The effect of alkaline pretreatment of waste-activated sludge, using two models to study the sequential hydrolysis rates of suspended (Sanders' surface model) and dissolved (Goel's saturation model) solids, on the mesophilic and thermophilic anaerobic digestion rate is evaluated. The pretreatment, which reduces the size of the solids, increases the reaction rate by increasing the surface area and the specific surface hydrolysis constant (KSBK); at thermophilic conditions from 0.45 × 10−3 kg m−2 d−1 for the fresh sludge to 0.74 × 10−3 kg m−2 d−1 for the pretreated sludge and at mesophilic conditions these values are 0.28 × 10−3 kg m−2 d−1 and 0.47 × 10−3 kg m−2 d−1 confirming the usefulness of a pretreatment for solids reduction. But for soluble solids, the thermoalkaline pretreatment decreases the reaction rates by inducing a competitive inhibition on the thermophilic anaerobic digestion rate while in the mesophilic range, a non-competitive inhibition is observed. A mathematical simulation of the consecutive reactions, suspended solids to dissolved solids and to methane in staged anaerobic thermophilic–mesophilic digestion, shows that with 4% suspended solids concentration it is better not to use a thermoalkaline pretreatment because overall solids reduction and total methane production are not as good as without pretreatment.

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