**4. Economic sustainability**

Economic sustainability of WWTPs refers to the economic factors affecting social, environmental and cultural aspects of the treatment systems. Economic efficiency of WWTPs presents the scenario of investments in terms of input and effluent quality as the output [70]. Decision and policy makers in developing countries are challenged with the fact that poor urban residents cannot afford costly conventional sewage treatment systems [71]. Fortunately, a broad range of costeffective technological options are extensively being studied to cater this category of people. Therefore, economic factors become vital to address these issues. For any WWTPs, economic indicators generally represent the costs associated with the construction and the operation of treatment management during its life time [72]. These are driving factors for decision making while selecting a technology in a practical situation.

For the economic assessment, the two most common indicators called capital expenditure (Capex) and operational expenditure (Opex) were calculated using the equations provided by [73]. The capital expenditure included cost of construction and life cycle costs. Operational expenditure included number of mechanical equipment, skilled workers, power consumption, labor, chemicals, and consumables. The data were taken from the state-of-art literatures [43, 74, 75] for calculating the construction and operating costs for economic evaluation for both the systems. For the comparison in the same scale few considerations were made:

Capex is annualized by taking the means of the initial investment costs by the life period of the project accounting for the time value of money while Opex is total discounted lifetime operational expenses.


The economic costs comparison **Table 4** showed that capex and opex costs of UASB+DHS system of 9740 p.e. is almost equal to the UASB+TF of 50,000 pe.


**Table 4.**

*Economic assessment of UASB+DHS system and UASB+TF system.*

While for the UASB+TF of 2337 p.e. UASB+DHS system expressed significantly less capex and opex values. For most of the UASB+TF, among the various costs, cost of personnel is the maximum for opex [50]. This could be also the reason for the decreased economic costs for UASB+DHS system. The another rationale for reduced economic costs of UASB+DHS system could be less manpower required due to the simple O&M processes including cleaning of the mechanical parts, chemical free operation and easy handling of sludge [76]. It is interesting to note that DHS inclination is towards negative value for economic assessment which means it is economically fit for the developing countries as capex and opex costs are negative indicators of sustainability and qualifies the criteria to be considered as self-sustainable WWTP system.
