Anaerobic Fermentation of Pig Manure to Increase Phosphorus Removal by Struvite Precipitation

Abstract

The precipitation reaction to form struvite (MgNH4PO46H2O) from nutrient-rich manure supernatants and slurries is limited by the concentration of phosphorus in dissolved form (P-PO4). P-PO4 constitutes only 10% to 25% of the total phosphorus (TP) in liquid pig manure, whereas the majority is bound in either organic or inorganic solids. In order to increase TP recovery from manure as struvite and reduce P loading to cropland and the environment, it is necessary to convert P from the solid phase to the dissolved phase. The purpose of this experiment was to investigate the effect of anaerobic fermentation and resulting acidification on the abundance of P-PO4 and other struvite constituents in pig manure. Liquid swine manure was incubated under anaerobic conditions in stirred and unstirred reactors for 4, 8, and 12 days of hydraulic retention time (HRT), with the struvite components and pH being monitored. In the two stirred reactors, anaerobic fermentation was effective in decreasing the manure pH (from 7.4 to 5.8) and increasing the proportion of P-PO4 to TP (from 10% to 27%) in the effluent of the 12 d HRT experiment. The abundance of dissolved P, Mg, and NH4 all increased with 12 d HRT. Maximum theoretical struvite potential increased by 2.5, 2.7, and 2.8 fold in the unstirred reactor and stirred reactors 2 and 1, respectively, with 12 d HRT. The unstirred reactor performed similarly to the stirred reactors, suggesting that constant agitation is not necessary for acidification to occur. These results can be applied to manure management when P recovery is a goal.