Anaerobic digestion of peel waste and wastewater for on site energy generation in a citrus processing facility

• Citrus peel waste digestion yields 0.116 m3 of methane at STP per kg as received.
• Citrus processing wastewater digestion yields 2.1 m3 of methane at STP per m3.
• 600 ton/day citrus factory can produce 3.2 MWe on site, using biogas from wastes.
• Biogas can supply all the energy (electricity and fuel) needs of the plant.

A citrus processing facility produces significant quantities of both peel waste and wastewater. A leach – bed batch anaerobic digester was operated successfully at thermophilic (55 °C) temperature for biogasification of citrus peel waste. The retention time in the digester was 25 days. The citrus processing wastewater was digested in a thermophilic DSFF (down flow stationary fixed film) anaerobic reactor fed continuously and operated for 76 days. At an average organic loading rate of 0.51 kg sCOD m−3 d−1 the hydraulic retention time was 16 days. No long term toxicity issues due to limonene were observed during digestion of both citrus waste feedstocks. Biogasification yielded, 0.116 m3 methane at STP kg−1 peel waste received and 2.1 m3 at STP m−3 wastewater received. An energy analysis showed that in a citrus processing plant handling 600 tons per day of fruits, the biogas produced from the waste streams is more than sufficient to meet all the electricity and fuel demands. Excess electricity generated from biogas may be sold generating estimated revenues of more than a $1 million annually (at electricity sale price of $0.04/kWhe).