Digestion of waste bananas to generate energy in Australia

This paper presents results from laboratory studies to measure the methane yield and rate of digestion of reject bananas. These parameters were determined in experiments that took into account the likely configuration of a full-scale plant in the banana growing region of north Queensland. The digestion was conducted in a 200-l reactor using fed-batch operation, relying entirely on the natural microbial consortia on the reject bananas to avoid reliance on external inocula such as sludge, an undesirable material around food packaging facilities. An enrichment culture was first established in a highly buffered 200-l batch digestion unit. The fed-batch digester was then started by exchanging leachate with the mature batch reactor. Under loading conditions of 0.6 kg VS m−3 d−1 over 70 days where the average working volume was 160 l, the digester produced 398 ± 20 l CH4 kg VS−1. Increasing the loading rate to 1.6 kg VS m−3 d−1 resulted in a reduced methane yield of 210 l CH4 kg VS−1 over 23 days of operation, with a concomitant accumulation of banana waste in the digester. The leachate at the end of digestion contained over 4000 mg l−1 K, 200 mg l−1 N and 75 mg l−1, levels that exceed acceptable limits for general agricultural irrigation.