Effect of low initial envelope material moisture content on swine tissue degradation in layered livestock mortality composting systems
A 12-week laboratory study was conducted to assess the minimum initial moisture content of compost bulking (envelope) materials necessary to sustain desired heat production and completion of carcass decomposition during emergency composting of swine carcasses. During full-scale field testing of a semi-enclosed emergency composting procedure, first developed and used by the Canadian Food Inspection Agency during an avian influenza outbreak in 2004, the ability of potential emergency compost envelope materials were evaluated on their ability to achieve elevated temperatures (>55 oC) necessary to inactivate pathogens and successfully decompose carcasses under a variety of initial moisture conditions, during cool and warm season trials. Two-way ANOVA modeling of results showed that envelope material type and envelope material initial moisture content had a significant effect on internal temperature production, with silage (52.5 oC) having the highest predicted internal temperature production. Counter to these findings, envelope material and initial moisture did not have a significant effect on carcass decomposition, and silage (72%) had the lowest predicted carcass decomposition. To corroborate and better understand these unexpected field test results, laboratory tests were carried out using the same envelope materials, under conditions of identical external temperature and a range of initial moisture contents. Results from the laboratory test showed that, when pre-moistened adequately, total oxygen uptake (and heat production potential) in ground cornstalks and similar materials were significantly higher (48 and 51 mg O2 respectively) than for moist silage (11 mg O2). Heat production potential increased significantly when initial moisture was increased from 15 to 35%, and no significant increase was noted when initial moisture content was raised to 60%. Animal tissue decomposition rankings observed in the lab agreed with those from field trials. Decomposition of tissue samples within cornstalks and oat straw exceeded 66% during the 10-day lab study, while decomposition in silage averaged only 54%. Animal tissue decomposition at initial moisture of only 25% was significantly improved over that observed at 15%, and no significant improvement in decomposition was noted when initial moisture was increased to 60%. These results are encouraging as they suggest modest increases in envelope material initial moisture can significantly improve mortality composting system performance. This is particularly important during emergency situations, as moisture addition can be a time consuming process and its practicality during emergency disposal operations will depend on the level of initial moisture necessary to achieve desired results.