Phosphorus transport and reduction in compact-bed filters simulations and experiments
Abstract: Excessive release of phosphorus from the effluent of on-site sanitation facilities to the environment is one source of aquatic pollution. The primary effect of excess phosphorus in the aquatic environment is eutrophication. In Sweden, there are about one million private on-site wastewater treatment facilities, half of which are considered unacceptable by the Swedish Environmental Protection Agency (SEPA). Recent guidelines issued by SEPA recommended total phosphorus reduction of 70-90 % in wastewater in small scale treatment facilities. Using reactive filter materials as compact filter systems following a biological pretreatment can be a convenient option for upgrading the present on-site sewage treatment facilities. Research on reactive filter materials has been undertaken in many different ways as there is no standard method for such investigations. Therefore, there is a need to develop methods and models to evaluate the reduction capacity and predict the longevity of potential reactive filter materials. In the first part of this thesis a filter material Filtra P (Nordkalk, Finland) is investigated. The phosphorus sorption characteristics of Filtra P were investigated in batch and column experiments. The batch test method was evaluated with respect to the effects of liquid-to-solid ratio and particle size on phosphorus sorption. Column experiments were carried out at different flow rates and phosphate (PO4-P) feed concentrations. The experimental data obtained from batch and column experiments were used to investigate the feasibility of modeling PO4-P breakthrough curves in fixed-bed columns by means of the convection-dispersion equation and a Langmuir isotherm. The model successfully predicted the PO4-P breakthrough curves for the columns with low influent concentration (11 mg dm-3), which is within the range normally found in wastewater (5-15 mg dm-3). Experimental observations however yield that the possibility to combine the convection-dispersion equation with a model accounting for precipitation of calcium-phosphate compounds in the bulk should be examined. The second part of the thesis focuses on dispersion in laminar flow through a randomly packed porous-medium model. The study of dispersion in flow through porous media is an important issue in many branches of science and engineering and has direct applications in several industries. For example, dispersion occurs in the disposal of sewage waste into aquifers and flow through reactive compact-bed filters that are used for on-site sanitation systems. In order to shed light on this phenomenon, a detailed longitudinal and transverse dispersion numerical experiments were performed in two-dimensional (2D) randomly packed beds of thousands of circular particles in a laminar flow regime. The calculated longitudinal and transverse dispersion coefficients were presented as Peclet numbers based on longitudinal and transverse dispersion coefficients versus Peclet number based on molecular diffusion coefficient, and were compared to three-dimensional (3D) experimental data available in the literature. It was found that at very low superficial liquid velocities both longitudinal and transverse dispersions were governed by molecular diffusion. Based on the comparisons between numerical results obtained from the 2D model to the 3D experimental data, it was found that the present 2D approach to study the longitudinal dispersion in flow through porous media can be considered to be acceptable. The present numerical data may be used to estimate the longitudinal and transverse dispersion coefficients in a 2D process by which one fluid is displaced from a porous medium by a second fluid which is miscible with the first in a laminar flow regime.
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