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Operational Limitations of Arctic Waste Stabilization Ponds: Insights from Modeling Oxygen Dynamics and Carbon Removal
Abstract
Presented here is a mechanistic model of the biological dynamics of the photic zone of a single-cell arctic waste stabilization pond (WSP) for the prediction of oxygen concentration and the removal of oxygen-demanding substances. The model is an exploratory model to assess the limiting environmental factors affecting treatment performance in arctic WSPs. A sensitivity analysis was used to provide a quantification of the relative uncertainties of parameters that exist within the described modeling framework. The model was able to qualitatively reproduce mesocosm experiment trends in phytoplankton growth, dissolved oxygen concentration, and the reduction of carbonaceous biochemical oxygen demand on Day 5 (CBOD5). These results demonstrated that CBOD5 reduction and oxygen state are very sensitive to organic loading regimes at low temperatures (5-15°C). The sensitivity analysis identified that it was the difference in phytoplankton growth rates, and the associated change in photosynthetic oxygen production, that mainly contribute to creating differences in CBOD5 removal rates and the development of aerobic conditions. The model was also sensitive to atmospheric aeration rates at low temperature, providing further evidence that low oxygen availability limits the treatment of CBOD5 in cold-climate WSPs. During the development process, it was discovered that common formulations of depth-integrated phytoplankton growth performed poorly for the arctic system to be modeled, which was a quiescent eutrophic environment. This paper presents a new phytoplankton growth formula within the paradigm of a poorly mixed eutrophic system that may find utilization in other eutrophic, colored, or turbid systems. The novel aspect of the approach is that the depth-integrated phytoplankton growth function was formulated upon the premise that the phytoplankton would be capable of orienting themselves to optimize their growth under poorly mixed conditions, and the average growth rate of the phytoplankton population must decrease as crowding puts pressure on shared resources. The general agreement of the model with the experiments, combined with the simplicity of the depth-integrated box model, suggests there is potential for further development of the model as a tool for assessing proposed arctic WSP designs. The sensitivity analysis highlighted the uncertainty and importance of the parameterization of bacterial and phytoplankton physiology and metabolism in WSP models.
... Literature that examines the effects of waste management in the Arctic has mostly been focused on marine biology and environmental science (Jing et al. 2012;Ragush et al. 2018), military impact including radioactive waste (Nuttall, 1998;Council, 2009;Hird, 2016;Rahbek-Clemmensen, 2017), Industrial waste including oil spills (Owens et al. 2009) and more recently individual indigenous communities (Hird & Zahara, 2017). However the impact of international cooperation with Russia on waste management within circumpolar indigenous communities is largely limited to partnerships in the area of radio-active/nuclear waste (Sawhill 2000(Sawhill , 2001Champ et al. 2001;Ortman, 2009;Vijgen, 2013;Koval et al. 2016). ...
Circumpolar communities continue to face huge challenges in waste management driven by global economic trends. Regional concerns over the increasing effects of climate change and environmental pollution has encouraged cross-border partnerships and joint project collaboration amongst Arctic member states. New incentives have also been introduced to foster regional investments in the waste management sector due to growing demands for sustainable solutions by indigenous communities. This paper provides insight to the existing cooperation programs between Russia and other Arctic member states specifically in the field of sustainable development and waste management. The study also presents a case study review on the extant municipal waste management challenges being faced by indigenous communities in the region and the attendant programs developed to ameliorate the negative impact. The research methodology applied comprises the analysis of related research, public documents and official reports.
... Another important consideration in evaluating Arctic lagoon performance could be related to the contribution of algae blooms to COD removal. This attribution is supported by Ragush et al. (2018) and Schmidt et al. (2017), who examined oxygen dynamics and organic degradation for lagoons during the ice-free period and demonstrated algae's contribution in supporting aerobic degradation. While the current version of the model does not consider algae growth, it appears to be indirectly accounted for by the oxygen transfer coefficient (k L ). ...
Waste stabilisation ponds (WSPs) are the method of choice for sewage treatment in most arctic communities because they can operate in extreme climate conditions, require a relatively modest investment, are passive and therefore easy and inexpensive to operate and maintain. However, most arctic WSPs are currently limited in their ability to remove carbonaceous biochemical oxygen demand (CBOD), total suspended solids (TSS) and ammonia-nitrogen. An arctic WSP differs from a 'southern' WSP in the way it is operated and in the conditions under which it operates. Consequently, existing WSP models cannot be used to gain better understanding of the arctic lagoon performance. This work describes an Arctic-specific WSP model. It accounts for both aerobic and anaerobic degradation pathways of organic materials and considers the periodic nature of WSP operation as well as the partial or complete freeze of the water in the WSP during winter. A uniform, multi-layer (ice, aerobic, anaerobic and sludge) approach was taken in the model development, which simplified and expedited numerical solution of the model, enabling efficient model calibration to available field data.
An artificial neural networks (ANN) model was developed for the prediction of annual nitrate-N (NO3-N) losses into the drain flow at Eugene F. Whelan Experimental Farm (Agriculture Canada, Woodslee, Ontario, Canada). Data consisted of daily measurements of nitrate-N taken from eight different soil conservation treatments during 1992-1994. The experiment consisted of four crop/tillage and two water table management systems. Due to the moderate size of the data set, a tenfold cross validation method was used for model validation. A sensitivity analysis was also performed to assess the effect of the input variables on the performance of the networks. The results of this study indicated that the performance of network predictions of nitrate-N was highly satisfactory for 6 of the treatments and acceptable for the remaining two. The sensitivity analysis demonstrated that network predictions of nitrate-N were not affected when either drain flow or evapotranspiration data were excluded from the network training files. Overall, this study reveals that, from adequate input information, Artificial Neural Networks could effectively predict loss of nitrate-N in drain outflows. While the ANN model itself is not transportable to any other site, it does provide another method of estimating nitrate-N losses from agricultural fields with fewer input parameters. In addition, they could also be used to identify the unnecessary parameters for ANN modeling and thus save valuable time and resources in data collection.
[1] To understand and quantify plankton community dynamics in the ocean, high-resolution models are needed to capture the temporal and spatial variations of physical, biological, and biogeochemical processes. However, ecosystem models often fail to agree with observations. This failure can be due to inadequacies in the data and/or inadequacies in the model formulation and parameterization. Here we parameterize and optimize a two-phytoplankton functional type model of phytoplankton growth rate and chlorophyll/carbon (Chl:C) ratio using data from the Lagrangian field measurements conducted during process cruises of the Long-Term Ecosystem Research–California Current Ecosystem (CCE) program. We parameterize the model based on a small coastal subset of the data and then extend and test it with the full data set, including data from offshore regions. The CCE process studies were focused on quantifying the size-resolved planktonic growth, grazing, production, and export rates while following water parcels. The resulting data therefore provided strong constraints for the model we employed. The modeled growth rates and Chl:C ratios were in good agreement with observations. Our results indicate that the model can accurately predict Chl:C ratios, biomasses, and growth rates of dominant functional types using relatively easily measured environmental variables (temperature, nutrients, and bulk chlorophyll). The model also accurately reproduces the subsurface maxima of growth rates, the spatial separation of carbon and chlorophyll maxima, and many other observations in the California Current coastal and pelagic ecosystems.
Light is never distributed homogeneously since it forms a gradient over biomass. As a consequence, the common theories on nutrient competition are not applicable to competition for light. In this paper, we investigate a model for light-limited growth and competition among phytoplankton species in a mixed water column. The model is based on standard assumptions such as Lambert-Beer's law of light absorption, a Monod equation for carbon uptake, and constant specific carbon losses. By introducing the concept of quantum return, we show that the dynamics of growth and competition can be quantified not only in terms of depth but also directly in terms of light availability. We argue that the crucial measure for phytoplankton growth is not a ''critical depth'' but a ''critical light intensity,'' I-out*. For each species, I-out* corresponds to the equilibrium light intensity at the bottom of a water column when the species is grown in monoculture. I-out* plays a role similar to the ''critical nutrient concentration'' R* used in models of nutrient-limited growth. For a constant light supply, the species with the lowest I-out* will competitively exclude all other species. There are, however, some important differences between R* and I-out*. Whereas R* reflects both the local and the total balance between nutrient uptake and nutrient losses, I-out* only reflects the total carbon balance. Moreover, I-out* decreases with increasing light supply, whereas R* is independent of the nutrient supply. As a consequence, (1) the outcome of competition for light may depend on the light supply, (2) the compensation point is not a good predictor for the outcome of competition, (3) the resource ratio hypothesis does not apply when species compete for both nutrients and light. The outcome of competition for nutrients and light may depend on the nutrient and light supply, on the mixing depth, and on the background turbidity due to inanimate substances.
In this paper an extended version of IWA's Activated Sludge Model No 3 (ASM3) was developed to simulate processes in waste stabilisation ponds (WSP). The model modifications included the integration of algae biomass and gas transfer processes for oxygen, carbon dioxide and ammonia depending on wind velocity and a simple ionic equilibrium. The model was applied to a pilot-scale WSP system operated in the city of Florianópolis (Brazil). The system was used to treat leachate from a municipal waste landfill. Mean influent concentrations to the facultative pond of 1,456 g(COD)/m(3) and 505 g(NH4-N)/m(3) were measured. Experimental results indicated an ammonia nitrogen removal of 89.5% with negligible rates of nitrification but intensive ammonia stripping to the atmosphere. Measured data was used in the simulations to consider the impact of wind velocity on oxygen input of 11.1 to 14.4 g(O2)/(m(2) d) and sun radiation on photosynthesis. Good results for pH and ammonia removal were achieved with mean stripping rates of 18.2 and 4.5 g(N)/(m(2) d) for the facultative and maturation pond respectively. Based on measured chlorophyll a concentrations and depending on light intensity and TSS concentration it was possible to model algae concentrations.
1. Nitrate reduction and assimilation have been studied in Chlorella pyrenoidosa under growth conditions by observing effects on the CO(2)/O(2) gas exchange quotient. 2. During assimilation of glucose in the dark, nitrate reduction is noted as an increase in the R.Q. to about 1.6 caused by an increased rate of carbon dioxide production. 3. During photosynthesis at low light intensity nitrate reduction is evidenced by a reduction in the CO(2)O(2) quotient to about 0.7 caused by a decreased rate of carbon dioxide uptake. 4. Chlorella will assimilate nitrogen from either nitrate or ammonia. When both sources are supplied, only ammonia is utilized and no nitrate reduction occurs. It is inferred that under the usual conditions of growth nitrate is reduced only at a rate required for subsequent cellular syntheses. The effect of nitrate reduction on the CO(2)O(2) quotient therefore provides a measure of the relative rate of nitrogen assimilation. 5. Over-all photosynthetic metabolism may be described from elementary analysis of the cells since excretory products are negligible. The gas exchange predicted in this way is in good agreement with the observed CO(2)/O(2) quotients.
Several sigmoidal functions (logistic, Gompertz, Richards, Schnute, and Stannard) were compared to describe a bacterial growth curve. They were compared statistically by using the model of Schnute, which is a comprehensive model, encompassing all other models. The t test and the F test were used. With the t test, confidence intervals for parameters can be calculated and can be used to distinguish between models. In the F test, the lack of fit of the models is compared with the measuring error. Moreover, the models were compared with respect to their ease of use. All sigmoidal functions were modified so that they contained biologically relevant parameters. The models of Richards, Schnute, and Stannard appeared to be basically the same equation. In the cases tested, the modified Gompertz equation was statistically sufficient to describe the growth data of Lactobacillus plantarum and was easy to use.
The paper considers factors that determine the oxygen balance in extreme climate waste stabilisation ponds during the critical spring warm-up period. At this time BOD load on the pond is a maximum, due to accumulation of wastewater under the ice during the winter. The paper describes the operation of a typical cold climate WSP and the events leading to a balanced steady state system as spring develops into summer. A mathematical model to simulate conditions within a batch fed experimental pond over the transient period is described. To model temperature changes in the water body experimental data were fitted to a generalised equation based on diurnal fluctuations in air temperature. The results are plotted in a normalised form and show the diurnal fluctuation and time lapse as the depth of the pond increases. Maximum daily water temperature lags behind maximum light intensity. Bacterial growth is simulated by a Monod kinetic model in which growth rate depends on initial substrate concentration; temperature compensation is applied using a temperature activity coefficient. Oxygen utilisation is calculated from substrate removal. Algal growth rate is more complicated as it is affected by temperature and light availability. Algal oxygen production potential is considered in terms of its primary metabolite yield, which is then used in a Monod equation to estimate the growth rate. The model uses a mass balance approach to determine dissolved oxygen concentration in the pond. The model is still in a simple form but shows reasonable agreement, in terms of events and time lapses, to measured parameters in experimental ponds recovering from ice cover.
The paper presents a brief review of the application of WSPs in extreme climates where ice formation occurs during winter. Design standards and methods are compared and different systems are described. Design equations developed by the US EPA and normative standards from the former Soviet Union are compared in a simple example using typical wastewater and performance characteristics. The results are similar except at low temperatures, where the Soviet method can give pond depths outside the prescribed limits. The paper examines construction and operational aspects of extreme climate WSPs, comparing North American, Northern European and Russian standards. It considers why WSP systems have not been widely adopted in Russia and the NIS, and looks at the advantages these systems may have in countries in economic transition.
A unified basis for design and operation of biological waste treatment systems employing suspensions of microorganisms is developed from microbial kinetic concepts and continuous culture of microorganisms theory. Biological solids retention time, θc, average time period a unit of biological mass is retained in the system, is suggested as an independent parameter for process design and control. Steady-state kinetic models are presented for three process configurations, i.e., completely mixed reactor without solids recycle, completely mixed reactor with solids recycle, and plug flow reactor with solids recycle. Reported values of kinetic coefficients are tabularized for: (1) aerobic treatment of carbonaceous wastes; (2) aerobic biological nitrification; (3) anaerobic methanogenic fermentation of carbonaceous wastes. These coefficient values are substituted into the models to determine lower limits, i.e., minimum values of θc, for each process. Minimum values of θc of models are compared with θc values for actual treatment systems.
Wastewater Stabilization Ponds (WSPs) are passive treatment systems influenced by both the ambient environment and the manner in which they are operated. Construction of wastewater treatment facilities is particularly challenging in the Canadian Arctic because of the remoteness of the communities and the extreme climate. Here, experimental bench scale WSPs were used to simulate single cell WSPs operating under summer treatment conditions in the Arctic. Experiments were performed to examine factors (temperature, irradiance, initial carbon concentrations, and organic loading rate) that may influence the oxygen state and carbon removal. All four factors were found to significantly (p < 0.05) affect oxygen state and carbon removal, but temperature (5 °C vs 15 °C) and initial carbon concentration (80 vs 240 mg/l) were found to be the most important factors. Final CBOD5 concentrations in the experimental columns ranged from <10 to 150 mg/l, and only experimental columns operated at low temperature and high initial carbon concentrations had final CBOD5 concentrations >30 mg/l. Phytoplankton growth and metabolism appears to have a prominent impact on CBOD5 removal and the oxygen status of the experimental WSPs. The findings suggest that WSPs operating in cool arctic climates are more sensitive to perturbation by weather or operational changes than similar systems in warmer climates, and greater consideration for process resiliency needs to be incorporated into the design of arctic WSPs. This study provides further evidence that WSPs are an appropriate municipal wastewater technology for the Arctic and can achieve effluent CBOD5 concentrations that meet secondary wastewater treatment standards, provided they are appropriately sized, designed, and operated for arctic conditions.
A model for the activated sludge process is presented. The bacterical metabolism is governed by the soluble substrate concentration. Suspended organics are hydrolysed before being available for the bacterial metabolism. Available soluble substrate is primarily used for the basal metabolism of the microorganism, and is for many practical cases rather insensitive to loading variations. The sludge age concept is revised, the sludge ages of the various sludge fractions are not identical. Predation and microbial decay - and thus loading variations - have significant impact upon the sludge age of the active biomass.
A non steady state process model of facultative stabilization ponds is presented. The model consists of 12 ordinary, first order, nonlinear differential equations that must be solved simultaneously. The equations represent mass balances for substrate, bacterial mass, algal mass, oxygen, organic nitrogen, ammonia, nitrate, organic phosphorus, inorganic carbon, alkalinity, and detritus. The behavior of major biochemical and biomass species is simulated subject to specific environmental constraints of wastewater characteristics, solar radiation, temperature, and hydrology. A fourth order Runge Kutta technique is used to solve the simultaneous differential equations. A published 1 year case history was used to evaluate the model. Model results were found to agree reasonably well with observed data.
Previous work on the solubility of oxygen in freshwater has been extended to seawater. Measurements of the Henry coefficient in the ranges 0°C<t<45°C and 0<S<50 fit the Setschenow relationship for the variation with salinity. The temperature dependance of the Setschenow coefficient for oxygen is found to be k=0.0225034 - 13.6083/T + 2565.68/T². The equation for the Henry coefficient as a function of temperature ans salinity is used to calculate values for unit standard atmospheric pressure concentrations (USAC) in freshwater and seawater in equilibrium with air at a total pressure of 1 atmosphere. It is estimated that the possible error in the new USAC values is no greater than +/-0.1% and probably less. Tables and equations are presented for obtaining accurate USAC values in the range 0°C<t<40°C and 0<S<40. Simple procedures are given for calculating standard atmospheric concentrations at pressures different from 1 atm. The presence of sea salt has a negligible effect on the fractionation of the oxygen isotopes during solution.
The rate of oxygen consumption by organisms in pond waters, as measured by a dark bottle technique for a 24-hr period, increased with increasing chemical oxygen demand (COD). Rate of oxygen consumption increased with temperature between 15 and 35 C (Q10values were 1.5 to 2.0). At the same level of COD, oxygen consumption was greater in unaltered pond water than in water in which the plankton was heat-killed and oxygen was utilized only by bacteria and other decomposers. COD was positively correlated with increasing concentrations of chlorophyll in pond waters. The amount of COD in soluble organic matter ranged from 20.2 to 95.0% of the total COD. The percentage of the COD in the soluble fraction decreased with increasing quantities of phytoplankton. The average amounts of oxygen (mg oxygen/mg dry wt) required to completely oxidize various types of biological material were; phytoplankton from pure cultures 1.29, macroscopic algae 1.00, higher aquatic plants 0.99, particulate matter from pond waters 0.98, and fish 1.19.
Two techniques of uncertainty analysis were applied to a mathematical model that estimates the dose-equivalent to man from the concentration of radioactivity in air, water, and food. The response-surface method involved screening of the model to determine the important parameters, development of the response-surface equation, calculating the moments using the response-surface model, and fitting a Pearson or Johnson distribution using the calculated moments. The second method sampled model inputs by Latin hypercube methods and iteratively simulated the model to obtain an empirical estimation of the cdf. Comparison of the two methods indicates that it is often difftcult to ascertain the adequacy or reliability of the response-surface method. The empirical method is simpler to implement and, because all model inputs are included in the analysis, it is also a more reliable estimator of the cumulative distribution function of the model output than the response-surface method.
The effects of water, plankton algae, and other suspended and dissolved substances on light attenuation for the total euphotic layer of the ocean were investigated. All three of these factors play different roles depending on the depth of the euphotic zone. When the euphotic zone is shallow (about 10 m) the dominant factor is plankton algae. In the deepest euphotic zones found, most of the light attenuation can be attributed to water itself. If the euphotic zone is of intermediate thickness (30–40 m), more than one-half of the light attenuation must be attributed to other paniculate and dissolved substances.
The maximum amount of chlorophyll (as a measure of phytoplankton biomass) that can be expected to be found under a m² of sea surface is 333 mg.
Lagoons are the most widely used form of wastewater treatment in Canada, particularly in the western provinces and northern territories. Existing lagoon design guidelines developed for warm climates must be modified for successful use in cold climates. The use of short-detention lagoons followed either by long-detention continuous flow lagoons or by 270 – 365 d storage lagoons will be required in most situations. Under special circumstances, the conversion of small lakes to engineered lagoon – lake systems may be an acceptable option. Detailed guidelines for the planning and design of these types of lagoons are presented with emphasis on the cold regions of Canada. Key words: wastewater, treatment, cold climate, lagoon, design, guidelines.
Phytoplankton often face the dilemma of living in contrasting gradients of two essential resources: light that is supplied from above and nutrients that are often supplied from below. In poorly mixed water columns, algae can be heterogeneously distributed, with thin layers of biomass found on the surface, at depth, or on the sediment surface. Here, we show that these patterns can result from intraspecific competition for light and nutrients. First, we present numerical solutions of a reaction-diffusion-taxis model of phytoplankton, nutrients, and light. We argue that motile phytoplankton can form a thin layer under poorly mixed conditions. We then analyze a related game theoretical model that treats the depth of a thin layer of phytoplankton as the strategy. The evolutionarily stable strategy is the depth at which the phytoplankton are equally limited by both resources, as long as the layer is restricted to the water column. The layer becomes shallower with an increase in the nutrient supply and deeper with an increase in the light supply. For low nutrient levels, low background attenuation, and shallow water columns, a benthic layer occurs; for intermediate nutrient levels in deep water columns, a deep chlorophyll maximum occurs; and for high nutrient levels, a surface scum occurs. These general patterns are in agreement with field observations. Thus, this model can explain many patterns of algal distribution found in poorly mixed aquatic ecosystems.
ABSTRACTA treatment based on Reichardt's formulation of the velocity profile in turbulent flow over a smooth plane surface is shown to give good agreement with published data on the boundary layer transfer of heat and mass over a wide range of Prandtl (or Schmidt) number, s̀. Applied to transfer to a water surface, agreement with published laboratory results is also good for low air speeds (smooth water). Comparison with observations for the sea shows there is little difference between the calculated evaporation coefficient and those reported for the sea with winds of ∼7 m s−1. This is consistent with sea trials having so far detected little increase of evaporation and heat transfer coefficients with wind speed over the range 4–10 m s−1.The air/water transfer of non-reactive gas is governed by the resistance of the viscous sublayer of water, and the smooth surface treatment gives the transfer velocity (VL) on a liquid phase basis asVL = 0.082 (pa/pw)1/2s̀−2/3u*where papw is the density ratio, air/water, and u* = friction velocity of the surface air flow. The agreement between this formula and published wind tunnel results is good for the smooth water condition. At higher wind speeds transfer exceeds the calculated value and appears then to increase roughly in proportion to the square of the wind speed.
Four algae of freshwater phytoplankton were studied in monospecific culture: Chlorella vulgaris, Fragilaria crotonensis, Staurastrum pingue and Synechocystis minima. Experiments were performed to determine the growth rate over a wide range of light intensities (5–800 E m–2 s–1, 15/9 light/dark photoperiod) and temperatures (10–35 C). The results provide a set of parameters (particularly the maximal growth rate associated to optimal conditions of light and temperature) for a three-equation model used to described the growth rate response of a non-nutrient-limited culture.
This paper presents a model for natural systems used in urban waste water treatment. The model includes thermal and biochemical submodels, derived from the physical and biochemical phenomena involved in the treatment process. The thermal submodel is based on a heat balance for the system and the numerical solution of the resulting differential equation. The biochemical submodel is based on the mass balances for the different variables, considering a one-dimensional flow within the ponds, which give rise to a system of linear equations subsequently solved by numerical methods. The model has been calibrated using experimental data from existing waste water treatment systems using both microphytes and macrophytes. The model may be applied to the design of new facilities and to improve operating conditions of existing facilities.
A dynamic mathematical model was developed to predict the effluent quality of facultative wastewater stabilization ponds. For a sound representation of sediment–water column, water column–atmosphere interactions and stratification due to variations in dissolved oxygen concentrations, a two-dimensional hydraulic model was employed considering dispersed flow and diffusion in horizontal and vertical directions, respectively. Resulting partial differential equation system was solved using finite difference methods and matrix manipulation techniques. The model has been calibrated and evaluated on the basis of collected data from a full-scale facultative stabilization pond in Selçuk, Izmir. Variations of COD, NH4-N, PO4-P, dissolved oxygen, bacteria and algae concentrations with time and the dimensions of the pond were estimated by using the dynamic model. The model can be used for design of new stabilization ponds and also, for improving the effluent quality of existing ponds.
A mathematical model is presented to describe the symbiotic growth of algae and bacteria in a high-rate wastewater treatment pond. Good agreement with available experimental results is demonstrated. The variation of process parameters (pH, DO and substrate concentration) with channel length and time of day is illustrated for a recirculated pond of “race-track” design. The influence of operating conditions on the algal yield obtainable is evaluated.
We describe a classification system for daily phytoplankton primary productivity models based on four implicit levels of mathematical integration. Depth-integrated productivity models have appeared in the literature on average once every 2 years over the past four decades. All of these models can be related to a single formulation equating depth-integrated primary production (2 PP) to surface phytoplankton biomass (C,,,J, a photoadaptive variable (PhO,,), euphotic depth (Z,,), an irradiance-dependent function (F), and daylength (DL). The primary difference between models is the description of F, yet we found that irradiance has a relatively minor effect on variability in X PP. We also found that only a small fraction of variability in 2 PP can be attributed to vertical variability in phyto- plankton biomass or variability in the light-limited slope for photosynthesis. Our results indicate that (1) differences between or within any model category have the potential to improve estimates of X PP by
What determines the vertical distribution of phytoplankton in different aquatic environments remains an open question. To address this question, we develop a model to explore how phytoplankton respond through growth and movement to opposing resource gradients and different mixing conditions. We assume stratification creates a well-mixed surface layer on top of a poorly mixed deep layer and nutrients are supplied from multiple depth-dependent sources. Intraspecific competition leads to a unique strategic equilibrium for phytoplankton, which allows us to classify the distinct vertical distributions that can exist. Biomass can occur as a benthic layer (BL), a deep chlorophyll maximum (DCM), or in the mixed layer (ML), or as a combination of BL+ML or DCM+ML. The ML biomass can be limited by nutrients, light, or both. We predict how the vertical distribution, relative resource limitation, and biomass of phytoplankton will change across environmental gradients. We parameterized our model to represent potentially light and phosphorus limited freshwater lakes, but the model is applicable to a broad range of vertically stratified systems. Increasing nutrient input from the sediments or to the mixed layer increases light limitation, shifts phytoplankton towards the surface, and increases total biomass. Increasing background light attenuation increases light limitation, shifts the phytoplankton towards the surface, and generally decreases total biomass. Increasing mixed layer depth increases, decreases, or has no effect on light limitation and total biomass. Our model is able to replicate the diverse vertical distributions observed in nature and explain what underlying mechanisms drive these distributions.
Results from studies of continuous cultures of Aerobacter aerogenes growing in chemically denned media indicate that specific growth rate (R) is a function of population density (P) as well as the concentration of the limiting nutrient (S). From these observations, and those of others, the following model for bacterial growth is derived:
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where um and B are growth parameters that are constants under defined conditions. This model is believed to have general applicability and to account for bacterial growth in both batch and continuous cultures.
Mass transfers of O2, CO2, and water vapor are among the key processes in the aerobic, autotrophic cultivation of moderate and extreme thermophiles. The dynamics and kinetics of these processes are, in addition to the obvious microbial kinetics, of crucial importance for the industrial desulfurization of high-pyritic coal by such thermophiles. To evaluate the role of the temperature on the gas mass transfer, kLa measurements have been used to supplement the existing published data. Oxygen mass transfer from gas (air) to liquid (5 mM H2SO4 in water) phase as a function of the temperature has been studied in a laboratory-scale fermentor. At 15, 30, 45, and 70°C, (kLa)o values (for oxygen) were determined under three different energy input conditions by the dynamic gassing in/out method. The (kLa)o was shown to increase under these conditions with increasing temperature, and straight lines were obtained when the logarithm of (kLa)o was plotted against the temperature. By multiplying the equilibrium concentration of O2 in water with (kLa)o maximal, O2 transfer capacities were calculated. It appeared that in finite of a decreased solubility of O2 at elevated temperature in mechanically mixed fermentors the calculated transfer capacities showed only minor changes for the range between 15 and 70°C. However, in an air-mixed fermentor the transfer capacity of O2 decreased slowly but steadily.
Carbon dioxide mass transfer was predicted by calculations on the basis of the data for oxygen transfer. The maximal CO2 transfer capacity, calculated as the product of the equilibrium CO2 concentration times (kLa)c, decreased slowly as the temperature increased over the range 15–70°C under all three energy input conditions. Subsequent process design calculations showed that for aerobic, autotrophic cultures, CO2 limitation is more likely to occur than O2 limitation.
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