Oxic Stage Research Articles

Effect of influent C/N ratio on nitrogen removal using PHB as electron donor in a post‐denitritation SBR

AbstractBackgroundThe post‐denitritation sequencing batch reactor (SBR) is widely‐used and can achieve high levels of nitrogen removal. In this study the effect of influent COD/TN (total nitrogen) ratio (i.e. C/N ratio) on nitrogen removal performance was investigated.ResultsThe experimental results showed that polyhydroxybutyrate (PHB) was the internal carbon source for denitritation, so PHB degradation rate following first‐order kinetics was the rate‐limiting step both for simultaneous nitritation–denitritation (SND) in the substrate famine period of the oxic stage and endogenous denitritation in the anoxic stage. Higher influent C/N ratio resulted in more PHB fractions in microorganisms, which facilitated a higher efficiency of SND and a faster endogenous denitritation rate (DNR). Consequently, mean TN removal ratio in oxic stage dropped from 32.81% to 8.61%, and average endogenous DNR in the anoxic stage fell from 1.50 to 0.27 mgN h‐1 gVSS‐1, when influent C/N ratio changed from 6.82 to 1.89. Furthermore, PHB fraction in the biomass did not drop drastically when influent C/N ratio dropped for a short‐term period, which facilitated better resistance to shock loads.ConclusionHigh influent C/N ration benefits nitrogen removal in this process, and an influent C/N ratio of 4.00 was suitable for advanced nitrogen removal. © 2013 Society of Chemical Industry

Modeling of the oxic stage in a HLW disposal cell in an argillaceous host rock

ABSTRACTDetermining the redox conditions in the near field of deep underground radioactive waste disposal cells is a key question regarding the performance of metallic components (e.g. waste overpack), which may undergo drastic corrosion processes in oxic conditions. This oxic transient is supposed to be short due notably to the oxygen consumption by corrosion and pyrite oxidation. However, the observed precipitation of Fe(III)-minerals as well as localized corrosion patterns on steel coupons placed during 6 years in a borehole drilled in the Toarcian argillite of Tournemire (France) may suggest that in-situ oxic conditions lasted several years, which is not consistent with reactive transport simulations performed with usual hypotheses (perfect contact between materials, high pyrite accessibility, water saturated conditions). Multicomponent reactive transport simulations considering gas diffusion were performed with the code HYTEC and reproduce correctly the observations made in Tournemire while considering imperfect interfaces and resaturation processes. The model was then applied to a disposal cell for high-level waste (HLW) representative of the design developed in France, putting into evidence the possibility of a redox contrast between the front and back of a disposal cell in an argillaceous medium, as well as a duration of the oxic stage within the cell as long as the ventilation of handling drifts is maintained.

Nitrogen Removal in a Real-Time Controlled Sequencing Batch Membrane Bioreactor

The performance of a real-time controlled Sequencing Batch Membrane Bioreactor (SBMBR) for removing organic matter and nitrogen from synthetic wastewater has been investigated in this study under two specific ammonia loadings of 0.0086 and 0.0045g NH4+-N gVSS−1 day−1. Laboratory results indicate that both COD and DOC removal are greater than 97.5% (w/w) but the major benefit of using membrane for solid-liquid separation is that the effluent can be decanted through the membrane while aeration is continued during the draw stage. With a continued aeration, the sludge cake layer is prevented from forming thus alleviating the membrane clogging problem in addition to significant nitrification activities observed in the draw stage. With adequate aeration in the oxic stage, the nitrogen removal efficiency exceeding 99% can be achieved with the SBMBR system. Furthermore, the SBMBR system has also been used to study the occurrence of ammonia valley and nitrate knee that can be used for real-time control of the biological process. Under appropriate ammonia loading rates, applicable ammonia valley and nitrate knee are detected. The real-time control of the SBMBR can be performed based on on-line ORP and pH measurements.

Environmental Redox Changes of the Ancient Sea in the Yangtze Area during the Ordo‐Silurian Transition

Abstract: Extensive organic‐matter (OM) rich facies (black shales) occur in the Ordo‐Silurian boundary successions in the Yangtze area, South China. To investigate the redox changes of the Yangtze Sea during the Ordo‐Silurian transition, two OM sections (Wangjiawan in Yichang, Hubei Province, and Sanjiaguan in Zhangjiajie, Hunan Province) straddling the Ordo‐Silurian boundary are studied. The measurements finished in this study include contents of the total organic carbon (TOC), pyrite sulphur, and different species of Fe, including dithionite‐extractable Fe (FeD), pyrite Fe (FeP), HCI‐extractable Fe (FeH), and total Fe (FeT), in black shales, as well as other redox proxies, such as the S/C ratio, the ratio between highly reactive Fe (FeHR = FeD + FeP) and FeT, and the Fep/(FeP + FeH) ratio, known as the degree of pyritization (DOP). In the Wangjiawan section, the Middle Ashgill sediments have high FeHR/FeT ratios (0.20‐0.77; avg. 0.45), high DOP values (0.21‐0.72; avg. 0.54), and a relatively constant sulfur content independent of the organic carbon content. By the contrast, the mid‐early Hirnantian deposits generally have low FeHR/FeT ratios (0.10‐0.35; avg. 0.21), low DOP values (0.11‐0.40; avg. 0.28), and S/C values are clustering on the normal marine value (S/C = 0.36). The late Hirnantian and early Rhuddanian deposits, similar to those of the Middle Ashgill deposits, are characterized by high FeHR/FeT ratios (0.32‐0.49; avg. 0.41), high DOP values (0.46‐0.68; avg. 0.53) and fairly constant sulfur contents. These data suggest the occurrences of marine anoxia on the Yangtze Sea shelf during intervals of the Mid Ashgill, Late Hirnantian and Early Rhuddanian, and ventilated and oxygenated marine conditions during the mid‐early Hirnantian time. The mid‐early Hirnantian ventilated event was concomitant with the global glacial period, likely resulted from the glacio‐eustatic sea‐level fall and subsequent circulation of cold, dense oxygenated waters upon the shelf seabed. Accordingly, the abrupt change from oxygenated to anoxic marine waters from the late Hirnantian and the early Rhuddanian were resulted from the post‐glacial rise of eustatic sea level. Combined with the data from Sanjiaguan section near the subemergent highland, salinity stratification of water columns are strongly evidenced by very low S/C ratios (0.00‐0.08, avg. 0.02), low FeHR/FeT ratios (0.10‐0.37, avg. 0.27), low DOP values (0.01‐0.27, avg. 0.10), and TOC contents (0.72%‐4.27%, avg. 2.55%). Under this circumstance, the anoxic water columns could have formed beneath the halocline, above which desalinized waters formed. In the Wangjiawan section, TOC contents are generally high (0.94‐9.32%, avg. 4.44%), but low (0.35%‐2.12%, avg. 1.29%) in the mid‐early Hirnantian that is coincident with the oceanic oxic stage, together with relation of the organic contents to the stratal thickness, suggesting that the accumulation of the organic matter was mainly controlled by the oxygen levels of the water columns; on the other hand, productivity and depositional rate may also have played a role in the organic accumulation and preservation.

Effect of biologically mediated pH change on phosphorus removal in BNR system for piggery waste treatment

Since applicable amount of animal waste to farm land has been greatly reduced because of the nutrient overload, nitrogen and phosphorus removal from animal waste has received a great attention. This study was conducted to evaluate how phosphorus was removed during biological nutrient removal (BNR) from piggery waste using laboratory and full scale units operated at 25 to 40°C. The phosphorus removal was performed by chemical precipitation with struvite and hydroxyapatite (HAP), cellular formation, it is basically related with pH and organic and nitrogen loads resulting in influent COD/N ratios. The removal efficiencies increased from 50 to 90% as COD/N ratios increased to 6 to 7, but carbon was not limited beyond this ratio for denitrification resulting in a stable pH. Overall, about 70% of the phosphorus removal was due to the precipitates of struvite and/or HAP, and the remaining removal was due to the cellular P formation. Any significant temperature effect on phosphorus removal was not observed within the operating temperature. In order to maximize phosphorus removal in BNR system, additional anoxic stage must be furnished prior to discharge its final effluent after oxic stage.

Performance of a hybrid SBR with fixed bed and suspended growth

A hybrid SBR system combined with fixed bed (media) and suspended growth zones was developed to improve both nitrogen and phosphorus removal. About 27% of the tank volume was filled with clay media to make a fixed bed in this system, and additional air was applied under the bed to wash the microbes from the media during the oxic stage to prevent the bed from clogging. This hybrid SBR system could eliminate the backwashing requirement for SBBR (sequencing batch biofilm reactor). This system showed a stable nitrification even at low temperature and shock load conditions. The specific reaction rates indicated the fixed bed zone had higher microbial activities for nitrification and denitrification, while the suspended growth zone had higher microbial activities for phosphorus release and uptake. In addition, the use of external sludge storage also increased both phosphorus and nitrogen removal. The effluent COD, nitrogen and phosphorus concentrations were respectively less than 15, 10.5 and 0.6 mg/L with weak sewage of 230 mg/L COD, 35 mg/L TKN and 5.3 mg/L TP.

Evaluation of the parameters affecting nitrogen and phosphorus removal in anaerobic/anoxic/oxic (A/A/O) biological nutrient removal systems

Considerable research has been performed on biological nutrient removal (BNR) systems which remove the problematic nutrients, nitrogen and phosphorus, that cause eutrophication. This research focussed on setting up two laboratory-scale anaerobic/anoxic/oxic (A/A/O) systems and investigating their reliability while undergoing various parameter changes. Pump failure, in the first trial, R1, led to a decrease in pH, exposure of the sludge to relatively low nitrate concentrations and reduction of the suspended solids concentration within the system. This adversely affected the phosphorus removal efficiency. Shock loading the system with increased influent phosphate concentrations for 56 days was shown to aid remediation of the phosphorus removal efficiency to values between 65 and 70% (w/w). The second trial, R2, highlighted the presence of bacteria capable of P-uptake under anoxic conditions (in the presence of nitrate). The characteristic anaerobic P-release was also evident. The bacteria responsible for phosphate uptake under anoxic conditions are thought to be the denitrifying phosphate removing bacteria (DPB). However, the presence of higher nitrate concentrations retarded the P-removal efficiency to some extent. Secondary release of P was evident in the clarifier of the A/A/O system during the R2 trial and especially during times of increased nitrate concentrations in the system. Between 20 and 40% (w/w) of the P taken up in the oxic stage of the system was released in the clarifier at various stages throughout the trial. © 2000 Society of Chemical Industry

The geochemical cycling of stable Pb, 210Pb, and 210Po in seasonally anoxic Lake Sammamish, Washington, USA

The geochemical processes controlling the behavior of stable Pb, 210Pb, and 210po in seasonally anoxic Lake Sammamish, Washington were identified from water column distributions and box model calculations. Total (sum of dissolved and particulate) inventories of stable Pb, 210Pb, and 210Po increased in the whole lake during the latter part of the oxic stage of the lake and were attributed to diffusion from sediments. Large decreases in the total inventories of these elements occurred during the transition from oxic to anoxic conditions, and the lowest inventories were observed during the sulfidic stage of stratification. The cycling of stable Pb and 210Pb during oxic periods appeared to be linked to Fe cycling while 210Po cycling was more closely linked to the cycling of Mn. The behavior of stable and radioactive Pb and, possibly, 210Po during anoxia was influenced by sulfur cycling. Thermodynamic calculations indicated that dissolved Pb concentrations might be controlled by PbS precipitation during anoxia. Flux balance calculations during stratification indicated that atmospheric deposition was the major source of both 210Pb and 210Po to the lake and fluvial input was more important for 210Po than for 210Pb. 210Pb and 210Po supplied by atmospheric deposition were scavenged and removed by sedimentation from the epilimnion. Residence times with respect to scavenging and sedimentation in the epilimnion were 1–3 and 617 days, respectively, for 210Pb and 9–22 and 9–26 days for 210Po. Model results in the hypolimnion indicated that at certain times the radionuclides diffused out of the sediments and at other times into the sediments. Residence times with respect to sedimentation of particulate nuclides in the hypolimnion were 2–124 days for 210Pb and 8–48 days for 210Po. The behavior of 210Pb and 210Po in Lake Sammamish was compared to that in B~' kford Reservoir, Massachusetts. Although the comparison indicated similarities (e.g., links to Fe and Mn cycling, larger scavenging rates for 210Pb than for 210Po), the major differences between the systems were that atmospheric inputs were greater than fluvial inputs, activities of the radionuclides were lower, and the presence of sulfide in the hypolimnion during anoxia affected the cycling of stable Pb, 210Pb, and 210Po in Lake Sammamish.

Monitoring of the Biological Nutrient Removal Process by an On‐Line NAD(P)H Fluorometer

AbstractAn on‐line NAD(P)H fluorometer has been used to investigate the fluorescence profile of a simulated anaerobic/anoxic/oxic biological nutrient removal (BNR) process and its dependency on operating parameters, such as the concentrations of organic substrate (acetate) and activated sludge. As expected from the different rates of NAD‐(P)H oxidation at different stages, the order of fluorescence levels was found to be anaerobic > anoxic > oxic. The effects of the acetate and sludge concentrations were observed most clearly at the anoxic stage: an increase in either concentration led to faster recovery of fluorescence from the drop caused by NO3− addition. A higher acetate concentration also resulted in a more significant, continuous rise in fluorescence, corresponding well with the enhanced PO43− release, at the anaerobic stage and a slower initial fluorescence decrease at the oxic stage, indicating the dependency of the decrease on substrate availability. The ratio of the two fluorescence drops caused by NO3− addition and aeration, Δ1/Δ2, obtained with the plant's sludge was found to be 0.7 × 0.1, which is similar to those of pure nitrate‐respiring cultures (e.g., Pseudomonas aeruginosa and Escherichia coli). The dominant majority of nonnitrifiers in the sludge therefore are capable of nitrate respiration, and the nitrifiers either contribute little to the overall fluorescence or perform nitrate respiration as the rest of the sludge population.

Effects of bioturbation in oxic and hypoxic conditions: a microcosm experiment with a North Sea sediment community

Sediment cores of 20 cm diameter contaning the natural benthic fauna were subjected to low oxygen conditions in a laboratory microcosm system. After several days of oxic conditions ('oxic stage') the oxygen content of the water was reduced to 25% saturation for 15 d ('hypoxic stage'), followed by a 'reoxygenation stage'. Effective solute transport rates were calculated using measurements with the conservative tracer ion bromide. Profiles of oxygen and ΣCO2 were measured and molecular diffusive as well as effective fluxes, account mg for effective solute exchange, were calculated. The overall response of the benthic community was to compensate for low oxygen content of the overlying water by increased pumping activity. On average, effective diffusion coefficients (Den} were 3 times higher in hypoxia than under oxic conditions. D eff reached 1.5 x 10^-4 cm2 s^-1, a value 30 times that of molecular diffusion. During hypoxia we observed low molecular diffusive O2 flux, higher effective O2 flux, as well as an increase in ΣCO2 within the sediment. We interpret this as a shift of transport away from diffusion within the bulk sediment interstices (oxic conditions) to the advective transport pathways along burrows during hypoxia. This facilitates fast transport of oxygen and bromide along burrows and contrasts with the slower transport of CO2 from the interstices governed by molecular diffusion. In this transient situation calulations based on gradients result in an unrealistic molar ratio of fluxes(CO2/O2)as high as 11.

A Scanning Electron Microscopy Evaluation of the Fabric and Mineralogy of Some Soils from Malaysia

AbstractFour pedons — an Aquic Dystropept, Typic Paleudult, Tropeptic Haplorthox, and Haplic Acrorthox — representing a sequence of soils on shales in Peninsular Malaysia were employed to evaluate the changes in plasmic fabric and related distribution patterns as a function of soil genesis. The decreases in sepic features when proceeding from the cambic to the oxic stage of soil formation is due to changes in colloid composition (micaceous to kaolinite‐sesquioxide), size of colloid (coarse to fine clay), and masking of the optical properties of the plasma by the free Fe. Complimentary SEM studies are presented to show the structure of the domains and the interdomain relationships which determine the plasmic fabrics and the related distribution patterns.Scanning electron microscopy (SEM) and thin‐section micrographs of secondary goethite and gibbsite are presented and based on the SEM morphologies, a silicification of the gibbsite to kaolinite is postulated in the surface horizons of the Acrorthox.