A novel Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process has recently been developed to treat saline/brackish wastewater. This process was shown to simultaneously remove organics, nitrogen and phosphorous with minimal sludge production. However, changes in environmental factors or operating conditions can cause instability in the system. In this study the major functional bacteria and their metabolism was investigated; sludge granulation was tested as a means of improving the process performance; and the effects of pH, temperature and carbon source on anaerobic metabolism and performance of this process were investigated to optimize the process’ operating conditions.

The functional bacteria and their metabolism were explored by m...[ Read more ]

A novel Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process has recently been developed to treat saline/brackish wastewater. This process was shown to simultaneously remove organics, nitrogen and phosphorous with minimal sludge production. However, changes in environmental factors or operating conditions can cause instability in the system. In this study the major functional bacteria and their metabolism was investigated; sludge granulation was tested as a means of improving the process performance; and the effects of pH, temperature and carbon source on anaerobic metabolism and performance of this process were investigated to optimize the process’ operating conditions.

The functional bacteria and their metabolism were explored by manipulating the conditions associated with their deterioration, failure and restoration. This involved changing the concentration of mixed liquor suspended solids and monitoring the relationships relating sulfate reduction and production with phosphorus removal, changes in microbial community structures and reaction stoichiometry. The major functional bacteria, i.e. sulfate-reducing and sulfide-oxidizing bacteria, were shown to both participate synergistically in the process and to be crucial for maintaining sulfur conversion at 15–40 mg S/L, corresponding to an optimal sludge concentration of 6.5g/L. Such sulfur conversion favored microbial community competition against glycogen accumulating organisms and various energy flows from internal polymers (i.e. polysulfide or elemental sulfur [poly-S^2-/S⁰] and poly-β-hydroxyalkanoates [PHA]) for phosphorus removal. Adding 25 ± 5 mg S/L of external sulfide at the beginning of the process’ anoxic phase was found efficiently to restore the process after failure.

Granular sludge gives superior biomass retention in wastewater treatment, better withstanding high-concentration wastewater and shock loadings. An effective strategy for the DS-EBPR sludge granulation under low organic loading conditions was sought by manipulating the organic loading rate, the superficial upflow velocity and the sludge settling time individually and together in a sequencing batch pump-lift reactor. Greater organic loading rate and superficial upflow velocity both promoted granules formation, but they simultaneously led to unstable and even degraded reactor performance. Gradually increasing the superficial upflow velocity (from 5.1 to 6.8 m/h) while keeping the loading rate at 112.4 mg COD/g volatile suspended solid/day was found to be the most effective technique for promoting granulation while maintaining stable reactor performance. The resulting 375–400 μm granules settled well and the sulfate-reducing and sulfide-oxidizing bacteria were enriched to 17.7% and 15.8%, respectively with granular sludge. An almost three-fold improvement in the P removal efficiency and a 25% cycle time reduction were observed compared with flocculent sludge.

The effects of representative pH (6.5, 7.0, 7.5, 8.0 and 8.5), temperatures (20, 25, 30 and 35 ℃) and influent acetate to propionate ratios (100–0, 75–25, 50–50, 25–75 and 0–100%, fractions of total organic carbon) on the anaerobic metabolism and performance were also investigated. A neutral pH (7.0–7.5), relatively warm conditions (25–30 ℃) and a mixture of carbon sources (75–25 and 50–50% of acetate to propionate ratios) were found to best support the anaerobic stoichiometry and kinetics conversion of the functional bacteria (i.e. sulfate-reducing and sulfide-oxidizing bacteria), thus maximizing their ability in competition against glycogen accumulating organisms. Consequently, sulfur driven-phosphorus removal was enhanced. The bacteria required 38–82% less energy for their maintenance under these conditions than competing glycogen-accumulating organisms, which allows them to cope more effectively with anaerobic starvation. Adversely, they showed much lower volatile fatty acids uptake rates than that of glycogen-accumulating organisms, which may explain the instability frequently observed in the operation of DS-EBPR systems.