Wood Ash as an Amendment in Municipal Sludge And Yard Waste Composting Processes

Wood ash from a wood-fired, electrical generating plant was examined as a potential amendment in municipal biosolids and yard waste composting applications. The rate of composting and the final compost quality (chemical, physical, and plant growth characteristics) were examined. Yard waste (leaves, grass, and wood chips) and a municipal biosolids-chip mixture were either not amended or amended with wood ash at eight percent or five percent by weight, respectively, and then composted outdoors in insulated, 1700L, aerated reactors. Yard waste piles heated rapidly to 60°C within six to seven days, whereas biosolid piles heated more slowly to a maximum of 52 to 57°C within nine to 11 days. Ash had little, if any, effect on the time-temperature response. In general, ash-amended compost had higher pi I, plant nutrient, and salt contents. Tomato plants (Lycopersicum esculentum) produced 100 percent more shoot biomass in biosolids than in yard waste compost media. Poor plant growth in the yard waste compost was likely due to the high initial pH and salt content of the growth medium. In yard waste media, tomato plants germinated and produced more shoot biomass in the control compost than in the ash-amended compost.

A pH neutralization study indicated that wood fly ash could be used as an economical substitute for lime which is commonly used to stabilize municipal biosolids prior to land filling or land application. Wood fly ash (pH = 13.2-13.4), when added to biosolids at a 2 to 1 ratio by weight, raised the pH of the mixture to 12.0.