A method to characterize precipitated minerals in organic waste and their transformation by anaerobic digestion: Focus on phosphorus

Unsustainable global phosphorus (P) management has led to the identification of digestate from anaerobic digestion (AD) as a potential source of P recovery. Understanding P interactions during AD is thus crucial for P recovery and modeling can help optimize recovery. Both understanding and modeling require accurate P fractionation into minerals that contain P, e.g., struvite, vivianite, amorphous calcium phosphate (ACP). The aim of this study was to develop a novel P fractionation method based on acidic leachate of P, Ca, Mg, Fe, K, and Al. Nine samples were acidified, three of which were digestate and six substrates used to feed a biogas unit. Acidification curves were obtained for each nutrient in each sample. Nutrient leachate was used to develop a fractionation algorithm based on P, allowing nutrient fractionation of all the samples. Dissolved nutrients during acidification were assumed to be issued from precipitated mineral origin. Precipitated P was found in all digestates, slaughterhouse sludge, and pig and cattle slurry and was originated from struvite, vivianite, and amorphous calcium phosphate (ACP) minerals. Struvite represents up to 70.6% of total P and ACP between 13.8% and 24.8% in digestates whereas vivianite was the main P precipitated form in slaughterhouse sludge (27.4%). Precipitated Mg were mainly found in samples where P was fractionated mainly as struvite. Precipitated Ca was fractionated into calcite and ACP while Fe was fractionated into vivianite and other ferrous minerals. No dissolved Al or K was found. Al was mainly fractioned as inert/organic, and K was mainly fractioned as soluble in all the samples. In conclusion, the acidification protocol and the fractionation algorithm developed in this study represent a useful, rapid and inexpensive P fractionation methodology for environmental samples during anaerobic digestion.