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Field Evaluation of a Two-Stage Liquid-Solid Separation System at a California Dairy

A two-stage mechanical separation system was installed on a large dairy farm near Tulare, California to reduce loading on the settling ponds and lagoon and to produce solids to be used as freestall bedding. The system included two inclined screens operated in series. The first separator used a 0.508 mm (0.020 in) screen followed by a low-pressure screw press to provide additional dewatering and a perforated stacking conveyor to stack the residual solids. The second separator was fitted with a 0.254 mm (0.010 in) screen and additional drying occurred on a perforated stacking conveyor. The two-stage system removed 59.7% of the total solids (TS), 65.7% of the volatile solids (VS), but only 22.8% to 34.8% of important major and minor plant nutrients (N, P, Ca, Mg, S). Removal by the first separator accounted for most of the solids and plant nutrient removal. The effluent from the separation system was still rich in valuable plant nutrients and 68.8% of the solids were VS. Therefore, the methane producing potential of the system effluent was still significant. The residue from the first separator had a C:N of 26.6 and would be a valuable substrate for composting. The residue from the second separator had a lower C:N (20.5), but contained a higher concentration of plant nutrients. Residue from the first separator was dried in open lots during the dry season prior to being stored in windrows. The moisture content was reduced from 77.3% to 9.2% prior to use as freestall bedding. Both N and C were lost from the residue during drying and storage.