Anaerobic Digestion of Food Waste: Effect of the Organic Load Variation in a Demo-Scale System

Abstract

This study aimed to evaluate the effect of organic loading rate (OLR) variation on the anaerobic treatment of food waste from a university restaurant in a demonstration-scale set-up. The system was submitted to the academic calendar and seasonal variation of temperature. During 330 days of semi-continuous operation in three different periods (i.e. school-period/fall, school-period/spring and vacation-period/summer), the reactor performance and stability process were analysed. The reactor temperature was not controlled and varied seasonally from 23 to 32 °C. Differently, the OLR and hydraulic retention time (HRT) were controlled by food waste available in the vacation period and by the reactor stability in the school periods, ranging from 0.15 to 0.51 kg VS m−3 day−1 for the OLR and from 121 to 230 days for the HRT. The highest average methane production rate and VS removal were 0.12 m3 CH4 m−3 day−1 and 81%, respectively. The results indicated that as the reactor had been operated using non-optimized mixing for six years before the experiment, probably the accumulation of inert materials inside affected its performance. In addition, despite the non-optimized mixing of the reactor, which represented up to 80% of the energy consumed, a net energy ratio of 1.98 was achieved. Throughout the experimental period a total of 15,970 kg of food waste was treated and 375 m3 of methane were produced. Despite the system limitations, the valorization of food waste through anaerobic digestion proved to be possible under simplified operating conditions, i.e. even without pH and temperature control, besides the organic loading rate reduction caused by the vacation period.