**Conflict of interest**

*New Advances on Fermentation Processes*

**142**

(2018MS051).

**6. Conclusions**

*(B) CO2 contents, (C) H2 contents, and (D) N2 contents.*

**Figure 1.**

ties, etc.

**Acknowledgements**

This book chapter reviewed the past findings in the impacts of metals on different stages of the anaerobic degradation process. The requirements of metals by the enzymes involved in the anaerobic process resulted in the different performances with varied metal species and bioavailability. In general, metals in certain concentrations were able to promote the lignocellulose degradation, the generation and consumption of organic components in the fermenters like LCFAs and VFAs, the biogas production, as well as the CH4 contents. The mechanisms of metals were studied by many scientists, focusing on the enzyme activities, microbial communi-

*Biogas composition under Cd stress with fermentation temperature of 55, 45, 35, and 25°C (A) CH4 contents,* 

Although a large amount of research has been carried out on individual stages or the entire anaerobic fermentation process, there are still challenges on controlling metal-stressed anaerobic degradation process for optimal utilization of metalcontaminated biowastes. Further work on bioavailability of metals during anaerobic fermentation process and detailed compositions of intermediary products, the relationships between microbial functions and metal species, etc. are recommended for better understanding of the metal-stressed anaerobic degradation process.

This work was funded by the Major Science and Technology Program for Water

Pollution Control and Treatment (No. 2017ZX07101-003, 2015ZX07204-007, 2015ZX07203-011), the Fundamental Research Funds for the Central Universities No conflict of interest.
