Valorization of water hyacinth to biomethane and biofertilizer through anaerobic digestion technology

Abstract

The purpose of this research study is to determine the effects of co-digestion of water hyacinth weed plant harvested from Lake Tana in Ethiopia and manures collected from two local farms, with Alkali-thermal pre-treatment on methane production and the value of the resulting digestates as fertilizer. In codigestion tests, the aerial part of water hyacinth resulted in the highest methane production of 159 NmL.gVS-1 at 25% poultry waste followed by the whole water hyacinth at 25% poultry waste with 147.73 NmL.gVS-1. The aerial part and the whole water hyacinth mixed with cow manure at 60:40 registered 120 and 114.47 NmL.gVS-1, respectively. Alkali-thermal pretreatment using (KOH at 50 °C for 5 h) at different concentrations of 1.5, 2 and 3% showed that 1.5% recorded the highest methane production. The results demonstrated the effectiveness of the (1.5% KOH, 50 °C, 5 h) pretreatment in improving mixtures solubilization, biodegradability, and methane production compared to the untreated ones. Alkali-thermal pretreatment reduces retention times compared to the untreated water hyacinth. The shortest retention time has been achieved for pretreated aerial part of water hyacinth at 25% poultry waste (8 days) and at 40% cow manure mixture (9 days) compared to the longest retention time of the whole untreated water hyacinth at 40% cow manure mixture (26 days). The gas chromatography (GC) analysis of biogas revealed that the highest percentage of CH4 (80%) was found for the whole pretreated water hyacinth at 25% poultry waste, followed by the untreated aerial part of water hyacinth at 25% poultry waste (60%). The application of the pretreatment method increased the concentration of calcium in the liquid digestate, making it more beneficial for soil use. The solid fraction, which comprises less than 10% of the digestate, exhibited a high phosphorus content compared to the liquid fraction. The use of Alkali-thermal pretreatment led to an increase in assimilable (P2O5) levels, and a significant reduction in the concentrations of Zn and Cu heavy metals in all pretreated mixtures. The levels of iron, zinc, and copper in the liquid digestate were within the acceptable range for soil fertilizer application in both untreated and pretreated mixtures. This work presents a novel technique for producing methane and appropriate digestate for soil application from water hyacinth, highlighting the potential for further research in this area.