Multiform Harvest (United States)

When manure is applied to crops on a nitrogen basis, it often creates a buildup of phosphorus (P) in the soil. Phosphorus recovery as struvite is one strategy to capture excess P prior to land application. To form struvite from dairy cow manure, an acid pH is required to break the calcium phosphate bonds present in the manure. Oxalic acid is desirable because, in addition to breaking bonds, its anion binds calcium. An experiment was designed to measure struvite precipitation from dairy manure after addition of commercial grade oxalic acid to result in varying pH points for maximum struvite production. Initially decreasing the pH of the dairy manure to 6.0 and later increasing the pH to 8.7 removed the most P (90%); however, this high pH can lead to magnesium phosphate precipitation and a pH of 6.0 was not low enough to completely disassociate the calcium from phosphate. Therefore, we recommend initially decreasing the pH to 5.5 and later increasing it to pH 8.2 which achieved 80%P removal.

When manure is applied to crops on a nitrogen basis, it often creates a buildup of phosphorus (P) in the soil. Phosphorus recovery by struvite precipitation is one strategy to capture excess P before land application. However, daily operating costs are expensive due to chemical inputs. Dairy cow manure contains larger concentrations of calcium compared to other livestock manure, which requires an acid addition to break the calcium phosphate bonds. Oxalic acid is desirable because, in addition to breaking bonds, its anion (oxalate) binds the calcium. This study investigated the production of a dilute oxalic acid solution by the fungus Aspergillus niger (A. niger) using whey permeate as a substrate. This study was based on the work by Strasser et al., which found that A. niger exclusively produces oxalic acid when fermenting a lactose carbon source. Whey permeate is a desired substrate due to its high lactose content, and because it can be viewed as a by-product of the dairy industry. Oxalic acid production was evaluated by independently incubating two strains of A. niger, ATCC 9029 and ATCC 6275, at two concentrations: 11% lactose whey permeate (no lactose added) or a 20% lactose whey permeate (lactose added), to see if either strain could produce the 1–1.5% oxalic acid solution necessary for struvite formation. Fungi were grown in two-liter vessels for 6 d at 30°C and a pH of 6. Samples were collected each day and analyzed for oxalic acid content. The first study compared oxalic production between the two strains when fermenting a 20% lactose whey permeate solution, and found that A. niger 9029 produced a 125mM oxalic acid solution and A. niger 6275 produced a 200mM oxalic acid solution over a period of 6 d. The production data were fitted with polynomial regression lines of y = -5.0x2+41.17x+37.95 (R2 = 0.72) and y = -2.80x2+36.88x+68.98 (R2 = 0.89), respectively. The second study compared oxalic production between the two strains when fermenting an 11% lactose whey permeate solution (no added lactose), and found that A. niger 9029 produced a 350mM oxalic acid solution and A. niger 6275 produced a 150mM oxalic acid solution over a period of 6 d. The data were fitted with polynomial regression lines of y = -17.4x2+147.7x+57.26 (R2 = 0.85) and y = -6.85x2+58.23x+28.81 (R2 = 0.75), respectively. These data indicate that A. niger can produce the necessary concentration of oxalic acid for struvite production when fermenting whey permeate with no added lactose.