Complementarity of Substrates in Anaerobic Digestion of Wastewater Grown Algal Biomass

Abstract

The objective of this study was to apply the concept of substrate complementarity. Algal biomass cultivated in effluent (domestic or from the brewing industry) in combination with olive mill wastewater was anaerobically digested, without any pre-treatment or correction of the substrate, allowing stability of the process and improvement of the digestion unit performance. For biomass produced in domestic sewage, the substitution of 10% of its volume by olive mill wastewater resulted in a better performance, with a yield of 0.10 m3 CH4 kg−1 volatile solids and an increase of 61% in methane production, when compared to the yield from the digestion of only algal biomass. Concerning algal biomass produced in domestic sewage, among others, the high content of ash (40%) and the low content of carbohydrates (3.6%) were the main factors to be overcome with the addition of complementary substrate. Regarding the algal biomass produced in brewing effluent, proportions of up to 20% of olive mill wastewater could be used without inhibiting anaerobic activity, however, a period of adaptation of the inoculum should be considered. Due to its more balanced chemical composition, biomass from the brewing industry alone presented better yield in methane production; 0.16 m3 CH4 kg−1 volatile solids, 14% higher than when digested together with 10% of the volume of olive mill wastewater substrate. The best performance of anaerobic digestion of algal biomass with another substrate was not related to the carbon–nitrogen ratio used. Therefore, other characteristics have influenced the algal biomass anaerobic process, such as the chemical constitution of algal biomass, mostly associated with the type of medium where it was cultivated.