Abstract
A computation model for inorganic composition of anaerobically digested sludge (ADS) was developed using 9 datasets taken from digesters at 9 different wastewater treatment plants. The model employed assumptions and calibration parameters based on stoichiometry and kinetics, combined with thermodynamic equilibrium computation. Through the model development process, the primarily precipitating solid species for Ca, Mg and Fe were determined to be CaHPO4, Mg(NH4)PO4･6H2O and FeCO3, respectively. Then, assuming that phosphorous (P), Ca and Mg contents in sludge biomass to be 1% of digested sludge volatile solid, respectively, that Al(OH)3 adsorb P with 0.05 mol-P/mol-Al(OH)3, and that CaHPO4 and FeCO3 to experience slow solid-to-solid reaction to CaCO3 and Fe3(PO4)2･8H2O with first-order reaction rates of 0.05 day–1 and 0.01 day–1, respectively, the developed model successfully reproduced aqueous P, Ca and Mg concentrations of the datasets. The model predicted the average inorganic composition of the tested ADS inside the digester of P (as 52% aqueous and 20% inorganic solid), Ca (as 5% aqueous and 65% inorganic solid) and Mg (as 52% aqueous and 6% inorganic solid).
