MST and MSC encompass two families of acetoclastic methanogens
(Karakashev et al., 2005). Results here imply progressive
operation of granule-based treatment reactors with shampoo
wastes results in reduced acetoclastic activity (Table 4), which explains
why methane production rates were low in the HUASB and
AHR units. Efficient anaerobic treatment and biogas production requires
both abundant and active acetoclastic and hydrogenotrophic
methanogens in the microbial treatment community (McHugh
et al., 2004). Hydrogenotrophic methanogens convert carbon dioxide
and di-hydrogen to methane, whereas acetoclastic methanogens
produce methane from acetate (Liu and Whitman, 2008).
Therefore, the lack of acetoclastic methanogens in the HUASB
and AHR units implies that COD removal was only occurring via
hydrolytic and fermentation reactions, and the activity of hydrogentrophic
methanogens. This suggests coating of the granules (see
Fig. S1) selectively restricts acetoclastic strains in the system. In
contrast, this was not prevalent in the AnCSTR unit where methane
production rates are relatively higher. Interestingly, COD removals
were higher in the HUASB and AHR reactors than the AnCSTR unit,
which may be due to limited optimisation of the AnCSTR reactor or
due to better mass transfer conditions for non-methane-producing
reactions the granule reactors