Selection of the Optimal Urban Wastewater Treatment Process in Various Climates by Using Analytical Hierarchy Process (AHP)

Introduction
With increasing development of urban communities and increase of population in towns, thus the need for water is also increased in the recent years. Production rate of municipal wastewater has also grown dramatically. On the other hand, increase of public awareness about the water pollution problems has followed the new and stricter environmental laws concerning wastewater discharge. Now producing of high volume of domestic wastewater and uncontrolled discharge into receptive water and underground resources, has faced the planners of the project with serious problems. Thus, that is anticipated until the year 1400 about 800 urban wastewater treatment plants arise in the country that enormous human and financial investment are included. Such investment must pay particular attention to the selected treatment process according to the economic and engineering criteria. In the recent years, many studies and optimization models is provided for finding the best options for wastewater treatment that most of them have only considered the cost of the investment and operation, while the best treating option is not always cheapest option. Selection of the optimization process of municipal wastewater treatment is an important and multidimensional issue. This is because of damages caused by failure of plan and waste of costs, Separate from satisfying Environmental standards and requirements, is requires to a systematic and completely scientific process for Selecting the process of optimal treating, To finally, has the minimal environmental impact with ability to perform in terms of technical and economical. In selecting the appropriate option of treating determine factors and decision criteria is important. And because of the complexity of communication between these criteria, and usually a criterion effect on the other criterion, making decision model and find the best options by using independent criteria is not easy. Therefore, the use of Multi-Criteria Decision-Making in this regard would be helpful. Since, several multi-criteria decisions methods such as weighted sum model, the TOPSIS method and Analytic Hierarchy Process (AHP) is presented. Analytic Hierarchy Process, is a useful method for working with multiple criteria and purposes in decisions, that Is able to evaluate the different objectives and determines difference between two options by priority vectors. The ultimate goal of this approach is identifying the best option and also determines rank of options by considering all decision criteria.
Methods 
This study is analytical- descriptive, that is conducted in the year 1392 in order to selecting the optimal municipal wastewater treatment and Evaluate matching the Frequency of wastewater treatment processes on the results of five different climates. The population of the study is consisted municipal wastewater treatment plants existing in the country (in operation and under construction) according to the latest figures released by the National Water and Wastewater Engineering Company in 1389.
Climatic division of the country
In any studies plan, investigate the area and Understanding them is important.in other words, whatever the information about knowing the area be provided more complete and more precise, Principles of plan has been more appropriate would be face fewer problems. In this research, due to the unique diversity of climate of the country and in order to selecting the optimal process of wastewater treatment in different climates, According to the climatic classification of Publication No. 3-117 (revised in 1392), the country is divided five climate, mountain, Khazari, Mediterranean, semi-desert and desert.
Treating Alternatives
According  to available statistics and information, currently the most common methods municipal wastewater Treatment in the country are four processes: activated sludge, stabilization pond, aerated lagoon and Trickling Filter that Currently, more than 90 percent of the municipal wastewater treatment plants in the country take advantage from one of these four processes. It alone could cause choosing these four processes as appropriate treatment options in different operating area. In addition to usual of these methods, another reason that these processes have chosen as optimal treatment options is that  facilities and most of the equipment needed to processes is made inside the country, and also access to technology of them is in the country. In present situation that the country is under different sanctions and lack of access to technology and equipment of a process, the process is virtually impossible to use, so different and new processes that are currently in developed countries are used as municipal wastewater treatment processes (such as membranous processes) were not in the list choosing the appropriate treatment process options.
Decision criteria
Based on studies and summarized conducted, Criteria for selecting the appropriate treatment process for each basin in terms of three criteria, environmental, economical and technical were determined, Also, due to the extensive dimensions of each of  the defined criteria, the following criteria were defined for each to achieve the best results.
 
Data collection and Mode of scoring to the parameters
In order to collect data and comments to resident experts in each climate, The five types of questionnaires were used that each of the questionnaire consisted of two parts, The first part
using a pair comparison matrices, priorities of criteria and sub criteria has investigated in target climates and the second part, using a pair comparison matrix, the level of importance of treatment options relative to each criterion and sub-criterion are evaluated on the same climates. After collecting the opinions of experts, information obtained are reviewed and evaluated by a team of experts.
 Results and discussion
In order to choose the optimal wastewater treatment process in the five state climates,
In this study, four  processes of urban wastewater treatment includes activated sludge, aerated lagoon, stabilization pond and trickling filter based on the environmental, economical and technical criteria and also related sub-criteria were evaluated by using AHP.
In the mountainous climate of the country (type 1), activated sludge, aerated lagoons, stabilization ponds and trickling filter processes have obtained these weights: 0.347, 0.269, 0.209 and 0.174 . Also in this climate, between the main criteria, environmental criteria with 0.558 weight has greater importance than other criteria and then technical criteria with 0.320 weight is in secondary importance.
In the Khazari climate of the country (type 2), activated sludge, trickling filter, aerated lagoons and stabilization ponds processes have obtained these weights: 0.340, 0.273, 0.204 and 0.183. Also in this climate, between the main criteria, environmental criteria with 0.683 weight has greater importance than other criteria and then technical criteria with 0.200 weight is in secondary importance.
In the Mediterranean climate of the country (type 3), activated sludge, aerated lagoons, trickling filter and stabilization ponds processes have obtained these weights: 0.334, 0.234, 0.231 and 0.201. Also in this climate, between the main criteria, environmental criteria with 0.540 weight has greater importance than other criteria and then technical criteria with 0.297 weight is in secondary importance.
In the semi-desert climate of the country (type 4), stabilization ponds, activated sludge, trickling filter and aerated lagoons processes have obtained these weights: 0.276, 0.273, 0.270 and 0.181. Also in this climate, between the main criteria, environmental criteria with 0.493 weight has greater importance than other criteria and then technical criteria with 0.311 weight is in secondary importance.
In the desert climate of the country (type 5), stabilization ponds, trickling filter, activated sludge and aerated lagoons processes have obtained these weights: 0.292, 0.266, 0.247 and 0.195. Also in this climate, between the main criteria, environmental criteria with 0.493 weight has greater importance than other criteria and then economical criteria with 0.311 weight is in secondary importance.
According to the results of Analytical Hierarchy Process in order to selecting the optimal urban wastewater treatment in different climates, environmental criteria as the most important criterion is known in all climates and the highest it's weight is in the Khazari climate(0.683) and the lowest it's weight is in the desert and semi-desert climates(0.493). the high importance of ecological criteria in the Khazari climate due to natural conditions of mentioned region and factors such as tourism industry, high groundwater levels, have numerous rivers and other factors related to the environment of region, Would be justified. also, technical criteria in four climate: mountainous, Khazari, Mediterranean and semi-desert of country has been second in importance and the highest it's weight is in the mountainous climate(0.320) and the lowest it's weight is in the desert climate(0.196).that the high importance of ecological criteria in the mountainous climate Due to the sensitive environmental conditions and the poor performance of most wastewater treatment processes in cold weather would be justified.
Based on the results in three climate mountainous, Khazari and Mediterranean activated sludge process Is introduced as the most appropriate treatment options. That the highest it's weight is in the mountainous climate (0.347) and the lowest it's weight is in the desert climate (0.247). Also, in two climate desert and semi-desert stabilization pond Is introduced as the most appropriate treatment options. That the highest it's weight is in the desert climate (0.292) and the lowest it's weight is in the khazari climate (0.183).finally, The highest and lowest weight of aerated lagoon process Is Respectively achieved in mountainous (0.269) and semi-desert (0.181) climates. And The highest and lowest weight of Trickling Filter process is respectively achieved in Khazari (0.273) and mountainous (0.174) climates.