Phosphorus distribution in untreated and composted solid fractions from slurry separation
Received for publication May 7, 2009. The distribution of phosphorus (P) (water-soluble inorganic P [Pi], acid-soluble Pi, and residual P) was determined in 40 samples of solids from solid–liquid separated slurry. These were collected from separation plants using different technologies, separating by simple mechanical means, by flocculation as pre-treatment before mechanical separation or by anaerobic digestion followed by separation and centrifugation. Simple mechanical separation yielded a low solid TP content (8–9 g P kg–1 dry matter [DM]) compared with separation by flocculation (26 g P kg–1 DM) or by anaerobic digestion-centrifugation (33.4 g P kg–1 DM). Acid-soluble Pi predominated in the high P-yielding solids, whereas organic-bound or residual P was a minor component in all slurry solids. Acid-soluble Pi and residual P were significantly correlated with total phosphorus (TP) content (R2 = 0.855 and R2 = 0.584), but water-soluble Pi was uncorrelated (R2 = 0.077). The relative distribution of Pi to TP in the solids showed a high proportion of water-soluble Pi in solids from simple mechanical separation, whereas the absolute concentrations were highest in solids from separation by flocculation and anaerobic digestion-centrifugation. Three solid fractions, representing the range of solids variability produced by the separation techniques, were composted for 30 d, and the P distribution was compared before and after composting. Total mass of P was conserved during composting, but water-soluble Pi as a proportion of TP decreased in most cases. The most pronounced decrease in water-soluble Pi was observed during composting of the solids separated using flocculation. However, changes in short- to medium-term bioavailability of P were modest, and thus the potential benefits of composting regarding storage and handling can presumably be realized without seriously compromising the P fertilization quality.