SPECIAL REPORT SR-2021-01
Denitrifying Leachfields: Enhanced Nitrogen Removal from Domestic Wastewater in Hawai‘i
Roger Babcock Jr., Micah Tang, Tracey Panlasigui, and Julien Sassi
January 2021, vi+21 pp.
ABSTRACT
Act 125 bans all cesspools in Hawaii by 2050, which means that at least 88,000 known cesspools on individual homesites must be upgraded. More than half of these cesspools are located in areas where there is the potential impact of nitrogen on drinking water and/or nearshore marine reef environments. These cesspools will need to be replaced by individual systems (< 1,000 gpd) that are capable of at least 50% nitrogen removal (as required by NSF254 and DOH rules). Denitrifying leachfields, also called denitrifying absorption beds or nitrogen reducing biofilters (NRBs), are a conventional absorption bed disposal system that is upgraded to achieve significant nitrogen removal in a passive, non-mechanical, no-energy manner. The NRB can allow a simple septic tank + absorption system to passively achieve nitrogen removal performance that would normally require a much more expensive advanced nitrifying/denitrifying aerobic treatment unit (ATU-N/DN). NRBs have been studied and developed in the eastern US, including Florida and New York, and one state already has a design criteria in their wastewater design rules. No testing/validation of NRBs has been conducted in Hawaii and thus their use is not yet possible for nitrogen removal. The soil conditions, available media, and temperature conditions (no cold winters) in Hawaii require that local design criteria needs to be developed via testing. As a first step, prior to full-scale field testing that must be conducted, a series of 14 laboratory-scale NRBs were constructed with different types of absorption bed media, with different depths, and continuously operated for 14months. The NRBs remained in operation, except for 7 columns that were disassembled for inspection after 14 months of operation. A series of 18 new columns with additional types of locally available media recently began operation (25 columns in total). The bench-scale testing is very important to frame the scope of the future full-scale field tests.