Separation of swine slurry into different concentration fractions and its influence on biogas fermentation

The raw swine slurry was separated into low solids content liquid and high solids content liquid in the experiment, and the influence of the concentration of raw slurry on separation effects was investigated based on evaluation index, such as recovery rate, concentration multiple and separation efficiency. The higher the concentration of the raw swine slurry, the higher the recovery rate of organic matter from the raw slurry to the high solids content liquid, the lower the concentration multiple for the organic matter. However, the separation efficiency of organic matter from the raw slurry with an intermediate solids content was higher than that from the raw slurry with either a high or low solids content. In terms of biogas production, separation efficiency increased as the concentration of the raw slurry declined. The high solids content liquid, whose volume accounted for 9.72–28.5% of that of the raw slurry, contained 52.7–70.6% of the chemical oxygen demand (COD), 57.6–80.1% of the biological oxygen demand (BOD5). In particular, it produced more than 75% of the total biogas yield. When heated using limited energy, the high solids content liquid, with lower volume, could achieve higher digestion temperature than the raw slurry. The high solids content liquid had 2.48–5.42times greater COD content and 2.81–5.92times greater BOD5 than the raw slurry and had a quicker degradation rate according to the kinetics equation. These two factors: enhanced digestion temperature and increased concentration of the substrate, could improve the efficiency of the entire biogas fermentation system in winter.