Biomethane production using ultrasound pre-treated maize stalks with subsequent microalgae cultivation

This study utilized a renewable energy source, agricultural waste, in anaerobic digestion (AD) at appropriate conditions to obtain biogas and biomethane as an energy carrier. Maize stalks underwent ultrasound (US) pre-treatment for better accessibility for microorganisms, as lignocelluloses have a stable structure, insoluble in water and resist both mechanical and enzymatic attack. The digestate after an anaerobic digestion process was used for cultivation of algae after adsorption with activated carbon for clarification. Photosynthetic microalgae have industrial and economic perspectives, so their low-cost cultivation has a great potential for many applications. The results showed the impact of US pre-treatment of maize stalks as a sole substrate and co-digested with algal biomass. The total yields were 1116 cm(3)/L, 1350.5 cm(3)/L and 1293.25 cm(3)/L for the untreated, ultrasonically pre-treated and microwaved maize stalks. The possibility of accumulating algal biomass using anaerobic digestate as a medium was demonstrated. US pre-treatment (400 W) showed high efficiency with respect to the extractives obtained per unit of energy input. Addition of 4 g/L of microalgal biomass as a co-substrate led to an increase in the biogas yield compared to native stalks. A small closed circle system, starting from anaerobic digestion of lignocellulosic substrates followed by microalgae cultivation in the digestate and subsequent return of microalgal biomass back in the bioreactor as a co-substrate was realized, encouraging circular economy. The suggested scheme is a simple and low-cost technology, as the substrate used is freely available and renewable, and algae proved to grow in a waste effluent as medium.