Intracellularly stored polysulfur maintains homeostasis of pH and provides bioenergy for phosphorus metabolism in the sulfur-associated enhanced biological phosphorus removal (SEBPR) process.

Sulfur-associated enhanced biological phosphorus removal has recently been developed for the removal of biological nutrients. In this new bioprocess, the polymeric sulfur compound (poly-S) is crucial to connecting sulfur conversions and polyphosphate accumulation; however, its mechanisms are still elusive. This study investigated the role of poly-S in maintaining the system stability by operating a lab-scale reactor for 720 d and conducting batch experiments with various initial pH values. The main findings were as follows: i) intracellular poly-S increased from 30 to 95 mg S (g VSS)-1, whereas polyhydroxyalkanoates increased from 8 to 22 mg C (g VSS)-1; ii) glycogen increased from 7.5 to 12.5 mg C (g VSS)-1 during the first 520 d before decreasing; and 3) P removal could be maintained at 8-12.5 mg P (L)-1. The decrease in glycogen was likely because the accumulation of enough poly-S could replace glycogen to provide reducing power and buffer the inner pH. The results of batch tests confirmed that poly-S could adjust the intracellular protons under anaerobic conditions (pH always returned to neutral or neutral levels at the end of anaerobic phase) and provide cellular bioenergy (adenosine triphosphate, for P uptake, thereby maintaining net P removal). The predominant microbial communities were facultative denitrifying Thauera (11%), sulfide-oxidizing Thiobacillus (8%), and sulfate-reducing Desulfobacter (9%). However, the conventional polyphosphate-accumulating organisms were detected at very low abundance (e.g. Tetrasphaera at only 0.02%). Overall, poly-S could regulate intracellular protons and energy balance and reduce glycogen accumulation, keeping good biological P removal performance.