Revisiting the evidence: an investigation, quantification and characterization of the biodegradation mechanisms and functional microbiology in conventional septic tanks




Shaw, Kelsey A.

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Decentralized sanitation technologies are considered simple and low-cost solutions to achieving the Sustainable Development Goal (SDG) 6.2: access to sanitation for all. This research specifically focuses on conventional septic tanks (ST), due to their wide-spread use and simplicity in design and operation. Despite the global prevalence of the ST, limited research has been performed to effectively characterize the relevant biodegradation mechanisms involved with sludge accumulation. It has been largely assumed that anaerobic digestion (AD) is the dominant biodegradation treatment mechanism in STs. However, evidence suggests these systems are primarily functioning as holding tanks as opposed to truly passive bioreactors. There is limited information available on the nature and basic characteristics of ST sludge and the impacts these systems have globally (i.e., greenhouse gas (GHG) emissions). These gaps in knowledge are a contributing factor to ST effluents polluting receiving water sources and endangering human health on a global scale. The overall objective of this research was to investigate and identify the extent of anaerobic activity occurring in conventional STs. This thesis revisits this underlying and poorly investigated assumption about these common useful and accessible sanitation variants from three different but complimentary perspectives. Although this research was significantly impacted by the unprecedented global health crisis (COVID19) that occurred for the majority of the project (>80%) it resulted in the following three manuscripts: 1. Manuscript #1 (Chapter 2): The first manuscript assessed available literature for evidence supporting the presence and relationship between anaerobic conditions, microbiology, and activity. The literature assessed presented no conclusive evidence of anaerobic microbiology or activity therefore anaerobic biodegradation mechanisms could not be confirmed. 2. Manuscript #2 (Chapter 3): This work further developed the understanding of the broader implications that non-sewered sanitation systems can have on the surrounding environment while highlighting the need for further in-field quantification of emission factors. It demonstrated that the current Intergovernmental Panel for Climate Change (IPCC) Model is overestimating GHG emissions for these systems. 3. Manuscript #3 (Chapter 4): The third manuscript laid the groundwork to continue field work and laboratory analysis that can quantify the extent and type of anaerobic activity present in these systems. Preliminary results indicate that acetoclastic methanogenesis may not be dominant type of activity in conventional ST sludge. By challenging commonly accepted assumptions, characterizing pathways for climate mitigation and establishing reliable experimental methods, this work can fundamentally change how so-called passive treatment sanitation systems are designed and operated. Providing the baseline for the accurate and detailed characterization of the microbial community and resulting activity present within these systems will allow for greater insight into their functioning and more rational design. Ultimately, this work will promote access to simple, effective, and sustainable sanitation solutions.



Non-sewered sanitation, Sustainable sanitation, Public Health, Environmental Engineering