Integrating Resiliency into Decision-making for Sea-Level Rise Adaptation of On-Site Wastewater Treatment Systems

Wastewater is handled under two broad categories: (1) decentralized systems and (2) centralized systems. Decentralized systems (a.k.a. onsite systems) include septic systems and are deployed where regional, centralized systems are not available or costly to install. In the septic systems, wastewater is partially treated as it flows from the property through a septic tank and subsequently a drain field where the clarified liquid is further treated by the unsaturated soil. Thus, by design, the drain field must be above the groundwater table and remain unsaturated to function effectively. As such, septic systems are vulnerable to the rising groundwater levels, which is a consequence of sea level rise (SLR) in coastal areas. With reduced soil buffer, wastewater may flow unimpeded to ground and surface waters in a saturated medium at faster rates allowing for less or no treatment, and therefore, contaminating groundwater. In this study, we address the SLR adaptation problems for coastlines and propose a comprehensive robust optimization model based on multi-objective mathematical programming for reformation of the septic system networks. The model jointly maximizes the resilience of the septic system in a given region and minimizes the investment costs over multiple periods under SLR risk scenarios. It proposes optimal combinations of adaptation approaches composed of on-site adaptation, building shared septic, sewer extension and installing micro plants and their implementation sequences.