Treatment of chocolate-processing industry wastewater in a low-temperature pilot-scale UASB: Reactor performance and in-situ biogas use for bioenergy recovery

Abstract The main objectives of this research were twofold: a) to evaluate the performance of a low-temperature pilot-scale upflow anaerobic sludge blanket (UASB) reactor during the treatment of a chocolate-processing industry wastewater (CIW) with high soluble chemical oxygen demand; and b) to evaluate the performance of two thermal conditioning devices (TCDs) for bioenergy recovery through the in-situ biogas use. For 275 days, the UASB reactor was operated at low temperature (17–19 °C), short hydraulic retention time (6.2 h) and variable total organic loading rate (OLRT) and soluble organic loading rate (OLRS) (10.3 ± 2.9 kg-CODΤ/m3/d and 7.9 ± 2.2 kg-CODS/m3/d, respectively). The produced biogas was burned in-situ with two TCDs, which mainly consisted of an insulated combustion chamber, a heat exchanger and a Bunsen burner. The OLRT and OLRS removal efficiencies were 59.5 ± 13.6% and 71.4 ± 13.2%, respectively, whereas the biogas production was in the range of 119–834 L/d. In-situ burning of the produced biogas with the TCDs increased the influent temperature between 0.8 and 5.8 °C, which improved the biogas production of the UASB reactor. The CIW energetic potential was calculated based on the UASB reactor operational parameters, the calorific value of methane in biogas and the daily CIW production (90 m3) of the studied factory. The estimated CIW energetic potential (13.59 MJ/m3CIW) represents a sustainable source of energy, which could be enough to satisfy the power consumption of 25–42 households in Toluca Valley Metropolitan Area, Mexico.