Treatment reliability of solar septic tank systems under comparative field conditions

Solar-assisted septic tanks integrated with evacuated-tube solar collectors have been proposed as a decentralized sanitation technology capable of enhancing wastewater treatment through solar-driven thermal processes. However, the influence of thermal storage capacity on treatment efficiency, thermal stability, and operational reliability under actual household conditions remains poorly understood. This study evaluated the long-term field performance of two solar-assisted septic tank systems installed in residential households in Thailand: one equipped with a 200-L insulated hot-water storage tank and another utilizing only a 5-L buffer tank without large thermal storage. Influent and effluent wastewater samples were collected between August 2017 and March 2018 and analyzed for solids, organic matter, nutrients, and microbial indicators. The system without large thermal storage achieved higher average removals for several organic and nutrient parameters, including total chemical oxygen demand (89%), total biochemical oxygen demand (82%), total Kjeldahl nitrogen (64%), and total phosphorus (77%), primarily due to higher peak temperatures reaching up to 54 °C. However, treatment performance exhibited substantial temporal variability and occasional negative removal events, indicating reduced operational stability and limited thermal retention. In contrast, the system equipped with a 200-L hot-water storage tank maintained more stable thermal conditions and achieved significantly improved microbial reduction, with an average Escherichia coli reduction of 2.88 log₁₀ units compared with 1.29 log₁₀ units in the system without thermal storage. Statistical analyses demonstrated that influent concentration and effluent temperature jointly influenced treatment performance, although their relative importance varied among parameters. The results indicate that sustained thermal exposure provided by hot-water storage contributes more effectively to treatment reliability and pathogen reduction than short-term temperature peaks alone. These findings provide practical evidence that thermal storage capacity is a critical design parameter for improving the reliability, stability, and public health performance of solar-assisted septic tank systems and may support the future development of decentralized sanitation technologies in tropical and resource-constrained settings.