Optimization of biogas production with water hyacinth

Abstract

The water hyacinth (Eichhornia crassipes [Mart.] Solms) due to its high growth rate and vegetative reproduction has been classified as an obnoxious freshwater weed especially in the tropics and subtropics. It has been reported that the presence of water hyacinths in waterways results in the displacement of native freshwater plants due to intense competition for light, space and essential resources such as nutrients, restricts navigation, fishing activities, cultural and social usage by affecting water quality, harbors diseasing causing vectors, etc. However the use of water hyacinth in the production of biogas has been an appealing solution to control the ever expanding infestation of this nuisance plant. This study therefore aimed at optimizing the biogas production from water hyacinth by enhancing the hydrolysis process. This study also attempted to investigate the optimum nutritional state (i.e. C/N and C/P ratios) required for effective biomethanation and to evaluate the kinetics of anaerobic digestion. This study was conducted using six numbers of batch bed barreled digesters each having a capacity of 451. For this study aerial parts (stems and leaves) of water hyacinth were used as the substrate with fresh cow dung mixed in the ratio of 2:3 by wet weight (w/w). Several parameters such as TS and VS were measured once a week while TOC, TN and TP were measured every 2 weeks. Biogas production rate, p H and temperature were measured on a daily basis. The study was conducted at ambient mesophillic temperature for a period of 4 months. Both C/N and C/P ratios decreased after mixing with cow dung having a very low C /N and C/P ratio o f 8 and 165, respectively. The C /N ratios did not approach to the optimum range of 20-30. Nevertheless the C/P ratios exceeded the optimum ratio of 167 required to enhance biogas production. However gas production commenced from all digesters within 3 days of the study (i.e. a production rate of 0.73-1.35 1/kg/day was recorded). This study showed that substrates having TS and VS contents in the range of 63-77 g/1 and 45-50 g/1, respectively produced biogas more efficiently. Higher gas production rates were obtained from the substrates in the f-1 digester (i.e. digester containing the hyacinths grown in a nutrient solution containing 28 TN mg/1 and 7.7 TP mg/1) particularly during the period of 14th - 27th day. f-1 digester had the highest C/N ratio of 16, with an optimum initial pH of 7.28 and temperature of 30.3°C to account for optimum biogas production. This study concluded that cow dung and water hyacinth mixtures produced biogas even though the C/N ratios were not with in the optimum range of 20-30. Nevertheless higher biogas productions were reported from substrates having higher C/N ratios of 16. Chen and Hashimoto model fitted well with the experimental data except for a few initial values since the correction for the temperature, pH and mass transfer were not accounted. Kinetic constants pm and K for the substrates were in the range of 0.0074-0.0332 day"1 and 0.0360-0.0386 day"1 , respectively.