We have been studying the effective use of aquatic plants, which have overgrown the southern basin of Lake Biwa, as biomass energy through methane fermentation, and application of the resulting liquid residue (digestate) as a nutrient solution in hydroponics. This study followed our earlier work and aimed to investigate the fertilizer properties of digestate obtained from anaerobic co-digestion of aquatic plants with vegetable waste from farmers and food waste. Digestate with or without nitrifying treatment (nitrified digestate and non-nitrified digestate, respectively) was prepared for the hydroponic experiment of leaf lettuce and Japanese mustard spinach. Since aquatic plants contain a large amount of Mn, Mn also residues in the digestate at excessive concentration to induce physiological disorders in plants, but co-fermentation reduced the Mn concentration in the digestate. Thus, leaf lettuce plants did not exhibit any Mn toxicity symptoms when cultivated hydroponically using non-nitrified digestate and nitrified digestate diluent, although Japanese mustard spinach plants did exhibit such symptoms. Leaf lettuce plants fed with nitrified digestate grew significantly better than plants fed with a common chemical nutrient solution. Furthermore, when grown with non-nitrified digestate in conditions that prevented a reduction in the pH of the nutrient solution, the nitrate content of the leaves decreased remarkably without decreasing plant growth, as compared with control plants. This showed that the digestate obtained from anaerobic co-digestion of aquatic plants has minimal risk in causing Mn toxicity symptoms and may be used as nutrient solution in hydroponic cultivation of leaf lettuce, with the possibility of improving the quality by using the non-nitrified digestate. However, we also found that the Mn concentration in the digestate should be reduced further if many plant species are to be cultivated using such a digestate, because Mn toxicity symptoms were still observed in Japanese mustard spinach.

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