2021-10-27 EMC Agenda item 5-b(i) Identifying wastewater management tradeoffs ... in Kona, Hawaii (RG)

Home

PLOS ONE Identifying wastewater management tradeoffs in Kona, Hawai'i Table 3. Life cycle costs for septic and ATU systems. htttltps //doll.o irg/1Q1.1371/joiunnall.Iporne,0257125:t003 option is assumed to have an annual O&M cost of $400 with a 60-year system replacement interval and the ATU option is assumed to have an annual O&M cost of $700 with a 30-year system replacement interval. The life cycle cost over 30 years of each system is presented in Table 3 for a range of capacities (1-5 bedroom (BR)), assuming a discount rate of 2.8%. Because the life cycle costs of both systems vary substantially by installed capacity, the size of each potential system upgrade was assigned based on the characteristics of parcels that are currently identified as having one or more cesspools. Using the State's GIS layer for OSDS on the island of Hawaii [57], we first assigned a system capacity equal to the number of bedrooms for all parcels in which the total number of bedrooms was less than or equal to five. Parcels with zero bedrooms were assigned a 1-BR capacity. For parcels with greater than five bed- rooms, the total number of structures on the property was divided by the total number of bed- rooms. Capacities were then assigned based on the average number of bedrooms per structure. For example, on a parcel with 100 structures and 200 bedrooms, each structure was assumed to have two bedrooms, meaning 100 2-BR systems would be installed. Using this approach, 233, 1,334, 4,088, 973, and 625 systems were assigned to 1-BR, 2-BR, 3-BR, 4-BR, and 5-BR capacities respectively, summing to a total of 7,253 systems. For each scenario, all, none, or a subset of the existing cesspools are converted to septic or ATU systems based on their risk scores and estimated required capacities. ,S1 '1.'able in the Sup- porting Information section summarizes the conversion totals for the eight scenarios. Total cesspool conversion costs were then calculated for each management scenario by multiplying the appropriate life cycle cost by the number of systems of each capacity requiring conversion for both septic and ATU upgrades. The proposed Kealakehe WWTP R-1 upgrade is estimated to cost roughly $160 million, including $35 million for soil aquifer treatment, $25 million for a subsurface wetland, and $20 million for piping, among other expenses [58]. Although the decision to invest in the upgrade is still up for debate —the Hawaii County's Department of Environmental Management has continued to research less costly alternatives like expanding infrastructure to deliver wastewa- ter treated at the (current) R-2level to golf courses and agricultural users for irrigation —the analysis presented in the current paper considers only the potential nearshore water quality impacts and costs of the proposed R-1 upgrade. 2.8 Analysis of tradeoffs Impacts of different management strategies, measured in terms of simulated changes in nutri- ents and potential changes to marine habitat quality, are spatially heterogeneous because the model and represented system change are, by design, not spatially uniform. On one hand, this ensures that nutrient or potential marine habitat quality hotspots are not inadvertently "smoothed out" as might occur when using a spatially lumped approach that focuses on aggre- gated or averaged values. On the other hand, with higher spatial granularity, comparing impacts of multiple scenarios simultaneously can be challenging, as it requires evaluating dis- tributions of impacts rather than single values. With that in mind, spider diagrams [591 sum- marizing results across each scenario were generated to supplement and aid in interpretation PLOS ONE I Ihuttttlps://doaa.oirg/10.1371/�OLuirirualll,lpoi ne,025712 a September 8, 2021 10 / 26