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In this paper, a destabilization episode in a semicontinuous dry thermophilic acidogenic reactor was induced. The reactor was fed with different mixtures of organic fraction of municipal solid waste (OFMSW) and food waste (FW). The aim was to determinate the effect of the organic loading rate (OLR) and cosubstrate ratio on biohydrogen production by dark fermentation. OLR was increased from 21.0 to 88.8 g TVS/L/day testing three mixture ratios (OFMSW:FW) from 80:20 to 50:50 and four hydraulic retention times (HRTs; 6.6, 4.4, 2.4, and 1.9 days). Results have shown that an increment in the OLR (from 21.0 to 59.8 g TVS/L/day) and FW proportion (from 80:20 to 50:50) improves the yield process in terms of solubilized organic matter. However, from 50:50 ratio combined with OLRs higher than 70.3 g TVS/L/day and HRTs lower than 4.4 days showed a clear destabilization episode due to an imbalance of the hydrolytic phase. This fact has been corroborated by the values of two indirect parameters, “nonsolubilized carbon” and “acidogenic substrate as carbon”.

The organic fraction of municipal solid waste (OFMSW) usually contains high lignocellulosic and fatty fractions. These fractions are well-known to be a hard biodegradable substrate for biological treatments and its presence involves limitations on the performance of anaerobic processes. To avoid this, thermochemical pretreatments have been applied on the OFMSW coming from a full-scale mechanical-biological treatment (MBT) plant, in order to pre-hydrolyze the waste and improve the organic matter solubilisation. To study the solubilisation yield, the increments of soluble organic matter have been measured in terms of dissolved organic carbon (DOC), soluble chemical oxygen demand (sCOD), total volatile fatty acids (TVFA) and acidogenic substrate as carbon (ASC). The process variables analyzed were temperature, pressure and NaOH dosage. The levels of work for each variable were three: 160–180–200 °C, 3.5–5.0–6.5 bar and 2–3–4 g NaOH/L. In addition, the pretreatment time was also modified among 15 and 120 min. The best conditions for organic matter solubilisation were 160 °C, 3 g NaOH/L, 6.5 bar and 30 min, with yields in terms of DOC, sCOD, TVFA and ASC of 176%, 123%, 119% and 178% respectively. Thus, predictably the application of this pretreatment in these optimum conditions could improve the H2 production during the subsequent Dark Fermentation process.