Peak Flow Conditions Research Articles

Fuzzy-control for improved nitrogen removal and energy saving in WWT-plants with pre-denitrification

In the last few years, numerous studies were carried out, dealing with the application of fuzzy-logic to improve the control of the activated sludge process. In this paper, fuzzy-logic based control strategies for wastewater treatment plants with pre-denitrification are presented that should lead to better effluent quality and, in parallel, to a reduction of energy consumption. Extensive experimental investigations on a large scale pilot plant as well as simulation studies (ASM1 with SIMBA) were carried out in order to design, evaluate and compare different fuzzy-controllers with each other and with comparable conventional control systems. The fuzzy-controllers were designed as high-level controllers that determine the DO-setpoints in the aerated zones and the ratio between aerated and non-aerated zones. Conventional PI-controllers were used to maintain the DO-concentration at the set-point levels. The ammonia and nitrate concentration in the effluent and the ammonia load in the influent were considered as input variables for the different fuzzy-controllers. Compared to the operation with fixed nitrification/denitrification zones and constant DO concentrations, the required air-flow could be reduced up to 24% by using fuzzy-logic based control strategies. In comparison with a more advanced conventional control strategy (relay controller with two thresholds and the NH4-N concentration in the effluent as single control variable) a reduction of air-flow-rate up to 14% could be achieved. At the same time, NH4-N peaks in the effluent that are normally caused by peak flow conditions could be reduced significantly. The large scale experiments show that the fuzzy-controllers can be easily implemented in modern control and supervision systems and that the control characteristics can be followed and modified during operation. It therefore can be expected that the developed fuzzy-control systems will be accepted by the operating personnel in wastewater treatment plants.

A particle image velocimetry study on the pulsatile flow characteristics in straight tubes with an asymmetric bulge

Flow characteristics in straight tubes with an asymmetric bulge have been investigated using particle image velocimetry (PIV) over a range of Reynolds numbers from 600 to 1200 and at a Womersley number of 22. A mixture of glycerine and water (approximately 40:60 by volume) was used as the working fluid. The study was carried out because of their relevance in some aspects of physiological flows, such as arterial flow through a sidewall aneurysm. Results for both steady and pulsatile flow conditions were obtained. It was found that at a steady flow condition, a weak recirculating vortex formed inside the bulge. The recirculation became stronger at higher Reynolds numbers but weaker at larger bulge sizes. The centre of the vortex was located close to the distal neck. At pulsatile flow conditions, the vortex appeared and disappeared at different phases of the cycle, and the sequence was only punctuated by strong forward flow behaviour (near the peak flow condition). In particular, strong flow interactions between the parent tube and the bulge were observed during the deceleration phase. Stents and springs were used to dampen the flow movement inside the bulge. It was found that the recirculation vortex could be eliminated completely in steady flow conditions using both devices. However, under pulsatile flow conditions, flow velocities inside the bulge could not be suppressed completely by both devices, but could be reduced by more than 80 per cent.

Pulsating Flow in a 90 Degree Bifurcation

The pulsating flow field in a 90 deg bifurcation was examined by performing LDV measurements in a model with square cross-sections and equal branch flow rates. The three-dimensional separation zones of both branches were studied revealing details of their temporal evolution. During acceleration the flow was attached, but close to the peak flow condition, separation initiated at both branches. The basic feature of the reverse flow zones at a given time instant was that these shrank streamwise in a direction perpendicular to the bifurcation plane and grew at the symmetry plane. Flow instabilities were strong in the horizontal branch during acceleration, in contrast to the vertical branch in which these appeared during deceleration. Comparison of this flow field for a particular time instant with the flow field under steady inlet conditions and similar Re, revealed that for the steady case the flow separated in the horizontal branch upstream of the unsteady case. [S0098-2202(00)03002-9]

Municipal landfill Leachate management

AbstractFrom 1995 to 1997, the Montgomery County Leachate Pretreatment Facility (MCLPF) has successfully pretreated in excess of 18,000,000 gallons of leachate generated by the county's municipal solid waste landfill. The collection system directs leachate from the original landfill, the new lined section, and the ash cell to the leachate pump station. The leachate, prior to being pumped to the leachate pretreatment system, is equalized in two storage lagoons with a combined capacity of more than 5,000,000 gallons.The innovative leachate treatment system, incorporating a biological reactor system equipped with a submerged fixed‐film reactor using a patented Matrix Biological Film (MBF) media, continues to provide excellent pretreatment results for the leachate generated at the Oaks Landfill in Montgomery County, Maryland. In 1995 and 1996, the system responded to the substantial challenges imposed by the changing characteristics of the material being landfilled and by the significant amounts of incinerator ash, received in 1995 from the county's resource recovery facility (RRF), which influenced the influent leachate characteristics. The extremely wet weather caused the collection and equalization system to be overloaded and required the biological reactor system to operate at peak flow and loading conditions for extended periods of time. In fact, the facility pretreated in excess of 12,500,000 gallons of leachate in 1996 compared to only 5,500,000 gallons for all of 1995, and continued to meet permit requirements. Although the landfill closed in 1997, technological challenges to the system will continue. The past performance indicates that this system will continue to provide excellent pretreatment results and, in future years, may provide an effluent suitable for direct discharge.

Numerical simulation of unsteady laminar flow through a tilting disk heart valve: Prediction of vortex shedding

Heart valves induce flow disturbances which play a role in blood cell activation and damage, but questions of the magnitude and spatial distribution of fluid stresses (wall shear stress and turbulent stress) cannot be readily addressed with current experimental techniques. Therefore, a numerical simulation procedure for flow through artificial heart valves is presented. The algorithm employed is based on the Navier-Stokes equations in generalized curvilinear coordinates with artificial compressibility for coupling of velocity and pressure. The algorithm applies a finite-difference technique on a body-conforming composite grid around the heart valve disk on which the numerical simulations are performed. Steady laminar flow over a backward-facing step and unsteady laminar flow inside a square driven cavity are computed to validate the algorithm. Two-dimensional, time-accurate simulation of flow through a tilting disk valve with a steady upstream Reynolds number as high as 1000 reveals the complex behavior of ‘vortex shedding’. By scaling the results at the Reynolds number of 1000 to peak systolic flow conditions, the maximum value of shear stress on the valve disk is estimated to be 770 dyn cm −2. The ‘apparent’ Reynolds stress associated with vortex shedding is estimated to be as high as 3900 dyn cm −2 with a vortex shedding frequency of about 26 Hz. The ‘apparent’ Reynolds stress value is of similar magnitude as reported in experiments but would not be expected to damage blood cells because the spatial scales associated with vortex shedding are much larger than blood cell dimensions.

Time-dependent laminar incompressible flow through a spherical cavity

A finite-difference numerical method for the solution of the unsteady flow of a viscous incompressible fluid through axisymmetric circular ducts of variable axial geometry is developed and applied to the flow in a spherical-cavity geometry approximating the human aortic valve. The presence and motion of the valve leaflets are considered only as long as they can be assumed to present negligible impedance to the flow. The numerical solution is based on the vorticity/streamfunction approach, and is carried out for the systolic acceleration phase of the heart beat. A hybrid-mesh design consisting of a fine cell structure in the region close to the solid walls and a coarser grid in the core region is used. An experimental flow-visualization study in an acrylic model of the spherical cavity shows good agreement with the numerical simulation. An early separation of flow occurs at the entrance to the cavity, and an annular eddy grows in the wake until it occupies most of the cavity. The use of the hybrid mesh also makes possible the simulation of fine secondary-flow features in the cavity under peak-flow conditions.