Municipal Sludge Application in Forests of Northern Michigan: a Case Study.
A large-scale operational demonstration and research project was cooperatively established by the US. Environmental Protection Agency, Michigan Department of Natural Resources, and Michigan State University to evaluate the practice of forest land application as an option for sludge utilization. Project objectives included completing (1) a logistic and economic assessment and demonstration of the technology available for conducting sludge applications in forest stands, and (2) several research studies that would augment knowledge in the areas of public involvement and acceptance, wildlife populations, food-chain transmission of potential toxicants, groundwater quality, nutrient cycling, and vegetation growth. Field trials in four forest types (aspen, oak, pine, northern hardwoods) were of a completely random design covering 54 ha, of which 18 ha were treated with nearly 4 million liters of anaerobically digested sludge. Average solids loading ranged from 8 to 10 Mg/ha,resulting in total nitrogen levels of 400 to 800 kg/ha. Differences in loading levels of nutrients, heavy metals, and trace elements were generally not significant among treated plots. Sludge was transported by truck a distance of 80 km to the study sites and sprayed by an all-terrain tanker on the forest floor at a cost of $48,576. The resulting unit cost of 1.3 cents per liter was comparable to typical operational costs for sludge application to farmland, considering the greater transport distance in this study. Preliminary findings indicate an enhanced nutritive quality of forage on fertilized plots and a resulting increase in use by both deer and elk. Increases in plant growth were related to elevated levels of soil nitrogen, phosphorus, calcium, and magnesium. Slight increases of nitrate were observed in soil percolate within one year of application, but these rapidly returned to near background concentrations. Analysis of sociological data provided new insights into public concerns and attitudes and outlined a process for constructive citizen involvement in program planning.