Impact of uranium on antibiotic resistance in activated sludge.

The emergence of antibiotic resistance genes (ARGs) and antibiotic-resistant bacteria (ARB) in the environment is well established as a human health crisis. The impact of radioactive heavy metals on ecosystems and ultimately on human health has become a global issue, especially for the regions suffering various nuclear activities or accidents. However, whether the radionuclides can affect the fate of antibiotic resistance in bacteria remains poorly understood. Here, the dynamics of ARB, three forms of ARGs-intracellular ARGs (iARGs), adsorbed extracellular ARGs (aeARGs), and free extracellular ARGs (feARGs)-and microbial communities were investigated following exposure to uranium (U), a representative radioactive heavy metal. The results showed that 90-d of U exposure at environmentally relevant concentrations of 0.05 mg/L or 5 mg/L significantly increased the ARB concentration in activated sludge (p < 0.05). Furthermore, 90-d of U exposure slightly elevated the absolute abundance of aeARGs (except tetO) and sulfonamide iARGs, but decreased tetracycline iARGs. Regarding feARGs, the abundance of tetC, tetO, and sul1 decreased after 90-d of U stress, whereas sul2 showed the opposite trend. Partial least-squares path model analysis revealed that the abundance of aeARGs and iARGs under U stress was predominantly driven by increased cell membrane permeability/intI1 abundance and cell membrane permeability/reactive oxygen species concentration, respectively. Conversely, the changes in feARGs abundance depended on the composition of the microbial community and the expression of efflux pumps. Our findings shed light on the variations of ARGs and ARB in activated sludge under U exposure, providing a more comprehensive understanding of antibiotic resistance risks aggravated by radioactive heavy metal-containing wastewater.

Metagenomics insight into the long-term effect of ferrous ions on the mainstream anammox system.

Anaerobic ammonium oxidation (anammox) bacteria have a high requirement for iron for their growth and metabolism. However, it remains unclear whether iron supplementation can sustain the stability of mainstream anammox systems at varying temperatures. Here, we investigated the long-term effects of Fe2+ on the mainstream anammox systems. Our findings revealed that the nitrogen removal efficiency (NRE) of the anammox system supplemented with 5 mg/L Fe2+ decreased from 76.5 ± 0.76% at 35 °C to 39.0 ± 9.9% at 25 °C. Notably, higher dosages of Fe2+ (15 mg/L and 30 mg/L) inhibited the anammox system, resulting in NREs of 15.9 ± 8.1% and 2.5 ± 1.1% at 25 °C, respectively. The results of microbial communities and function profiles suggested that the high Fe2+ dosage seriously affected the iron assimilation and utilization in the mainstream anammox system. This was evident from the decreased abundance of genes associated with Fe(II) transport and uptake, which in turn hindered the biosynthesis of intracellular iron-cofactors, resulting in decrease in the absolute abundance of Candidatus Brocadia, a key anammox bacterium, as well as a decline in NRE. Furthermore, our results showed that the anammox process was more susceptible to iron supplementation at 25 °C compared to 35 °C, which may be due to the oxidative stress reactions induced by combined lowered temperature and a high Fe2+ dosage. Overall, these findings offer a deeper understanding of the effect of iron in mainstream anammox systems, which can contribute to improved stability maintenance and effectiveness of anammox processes.

Screening of phosphorus solubilizing microorganisms in cold environment and their effects on the growth of Brassica napus.

To screen out phosphorus solubilizing strains that can adapt to cold climate in Qinghai Province, Bacillus mucilaginosus, B. megaterium, B. cereus, Streptomyces violovariabilis, S. cinnamofuscus, and S. flavoagglomeratus were screened with solid plate medium as the primary and liquid medium as the secondary screening, with calcium phosphate, lecithin, and phytic acid as the single source of phosphorus. By comprehensively comparing the size of phosphate solubilizing circle in the solid plate medium and the soluble phosphorus content in the liquid medium, three strains of phosphate solubilizing bacteria with good phosphate solubilizing effects were screened, S. violovariabilis, S. cinnamofuscus, and B. mucilaginosus. The three phosphate solubilizing bacteria were made into liquid ino-culants, and the small rapeseed pot experiment was carried out with two soils with different fertilities in a cold climate in September. Compared with the control, plant height, fresh weight, root length, and root weight of rapes in high-fertility cultivated soil increased by 35.5%, 191.0%, 26.2%, and 282.7%, while plant phosphorus absorption, total phosphorus and available phosphorus contents in the rhizosphere soil increased by 968.9%, 5.1%, and 2.1%, respectively. In low-fertility soil, plant height and fresh weight was increased by 45.8% and 61.3%, root length and weight was decreased by 2.6% and 4.4%, while plant phosphorus absorption and the contents of total P and available P in rhizosphere soil were increased by 91.5 %, 29.1%, and 213.7%, respectively. The effect of the other two inoculants treatments was less significant than S. violovariabilis. Therefore, S. violovariabilis was the phosphate solubilizing strain suitable for the cold climate in Qinghai.

Antibiotic-resistant bacteria and antibiotic resistance genes in uranium mine: Distribution and influencing factors.

Both heavy metals and radiation could affect the proliferation and dissemination of emerging antibiotic resistance pollutants. As an environmental medium rich in radioactive metals, the profile of antibiotic resistance in uranium mine remains largely unknown. A uranium mine in Guangdong province, China was selected to investigate the distribution and influencing factors of antibiotic-resistant bacteria (ARB) and antibiotic resistance genes (ARGs) including intracellular ARGs (iARGs), adsorbed-extracellular ARGs (aeARGs), and free extracellular ARGs (feARGs). The result indicated that sulfonamide and tetracycline ARB could be generally detected in mining area with the absolute concentrations of 7.70 × 102-5.18 × 105 colony forming unit/g. The abundances of aeARGs in mine soil were significantly higher than those of iARGs (p < 0.05), highlighting the critical contribution of aeARGs to ARGs spread. The feARGs in mine drainage and its receiving river were abundant (3.38 × 104-1.86 × 107 copies/mL). ARB, aeARGs, and iARGs may correlate with nitrogen species and heavy metals (e.g., U and Mn), and feARGs presented a significant correlation with chemical oxygen demand (p < 0.05). These findings demonstrate the occurrence of ARB and ARGs in uranium mine for the first time, thereby contributing to the assessment and control of the ecological risk of antibiotic resistance in radioactive environments.

Intensified nitrogen removal by endogenous denitrification in a full-scale municipal wastewater treatment plant.

In this study, for the first time, endogenous denitrification (ED) was enhanced in a practical anaerobic-anoxic-oxic-[post-anoxic]-[post-oxic] (AAO-AO) process, contributing to a remarkable increase in the nitrogen removal efficiency (NRE). The long-term operation (203 days) result showed that the NRE was improved by 7% compared to the theoretical maximum NRE (68-70%) of AAO processes, with the effluent total nitrogen (TN) decreasing from 13.7 (1 d) to 6.1 mg/L (203 d). Approximately 99.4% of the influent COD was transformed to poly-ß-hydroxyalkanoates (PHAs) in the anaerobic zone. The synthesized PHAs were consumed in the following zones and the secondary sedimentation tank accompanied by over 32.5% N-loss, indicating that the ED process could be responsible for the enhanced NRE. 16S rRNA amplicon sequencing results further confirmed that denitrifying glycogen-accumulating organisms, which are capable of ED, were enriched with the relative abundance of 2.10%. Our findings provide a novel cost- and energy-efficient strategy to improve nitrogen removal without external carbon additions but by enhancing ED performance.

Advances in sulfur conversion-associated enhanced biological phosphorus removal in sulfate-rich wastewater treatment: A review.

Recently an innovative sulfur conversion-associated enhanced biological phosphorus removal (S-EBPR) process has been developed for treating sulfate-rich wastewater. This process has successfully integrated sulfur (S), carbon (C), nitrogen (N) and P cycles for simultaneous metabolism or removal of C, N and P; moreover this new process relies on the synergy among the slow-growing sulfate-reducing bacteria and sulfur-oxidizing bacteria, hence generating little excess sludge. To elucidate this new process, researchers have investigated the microorganisms proliferated in the system, identified the biochemical pathways and assessed the impact of operational and environmental factors on process performance as well as trials on process optimization. This paper for the first time reviews the recent advances that have been achieved, particularly relating to the areas of S-EBPR microbiology and biochemistry, as well as the effects of environmental factors (e.g., electron donors/acceptors, pH, temperature, etc.). Moreover, future directions for researches and applications are proposed.

Comparison of short-term dosing ferrous ion and nanoscale zero-valent iron for rapid recovery of anammox activity from dissolved oxygen inhibition.

As obligate anaerobes, anammox bacteria are sensitive to oxygen, which might hinder the maximization of anammox activity. However, there are very few effective strategies to rapidly recover anammox activity after its deterioration under exposure of oxygen. In this study, the activity recovery of anammox bacteria encountering dissolved oxygen (DO) exposure (0.2 and 2.0 mg L-1) were compared by three strategies in short-term experiments, nZVI, Fe(II) dosing, and N2 purging. nZVI is more effective in recovering anammox activity with a high DO exposure (2 mg L-1), compared to a low DO exposure (0.2 mg L-1). After inhibiting by 2.0 mg L-1 DO, anammox activity recovery (normalized to the control) was ranked in the order of nZVI (5 mg L-1) addition (63 ±â€¯8.2%) > Fe(II) (5 mg L-1) addition (41 ±â€¯8.0%) >N2 purging (39 ±â€¯4.0%). In contrast to Fe(II) ion additions, the shell structure of nZVI combined with the buffering effect of biomass-extracellular polysaccharide (EPS) prevented the sharp pH variation and excessive dissolved Fe(II)/Fe(III) in solution. Under such circumstances, nZVI addition (5 and 25 mg L-1) increased the intracellular reactive oxygen species (ROS) to a moderate level (<200%), which might be responsible for the better activity recovery of anammox than that of Fe(II) addition and N2 purging. Specifically, 5 mg L-1 nZVI dosage moderately enhanced the intracellular O2- production (∼150% of the control) after scavenging 2.0 mg L-1 DO, and the anammox activity recovered better than that of both 5 and 25 mg L-1 Fe(II) ions additions. However, high dosage nZVI (75 mg L-1) inhibited anammox activity in spite of low or high DO exposure. Our findings elucidate that appropriate amount of nZVI (short-term dosing) can rapidly recover anammox activity when anammox bacteria encountering oxygen exposure accidentally and could be useful in facilitating the robust operation of anammox-based processes.

Identification of the function of extracellular polymeric substances (EPS) in denitrifying phosphorus removal sludge in the presence of copper ion.

Industrial wastewater containing heavy metals that enters municipal wastewater treatment plants inevitably has a toxic impact on biological treatment processes. In this study, the impact of Cu(II) (0, 1.5, 2, 2.5, 3 mg/L) on the performance of denitrifying phosphorus removal (DPR) and microbial community structures was investigated. Particularly, the dynamic change in the amount and composition of extracellular polymeric substances (EPS), and the role of EPS in P removal, were assessed using three-dimensional excitation-emission matrix fluorescence spectroscopy combined with parallel factor (PARAFAC) analysis. The results showed that, after long-term adjustment, the P removal efficiency was maintained at 95 ± 2.7% at Cu(II) addition up to 2.5 mg/L, but deteriorated when the Cu(II) addition was 3 mg/L. The EPS content, including proteins and humic substances, increased with increasing Cu(II) additions at concentrations ≤2.5 mg/L. This property of EPS was beneficial for protecting phosphate-accumulating organisms (PAOs) against heavy metals, as both proteins and humic substances are strong ligands for Cu(II). Therefore, the PAOs abundance was still relatively high (67 ± 3%) when Cu(II) accumulation in sludge was up to 10 mg/g SS. PARAFAC confirmed that aromatic proteins could be transformed into soluble microbial byproduct-like material when microorganisms were subjected to Cu(II) stress, owing to their strong metal ion complexing capacity. The increase in the percentage of humic-like substances enhanced the detoxification function of the sludge EPS. EPS accounted for approximately 26-47% of P removed by adsorption when Cu(II) additions were between 0 and 2.5 mg/L. The EPS function, including binding toxic heavy metals and P storage, enhanced the operating stability of DPR systems. This study provides us with a better understanding of (1) the tolerance of DPR sludge to copper toxicity and (2) the function of sludge EPS in the presence of heavy metals in biological P removal systems.

Comparison of endogenous metabolism during long-term anaerobic starvation of nitrite/nitrate cultivated denitrifying phosphorus removal sludges.

Denitrifying phosphorus removal (DPR) by denitrifying phosphorus-accumulating organisms (DPAOs) is a promising approach for reducing energy and carbon usage. However, influent fluctuations or interruptions frequently expose the DPAOs biomass to starvation conditions, reducing biomass activity and amount, and ultimately degrading the performance of DPR. Therefore, a better understanding of the endogenous metabolism and recovery ability of DPAOs is urgently required. In the present study, anaerobic starvation (12 days) and recovery were investigated in nitrite- and nitrate-cultivated DPAOs at 20 ± 1 °C. The cell decay rates in nitrite-DPAOs sludges from the end of the anaerobic and aerobic phase were 0.008 day⁻¹ and 0.007 day⁻¹, respectively, being 64% and 68% lower than those of nitrate-DPAOs sludges. Nitrite-DPAOs sludges also recovered more rapidly than nitrate-DPAOs sludge after 12 days of starvation. The maintenance energy of nitrite-DPAOs sludges from the end of the anaerobic and aerobic phase were approximately 31% and 34% lower, respectively, than those of nitrate-DPAOs sludges. Glycogen and polyphosphate (poly-P) sequentially served as the main maintenance energy sources in both nitrite-and nitrate-DPAOs sludges. However, the transformation pathway of the intracellular polymers during starvation differed between them. Nitrate-DPAOs sludge used extracellular polymeric substances (EPS) (mainly polysaccharides) as an additional maintenance energy source during the first 3 days of starvation. During this phase, EPS appeared to contribute to 19-27% of the ATP production in nitrate-DPAOs, but considerably less to the cell maintenance of nitrite-DPAOs. The high resistance of nitrite-DPAOs to starvation might be attributable to frequent short-term starvation and exposure to toxic substances such as nitrite/free nitrous acids in the parent nitrite-fed reactor. The strong resistance of nitrite-DPAOs sludge to anaerobic starvation may be exploited in P removal by shortcut denitrification processes.

Nitrite survival and nitrous oxide production of denitrifying phosphorus removal sludges in long-term nitrite/nitrate-fed sequencing batch reactors.

Nitrite-based phosphorus (P) removal could be useful for innovative biological P removal systems where energy and carbon savings are a priority. However, using nitrite for denitrification may cause nitrous oxide (N2O) accumulation and emissions. A denitrifying nitrite-fed P removal system [Formula: see text] was successfully set up in a sequencing batch reactor (SBR) and was run for 210 days. The maximum pulse addition of nitrite to [Formula: see text] was 11 mg NO2(-)-N/L in the bulk, and a total of 34 mg NO2(-)-N/L of nitrite was added over three additions. Fluorescent in situ hybridization results indicated that the P-accumulating organisms (PAOs) abundance was 75 ± 1.1% in [Formula: see text] , approximately 13.6% higher than that in a parallel P removal SBR using nitrate [Formula: see text] . Type II Accumulibacter (PAOII) (unable to use nitrate as an electron acceptor) was the main PAOs species in [Formula: see text] , contributing 72% to total PAOs. Compared with [Formula: see text] , [Formula: see text] biomass had enhanced nitrite/free nitrous acid (FNA) endurance, as demonstrated by its higher nitrite denitrification and P uptake rates. N2O accumulated temporarily in [Formula: see text] after each pulse of nitrite. Peak N2O concentrations in the bulk for [Formula: see text] were generally 6-11 times higher than that in [Formula: see text] ; these accumulations were rapidly denitrified to nitrogen gases. N2O concentration increased rapidly in nitrate-cultivated biomass when 5 or 10 mg NO2(-)-N/L per pulse was added. Whereas, N2O accumulation did not occur in nitrite-cultivated biomass until up to 30 mg NO2(-)-N/L per pulse was added. Long-term acclimation to nitrite and pulse addition of nitrite in [Formula: see text] reduced the risk of nitrite accumulation, and mitigated N2O accumulation and emissions from denitrifying P removal by nitrite.

Long-term impact of anaerobic reaction time on the performance and granular characteristics of granular denitrifying biological phosphorus removal systems.

Removal of nitrogen and phosphorus (P) from wastewater is successfully and widely practiced in systems employing both granular sludge technology and enhanced biological P removal (EBPR) processes; however, the key parameter, anaerobic reaction time (AnRT), has not been thoroughly investigated. Successful EBPR is highly dependent on an appropriate AnRT, which induces carbon and polyphosphate metabolism by phosphorus accumulating organisms (PAOs). Therefore, the long-term impact of AnRT on denitrifying P removal performance and granular characteristics was investigated in three identical granular sludge sequencing batch reactors with AnRTs of 90 (R1), 120 (R2) and 150 min (R3). The microbial community structures and anaerobic stoichiometric parameters related to various AnRTs were monitored over time. Free nitrite acid (FNA) accumulation (e.g., 0.0008-0.0016 mg HNO2-N/L) occurred frequently owing to incomplete denitrification in the adaptation period, especially in R3, which influenced the anaerobic/anoxic intracellular intermediate metabolites and activities of intracellular enzymes negatively, resulting in lower levels of poly-P and reduced activity of polyphosphate kinase. As a result, the Accumulibacter-PAOs population decreased from 51 ± 2.5% to 43 ± 2.1% when AnRT was extended from 90 to 150 min, leading to decreased denitrifying P removal performance. Additionally, frequent exposure of microorganisms to the FNA accumulation and anaerobic endogenous conditions in excess AnRT cases (e.g., 150 min) stimulated increased extracellular polymeric substances (EPS) production by microorganisms, resulting in enhanced granular formation and larger granules (size of 0.6-1.2 mm), but decreasing anaerobic PHA synthesis and glycogen hydrolysis. Phosphorus removal capacity was mediated to some extent by EPS adsorption in granular sludge systems that possessed more EPS, longer AnRT and relatively higher GAOs.

A comparison of endogenous processes during anaerobic starvation in anaerobic end sludge and aerobic end sludge from an anaerobic/anoxic/oxic sequencing batch reactor performing denitrifying phosphorus removal.

The endogenous processes in anaerobic end and aerobic end sludge responsible for biological denitrifying phosphorus removal were compared during a 7-d starvation under anaerobic conditions. The results showed that polyphosphate and glycogen were utilized simultaneously to generate maintenance energy for both the anaerobic and aerobic end sludge. During the 7-d starvation, the decay rate of denitrifying-phosphorus-accumulating-organisms (DPAOs) was higher for the aerobic end sludge than for the anaerobic end sludge. More energy was required for maintenance in the aerobic end sludge than for the anaerobic end sludge, with the greater amount of phosphorus release and glycogen degradation occurring in the aerobic end sludge. Moreover, different metabolic pathways for the endogenous processes were observed for the anaerobic and aerobic end sludge. After the 7-d starvation, the activity of DPAOs decreased more for the aerobic end sludge than that for the anaerobic end sludge.

Effect of anaerobic reaction time on denitrifying phosphorus removal and N2O production.

Nitrous oxide (N(2)O) is a highly potent greenhouse gas; however, the characteristics of N(2)O production during denitrification using poly-ß-hydroxyalkanoates (PHA) as a carbon source are not well understood. In this study, effects of anaerobic reaction time (AnRT) on PHA formation, denitrifying phosphorus removal and N(2)O production were investigated using a laboratory-scale anaerobic/anoxic/oxic sequencing batch reactor (An/A/O SBR). The results showed that operation of the An/A/O SBR for 0.78 SRT (47 cycles) after the AnRT was shortened from 90 min to 60 min resulted in anaerobically synthesized PHA improving by 1.8 times. This improvement was accompanied by increased phosphorus removal efficiency and denitrification. Accordingly, the N(2)O-N production was reduced by 6.7 times. Parallel batch experiments were also conducted with AnRTs of 60, 90 and 120 min. All results indicated that in addition to the amount of anaerobically synthesized PHA, the kinetics of PHA degradation also regulated denitrifying phosphorus removal and N(2)O production.

Effect of copper ion on the anaerobic and aerobic metabolism of phosphorus-accumulating organisms linked to intracellular storage compounds.

The shock load effect of heavy metals (Cu (II)) on the behavior of poly-phosphate-accumulating organisms (PAOs) was investigated with respect to the transformations of poly-P, intracellular polyhydroxyalkanoates (PHAs) and glycogen. The PAOs biomass was exposed to different concentrations of Cu (II) at various pH and biomass levels. The results showed that when the mixed liquor suspended solid (MLSS) concentration was 2500-4000 mg/L, the P removal was not adversely affected by spiking with 2 mg Cu(2+)/L; however, it deteriorated completely after a Cu (II) shock concentration of 4 mg/L. Nevertheless, the tolerance of PAOs biomass to Cu (II) shock could be enhanced by increasing the MLSS. Moreover, in the presence of 2 mg Cu(2+)/L, the P removal efficiency was highest at an initial pH of 6.2 and lowest at an initial pH of 6.9, indicating that the Cu inhibitory effect was reduced by increasing the pH to 7.6. The inhibition by Cu (II) was related to the transformation of intracellular storage compounds of PAOs. Specifically, poly-P degradation might be inhibited, which reduced the energy available for PHA production and eventually led to poor P removal.

The long-term effect of carbon source on the competition between polyphosphorus accumulating organisms and glycogen accumulating organism in a continuous plug-flow anaerobic/aerobic (A/O) process.

Laboratory experiments were conducted in a continuous plug-flow anaerobic/aerobic (A/O) process to kinetically investigate the long-term effect of the different carbon sources (i.e., acetate, acetate/propionate, propionate and glucose) on the competition between polyphosphorus accumulating organisms (PAOs) and glycogen accumulating organisms (GAOs). It was found that propionate was more benefit than acetate for PAOs even in the A/O process, and PAOs enriched with acetate were readily able to metabolize propionate without the requirement of adaptation. Glucose gave GAOs metabolic advantage in the PAOs-GAOs competition, which thereby worsened the EBPR performance. Nevertheless, the EBPR capacity could recover by returning carbon to acetate, with the acclimation time of approximately 2-SRTs. This suggests that the varying of carbon can be an effective approach to provide PAOs a competitive advantage over GAOs. Additionally, MLVSS/MLSS could indicate the shift of the microorganism between GAOs and PAOs, but it was not as precise as the biomass-P content.

Effect of influent nutrient ratios and hydraulic retention time (HRT) on simultaneous phosphorus and nitrogen removal in a two-sludge sequencing batch reactor process.

A laboratory-scale anaerobic-anoxic/nitrification sequencing batch reactor (A(2)N-SBR) fed with domestic wastewater was operated to examine the effect of varying ratios of influent COD/P, COD/TN and TN/P on the nutrient removal. With the increased COD/P, the phosphorus removals exhibited an upward trend. The influent TN/P ratios had a positive linear correlation with the phosphorus removal efficiencies, mainly because nitrates act as electron acceptors for the phosphorus uptake in the A(2)N-SBR. Moreover, it was found that lower COD/TN ratio, e.g. 3.5, did not significantly weaken the phosphorus removal, though the nitrogen removal first decreased greatly. The optimal phosphorus and nitrogen removals of 94% and 91%, respectively were achieved with influent COD/P and COD/TN ratios of 19.9 and 9.9, respectively. Additionally, a real-time control strategy for A(2)N-SBR can be undertaken based on some characteristic points of pH, redox potential (ORP) and dissolved oxygen (DO) profiles in order to obtain the optimum hydraulic retention time (HRT) and improve the operating reliability.

[Characteristics and affecting factors of denitrifying phosphorus removal in two-sludge sequencing batch reactor].

The characteristics of denitrifying phosphorus removal in a lab-scale two-sludge anaerobic-anoxic/nitrification SBR (A2 NSBR) system were studied fed with domestic wastewater. The influence of some key operation parameters, like C/P, C/N, and HRT, were examined using parallel tests, pH, dissolved oxygen (DO) and redox potential (ORP) were monitored on line to validate whether they could be used as the control parameters for this denitrifying phosphorus removal process. Results indicated that P removal efficiency showed an increased trend on the whole with the increase of the C/P. When the influent C/P was greater than 19.39, good phosphorus removal efficiency was achieved. However, the phosphorus removal efficiency deteriorated once the influent C/P decreased less than 15.36. On the other hand, relatively good phosphorus removal efficiency could be maintained in the A2 NSBR system even at a low C/N ratio, though the denitrification efficiency decrease instead. It is also found that increasing the influent C/N increased the PHB amount stored by polyphosphate accumulating organisms (PAO) and therefore the ultimate denitrification and phosphorus removal efficiency were both improved. For an excessively high C/N, the incompletely reacted COD will be residual to anoxic stage. Thus, the pure denitrification reaction, which preferentially supports OHOs, becomes the dominant reaction. This decreases the amount of available electron acceptors for denitrifying polyphosphate accumulating organisms (DNPAOs) at the anoxic stage which eventually impacts the anoxic phosphorus removal capacity. In addition, since A2 NSBR has two completely independent SBR systems, it benefits to establish a process control system in terms of the parameters DO, ORP, and pH.

Characteristics of anoxic phosphorus removal in sequence batch reactor.

The characteristics of anaerobic phosphorus release and anoxic phosphorus uptake were investigated in sequencing batch reactors using denitrifying phosphorus removing bacteria (DPB) sludge. The lab-scale experiments were accomplished under conditions of various nitrite concentrations (5.5, 9.5, and 15 mg/L) and mixed liquor suspended solids (MLSS) (1844, 3231, and 6730 mg/L). The results obtained confirmed that nitrite, MLSS, and pH were key factors, which had a significant impact on anaerobic phosphorus release and anoxic phosphorus uptake in the biological phosphorous removal process. The nitrites were able to successfully act as electron acceptors for phosphorous uptake at a limited concentration between 5.5 and 9.5 mg/L. The denitrification and dephosphorous were inhibited when the nitrite concentration reached 15 mg/L. This observation indicated that the nitrite would not inhibit phosphorus uptake before it exceeded a threshold concentration. It was assumed that an increase of MLSS concentration from 1844 mg/L to 6730 mg/L led to the increase of denitrification and anoxic P-uptake rate. On the contrary, the average P-uptake/N denitrifying reduced from 2.10 to 1.57 mg PO4(3-)-P/mg NO3(-)-N. Therefore, it could be concluded that increasing MLSS of the DEPHANOX system might shorten the reaction time of phosphorus release and anoxic phosphorus uptake. However, excessive MLSS might reduce the specific denitrifying rate. Meanwhile, a rapid pH increase occurred at the beginning of the anoxic conditions as a result of denitrification and anoxic phosphate uptake. Anaerobic P release rate increased with an increase in pH. Moreover, when pH exceeded a relatively high value of 8.0, the dissolved P concentration decreased in the liquid phase, because of chemical precipitation. This observation suggested that pH should be strictly controlled below 8.0 to avoid chemical precipitation if the biological denitrifying phosphorus removal capability is to be studied accurately.

[Effect of carbon source and nitrate concentration on denitrifying dephosphorus removal and variation of ORP].

Effect of added carbon source and nitrate concentration on the denitrifying phosphorus removal by SBR process was systematicaly studied, at the same time the variation of oxidation reductiun potential (ORP) was investigated. The results showed the phosphate release rate and the denitrifying and dephosphorus uptake rate in anoxic phase increased with the high carbon source concentration under anaerobic condition (100-300mg/L). However when the carbon source added in anaerobic phase was high to 300mg/L, the residual COD inhibited the succeed denitrifying dephosphorus uptake. High nitrate concentration (5, 15, 40mg/L) in anoxic phase increased the initial denitrifying dephosphorus rate. Once the nitrate depletes, phosphate uptake changed to phosphate release. Moreover, the time of the turning point occurred later with the higher nitrate addition. ORP can be used as a control parameter of phosphorus release, and it can also indicate the denitrificaiton react degree during the anoxic phosphorus removal but can't be used as control parameter of phosphorus uptake.

Effect of carbon source and nitrate concentration on denitrifying phosphorus removal by DPB sludge.

Effect of added carbon source and nitrate concentration on the denitrifying phosphorus removal by DPB sludge was systematically studied using batch experiments, at the same time the variation of ORP was investigated. Results showed that the denitrifying and phosphorus uptake rate in anoxic phase increased with the high initial anaerobic carbon source addition. However once the initial COD concentration reached a certain level, which was in excess to the PHB saturation of poly-P bacteria, residual COD carried over to anoxic phase inhibited the subsequent denitrifying phosphorus uptake. Simultaneously, phosphate uptake continued until all nitrate was removed, following a slow endogenous release of phosphate. High nitrate concentration in anoxic phase increased the initial denitrifying phosphorus rate. Once the nitrate was exhausted, phosphate uptake changed to release. Moreover, the time of this turning point occurred later with the higher nitrate addition. On the other hand, through on-line monitoring the variation of the ORP with different initial COD concentration, it was found ORP could be used as a control parameter for phosphorus release, but it is impossible to utilize ORP for controlling the denitrificaion and anoxic phosphorus uptake operations.