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 nutrient distribution in manure slurries. The aim of this project was to study particle size, COD, nutrient, micro-nutrient, and heavy metal distributions in raw and anaerobically digested (AD) manure from a growing-finishing swine operation. Anaerobic digestion was performed in sequencing batch reactors (ASBRs) operated at 25 degrees C. Anaerobic treatment reduced manure solids concentration by 70%, through both biological degradation and physical sedimentation of large particles in the ASBRs, and significantly reduced particle size. Particles smaller than 10 micrometer represented 64% and 84% of dry matter (DM) in raw and AD manure, respectively. An analysis of particles between 0.05 and 10 micrometer in AD manure indicated a bimodal distribution, on a count basis, with peaks around 0.1 and 1 micrometer. No particles larger than 2 micrometer were detected in the 0.05 to 10 micrometer size range. The removal of particles larger than 10 micrometer may thus be sufficient as a pretreatment for membrane microfiltration of digested swine manure. In AD and raw manure, approximately 20% of total P was soluble, while another 50% was associated with particles between 0.45 and 10 micrometer. Only 30% of the phosphorous was linked to particles larger than 10 micrometer. Approximately 80% of Zn and over 95% of organic N and Cu were associated with particles between 0.45 and 10 micrometer. A solid-liquid separation system must thus remove a sizeable portion of the solids between 0.45 and 10 micrometer to substantially reduce phosphorous and heavy metal concentrations in raw and AD manure. Most commercial solid-liquid separators will be able to remove a considerable fraction of raw manure DM, but except for the decanter centrifuge, they will not be efficient in terms of nutrient and heavy metal separation.