Synergistic effects of Fe salts and composite additives on anaerobic digestion of dairy manure

Abstract

Anaerobic digestion (AD) can be accelerated through the addition of effective additives. A batch assay was conducted to determine an optimized organic composite additive (OOCA) consisting of urea, diammonium phosphate (DAP), and citric acid (CA). Fe salts (Fe2(SO4)3, Fe(NO3)3, FeCl3 and FeCl2) were then introduced to explore their synergistic effects with the OOCA during AD. The use of composite additives consisting of Fe salt and the OOCA increased biogas production (505–568 mL/g VS), shortened digestion periods (15–21 days), and promoted total COD removal rates (41.4–69.3%), compared with the control reactor without additives. The maximum biogas yield of 568 mL/g VS was obtained for the treatment receiving composite additive Fe(NO3)3@OOCA. Elemental analysis and fertility tests further demonstrated that the fertility of the digestate was improved considerably by using the composite additives. These results strongly indicated that improved AD performance could be attributed to the synergistic effects of trace element Fe and the OOCA. This finding is expected to have a positive effect on treatment process and open a new avenue for the use of composite Fe salt additives in future biomass energy production.