First stage hydrolysis/acidification, using maize as the substrate, was carried out both in single pass (SP) reactors at different hydraulic retention times (HRT) and in hydraulic flush (HF) reactors in which the solids and liquid retention time were uncoupled. The HF reactors were operated at two different loadings (2 and 4 g VS l−1 d−1) and at liquid retention times between 2–16 days with solids held for 20 days. The volatile solids destruction (VS destroyed d−1) and specific methane potential (l CH4 g VSadded d−1) of the intermediate products formed in the SP reactors both decreased with increasing retention and showed a maximum VS destruction of 55.6% at a 2-day HRT. The HF reactors showed a slightly lower VS destruction at the same loading but the methane potential was slightly higher as the intermediate COD produced had a greater proportion of volatile fatty acids (VFA) present. At the loading used it was necessary to operate the HF reactors at a HRT of less than 8 days to have any benefit over the SP reactors.

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