Relative efficacy of a drinking-water treatment residual and alum in reducing phosphorus release from poultry litter

Abstract

Amending poultry litter with alum (aluminum sulfate) effectively reduces soluble phosphorus (P) concentrations, but the practice can be expensive. Aluminum (Al)‐based drinking‐water treatment residuals (WTRs) can be obtained free of charge and are enriched in Al hydr(oxides) that make them efficient P sorbents. Substituting Al‐WTRs for alum would be a cost‐effective practice to reduce soluble P in manures when compared with alum‐only use. The research studied the reductions in soluble P, Al, and total organic carbon (TOC) concentrations in suspensions prepared by mixing variable Al‐WTR and alum rates (0 to 25% by weight) with poultry litter. Suspensions were maintained at pH of 6.5 during the sorption step, and allowed to react up to 50 d, without shaking. On a per‐mole of oxalate‐extractable Al basis, the Al‐WTR was nearly as effective as alum in reducing P release. Increasing mixed alum/WTR mass loads resulted in greater soluble P reduction, simply due to increased molar Al/P ratios. Contact time did not significantly influence soluble P reduction. Two significant advantages of Al‐WTRs compared to alum indicated by this study are cost effectiveness and significantly less release of dissolved Al. Soluble Al and TOC concentrations were least for suspensions with the lowest soluble P levels, suggesting that P was removed from solution as an organo‐Al‐P amorphous phase. The amount of P desorbed from the mixtures decreased to <1% with increasing molar Al/P ratios ≥1. Results suggest that Al‐WTR application with or without alum can reduce soluble P in poultry litter; however, field validation of the amendment effectiveness is needed.