Effect of additives on Phosphorus, Copper and Zinc separation in animal raw and digestate slurries

To maintain a sustainable local and global environmental balance and secure a renewable phosphorus (P) source, it is necessary to remove and recover P from animal manure. Some concerns are also related to heavy metals contained in manure, especially copper (Cu) and zinc (Zn) that, when applied to land, can impair soil and water quality. A suitable technology to remove these elements from slurries can be solid-liquid separation. However, adequate removal efficiencies can be obtained only with the use of additives. The objective of this study was to evaluate the effect of two additives (calcium hydroxide Ca(OH)2 and aluminum sulfate Al2(SO4)3) on the P, Cu and Zn separation efficiencies using raw slurry and co-digested slurry (before and after a physico-chemical treatment of ammonia stripping). After the addition of the chemicals, slurries were mixed and then separated using a static filter with a 0.25-mm mesh after 30 min of sedimentation. All tests were conducted in duplicate. Liquid samples were analysed for pH, dry matter content and volatile solids, which were determined using standard methods. The contents of P, Cu and Zn were obtained by ICP-MS analysis. As expected, the addition of additives effectively improved separation efficiencies of P, Cu and Zn. The maximum efficiencies obtained depended on the type of slurry and additive used. For example, the P separation efficiencies ranged from 57 to 88% using Al2(SO4)3 and from 39 to 74% using Ca(OH)2. The use of Al2(SO4)3 had a more consistent effect on removal efficiencies of P, Cu and Zn than did Ca(OH)2.