Phosphorus distribution in untreated and composted solid fractions from slurry separation

The distribution of phosphorus (P) (water-soluble inorganic P [P(i)], acid-soluble P(i), 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 P(i) predominated in the high P-yielding solids, whereas organic-bound or residual P was a minor component in all slurry solids. Acid-soluble P(i) and residual P were significantly correlated with total phosphorus (TP) content (R(2) = 0.855 and R(2) = 0.584), but water-soluble P(i) was uncorrelated (R(2) = 0.077). The relative distribution of P(i) to TP in the solids showed a high proportion of water-soluble P(i) 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 P(i) as a proportion of TP decreased in most cases. The most pronounced decrease in water-soluble P(i) 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.