Alkaline Wood Ash, Turbulence, and Traps with Excess of Sulfuric Acid Do Not Strip Completely the Ammonia off an Agro-waste Digestate

Abstract

The present study combined two nutrient management strategies to improve the marketability of a waste-derived fertilizer: (a) isolation of ammoniacal nitrogen and (b) preparation of a bulk soil amendment. The wood fly ash with low content of pollutants was added to an agrowaste anaerobic digestate as alkaline stabilizer, which promoted the volatilization of ammonia and adsorption processes, and as nutrient supplement. The 39.71 ± 1.44 g blend was incubated for 60 hours at 20°C and 100 rpm in a closed chamber (250-mL Schott Duran® bottle) with a 5.21 ± 0.10 mL sulfuric acid trap of 10 different concentrations (0.11, 0.21, 0.32, 0.43, 0.54, 0.64, 0.75, 0.86, 0.96, and 1.07 mol/L). For analytical purposes, the sulfuric acid, water-soluble, and water-insoluble fractions of the blend were isolated after the incubation. The 1.07 mol/L sulfuric acid solution contained 23.69 ± 5.72 % more of ammonical nitrogen than the 0.11 mol/L solutions. However, in all cases the amount of nitrogen in the H2SO4 compartment was lower than the one in the water-soluble and water-insoluble fractions. Only the 15.52 ± 2.13 % of the nitrogen accounted after the incubation was found in the H2SO4 trap. The bottleneck of the NH3 stripping process was the rate of mass transfer at the interface between the blended fertilizer and the headspace of the closed chamber. The organic phosphorus was more susceptible to be adsorbed during the alkaline treatment with non-intrusive acidification than the nitrogen and carbon. Activation of the ash as adsorbent before mixing with the digestate should improve the properties of the blend as slow release fertilizer, since more nutrients would end in the water-insoluble fraction.