Size Distribution and Composition of Particles in Raw and Anaerobically Digested Swine Manure

The design of efficient manure treatment systems requires a better characterization of particle and nutrientdistribution in manure slurries. The aim of this project was to study particle size, COD, nutrient, micro-nutrient, and heavymetal distributions in raw and anaerobically digested (AD) manure from a growing-finishing swine operation. Anaerobicdigestion was performed in sequencing batch reactors (ASBRs) operated at 25C. Anaerobic treatment reduced manure solidsconcentration by 70%, through both biological degradation and physical sedimentation of large particles in the ASBRs, andsignificantly reduced particle size. Particles smaller than 10 .m represented 64% and 84% of dry matter (DM) in raw andAD manure, respectively. An analysis of particles between 0.05 and 10 .m in AD manure indicated a bimodal distribution,on a count basis, with peaks around 0.1 and 1 .m. No particles larger than 2 .m were detected in the 0.05 to 10 .m size range.The removal of particles larger than 10 .m may thus be sufficient as a pretreatment for membrane microfiltration of digestedswine manure. In AD and raw manure, approximately 20% of total P was soluble, while another 50% was associated withparticles between 0.45 and 10 .m. Only 30% of the phosphorous was linked to particles larger than 10 .m. Approximately80% of Zn and over 95% of organic N and Cu were associated with particles between 0.45 and 10 .m. A solid-liquidseparation system must thus remove a sizeable portion of the solids between 0.45 and 10 .m to substantially reducephosphorous and heavy metal concentrations in raw and AD manure. Most commercial solid-liquid separators will be ableto remove a considerable fraction of raw manure DM, but except for the decanter centrifuge, they will not be efficient in termsof nutrient and heavy metal separation.