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

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PLOS ONE Identifying wastewater management tradeoffs in Kona, Hawai'i Table S. Return on investment for each management strategy in terns of N, P, and habitat quality (change in physical units per million dollars invested). Scenario N (kg/d/$MM) P (kg/d/$MM) Habitat (ha, All ATU upgrade 0 451 0 117 1.732 WWTP upgrade 1.103 0.342 1.931 Ta get low efficiency 0 113 0 004 2.460 Target high efficiency 0 202 0.020 2.701 Target low efficiency+ WWTP upgrade 0.483 0.130 LK4 Target high efficiency + N-VWTP upgrade 0.539 0.140 2.272 hi tlp,;://dr.al.rbrrl/10.1371/po[irrpall.lpoinie.1125712a:100a effectiveness, upgrading only the WWTP (FE g 911) achieved better total nutrient reduction than targeted cesspool conversion (Mg 9C and 91)) at a fraction of the cost. However, that nutrient reduction did not correspond to a higher ranking in overall marine habitat quality because all of the nutrient reduction occurred only in the immediate vicinity of the WWTP, while nutrient reduction under targeted cesspool conversion was more evenly distributed along the coast. The ROI for N was lowest for the target low efficiency scenario (0.113 kg/d reduction per million dollars) and highest for the WWTP upgrade scenario (1.103 kg/d reduction per million dollars). The targeted conversion scenarios with the WWTP upgrade (0.483-0.539 kg/d reduc- tion per million dollars) generally tended to have higher ROI than their counterparts without the WWTP upgrade (0.113-0.202 kg/d reduction per million dollars) due to the relatively high benefit -cost ratio of the WWTP upgrade. Although the all-ATU upgrade scenario generated the largest simulated N-reduction benefit in absolute terms, its high cost resulted in a below - median ROI (0.451 kg/d reduction per million dollars). ROI rankings across scenarios were similar for P, ranging from a 0.004 kg/d reduction (target low efficiency) to a 0.342 kg/d reduc- tion (WWTP upgrade) per million dollars. However, the ROI for potential habitat quality improvement followed a different pattern. Because the habitat quality benefits of the WWTP upgrade were concentrated within a relatively small area, the overall ROI for each scenario was less affected by whether the WWTP upgrade occurred or not. Thus, the target high efficiency scenario had the highest ROI for potential habitat quality improvement (2.701 ha per million dollars), while the all ATU upgrade scenario had the lowest (1.732 ha per million dollars). ROI values for all environmental metrics across each scenario are summarized in "1Tablle' 4. Discussion and conclusions The choice of wastewater management scenario will involve tradeoffs among cost, nutrient loading, and marine outcomes. While a full upgrade and highest -level conversion plan will guard against the most nutrients and provide the highest protection of the near shore marine environment measured here, at $569 million in present value terms, this scenario is twice as expensive as following the targeted cesspool upgrade plan (without upgrading the WWTP) and over $100 million more expensive than the targeted cesspool conversion approach, includ- ing the WWTP upgrade. While these tradeoffs express the cumulative effect of the scenarios on these dimensions, spatial differences in impacts are obscured by these comparisons. For example, potential habitat loss from upgrading the WWTP but leaving the cesspools in place is geographically very different from the potential consequence of upgrading the cesspools but leaving the WWTP as is. Upgrading the WWTP but leaving the cesspools in place moves potential marine habitat loss from the area surrounding the plant towards the south, while upgrading the cesspools while preserving the current WWTP technology shifts the majority of the potential marine ecological losses to the area immediately offshore of the WWTP. One PLOS ONE I Ihii 1tlps://doaa.oirg/10.1371/�OLuirirualll,lpoi ne,025712 a September 8, 2021 19 / 26