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Graphical abstract Highlights • Sophorolipid enhanced the SCFA production from anaerobic fermentation of WAS. • Sophorolipid facilitated the WAS solubilization. • Sophorolipid enhanced release of biodegradable organics. • Inhibition of methanogens by sophorolipid led to accumulation of SCFA. • Sophorolipid increased the abundance of hydrolytic microbes and SCFA producers. Abstract It was found in this study that the presence of sophorolipid (SL) enhanced the production of short-chain fatty acid (SCFA) from anaerobic fermentation of waste activated sludge (WAS). Experimental results showed that with an increase of SL addition from 0 to 0.1 g/g TSS, the maximal SCFA yield increased from 50.5 ± 4.9 to 246.2 ± 7.5 mg COD/g VSS. The presence of SL reduced the surface tension between hydrophobic organics and fermentation liquid, which thereby accelerated the disintegration of WAS and improved the biodegradability of the released organics. SL promoted the carbon/nitrogen ratio of the fermentation system, enhancing the conversion of proteins in WAS. Moreover, SL suppressed severely the activities of methanogens, probably due to the drop of pH caused by SL addition. Amplicon sequencing analyses revealed that SL increased the abundance of hydrolytic microbes such as Bacteroides sp. and Macellibacteroides sp., and SCFA producers (e.g., Acinetobacter sp.). [ABSTRACT FROM AUTHOR]

[Display omitted] • Short-term FA and FNA treatment on FN sludge instantly reduce N 2 O at all DO levels. • The dependency of N 2 OR on DO was altered by exposing FN sludge to high FNA. • Short-term exposure of PN sludge to high FNA abolished N 2 O production. • FNA treatment on sludge led to 3.5% more cost, while FA treatment saved 2% cost. The potentials of using endogenous free ammonia (FA) and free nitrous acid (FNA) as nitrous oxide (N 2 O) mitigators were investigated in treatment of both mainstream and sidestream wastewater. Although the N 2 O emission factor of a sidestream partial-nitritation (PN) reactor (averaged 1.70 % ± 0.39 %, n = 30) was about 2.4 times higher than a mainstream full-nitrification (FN) reactor (averaged 0.72 % ± 0.24 %, n = 30) (P < 0.01), one-hour exposure of PN sludge to 1.5 mg HNO 2 -N/L FNA could virtually abolish N 2 O emission. As for FN sludge, both 45 mg NH 3 -N/L FA and 0.015 mg HNO 2 -N/L FNA successfully mitigated N 2 O production at varying dissolved oxygen (DO) levels (50 % vs 61 %), while 1.5 mg HNO 2 -N/L FNA not only reduced more N 2 O (92 %) but also altered the N 2 O dependency on DO. Both FNA and FA sludge treatment were effective N 2 O mitigation strategies with FNA toward the end of carbon neutrality and FA being more economically appealing (2 % cost saving). [ABSTRACT FROM AUTHOR]