Limitations of orthophosphate removal during aerobic batch treatment of piggery slurry

Previous studies on removal of orthophosphates (ortho-P) from piggery slurry during aeration treatments show performances ranging between 30 and 91%. This study investigated three factors believed to be potentially responsible for the wide variations as well as limiting even higher removals of ortho-P from the slurry during aeration treatments: (i) levels of insoluble P, (ii) inadequate pH levels, and (iii) inadequate metal ions. Results indicated that, removal of insoluble P by prior aeration solids/liquid (PASL) separation and by mid-aeration solids/liquid (MASL) separation (4 days into aeration process) resulted in an additional removal of 3 and 8%, respectively. Removal of insoluble P by MASL separation, therefore, significantly improved soluble ortho-P removal more than removal by PASL separation most probably because more insoluble P is removed in the former process than in the latter process. Results also showed that upward adjustments of pH beyond that obtained from the aeration process did not significantly improve the removal of ortho-P suggesting that the pH level achieved during aeration process may not significantly limit further removal of soluble ortho-P during the aeration treatment of piggery slurry. Significant additional removals of 11, 9, and 5% were observed on the addition of a single dose of Ca2+ ions (80 mmol [Ca2+] l(-1) [slurry]) to the piggery slurry, improving removal levels to 95, 96, and 97% for the control, PASL separation, and MASL separation, respectively. Subsequent double and triple doses did not significantly improve the removal of ortho-P in the control or in the PASL separation treatments. Based on the results of this study, depletion of Ca2+ ions (or/and other similarly acting metal ions) and accumulated insoluble P, are most probably the two main factors limiting further removal of soluble ortho-P during aeration treatments of piggery slurry. These two. factors are also likely to be responsible for the wide variations observed in the removal efficiencies because their, content will vary with both the feed mineral-supplementation and the manure management practices. (C) 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Ltd.