Particle Size Distribution and Characteristics of Raw and Anaerobically Digested Swine Manure Slurry

Abstract

The design of efficient solid-liquid separator systems requires a better characterization of particle and nutrient distribution in manure slurries. The aim of this study was to establish particle size, nutrient, micronutrient and heavy metal distribution in raw and anaerobically digested (AD) growing-finishing swine manure slurry. Digestion was performed in anaerobic sequential batch reactors operated at 25°C. Anaerobic digestion reduced solids concentration by 70% and had a significant effect on particle size distribution. Particles smaller than 10 µm represented 81% and 51% of dry matter (d.m) in AD and raw manure, respectively. However, the concentration of soluble solids (< 0.45 µm) may have been underestimated in raw manure because of volatile fatty acid (VFA) volatilization during solids measurement. An analysis of particles between 0.05 and 10 µm in AD manure indicated a bimodal distribution, on a count basis, with peaks around 0.1 and 1 µm. No particles larger than 2 µm were detected in the 0.05 to 10 µm size category. The removal of particles larger than 10 µm will thus be the main objective to prepare AD manure for membrane filtration. In AD and raw manure, approximately 20% of total P was soluble, while another 50% was associated with particles between 0.45 and 10 µm. Only 30% of the phosphorous was linked to particles larger than 10 µm. Approximately 80% of Zn and over 95% of organic N and Cu were associated with particles between 0.45 and 10 µm. Therefore, a solid-liquid separation system must remove a sizeable portion of the solids between 0.45 and 10 µm to reduce phosphorous and heavy metal concentration in raw and AD manure. Except for the decanter-centrifuge, most commercial solid-liquid separators will be able to remove a considerable fraction of raw manure d.m., but they will not be efficient in terms of nutrient and heavy metal separation.