Microbial response and spatial profiling of nitrite-producing denitrification granular sludge in-situ exposed to high nitrite stress

Abstract

Nitrite-producing denitrification granular sludge (NPG) is emerging as a novel approach to employing anammox for cost-efficient nitrogen removal from high-strength nitrate-containing wastewater. However, the underlying impact of increasing stress of nitrite (NO2–-N) accumulation on microbial metabolism and cooperation of denitrifying bacteria and heterotrophs in NPG was still not clear. This study first demonstrated a high-rate NPG system with elevating nitrate (NO3–-N) from 52.4 mg/L to 613.2 mg/L. During this operation, a rapid increase in granular size from 322.8 µm to 1124.0 µm was observed with a relatively stable nitrate-to-nitrite transformation ratio (NTR) as high as 83.0 %, and effluent NO2–-N of increasing from 16.5 mg/L to 331.9 mg/L. Such an increasing NO2–-N stress improved the capability of the TCA cycle with rising intermediates of amino acid metabolism. This further stimulated the production of exocellular organic matter that replenished as electron donors and promoted the growth of granular size. The homogeneously spatial distribution of key bacteria (Thauera) surrounding the NPG with a calcium-dominated inorganic core inside was revealed to play a key role in enhanced substrate transfer capability. Genes nar and nap encoding NO3–-N reductase dominated by key species belonging to Thauera showed much higher abundance than that encoding NO2–-N reductase, which was the key reason for relatively stable NO2–-N accumulation. It illustrated the excellent persistence of NPG to high NO2–-N stress with special structure of functional bacteria inside granules, providing a great potential for the treatment of high-strength nitrate-containing wastewater.