The demand for water has been increasing dramatically recently in domestic, industrial and agricultural sectors, due to economic development and population growth. Therefore, wastewater may be considered as a valuable water resource for some of the mentioned sectors. The main aim of this paper is to introduce a new method to determine the best applications of effluents. In order to achieve this goal, 60 monthly effluent water quality data, sample data fields which were collected and analyzed during the years 2013–2017 by the wastewater treatment plant of Arak city, Iran, were used. Two indices are developed and applied, which involved the entropy and fuzzy logic-based approaches. Six possible effluent applications were considered including industrial, artificial groundwater recharges, environmental purposes, irrigation usages for green spaces, oilseed and fodder production. The results showed that the quality of studied effluents has been improved during the five mentioned years. As in the last year, according to the indices, the effluents could be used for industrial, environmental, fodder production, oilseed production, and artificial groundwater recharge applications. Also, the fuzzy effluent quality index has produced larger values than the entropy index, such that their largest difference is equal to 32.9, and under similar conditions offered fewer possible applications.