Full-scale applications of a sulfur-based autotrophic denitrification biofilter for polishing secondary effluent: annual performance, improved winter performance, and microbial community succession

• Annual performance of full-scale SADeN biofilters was reported for the first time. • Dosing thiosulfate against low-temperature stress enhanced 3-fold denitrification. • Thiosulfate rapidly removed DO and amplified the electron flux of S 0 -based filler. • Seasonal variations drove microbial community succession in SADeN biofilters. • Thiosulfate improved autotrophic denitrifiers and widened their econiche in winter. Sulfur-based denitrification (SADeN) technology is a cost-saving and low-carbon alternative for treating organic-deficient wastewater. However, the use of this technology in full-scale applications is still limited. Crucial concerns for biological processes, such as changes in seasonal performance and the ways in which it responds to low temperatures, have yet to be studied in real SADeN processes. Herein, two SADeN biofilters in parallel (downflow, with a design water treatment capacity of 8000 m 3 ·d −1 each) were systematically investigated over a period of more than 400 days. Seasonal variations in denitrification rates were observed, with an average difference of up to 2 times between summer and winter. The use of thiosulfate was verified as an efficient strategy to improve the performance of SADeN biofilters in winter, which was found to have an overstoichiometric enhancement effect (OSEE), with 24.79–331.50 % more nitrate removal than that calculated according to thiosulfate dosages. Analyzing the biofilter vertically revealed that the OSEE occurred because thiosulfate rapidly consumed dissolved oxygen in the upper zone of the bed and activated the electron flux of the sulfur-based reactive filler (SReF) in the lower zone. Microbial community analysis further suggested that this increase in electron flux may be associated with the abundance recovery of sulfur autotrophic denitrifiers and the stability recovery of the microbial eco-networks. This study offers a paradigm for the full-scale application of SADeN technology in real wastewater treatment plants, providing an in-depth understanding of the role of dosing thiosulfate in tackling low-temperature challenges.