TREATMENT OF DIGESTATES FOR AMMONIA NITROGEN RECOVERY AS STRUVITE

Abstract

The use of digestate as fertilizer requires a careful management in order to prevent the uncontrolled release of ammonia nitrogen. In this regard, the NH4+ recovery as struvite (MgNH4PO4·6H2O) is a suitable option to reduce the drawbacks of digestate exploitation. This process requires the dosage of magnesium and phosphorus reactants and it is strongly affected by the applied operating conditions. In this paper, the struvite precipitation processes for the treatment of digestates, obtained by the co-digestion of sewage sludges and agro-industrial wastes, was developed. Several experiments were carried out by using both commercial reactants and by-products. The experimental results allowed to identify the operating conditions able to maximize the ammonium abatement and recovery. The best performances were obtained with magnesium sulphate and phosphoric acid. By dosing these compounds at molar ratios of N/Mg/P =1/1,3/1,3 and by setting the pH at 9, NH4+ abatements of 95% were reached. The efficiencies decreased of about 20% by using salts containing cations such as calcium, sodium and potassium, which can hinder the formation of struvite crystals. Satisfactory abatements, of about 85%, were obtained by exploiting bone meal and marine magnesium chloride as low cost reagents. The use of these unconventional reactants could be particularly advantageous as the treatment costs would be notably reduced.