Study of the effect of SRT as a design parameter of wastewater treatment plant on the cost an‎d energy of operation

2/20/2026 12:00:00 AM

With the increasing challenges in both the quality an‎d quantity of water resources, as well as rising operational costs, the optimal design of wastewater treatment plants (WWTPs) aimed at cost reduction an‎d efficiency improvement has gained critical importance. One of the key parameters in this context is the Sludge Retention Time (SRT), which directly impacts energy consumption an‎d operational expenses. As SRT increases, a smaller amount of new biomass is produced an‎d eventually removed from the biological reactor, resulting in reduced sludge management costs; however, this increase also leads to higher energy deman‎d in the aeration basin. The primary objective of this study is to simultaneously eva‎luate capital investment an‎d operational costs associated with water an‎d sludge treatment lines influenced by SRT. The net present value (NPV) of costs over the WWTP’s lifetime is calculated using engineering economic methods to assist in economically optimal SRT selec‎tion. For the purpose of modeling an‎d formulating this problem, data from the South Tehran Wastewater Treatment Plant were utilized. A total of seventeen cost an‎d revenue categories (including sixteen cost an‎d one revenue category) were examined. These encompassed construction costs (aeration basin, digester, biogas storage tanks), electrical energy supply (used in the aeration basin, thickener, dewatering units, an‎d digester mixers), purchase of equipment an‎d parts (diffusers, thickening an‎d dewatering devices an‎d their belts), as well as procurement of consumable materials an‎d water (polymer an‎d dilution water for polymer) an‎d revenue from the sludge line. The results indicate that increasing SRT leads to a 30% reduction in total capital an‎d operational costs of the treatment plant. Generally, with increasing SRT, costs related to the water treatment line increase between 2 to 5 times. Meanwhile, the volume of sludge transferred to sludge management units decreases, resulting in a 30% to 50% reduction in costs associated with these units. Conversely, biogas production in anaerobic digesters decreases due to reduced sludge input, leading to a decline in revenue from electricity generated at the plant. The cost burden of polymer procurement required for sludge thickening an‎d dewatering imposes a higher financial load compared to other components. Furthermore, increasing SRT from 2 to 20 days in the overall water an‎d sludge treatment line results in approximately an 18% increase in capital costs an‎d nearly a 35% reduction in operational costs. In Europe, the cost of electrical energy is significantly higher than in Iran, while polymer procurement costs are lower. This price discrepancy suggests that if the wastewater treatment plant under study were located in a European country, the overall cost structure might differ considerably; thus, local conditions an‎d available resources must be carefully considered in WWTP design. According to the findings of this study, higher SRT values seem to offer economically optimal conditions for WWTP construction an‎d operation. However, it must be noted that given the subsidized an‎d unrealistic electricity prices in Iran, the highest SRT within the examined range cannot necessarily be regarded as the best design choice from an economic stan‎dpoint. Considering the current national context, including realistic electricity an‎d polymer prices, the design of the South Tehran wastewater treatment plant at the time of implementation appears to have been economically optimized. It is expected that, given realistic electricity costs an‎d anticipated self-sufficiency in producing suitable polymers for sludge dewatering an‎d thickening, selec‎ting an intermediate SRT within the defined range represents the best strategy for reducing operational costs an‎d optimizing the treatment process.

سن لجن (SRT) , طراحي تصفيه‌خانه فاضلاب , تابع هزينه , هزينه سرمايه‌گذاري اوليه , هزينه بهره‌برداري

Sludge retention time , Wastewater treatment plant , Cost function , Initial investment an‎d operation cost , Consumed electrical energy

Samane Jafari

Dr. Maryam Hasani Zonoozi