The good degradation of Pistia stratiotes (83-99% of VS within 30 days digestion time) indicates that this plant is an excellent substrate for biogas production. Furthermore, a digestion time of 10 days is sufficient since a large proportion of the gas has then been produced. A temperature of 37.5°C may not always be taken as an optimum although, at this temperature, gas production is the highest. The optimum temperature can only be calculated from the total energy balance of a particular biogas plant. The higher gas yield at 37.5°C would necessitate a higher heating demand for the digester in temperate climate zones. Therefore, the optimum temperature of digestion can only be determined when the ambient temperature is fully taken into consideration.