SOLID–LIQUID SEPARATION OF FLUSHED SWINE MANURE WITH PAM: EFFECT OF WASTEWATER STRENGTH

Abstract

Organic polymers are useful to increase separation of suspended solids and reduced carbon compounds from liquid swine manure. Along with the solids, there is a capture of the nutrients associated with small particles typical of these wastes. The combined effect increases the amount of materials available for value–added products, reduces the size of process units necessary to treat the liquid, and provides needed alternatives to land application. In this work, we evaluated the effect of solids strength typical of flushing systems on optimum polymer dose requirement and chemical use efficiency. Seven flush samples of varied strength were obtained two weeks apart during a three–month period in a feeder–to–finish operation (22.7 to 100 kg weight) in Bladen County, North Carolina. Treatments consisted of eight rates (0 to 140 mg L–1) of polyacrylamide (PAM) followed by screening. Manure samples were characterized for solids, nutrients, and oxygen–demanding compounds amenable for separation. Their concentration varied greatly: 0.4% to 2.5% total solids (TS), 0.1% to 1.6% total suspended solids (TSS), 5.9 to 31.3 g L–1 chemical oxygen demand (COD), 0.7 to 10.6 g L–1 biochemical oxygen demand (BOD5), 749 to 2442 mg total Kjeldahl (TKN) L–1, and 96 to 585 mg total phosphorus (TP) L–1. About 87% of P and 45% of N were organic forms, and 80% of TSS were volatile (VSS). Separation by screening alone was not effective; efficiencies were <20% TSS and VSS, <10% COD and BOD5, and <15% N and P. Mixing with PAM before screening substantially increased separation; efficiencies using 140 mg L–1 rate were 95% TSS and VSS, 69% COD, 59% BOD5, and 67% carbonaceous BOD5. Inorganic P and N were not reduced by treatment. However, PAM significantly enhanced removal of both organic P and N (92% and 85%, respectively). For every 100 g of TSS removed, there was a 1.32–g reduction of COD, 3.32–g reduction of organic P, and 7.26–g reduction of organic N. The N:P nutrient ratio was improved from 4.79 to 12.11, resulting in a more balanced effluent for crops. It was more economical to treat flushed manure with the higher strength. Changes in optimum PAM rate were small (70 to 110 mg L–1) and, consequently, polymer usage rate based on solids produced decreased significantly (from 5.34 to 0.75 g PAM/100 g dry solids separated) with increased wastewater strength. Therefore, reduction of total water volume to clean the houses can result in significant savings (about 700%) in total polymer cost. Chemical cost to capture 95% of the suspended solids was estimated to be $1.37 to $1.27 per finished pig for liquid waste containing 2% to 2.5% TS, respectively. Our results indicate that PAM technology can be enhanced for better liquid manure handling systems and associated management of nutrients.