It can be assumed that carbohydrates are the main source of biogas from P. stratiotes. Thus, the theoretical gas yield (of 50% methane) can be calculated as being 830 litres kg-1 VS of substrate if carbohydrates are fully degraded. In comparing the results of these tests with the theoretically possible gas yield, the degradation of VS can be calculated. Therefore, the total methane production was recalculated to a 50% methane content in the biogas. This calculated biogas production was then compared with the calculated value for 100% degradation (Table 4). Literature data (Table 5) pertaining to the gas yield from different water plants show that P. stratiotes yields relatively high amounts of gas.