and disadvantages for a rational evaluation. The demands in production practices, such as economic costs and stable operation, also need to be considered. Integrating the above reflections and the AHP basic model, the hierarchy index system (represented in Figure 1.) for evaluating leachate concentrate treatment, which engaged at the highest level (Goal level), was constituted. On criteria level, the objectives were general aspects appraising pollution control technology: B1 - Environmental benefit, characterizing pollutant treatment efficiency of the techniques in alternative level, B2 - Economic benefit, presenting fund occupancy and cost expenditure indicators B3 - Management benefit, conversing stability in operating and convenience in management. On sub-criteria level under B1 (Environmental benefit) the seven parameters which intented to convey the removal rate of contamination are chosen: C1 - Chemical Oxygen Demand (COD), C2 - Biochemical Oxygen Demand (BOD), C3 - Chromaticity, C4 - suspended solids (SS), C5 Total Nitrogen (TN), C6 - ammonia nitrogen(NH3-N), C7 - Total Phosphorus (TP). The selection of these seven indexes is in accord with the limitation of national standard in China and the characteristics of effluent after disposed.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Figure 1. AHP tree for evaluating the leachate concentrate treatment
On sub-criteria level under B2 (Economic benefit) three factors were opted:C8 - Investment cost per ton of concentrate, including construction costs, equipment costs and installation costs, C9 - Operation cost per ton of concentrate contain medicament costs added to the treatment process to promote treatment and removal of rust and clogging, comprehensive energy consumption for electricity, coal (or steam), natural gas, diesel and other energy during runtime and equipment maintenance and depreciation expenses, C10 - Floor space per ton of concentrate. On sub-criteria level under B3 (Management benefit) two factors were selected: C11 Convenience of operation, measuring automation, stability and complexity of manual management, C12 – Technology Readiness Level(TPL), estimating technology maturity and popularity.
2.3 Evaluation process of index weight 2.3.1 Group decision making The method to determining the weights of each layer was one of the decisive indexes to determine whether the assessment is scientific and accurate. If the traditional single expert evaluation approach was applied simply, the pairwise comparison judgment matrices of different levels would be obtained, which would be of great subjectivity. In addition, by referring to the weight of the previous study, the inconsistent objects and experts would lead that the settled weights are not in conformity with the actual situation. Group decision making was adopted to determine the index weight, and the group was composed of experts engaged in leachate treatment from universities, enterprises, authorities and so on. Proficients participating in this paper were not limited to a small institution and were involved in scientific research or practical work related to leachate for many years. They had a comprehensive understanding of the characteristics of leachate concentrate and its main process and the conclusion had certain reference value. In order to facilitate data collection and comprehension of respondents, the expert evaluation table was established,and distributed by network. In the web-based questionnaire, experts could make pairwise comparisons using a 1–9 preference scale to determine the relative weights of two criteria, shown in Table 3.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 3. Comparative scale of 9 points used in the paired comparison(Saaty, 1977) Scale
Definition
aij=1
I is equally important with j
aij=3
I is moderately more important than j
aij=5
I is strongly more important than j
aij=7
I is very strongly more important than j
aij=9
I is extremely more important than j
aij=2,4,6,8,
Intermediate values
Reciprocal
aij= 1/aij
2.3.2 Evaluation process Each index weight(Wi) took the average value evaluated by all experts(Wik) that was calculated by the pairwise comparison judgment matrix. Nine questionnaires were selected by Internet background ascertaining relations among factors and there will be nine groups of matrices. The normalized weight vector w=(w1,w2,…wn) was obtained by solving the following matrix equation based on pairwise comparison data Eq.(1): Aw=λmaxw (1) λmax is maximum eigenvalue of the matrix A. However, the above vectors could be considered as weight after checking the consistency.The value of Consistency Index(CI) was defined as an index measuring the uniformity of matrix and calculated by Eq.(2): CI =
!!"# !! !!!
(2)
n is the number of level. To estimate the consistency of pairwise comparisons, Consistency ratio (CR) was given by Eq.(3): !" CR = ......(3) !"
Random Index(RI) was the random consistency index and shown in Table 4.If CR was less than 0.1, the inconsistency are acceptable, otherwise the judgment matrix will need adjustment. Table 4. Consistency random index RI(Saaty, 1977) n RI
3 0.58
4 0.90
5 1.12
6 1.24
7 1.32
8 1.41
9 1.45
10 1.49
3. RESULTS AND DISCUSSION 3.1 Survey results analysis Results of data survey were processed by benchmark ratio method, D1 was set as standard and all of its indicators as 1. The ratio of others to D1 took place the original data and were substituted in the AHP model(Table 5). Due to some data missing, simplified processing is done.
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
Table 5. Datum ratio table of sub-criteria No.
C1
C6
C8
C9
C10
C11
C12
1
1
1
1
1
1
1
1
2
2.256
1.333
0.291
0.573
0.323
1
1
3
2.326
7.724
0.152
5.867
0.593
1.244
1.104
4
2.332
7.968
0.16
5.867
0.635
1.244
1.104
5
2.31
5.333
0.827
0.49
0.622
0.905
0.941
6
2.294
7.973
0.286
0.497
0.489
0.905
0.941
In terms of the above research, actually, it was arduous to seek continuous cases of AOP or MVR processes for concentrate and the data were obtained in the previous run-time. SCE was the only technique which made it possible to stable operation in current China. Compared with other influents of Reverse Osmosis(RO) concentrate, MVR were more inclined to dispose leachate and Nanofiltration(NF) concentrate which had low salt content and might apparently decreased the impact of scaling and corrosion. 3.2 Index weight Following the AHP process, nine groups of comparison judgment matrices were constructed and owing to consistency ratio ( NH3-N removal rate> Investment cost> Operation cost> Convenience of operation> Technology Readiness Level. Although RO concentrate is hard-pressed to deal with, SCE manifests the optimum benefit in all three aspects, and has distinct advantages in Economic perspective. MVR is more suitable to evaporate raw leachate, which gets higher total score than to treat NF concentrate with the same process. Coupled with coagulation and sedimentation, AOP could have higher efficiency of controlling pollutants than its own. There is not much difference between three techniques for management benifits as they are still in developing period in this field. The sequence of the comprehensive evaluation was obtained: SCE>MVR>AOP. AKNOWLEDGEMENTS The authors are grateful for the financial support from the Special Fund of Environmental Protection Research for Public Welfare of China (No. 201509055).
Sardinia 2017 / Sixteenth International Waste Management and Landfill Symposium / 2 - 6 October 2017
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