High Dissolved Oxygen Concentration Research Articles

Effects of drip irrigation systems on the recovery of dissolved oxygen from hypoxic water

The dissolved oxygen concentration (DOC) is an important irrigation water quality parameter that can become a limiting factor in some intensive agriculture systems. A low DOC in the irrigation water may have critical consequences because it causes root oxygen deficiency, which in turn can result in agronomic problems. The aim of this study was to improve the understanding of the dynamics of the DOC in hypoxic irrigation water when passing throughout a surface drip irrigation system (DIS) and seeping into the soil. To this end, an experimental DIS consisting of three types of commercial emitters and a venturi air injector, installed in-line, was set up for evaluation. Furthermore, subsurface water samplers were buried to catch the water in the soil. The trials were conducted with water from two different sources. The control treatment was performed with fresh channel water, which had a high DOC (7.54 mg L−1; 92.2% saturation), and the low DOC treatments were supplied from a covered agricultural reservoir and had DOC values less than 1.08 mg L−1 (10.8% saturation). After the low DOC treatments, the final DOC in the soil 24 h after irrigation ranged from 3.77 mg L−1 to 5.31 mg L−1 (47.2% to 65.2% saturation). There was an increase in the DOC in all stages of the experimental DIS, which was more important in the water passing through the emitters. The main factor determining the final DOC was the type of emitter, where DOC differences were correlated to their flow performance. The control treatment reached a similar DOC in the soil 24 h after irrigation, indicating that using hypoxic water under DIS does not affect the final soil DOC. Finally, the application of a venturi air injector increased the DOC in the low DOC source up to values typically found in open channels and reservoirs.

Diel cycling and stable isotopes of dissolved oxygen, dissolved inorganic carbon, and nitrogenous species in a stream receiving treated municipal sewage

Diel and synoptic studies were undertaken in Silver Bow Creek, a small, highly eutrophic stream receiving municipal sewage from the city of Butte, Montana, USA. During mid-summer baseflow conditions, oxidation of ammonium from the Butte wastewater treatment plant created a 2-km long reach marked by nightly hypoxia and extreme growth of aquatic plants and algae. Large diel cycles in the concentrations and isotopic compositions of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were inversely related, and are explained by the daily cycle of photosynthesis and respiration, modified by the effects of nitrification. Nitrification rates were higher during the day than at night due to a combination of higher water temperatures and higher DO concentrations. Changes in nitrification rate imparted diel cycles in the concentrations and N-isotopic compositions of NO 3 − and NH 4 +, whereas total dissolved inorganic nitrogen (DIN) concentrations and δ 15N-DIN values showed minimal diel variation. δ 15N-NH 4 steadily increased with distance downstream over a 5 km reach, whereas δ 15N-NO 3 showed a more complex spatial pattern. Plant assimilation caused downstream decreases in DIN and soluble reactive phosphorous: however, it was not possible to determine whether the plants assimilated NO 3 −, NH 4 +, or both. An important new finding of this study is the recognition that synoptic and diel changes in δ 18O-DO in the zone of active nitrification imparted a corresponding change in δ 18O-NO 3. Future studies examining the O-isotope composition of nitrate need to consider whether DO has a constant or shifting isotopic composition at the source of nitrification. The results of this study underscore the complex and dynamic interactions between biologically-active solutes in nutrient-rich streams, and also emphasize the importance of recognizing these cycles when collecting samples for scientific or monitoring purposes.

Some aspects of water quality characteristics in small shallow tropical man-made reservoirs in Kenya

The water quality in eight small reservoirs (0.065-0.249 km2) in both the rugged escarpment landscape above the rift valley floor and the stepped plateau above them were studied between 1998 and 2000. Water transparency was measured using a 20cm Secchi visibility disk. Total dissolved solids (TDS) and electrical conductivity (specific conductance) with temperature compensation at 25oC were recorded in situ from a portable Jenway probe model 4075. Water pH, dissolved oxygen (DO), percentage oxygen saturation (POS) were recorded on site from a portable WTW probe model ProfiLine Oxi 197/197-S. The overall range of mean water pH was 7.0 – 8.4 while the overall rangeof mean TDS was 29-82 mg/l. The TDS was slightly higher in the plateau reservoirs. The overall range of specific conductance was 37-101 μS/cm. The range of TDS and specific conductance in the reservoirs was quite low compared to other reservoirs in Kenya. The mean range of dissolved oxygen and percentage oxygen saturation was 2.0-7.2 mg/l, 23.4.2-33.6%, respectively. The level of dissolved oxygen was quite low in most reservoirs with dry season hypolimnial oxygen deficits in some sites and higher dissolved oxygen content in the more transparent reservoirs. The summary results showed a cleardifference in water quality between the plateau and escarpment reservoirs in the study area. The water quality in the reservoirs was found to be suitable for domestic and livestock utilization.However, additional water quality parameters are required to confirm this conclusion. The reservoirs were found to possess a good potential for multipurpose development.

Effect of dissolved oxygen on nitrogen removal and process control in aerobic granular sludge reactor

A sequencing batch reactor (SBR) with aerobic granular sludge was operated to determine the effect of different DO concentrations on biological nitrogen removal process and to investigate the spatial profiles of DO, ORP and pH as online control parameters in such systems. The results showed that DO concentration had a significant effect on nitrification efficiencies and the profiles of DO, ORP and pH. The specific nitrification rate was decreased from 0.0595 mgNH(4)(+)-N/(gMLSS min) to 0.0251 mgNH(4)(+)-N/(gMLSS min) after DO concentration was dropped off from 4.5mg/L to 1.0mg/L. High DO concentration improved the nitrification and increased the volumetric NH(4)(+)-N removal. Low DO concentration enhanced TIN removal, while prolonged the nitrification duration. Also there existed a good correlation between online control parameters (ORP, pH) and nutrient (COD, NH(4)(+)-N, NO(2)(-)-N, NO(3)(-)-N) variations in aerobic granular sludge reactor when DO was 2.5mg/L, 3.5mg/L and 4.5mg/L. However it was difficult to identify the end of nitrification and denitrification when DO was 1.0mg/L, due to no apparent bending points on ORP and pH curves. In conclusion, the optimal DO concentration was suggested at 2.5mg/L as it not only achieved high nitrogen removal efficiency and decreased the reaction duration, but also saved operation cost by aeration and mixing.

Effect of dissolved oxygen on oxidation and hydrogen pick up behaviour—Zircaloy vs Zr–Nb alloys

Upcoming advanced nuclear power reactors contemplate partial boiling inside the core and it is important to establish the effects of partial boiling on oxidation and hydrogen pick up for sufficiently long periods of time. The effect of partial boiling on oxidation behaviour was assessed in this study in terms of change in the level of dissolved oxygen (DO) in static autoclaves. The results clearly demonstrated that the oxidation behaviour of Zircaloy-2 is quite immune to the level of dissolved oxygen, while Zr–Nb alloys showed very different oxidation behaviour with change in the dissolved oxygen level. The weight gain due to oxidation in Zr–Nb alloys during long term exposure was almost doubled with increase in the DO level. Nevertheless, in case of all the three alloys, the corresponding hydrogen pick up was found to be lower for materials oxidized at higher DO levels irrespective of the effect of DO level changes on weight gain. This is attributed to the preferential reaction of the metal with free oxygen rather than water to produce the oxide when the DO levels are high. Further, the oxidation behaviour of Zr–1 wt%Nb alloy, as a function of dissolved oxygen content in lithiated water, was monitored in situ by electrochemical impedance spectroscopic (EIS) measurements using controlled displacement electrochemistry (CDE) technique in a recirculation loop at reactor operation temperature. The oxidation behaviour (for upto 90 h duration at 310 °C) in high DO content of water was in good correlation with the results obtained from accelerated oxidation experiments during short term exposure (up to 24 h at 400 °C) in static autoclaves.

CENTRALIZED DISSOLVED OXYGEN TRACKING AT WASTEWATER TREATMENT PLANT: NOWY DWOR GDANSKI CASE STUDY

The aeration is used in different operations at wastewater treatment plants. The biological processes that are carried out at require sufficiently high dissolved oxygen concentration in order to maintain microorganisms in an activated sludge. The paper proposes a centralized nonlinear model predictive controller of dissolved oxygen tracking and aeration system control. The controller is applied to wastewater treatment plant at Nowy Dwor Gdanski and its performance is compared with the performance of the control system that is currently applied at this site.

Development and evaluation of a spatially-explicit index of Chesapeake Bay health

In an effort to better portray changing health conditions in Chesapeake Bay and support restoration efforts, a Bay Health Index (BHI) was developed to assess the ecological effects of nutrient and sediment loading on 15 regions of the estuary. Three water quality and three biological measures were combined to formulate the BHI. Water quality measures of chlorophyll-a, dissolved oxygen, and Secchi depth were averaged to create the Water Quality Index (WQI), and biological measures of the phytoplankton and benthic indices of biotic integrity (P-IBI and B-IBI, respectively) and the area of submerged aquatic vegetation (SAV) were averaged to create the Biotic Index (BI). The WQI and BI were subsequently averaged to give a BHI value representing ecological conditions over the growing season (i.e., March–October). Lower chlorophyll-a concentrations, higher dissolved oxygen concentrations, deeper Secchi depths, higher phytoplankton and benthic indices relative to ecological health-based thresholds, and more extensive SAV area relative to restoration goal areas, characterized the least-impaired regions. The WQI, P-IBI and BHI were significantly correlated with (1) regional river flow ( r = −0.64, −0.57 and −0.49, respectively; p < 0.01), (2) nitrogen (N), phosphorus (P) and sediment loads (all positively correlated with flow), and (3) the sum of developed and agricultural land use (highest annual r 2 = 0.86, 0.71 and 0.68, respectively) in most reporting regions, indicating that the BHI is strongly regulated by nutrient and sediment loads from these land uses. The BHI uses ecological health-based thresholds that give an accurate representation of the health conditions in Chesapeake Bay and was the basis for an annual, publicly released environmental report card that debuted in 2007.

Methane and nitrous oxide emissions from an integrated rainfed rice–fish farming system of Eastern India

Integration of fish stocking with rice ( Oryza sativa L.) cultivation promises an ecologically sound and environmentally viable management of flooded ecosystem. Rice agriculture contributes to the emission of greenhouse gases CH 4 and N 2O, but little is known on the effect of fish rearing in fields planted to rice on the emission of these two greenhouse gases. In a field study, CH 4 and N 2O fluxes were measured from a sub-humid tropical rice field of Cuttack, eastern India, as affected by integrated rice–fish farming under rainfed lowland conditions. Three Indian major carps, Catla catla H., Labeo rohita H. and Cirrhinus mrigala H., and Puntius gonionotus B. were stocked in rice fields planted to two rice cultivars in a split-plot design with no fish and fish as the main treatments and two rice varieties as sub-treatments with three replicates each. Fish rearing increased CH 4 emission from field plots planted to both the rice cultivars with 112% increase in CH 4 emission in cv. Varshadhan and 74% in case of cv. Durga. On the contrary, fish stocking reduced N 2O emission from field plots planted to both the rice varieties. Movement of fish and associated bioturbation coupled with higher dissolved organic-C and CH 4 contents, and lower dissolved oxygen could be the reasons for release of larger quantities of CH 4 from rice + fish plots, while higher dissolved oxygen content might have influenced release of more N 2O from the rice alone treatment. The total greenhouse gas emission, expressed as CO 2 equivalent global warming potential (GWP), was considerably higher from rice + fish plots with CH 4 contributing a larger share (91%) as compared to rice alone plots (78–81%). On the contrary, N 2O had a comparatively lesser contribution with 19–22% share in rice alone plots that was further reduced to 9% in rice + fish plots. However, considering the profit-loss analysis based on the market price of the produce, rice–fish system provided a net profit of $453.36 ha −1 over rice alone system in spite of higher carbon credit compliance of a rice–fish ecosystem due to larger cumulative GWP.

Effect of salt-marsh plants on the mobility of Cr in sediments

Depth variation (1-cm resolution) of total Fe, Mn, and Cr concentrations was determined in below-ground biomass of Halimione portulacoides and in root sediments from two Tagus Estuary salt marshes (Portugal). The metal fractions extracted by hydroxylamine (HYD) and HCl were also quantified in sediment samples. Edaphic conditions of the sediment (O2, pH, EH, acidvolatile sulphides [AVS], and root biomass) were characterized in both marshes. Higher dissolved oxygen concentrations were found at depths with increased root biomass, while AVS were nearly absent in the root zone increasing with depth. Sediments colonized by H. portulacoides showed higher variability of total metal concentrations with depth than non-vegetated sediments, possibly due to intense exchanges between the below-ground plants and surrounding sediments. Concentrations of FeHYD (mainly Fe oxyhydroxides) were elevated in the root sediments since Fe(II) is oxidized by the O2 released from roots. The normalization of CrHYD levels to FeHYD showed a sharp increase when oxygen is depleted, suggesting that Cr is sequestered in the Fe oxyhydroxide fraction. Roots of H. portulacoides from both marshes showed little ability to take up and accumulate Cr. A possible explanation is the reduction of soluble Cr(VI) to the stable Cr(III) and subsequent retention in the Fe oxyhydroxide fraction. The inability or limited defence mechanisms of H. portulacoides to accumulate Cr indicates that this halophyte is not adequate for successfully phytostabilizing Cr-polluted marshes.

High altitude mountain streams as a possible refuge habitat for the catfish Amphilius uranoscopus

Amphilius uranoscopus is a catfish species, restricted to rivers and streams in east, southern and central Africa. It is likely to be displaced due to both competition and predation by exotic trout and other introduced fish. In high altitude mountain streams it can be the only species occurring, which means that this habitat may act as a refuge for this species. Ecosystems like this are threatened by habitat alteration and are therefore in need of protection. The abiotic environment, population structure, behavior and feeding biology of Amphilius uranoscopus were studied in a small, high-altitude perennial tributary of the Limpopo River in the Soutpansberg mountain range, Limpopo Province, South Africa, during 2005–2006. Here A. uranoscopus showed nocturnal behavior. It used dark hollow crevices in rapids as shelters during the daytime. The rapids are characterized by a high flow rate, high dissolved oxygen content and coarse riverbed substrate consisting mainly of boulders without fallen leaves. In contrast to the adults, juveniles found shelter among the fallen leaves in pools. At night, A. uranoscopus moved out of the rapids into the open water of the pools. The main food of A. uranoscopus consisted of macroinvertebrates, mainly Trichoptera larvae. Amounts of algae and detritus in its diet were negligible. A. uranoscopus foraged mainly in rapids and on rock surfaces, spending less time foraging between the fallen leaves at the bottom of pools, in open water, at the surfaces of bottom and bank and near the water surface. At the water surface, it also fed opportunistically on terrestrial insects that dropped into the stream, like flying termites. A. uranoscopus was the top predator of the stream system and reached a high density (0.71 fish m−2). There was no competition from other fish species at the study site as they were lacking. A. uranoscopus showed a striking ability to climb and cling on to vertical substrates to conquer waterfalls and very shallow streams that sometimes become dry, using its fins, adhesive body and protruding mouth teeth.

Influence of dissolved oxygen concentration and aeration time on nitrite accumulation in partial nitrification process

This study investigated the influence of dissolved oxygen concentration and aeration time on nitrification and nitrite accumulation in an attempt to optimize the recently developed biological-partial-nitritation process for the treatment of strong nitrogen wastewaters. Investigation of dissolved oxygen concentration on ammonium and nitrite oxidation was carried out in a batch reactor. The dissolved oxygen concentration of 0.5 mg O 2 /L inhibited both ammonium as well as nitrite oxidation, while increase of dissolved oxygen concentration to ~1 mg O 2 /L increased the ammonium oxidation rate and was comparable to that at higher dissolved oxygen concentrations. Experiments were carried out in a sequencing batch reactor for more than 100 days to investigate the influence of aeration time on nitrite accumulation. The dissolved oxygen concentration was controlled at ~1.0 mg O 2 /L (in the range of 0.8-1.5 mg/L) during the aeration stage, and volatile suspended solid was maintained at 2.0 g/L while temperature and pH were 30±1oC and 8.3±0.1, respectively. In a typical cycle, complete nitrification occurred at aeration time longer than 6 h. When the aeration time was reduced to 4 h., ~80 % of partial nitritation was achieved. With a further reduction in aeration time to 3 h., nearly 1:1 nitrite/ammonium ratio was yielded. This result revealed that for the reactor design, aeration time determined by feasibility experiments must be considered based on the nitrogen strength in wastewater and biomass concentration in the reactor with dissolved oxygen concentration of ~1.0 mg O 2 /L for satisfactory partial nitrification with subsequent processes such as anaerobic ammonium oxidation.

Analysis of nitrifying bacterial communities in aerobic biofilm reactors with different DO conditions using molecular techniques

In order to assess the relationship between the dissolved oxygen (DO) concentration and the characteristics of nitrifying bacterial communities in an aerobic biofilm reactor, molecular techniques including denaturing gradient gel electrophoresis (DGGE)/cloning based on PCR targeting 16S rRNA and the amoA gene and fluorescence in situ hybridisation (FISH) were conducted. The D-1, D-2, D-3 and D-4 reactors with different DO concentrations (1, 3, 5 and 7 mg/L, respectively) were set up in the thermostat and acclimated. The optimal DO concentration with stable nitrification efficiency was above 5.0 mg/L. As was shown by the results of DGGE and cloning, the community of ammonia-oxidising bacteria (AOB) and the ratio of Nitrosomonas sp. changed only slightly despite their differing nitrification efficiencies. The results of FISH indicated that higher DO concentrations resulted in an increase in AOB and nitrite-oxidising bacteria (NOB), and a reduction in heterotrophic microorganisms. The INT-dehydrogenase activity (DHA) test demonstrated that the activity of AOB decreased with reductions in the DO concentration. This means that the DO concentration does not influence the community of AOB, but rather the activity of AOB. In the relationship between the attached biomass and the nitrification efficiency, only the active biomass affected the nitrification efficiencies.

Nitrification of human urine for its stabilization and nutrient recycling

Nitrification of human urine performed for its stabilization, and culture of Spirulina platensis in the nitrified human urine were investigated for nutrient recovery. With daily adjusting to pH 8 and keeping high dissolved oxygen concentration, mean 95.0% of NH 4-N in human urine can be finally stabilized and oxidized to NO 3-N. Furthermore, this nitrified human urine seems to be an ideal culture medium for S. platensis. Without pH adjustment, only about 50.0% NH 4-N could be converted, i.e. NH 4NO 3 would be formed. Under low dissolved oxygen concentration, mainly short nitrification (from NH 4-N to NO 2-N) occurred.

Efficacy of algal metrics for assessing nutrient and organic enrichment in flowing waters

Summary1. Algal‐community metrics were calculated for periphyton samples collected from 976 streams and rivers by the U.S. Geological Survey’s National Water‐Quality Assessment (NAWQA) Programme during 1993–2001 to evaluate national and regional relations with water chemistry and to compare whether algal‐metric values differ significantly among undeveloped and developed land‐use classifications.2. Algal metrics with significant positive correlations with nutrient concentrations included indicators of trophic condition, organic enrichment, salinity, motility and taxa richness. The relative abundance of nitrogen‐fixing algae was negatively correlated with nitrogen concentrations, and the abundance of diatom species associated with high dissolved oxygen concentrations was negatively correlated with both nitrogen and phosphorus concentrations. Median algal‐metric values and nutrient concentrations were significantly lower at undeveloped sites than those draining agricultural or urban catchments.3. Total algal biovolume did not differ significantly among major river catchments or land‐use classifications, and was only weakly correlated with nitrate (positive) and suspended‐sediment (negative) concentrations. Estimates of periphyton chlorophyll a indicated an oligotrophic–mesotrophic boundary of about 21 mg m−2 and a mesotrophic–eutrophic boundary of about 55 mg m−2 based on upper and lower quartiles of the biovolume data distribution.4. Although algal species tolerance to nutrient and organic enrichment is well documented, additional taxonomic and autecological research on sensitive, endemic algal species would further enhance water‐quality assessments.

제주도 주변해역의 해양환경요인과 멸치 난자치어 분포

Anchovy spawn from the end of May to mid-October, when the water temperature is and the salinity is 26.0-33.6 psu. The main spawning season is between July to August, when the water temperature is and the salinity is between 26.0-32.2 psu. The main spawning grounds of anchovy are coastal areas shallower than 50 m around the islands located in the Jeju Strait. Anchovy larvae are distributed near the fronts between Chuja-do, Jangsu-do, Yeoseo-do, and the open sea rather than in the spawning grounds. Anchovy eggs and larvae density increased in accordance with the high level of during the summer season (July-August). In terms of the suspended sediment (SS) levels along the northern coast of the Jeju Strait, high densities of anchovy eggs (12.0-18.0 mg/L) were observed, mainly in the area affected by the coastal waters of the southern sea with high SS levels, while larvae (10.0-19.0 mg/L) tended to be distributed over a wide area with high SS levels, including the open sea. In terms of the dissolved oxygen (DO) content, eggs (5.4-6.8 mg/L) were observed in coastal areas with a high DO content, while larvae (4.2-6.4 mg/L) were distributed widely in areas with a relatively low DO content, from the southern coast to the open sea.

Influence of mixed liquor recycle ratio and dissolved oxygen on performance of pre-denitrification submerged membrane bioreactors

The conflicting influence of mixed liquor recycle ratio and dissolved oxygen on nitrogen removal and membrane fouling of a pre-denitrification submerged MBR was investigated in this study. It was found that a high aeration rate of 10 L air/min was able to minimize membrane fouling as compared with lower aeration rates of 5 and 2.5 L air/min in this study. Faster fouling at lower aeration rate was due to the decrease in cross-flow velocity across the membrane surface. However, high DO concentration (average of 5.1±0.5 mg O 2/L) present in the recycle mixed liquor at an aeration rate of 10 L air/min deteriorated the TN removal efficiency when operating at a recycle ratio of more than 3. A lower aeration rate of 5 L air/min, resulting in an average DO concentration of 3.4±0.7 mg O 2/L in the recycle mixed liquor, led to an improvement in TN removal efficiency: 63%, 80%, 84% and 89% for mixed liquor recycle ratio of 1, 3, 5 and 10, respectively. Further decrease in aeration rate to 2.5 L air/min, resulting in an average DO concentration of 1.9±0.8 mg O 2/L, did not improve the TN removal efficiency. Using a newly developed simplified nitrification–denitrification model, it was calculated that the COD/NO 3 −-N required for denitrification at 10 L air/min aeration rate was higher than those associated with 5 and 2.5 L air/min aeration rates. The model also revealed that denitrification at an aeration rate of 10 L air/min was limited by COD concentration present in the wastewater when operating at a mixed liquor recycle ratio of 3 and higher.

Simultaneous Nitrification, Denitrification, and Phosphorus Removal in Single‐Tank Low‐Dissolved‐Oxygen Systems Under Cyclic Aeration

Simultaneous nitrification and denitrification (SND or SNdN) may occur at low dissolved oxygen concentrations. In this study, bench-scale (approximately 6 L) bioreactors treating a continuous feed of synthetic wastewater were used to evaluate the effects of solids retention time and low dissolved oxygen concentration, under cyclic aeration, on the removal of organics, nitrogen, and phosphorus. The cyclic aeration was carried out with repeated cycles of 1 hour at a higher dissolved oxygen concentration (HDO) and 30 minutes at a lower (or zero) dissolved oxygen concentration (LDO). Compared with aeration at constant dissolved oxygen concentrations, the cyclic aeration, when operated with proper combinations of HDO and LDO, produced better-settling sludge and more complete nitrogen and phosphorus removal. For nitrogen removal, the advantage resulted from the more readily available nitrate and nitrite (generated by nitrification during the HDO period) for denitrification (during the LDO period). For phosphorus removal, the advantage of cyclic aeration came from the development of a higher population of polyphosphate-accumulating organisms, as indicated by the higher phosphorus contents in the sludge solids of the cyclically aerated systems. Nitrite shunt was also observed to occur in the LDO systems. Higher ratios of nitrite to nitrate were found in the systems of lower HDO (and, to less dependency, higher LDO), suggesting that the nitrite shunt took place mainly because of the disrupted nitrification at lower HDO. The study results indicated that the HDO used should be kept reasonably high (approximately 0.8 mg/L) or the HDO period prolonged, to promote adequate nitrification, and the LDO kept low (< or =0.2 mg/L), to achieve more complete denitrification and higher phosphorus removal. The above findings in the laboratory systems find strong support from the results obtained in full-scale plant implementation. Two plant case studies using the cyclic low-dissolved-oxygen aeration for creating and maintaining SND are also presented.

Temporal and spatial variation of physical, biological, and chemical parameters in a large waste stabilisation pond, and the implications for WSP modelling

The spatial and temporal variation of physical, chemical, and biological parameters was determined, in summer and winter, at nine sites in a large (112 ha) waste stabilisation pond (WSP) at the Bolivar Wastewater Treatment Plant. Each site was extensively sampled over the course of one day, with the nine sites sampled over successive days at exactly the same times of day, progressing in the direction of bulk flow through the pond. Analyses of covariance were used to test the independent impact of site and climate on the way in which the mean values and stratification gradient of the physical, chemical, and biological parameters varied diurnally at each site. In both winter and summer studies there was a very strong correlation at all sites between changes in temperature, pH and dissolved oxygen (DO). Mean pond temperatures were higher in summer than winter, and thermal stratification was more common in summer. In summer, during the day at each site, concentrations of chlorophyll-a, DO, suspended solids and pH increased with higher solar radiation levels. This relationship was less evident in winter. There was no systematic depth or temporal variation identified in either the summer or winter study for the broad range of chemical parameters measured. Mean values for these parameters, and to a lesser extent their stratification gradients, increased by varying extents throughout the day at the different sites in both summer and winter, irrespective of changes in climate when the different sites were sampled. Sites nearer the inlet to the WSP recorded lower NH4N and higher NO2N and NO3N concentrations than the rest of the WSP. This was indicative of nitrification. Somewhat surprisingly, high DO concentrations were also recorded at these sites near the inlets. Computational fluid dynamics (CFD) modelling, incorporating the predominant wind conditions, offers a rationale for these observations. Recirculation was evident, which may increase the residence time for the slow growing autotrophic nitrifying bacteria and recirculate oxygen rich water around these sites - conditions which would enhance nitrification. Understanding the effect of these variations, overlaid by the influence of hydraulic and temporal scenarios, assists in developing a mechanistic understanding of pond operation.

Contribution of endo- and exocellular glucose oxidase to gluconic acid production at increased dissolved oxygen concentrations

For gluconic acid fermentation with Aspergillus niger oxygen-enriched air was used to increase dissolved oxygen concentration during the production phase from c. 30% to 100% saturation related to air at 1 bar. At the highest dissolved oxygen concentration the activity of glucose oxidase increased more than three-fold. At the same time, the gluconic acid production rate increased by a factor of two. During the fermentations, glucose oxidase was present endocellularly as well as exocellularly; the presence of this enzyme in the broth was due to a controlled secretion. It was found that the two fractions of the enzyme showed different activities in their contribution to overall acid production. Due to a limitation of oxygen diffusion into the mycelial pellets, activity of endocellular enzyme was reduced by 50–80%. However, since the major part of the enzyme was present in the pellets the action of endocellular glucose oxidase is not negligible. This latter factor must be taken into consideration when optimizing bioreactor design.

Estimation of kinetic parameters of a model for deammonification in biofilms and evaluation of the model

A systematic approach to estimate and evaluate parameters for deammonification in biofilms from available experimental data was evaluated. Parameter estimation was based on a regional steady state sensitivity analysis to select relevant parameters and design of experiments based on a local identifiability analysis. The calibrated model was evaluated under different experimental conditions. Nine of the 16 kinetic and stoichiometric parameters had a significant influence on model predictions. Of these nine parameters only six kinetic parameters were identifiable from batch experiments regardless of the experimental design. More parameters were not identifiable due to high correlations between growth rates and the corresponding affinity constant for oxygen. Data from a batch experiment at 2 mg/L dissolved oxygen (DO) were used to estimate inactivation rates and affinity constants for oxygen for ammonium oxidisers (AO), nitrite oxidisers (NO) and anaerobic ammonium oxidisers (AN). In addition, it was found that not only kinetic and stoichiometric parameters but also the external mass transfer resistance significantly affected model predictions. The resulting model was able to reproduce batch test and continuous reactor operation where DO concentrations were similar to those in the batch experiment used for parameter estimation. However, the model overestimated deammonification for a batch experiment at a much higher DO concentration (5 mg/L). Thus, parameter values that are identifiable and are estimated for given environmental conditions may not necessarily be valid for significantly different experimental conditions.