Phosphorus transformation in pig slurry due to diet and intermittent aeration treatments

The experiment was conducted to study the transformation of phosphorus (P) in slurry from pigs fed two different diets, i.e. the regular maize-soya bean meal diet with or without a supplement of 9% sugar beet pulp (SBP). The slurry was collected and treated either without or with aeration and intermittent aeration (aeration on/off ratio=10 h:14 h) at an airflow rate of 0·6 l m ^-3 s ^-1 for 15 days. Slurry sources and treatments were composed of a 2 by 2 factorial design. The results indicated that aeration increased (probability P<0·01) the slurry pH by 0·5-0·8 within 24 h, from 6·5 to 7·0 for the SBP slurry, and from 6·9 to 7·7 for the control slurry, but the pH reached 7·6 and 8·0 for the SBP and control slurry in 3-4 days. The average pH of the SBP slurry was lower (P<0·01) than that of the slurry from the control diet (6·67 versus 7·38). However, little change in pH was observed in the non-aerated slurry. Aeration decreased total inorganic P, insoluble inorganic P, and soluble P, but increased organic P by approximately 30 mg l ^-1. The average organic P in the slurry for both diets with aeration was about 17·4% higher than that in the same slurry without aeration. Aeration decreased insoluble inorganic P by about 7·2% and soluble P by about 4·5%. The mass balance of P fractions showed transformation of insoluble inorganic P into organic forms during the aeration stage. The insoluble inorganic P took about 68% of the total P in the slurry, so it is essential to perform solid-liquid separation prior to aeration to enhance the efficiency of soluble P removal because insoluble inorganic P is mainly contained in the slurry solids.