Sludge Retention Time in Activated Sludge Systems: A Key Operational Parameter with Unresolved Challenges

SRT links microbial growth requirements, reactor design, and activated sludge operation. However, the relationship among SRTmin, SRTdesign, and SRTop is not always clearly defined in the literature. This study combines a literature review of 58 publications with comparisons of publication approaches, an illustrative design-based case study, and full-scale plant data to examine the practical importance of these distinctions. Three SRTmin formulations were compared over a temperature range of 3–35 °C using literature-based kinetic parameters and fixed operating assumptions. Also, a sensitivity analysis was performed to assess how variations in the assumed literature-based kinetic parameters affected the calculated SRTmin. All three equations predicted a decrease in SRTmin with increasing temperature. The sensitivity analysis identified the maximum specific growth rate as the most influential parameter: a 20% reduction increased SRTmin by approximately 29–32%. We also compared the calculations of SRTop equations with and without effluent solids losses and considered temporal averaging. Excluding effluent solids increased the calculated daily SRTop by 3.16% on average, with individual differences reaching 16.54%. Calendar-week means and two-week averages smoothed the fluctuations in calculated SRTop pattern by combining higher and lower values, making short-term peaks less apparent. Moreover, we investigated whether adapting SRTop to seasonal wastewater temperature could offer benefits in terms of energy saving. Two design scenarios were compared to examine the potential effects of temperature-dependent SRTop adjustment on oxygen demand and sludge production. In the design-based comparison, the scenario without seasonal SRT adaptation had 10.4% higher calculated summer oxygen demand than the temperature-adapted scenario. To explore whether a similar relationship was evident in actual plant operation, full-scale data were analyzed for associations between blower electricity consumption, operational SRTop, and its difference from a temperature-dependent SRT reference. The analysis showed that monthly SRTop above a temperature-dependent reference was strongly associated with daily blower electricity consumption (r = 0.83, p = 0.001), whereas the association with specific blower electricity consumption was weaker (r = 0.54, p = 0.071). These findings highlight the importance of clearly defined calculations and interpreting operational SRT against biological requirements. They also support further investigation of temperature-dependent SRTop adjustment alongside aeration control but do not demonstrate achievable electricity savings at unchanged treatment performance.