Enhanced flocculation of two bioflocculation-producing bacteria by secretion of Philodina erythrophthalma

Bdelloid rotifer are reported to play a promoting role in microbial aggregation and floc formation in activated sludge systems; however, the mechanisms involved in this process are unclear. This study explores the effect of a rotifer secretion (RS) from the species Philodina erythrophthalma on the flocculation and growth of two bioflocculation-producing bacteria isolated from activated sludge. Results show that although the secretion has weak bioflocculability in itself, it can significantly enhance the flocculability of bioflocculation-producing bacteria and promote formation of microbial aggregation and floc. The possible mechanism is that the RS causes an increase in the bacteria densities and extracellular polymeric substance contents. The improvement of flocculability using RS shows an S-curve changing tendency with collection time, and corresponds with the first-order model with secretion dosage. Chemical composition analysis shows that low contents of non-protein organic nitrogen and polysaccharides are found in the RS, which implies that RS acts more like a growth-promoting substance or infochemical than as a nutrient in the promotion of bacterial growth. In conclusion, the findings provide a novel and potential strategy for promoting sludge floc formation using the infochemical secreted by this rotifer.