Effect of Iron Addition on Thermophilic-Mesophilic Anaerobic Digestion

Under some conditions, thermophilic anaerobic digesters produce effluent with elevated volatile fatty acids concentration (i.e., ≥1500 mg/L). The acids can be problematic since they cause odor, represent residual substrate that has not been converted to methane, and can exert a significant BOD5 when conveyed to an aeration tank via filtrate/centrate from biosolids thickening/dewatering. Therefore, methods to reduce the acids concentration would be helpful. In the work described, the direct addition of ferric chloride to the thermophilic stage of a full-scale temperature-phased anaerobic digestion system correlated with a sharp drop in volatile fatty acids (VFA) concentrations in the digesting biosolids. Addition of a blend of metallic micronutrients, including iron, produced a similar drop in VFA level observed in the pre-design pilot study of the temperature-phased system. Several metals may provide a stimulatory effect to reduce acids concentration. Iron, in the form of ferric chloride also provides struvite deposition control and reduces hydrogen sulfide (H2S) in the digester gas. In this paper, the practical benefits of adding iron and other trace nutrients to TPAD systems is described. In addition, the work serves as an example of linking university laboratory studies and full-scale operations to improve biosolids processing.