Lowering the phosphorus (P) content of animal manure is one approach of addressing concerns over surplus P accumulation in soils resulting from land application of animal manure. We sought to develop a treatment system for liquid manures that conserves manure nitrogen (N) while removing most of the manure P. Initial evaluation of a treatment system involving manure solid separation and precipitation of dissolved P with an alkaline salt (Ca(OH)2) resulted in poor liquid/solid separation and poor dissolved P removal and created conditions promoting ammonia-N (NH3-N) volatilization. As a result, we developed a three-stage system with iterative solid removal and acid salt (ferric sulfate - Fe2(SO4)3) sorption of dissolved P: (1) removal of bulk and intermediate sized solids (>25 μm); (2) chemical treatment to convert dissolved P; and (3) final removal of fine solids and chemically sorbed P. When tested on manure slurries from 150 and 2700 cow dairies, 96% to 99% aqua regia extractable P was removed along with 92% to 94% of the solids, resulting in liquid manure filtrates with N:P ratios greater than 19:1. While costs of treatment were roughly $38 kg-1 P removed, equivalent to $750 cow-1 yr-1, we anticipate that refinement of the process and beneficial uses of the solid materials (bedding, compost, etc.) will improve cost-efficacy considerably.
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