The effect of catchment characteristics on sewage settling velocity grading

A procedure has been developed which measures the settling velocity distribution of particles within a complete sewage sample. The development of the test method included observations of particle and liquid interaction using both synthetic media and sewage. Comparison studies with two other currently used settling velocity test procedures was undertaken. The method is suitable for use with either DWF or storm sewage. Information relating to the catchment characteristics of 35 No. wastewater treatment works was collected from the privatised water companies in England and Wales. 29 No. of these sites were used in an experimental programme to determine the settling velocity grading of 33 No. sewage samples. The collected data were analysed in an attempt to relate the settling velocity distribution to the characteristics of the contributing catchment. Statistical analysis of the catchment data and the measured settling velocity distributions was undertaken. A curve fitting exercise using an S-shaped curve which had the same physical characteristics as the settling velocity distributions was performed. None of these analyses found evidence that the settling velocity distribution of sewage had a significant relationship with the chosen catchment characteristics. The regression equations produced from the statistical analysis cannot be used to assist in the design of separation devices. However, a grading curve envelope was produced, the limits of which were clearly defined for the measured data set. There was no evidence of a relationship between settling velocity grading and the characteristics of the contributing catchment, particularly the catchment area. The present empirical approach to settling tank design cannot be improved upon at present by considering the variation in catchment parameters. This study has provided a basis for future research into the settling velocity measurement and should be of benefit to future workers within this field.

Chemical constituents associated with sewage settling velocity profiles

A methodology has been developed to measure the chemical constituents associated with the settling velocity fractions that comprise a wastewater settling velocity profile (SVP). 31 wastewater samples were collected from fifteen different catchments in England and Wales. For each catchment, settling velocity and associated chemical constituent profiles were determined. The results are mainly for Suspended Solids (SS), Chemical Oxygen Demand (COD), Phosphorus (P) and Total Kjeadahl Nitrogen (TKN), however these are supplemented by the results from 5 events for a suite of heavy metals. COD, P, Hg, Mn and Pb were found to be predominantly associated with the solid phase and TKN, Al, Cu and Fe with the liquor phase of the wastewater samples. The results in the thesis are expressed as mass of pollutant (g) per mass total SS (kg). COD and P were found to be mainly associated with the sinkers and had a particular affinity for solids with settling velocities in the range 0.9-9.03mm/sec. TKN was mainly associated with the soluble phase, however of the solids that did settle, a peak was found to be associated within the settling velocity range 0.9-9.03mm/sec. The relationships identified for COD and P were generally found to be unaffected by flow conditions and catchment characteristics. However, TKN was found to be affected by catchment type. Data on the distribution of heavy metals was limited, and no specific relationships with solids were identified. 16 mean pollutant profiles are presented in the thesis. Presentation of the data in this form will enable the results to be of use in the design of sedimentation devices to predict removal efficiencies for solids and associated pollutants. The findings of the research may also be applied to modelling tools to provide further characteristics on the solids that are modelled than is currently used. This would enhance the overall performance of tools used in integrated catchment modelling.

Hydrodynamic modeling of mineral wool fiber suspensions in a two-dimensional channel flow

A consequence of a loss of coolant accident is the damage of adjacent insulation materials (IM). IM may then be transported to the containment sump strainers where water is drawn into the ECCS (emergency core cooling system). Blockage of the strainers by IM lead to an increased pressure drop acting on the operating ECCS pumps. IM can also penetrate the strainers, enter the reactor coolant system and then accumulate in the reactor pressure vessel. An experimental and theoretical study that concentrates on mineral wool fiber transport in the containment sump and the ECCS is being performed. The study entails fiber generation and the assessment of fiber transport in single and multi-effect experiments. The experiments include measurement of the terminal settling velocity, the strainer pressure drop, fiber sedimentation and resuspension in a channel flow and jet flow in a rectangular tank. An integrated test facility is also operated to assess the compounded effects. Each experimental facility is used to provide data for the validation of equivalent computational fluid dynamic models. The channel flow facility allows the determination of the steady state distribution of the fibers at different flow velocities. The fibers are modeled in the Eulerian-Eulerian reference frame as spherical wetted agglomerates. The fiber agglomerate size, density, the relative viscosity of the fluid-fiber mixture and the turbulent dispersion of the fibers all affect the steady state accumulation of fibers at the channel base. In the current simulations, two fiber phases are separately considered. The particle size is kept constant while the density is modified, which affects both the terminal velocity and volume fraction. The relative viscosity is only significant at higher concentrations. The numerical model finds that the fibers accumulate at the channel base even at high velocities; therefore, modifications to the drag and turbulent dispersion forces can be made to reduce fiber accumulation.