Toward Sustainable Water Treatment: Hydraulic Retention Time Effects on Natural and Chemical Coagulants in Continuous-Flow Helically Coiled Tube Flocculators

Access to safe drinking water remains a global challenge, particularly in decentralized and low-resource settings where conventional treatment technologies may be economically or operationally impractical. Combining low-energy hydraulic flocculation systems with biodegradable natural coagulants represents a promising strategy for sustainable water treatment. This study investigated the influence of hydraulic retention time (HRT) on the clarification performance of aluminum sulfate, Moringa oleifera, and Aloe vera in continuous-flow helically coiled tube flocculators (HCTFs). Four HRTs (1.71–6.84 min) were obtained by varying reactor length while maintaining a constant flow rate of 0.5 L/min. Synthetic water with an initial turbidity of approximately 100 NTU was treated, and clarification performance was evaluated by residual turbidity and turbidity removal efficiency. Two-way ANOVA revealed significant effects of HRT and coagulant type, as well as a significant interaction between these factors (p < 0.001), indicating that the influence of HRT depended on the dominant coagulation mechanism. Aluminum sulfate maintained consistently high turbidity removal efficiencies (96.3–98.1%) regardless of HRT. In contrast, Moringa oleifera achieved the highest clarification efficiency (99.5%) and a final turbidity of 0.49 NTU at the longest HRT, whereas Aloe vera exhibited the greatest hydraulic sensitivity, with turbidity removal increasing from 78.9% to 96.1% as HRT increased. The results demonstrate that HRT should be selected according to the dominant coagulation mechanism of the coagulant rather than adopted as a fixed design parameter. This mechanism-based approach provides a rational basis for designing and optimizing continuous-flow flocculators and supports the development of compact, efficient, and sustainable decentralized water treatment systems.