Sensitivity analysis of parameters affecting suspended growth in industrial wastewater treatment plants; with emphasis on economic performance criteria

Industrial wastewater treatment is of high priority due to the presence of an extraordinary concentration of dangerous pollutants. Similar to urban wastewater treatment plants, there are plenty of options while designing industrial ones which need various analyses and researches to choose correctly. One of the most effi cient ways to solve this problem is to implement Economic and Sensitivity Analysis. This research has studied designing an industrial wastewater treatment plant utilizing three different biological treatment methods (including Sequential Batch Reactors (SBR), plug-fl ow activated sludge with a secondary clarifi er, and step aeration activated sludge with a secondary clarifi er). It also measures the sensitivity of performance cost parameters to factors affecting the suspended growth unit. The results of the economic analysis showed that using SBR has the highest construction cost of $ 70,200,000 and the highest total cost of 96,900,000. Sensitivity analysis showed that by adjusting design retention time and variance between heterotrophic microorganism decay rate and the reality, using activated carbon-based systems could signifi cantly reduce total annual costs.


Introduction
A study of production policies in industrial countries around the world shows that strict and complex rules have been put in place to control environmental pollution to which paying attention and adhering, leads to the creation of a sustainable industry and economy. One of the most important tasks of managing industrial estates is to design, construct, and operating wastewater treatment plants (WWTPs) economically effi ciently which is getting more understandable by looking at the lack of resources and the increase of industrial units [1][2][3][4][5].
Modeling, simulation, anticipation, and analysis of earlier data are some of the operational solutions that can help to optimize the economic costs of WWTPs which could get complicated by unpredictable factors such as operational problems, especially in biological methods [6][7][8]. Every one of these complexities necessitates the sensitivity analysis of parameters affecting the economic performance of treatment plants to form a correct managerial perspective [9][10][11].
No one doubts the necessity of wastewater treatment due to its high amount of physical, chemical, and biological pollution which is only reachable through treatment plants. However, the high costs of these large-scale projects have led experts https://www.peertechzpublications.com/journals/annals-of-environmental-science-and-toxicology Citation: Gheibi  to fi nd the most appropriate process that, besides meeting quality parameters, optimally satisfi es the technical, economic, and environmental goals [12][13][14][15][16]. On the other hand, each of these treatment plants and related processes has special characteristics and attitudes. Determining the strengths and weaknesses of each unit and process gives a proper outlook to managers and consultants through the design process [17][18][19][20].
One of the appropriate steps to create this attitude is to analyze the sensitivity of sub-indicators affecting the performance of each unit and its associated costs. The results of this analysis, along with comprehensive information on the current situation, lead to a smarter choice; An option that shows the least sensitivity in case of changes in delicate parameters [21][22][23][24].

Materials and methods
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 [29]. There are different methods to perform each of these steps, which are associated and in combination with other steps during the design process [30]. Biological processes used for wastewater treatment are divided into two important groups of suspended growth and attached growth (biological layer). In suspended growth processes, microorganisms in charge of the treatment process are kept suspended in a liquid by proper mixing [31].
Three suspended growth processes studied in this research are Sequential Batch Reactor (SBR), plug-fl ow activated sludge with a secondary clarifi er, and step aeration activated sludge with secondary clarifi er which is shown in Figures 1a-c respectively. The fl ow process diagram for plug-fl ow and step aeration-activated sludge is similar to the SBR diagram except for the suspended growth unit which is shown in Figure 1a.
For further information, designing the treatment process and The sensitivity of the sub-indicators was also checked by the sensitivity analysis tool embedded in the mentioned software.

Results and discussion
Economic and sensitivity analysis was performed by establishing process diagrams for the three selected methods in Figure 1

Another economic criterion considered in this study is Present
Value, which is one of the standard methods of evaluating Citation: Gheibi  Economic analysis of these parameters shows that a large part of annual operating costs is because of depreciation (equipment and structure), which costs $ 3,370,000 per year if the SBR is used, which is the highest among the annual depreciation costs of the three examined methods (Figure 3). The lowest annual cost has been allocated to required chemicals because the designed processes have little need for expensive chemicals.
The plan related to using SBR in the suspended growth process has the highest depreciation, energy, performance, and maintenance cost among the 6 parameters of operating costs. Also, the plug-fl ow activated sludge system reached the highest cost of used materials annually. Furthermore, the highest cost of chemicals despite low amount usage is owed by step aeration activated sludge by 18,900 $ annually (Figure 3).
In the sensitivity analysis section, the dependence and sensitivity of 6 operating cost parameters are examined concerning the factors affecting the performance of the suspended growth unit. In the process sensitivity analysis, fi ve factors of design retention time, maximum specifi c growth coeffi cient of heterotrophic microorganisms, heterotrophic microorganism degradation rate, maximum autotrophic microorganism specifi c growth rate, and autotrophic microorganism degradation rate were identifi ed as factors affecting the suspended growth unit. Sensitivity analysis of 6 operating cost parameters using SBR showed that energy has the highest sensitivity (in this case, the most changes in the slope of the graph) to changes in design retention time ( Figure 4). The highest sensitivity to the rate of heterotrophic microorganism decay is also caused by energy cost ( Figure 5).
Also, studies showed that the operating cost parameters were insensitive to other factors. Sensitivity analysis of plug fl ow-activate carbon unit showed that operating cost parameters have the highest vulnerability to the changes in retention time and heterotroph microorganisms' decay rate. Among all, depreciation has the most sensitivity to the mentioned factors (Figures 6,7).
Similarly, for the last proposed WWTP scheme (step aeration-activated carbon) the same circumstances have happened. Depreciation showed the most sensitivity to retention time and heterotroph microorganisms' decay rate (Figures 8,9). by reviewing all these analyzes, it can be concluded that

Conclusions
Wastewater treatment has seen different methods and systems due to its different characteristics. Therefore, environmental engineers see a wide variety of options in designing these infrastructures. Choosing the right option requires careful research and thorough familiarity with the processes in the fi rst step. for the second step, they should make appropriate comparisons and analyses for the selected options to fi nd a suitable estimation and strategy over the future       conditions. Economic and sensitivity analysis is one of the most effi cient ways to mentioned strategies. The present study deals with economic and sensitivity analysis when using three different suspended growth methods. The annual operating costs for the proposed projects are divided into six categories including performance (manpower), maintenance, materials, chemicals, energy, and depreciation. By implementing sensitivity analyzes, it was also observed that by using activated carbon-based systems, changes in design retention time, and a contradiction of heterotrophic microorganism degradation rate to the reality, annual operating costs can be subject to major changes. Most of these changes happen in energy and annual depreciation costs.