Variation in septic system effluent inputs to tributaries in multiple subwatersheds and approaches to distinguish contributing pathways and areas

Review of linear and circular approaches to on-site domestic wastewater treatment: Analysis of research achievements, trends and distance to target

This comprehensive review critically assesses traditional and emerging technologies for domestic wastewater treatment and reuse, focusing on the transition from conventional centralised systems to innovative decentralised approaches. Through an extensive literature search on domestic wastewater systems serving a population equivalent of less than or equal to 10, the study juxtaposes linear and circular methods and highlights their impact on urban water management and the environment. The papers reviewed were classified into five categories: Environmental studies, economic studies, social studies, technological studies, and reviews and policy papers. The analysis was carried out separately for linear and circular approaches within each category. In addition, the maturity of the technology (lab/pilot or full-scale application) was taken into account in the analysis. The research landscape is shown to be evolving towards circular methods that promise sustainability through resource recovery, despite the dominance of linear perspectives. The lack of clear progress in decentralised technologies, the scarcity of circularity assessments and the challenges of urban integration are highlighted. Operational reliability, regulatory compliance and policy support are identified as key barriers to the adoption of decentralised systems. While conventional pollutants and their environmental impacts are well addressed for linear systems, the study of emerging pollutants is in its infancy. Conclusions on the impact of these hazardous pollutants are tentative and cautious. Social and economic studies are mainly based on virtual scenarios, which are useful research tools for achieving sustainability goals. The conceptual frameworks for assessing the social dimension need further refinement to be effective. The paper argues for a balanced integration of centralisation and decentralisation, proposing a dual strategy that emphasizes the development of interoperable technologies. It calls for further research, policy development and widespread implementation to promote decentralised solutions in urban water management and pave the way for sustainable urban ecosystems.

Eliciting expert judgements to underpin our understanding of faecal indicator organism loss from septic tank systems

Septic tank systems (STS) in rural catchments represent a potential source of microbial pollution to watercourses; however, data concerning the risk of faecal indicator organism (FIO) export from STS to surface waters are scarce. In the absence of empirical data, elicitation of expert judgements can provide an alternative approach to aid understanding of FIO pollution risk from STS. Our study employed a structured elicitation process using the Sheffield Elicitation Framework to obtain expert judgements on the proportion of FIOs likely to be delivered from STS to watercourses, based on 36 scenarios combining: (i) septic tank effluent movement risk, driven by soil hydro-morphological characteristics; (ii) distance of septic tank to watercourse; and (iii) degree of slope. Experts used the tertile method to elicit a range of values representing their beliefs of the proportion of FIOs likely to be delivered to a watercourse for each scenario. The experts judged that 93 % of FIOs would likely be delivered from an STS to a watercourse under the highest risk scenario that combined (i) very high STS effluent movement risk, (ii) STS distance to watercourse <10 m, and (iii) a location on a steep slope with gradient >25 %. Under the lowest risk scenario, the proportion of FIOs reaching a watercourse would likely reduce to 5 %. Expert confidence was high for scenarios that represented extremes of risk, while uncertainty increased for scenarios depicting intermediate risk conditions. The behavioural aggregation process employed to obtain a consensus among the experts proved to be useful for highlighting both areas of strong consensus and high uncertainty. The latter therefore represent priorities for future empirical research to further improve our understanding of potential pollution risk from septic tanks and in turn enable better assessments of potential threats to water quality in rural catchments throughout the world where decentralised wastewater systems are common.