Response of granular anammox process under mainstream conditions to continuous and transient organic loads

Abstract

Even small fluctuations in the amount of organic matter in wastewater significantly affect the structure and function of anammox microbial communities under mainstream conditions with low ammonium concentrations, because a high organic C/N ratio causes heterotrophic denitrifying bacteria to outcompete anammox bacteria. This paper presents a comprehensive investigation of the effects of different levels of continuous and transient organic loads on nitrogen removal and sludge characteristics in upflow granular anammox reactors under mainstream conditions. Continuous organic load at 30 mg chemical oxygen demand (COD)/L influent improved nitrogen removal efficiency without compromising anammox performance. However, exposure to higher levels of continuous organic load (>= 60 mg COD/L) caused significant inhibition of anammox activity (complete inhibition at 150 mg COD/L). The performance degradation was accompanied by the destabilization of anammox granular sludge, with a decrease in tightly bound protein and increase in loosely bound polysaccharide contents in the sludge extracellular polymeric substances. Further, DNA sequencing analysis showed that heterotrophic denitrifying Rhodocyclaceae and Pseudomonadaceae bacteria collectively accounted for 36.8-61.3% of the total bacterial reads. These bacteria emerged and outcompeted anammox Ca. Brocadiaceae (accounting for less than 8.4% of the total reads) with increasing organic load, resulting in a significant decrease in anammox activity. Meanwhile, the inhibitory effect of transient organic load (one-day shock loading) was fully reversed within one day at both 150 and 300 mg COD/L shock levels, although the latter caused a sudden fall in anammox performance. The overall results suggest that loading an appropriately low amount of organic matter can provide a means to improve the nitrogen removal performance of anammox granular sludge without sacrificing anammox activity and granule stability under mainstream conditions.