Abstract
Effective energy recovery systems are required to handle organic waste produced at individual farms and industrial plants. High rate anaerobic digestion (AD) reactors (e.g. UASB) are efficient but they require low particle content in the feed, whereas manure and other relevant feeds contain a variety of particles.
 A two stage biogas process consisting of a storage tank (ST) and a high rate AD reactor can meet the above challenges. The ST serves three purposes: Store the waste in a submerged suspension, disintegration and hydrolysis of the waste to produce dissolved substrate and separate a liquid without large particles as AD feed. A pilot system consisting of a 20 L hybrid AD reactor together with a 200 L ST was constructed to test the two stage concept. The ST contained a filter system to extract only the liquid phase of the organic waste, which was solid waste from apple juice production. The AD liquid effluent was returned to the ST to maintain the systems liquid balance. A hydrogen sensor on the produced biogas was used to monitor the AD and thereby automatically control AD feeding for safe operation. Hydrogen set points of 20-40 ppm were tested.
 The apple waste first underwent a natural conservation in the ST by production of organic acids and alcohol. The liquid feed to the AD extracted from the ST had a pH of 4.5 and a total organic concentration of 0-50 g COD/L through the months of operation reported. The temperature in the system was ambient; 18-23 ºC. The production of biogas was 10-25 litres per day, and the biogas methane content was maintained at ≈ 70 (v/v %). The hydrogen based AD feeding control operated well but with some limitations.
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