Direct filtration using coarse media and dual media filters in series
A filtration system consisting of coarse media filters in series with standard dual media filters was proposed and evaluated as an alternative for direct filtration of high turbidity waters. The coarse media filters utilized a 3.08 mm uniform sand with a media depth of 1.32 m;The study was divided into three phases. The first phase involved the use of jar tests to determine optimum coagulants and approximate dosages for direct filtration. The jar test results were used in the second phase of study which involved laboratory pilot filter studies. The optimum alum dosage in the pilot plant studies was less than that determined in the jar test. Successful filter runs were possible with influent solids concentrations of 100 mg/l of kaolin or less. Headloss development (3 m maximum) limited the filter run lengths at kaolin concentrations of 25 and 50 mg/l. With 100 mg/l of kaolin, filter runs were terminated due to turbidity breakthrough. The third phase of the study involved a pilot plant field investigation using Des Moines river water as the raw water source. The chemical coagulants used in the laboratory did not produce acceptable results during the field study. The difference in chemical requirements was attributed to the nature of the raw water turbidity. The turbidity in the Des Moines river water was attributed mainly to the presence of diatoms;Particle deposition in the coarse media filters was recorded photographically during the laboratory filter runs. The particles were deposited in the filters by first coating the media. Next, the particles filled the media void spaces. The filling of the void spaces resulted in the development of narrow channels through which the suspension was transferred deeper into the filter;Two mathematical models were applied to the laboratory pilot filter data. Both models agreed reasonably well with the experimental data and were subsequently used to develop design curves. Design considerations for both small and large system applications were evaluated;The results of the study indicated that coarse media filters in series with standard dual media filters can be used successfully in extending the process of direct filtration to waters of higher turbidity than presently acceptable.