Integration of microalgae production with anaerobic digestion of dairy cattle manure: an overall mass and energy balance of the process

This work aimed to investigate the possibility of integrating microalgae production with anaerobic digestion of dairy cattle manure and subsequent digestate treatment, thus helping to reduce the cost of slurry treatment and improving the energy balance of the process. Real biogas and digestate-treatment units were monitored for energy, mass and nutrient balances. The existing system produces electrical energy (2182 ekWh d-1) and organic fertilizers/amendments (11,655 t y-1), among others. Microalgae production was integrated with this system by using untreated ultra-filtered digestate as the growth medium for the production of Scenedesmus sp. The tolerance of this strain to digestate was evaluated and results demonstrated that a percentage of digestate of over 10% inhibited the growth of this microalga, but below this value productivity of up to 124 mg L-1 d-1 could be obtained. The composition of the culture medium also influenced the biomass composition, with protein, carbohydrate and lipid content being direct functions of ammonia concentrations. Integrating microalgae production with anaerobic digestion, it is possible to produce 166-190 t y-1 of valuable microalgal biomass. Overall energy and mass balances of the process allow us to show that the integration of both anaerobic digestion and microalgae production steps are positive and technically feasible.