Precipitation, chelation, and the availability of metals as nutrients in anaerobic digestion. II. Applications

The relative importance of the individual effects of precipitation and chelation of metal ions in anaerobic digestion is assessed. Experimentally determined soluble metal ion levels are compared with predicted levels obtained by using a previously described methodology.¹ It is found that soluble metal complexes may increase the level of soluble metals in the presence of CO 32− and S²⁻ by a factor of up to 10⁴. The formation of a soluble complex may increase or decrease the availability of the metal ion in question for microbial uptake. Two case studies are presented, one using a defined medium and one a complex medium. It is possible, in the case of the defined medium, to accurately predict the free metal ion concentration using the methodology previously developed.¹ While the identification of the presence of natural chelating compounds in a complex medium is not routinely possible, the significant discrepancy between the measured level of the soluble metal ion Fe²⁺ and the calculated level in the case studies presented indicates that natural chelating compounds may play a vital role in providing available metal ions to the microorganisms of an anaerobic digester.