Due to endogenous contaminants, treatment methods of crop residues from contaminated sites must be carefully selected considering contaminant separation, environmental impact, and economical concerns. Contaminated residues are generally disposed of by composting, pyrolysis, direct disposal, incineration, ashing, and anaerobic digestion. Anaerobic digestion is a biological process in which microorganisms degrade organic matter and convert into biogas as the end product. Agricultural crop residues are an important source of biomass that can be utilized as a substrate in anaerobic digestion. Anaerobic digestion for crop residues has been applied as an effective technology in terms of renewable energy production, byproduct utilization, and agricultural waste reduction. For these reasons, anaerobic digestion could be the appropriate option for crop residues from heavy metal contaminated sites with considerations in terms of the aforementioned categories (i.e., contaminant separation, environmental impact, and economical concerns) among various treatment methods. However, heavy metals have been known to adversely affect the anaerobic digestion process, and the fate and effect of heavy metals in crop residues during anaerobic digestion needs to be addressed.

  Firstly, biochemical methane potential (BMP) tests using sunflowers (i.e., Helianthus annuus) harvested from four differential levels of heavy metals containing soils were conducted to investigate the applicability of anaerobic digestion for heavy metal containing crop residues. According to the results, the methane gas production of crop residues from heavy metals containing soils were comparable to that of the control test, which was not contaminated with heavy metals. Significant adverse effects of heavy metals in crop residues on methane gas production were not observed under the experimental conditions of this study. Even though anaerobic bacterial activities are known to be typically affected by the amounts of heavy metals in the form of liquid phase, all of the observed amounts of heavy metals in this study were not only similar between the test conditions but also below the reported inhibitory levels. These findings revealed that anaerobic digestion can be an alternative to the treatment method of heavy metal-containing crop residues from phytoremediation sites.

  In order to investigate the long-term stability on the performance of the anaerobic digestion process, a laboratory-scale continuous stirred-tank reactor (CSTR) was operated for 1,100 days with sunflower harvested in a heavy metal contaminated site. Changes of microbial communities during digestion were identified using pyrosequencing. According to the results, soluble heavy metal concentrations were lower than the reported inhibitory level and the reactor performance remained stable up to OLR of 2.0 g VS/L/day at HRT of 20 days. Microbial communities commonly found in anaerobic digestion for cellulosic biomass were observed and stably established with respect to the substrate. Thus, the balance of microbial metabolism was maintained appropriately and stability on the performance of the anaerobic digestion was confirmed by long-term operation of laboratory-scale CSTR operation.

  Although the applicability and stability of anaerobic digestion for heavy metal containing crop residues were ascertained with the conducted tests, inconsistency between biodegradation ratio of biomass and releasing characteristics of heavy metals through biodegradation of biomass was observed. For better understanding of anaerobic digestion of crop residues from heavy metal phytoremediation sites without the adverse effects of heavy metals, the releasing characteristics of endogenous heavy metals should be considered for stable anaerobic digestion process. This study was conducted to examine the releasing characteristics of heavy metals from biomass and the fate of heavy metals after release. According to the volatile solids and carbon balance analyses of anaerobic batch test results, maximum of 60% by wt. of biomass was degraded. During the biodegradation, among Cd, Cu, Ni, Pb, and Zn, only Cu and Zn were observed in soluble form (approximately 40% by wt. of input mass). The results concluded the irrelevancy between degradation ratio of biomass and ratio of released heavy metals amounts from biomass. It was shown that this discordance was caused by the fate (i.e., precipitation and adsorption) of heavy metal species in solutions after being released from biomass. Thus, ultimate heavy metal concentrations in solutions, which can exert adverse effects on anaerobic digestion performance, were strongly dependent upon not only released heavy metal amounts but also their fate in solution after release.

  A model of the anaerobic digestion process which attempts to explain the complex patterns of the anaerobic digestion process is required to better understanding and design anaerobic digestion process. Mathematical models have provided an understanding of important inhibition patterns and have given guidelines for operation and optimization of anaerobic digesters. However, a mathematical model for prediction in change of heavy metal concentrations in anaerobic digestion process according to the degradation of heavy metal containing biomass has not been studied in previous research. For this reason, developing a mathematical model is needed for better understanding of anaerobic digestion of crop residues from heavy metal phytoremediation sites without the adverse effects of heavy metals. In this study, to simulate the change of soluble heavy metals in anaerobic digestion system, a mathematical model based on mass balance is developed. The model can describe the soluble heavy metal concentrations in anaerobic digester according to degradation of heavy metal containing crop residues. From the sensitivity analysis for the variables used in the model, OLR has the highest sensitivity with gradient of trend line. Although substrate degradation kinetic (k) has relatively low sensitivity to the change of heavy metal concentrations in liquid phase, the k value can be an important input parameter due to its variation with type of substrate. The developed model will provide useful information on anaerobic digestion process design for heavy metal containing substrate and will expand the substrate types using simple batch test for substrate degradation kinetics. Several application examples and required improvements were also discussed. However, the model developed in this study includes several uncertain assumptions for the convenience of calculation (i.e., MLSS is constant during digestion, heavy metal adsorption occurs only to MLSS, etc.). Consequently, upgrading the developed model should be accompanied by verification and improvement of the uncertain assumptions for degree of completion.

  Keywords: Anaerobic digestion; Heavy metal-containing crop residues; Treatment of crop residues; Biodegradation; Releasing characteristics of endogenous heavy metal; Model development