Effect of suspended solids from anaerobic digested wastewater on performance and microbial community of autotrophic nitrogen removal process

Most anaerobically digested wastewaters have high concentrations of suspended solids, while little is known about whether they adversely affect the performance of the autotrophic nitrogen removal process. In this work, using air-lift internal circulation reactor we studied the effect of suspended solids on the autotrophic nitrogen removal process by adding 600–900 mg/L suspended solids from digested swine wastewater to the influent (R–SS). The results showed that during the formation of autotrophic nitrogen removal granular sludge, suspended solids containing Ca, Mg, Cu P, and Pb can enter the granular sludge, occupy the ecological niche of anammox bacteria, lead to loose internal structure, and damage the internal anoxic environment. It then reduces the settling performance in the R–SS, causes heavy wash-out of granular sludge from the R–SS reactor (2.5 times more than the control, R–CK), and finally reduces the maximum nitrogen removal rate of the autotrophic nitrogen removal process from 5.16 Kg N/m3/d to 4.31 Kg N/m3/d. Meanwhile, the metagenomic analysis showed that suspended solids reduce the relative abundance of reads from key nitrogen genes of anammox bacteria yet increases reads from both nitrite-oxidizing and denitrifying bacteria, especially nir and nor genes. Further analysis of 16 highly abundant genomes assembled via metagenomics showed that suspended solids decrease the relative abundance of Kuenenia_seed103, an anammox bacterium, from 13.36% (R–CK) to 11.03% (R–SS). Since suspended solids can act as a carbon source for denitrifiers to compete with nitrite or nitric oxide with anammox bacteria, the denitrifier community has progressed from a simple denitrifier consisting of only one or two denitrification steps (nar or nos) that acts as an ammonia and nitrite sink to a complete one that directly reduces nitrite to nitrogen gas. This study will lay a good foundation for its application in wastewater treatment.