Cultivation of Filtered Anaerobically Digested Sewage Sludge under Light Illumination for Improvement of Fertilizer Quality

The anaerobic co-digestion of sewage sludge and various types of organic waste produces digestate, which can be used directly as fertilizer. The cultivation of photosynthetic bacteria with digestate is focused on to improve the fertilizer quality. After eight months of enrichment with artificial culture media from a seed of sewage sludge, the accumulation of Rhodospirillaceae, which are known to be photosynthetic bacteria, was observed, and the detection ratio to the total number of reads was 55% by sequencing. Filtered digestate liquor from the anaerobic digestion of sewage sludge was continuously cultivated under light illumination, with the seeding of the enrichment culture only at the beginning. The cultivated liquor had a higher pufM gene copy number, which is an indicator of photosynthetic bacteria. Pot experiments showed that grown komatsuna (Brassica rapa var. perviridis) had significantly higher (t-test, p < 0.05) fresh weight and ratios of chlorophyll and carotenoid. These results confirmed that the cultivation of filtered digestate liquor under light illumination improves the fertilizer quality. On the other hand, the cultivation of digestate by the anaerobic digestion of sewage sludge under light illumination with the seeding of the enrichment culture only at the beginning resulted in no significant difference in pot experiments. This showed that the cultivation of filtered digestate liquor under light illumination is practical.