Abstract
Septic tanks are widely deployed for off-grid sewage management but are typified by poor treatment performance, discharge of polluting effluents and the requirement for frequent de-sludging. The Solar Septic Tank (SST) is a novel septic tank design that uses passive heat from the sun to raise in-tank temperatures and improves solids degradation, resulting in a cleaner effluent. Treatment has been shown to exceed conventional systems, however, the underlying biology driving treatment in the system is poorly understood. We used next generation sequencing (Illumina Miseq (San Diego, CA, USA), V4 region 16S DNA) to monitor the microbiology in the sludge and effluent of two mature systems, a conventional septic tank and an SST, during four months of routine operation in Bangkok, Thailand, and evaluated the ecology against a suite of operating and performance data collected during the same time period. Significant differences were observed between the microbiome of the sludge and effluent in each system and the dominant taxa in each appeared persistent over time. Furthermore, variation in the microbial community composition in the system effluents correlated with effluent water quality and treatment performance parameters, including the removal of chemical and biochemical oxygen demand and the concentration of fecal and total coliforms in the effluent. Thus, we propose that a wide-scale survey of the biology underlying decentralised biotechnologies for sewage treatment such as the SST could be conducted by sampling system effluent rather than sampling sludge. This is advantageous as accessing sludge during sampling is both hazardous and potentially disruptive to the anaerobic methanogenic consortia underlying treatment in the systems.
Highlights
Across the global north and south alike, septic tanks are amongst the most widely used household-scale sewage management systems employed in the absence of centralised sewerage networks [1]
This may relate to diurnal temperature fluctuations in the Solar Septic Tank (SST) arising from natural diurnal changes in solar radiation, which we propose should be investigated in future studies of this system
Microbiology in small-scale anaerobic digestion (AD): Culture independent methods that enable interrogation of the underlying biology in large-scale AD technologies are widely recognised for their potential to deliver better process management [34,35], yet these methods have rarely been applied to septic tanks
Summary
Across the global north and south alike, septic tanks are amongst the most widely used household-scale sewage management systems employed in the absence of centralised sewerage networks [1]. The septic tank design targets low-cost treatment of sewage primarily by separation of the solid and liquid fractions of the waste and passive anaerobic digestion (AD) of retained solids [2]. The liquid fraction may be discharged to the environment, typically after a retention separation, liquid fraction may be discharged to the environment, typically after asolids, retention of only of only 1–2the days when serving combined grey and blackwater sources. 1–2 days when serving combined grey and blackwater sources. In so doing, accumulated solids gradually serve to reduce effective tank over time.liquid
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