A thesis submitted in partial fulfillment of the requirements for the degree of Master of Engineering

Thesis (M.Eng.) - Asian Institute of Technology, 2000

This study was carried out to evaluate and determine the suitable working conditions for enhancement of acids production with the stabilization of solid pineapple waste simultaneously when operating with two-phase anaerobic digestion. A batch study was conducted on the lab scale units using solid pineapple waste (peel) as feed. It was found that, to enhance acids production, the operation of a digester at neutral pH is effective. In a period of two weeks, volatile fatty acids, which mainly consisted of acetic and butyric acid, could be obtained at about 50 g/kg pineapple waste. Overall volatile solid reduction of74 - 80% was obtained with 51-57% of volume reduction. The potential energy that could be recovered was calculated based on volatile fatty acids generated from digestion. The energy about 800 kJ/kg of solid pineapple waste could be gained from two phase anaerobic digestion. Apart from the objectives of the study, the inhibition from unionized acids to the activity of acetogenic bacteria was observed. The threshold limit of unionized acids concentration for acidogenesis of 600 mg/L was set and used as a tool to describe phenomena, which occurred during the experiment. Based on the hypothesis that unionized acids are the inhibitory factors to bacteria, the combination of volatile fatty acids and pH should be considered to identify suitable conditions for acidogenesis and methanogenesis in anaerobic digestion process.