Thermophilic biodigestion of fermented sugarcane molasses in high-rate structured-bed reactors: Alkalinization strategies define the operating limits

Abstract

Energetic exploitation of sugarcane-derived byproducts via anaerobic digestion (AD) has recently been highlighted as an alternative to the conventional ethanol-producing approach. In this context, the thermophilic (55 °C) methane production from fermented molasses was assessed in the long-term (>200 d) continuous operation of two anaerobic structured-bed reactors. Two types of alkalinization strategies, namely, NaHCO3 dosing (RM1) and NaOH dosing coupled to effluent recirculation (RM2) were compared, as well as different levels of organic loading rate (OLR) and hydraulic retention time (HRT) were applied to reach limiting operating conditions. Both alkalinization strategies provided an equivalent buffer control up to an OLR of 7.5 kg-CODt m−3 d−1, from which unwanted accumulation of organic acids was observed in RM2. NaHCO3 dosing enabled doubling the OLR (15.0 kg-CODt m−3 d−1) without performance deterioration, i.e., organic matter removal > 80.0% and maximum methane yield values. A detailed assessment of the food-to-microorganism ratio demonstrated the maintenance of stable operating conditions at excess substrate availability (>3.0 g-CODt g−1VSS d−1) in the feeding chamber (FDC) of the reactors, in which substrate conversion exceeded 70.0%. Organic overloads occurred only when biomass retention reached a saturation level in the FDC, hampering the uptake of volatile organic acids, regardless of the operating condition. In any case, the wide range of operating conditions (OLR = 2.5–15.0 kg-CODt m−3 d−1 and HRT = 24.0–36.0 h) associated with maximized methane production characterized a highly flexible process, which enables varied design arrangements in full-scale AD plants.