Anaerobic digestion of whole cells and post-extracted algae residues of Scenedesmus obliquus in immobilized batch reactor

Conventional anaerobic digesters are operated by mixing microbes with substrates under anaerobic conditions to generate biomethane (bioCH4). After anaerobic digestion (AD), digestates are inseparable from spent microbes and in some cases, the digester fails due to the accumulation of organic acids. This study demonstrates an immobilized bacteria reactor system adopting digested sludge (DS) encapsulated in calcium-alginate to digest algae and post-extracted algae residue (PEAR). This has not been reported elsewhere. From this study, it was established that a mixing speed of 100 rpm and 10 days of AD at a temperature of 37 °C are the best conditions for stable encapsulation of DS, while at a concentration of 4% w/v sodium alginate, bioCH4 was maximized. Relative to the conventional approach, 24% and 58% more bioCH4 was produced, digesting algae and PEAR of Scenedesmus obliquus, respectively. Spent encapsulated DS was reused and a performance of 85% was achieved, based on the bioCH4 production employing spent encapsulated DS compared to fresh encapsulated DS. Improved bioCH4 production from the immobilized reactor suggests that alga-to-biofuel can be more economically viable while separability of digestates at the end of AD indicates a reduction in waste generation and digestates can be processed for other uses such as animal feed or fertilizer.