Modification of a conventional anaerobic digester for improving the effluent and sludge characteristics

The aim of this study was to enrich the composition of anaerobic digester sludge in terms of nitrogen and phosphorus by struvite (MgNH₄PO ₄) formation. Waste activated sludge was anaerobically digested in batch reactors under the conditions that the minimum stoichiometric requirement for struvite formation was satisfied in all reactors. For this purpose, different amounts of magnesium (Mg²⁺) and phosphate (PO ₄^3-) ions were added initially to the batch reactors. The results showed the effects of adding Mg²⁺ and PO₄ ^3- ions on the performance of anaerobic digestion and on the amounts of ammonium (NH₄⁺) and PO₄^3- ions released during digestion. The results indicated that the performance of the anaerobic digestion in chemical oxygen demand removal changed between -4.0% and 15.4% for a Mg²⁺ concentration range of 277.2 to 529.3 mg/L and a PO₄^3- concentration range of 377.4 to 2372.4 mg/L. There was an increase in the concentration of NH₄⁺ in all reactors during digestion, but the extent of concentration increase could be controlled by increasing the initial concentrations of Mg²⁺ and PO₄^3-. The maximum removal of NH₄⁺ ions (33.7%) was observed in the reactor dosed initially by Mg²⁺ and PO₄^3- concentrations of 467.2 and 2123.3 mg/L, respectively. The concentration of PO₄^3- ion was lower than its initial concentration in all reactors, indicating considerable fixation of PO₄^3- as it was released, by struvite and other solid species formation. The decrease in the concentration of Mg²⁺ ions similar to PO₄^3- was accepted as evidence for the formation of solid species composed of Mg²⁺ and PO₄ ³, in addition to struvite.