TE supplementing Grass silage Anaerobic Digestion (TE-GAD). Grass Silage AD at increasing rates met instability above OLR 2.5; rescue affected with Fe-Co-Ni supplement at OLR 4, with analagous reactor digesting grass silage-slurry at identical rates.

Wall et al. (2014) attempted to determine the optimal mix of silage/slurry in biomethane based AD. Trials showed that although BioMethane Potential (BMP) was highest in 100% silage tests, generation of biomethane was more stable with a 20% inclusion of dairy slurry (by VS%). At higher loading rates BM output in 100% silage (R-G) and 80%silage:20% slurry (R-SG) reached parity, possibly an effect of incomplete degradation of solids within the retention time, but may also be imformed by the microbial community created by increased rate of feedstock addition.Another trend documented both in the original study and more broadly is the decline in reactor stability with increased organic loading rate (OLR), as measured by the FOSTAC ratio (effectively the reactor's buffering capability and frequently an indicator for process stability). Increases in OLR from 2.5 to 3 kgVS/m3/d led to rapid accumulation of VFAs (AcO, PrO) in R-G, while no issue was observed over the course of R-SG operation. Due to the constant turnover of organic material and active biology of the reactor community, issues with reactor operation are frequently linked to limiting concentrations of trace elements such as Fe, Co, Ni, etc.. However it is not externally apparent which portions of the community are restricted by the lack of trace elements (TE) - this study aims to characterise the communities stressed by Wall et al., 2014, and determine which populations relate to bottlenecking of (TE).The two reactors characterised in this study (R-G,R- SG) were sampled over 68 and 65 weeks respectively after being seeded from the same sludge inoculum, fed 100% silage and 80:20% silage:slurry, and operated along the same design principles. R-G was sampled at week 1, 20, 37, 40, 43, 49, 54, 64, 65, 68. R-GS was sampled at week 01, 20, 37 and 65 to act as a positive control for R-G.