Effects of loading rate and temperature on anaerobic co-digestion of food waste and waste activated sludge in a high frequency feeding system, looking in particular at stability and efficiency

A continuously stirred tank reactor (CSTR) with a high feeding frequency (HFF) of once every 15 min was employed in order to ease the loading shock frequently occurred in digester with a low feeding frequency. The effects of the organic loading rate (OLR) and temperature on the co-digestion of food waste and waste activated sludge was evaluated in a 302-day long-term experiment. Due to the high hydrolysis rate, the maximum CH4 yield in a thermophilic reactor was 407 mL CH4/gVSadded, a value that was significantly higher than the 350 mL CH4/gVSadded that occurred in a mesophilic reactor. Although the alkalinity declined when HRT was shorted than 10 d, caused by the decrease of conversion ratio from protein to ammonium, the increase of specific methanogenic activity helped HFF system to achieve stable performance at an OLR of 11.2 (HRT 7.5 d) and 30.2 gVS/L/d (HRT 3 d) under mesophilic and thermophilic conditions, respectively.