WRRCTR No.27 Anaerobic Digestion of Pineapple Mill Wastes

Abstract

To determine the response of pineapple mill waste to anaerobic digestion, a simulated mill waste with a COD value equal to actual mill waste was made from commercial pineapple juice for this study. Ammonium hydroxide was used as a pH control and a nitrogen supplement. The study utilized batch-anaerobic digesters in initial stages to find optimum conditions for long-term studies which were conducted in 9-liter bottles with periodic removal of digested waste and addition of fresh waste. Seed was composed of digested domestic sewage sludge, bottom sediment from a canal used for disposal of pineapple mill waste, and fermented pineapples and pineapple juice. Principal studies involved three digesters maintained at ambient temperature as controls, two digesters with 75 mg/l of phosphorus added, two digesters heated to 90°F, and one digester whose contents were mixed by recirculation of digester gases. The digesters were operated for sixty days. Analytical parameters were COD, sugar, pH, ORP, volatile acids, gas production, gas composition, alkalinity, and total bacteria by the Breed Counting Method. The results of the study showed that pineapple mill waste undergoes a rapid initial decomposition. A reduction in COD occurred along with a rapid drop in sugar concentration and subsequent formation of volatile acids. Following initial activity, there is a period of inactivity after which recovery from the acid stage occurs and a steady reduction of waste material begins. The test data collected indicated that raw pineapple waste liquor is unsuitable for anaerobic digestion unless it is heated, stirred, or treated with phosphate. After sixty days of operation, digesters at ambient temperature, those with added phosphorus, and those that were stirred produced a COD reduction of 40, 51, and 53 percent, respectively.There was strong evidence that pineapple mill waste may be satisfactorily treated anaerobically if it is heated and optimum conditions exist. Digesters operated at 90°F effected 66 percent reduction in COD with a steady production of gas containing at least 75 percent CH4.