In-situ recovery of elemental sulfur (S0) and maintenance of functional microbial activity via polysulfide conversion in DSSADN sludge

Abstract

Double short-cut sulfur autotrophic denitrification (DSSADN) with elemental sulfur (S0) and NO2−-N accumulation not only enhances anaerobic ammonium oxidation (Anammox) for nitrogen removal but also reduces SO42− pollution in sulfide-driven denitrification. However, the accumulation of microbially produced S0 in sludge leads to biomineralization and reduces system stability. This study investigated the key operational parameters and underlying mechanisms for sludge S0 extraction based on polysulfide transformation characteristics, aiming to prevent sludge mineralization and achieve stable DSSADN performance. The results showed that S0 accumulation reached 48% within the DSSADN reactor, decreasing denitrification performance by 47%. Efficient S0 extraction and maintained biological activity were accomplished under the conditions of pH 10–11, 2 h extraction duration, 300 mg/L S2− concentration, and 2–3 g/L mixed liquor suspended solids concentration. After online extraction with S2−, the S0 accumulation ratio in the DSSADN reactor sludge decreased to below 7%. Nitrate and sulfide conversion rates remained stable at 1.3 and 2.8 kg/(m3·d), respectively. The extraction process reduced the S0 levels both inside and outside the cells, ensuring the dominance of Thiobacillus. The abundance of core functional genes, including sulfur oxidation genes (sqr, fccB) and nitrate reduction genes (narG/narZ), was suppressed by S0 accumulation, but high relative abundance again after S0 extraction, while the nitrite over-reducing gene (nirK) maintained low abundance. This study provides a novel sulfur recovery strategy for the stable operation of coupled DSSADN and PN/Anammox processes, which is conducive to promoting the engineering application of sulfur autotrophic denitrification technology and sulfur resource recovery.