The effectiveness of divalent cation addition for highly saline activated sludge cultures: Influence of monovalent/divalent ratio and specific cations.

Saline wastewaters are prevalent in various industries and pose challenges to stable biological treatment. Increasing monovalent cation concentrations are commonly reported to deteriorate treatment and settling performance, while divalent cations can enhance flocculation and settling. However, many previous studies were performed at relatively low salinities and reports conflict on whether concentrations of monovalent cations, divalent cations, or their ratio (M/D) are most critical. This study investigates whether addition of divalent cations shows the same benefits at high salinity (∼40 g NaCl.L-1) and whether divalent ion concentration or M/D is a better predictor of enhancement. Nine sequencing batch reactors were operated at 0.8 M NaCl or KCl monovalent salt concentration, and the concentration of divalent cations (Ca2+ and Mg2+) was varied. M/D was found to be the critical factor that consistently influenced sludge characteristics. It was particularly important in describing hydrophobicity, sludge volume index (SVI) and specific oxygen uptake rate (SOUR), with rpartial of -0.879, 0.971 and 0.966 respectively in models that had an r2adj greater than 0.93. Lower M/D also increased biomass concentrations and reduced extracellular polysaccharides, the latter which in turn correlated strongly with many shape and surface charge measures. The specific monovalent salt (Na+ or K+) influenced treatment performance, biomass concentrations, hydrophobicity, SOUR, extracellular protein and SVI. The specific divalent cation was only important in describing SVI, where Mg2+ was beneficial. Overall, this study shows that addition of divalent cations can greatly benefit high salinity activated sludge systems by improving the sludge structure, settling and organic removal.

Shock effects of monovalent cationic salts on seawater cultivated granular sludge.

Saline wastewater is commonly encountered in various industries, posing challenges to biological treatments. The application of seawater as a seed source provides a media of diverse halophilic organisms for rapid startup. However, effects of transitioning from a mixed salt source to monovalent salt solutions prevalent in industry remains unexplored. Hence, seed sludge was cultivated using seawater and later granulated under a mixed-salt synthetic medium comprising a mixture of NaCl, KCl and Na2SO4 at a combined concentration of 0.8 M (0.27 M each). The stable, acclimated granules were then tested against single salt media of 0.8 M NaCl, KCl, or Na2SO4. Shift to single salt media resulted in granule disaggregation, poor settling, sludge washout and development of fluffy or slimy flocs. Changes in exopolysaccharides composition after the single salt shift was the predominant reason for the large changes in sludge morphology. The impacts of KCl and Na2SO4 were more significant than the shift to NaCl. The resulting impacts also had a major influence on the treatment performance. A complex mechanism involving monovalent cation stimulation of proteins; ionic strength impacts on exopolysaccharides and morphology; solution density influence on sludge density and settling; and tonicity impacts on cell viability and treatment is described.

Recent advances in dissimilatory sulfate reduction: From metabolic study to application.

Sulfate-reducing bacteria (SRB) are a group of diverse anaerobic microorganisms omnipresent in natural habitats and engineered environments that use sulfur compounds as the electron acceptor for energy metabolism. Dissimilatory sulfate reduction (DSR)-based techniques mediated by SRB have been utilized in many sulfate-containing wastewater treatment systems worldwide, particularly for acid mine drainage, groundwater, sewage and industrial wastewater remediation. However, DSR processes are often operated suboptimally and disturbances are common in practical application. To improve the efficiency and robustness of SRB-based processes, it is necessary to study SRB metabolism and operational conditions. In this review, the mechanisms of DSR processes are reviewed and discussed focusing on intracellular and extracellular electron transfer with different electron donors (hydrogen, organics, methane and electrodes). Based on the understanding of the metabolism of SRB, responses of SRB to environmental stress (pH-, temperature-, and salinity-related stress) are summarized at the species and community levels. Application in these stressed conditions is discussed and future research is proposed. The feasibility of recovering energy and resources such as biohydrogen, hydrocarbons, polyhydroxyalkanoates, magnetite and metal sulfides through the use of SRB were investigated but some long-standing questions remain unanswered. Linking the existing scientific understanding and observations to practical application is the challenge as always for promotion of SRB-based techniques.

Denitrifying sulfur conversion-associated EBPR: Effects of temperature and carbon source on anaerobic metabolism and performance.

The recently developed Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process has demonstrated simultaneous removal of organics, nitrogen and phosphorus with minimal sludge production in the treatment of saline/brackish wastewater. Its performance, however, is sensitive to operating and environmental conditions. In this study, the effects of temperature (20, 25, 30 and 35 °C) and the ratio of influent acetate to propionate (100-0, 75-25, 50-50, 25-75 and 0-100%) on anaerobic metabolism were investigated, and their optimal values/controls for performance optimization were identified. A mature DS-EBPR sludge enriched with approximately 30% sulfate-reducing bacteria (SRB) and 33% sulfide-oxidizing bacteria (SOB) was used in this study. The anaerobic stoichiometry of this process was insensitive to temperature or changes in the carbon source. However, an increase in temperature from 20 to 35 °C accelerated the kinetic reactions of the functional bacteria (i.e. SRB and SOB) and raised the energy requirement for their anaerobic maintenance, while a moderate temperature (25-30 °C) resulted in better P removal (≥93%, 18.6 mg P/L removal from total 20 mg P/L in the influent) with a maximum sulfur conversion of approximately 16 mg S/L. These results indicate that the functional bacteria are likely to be mesophilic. When a mixed carbon source (75-25 and 50-50% acetate to propionate ratios) was supplied, DS-EBPR achieved a stable P removal (≥89%, 17.8 mg P/L for 400 mg COD/L in the influent) with sulfur conversions at around 23 mg S/L, suggesting the functional bacteria could effectively adapt to changes in acetate or propionate as the carbon source. The optimal temperatures or carbon source conditions maximized the functional bacteria competition against glycogen-accumulating organisms by favoring their activity and synergy. Therefore, the DS-EBPR process can be optimized by setting the temperature in the appropriate range (25-30 °C) and/or manipulating influent carbon sources.

Elucidating the stimulatory and inhibitory effects of dissolved sulfide on sulfur-oxidizing bacteria (SOB) driven autotrophic denitrification.

Autotrophic denitrification has been widely studied for odor mitigation, corrosion control and nitrogen removal in recent years. This paper examines the response of sulfur-oxidizing bacteria (SOB) driven autotrophic denitrification under short-term stress of dissolved sulfide. A series of batch tests were conducted to investigate the effect of different sulfide concentrations (0-1600 mg-total dissolved sulfide (TDS)/L) on autotrophic denitrification and sulfide oxidation by SOB-enriched sludge. Our results show that autotrophic denitrification (NO3- to N2) was stimulated up to 200 mg-TDS/L with a maximum denitrification rate of 9.4 mg-N/g-volatile suspended solids (VSS)/h, and the nitrite reduction was a rate limiting step. When sulfide concentration was higher than 200 mg-TDS/L, it inhibited nitrate reductase, and nitrate reduction became the rate limiting step according to Edwards and Aiba inhibition models. Sulfide oxidation, however, was not inhibited and the maximum rate of 100.3 mg-TDS/g-VSS/h was obtained at sulfide concentration of 1000 mg-TDS/L. It is important to point out that the transient inhibition on autotrophic denitrification caused by high sulfide stress was resilient and non-lethal with no significant changes in cell viability even under sulfide concentration of 1000 mg-TDS/L. This study reveals the stimulatory and inhibitory effects of dissolved sulfide on SOB driven autotrophic denitrification and its possible underlying mechanism with discussion on engineering implications.

Denitrifying sulfur conversion-associated EBPR: The effect of pH on anaerobic metabolism and performance.

The performance of the denitrifying sulfur conversion-associated enhanced biological phosphorus removal (DS-EBPR) process tends to be unstable and requires further study and development. This in turn requires extensive study of the anaerobic metabolism in terms of its stoichiometry and kinetics. This study evaluates the corresponding responses of DS-EBPR to pH, as it significantly influences both stoichiometry and biochemical kinetics. The impacts of five representative pH values ranging between 6.5 and 8.5 on the anaerobic metabolism were investigated, followed by identification of the optimal pH for performance optimization. A mature DS-EBPR sludge was used in the study, enriched with approximately 30% sulfate-reducing bacteria (SRB) and 33% sulfide-oxidizing bacteria (SOB). Through a series of batch tests, the optimal pH range was determined as 7.0-7.5. In this pH range, the anaerobic stoichiometry of phosphorus released/volatile fatty acid (VFA) uptake ratio, sulfate reduction, and internal polymer production (including poly-ß-hydroxyalkanoates and polysulfide and/or elemental sulfur) all increased along with the anaerobic kinetics of the VFA uptake ratio. Consequently, phosphorus removal was maximized at this pH range (≥95% vs. 84-93% at other pH values), as was sulfur conversion (16 mg S/L vs. 10-13 mg S/L). This pH range therefore favors the activity and synergy of the key functional bacteria (i.e. SRB and SOB). Anaerobic maintenance tests showed these bacteria required 38-61% less energy for maintenance than that reported for GAOs regardless of pH changes, improving their ability to cope with anaerobic starvation. Adversely, both bacteria showed much lower VFA uptake rates than that of GAOs at all tested pH values (0.03-0.06 vs. 0.2-0.24 mol-C/C-mol biomass/h), possibly revealing the primary cause of frequent instability in the DS-EBPR process.

Alkaline textile wastewater biotreatment: A sulfate-reducing granular sludge based lab-scale study.

In this study the feasibility of treating dyeing wastewater with sulfate reducing granular sludge was explored, focusing on decolorization/degradation of azo dye (Procion Red HE-7B) and the performance of microbial consortia under alkaline conditions (pH=11). Efficiency of HE-7B degradation was influenced strongly by the chemical oxygen demand (COD) concentration which was examined in the range of 500-3000mg/L. COD removal efficiency was reduced at high COD concentration, while specific removal rate was enhanced to 17.5 mg-COD/gVSSh-1. HE-7B removal was also improved at higher organic strength with more than 90% removal efficiency and a first-rate removal constant of 5.57h-1 for dye degradation. Three dye-degradation metabolites were identified by HPLC-MS. The granular structure provided enhanced removal performance for HE-7B and COD in comparison to a near-identical floc SRB system and the key functional organisms were identified by high throughput sequencing. This study demonstrates an example of a niche application where SRB granules can be applied for high efficient and cost-effective treatment of a wastewater under adverse environmental conditions.

Granulation of susceptible sludge under carbon deficient conditions: A case of denitrifying sulfur conversion-associated EBPR process.

Sludge granulation has been recognized as a promising biotechnology in wastewater treatment. Whereas the granulation of susceptible sludge in particular with a very low organic loading rate (OLR) (≤0.6 kg COD/m(3)/day or ≤ 120 mg COD/g VSS/day) is a difficult task that has not been achieved in activated sludge systems yet. This study was aimed at exploring an effective strategy for sludge granulation in the recently developed Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) process using a sequencing batch pump-lift reactor. Four strategies were studied by manipulating the factors of organic loading rate (OLR), superficial upflow velocity and sludge settling time individually or collectively. Increasing both the OLR and the superficial upflow velocity effectively promoted granule formation but at the same time led to unstable and even deteriorated reactor performance. The development of granules proceeded via several stages: formation, dispersion, reformation and stabilization. Gradually increasing the superficial upflow velocity from 5.1 to 6.8 m/h and keeping the OLR at 112.4 mg COD/g VSS/day proved to be most effective strategy for accelerating granulation while simultaneously achieving stable reactor performance. Under these conditions, the granules became stable with a diameter of 375-400 µm and displayed excellent settleability. The two major microbial groups, sulfate-reducing bacteria and sulfide-oxidizing bacteria, in the microbial community of the DS-EBPR granular sludge were enriched to 17.7% and 15.8% respectively. The newly developed DS-EBPR granular system was able to achieve an almost threefold improvement in phosphorus removal efficiency and 25% reduction in the operating cycle time compared with a flocculent DS-EBPR system.

Functional bacteria and process metabolism of the Denitrifying Sulfur conversion-associated Enhanced Biological Phosphorus Removal (DS-EBPR) system: An investigation by operating the system from deterioration to restoration.

A sulfur conversion-associated Enhanced Biological Phosphorus (P) Removal (EBPR) system is being developed to cater for the increasing needs to treat saline/brackish wastewater resulting from seawater intrusion into groundwater and sewers and frequent use of sulfate coagulants during drinking water treatment, as well as to meet the demand for eutrophication control in warm climate regions. However, the major functional bacteria and metabolism in this emerging biological nutrient removal system are still poorly understood. This study was thus designed to explore the functional microbes and metabolism in this new EBPR system by manipulating the deterioration, failure and restoration of a lab-scale system. This was achieved by changing the mixed liquor suspended solids (MLSS) concentration to monitor and evaluate the relationships among sulfur conversion (including sulfate reduction and sulfate production), P removal, variation in microbial community structures, and stoichiometric parameters. The results show that the stable Denitrifying Sulfur conversion-associated EBPR (DS-EBPR) system was enriched by sulfate-reducing bacteria (SRB) and sulfide-oxidizing bacteria (SOB). These bacteria synergistically participated in this new EBPR process, thereby inducing an appropriate level of sulfur conversion crucial for achieving a stable DS-EBPR performance, i.e. maintaining sulfur conversion intensity at 15-40 mg S/L, corresponding to an optimal sludge concentration of 6.5 g/L. This range of sulfur conversion favors microbial community competition and various energy flows from internal polymers (i.e. polysulfide or elemental sulfur (poly-S(2-)/S(0)) and poly-ß-hydroxyalkanoates (PHA)) for P removal. If this range was exceeded, the system might deteriorate or even fail due to enrichment of glycogen-accumulating organisms (GAOs). Four methods of restoring the failed system were investigated: increasing the sludge concentration, lowering the salinity or doubling the COD loading, non of which restored SRB and SOB activities for DS-EBPR; only the final novel approach of adding 25 ± 5 mg S/L of external sulfide into the reactor at the beginning of the anoxic phase could efficiently restore the DS-EBPR system from failure. The present study represents a step towards understanding the DS-EBPR metabolism and provides an effective remedial measure for recovering a deteriorating or failed DS-EBPR system.