Anoxic Fraction Research Articles

Tc(VII) and Cr(VI) Interaction with Naturally Reduced Ferruginous Smectite from a Redox Transition Zone.

Fe(II)-rich clay minerals found in subsurface redox transition zones (RTZs) can serve as important sources of electron equivalents limiting the transport of redox-active contaminants. While most laboratory reactivity studies are based on reduced model clays, the reactivity of naturally reduced field samples remains poorly explored. Characterization of the clay size fraction of a fine-grained unit from the RTZ interface at the Hanford site, Washington, including mineralogy, crystal chemistry, and Fe(II)/(III) content, indicates that ferruginous montmorillonite is the dominant mineralogical component. Oxic and anoxic fractions differ significantly in Fe(II) natural content, but FeTOTAL remains constant, demonstrating no Fe loss during its reduction-oxidation cyclings. At native pH of 8.6, the anoxic fraction, despite its significant Fe(II), ∼23% of FeTOTAL, exhibits minimal reactivity with TcO4- and CrO42- and much slower reaction kinetics than those measured in studies with biologically/chemically reduced model clays. Reduction capacity is enhanced by added/sorbed Fe(II) (if Fe(II)SORBED > 8% clay Fe(II)LABILE); however, the kinetics of this conceptually surface-mediated reaction remain sluggish. Surface-sensitive Fe L-edge X-ray absorption spectroscopy shows that Fe(II)SORBED and the resulting reducing equivalents are not available in the outermost few nanometers of clay surfaces. Slow kinetics thus appear related to diffusion-limited access to electron equivalents retained within the clay mineral structure.

Potential of hybrid processes for nitrogen removal from swine wastewater

A comprehensive investigation was carried out using a bench-scale plant (70l) with the aim of evaluatingthe potential of two hybrid processes that use plastic moving carriers for nitrification and denitrification, given the restriction that low and stable values of ammonium and nitrate had to be met. In one process, the nitrification aerobic reactor operated with the biomass in biofilm exclusively and achieved a nitrification rate of 0,8 gN/m2·d. In the other process, nitrification operated as a hybrid process and a similar nitrification rate in the biofilm was found. Batch tests revealed that suspended solids in the hybrid reactor provided an additional 20% nitrification rate. This resulted in a potential volumetric nitrification rate of 380 gN/m3·d at 20°C, which is 2.5 times the expected rate in activated sludge plants under the same conditions. A high anoxic fraction (55%) was used in both processes with hybrid denitrification, since it minimized inhibition of nitrification in the biofilm due to the biodegradable COD applied load. Batch tests undertaken with high suspended solids (4 g/l) revealed that 60% of the denitrification took place in the biomass in suspension, and 40% in biofilm, providing a high level and robust performance. The dosage of methanol was crucial in regulating effluent nitrate. It is concluded that both processes offer great potential regarding the design of more compact new plants and the upgrading of existing facilities.

Multi-criteria analysis of wastewater treatment plant design and control scenarios under uncertainty

Wastewater treatment plant control and monitoring can help to achieve good effluent quality, in a complex, highly non-linear process. The Benchmark Simulation Model no. 2 (BSM2) is a useful tool to competitively evaluate plant-wide control on a long-term basis. A method to conduct scenario analysis of process designs by means of Monte Carlo (MC) simulations and multi-criteria evaluation is presented. It is applied to the open loop version of BSM2 and to two closed loop versions, one with a simple oxygen controller and the other one with an ammonium controller regulating the set-point of the oxygen controller (cascade controller). The results show a much greater benefit of the cascade controller compared to the simple controller, both in environmental and economic terms. From an optimal process design point of view, the results show that the volume of the primary clarifier and the anoxic fraction of the reactor volume have an important impact on process performance. The uncertainty analysis of the optimal designs, also performed with MC simulations, highlights the improved and more stable effluent under closed loop control.

A benthic–pelagic coupled ecosystem model to estimate the hypoxic estuary including tidal flat—Model description and validation of seasonal/daily dynamics

A new ecosystem model – the Ecological Connectivity Hypoxia Model (ECOHYM) – was developed and applied to Tokyo Bay, a hypoxic estuary in Japan. ECOHYM permits study of the dynamics and mechanisms of hypoxia from two perspectives: (1) the whole estuary, composed of temporal-spatial mutual linkage of benthic–pelagic or central bay-tidal flat ecosystems (holistic approach), and (2) each biochemical and physical process, contributing to oxygen production/consumption (elemental approach). ECOHYM evaluates the oxygen–carbon–nitrogen–phosphorus-coupled cycle driven by physical and biochemical processes. The processes mechanically describe the ecosystem network consisting of benthic–pelagic and central bay–tidal flat ecosystem coupling. The model also describes the early diagenetic processes of the benthic ecosystems by setting a micro-scale vertical spatial resolution due to the drastic vertical change of benthic biological metabolism and its high contribution to total oxygen consumption. Model variables were mainly selected based on low trophic level; namely, phytoplankton, zooplankton, detritus, dissolved organic matter, NH 4–N, NO 3–N, PO 4–P, benthic algae, suspension feeders, deposit feeders (benthic faunas), dissolved oxygen (DO) and oxygen demand units (ODU, representing the stoichiometric expression of Mn 2+, Fe 2+ and S 2− oxygen demand). Detritus was divided into fast-labile, slow-labile and refractory parts. Bacterial mineralization processes were divided into oxic, suboxic and anoxic fractions. The model was applied for a time frame of 100 years under seasonal forcing functions to achieve an annual periodical steady state. The model results revealed that the seasonal dynamics were more noticeable than the daily/tidal dynamics in the central bay area. In contrast, the daily/tidal dynamics were determinant in the tidal flat area. Calculated seasonal dynamics in the central bay and daily/tidal dynamics in the tidal flat were in good agreement with the observed data. In particular, the model succeeded in reproducing the following vertical DO profiles during the hypoxic season (summer): (1) high oxygen at the sea-surface and hypoxia at the sea-bottom in the central bay area, (2) the reduced thickness of the benthic oxic layer in the central bay area, and (3) the drastic change of DO micro-profiles between daytime and nighttime of the benthic system in the tidal flat areas. Oxygen fluxes at the sediment–water interface in the model also reproduced the in situ observed values both in the central bay and tidal flat areas.

The influence of organic loading and anoxic/oxic times on the removal of carbon, nitrogen and phosphorus from a wastewater treated in a sequencing batch reactor

In this study, 10 L sequencing batch reactors (SBRs) were operated at a 12-h cycle length (four alternating anoxic/oxic conditions) to assess the biological nutrient removal potential of a domestic wastewater treated at the Huay Kwang plant, Bangkok, Thailand. The wastewater was found to be carbon-limited (chemical oxygen demand (COD) to total Kjeldahl nitrogen (TKN) (i.e., COD:TKN) ratio of 6.4:1). This ratio was insufficient to support good phosphorus removal. Glucose was therefore added to increase the COD:TKN ratio ultimately to 10:1 and the COD, TKN and total phosphorus (TP) removals at this ratio were all in excess of 95%. An alternative carbon source from a local fruit canning industry was then added at the same COD:TKN ratio; and, in order to increase the throughput of the waste treated, the cycle length was simultaneously shortened to 8 h keeping approximately the same anoxic/oxic fractions. The COD removal remained high (> 95%), however the TKN and TP removals were substantially reduced (79% and 66%, respectively), indicating that the shortened cycle length was sub-optimum. The last phase of the research involved changing the anoxic/oxic fractions of the cycle time to maximize performance. It was found that for the conditions studied in this research, the performance improved in proportion to the increase in the first anoxic fraction, being most stable at the highest anoxic fraction of the cycle length (0.33).

Controlling sludge settleability in the oxidation ditch process

This paper describes an investigation aimed at developing an operating technique for controlling sludge settleability in the oxidation ditch form of the nitrification denitrification activated sludge process. It was hypothesized that specific sludge volume index (SSVI) is lowest at an optimum process anoxic fraction and increases at higher and lower fractions. Using effluent ammonia:nitrate ratio as a surrogate for anoxic fraction, it was found that a simple empirical model based on a three solids retention time moving average nitrogen ratio was able to replicate the long-term SSVI variations in two independent oxidation ditches at a full-scale plant. Operating data from a second oxidation ditch plant during periods when a prefermenter was on- or off-line showed that SSVI also varies with RBCOD, greater RBCOD giving lower SSVI. It was concluded that best settleability occurs at about the same anoxic fraction as lowest effluent total nitrogen concentration, with an ammonia:nitrate ratio of about 1. An operating rule of thumb is to use dissolved oxygen control to maintain effluent ammonia and nitrate nitrogen concentrations about equal. A third oxidation ditch plant deliberately operated in this manner achieved 15-month median operating values for SSVI of 60 mL/g and for effluent ammonia, nitrate and total N, respectively, of 0.2, 0.3 and 2.0 mg N/L.

Appropriate Design of Activated Sludge Systems for Nitrogen Removal from High Strength Wastewaters

The study is focused on defining a conceptual approach for the design of activated sludge systems treating strong wastewater for nitrogen removal. The effect of major factors such as COD/N ratio, denitrification potential, available nitrate, anoxic volume fraction, and recycle ratio is evaluated in terms of basic stoichiometry. An algorithm for appropriate design is developed for a continuous flow activated sludge and sequencing batch reactor, both operated in a pre-denitrification mode. The design approach is tested for two types of strong industrial wastewaters with different N contents and COD/N ratios and a typical domestic sewage for comparative evaluation.

Improving the predictions of ASM2d through modelling in practice

The paper presents the adjustments carried out on the structure and in some default values of the kinetic coefficients of the ASM2d model in order to get an improved prediction for the experimental results obtained in pilot scale plants studies with different activated sludge treatment processes for carbon, nitrogen and phosphorous removal. In order to predict the high effluent filtered COD experimentally observed in high rate processes for carbon removal, a new model structure has been proposed, incorporating into the carbon model a soluble fraction of the slowly biodegradable substrate. Studies with the step feed and the alternating processes showed simultaneous nitrification-denitrification in aerated reactors which was predicted with increased values in the oxygen saturation coefficients for heterotrophic and autotrophic biomass. Both processes also showed loss of the denitrification capacity under unfavourable conditions, such us rains and low anoxic fraction, which required a very large decrease in the value of the reduction factor for denitrification so as to improve the predictions for effluent nitrate experimental results. Regarding phosphorus removal, the ASM2d model showed a satisfactory predictive capacity. For improved predictions of phosphorus release in anaerobic conditions, high values of the rate constant for storage of X(PHA) and low values of the anaerobic hydrolysis reduction factor were used. For phosphorus uptake in aerobic and anoxic conditions satisfactory predictions were obtained using the default values.

Modelling belowground processes to explain field-scale emissions of nitrous oxide

Globally, soils are the most important source of nitrous oxide (N 2O). In several recent studies, N 2O fluxes from soils have been monitored. Regression relationships between N 2O fluxes and variables like average soil moisture content, nitrogen content and temperature did in general not explain more than 50% of the flux variance. The quantitative insight into the relation between measured fluxes on the one hand, and the underlying basic processes and their determining factors on the other, is still limited. This study aims to contribute to this insight by modelling underground processes to explain N 2O profiles in the soil. On sandy grassland soils in the Wageningen Rhizolab, we monitored aboveground N 2O fluxes as well as underground profiles of several variables. A process-based model was adapted to simulate the relevant processes. In the model we assumed that the sandy soil profile consisted of homogeneous soil layers. The modelled processes included biological and physical processes. Nitrous oxide is produced during denitrification and is transported via diffusion. The simulated O 2 and CO 2 profiles were satisfactory, indicating that the (bulk) respiration rates used in the model were realistic. The N 2O profiles, however, were less well simulated. With respect to N 2O production, the assumption of homogeneity within the soil layers was probably not correct for the sandy soil. We discuss modelling approaches to explain observed N 2O profiles by describing partial anaerobicity in soil layers. Of these, the so-called ‘randomly distributed-pore model’ [Eur. J. Soil Sci. 46 (1995) 507] and an adapted version for regularly distributed pores appear good candidates for incorporation in field-scale models for a layered sandy soil. Such models would describe processes like bulk respiration and the transport and plant uptake of water and nutrients. The randomly distributed-pore model is a relatively simple model for a steady state situation. For sandy soil layers, the model predicts that a substantial part of the volume can be anoxic under a range of conditions including conditions that often occurred in our experiment: a ratio (volumetric moisture content)/(saturated volumetric moisture content)≥0.7 and oxygen percentages in the air-filled pores between 5 and 21%. The regularly distributed-pore model also predicts the occurrence of anoxic fractions under common experimental conditions, but these fractions are significantly smaller than those according to the randomly distributed-pore model.

The distribution of anoxic volume in a fractal model of soil

A simple description of soil respiration is combined with a three-dimensional random fractal lattice as a model of soil structure. The lattice consists of gas-filled pores and soil matrix that is a combination of the solid phase and water. A respiration process is assumed to take place in the soil matrix. Oxygen transport occurs by diffusion in the gas-filled pores and, at a much slower rate, in the soil matrix. The stationary state of this process is characterized by the fraction of the matrix that has zero oxygen concentration, i.e., the anoxic fraction. The anoxic fraction of a three-dimensional lattice appears to be largely determined by the presence and distribution of pores that are not connected to the surface of the lattice. Local gradients in connected gas-filled pores play an insignificant role due to the enormous difference in diffusion coefficient between the gas-filled pores and the saturated soil matrix. Analytical and numerical results for the fractal model are compared with calculations for a dual-porosity model comprising spherical aggregates with a lognormal radius distribution. A one-dimensional fractal lattice and the dual-porosity model yield qualitatively similar predictions, suggesting an anoxic fraction that decreases exponentially with the square root of the local oxygen concentration. However, the anoxic fraction of a three-dimensional fractal lattice decreases much faster than exponentially, implying that large clumps of soil matrix are comparatively rare. We propose that this is due to aggregation of soil particles in more than a single dimension, which has important consequences for anaerobic processes in soil. The fractal model accounts for the geometrical implications of three dimensions. A lognormal radius distribution is essentially a one-dimensional structure model.

Evaluation of nitrogen removal by step feeding in large treatment plants

Step feeding flow configuration is an attractive process alternative for large treatment systems designed for nitrogen removal, eliminating the need for internal recycling and optimizing organic carbon utilization for denitrification. Volume ratios of anoxic and aerobic volumes and wastewater fraction to be diverted from the inlet of the system are important parameters to be considered in the design of the step feeding process. Wastewater characterization and especially C/N ratio significantly affect the design. The effect of wastewater characterization is illustrated for the Riva Plant in Istanbul, with a capacity of 4 million population equivalent, designed as a two-stage step feeding system for biological nutrient removal. It is shown that the proposed design cannot meet the effluent standard of 10 mgl−1 total N for the relevant sewage characterization assessed on the basis of an extensive experimental study, totally different from the one adopted for design.

Simplified models of anoxia and denitrification in aggregated and simple‐structured soils

SummaryTwo complementary approaches to modelling soil anoxia and denitrification are compared. The first postulates a structurally and biologically heterogeneous soil matrix (physical aggregates) and Michaelis—Menten kinetics; the second a random distribution of cylindrical air‐filled pores, a uniform metabolic activity and zero‐order reaction kinetics. Simple functional approximations to both models are developed, allowing their differences to be explored. At water contents corresponding to aggregate saturation the anoxic fractions and denitrification rates predicted by the aggregate model exceed those predicted by the simple‐structure model. In drier soils the predictions of the latter model typically exceed those of the former by between one and three orders of magnitude, the discrepancy lessening as the oxygen reaction potential and the mean radius of the aggregates increase. The aggregate model is much more sensitive to air‐filled porosity, and shows a decreased denitrification efficiency when nitrate concentrations are low. It is likely to predict sharper rainfall‐induced denitrification events, and a smaller background activity, than the distri‐buted‐pore treatment. Whichever of these or other process‐based treatments of denitrification is adopted, simple approximations like those presented here greatly facilitate inclusion in larger‐volume systems models.

Influences of wastewater composition and operating conditions on activated sludge bulking and scum formation

Bulking and scum formation (scumming) are common phenomena in activated sludge plants. Four main problem groups with filamentous microorganisms with different growth requirements are distinguished: (1) “Aerobic Bulking”, (2) “Scumming due to Actinomycetes”, (3) “Low F/M Bulking and Scumming” and (4) “Bulking due to Sulphide Oxidising Bacteria”. Various wastewater fractions, such as readily biodegradable substrate, surfactants, hydrophilic and lipophilic slowly-biodegradable substrate, inoculated filamentous microorganisms and sulphide, strongly influence the biocenosis composition. Apart from wastewater composition, operating conditions, such as solids retention time, reactor configuration, mechanical stress by recirculation pumps and insufficient oxygen supply, are often decisive for the occurrence of a bulking or scumming problem, as well. A predictive scheme for the possible occurrence of the four main problem groups is presented for different combinations of wastewater compositions, operating conditions and reactor design. All four problem groups can occur in aerobic nitrifying systems. In non-nitrifying systems, only “Aerobic Bulking” and “Bulking due to Sulphide Oxidising Bacteria” are expected, depending on wastewater composition and operating conditions. In systems with a large anoxic mass fraction, “Low F/M Bulking and Scumming” is typically the only problem group. Since wastewater fractions can hardly be manipulated in practice, at least operating conditions and reactor design should be optimised in order to obtain a better performance of the plants.

Anoxia induces phospholipase A2 activation in rabbit renal proximal tubules.

Phospholipase A2 (PLA2) activation during anoxic cell injury was determined by use of a variety of approaches in rabbit proximal renal tubules. Arachidonic acid (AA) mass release increased from 4 +/- 1 (normoxia control) to 40 +/- 6 ng/mg protein after 20 min and 106 +/- 16 ng/mg protein after 40 min of anoxia. PLA2 activity was measured by estimating the amount of sn-2 fatty acid released from either 14C-labeled Escherichia coli membranes or [14C]phosphatidylethanolamine (PE) micelles incubated with membrane and cytosolic fractions obtained from normoxic or anoxic tubules. At pH 7.4 and 1 mM Ca, PLA2 activity increased in the 20-min anoxic membrane fractions from 8.1 +/- 2.3 (normoxic) to 15.2 +/- 2.1 pmol.min-1.mg protein-1 (anoxic). When the proximal tubules were homogenized in the absence of Ca, the anoxia-induced PLA2 activity was found to be soluble. Preincubation with pancreatic PLA2 antibody inhibited 50% of both basal and anoxia-stimulated PLA2 activity. Two protein bands (40- and 21-kDa species) immunoreactive to PLA2 antibody were detected in the membrane fraction. A sixfold increase in the immunoreactivity of the 40-kDa band was detected after 40 min of anoxia of proximal tubules. These results suggest that anoxia induces an intracellular PLA2 activity in kidney cells that could be immunologically related to pancreatic PLA2.

Operational Strategies for Predenitrification Process

A detailed mathematical model of the predenitrification process was employed to study the effects of key operational variables on nitrogen removal and secondary clarification. Effects of influent flow rate and solids residence time on thickening‐imposed capacity were elucidated. The dependence of optimal settings of dissolved oxygen concentration, anoxic volume fraction, and mixed liquor recirculation ratio on solids retention time, influent flow rate, influent wastewater characteristics, and temperature was determined. The influence of clarifier bio‐reactions was also examined.

Theories for estimation of the fraction of denitrifiers in combined nitrifying-denitrifying treatment plants

Theories for the estimation of the fraction of denitrifiers in active biomass are proposed and discussed. Two concepts are used for decay. One is the decay-growth concept, where organic matter cycles between substrate and biomass, and the second is the traditional decay concept, where biomass decays without any recycling of substrate. It is shown that the fraction of denitrifiers is a function of three variables, the potential inlet fraction of denitrifiers (the fraction of the influent material which after primary growth upon the substrate is denitrifiers) the anoxic solids retention time ratio and the total solids retention time. The potential inlet fraction of denitrifiers is the dominating variable. In order to maximize the fraction of denitrifiers in the activated sludge, the influent wastewater should be anoxic treated as the first step of the process and the anoxic fraction of the total solids retention time should be as high as possible.

Microoxic-Anoxic Niche of Beggiatoa spp.: Microelectrode Survey of Marine and Freshwater Strains

Beggiatoa spp. grow optimally in media containing opposed gradients of oxygen and soluble sulfide, although some strains also require an organic substrate. By using microelectrodes, we characterized oxygen and sulfide gradients during their initial development in uninoculated media and in cultures of marine and freshwater strains. In gradient media, Beggiatoa strains always grew some distance below the air/agar interface as a dense "plate" of constantly gliding filaments with sharply demarcated upper and lower boundaries. Within established plates, the maximum oxygen partial pressure was 0.6 to 6.0% of air saturation and not significantly lower if filaments were fixing nitrogen. Oxygen penetrated only 100 to 300 mum into the plate, and the anoxic fraction increased from less than 10% to approximately 90% during later stages of growth. For lithoautotrophically grown marine strains, the linearity of the oxygen profile above the plate plus its drop to zero therein indicated that oxygen uptake for the entire tube occurred only within the Beggiatoa plate. Consequently, oxygen consumption could be predicted solely from the distance between the air/agar interface and the top of a plate, given the diffusion coefficient for oxygen. By contrast, for freshwater strains grown heterotrophically (with sulfide also in the medium), oxygen profiles were frequently nonlinear because of nonbiological reaction with sulfide which had diffused past the aggregated filaments. For all strains tested, microoxic aggregation also occurred in the absence of sulfide, apparently reflecting a step-up phobic response to oxygen.

Nitrogen removal in a semi-continuous process

Laboratory tests using synthetic feed were conducted to evaluate the nitrogen removal capabilities of the continuously fed intermittently decanted (CFID) process. Experimental results were in general agreement with theoretically predicted values. Ammonia removal is governed by the extent of nitrification during aeration, and the level of leakage during settling and decantation. The former is controlled by the aerobic SRT, with the critical value being approx. 5 days at 13–18°C and <2.5 days at 25–26°C. Ammonia leakage is controlled by hydraulic parameters. For a given nominal hydraulic residence time, short cycle, short settling and short decantation times are conducive to low leakage. Total removal of nitrogen can be obtained if a well nitrifying system is allowed to denitrify. Denitrification is affected primarily by the anoxic fraction of the cycle. The specific denitrification rate ranged from 0.014 to 0.037 g N g SS −1 day −1, which is comparable to reported values for internal or endogenous denitrification in conventional systems.