Economic evaluation of different biological municipal wastewater treatment systems and implementation of AHP method based on operating costs

Water is the main matter for life and living organisms are highly dependent on it in a way that more than 50% of each living organism is consists of water. Freshwater resources make up a very small part of the planet’s water supply (about 3%) and of this limited amount, a very small percentage (about 10%) is reachable [1,2]. This chemical is naturally associated with impurities, and pure water is only reachable by its main reservoirs in polar glaciers because water dissolves or suspends organic and inorganic matter by passing through soil and air or mixing with other liquids. Drinking water is no exception to this rule and contains salts and minerals, but in limited quantities and in a controlled manner; the reduction of these fi gures to the allowable range takes place in water treatment plants [3-6].


Introduction
Water is the main matter for life and living organisms are highly dependent on it in a way that more than 50% of each living organism is consists of water. Freshwater resources make up a very small part of the planet's water supply (about 3%) and of this limited amount, a very small percentage (about 10%) is reachable [1,2]. This chemical is naturally associated with impurities, and pure water is only reachable by its main reservoirs in polar glaciers because water dissolves or suspends organic and inorganic matter by passing through soil and air or mixing with other liquids. Drinking water is no exception to this rule and contains salts and minerals, but in limited quantities and in a controlled manner; the reduction of these agriculture lead to the production of wastewater because by the augmentation of waste materials such as urine, feces, detergents, fertilizers, and other pollutants in water and exceeding the standard amount of organic and mineral substances, water becomes useless for previous purposes [7,8]. Pollutants existing in wastewater, have the potential to spoil these few resources if released to nature [9][10][11][12]. therefore, the amounts of pollutants should be adjusted even for disposal in nature. This is where the importance and necessity of wastewater treatment and the construction of treatment plants become clear. According to the source of its production, wastewater is generally divided into three general categories: urban, industrial and agricultural and it should be noted that the source of production affects the kinds and amount of existing materials and pollutants [4]. Therefore, the treatment plant is designed based on the type of incoming wastewater. One of the most important issues before designing and constructing any wastewater treatment plant is choosing the right treatment process, in a way that a wrong choice can result in a huge increase of costs and even in some cases not achieving the desired outcome [13][14][15][16][17]. The United Nations Environment Program (UNDP) (2015) has evaluated wastewater treatment policies in a report with a comparative approach [18].
In the fi rst step, this research calculates the operating costs of the treatment plant for each of the three common methods of biological treatment, including Complete Mixed Activated Sludge (CMAS), Aerated lagoon, and Oxidation Ditch, and in the next step, the value of each method and the optimal choice is determined. In this regard, the Analytic Hierarchy Process (AHP) is one of the most comprehensive systems designed for decision-making with multiple criteria. This analysis is a fl exible and quantitative method for selecting options based on their relative performance for a number of criteria. The purpose of using AHP in the present study is to identify reference options and also to determine their ranking by considering all decision criteria, which are examined in detail in the following sections of the article.

Wastewater treatment processes
Wastewater treatment means lowering the amount of existing organic, mineral, and biological agents in water to the allowable range which includes primary, secondary (biological treatment), and advanced treatment. The designation of these stages may be shorter or longer depending on the incoming wastewater and the selected treatment methods, but the existence of primary and secondary ones is defi nite [19]. There are different methods to perform each of these steps, which are associated and in combination with other steps during the design process [20].
The biological processes used for wastewater treatment are divided into two important groups: suspended growth and attached growth (biological layer). In the fi rst method, the microorganisms in charge of the purifi cation process are kept suspended in the liquid by proper mixing [21]. The most common suspended growth process used for municipal wastewater treatment is activated sludge, which makes oxidation ditch and aerated/non-aerated lagoons the next priority. Therefore, the studied processes are limited to three processes of Complete mixed activated sludge (CMAS), oxidation ditch, and aeration lagoons, which are most used in Iran. Flowchart Process for the three under consideration processes is shown in Figure 1a-c.
The CMAS process has a continuous fl ow and has an initial settling tank to remove coarse sediments and apply additional load to the aeration tank ( Figure 1a

Economic simulation
Performing economic calculations before decision-making processes (especially in infrastructural projects which involve large-scale costs) is necessary [23]. In order to perform these calculations, besides using instructions and performing calculations manually, utilizing provided models and software  [2]. The method is based on a hierarchical structure to assist the analyst in managing critical aspects of the problem, within a tree-like hierarchical structure that reduces complex decisions to some simple comparisons and rankings [12]. The main reasons for using this method can be summarized as follows [4]: • Having an effective and operational approach to consider non-structural and complex decisions; • Pairwise comparisons in the AHP method allow decision-makers to extract the weight of any criteria or the ranking of each option from paired matrices, and a large number of criteria are able to be considered; • Helping decision-makers to incorporate critical aspects of the problem into a hierarchical structure to shape fl exibility based on the problem; • The hierarchical structure of the AHP method resulted from pairwise comparisons of independent judgments, is preferable to efforts that prioritize all decisions and criteria simultaneously.
The purpose of using the analytic hierarchy process is to identify reference options as well as determining their ranking by considering all decision criteria simultaneously [2]. This analysis is a fl exible and quantitative method for selecting options based on their relative performance over a number of criteria. Overall, the AHP method typically involves the following six steps:

Results and discussion
Economic simulation based on municipal wastewater input with the specifi cations presented in Table 1 (which is equivalent to the average specifi cations of fi ve wastewater treatment plants in Mashhad) and other required data has been chosen in accordance with US design standards which are almost the same as software default data.
Economic analysis was performed after defi ning three  target processes, to which their fl ow process diagrams are referred in Figure 1(a-c), and assigning the required software parameters in relation to the fl ow characteristics and general conditions of the treatment plant, the most important of which mentioned in Table 1. The results are refl ected in Figure 2 and according to the economic calculation chart; operating costs for wastewater treatment plants are divided into six categories including performance, maintenance, materials, chemical, energy, and amortization. As presented in the diagram, the aeration lagoons have the highest energy consumption and the lowest cost for materials, performance, and maintenance. On the other hand, the Complete mixed activated sludge process, which is briefl y represented by CMR, meaning complete mixing reactor, has the highest amortization and operating costs. Also, the oxidation ditch process has the lowest amortization and energy consumption.
Another important cost parameter that is always considered for comparison is the construction cost [24], which according to the results of economic analysis, for each of the three treatment processes; the construction cost is as shown in Figure 3. The highest construction cost is allocated to the CMAS activated sludge treatment plant with a fi gure of 84,600,000 dollars, and the other two processes require approximately the same and 34 million dollars. Therefore, due to the variety of factors affecting operating costs, in order to make the right decision for choosing the optimal option, decision-making methods should be implemented.
Meetings with experienced experts of Mashhad Water and Sewerage Department using brainstorming techniques were conducted to determine the importance and evaluation coeffi cients of the six operating cost parameters (operating costs, maintenance, materials and consumables, chemicals, energy, and amortization). As shown in Figure 4b, the highest value was assigned to the maintenance with 26.9% and the lowest to amortization costs with 1.8%.
The Analytic Hierarchy Process (AHP) was implemented using Expert Choice 11 software and using the valuation fi gures obtained in joint meetings and the calculated costs shown in Figure 2. The results which are depicted in Figure 4a show that the highest value among the biological treatment plants belongs to aeration lagoons with a value of 47.3%. Oxidation stream and CMAS with values of 27.9% and 24.9% respectively are in the next priorities. Of course, as shown in Figure 5, these values are based on the sensitivity defi ned for each operating cost parameter, and if they change at another time or place, or any other Involved factors, the value of each option fl uctuates consequently.

Conclusion
The need for wastewater treatment is not hidden from anyone due to the high degree of physical, chemical, and biological pollution, and this is completely possible by wastewater treatment plants. However, the high cost of these large-scale projects has led experts to fi nd the most appropriate option that optimally meets the technical, economic, and environmental goals. The results of this research show that the highest construction cost was allocated to the CMAS