Effects of simulated extreme precipitation flooding on the degradation of anaerobic digestion effluent by algal-bacterial symbiosis system

Precipitation is directly affected by global warming among different extreme climate types. The moisture-holding capacity of the atmosphere increases by 7% with a temperature increase of 1 degrees C. A higher moisture content results in frequent and violent rainfall events, and the frequency and intensity of heavy rainfall and flooding events in certain countries and regions is gradually aggravated. Simulated extreme precipitation experiments with precipitation intensities of 50, 100, and 200 mm and precipitation time on 3th, 6 th, and 9 th day were set to study the effects of extreme precipitation and flooding events on the degradation performance of open algae-bacteria symbiotic (ABS) system for anaerobic digestion effluent (ADE). The experimental results showed that the precipitation time had an obvious impact on the open algae-bacteria symbiotic system. The daily COD (chemical oxygen demand) degradation rates in the algae-bacteria symbiotic system on the fourth day decreased from 12.64% (unaffected by precipitation) to 2.94, 2.38, and 1.90%, respectively, under the conditions of rainfall on the third day and precipitation intensities of 50, 100, and 200 mm. At the end of the experiments, the COD contents in the ADE increased from 355.09 mg/L (unaffected by precipitation) to 435.77, 413.67, and 403.54 mg/L, respectively. The degradation effects of COD, TN (total nitrogen), and TP (total phosphorus) in the open algae-bacteria symbiotic system were affected by extreme precipitation (200 mm) and flooding. On the third day of the heavy rainfall event, Actinobacteria, Proteobacteria, and Bacteroidetes were the most dominant phyla in the samples of the symbiotic system between algae and bacteria. Among them, the abundance of Proteobacteria and Bacteroidetes was the highest, accounting for 15.33, 21.33 and 19.97%, respectively. As the symbiotic system operated stably, their proportions decreased to 13.58, 12.13, and 17.85% on the ninth day. At the class level, the abundance of Beta-proteobacteria and Gamma-proteobacteria was relatively high in the samples of the symbiotic system. On the third day of the heavy rainfall event, they accounted for 8.99 and 7.11%, respectively. With the stable operation of the system, their proportions showed a decreasing trend, and on the sixth and ninth days, they were 7.03% and 6.01, 4.87 and 4.39%, respectively.