The digestion of pure algal AICAR resulted in approximately 57% VSR , which was greater than the 47% VSR observed for WAS alone . Anaerobic co-digestion studies showed that the addition of any algal species to WAS improved VSR compared to digestion of WAS alone. These results indicate that microalgae addition will improve the digestibility of wastewater sludge and possibly generate more useful biogas. It has been observed in all the experiments that higher proportions of microalgae (25% and 37%) slightly decrease the gas production rate to 12% compared to using food waste alone. The low yield is due to several reasons which comprises of recalcitrance of few species of algae to biodegradation and inhibition of microbiological conversion process by ammonia released from biomass. Ammonia toxicity might be counteracted by co-digesting algae with high carbon organic wastes. Carbon-rich feedstock include primary and secondary municipal sludge, sorted municipal organic solid waste, FOGs, food industry waste, waste paper, and various agricultural residues. The co-digestion of algae (Scenedesmus sp. and Chlorella sp. below) with wastepaper, found that the addition of 50% wastepaper (based on volatile solids) increased methane production rate to 1170±75 ml/l day, as compared to 573±28 ml/l day of algal sludge digestion alone, both operated at 4 g VS/l day, 35 °C for 10 days. All the experimental studies proved that the algal biomass was well digested by itself under anaerobic conditions. Co-digestion of algae with other substrates has been shown to improve anaerobic digestibility of the algae by improving the feed composition .