  Harvesting organic matter from wastewater is widely applied to maximize
energy recovery; however, it limits the applicability of secondary treatment
for acceptable effluent discharge into surface water bodies. To turn this
bottleneck issue into an opportunity, this study developed oxygen-induced
thiosulfatE production duRing sulfATe reductiOn (EARTO) to provide an efficient
electron donor for wastewater treatment. Typical pretreated wastewater was
synthesized with chemical oxygen demand of 110 mg/L, sulfate of 50 mg S/L, and
varying dissolved oxygen (DO) and was fed into a moving-bed biofilm reactor
(MBBR). The MBBR was operated continuously with a short hydraulic retention
time of 40 min for 349 days. The formation rate of thiosulfate reached
0.12-0.18 g S/(m2.d) with a high produced thiosulfate-S/TdS-S ratio of 38-73%
when influent DO was 2.7-3.6 mg/L. The sludge yield was 0.23-0.29 gVSS/gCOD,
much lower than it was in conventional activated sludge processes. Then, batch
tests and metabolism analysis were conducted to confirm the oxygen effect on
thiosulfate formation, characterize the roles of sulfate and microbial
activities, and explore the mechanism of oxygen-induced thiosulfate formation
in ERATO. Results examined that oxygen supply promoted the
thiosulfate-Sproduced/TdS-Sproduced ratio from 4% to 24-26%, demonstrated that
sulfate and microbial activities were critical for thiosulfate production, and
indicated that oxygen induces thiosulfate formation through two pathways: 1)
direct sulfide oxidation, and 2) indirect sulfide oxidation, sulfide is first
oxidized to S0 (dominant) which then reacts with sulfite derived from
oxygen-regulated biological sulfate reduction. The proposed compact ERATO
process, featuring high thiosulfate production and low sludge production,
supports space- and energy-efficient secondary wastewater treatment.
