Examining the capacity for cultivating marine macroalgae using process liquids from biogas digestate as nutrient source and cultivation medium

The land-based and seawater-independent production of macroalgae as a source of renewable biomass with a multitude of potential applications increasingly attracts the attention of science and industry. However, the availability of a feasible nutrient source acceptable for supplying a large-scale cultivation facility on land remains a persisting obstacle that has not yet been overcome. Hence, this study aimed to test an uncomplicated technological approach to sufficiently reduce the nutrient load of the biogas digestate and to identify an optimal ratio of pretreated digestate to volume of cultivation medium. Based on an initial solid-liquid separation of biogas digestate, the liquid phase was treated with flocculation agent and fine separation (liquid A), followed by filtration through an organic sorption filter (liquid B) and finally through a high-performance soil filter (liquid C). The results showed notable differences regarding the nutritional value of the different process liquids. The mixtures of process liquids A to C with artificial seawater (ASW) optimal for biomass increase were 1:50, 1:10 and 1:1, respectively. Maximum biomass increase was achieved by using a mixture of process liquid A and ASW with the ratio 1:50, which resulted in a growth rate of 7.02% FW d⁻¹ in comparison to a growth rate of 5.71% FW d⁻¹ of the untreated control group that used f/2-standard nutrient supply. Regarding the attempt of using a maximum amount of digestate and of reducing the necessity of diluting the process liquid with ASW, process liquid C with a mixing ratio of 1:1 appeared promising for a large-scale land-based cultivation of marine macroalgae. However, the transfer of the data to other agricultural biogas plants needs to be adapted to the respective case as changing feeding strategies of the biogas fermenters with different biogas substrates might lead to a large variation of nutrients in the biogas digestate resulting in changing quality and quantity of nutrients in the produced process liquids.