When the Hawaii Department of Health tightened its restrictions on landfilling biosolids by requiring the lining of sludge drying beds, the U.S. Navy Public Works Center (PWC) at Pearl Harbor decided to look for a less expensive alternative to landfilling. Biosolids composting became the Navy’s answer. A pilot project was started in July, 1997, and the full-scale program became operational in 1999.
The composting facility at the PWC receives biosolids from the Navy’s wastewater treatment facility (WWTF) at Fort Kamehameha and the Army’s Schofield WWTF, as well as the City and County of Honolulu’s Honouliuli WWTF. Green materials are collected from the Navy’s tree trimming and ground maintenance contractors. The city and county also provide yard trimmings to the facility as part of its agreement with the Navy. The facility has a design capacity of treating 15,600 wet tons of biosolids annually.
Located in an old coral pit, the site has a 300 foot by 300 foot lined pad for aeration and processing. The pad was constructed with a coral surface working layer on top of a more impermeable clay barrier. There are approximately 15 acres of curing and compost storage area. Biosolids from the Navy and Army WWTFs are handled separately from the Honolulu material, since the finished compost from the Navy and Army is permitted for application on federal Department of Defense (DOD) lands, while the city/county feedstocks are used for landscaping public areas.
Yard trimmings chipped prior to delivery at the site are mixed at an approximate ratio of three parts green material to one part biosolids. (The actual ratio is determined by the operator based upon moisture levels and other factors.) Combined materials are placed in a ten-foot-high static pile on top of air supply lines, where they remain for about three weeks. After trommel screening, chips larger than a half-inch go back into an aerated pile, while the screened compost is transferred to a curing windrow where it is watered and turned for three to four months. Temperatures and moisture content are monitored regularly to ensure pathogen and vector reduction. Leachate is collected and used to provide added moisture to windrows and aerated static piles.