Valorization Phragmites australis biomass enhancing biomethane production: A comparison of electrohydrolysis vs ultrasonic and microwave pretreatments

This work studies the pretreatment of milled Phragmites Australis biomass straw suspensions, to maximize biomethane production through subsequent anaerobic digestion. Three pretreatment techniques as Ultrasound (US), Microwave (MW) and Electrohydrolysis (EH) were assessed using batch tests in an ultrasonic bath, a microwave digester and an electrochemical cell lab-scale systems respectively and using the same theorical power applied (10000 to 30000 kJ·kg -1 TS). Subsequently, batch anaerobic digestion assays were carried out during 30 days measuring biochemical methane production. Special attention was paid on checking the possible advantages of EH option vs the other two alternatives. None of the three pretreatments caused significant changes in pH nor electrical conductivity which could be detrimental to subsequent anaerobic digestion process. Increasing energy dose in the US and MW cases produced an improvement in methane production from 70% (without pretreatment) to approximately 90% of the total theoretical value for energies of 30000 kJ kg -1 TS. In the EH case, a clear positive effect was observed, in which 100% anaerobic biodegradability was achieved using only 15000 kJ kg -1 TS. However, for higher EH energies, there was a clear decrease in methane production. It was observed that EH seems to be an aggressive technique that caused important structural changes in lignocellulosic material, reducing their lignin content while slightly solubilizing particulate carbon. Aerobic biodegradability peaked at around 15,000 kJ kg⁻¹TS, after which unidentified biological inhibitors generated mild toxicity. EH appears to be promising pretreatment, although the mechanisms leading to inhibition of biomethanation at elevated energy levels require further investigation. • Ultrasounds and microwaves increased 20% CH 4 production under 30000 kJ kg TS -1 • Electrohydrolysis increased 30% CH 4 production applying just 15000 kJ kg TS -1 • Electrohydrolysis energy higher than 15000 kJ kgTS -1 inhibited biomethanation • Electrohydrolysis would be promising but inhibitors still need to be identified