The effects of heavy metals on anaerobic digestion processes

The optimisation and wider utilisation of renewable energy technologies is essential for the sake of the global climate and all life on earth. Anaerobic digestion of organic matter is one such technology that has its place as one of the most important methods of waste reduction and sustainable energy generation. Already being utilised in a multitude of contexts worldwide, anaerobic digestion still has vast unrealised potential. A major cause of this is process inhibition, and one of the main groups of chemical inhibitors is heavy metals. However, the literature is currently lacking in breadth and depth of understanding of the role of heavy metals in anaerobic digestion processes. Furthermore, the addition of trace nutrients to reactors in large-scale applications is becoming a more common practise for the purposes of biogas enhancement and improved reactor stability. However, the detailed effects of specific trace elements on different feedstocks and in various applications has not been widely studied. This work investigated the effects of Co, Fe, Hg, W and Zn on anaerobic digestion processes through a series of laboratory experiments. Sewage sludge spiked with these metals was used as the feedstock for 9L continuously-fed anaerobic digesters and biochemical methane potential tests. Spiked heavy metal concentrations were carefully chosen for the purpose of measuring the impact of selected metals on anaerobic processes. Biogas quantity and quality were continuously recorded, as were the reactor parameters, TS%, VS%, pH, acid/alkalinity ratio, VFA and ammonia concentrations, in order to closely monitor metal-effects over time. Reactor stability and biogas production were negatively affected by Hg, Zn and Fe, with Hg exhibiting extremely toxic effects even at low concentrations. Co had both negative and positive effects on anaerobic processes, depending on the spiking concentration. W exhibited biogas promotion effects at concentrations between 0.6-10ppm, and its addition did not negatively affect reactor stability. Recommendations were made for users of anaerobic digestion technologies regarding the potential damage or enhancement effects certain metals would be expected to have on their reactors.