Working group 3: Engineering

The influence of reactor features, such as mixing, hydraulics or operational mode (e.g. continuous or batch), has not yet been properly investigated. In completely mixed anaerobic reactors, precipitation of metals confers no advantage in terms of metals retention in the digester and hence dosing of metals as soluble complexes could be advantageous to improve their availability for microbial uptake before the metals are washed out. Conversely, anaerobic reactors that decouple the biomass retention time from the hydraulic retention time, such as UASB (Upflow Anaerobic sludge Blanket) and SBR (Sequencing Batch Reactor) digesters, are likely to benefit from being dosed with metals in a non-complexed form so that the metals will precipitate and accumulate within the biomass sludge fraction, from which they can be assimilated more slowly by the microorganisms as they are required. This is highly likely to be more cost effective than dosing complexed metals that remain in the soluble phase and are washed out of the reactor in a matter of hours, even if there is a trade-off in bioavailability. Another engineering aspect that is particularly relevant to CSTR-type digesters (representing the vast majority of sewage sludge and agricultural slurry digesters) is the mixing pattern present in the vessel. These reactors are rarely well-mixed and hence often consist of different zones; research is in its infancy to understand how mixing patterns might influence the microbiology of a digester and to relate this to biogas output. It is likely that the creation of micro-environments within a digester affects the microbiology of that mixing zone and also the chemistry and hence, logically it could be proposed that the speciation and bioavailability of metals within a CSTR will differ depending on the mixing regime and the particular mixing zones created within.