Municipal and industrial by-products are often treated as waste and are disposed in landfills despite containing nutrients that can be used for beneficial purposes. This study examines the impact that two waste products, Flue Gas Desulfurization (FGD) gypsum and municipal biosolids, have on the growth and biofeedstock quality of a promising bioenergy crop, Miscanthus x giganteus when grown on marginal lands. This study also evaluated any improvements in soil quality of marginal lands that occurred from the application of these two soil amendments.
Biosolids significantly increased Miscanthus biomass height, and density on marginal lands, and resulted in dry biomass yields that were three times higher than in the other treatments. FGD gypsum had no impact on Miscanthus growth, but it is possible that a longer study period is needed to see significant differences in yield.
Nutrient and heavy metal content of the Miscanthus feedstock were measured to determine if there were any significant deficiencies or toxicities in the feedstock from biosolids and FGD gypsum application onto the soil. The results indicated that the nutrient and metal content of feedstock was not affected by the application of these soil amendments.
There were no significant differences in lignocellulosic composition of plants due to biosolids or FGD gypsum application. Pretreatment method had a much greater impact on the recovery of fermentable sugars than the use of soil amendments. Acid hydrolysis was used to determine theoretical sugar recovery from Miscanthus feedstock. This theoretical data was used to evaluate a method that is adaptable to an industrial scale for recovery of fermentable sugars from Miscanthus that utilized microwave pre-treatment and enzyme saccharification. Several different catalyst and temperature combinations were tested during pre-treatment; it was determined that microwave pretreatment and enzyme hydrolysis can reach a maximum recovery rate of 65.2 g/100 g dry biomass, a cellulose recovery of 34.0 g/100 g dry biomass, and a hemicellulose recovery of 14.5 g/100 g dry biomass when the High Base, 150°C pretreatment is used.
The theoretical ethanol yield from Miscanthus plants is promising. Using microwave pretreatment and enzyme saccharification results in a theoretical ethanol yield of 94.4 gal/dry ton biomass, meaning that Miscanthus fields that have been treated with biosolids could reach ethanol outputs of 1572 L/ha.
A one-time addition of biosolids and FGD gypsum did not improve the soil quality of marginal lands over an 18-month time period. Improvements in soil physical and chemical properties and soil nutrient content will take a longer time period to become evident and it is likely that reapplication of these nutrients will help to further improve soil properties and nutrient contents. Further study of this system is necessary to determine the long-term impacts of the application of these two soil amendments.