| The focus of this study was to investigate, using laboratory-scale batch and on-farm digesters, the effect of anaerobic digestion on the speciation and water-extractability of inorganic phosphorus (P) in dairy manure. Water extraction, equilibrium chemical modeling, and spectroscopic methods were employed to determine the nature of inorganic P solid phases. Water-extractable P (WEP), which refers to the percentage of total P (TP) released as dissolved reactive P (DRP) following water extraction, was used as a stability indicator of inorganic P solid phases.; Batch digestion of dairy manure showed that inoculum-to-substrate ratio (ISR) could influence WEP levels at low extractant (i.e., water)-to-manure ratios. Higher volatile solids (VS) destruction and lower WEP levels were obtained for the low ISR (0.3 g VS g-1 VS) batch digesters. Using a geochemical equilibrium model (Minegl+), struvite, dicalcium phosphate dihydrate (DCPD or brushite), dicalcium phosphate anhydrous (DCPA or monetite), octacalcium phosphate (OCP), and beta-tricalcium phosphate (beta-TCP) were determined as the probable inorganic P solid phases in the water extracts of raw and anaerobically digested manure.; Analysis of manure samples from six on-farm digesters in Wisconsin showed that effluent WEP was significantly (22 to 47%) lower than that of the influent. Chemical indicated that newberyite, struvite, DCPD, DCPA, and OCP were the probable inorganic P solid phases in both influent and effluent. Using x-ray powder diffraction, struvite was identified in both influent and effluent samples from a selected on-farm digester. Analysis of the P K-edge x-ray absorption near-edge structure (XANES) spectra revealed that Mg phosphates were the predominant solid phases in the on-farm digester samples; Ca phosphates were present at lower concentrations in both the influent and effluent samples.; The overall study results are strongly suggestive of struvite being the most predominant inorganic P solid phase in both raw and anaerobically digested dairy manure. Anaerobic digestion appears capable of increasing the stability of P-containing solid phases via destruction of organic solids, thereby decreasing the water extractability of manure P. Land application of anaerobically digested manure could lower P losses from agricultural fields at least in a short-term basis. |
