Effects of Polymer Charge Density and Molecular Weight on Flocculation Treatment of Swine Manure at Various Dry Matter Contents.

This study investigated the effects of polymer charge density (CD) and molecular weight (MW) on the efficiency of solid‐liquid separation of swine manure at various dry matter (DM) contents. Flocculated manures were filtered through 1 mm screens. The dose of high MW polymers required for optimum suspended solids (SS) removal increased with increasing CD. At manure DM content ranging from 29 to 90 g L⁻¹, optimum dose averaged 22, 29, 63, and 85 mg L⁻¹ with the 2%, 10%, 35%, and 55% CD polymers, respectively. Chemical cost (in U.S. dollars) was estimated at $0.12, $0.18, $0.47, and 0.69 per m³ of manure for the four polymers, respectively. However, maximum SS and P reductions tended to increase with increasing CD and averaged 84%, 89%, 94%, and 95% for SS and 62%, 67%, 70%, and 71% for P with the 2%, 10%, 35%, and 55% CD polymers, respectively. High MW polymers required lower doses and were more efficient in terms of SS and P removals than medium MW polymers, while similar results were obtained with high and very high MW polymers. Results suggested that bridging was the main mechanism involved in manure flocculation, but high SS removal required the bridging as well as the charge neutralization effect of high CD polymers. There was no consistent correlation between optimum dose and manure DM content between 29 and 106 g L⁻¹ for the 10% and 35% CD, high MW polymers. Manure dilution at the farm would thus increase cost because of higher polymer requirements and the need for equipments able to handle larger volumes.