Characteristics of dissolved organic nitrogen in effluent from a biological nitrogen removal process using sludge alkaline fermentation liquid as an external carbon source.

The addition of sludge alkaline fermentation liquid (SAFL) to biological nutrient removal (BNR) processes has been widely shown to reduce the dissolved inorganic nitrogen (DIN) in the effluent. However, knowledge regarding the effect of using SAFL in a BNR as an additional carbon source on the characteristics of dissolved organic nitrogen (DON) in the effluent is limited. This study investigated the effect of SAFL addition on effluent DON features and microbial community dynamics in a BNR process treating municipal wastewater. The performance of SAFL was compared with other two reactors (i.e., without an external carbon source and with the addition of sodium acetate). The results showed that the addition of SAFL can significantly reduce effluent DON (p < 0.05). The effluent DON was slightly higher with SAFL than with sodium acetate, but the bioavailable DON of the two reactors was similar (1.06 ± 0.11 vs 1.04 ± 0.12 mg/L, respectively, p > 0.05). The SAFL addition led to a decreased percentage of low molecular weight DON (p < 0.05) as well as an increased ratio of fulvic-like and humic-like substances to proteins-like substances. Moreover, the SAFL addition resulted in a lower percentage of substances resembling proteins/amino sugars and a higher percentage of lignin-like molecules than sodium acetate. These features accounted for the low DON bioavailability. The SAFL promoted the increased abundance of Bacteroidetes, Chloroflexi, Comamonadaceae and Rhodocyclaceae, which could be associated with the decreased effluent DON and its bioavailability. This study indicates that using SAFL as a BNR carbon source not only improves the removal of DIN but also reduces effluent DON and specifically DON bioavailability. These results facilitate the acquisition of comprehensive knowledge regarding the use of SAFL as an alternative external carbon source in the BNR process.