Phosphorus Fractions in Solid and Liquid Separates of Swine Manure Following Liquid-Solid Separation Using Different Technologies.

 

Continuous application of manure to agricultural lands in excess of crop removal results in a build up of soil phosphorus (P), increasing the risk of P loadings to water bodies. Over supply of P is more of a concern with liquid swine manure due to its low N: P ratio. Solid-liquid separation is an approach where N-rich liquid fraction is separated from P-rich solid fraction, allowing the liquid fraction to be used to supplement N to crops without over-supplying P. During this separation of solid and liquid, how different P fractions in manure are partitioned is not well known. We characterized liquid and solid separates for P fractions after separating liquid swine manure using three different techniques, centrifuging without flocculant, centrifuging with a flocculant and rotary press with a flocculant, to provide additional information in making choices regarding suitable separation methods and to make the best use of both solid and liquid separates.  Samples of raw manure (RM), separated solid (SS), and separated liquid (SL) using different separation techniques were analyzed for P fractions using modified Hedley fractionation scheme in triplicates. Proportion of RM recovered in SS was greater when a flocculant was used irrespective of the separation method. Concentrations of molybdenum reactive P (MRP) and total P (TP) extracted with water, NaHCO₃, NaOH, and HCl fractions on a fresh weight basis were greater in SS than SL for all techniques.  Total labile (water + NaHCO₃ extractable) P concentrations were 30- to 40-fold greater in SS than SL. Total labile P partitioned to SS fraction was 62.6, 81.6 and 81.4 % for centrifuging without flocculant, centrifuging with flocculant and rotary press with flocculant, respectively. Therefore both centrifuging and rotary press with flocculant were equally effective in partitioning total labile P to SS, and were superior to centrifuging without a flocculant.