OPPORTUNITIES OF ADVANCED BIOMETHANE PRODUCTION FROM MICROALGAE GROWN ON BIOGAS PLANT DIGESTATE. Part 1.

The purpose of the work is to review technologies and analyze the possibility of introducing microalgae cultivation at biogas/biomethane plants while simultaneously obtaining of additional biomethane yield and utilization of the liquid fraction of the digestate. Modern technologies, types and equipment for growing microalgae on the liquid fraction of digestate of biogas plants were considered and analyzed. The current situation of biogas/biomethane production in Ukraine and disposal of the liquid digestate fraction is presented. The chemical composition of the digestate and the methods of its preparation for the cultivation of microalgae were analyzed. Recommendations and a block scheme for technology implementation at a biogas/biomethane plant have been developed. Today, one of the most economically profitable options for Ukraine is the production of biomethane. On the other hand, there is a clear need for innovative, low-cost, safe production of biomethane using alternative, high-yield biomass that can utilize waste feedstock and also sequester carbon. Microalgae is a promising biomass that can convert nutrient-rich digestate into valuable biomass for biofuel and biodiesel production and is considered as a double-counting feedstock (RED II Directive, Annex 9). Microalgae demonstrate high photosynthetic efficiency and productivity (almost twice that of terrestrial plants) and are promising for advanced biomethane production. Integrating microalgae harvesting into an anaerobic digestion plan is the most efficient and promising way to harvest microalgae, as most of the components required for microalgae harvesting are almost free under the following conditions: process heat, CO₂, nutrients, water, transport of microalgae to end use. Electricity alone for lighting is not free, but can be minimized by using solar energy, energy storage and biogas electricity. Algae biomass can potentially increase the annual production of biomethane at an existing plant by 9.4%, as was achieved for a similar project in Sweden [33].