The application of the analytical hierarchy process (AHP) in determining organic waste process technology-oriented-reducing greenhouse gas

. The existence of waste in the environment, which is not managed well, could contribute to global warming and cause climate change on the earth. This research aimed to determine the suitable management preference for organic waste treatment produced in small cities. The methods used are based on pair comparison in hierarchy decisions developed through the analytical hierarchy process. The hierarchy consisted of goals, criteria, sub-criteria and alternatives applied to waste treatment engineering. The expert suggestion was used in a pair comparison matrix to determine the level of technology. The comparison was used to get the significance level of decision criteria and the relative performance of the options. The city’s waste managed by the Buleleng government was used to demonstrate the application of the analytical hierarchy process in that region. The result showed that the important factor in deciding on waste treatment for the small city is environment and engineering, with each eigenvector priority (0.28), sociocultural (0.24) and economy (0.20). According to the recruitment preference for Singaraja waste treatment, the analytical hierarchy process showed that controlled landfill and composting are the most suitable treatment, followed by incineration, anaerobic digestion, and mechanical and biological treatment.


Introduction
According to data from The National Waste Management Information System (SIPSN), for 2022, 67.23% of the composition of waste generated in urban areas in Indonesia is organic waste [1].Among them, 40.7% are food waste, 13% are wood/twigs, and 11.31% are paper/cardboard.High organic waste production should be managed well to prevent greenhouse gas emissions.Biodegradable organic waste landfilled at the final processing location will release methane gas.To manage the waste well, the government must provide good infrastructure.Today, most of the waste is managed by landfilling system [2].However, this method should be improved and replaced by other processes due to limited land area and several environmental problems, such as leachate production besides methane.On the other hand, there are various possible waste processing technology options.Various criteria must be considered in determining the appropriate choice.Multi-criteria analysis is one way to achieve that goal.In this multi-criteria decision maker (MCDM), the aims and criteria wanted were combined in the decision framework provided.This method has been widely used in ecology evaluation by considering complex and interactive environmental problems, sociocultural, technical, monetary and economic.Besides that, the interchange of the goals and competitive criteria was also considered.The AHP is a multi-criteria analysis which supports the decision-maker in solving and making complex decisions [3,4,5].The AHP is suitable for waste treatment because it can assist in choosing the specific and effective waste treatment [4,5].Principally, the AHP is analytical, which supports the process in various criteria and complex problems into easy problem solvers.This study examines municipal waste treatment options using AHP in determining the best and appropriate technology for the organic fraction of municipal solid waste (OFMSW) management.

Materials and method
The MCDM framework based on the AHP approach was used to compile and analyze the problem with the decision-maker view hypothesis.The criteria and alternatives used were from experts, waste management regulation and literature.The sub-criteria was confirmed from the literature, interaction and discussion with the TPA manager.

Research area
The research area was the waste produced in Singaraja, northern Bali.Singaraja is on 8º3'40" -8º23'00" LS and 114º25'55" -115º27'28" BT geographically, have 82527 people.Administratively, Singaraja consists of 18 districts and one village.The waste produced is estimated at 58000 kg per day with 0.7 /capita/day.The waste comprised more than 68% organic fraction, including 35% part of the plant, 15% food waste and 10% cardboard, respectively.The waste produced was filled by controlled landfill technology at the Bengkala End of Waste Processing (TPA), Kubutambahan, Buleleng Regency, Bali.The TPA has been open since 2006 with an area of 4.8 Ha.

Data collection
The primary and secondary data were used in this research.The primary data was collected by questionnaire spreading with a non-probability sampling technique.The respondents with knowledge and experience in waste management were chosen, including two academics and three practitioners.Besides the questionnaire, the knowledge acquisition process for structuring the criteria, sub-criteria and waste management technology option was obtained by interviewing the municipal solid waste manager, literature, and other secondary information.The criteria included environment, sociocultural, technique and economic.The environmental criteria included air pollution, pathogenic exposure, and water and soil pollution.Sociocultural criteria included acceptability, community engagement, and susceptibility.The technical criteria consisted of possibilities and sustainability, ease and effectiveness in operation and energy and nutrition recovery.The economic criteria included investment cost and operation and maintenance cost.

Identification of waste management technology option
This research analyzed four alternatives of OFMSW options: sanitary landfills, composting, incineration, anaerobic digestion, and mechanical and biological treatment.These methods were relevant to waste management in developing countries [5,6].According to the AHP method, alternatives, sub-criteria, and criteria were distributed into a multilevel hierarchy structure, as presented in Figure 1.

AHP steps
Based on the questionnaire responses by the experts, the first step was to create a decision matrix to identify priority weights, which consisted of objectives and sub-criteria, and the expert's scores were collected using the geometric mean.Second, every element was compared in pairs to know one important relationship level to get the goals.The comparison was input into the matrix.The equation determined the priority: A is the comparison matrix, λmax is the eigenvalue, and W is the priority vector.Third, the feedback into the decision-making according to the value input was determined through consistency measurement ratio (CR): with CI as the consistency index, n as the dimension of the comparison matrix, λmax as the primary eigenvalue, and RI as the index ratio.If the consistency ratio is less than 0,1 (<10%), the matrix is assumed to be consistent.Otherwise, the measurement is continued by modifying comparison to minimize the inconsistency.

Results and discussion
The pairwise matrix comparison between the main criteria: environment, technique, social acceptance and economic to goal with the geomean value from all experts was shown in Table 1.To produce a priority vector or eigenvector (right column), the vector in each matrix column (Table 1) was normalized (every column element was divided with the total column value), and the mean of every row of the matrix was the result, as presented in Table 2.The mean of every row was an eigenvector or priority vector of each criterion.The following procedure was adopted to check the consistency manually.First, the eigenvalue (λmax) was calculated using the equation ( 1 The mean value (λmax) is obtained from the matrix operation given above, (λmax) average = 4.1913.So, by using equation ( 3), the consistency index (CI) was calculated by: CI = (λmax -n)/(n-1) = (4.1913-4)/(4-1) = 0.064 Therefore, the consistency ratio (CR) is calculated by equation (2): CR = CI/RI = 0.064/0.90= 0,07 < 0,1 The RI value of 0.90 was obtained from Table 2 for n = 4 (matrix size).Because the CR value (0.07) was less than 0,1 (maximum limit permission), the assessment of the matrix (Table 2) was assumed to be consistent and satisfying logically.The next assessment involved comparing the effect of sub-criteria on the main criteria.The assessment on economics showed that the effect of investment cost, operation and maintenance costs had the same role on waste management treatment options.Pathogenic exposure had the biggest effect, followed by air, water and soil pollution, considering the main criteria of the environment.The main criteria of the matrix technique showed that the sub-criteria of energy and nutrition recovery were the highest influenced in deciding than easy and effectiveness, possibility and sustainable operation.The effect of acceptability on the main sociocultural criteria had more effect than community engagement and area vulnerabilities.All the assessments in the comparison matrix were consistent because the third matrix's CR value was lower than 0.1.Furthermore, a comparison value is made to the waste treatment technology options based on each criterion and sub-criteria.The rank of waste management option based on the effect of all the criteria and sub-criteria was presented in Figure 2. Figure 2 showed that anaerobic digestion and incineration were suitable for waste technology based on air, water and soil pollution sub-criteria.Anaerobic digestion was the best choice based on the impact of pathogenic exposure potential and vulnerability.From the land usage view, mechanical and biological treatment was chosen.Incineration was assumed to be the best choice based on air, water and soil pollution, technology sustainability and energy and nutrition recovery potential.According to acceptability, community engagement, investment, operation and maintenance cost, and easy and effective operation, the controlled landfill and composting were assumed to be the proper waste treatment technology.
Where: L1 = Air pollution, L2 = Water and soil pollution, L3 = Pathogenic exposure, L4 = Utilization, S1 = Acceptability, S2 = Community engagement, S3 = Susceptibility area, E1= Investment cost, E2= Operating and maintenance cost, T1= Probability and sustainability, T2= ease and effectiveness in operation, dan T3= Energy recovery and nutrients.According to the analysis obtained from this research, the suitable waste management technology for Singaraja municipal solid waste was the controlled landfill and composting or incineration followed by anaerobic digestion and mechanical biological treatment.The environment and technique were the factors which informed the waste management option for small cities.This result differed from [7], which reported that anaerobic digestion was the best waste management technology using the AHP method.
Controlled landfill and composting were suitable for the overall aim, sociocultural acceptance criteria, economics and technique [2, 8,9].Besides that, those technologies were cheap and had minimized negative impacts.Contrary to environmental criteria, controlled landfill was not the first rank.Incineration was assumed to be the suitable option for the environmental criteria.Overall, incineration was also equal to controlled landfill and composting.Incineration was not on the first rank in sociocultural, economic and technique, although it was suitable for sustainability sub-criteria and energy recovery in technique criteria.Incineration was considered less easy and more effective than controlled landfill and composting.
The next rank was anaerobic digestion, which was suitable considering to environment criteria.It was unsuitable for comparing the other three criteria to controlled landfill and incineration.Although anaerobic digestion had the potential for energy and nutrition recovery, it was unsuitable for technique criteria compared to incineration.The sustainability and effectiveness of the operation were lower than incineration.The anaerobic digestion processed biodegradable waste.About 80% of the Singaraja waste composition was managed by anaerobic digestion or composting.Incineration was the most suitable technology for quantity and quality.The incineration could process more than 99% of total waste except metal and glass (0.34%).

Conclusion
The case study was the quantity and quality of Singraja waste managed by Buleleng municipalities.AHP achieved the preference of criteria and sub-criteria.The result showed that the most decision-making for small cities' waste management was environment and technical, with each eigenvector priority (0.28), followed by sociocultural (0.24) and economic (0.20).Based on the technology options, the controlled landfill and composting were relevant, followed by incineration, anaerobic digestion, and mechanical and biological treatment.Therefore, the Buleleng government should apply the composter technology for the organic waste fraction of municipal solid waste (food, garden, and organic waste) from sources or households, by community groups or centralized composting.Besides, the garbage bank should be managed effectively to minimise the amount of plastic and organic waste in the controlled landfill.This option technology was good not only in economic, socio-cultural and technology but also in environmental aspects.To get high biodegradable waste quality, the government should invest the waste collection and separation from door to door.

Figure 2 .
Figure 2. Arrangement of waste management alternative based on sub-criteria.

Table 1 .
Pair comparison matrix of main criteria related to the goals.

Table 2 .
Calculation of priority vector.