Macrophytes as a Digestion Substrate. Assessment of a Sonication Pretreatment

Biomethane potential (BMP) test was performed to access the anaerobic degradation of three macrophytes (Eichhornia crassipes, Pistia stratiotes and Salvinia molesta). Sonication pretreatment was evaluated in order to pre-hydrolyze the plants, aiming to remove the lag time and potentially increase the methane production. Two sonication energies were tested: 40,000 and 80,000 kJ kgTS− 1. Results indicated that not all the aquatic plants presented the same behavior. Raw biomass of Eichhornia and Pistia required a long lag-phase (11 and 8 days, respectively), while Salvinia did not, being the faster and most productive raw biomass (64% of biomethanization factor). Pretreated biomass was solubilized, however the results did not have a direct translation to the digestion performance. In fact, no substantial improvement on the amount of methane production occurred. The main experimental evidence was the full reduction of the lag-phase for Eichhornia and Pistia, while Salvinia did not exhibited a substantial change neither in methane production or in kinetics. The high biodegradability of Salvinia biomass without pretreatment indicated that ultrasound pretreatment is not required. Energetic Sustainability Index evidenced that sonication pretreatment process was not sustainable, due to the required high water content of the bulk.