A statistically designed range of tests was used in order to map the impact of time, temperature and pH on the acidogenic fermentation of sewage sludge with the addition of waste fly ash. The main factors investigated were temperature (35, 55 and 65 °C), pH (7, 8 and 8.5) and retention time (1, 2 and 4 days). The initial pH was adjusted by adding ash. Up to about a third of the volatile solids could be solubilized in less than two days retention time. Higher temperatures (55 and 65 °C) and adjusted pH (7 and 8) favored hydrolysis whereas fermentation producing organic acids was faster at lower temperatures (35 °C). Sludge hydrolysis occurred fast at 55 and 65 °C, reaching a solubilized total organic carbon (TOC) concentration of 3.84 g TOC L^-1 after one day. Thermophilic conditions (55 and 65 °C) resulted in a lower volatile fatty acids (VFA) concentration compared to mesophilic conditions (35 °C). At 35 °C, the highest VFA concentration was measured after 4 days and initial pH 7 (10.0 ± 0.2 g COD L^-1). This study showed the potential of using a waste stream to increase and hasten the hydrolysis of particulate organics, resulting in higher TOC solubilized in 2 days, while promoting a higher VFA production measured as g COD L^-1.