The spatial significance of water quality indicators in waste stabilization ponds--limitations of residence time distribution analysis in predicting treatment efficiency.

Over the past fifty years, considerable research in waste stabilization pond operation has led to the development of a number of models used to describe the hydraulic regime and predict treatment efficiency. Models range in complexity from plug or completely mixed simplifications to computational fluid dynamics (CFD) models which are able to predict flow hydraulics at a local level. Information about the exit age of pond effluent can be used to estimate pollutant decay. However, a mechanistic approach to understanding pond operation highlights the importance of knowing both the time and spatial history of pond effluent. A CFD model of a large pond system was constructed to demonstrate various hydraulic scenarios under different boundary conditions. Two scenarios were compared to visually demonstrate the effects of differing hydraulic conditions. Typical mechanistic models were applied to each condition to quantify biological differences. This simple example indicates that integrating biological and localised flow models will lead to a more holistic understanding of pond operation and treatment efficiency.

Investigation of anaerobic zone short-circuiting using computational fluid dynamics.

Flow characteristics of anaerobic zone was investigated using the computational fluid dynamics technique. The tank is an inlet/outlet baffle tank with two impellers to provide additional energy to prevent solid settlement. The turbulent flow was modelled using the standard k-e model and the rotation of the impellers was modelled using the multiple rotating frame of reference technique. The model simulated tracer results and was in reasonable agreement with measurement. Solid settlement was tracked using the Lagrangean disperse phase tehnique. Flow in anaerobic zone exhibited completely mixed behaviour with short-circuiting. A number of options were investigated. All exhibited the general completely mixed system, which resulted from the flow circulation around the central baffle. Some options, however, led to major short circuiting and were avoided. The function of the impellers was to maintain the flow circulation and to prevent solid settlement.

A multiphase CFD model of DAF process.

A Eulerian-Eulerian multiphase CFD model is employed for the air/water flow. A 3D structure grid is used to incorporate the air nozzle and tank geometry. The fixed frictionless wall boundary approximating the free surface acts as a sink to allow the air bubbles to escape. The air/water volume fraction in the flotation tank is evaluated to determine the effective air/water fluid density. The floc particle is then introduced and is tracked in the air/water fluid using a disperse Lagrangean model. Fate of these flocs depends on their sizes and density. Flocs therefore can either escape through the top water surface, settles in the main tank or breakthrough under the outlet weir. The CFD model is developed for a full scale DAF tank to predict the flow dynamic, particle removal and settled solid profile. The general flow pattern is compared with flow visualisation using the underwater camera. Comparison of average fluid velocities is carried out using acoustic Doppler velocimetry ADV measurement.

Small scale model for CFD validation in DAF application.

A laboratory model is used to measure the generic flow patterns in dissolved air flotation (DAF). The Perspex model used in this study allows the use of laser Doppler velocimetry (LDV), a non-invasive, high-resolution (+/- 2 mm s-1) laser technique of flow velocity measurement. Measurement of flow velocity in the single-phase situation was first carried out. Air-saturated water was then supplied to the tank and measurements of bubble velocity in the two-phase system were made. Vertical flow re-circulation was observed in the flotation zone. In the bottom of the flotation zone (near the riser) secondary flow re-circulation was observed, but only in the two-phase system. Another phenomenon was the apparent movement of flow across the tank width, which may be due to lateral dispersion of the bubble cloud. Data from preliminary computational fluid dynamics (CFD) models were compared against this measured data in the case of the single-phase system. The CFD model incorporating a k-epsilon model of turbulence was found to give closer agreement with the measured data than the corresponding laminar flow model. The measured velocity data will be used to verify two-phase computational fluid dynamics (CFD) models of DAF.

Nitrogen metabolism in the stalk tissue of maize.

During ear development in maize (Zea mays L.), nitrogenous compounds are translocated from vegetative organs to the kernels. At anthesis, the stalk contains approximately 40% of the total plant N, and contributes 45% of the N remobilized to the ear. Therefore, the stalk has an important function as a temporary reservoir for N. Little is known of the metabolism of maize stalks, and this paper describes initial studies of enzymes of N metabolism. High in vitro activity of glutamine synthetase (GS) in maize stalk samples throughout ear development contrasted with a peak in activity of glutamate synthase soon after anthesis and negligible nitrate reductase. With fresh sections of stalk tissue collected at anthesis, (15)N-feeding experiments confirmed high GS and low nitrate reductase activities. Two isoforms of GS were separated from extracts from stalk tissue: GS1, the cytoplasmic form, increased to maximum levels at 2 weeks postanthesis and remained fairly high thereafter; whereas the plastidic form, GS2, declined progressively during kernel development. Western blot analysis confirmed the presence of constantly high levels of GS protein after anthesis. The levels of GS proteins decreased after transfer of N-starved, hydroponically grown plants to N-rich conditions in order to restrict remobilization of N. In contrast, transfer of plants grown under abundant N conditions to N-free medium, which encourages N remobilization, resulted in a relative increase in GS protein. Because glutamine is the major form of N transported in maize, the results indicate that GS, specifically the GS1 isoform, has a central role in the remobilization on nitrogenous compounds from the stalk to the ear.