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 result ing unit cost of 1.3 cents per l i ter 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 nitrateN were observed in soil percolate within one year of application, but these rapidly returned to near background concentrat ions. Analysis of sociological data provided new insights into public concerns and attitudes and outlined a process for constructive citizen involvement in program planning. Implementation of secondary treatment standards for wastewater discharges in the Great Lakes Region has resulted in increased volumes of wastewater sludge requiring removal from treatment facilities. In Michigan, combined residential and industrial water use and mandatory effluent phosphorus removal have resulted in the annual generation of 202,500 dry Mg (223,218 tons) of sludge by municipal wastewater treatment plants (MDNR 1983). While traditional strategies for dealing with this residual waste have emphasized options for disposal such as incineration and use as landfill, sludge management programs developed since 1978 have increasingly turned to nutrient and organic matter utilization through the practice of land application. Numerous concerns have been raised about the hazards of land application of sludge. The concerns of state residents center on public health and environmental quality (Peyton et al. 1983). The potential presence of pathogens, heavy metals, and toxic organic compounds in sludge is a leading health concern. Nutrient enrichment of groundwater