Anaerobic digestion of citrus industry effluents using an Anaerobic Hybrid Reactor

Mexico is one of the main citrus producers, ranking fourth in the world with approximately 7.4 million tons per year, with oranges contributing 4.08 million tons to that total. The effluents from the citrus industry have high loadings of organic matter and low pH levels. These effluents also are suitable for treatment by anaerobic digestion to reduce the organic matter and to obtain a value-added product, such as biogas. The aim of this work was to perform the anaerobic digestion of effluents from the citrus industry by using a high-rate reactor, evaluating the biogas production and use as an alternative energy source. An Anaerobic Hybrid Reactor (AHR) was used to treat the effluent from a citrus processing plant in Martinez de la Torre, Veracruz, Mexico. The AHR was operated at an organic loading rate (OLR) of 8 gCOD(T)/Ld at steady state, obtaining a total and soluble COD removal of approximately 85%, with methane yields around 0.15 LCH4 to STP/gCOD(rem). An inhibitory effect was observed by the presence of D-limonene in the citrus effluent, although no pretreatment was used for elimination of this essential oil, a decrease in the COD removal and methane yields was observed due to the higher OLR. An energy estimate was obtained with the biogas, and it was found that up to 2.2% of the electricity required for the operation of a citrus processing plant can be generated using anaerobic treatment.