Bahman Heydari - Department of Agricultural Machinery Engineering, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran
Shahin Rafiee - Department of Agricultural Machinery Engineering, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran
Elham Abdollahzadeh Sharghi - b Environmental Group, Department of Energy, Materials and Energy Research Center (MERC), MeshkinDasht, Alborz, Iran.
Seyed Saeid Mohtasebi - Department of Agricultural Machinery Engineering, Faculty of Agricultural Engineering and Technology, College of Agriculture and Natural Resources, University of Tehran, Karaj, Iran.

The aromatic and dark-colored spearmint essential oil wastewater (SEOW) generally contains a large amount of organic matter, including chemical oxygen demand (COD), phenolic compounds, and inorganic contents. In this study, the pollutant removal performance and biogas production rate of an up-flow anaerobic sludge blanket (UASB) reactor used for the treatment of SEOW were investigated. During the 102 days UASB operation at hydraulic retention time of 60 hours, the organic loading rate (OLR) was increased from 0.14 to 2.69 kg COD/m3.d by increasing the influent SEOW concentration. With increasing OLR from 0.14 to 2.69 kg COD/m3.d, the concentrations of COD and phenol in the influent of the UASB reactor increased to 6720±383 mg/L and 383±88 mg/L, respectively. At OLR equal to 2.69 kg COD/m3.d, the steady-state average removal efficiencies of COD and phenol were 72.0±1.4 and 63.1±6.7 %, respectively. The stability of the anaerobic system was confirmed by the average steady-state ratios of the volatile fatty acid/alkalinity and pH in the UASB reactor, which were less than 0.4 and 7.5±0.1, respectively, at different OLRs. The optimum OLR was found to be 2.69 kg COD/m3.d, where 26.9±1.7 L/d production of biogas containing 63.0±5.2 % and 22.4±4.2 % methane and carbon dioxide, respectively, was obtained. Moreover, at OLR equal to 2.69 kg COD/m3.d, the biogas yield and net heating value were 462.2±46.9 L/kg CODremoved and 24.7±5.2 MJ/m3, respectively. The results of the current study reveal the substantial potential of the UASB reactor in terms of pollutant removal performance and biogas production for the treatment of SEOW.

Spearmint Essential Oil Wastewater (SEOW), Up-flow Anaerobic Sludge Blanket (UASB), Biogas, Phenol Chemical Oxygen Demand (COD)