Trade-offs across values in cesspool management highlight challenges to policy making

Area of Science:

• Environmental Science
• Public Health
• Decision Analysis

Background:

• On-site Sewage Disposal Systems (OSDS) are prevalent globally and pose contamination risks to water sources in Hawai'i.
• State legislation mandates the conversion of thousands of cesspools by 2050, necessitating urgent planning for alternatives.
• High-elevation areas in north Maui face particular challenges for cesspool replacement and wastewater management.

Purpose of the Study:

• To collaboratively design cost-effective and equitable solutions for cesspool replacement in north Maui.
• To utilize a participatory structured decision-making (SDM) approach involving diverse stakeholders.
• To model environmental (groundwater nutrients), economic (costs), and social (equity) factors of various alternatives.

Main Methods:

• Conducted participatory workshops with stakeholders to define objectives and brainstorm alternatives.
• Employed technical modeling to assess groundwater nutrient impacts, costs, and equity of 33 proposed solutions.
• Utilized multi-objective analysis and efficiency frontiers to evaluate trade-offs and identify optimal strategies.

Main Results:

• Composting toilets showed promise across most objectives but presented high maintenance burdens.
• Cluster sewering for dense areas, followed by incentivized septic systems for high-flow properties, emerged as a cost-effective strategy.
• Aerobic treatment units and passive absorption systems were identified as less cost-effective, with higher costs and lower nutrient reduction.

Conclusions:

• A hybrid sewer-septic approach is recommended for north Maui, with estimated project costs of $183-258 million.
• Participatory SDM provides a transparent, evidence-based framework for complex sanitation planning.
• The SDM approach can be replicated statewide and in other regions facing similar resource management challenges.