Biogas production from water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nitrogen concentrations

Abstract

This paper reports the biogas production from water hyacinth (Eichhornia crassipes (Mart.) Solms) grown under different nitrogen concentrations of 1-fold [28 mg/L of total nitrogen (TN)], 2-fold, 1/2-fold, 1/4-fold and 1/8-fold and plants harvested from a polluted water body. This study was carried out for a period of 4 months at ambient mesophilic temperatures of 30.3–31.3°C using six 3-barreled batch-fed reactors with the innermost barrel (45 L) being used as the digester. There was no marked variation in the C/N ratios of the plants cultured under different nitrogen concentrations. The addition of fresh cow dung having a low C/N of 8 resulted in a significant reduction in the C/N ratios of the water hyacinth substrates. However, gas production commenced 3 days after charging the reactors and gas production rates peaked in 4–7 days. The volatile solids (VS) degradation and gas production patterns manifested that in conventional single-stage batch digesters acidogenesis and methanogenesis of water hyacinth requires a retention time of around 27–30 days and 27–51 days, respectively. Substrates in the f-1 digester (i.e., the digester containing plants grown under 28 TN mg/L) having the lowest VS content of 45.3 g/L with a highest C/N ratio of 16 showed fairly higher gas production rates consistently (10–27 days) with higher gas yields containing around 50–65% of CH₄ (27–51 days). Moreover the highest overall VS (81.7%) removal efficiencies were reported from the f-1 digester. Fairly higher gas production rates and gas yields with fairly higher CH₄ contents were also noticed from the f-2 digester containing substrates having a C/N of 14 and f-out digester (containing the plants harvested from the polluted water body) having the lowest C/N ratio of 9.7 with a fairly high VS content of 56 g/L. CH₄ production was comparatively low in the f-1/8, f-1/4 and f-1/2 digesters having VS rich substrates with varying C/N ratios. We conclude that water hyacinth could be utilized for biogas production irrespective of the fact that the plants are grown under higher or lower nitrogen concentrations and that there is no necessity for the C/N ratio to be within the optimum range of 20–32 required for anaerobic digestion. Further it is concluded that several biochemical characteristics of the substrates significantly influences biogas production besides the C/N ratio.

Keywords:

• Biogas
• C/N ratio
• digester
• water hyacinth (Eichhornia crassipes (Mart.) Solms)

Acknowledgments

We are grateful to Swedish International Development Cooperation Agency (Sida) and Asian Development Bank (ADB) for granting funds for this project. We are also grateful to Asian Institute of Technology (AIT), Thailand for coordinating this project, the NERD Centre for the assistance provided in the manufacturing of the digesters, Professor Ajith de Alwis, Professor Samir K. Khanal (Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA 50011-3232, USA) and Professor Ju-Chang Huang (Department of Civil Engineering, Hong Kong University of Science and Technology, Kowloon, Hong Kong) for their valuable comments, and our Research Students Mr. Sudesh Gunasekara, Mr. P.D.C. Botheju and Ms Anusha Kasige for their assistance on numerous works. Very special thanks are given to Dr. Jagath Manatunge (Saitama University, Japan) for reviewing this manuscript.