The best-studied macrophyte is the water hyacinth (Eichhornia crassipes) (Gass — mann et al. 2006; Gunnarsson and Petersen 2007). It has been found to produce up to 140 Mgha-1 year-1 of biomass (dry weight) (Gunnarsson and Petersen 2007). Two energetic applications of Eichhornia crassipes which may produce transport bio­fuels have been studied. The first is ethanol production from hemicellulose present in water hyacinths. A yield of 0.14-0.17 (g ethanol) (g dry weight)-1 has been re­ported (Mishima et al. 2008). However, studies of the overall energy efficiency of the production of ethanol from the water hyacinth have so far suggested that the en­ergy balance is negative (Gunnarsson and Petersen 2007). An alternative option is the anaerobic conversion of water hyacinthbiomass into CH4. Though the feasibility thereof has been demonstrated, the process is complicated, among other things by the floating behaviour of water-hyacinth-derived material (Malik 2007). Moreover, the water hyacinth is very effective in adsorbing pollutants (Gunnarsson and Pe­tersen 2007; Malik 2007), and these may interfere with, for example, the sustainable use of residuals (‘digestate’) remaining after anaerobic conversion. More limited re­search has been done regarding another invasive macrophyte: water lettuce (Pistia stratiotes L.), which has growth characteristics similar to water hyacinth (Mishima et al. 2008). The yield of ethanol from hemicellulose conversion is 0.15-0.16 g per gram of dry weight (Mishima et al. 2008); no study has been found regarding the overall energy efficiency of this conversion.