A model of localised oxygen sinks around bacterial colonies within activated sludge

A new mathematical analysis of diffusional resistances of oxygen in activated sludge is developed. It assumes that the distribution of bacteria within activated sludges flocs are in colonies rather than the usual assumption of a homogeneous distribution of bacteria throughout the sludge. The solutions are for steady-state. The bacterial colony model results in a completely different shape of dissolved oxygen gradient in the floc when compared to the homogeneous bacterial distribution model. The new “bacterial colony” model predicts highly localised oxygen demands around the colony with maximum dissolved oxygen deficits in a 20–40 μm dia floc of 2–3 mg l⁻¹.     

The intrinsic kinetics of nitrification in a continuous flow suspended growth reactor

The intrinsic rate of nitrification was observed in a continuous flow reactor by eliminating external and internal diffusional resistances. The former were minimized by means of intense agitation, and the latter by mechanical rupture of the activated sludge floc using high impeller rotational speeds. The experimental results obtained from the continuous flow experiments confirmed the applicability of Michaelis-Menten kinetics to the activated sludge nitrification process. A possible dependency of k on contact time was found, larger values of k being observed under shorter contact times. The Michaelis constant K, was found practically unaffected by the contact times used in this study.     

Micro-profiles of activated sludge floc determined using microelectrodes

The microbial activity within activated sludge floc is a key factor in the performance of the activated sludge process. In this study, the microenvironment of activated sludge flocs from two wastewater treatment plants (Mill Creek Wastewater Treatment Plant and Muddy Creek Wastewater Treatment Plant, with aeration tank influent CODs of 60-120 and 15-35 mg/L, respectively) were studied by using microelectrodes. Due to microbial oxygen utilization, the aerobic region in the activated sludge floc was limited to the surface layer (0.1-0.2mm) of the sludge aggregate at the Mill Creek plant. The presence of an anoxic zone inside the sludge floc under aerobic conditions was confirmed in this study. When the dissolved oxygen (DO) in the bulk liquid was higher than 4.0mg/L, the anoxic zone inside the activated sludge floc disappeared, which is helpful for biodegradation. At the Muddy Creek plant, with its lower wastewater pollutant concentrations, the redox potential and DO inside the sludge aggregates were higher than those at the Mill Creek plant. The contaminant concentration in the bulk wastewater correlates with the oxygen utilization rate, which directly influences the oxygen penetration inside the activated sludge floc, and results in redox potential changes within the floc. The measured microprofiles revealed the continuous decrease of nitrate concentration inside the activated sludge floc, even though significant nitrification was observed in the bulk wastewater. The oxygen consumption and nitrification rate analyses reveal that the increase of ammonia flux under aerobic conditions correlates with nitrification. Due to the metabolic mechanisms of the microorganisms in activated sludge floc, which varies from one treatment plant to another, the oxygen flux inside the sludge floc changes accordingly.     

Model system of bulking and flocculation in mixed culture of Sphaerotilus sp. and Pseudomonas sp. for dissolved oxygen deficiency and high loading

A mixed continuous culture system was made up as a model for bulking and flocculation phenomena of the activated sludge to study the effect of dissolved oxygen (DO) deficiency and the effect of high organic loading. The system consisted of a floc forming bacterium and a filamentous bacterium which were isolated from the activated sludge and were identified as Pseudomonas sp. and Sphaerotilus sp., respectively. Sphaerotilus sp. had potential to cause a filamentous bulking phenomenon on the activated sludge. It was observed that the filamentous microorganism showed three kinds of growth form, filamentous form, pellet form and dispersed form, and that the floc former showed two kinds of growth form, good floc form and dispersed form. In the model system, these changes of growth form of two microorganisms, which could be thought as the cause of settling characteristics changes, depended on the DO level and the dilution rate (as a substitution for organic loading). The DO level also influence the aggregative ability of each microorganism and the maximum oxygen uptake rate, Q_O2max, of filamentous microorganism. The proportions of both microorganisms in model system were inverted reversibly by the DO level or the dilution rate changes.     

Demonstration of mass transfer and pH effects in a nitrifying biofilm

A bench-scale nitrifying trickling filter (surface AREA = 0.5 m²) was developed to permit evaluation of diffusion of oxygen within a biofilm, the pH dependence of ammonium oxidation and external mass transfer. In addition, a biofilm model was developed and verified for homogeneous nitrifying biofilms of varied thickness and for thin nitrifying biofilms covered by heterotrophic biofilms. The model uses literature values for the pH dependence of Monod coefficients for Nitrosomonas and Nitrobacter.

The diffusion coefficient of oxygen in the biofilm was found to be 40–80% of the value in pure water. Due to mass transfer resistance, the biomass ·sees” a lower pH than is measured in the water film passing over it. The surface uptake rate of ammonia is used as an indicator of pH gradients within the biofilm system. With the help of oxygen limitation experiments, the location of nitrifying biomass within mixed biofilms (heterotrophic, autotrophic) can be determined.

The biofilm model predicts ammonium uptake rate of a trickling filter as a function of the bicarbonate concentration in the water film.     

Stream productivity analysis with dorm—I: Development of computational model

A numerical dissolved oxygen routing model (DORM) is developed to determine total stream community photosynthesis (P) and community respiration rates (R) through iterative routing of two-station diel DO measurements. The model differs from existing procedures for diel curve productivity analysis; it uses the complete stream DO transport equation, including longitudinal dispersion, dependence of respiratory rate on water temperature and dissolved oxygen and wind dependent oxygen transfer through the water surface. The model, DORM, is more accurate than the traditional graphical procedures of diel curve analysis and is simple to apply. DORM is also valuable in an analysis of the results' sensitivity to the numerous coefficient and parametric assumptions required in diel curve analysis. The model is not a predictive DO model, but is designed for the interpretation of DO measurements in a stream, channel or river.     

Algal growth in warm temperate reservoirs: kinetic examination of nitrogen, temperature, light, and other nutrients

Nutrient limited growth of the phytoplankton assemblage in two Texas reservoirs was studied by a combination of nutrient addition experiments and statistical modeling. Dilution bioassays were run to ascertain the qualitative and quantitative patterns in nutrient limitation. Algal growth was frequently and strongly nutrient limited, particularly when temperature was >22°C. By itself, N was more often stimulatory than P, though strong additional enhancement of growth by P and trace nutrients was often detected. Monod growth kinetics indicated that half-saturation constants for N limited growth for the entire algal assemblage were in the range 20–200 μg N/L, relatively high compared to literature values, and increased with increasing temperature. Maximal growth was also an increasing function of temperature. A single temperature-dependent model was fit to the growth dynamics for all experiments showing N-limitation. The model μ=0.0256·T([DIN]/66.0+[DIN]) where μ is specific growth rate (d⁻¹), T is temperature (°C) and [DIN] is dissolved inorganic N (μmol/L) fit the experimental results reasonably well (r²=0.82). However, only a modest predictive power for growth in the controls (our best estimate of growth in situ) was achieved (r²=0.26). Thus, even with unusually detailed, site-specific fitting of model parameters, accurately modeling algal growth in natural ecosystems can remain a challenge.     

Variation of manganese, dissolved oxygen and related chemical parameters in the bottom waters of Lake Mendota, Wisconsin

The increase of Mn concentration and the simultaneous depletion of dissolved oxygen in the bottom waters (hypolimnion) of Lake Mendota, Wisconsin were studied in situ. Considerably higher Mn concentrations were found in the hypolimnion compared to Fe, despite a sedimentary Fe/Mn weight ratio of 19 : 1. Laboratory equilibration studies under anoxic conditions indicated that pH was an important parameter involved in the release of Mn from the sediments. This was also implied by the Mn and pH data measured in situ. Possible mechanisms involved in the release of Mn from the sediments including desorption of Mn from oxide substrates, reduction of Mn oxides and dissolution of carbonates and sulfides are discussed.     

Activity of Zoogloea ramigera growing in flocs and in suspension

A comparison was made between cells of Zoogloea ramigera occurring in flocs and in suspension. Suspended cells were obtained by adding cellulose to the growth medium, which prevented floc formation without damaging the cells. Differences in protein synthesis and poly-β-hydroxybutyrate production of cells in suspension and cells in flocs were only found between slowly shaken cultures. Increasing the oxygen content of such cultures did not raise the growth limitation of the cells in flocs. The oxygen uptake of the suspended cells was found to represent a zero order reaction at an oxygen concentration of the culture above 0.1 mg/l. and that of cells in flocs above 1.5 mg/l. Below 20 mg/ml glucose the glucose respiration rate of cells in flocs was lower than that of suspended cells. Above approx 20 mg/l. glucose, the glucose respiration of both types of cells was almost a zero order reaction.     

A preliminary modeling analysis of water quality in Lake Okeechobee, Florida: Diagnostic and sensitivity analyses

A deterministic mass balance model for nutrient and phytoplankton dynamics was previously developed and calibrated to a comprehensive set of field data for Lake Okeechobee. In the present study, diagnostic and sensitivity analyses were conducted with the calibrated model to better understand factors controlling phytoplankton and load-response dynamics in the lake. Phytoplankton growth rate limitation due to underwater light attenuation appears to be substantially greater than growth rate limitation due to non-optimal phosphorus concentrations. Phytoplankton biomass appears strongly controlled by the supply rate of dissolved available phosphorus to the water column. The dynamics of total phosphorus and chlorophyll a concentrations in the lake are strongly influenced by sediment-water phosphorus fluxes. There is a wide range of uncertainty in responses of total phosphorus and cholorophyll a concentrations to changes in tributary phosphorus loadings. Much of this uncertainty is due to a lack of quantitative understanding of sediment responses to changes in tributary loadings. Other important factors are inter-annual variability in hydrometeorological conditions and the potential influence of wind-induced resuspension of particulate phosphorus. Responses of total phosphorus and chlorophyll a concentrations for a given change in tributary loading depend not only on the magnitide of the loading change, but also on the time frame after the loading change due to a lag in sediment response. Load-response predictions for Lake Okeechobee must take into account changes in available phosphorus loadings to the water column, and must be premised on assumptions for changes in internal phosphorus loadings from the sediments. Results from this preliminary modeling analysis are provisional in that they do not include potential nitrogen limitation, potential interactions between phosphorus and nitrogen, or phytoplankton responses to potential nitrogen fixation.     

Some factors affecting floc formation by Zoogloea ramigera, strain I-16-M

A comparison was made between cells of Zoogloea ramigera occurring in flocs and in suspension. Suspended cells were obtained by adding cellulose to the growth medium, which prevented floc formation without damaging the cells. Differences in protein synthesis and poly-β-hydroxybutyrate production of cells in suspension and cells in flocs were only found between slowly shaken cultures. Increasing the oxygen content of such cultures did not raise the growth limitation of the cells in flocs. The oxygen uptake of the suspended cells was found to represent a zero order reaction at an oxygen concentration of the culture above 0.1 mg/l. and that of cells in flocs above 1.5 mg/l. Below 20 mg/ml glucose the glucose respiration rate of cells in flocs was lower than that of suspended cells. Above approx 20 mg/l. glucose, the glucose respiration of both types of cells was almost a zero order reaction.     

Effect of sludge retention time on inclined plate function,treatment performance and sludge characteristics of inclined plate membrane bioreactors treating municipal wastewater

Two identical bench scale inclined plate membrane bioreactors (ip‐MBRs) were operated for the treatment of real municipal wastewater for 1 year. Sludge retention time (SRT) was varied over the course of operation to investigate the effects on inclined plate function, treatment performance and sludge characteristic. Removal rates of chemical oxygen demand and ammonia over 90% and of total nitrogen (TN) more than 70% were achieved when ip‐MBRs were operated under SRTs between infinite and 40 days while short SRT (20 days) negatively affected TN removal. When the sludge concentration in anoxic tank exceeded 15 g/L, the failure of the inclined plate function was observed, resulting in no difference in sludge concentrations between aerobic and anoxic tanks. To avoid severe effects on inclined plate function and treatment performance, an SRT range of 40–80 days was recommended for ip‐MBRs. Moreover, sludge floc size under prolonged SRT became smaller than that under short SRT due to increased attrition among the sludge floc particles caused by strong aeration needed for keeping a sufficient dissolved oxygen level.     

Diffusion into microbial aggregates

Theoretical work in the biological waste treatment field has been directed at modeling substrate removal processes in fluidized and fixed film microbial systems in terms of the basic rate processes. Much of the research has been directed at delineating the rate limiting steps to simplify the problem. Various researchers have shown that the rate limiting step can be mass transfer through the microbial aggregate to the active sites at the cells. Therefore, any mechanistic model that incorporates mass transfer must be sensitive to variations in the reactant diffusion coefficient through floc material. A direct measure of mass flux has been developed to determine the variations in the diffusion coefficients of glucose and oxygen through microbial aggregates grown under various experimental conditions. A factorial analysis indicated significant changes in the molecular diffusion coefficient with variations in sludge age and carbon-nitrogen ratio in the growth media. Oxygen diffusivity varied from 20 to 100% of its value in water, glucose from 30 to 50%. A simple zero order diffusion-reaction kinetic model for spherical floc was constructed. It indicated that oxygen diffusion limitations are possible in the high rate activated sludge processes with large floc particles.     

The impact of pH on floc structure characteristic of polyferric chloride in a low DOC and high alkalinity surface water treatment

The adjustment of pH is an important way to enhance removal efficiency in coagulation units, and in this process, the floc size, strength and structure can be changed, influencing the subsequent solid/liquid separation effect. In this study, an inorganic polymer coagulant, polyferric chloride (PFC) was used in a low dissolved organic carbon (DOC) and high alkalinity surface water treatment. The influence of coagulation pH on removal efficiency, floc growth, strength, re-growth capability and fractal dimension was examined. The optimum dosage was predetermined as 0.150 mmol/L, and excellent particle and organic matter removal appeared in the pH range of 5.50-5.75. The structure characteristics of flocs formed under four pH conditions were investigated through the analysis of floc size, effect of shear and particle scattering properties by a laser scattering instrument. The results indicated that flocs formed at neutral pH condition gave the largest floc size and the highest growth rate. During the coagulation period, the fractal dimension of floc aggregates increased in the first minutes and then decreased and larger flocs generally had smaller fractal dimensions. The floc strength, which was assessed by the relationship of floc diameter and velocity gradient, decreased with the increase of coagulation pH. Flocs formed at pH 4.00 had better recovery capability when exposed to lower shear forces, while flocs formed at neutral and alkaline conditions had better performance under higher shear forces.     

Stream productivity analysis with dorm—II: Parameter estimation and sensitivity

The dissolved oxygen routing model DORM, which determines productivity and respiration of a stream biological community, requires in addition to stream geometry and stream flow, parameter values for reaeration coefficients and temperature and dissolved oxygen (DO) limitations on respiration. Illustrated are the methods used to obtain numerical values in a field study at the U.S. EPA Monticello Ecological Research Station. The calculated community productivity and respiration rates are sensitive to certain input parameters and assumptions. Analysis showed that, among the many parameters needed, hydraulic residence time and reaeration coefficients should be determined with utmost care. Other sensitivity tests included temperature effects, half-saturation values and longitudinal dispersion.     

Predicting the settling velocity of flocs formed in water treatment using multiple fractal dimensions

Here we introduce a distribution of floc fractal dimensions as opposed to a single fractal dimension value into the floc settling velocity model developed in earlier studies. The distribution of fractal dimensions for a single floc size was assumed to cover a range from 1.9 to 3.0. This range was selected based on the theoretically determined fractal dimensions for diffusion-limited and cluster-cluster aggregation. These two aggregation mechanisms are involved in the formation of the lime softening flocs analyzed in this study. Fractal dimensions were generated under the assumption that a floc can have any value of normally distributed fractal dimensions ranging from 1.9-3.0. A range of settling velocities for a single floc size was calculated based on the distribution of fractal dimensions. The assumption of multiple fractal dimensions for a single floc size resulted in a non-unique relationship between the floc size and the floc settling velocity, i.e., several different settling velocities were calculated for one floc size. The settling velocities calculated according to the model ranged from 0 to 10 mm/s (average 2.22 mm/s) for the majority of flocs in the size range of 1-250 μm (average 125 μm). The experimentally measured settling velocities of flocs ranged from 0.1 to 7.1 mm/s (average 2.37 mm/s) for the flocs with equivalent diameters from 10 μm to 260 μm (average 124 μm). Experimentally determined floc settling velocities were predicted well by the floc settling model incorporating distributions of floc fractal dimensions calculated based on the knowledge of the mechanisms of aggregation, i.e., cluster-cluster aggregation and diffusion-limited aggregation.     

Physical and chemical properties of activated sludge floc

Physical and chemical characteristics of activated sludge such as floc size, density, specific surface, carbohydrate content, dehydrogenase activity and settleability were investigated by seven parallel bench scale activated sludge units operated under different sludge ages (1.1–17.4 days). The analytical methods used included a dye adsorption technique for specific surface area determinations, the Coulter Counter method for floc size measurements and interference microscopy for floc density determinations. The typical floc sizes were found to be in the range 10–70 μm with floc densities in the range 1.015–1.034 g cm⁻³. A strong correlation between floc density and size was obtained. The specific surface areas measured (typically 100–200 m² g⁻¹ dry sludge) were found to be one to two orders of magnitude higher than the corresponding geometric floc surface areas, indicating a porous floc structure. Sludge settleability, for non-filamentous sludges, was well correlated to both floc size, density and specific surface area, but not to the sludge carbohydrate content, which was found to vary between 6 and 18%.     

Photo-dissolution of flocculent, detrital material in aquatic environments: contributions to the dissolved organic matter pool

This study shows that light exposure of flocculent material (floc) from the Florida Coastal Everglades (FCE) results in significant dissolved organic matter (DOM) generation through photo-dissolution processes. Floc was collected at two sites along the Shark River Slough (SRS) and irradiated with artificial sunlight. The DOM generated was characterized using elemental analysis and excitation emission matrix fluorescence coupled with parallel factor analysis. To investigate the seasonal variations of DOM photo-generation from floc, this experiment was performed in typical dry (April) and wet (October) seasons for the FCE. Our results show that the dissolved organic carbon (DOC) for samples incubated under dark conditions displayed a relatively small increase, suggesting that microbial processes and/or leaching might be minor processes in comparison to photo-dissolution for the generation of DOM from floc. On the other hand, DOC increased substantially (as much as 259 mgC gC⁻¹) for samples exposed to artificial sunlight, indicating the release of DOM through photo-induced alterations of floc. The fluorescence intensity of both humic-like and protein-like components also increased with light exposure. Terrestrial humic-like components were found to be the main contributors (up to 70%) to the chromophoric DOM (CDOM) pool, while protein-like components comprised a relatively small percentage (up to 16%) of the total CDOM. Simultaneously to the generation of DOC, both total dissolved nitrogen and soluble reactive phosphorus also increased substantially during the photo-incubation period. Thus, the photo-dissolution of floc can be an important source of DOM to the FCE environment, with the potential to influence nutrient dynamics in this system.     

Field evidence for copper mobilization by dissolved organic matter

Fluxes of copper from the sediments to the water column in response to the presence of dissolved organic matter were measured in field enclosures placed on the floor of the Contrary Creek arm of Lake Anna, Virginia. Experimental chambers received a spike input of either sodium humate or phytoplankton; unamended chambers served as controls. Over the subsequent five days, water samples were withdrawn from the chambers and analyzed for total and dissolved copper, total and dissolved organic carbon (DOC) Eh, pH, dissolved oxygen, dissolved sulfides and heterotrophic microbial activity. Chambers amended with phytoplankton did not experience a rise in dissolved copper concentrations. Furthermore, the concentrations of dissolved copper in these chambers were not well-correlated with DOC concentrations (r = 0.1175, P = 0.642). Dissolved copper concentrations in chambers amended with sodium humate rose approximately 500% and remained elevated for the duration of the experimental trial (5 days). In this treatment, dissolved copper concentrations were well-correlated with DOC concentrations (r = 0.798, P < 0.005). The increase in dissolved copper in the chambers amended with sodium humate was observed to persist despite reducing conditions which were confirmed by redox measurements and the occurrence of dissolved sulfides. The addition of sodium humate to the chamber water resulted in a net mobilization of copper to the water column from the sediments. In contrast, dissolved organic carbon generated by the decomposition of phytoplankton did not appear to mobilize copper over the time interval examined. None of the changes in soluble copper concentrations could be related to Eh or pH effects. Supplementary laboratory experiments corroborated the field trials. The results suggest that long term (but not short term) products of decomposition processes may enhance the concentration of dissolved copper in the water column by the formation of soluble organometallic complexes.