Algae biofilm emerges as an attractive alternative to activated sludge for wastewater treatment because of its advantages in nutrient recovery, energy-saving, and value-added biomass production. Since regular harvest of the biomass is needed, the harvest interval can be an important operating parameter; however, it is seldom optimized, and its effects on algae biofilm remain largely unexplored. In this study, we investigate the nutrient removal patterns and microbiomes in algae biofilm reactors under three different harvest intervals (7, 14, and 28 days). Under longer harvest intervals, the biofilm accumulates higher amounts of biomass and higher proportions of cyanobacteria. Nevertheless, the dispersion of cyanobacteria into the plankton is reduced by up to 99%. High throughput sequencing shows that Leptolyngbya proliferates over Limnothrix under longer harvest intervals, which can limit the latter's escape from the biofilm. Besides, nitrogen removal is promoted by the enhanced ammonia assimilation under longer harvest intervals, with the total nitrogen removal rate increasing from 80% to 94%. In contrast, the nitrification and denitrification activities are inhibited because nitrifiers (Nitrosomonas and Nitrospira) and denitrifiers (e.g., Zoogloea) tend to be washed out under longer harvest intervals. This is also confirmed by the reduced production of corresponding enzymes. Ecological modeling indicates that deterministic processes dominate the community assembly in both biofilms and planktons, underpinning the importance of the harvest interval in manipulating the algal-bacterial symbiosis. Our study suggests that the future optimization of algae biofilm should take the harvest interval into consideration and recommends a longer harvest interval for wastewater treatment.

Extending algae biofilm's harvest interval promotes nitrogen recovery and attenuates cyanobacteria dispersion

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