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 concentrations were assumed to remain the same, but if water demand was > 1.5 m;/d, nutri- ent concentrations were assumed to increase from medium to high intensity. 2.3.2. Wastewater treatment. A number of cesspool conversion technologies, each with its own set of costs and range of nutrient removal efficacy, could be implemented to meet the requirements of Act 125. In general, most upgrade options consist of a treatment system and a disposal system. The former provides primary or secondary treatment of household wastewa- ter. Examples include septic tanks and ATU, some of which are capable of denitrification. The latter are paired with a treatment system to dispose of the treated wastewater. Examples include absorption systems and seepage pits. Based on discussions with the Cesspool Conver- sion Working Group, the model assigns one of two technologies to existing cesspools depend- ing on risk scores that incorporate groundwater and drinking water risk, stream and watershed risk, coastal water risk, and risk related to soil type [25]. Cesspools located within the same grid cells are aggregated into a single representative cesspool for each cell with an averaged risk score (Fig ➢.C). OSDS risk scores range from 7 to 18 [25], and the median score of 12 was selected as the conversion threshold score (see S1 ]Frig in the Supporting Information section). Aggregated cesspools with an average risk score G 12 are upgraded to a septic tank and absorption bed/trench system in the targeted conversion scenarios. Aggregated cesspools with an average risk score > 12 are upgraded to an ATU paired with an absorption system in the targeted and all-ATU scenarios. The majority of OSDS units (52%) are assigned a risk score of 12. Therefore, with a conversion threshold score of 12, 60% of the OSDS units will be upgraded to an ATU while 40% will be upgraded to a septic tank. If a risk score of 11 was instead selected as the conversion threshold, 84% of the OSDS units would be upgraded to an ATU system, whereas only 8% would be converted with a threshold risk score of 13. It is there- fore important to acknowledge that total nutrient mass load can easily fluctuate, depending on the proportion of OSDS units that are converted to ATU systems. Given that the effectiveness of a particular conversion technology will depend on a variety of site -specific factors, low and high efficiency average nutrient removal rates are considered for both septic and ATU upgrades, in accordance with the range of values presented in [42]. Nutrient concentration reduction assumptions for cesspool upgrade options are summarized in Talaie 2. The Kealakehe WWTP, the only municipal WWTP in the study area, currently treats approximately 6,400 in of sewage per day from the North Kona sewerage system to secondary treatment classification through the use of aerated lagoons. The treated effluent is disposed of in a nearby percolation basin (]Frig 1C), after which it eventually discharges into coastal waters. Typical nutrient concentrations of this effluent are 22.6 mg/L for total N and 7.01 mg/L for total P [38]. Given concerns about the potential impacts the released effluent may have on nearshore water quality, an upgrade to the WWTP has been proposed in recent years. Through a combination of R-1 treatment, subsurface flow constructed wetlands, and soil aquifer Table 2. Assumptions regarding nutrient concentrations in released effluent for wastewater treatment alternatives. ols (low effi ATU_ upgrade (low ATU uperade (hiel N (mg/L) 60.5 45.4 (25% reduction) 27 2 (55% reduction) 27 2 (55% reduction) 9.1 (85% reduction) Current WWTP ---------------- 22 6 Upgraded WWTP 2 3 (90 /o reduction) h ps:!(dc1,org/10..�'71/�Ourr)all,przinr,025712a,1002 P (mg/L) Source 16.5 [42] 16.5 (0% reduction) [4ll] 13.2 (20% reduction) [4Il] 14.9 (10% reduction) [411 ] 11.6 (30% reduction) [4111 7 0 [4 3] 0.70 (90% reduction) [431 PLOS ONE I Ihii tlpws://doaa .oirg/ 10.1371/�oauirirns.Il,lporne,025712 a September 8, 2021 7 / 26