Gypsum Additions Reduce Ammonia Nitrogen Losses During Composting of Dairy Manure and Biosolids

Abstract

Composting of N rich organic materials often leads to N loss via ammonia volatilization. Literature references from as early as 1922 have suggested gypsum can prevent N loss from manure. Millions of tons of high quality by-product gypsum are produced each year in the United States as a result of flue gas desul-furization (FGD) scrubbing of sulfur dioxide during combustion of coal. Our objective was to determine the impact of this gypsum on N release when mixed with dairy manure and biosolids during composting. A preliminary experiment was conducted involving 4-liter vessels containing 1.1 kg of dairy manure mixed with by-product gypsum at dry weight rates (w/w) of 0, 6%, 13% and 23% and composted for 18 days. The ammonia-N released in the off gas was trapped in 0.67 M boric acid solution. Loss of ammonia-N was essentially complete after seven days. When expressed as percent of initial N in the mixes, the amount of N lost ranged from 6.4% for the zero rate control to 2.6–2.8% for the gypsum treatments. Composting studies were also conducted in insulated 210-liter stainless steel vessels over a 28-day period using dairy manure and biosolids treated with or without 17% gypsum (dry weight, w/w). Results revealed the amount of N lost, as a percentage of that originally present in the compost mix, was 7.27% and 15.6% without gypsum for dairy manure and biosolids, respectively, and 3.62% and 13.6% with gypsum. The difference between the dairy manure and biosolids results is attributed primarily to a lower C:N ratio of the biosolids compared to the dairy manure. The final composts were found to contain significant amounts of plant nutrients while heavy metals were well below values considered to be harmful to surface water quality or for crop production. We conclude that combining organic waste streams, especially N-rich streams, with by-product gypsum produces a quality product while also decreasing the loss of N and reducing odors associated with the volatilization of ammonia during the composting process.