Packed bed filtration in potable water treatment: Pretreatment chemistry for the removal of particles and natural organic matter.

摘要

The goal of this research was to investigate experimentally the relationship between the concentrations of colloidal particles and natural organic matter and corresponding chemical pretreatment conditions for coagulation and filtration, and assess the impacts of these pretreatment techniques on the performance of a filtration system. The experimental studies in this research were conducted in two phases. In the first phase, the relationships between the raw water characteristics and corresponding minimum effective alum doses (MEADs) were investigated. To test the effects of raw water characteristics on minimum effective chemical conditions for coagulation and subsequent filtration, the concentrations of colloidal silica particles and natural organic matter (NOM) in model raw waters were systematically varied. Jar tests were performed for each combination of colloidal and NOM concentrations and the minimum effective coagulant doses for the removals of settled and filtered turbidity and DOC were observed.; The MEAD for the coagulation of low silica (low turbidity) waters without NOM decreased as silica concentration increased, whereas it increased proportionally with increasing silica concentration for high turbidity waters. In low silica waters, contact opportunities for floc formation are limited by low solid volume and removal is achieved via a sweep flocculation mechanism. Increasing silica concentration within this range provides additional floc volume thereby reducing the alum dose required to induce sweep flocculation. Once the requirement for effective flocculation is met by sufficient silica concentration, the MEAD increases in direct proportion with increasing silica.; In the second phase of this research, laboratory dual media filtration experiments were conducted (a) in direct filtration mode using a model raw water moderate in turbidity and low in DOC and (b) in conventional filtration mode treating a water moderate in turbidity and high in DOC. This study focused on the effectiveness of these filtration processes for the removals of turbidity, NOM, and 4 mum particles used as a surrogate for Cryptosporidium oocysts. Based on the conceptual model developed in the first phase for the relationships of colloidal and NOM concentrations and corresponding coagulant requirements, the impacts of some of the chemical pretreatment conditions on the filter performance were investigated at conventional and high filtration rates. Simulations of filter performance for the removal of particles were made using existing models of clean bed efficiency and filtration dynamics (ripening). (Abstract shortened by UMI.)