Struvite Crystallization for Anaerobic Digested Supernatants: Influence on the Ammonia Efficiency of the Process Variables and the Chemicals Dosage Modality

The treatment or recovery of nutrients (N and P) from anaerobically digested supernatants is now a central topic, related both to the zootechnical wastewaters and the supernatants from the sludge lines of urban wastewater treatment plants (WWTP), applying codigestion of excess sludge and organic fraction of municipal solid waste (OFMSW). In both cases, the crystallization and precipitation of Struvite (Magnesium Ammonium Phosphate) is a hypothetical solution to reduce the high concentrations of ammonia nitrogen and to reuse the final product. This study shows the effects of the main variables of Struvite crystallization (pH, reaction time and dosage modality) applied in bench scale to anaerobic zootechnical and codigested urban supernatants. The matrices with initial concentrations of NH4-N from 1000 to 2000 mgL, were added to phosphoric acid (source of P) and magnesium oxide (source of Mg), pure and of industrial origin, to achieve the correct molar ratios. Adjustment of the pH at 8.5 was provided through NaOH dosage. The experimental tests, at increasing ratios (Mg:N:P) from 1:1:1 to 2:1:1, have been carried out at reaction times from 0 to 60 minutes and at two distinct dosage modalities. Indeed, the dosing tests were firstly provided by direct addition of the different reagents and next by indirect addition, with the prior dilution of the magnesium source in the phosphoric acid. The results defined as the different sequence of reagents addition modifies the percentage of struvite formation and, consequently, the ammonia decrease. Specifically, compared to the obtained efficiencies range between 50 % and 70 % with direct dosage, the tests with indirect dosage allow the recovery of nitrogen greater than 20 %, at the same molar ratios and reaction times. The time factor does not affect the precipitation more than 10 %, regardless of the dosage carried out. Further, the use of a thermodynamic model allowed to assess and justify the transformation of other ions in solution (K and Ca) highlighting the formation of other secondary compounds (k-struvite, hydroxyapatite).