Comparison of suspended versus immobilized digested sludge for 2-stage anaerobic digestion of algae

Abstract

A 2-stage anaerobic digestion (AD) approach to extract biogas from algae using microbes immobilized in calcium-alginate (CA) beads is demonstrated here for the first time. The biomethane released from the 2nd stage of AD is 61% and 72% of the yield from single-stage AD, digesting whole alga and Post-Extracted Algae Residue (PEAR) of S.obliquus, respectively. However, additional 56scc/gVS and 32scc/gVS of biohydrogen produced from the 1st stage of AD makes the 2-stage AD more energy efficient. Adopting glucose as a model compound, the mass transfer behavior of the substrates in CA was studied. Diffusion time and diffusivity of glucose increased with increase in sodium alginate concentration while higher diffusivities of glucose were obtained in digested-sludge-loaded beads due to exclusion effect. The AD reaction is rate-limiting. The data in this work can be used to predict the residence time of substrates in packed bed reactors filled with microbes encapsulated in CA.