Extended ADM1 model to study trace metal speciation and its effects on anaerobic digestion

Abstract

Trace metals (TM) are often added to anaerobic digestors (AD) to improve digestor performance and methane yield. Though numerous experimental studies are done to study TM effects, a model based on ADM1 will help plant operators in case of metal deprivation as there are analytical limitations in quantifying metal speciation which controls TM effects on AD. This study developed a complete dynamic model based on ADM1 with least and unavoidable model input information to study TM speciation and its influence on AD. In addition to the biochemical and gas stripping processes described in ADM1, the model introduced new physicochemical processes with effects of operational conditions like pH, temperature and ionic strength. The biogeochemical processes influencing TM speciation include microbial uptake, precipitation (sulphides, phosphates, carbonates), adsorption (onto biomass, soluble inerts and precipitates), inorganic complexation and organic complexation (with EDTA, VFAs, amino acids (AAs)). The model was tested under different scenarios: variable EDTA concentration and dosing form, variable substrate and variable ionic strength. Complexation of TMs with EDTA and AAs influences metal bioavailability and methane yield. Supplementing EDTA reduces TM dosing and dosing EDTA as metal-EDTA complex is more effective than supplementing metals and EDTA separately. TM-AA complexation depends on substrates as amino acid compounds vary with substrates. Ionic strength influences TM adsorption and precipitation processes and hence is an important operational parameter controlling the system like pH and temperature. The model has been compared to experimental data obtained from this study and has been successfully applied to predict cobalt deprivation in an anaerobic digestor.