The Changes in Total and Available Heavy Metals During Manure Composting

摘要

Composting of livestock manure is a viable and environmentally sound method of waste management. It is usually an aerobic process, but anaerobic composting through use of digesters is also a widely practiced waste management option, due to its cost efficiency. Although physicochemical changes during composting have been documented, information on the behaviour of total and available heavy metals during composting of livestock manure is scanty. Such information is necessary for an efficient compost use and environmental management practice. In the present investigation, we compared the behaviour of total and available heavy metals in a mixed (cattle + swine + chicken) manure which was composted for 195 days, with and without air, using rice straw asa bulking agent. Marked increases due mainly to concentration effect, in total Fe, Zn, Cu and Mn, especially under anaerobic conditions were noted during composting. Conversely, aerobic composting significantly decreased available Fe by 36%, Zn by 50%, Cu by 14% and Mn by 27% at 195d. The decreases could be due to complexation with other ions and/or organic matter. The available Fe increased by 59% and Zn by 87%, while available Cu decreased by 16% and Mn by 22% under anaerobic conditions. Whereas 76 and 56% of the variations in available Fe and Zn were due to the changes in their total contents under anaerobic conditions, only 14 and 2% of such variations were so attributed under aerobic conditions. For available Cu and Mn, 65 and 39% of the variations in their levels under aerobic composting could be explained by the changes in total Cu and Mn compared with only 3 and 14% under anaerobic conditions. On the basis of these results, manures from sources such as anaerobic digesters may have a higher potential for metal, especially, Fe and Zn export to the environment upon application to soils. Factors other than total content may be responsible for the availability of Fe and Zn in aerobic compost. Since aerobically composted manure had low metal levels in available forms, it could ensure a slow release upon application to soils, which could be managed to synchronise with plant requirement.