Improving the stability and efficiency of anaerobic hybrid reactor in treating citric acid wastewater using syntrophic methanogenic consortia

Due to the high concentrations of chemical oxygen demand (COD) and total volatile acids (TVA) in the range of 18,000-20,000 mg/L and 4,900-5,600 mg/L, respectively, the treatment of citric acid production wastewater is a challenge. An anaerobic hybrid reactor (AHR) was used to treat this wastewater. The AHR operated at the maximum organic loading rate (OLR) of 7.6 kg COD/m³/d and a hydraulic retention time (HRT) of 2.5 days without any inhibition. The COD removal efficiency, methane yield, and methane content were 90%, 0.25 Nm³ CH₄/kg COD removed, and 65%, respectively. Moreover, the activities of acetoclastic and hydrogenotrophic methanogens were approximately 108% and 63% higher than those of the initial inoculum, respectively. After increasing OLR to 8.3 kg COD/m³/d, TVA was accumulated at 5,200 mg/L, leading to failure. The main organic acids were acetic acid (AA) and propionic acid (PA) with concentrations of 2,800 and 1,300 mg/L, respectively. Syntrophic methanogenic consortia (SMC) were augmented into the AHR to recover the system. After 4 days of bioaugmentation, the TVA concentration decreased to less than 500 mg/L. Bioaugmentation also increased the ratio of the methanogens and acetogens to total microorganisms at the end of the recovery period to 32% and 23%, respectively. The AHR can be recovered to operate at OLR of 7.6 kg COD/m³/d within 18 days with good performance and microbial balance.