Optimization of Phosphorus Partitioning in Dairy Manure Using Aluminum Sulfate with a Mechanical Solids Separator

The over application of phosphorus as land applied animal manure is a problem on many livestock farms in the United States. The use of liquid aluminum sulfate (alum) to precipitate soluble phosphorus from dairy manure was investigated in conjunction with a mechanical screw-press solids separator. In the molar ratio optimization studies aluminum to soluble phosphorus ratios of 2.5, 5 and 7.5:1 were investigated in settling tests using Imhoff cones. The total suspended solids in the chemically amended supernatant were 56, 86 and 93% less than in the supernatant from the control. Also, the soluble phosphorus concentration in the alum-amended supernatant was 68, 95 and 98% less. For the screw press separator tests, alum was added at a aluminum to soluble phosphorus molar ratio of 5.5:1. The alum addition increased the press liquor flow rate by 35%, but it did not increase the dry mass capture efficiency. The soluble phosphorus concentration was immediately reduced in the screw press influent after the addition of chemical where it remained in the precipitate form throughout the separation process.