Review Impacts of trace element supplementation on the performance of anaerobic digestion process: A critical review Yee Yaw Choong a , Ismail Norli a,⇑ , Ahmad Zuhairi Abdullah b , Mohd Firdaus Yhaya c a Environmental Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia b School of Chemical Engineering, Universiti Sains Malaysia, Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia c Bioresource, Paper and Coatings Technology Division, School of Industrial Technology, Universiti Sains Malaysia, 11800 Penang, Malaysia highlights  Trace element supplementation improves the anaerobic digestion performance.  Fe, Ni, Co are the most desirable trace elements in the anaerobic digestion system.  Multi-elements supplementation can be more beneficial.  Response of trace element supplementation is uncertain.  Future research should consider crucial aspect of trace element bioavailability. article info Article history: Received 11 January 2016 Received in revised form 1 March 2016 Accepted 3 March 2016 Available online 10 March 2016 Keywords: Trace element supplementation Anaerobic digestion Metalloenzyme Methanogenesis Bioavailability abstract This paper critically reviews the impacts of supplementing trace elements on the anaerobic digestion per- formance. The in-depth knowledge of trace elements as micronutrients and metalloenzyme components justifies trace element supplementation into the anaerobic digestion system. Most of the earlier studies reported that trace elements addition at (sub)optimum dosages had positive impacts mainly longer term on digester stability with greater organic matter degradation, low volatile fatty acids (VFA) concentration and higher biogas production. However, these positive impacts and element requirements are not fully understood, they are explained on a case to case basis because of the great variance of the anaerobic digestion operation. Iron (Fe), nickel (Ni) and cobalt (Co) are the most studied and desirable elements. The right combination of multi-elements supplementation can have greater positive impact. This mea- sure is highly recommended, especially for the mono-digestion of micronutrient-deficient substrates. The future research should consider the aspect of trace element bioavailability. Ó 2016 Elsevier Ltd. All rights reserved. Contents 1. Introduction ......................................................................................................... 370 2. Roles of trace elements in anaerobic digestion ............................................................................. 370 2.1. Trace element composition of methanogens .......................................................................... 370 2.2. Metalloenzymes in methanogenesis ................................................................................ 371 2.3. Trace elements in hydrolysis, acidogenesis, and acetogenesis ............................................................ 371 3. Impacts of single trace element supplementation on anaerobic digestion performance ............................................. 372 3.1. Iron (Fe) ....................................................................................................... 372 3.2. Nickel (Ni) ..................................................................................................... 372 3.3. Cobalt (Co) ..................................................................................................... 372 3.4. Influential factor that causes the impacts variation .................................................................... 372 4. Impacts of multiple trace elements supplementation on anaerobic digestion performance .......................................... 373 5. Crucial consideration: Trace element bioavailability ......................................................................... 374 http://dx.doi.org/10.1016/j.biortech.2016.03.028 0960-8524/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. Tel.: +60 4 6532824; fax: +60 4 6573678. E-mail address: norlii@usm.my (I. Norli). Bioresource Technology 209 (2016) 369–379 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech