Filtration Removals of Microorganisms and Particles

Filtration log removals (log Rʹs) were determined by pilot plant seeding experiments for a selected array of organisms, i.e., protozoan cysts, algae, bacteria, and viruses, as well as turbidity/particles. Removals of these organisms and particles varied from one species to another within the range 1.4 <= log R <= 5.1 for "reference," i.e., specified, conditions. For other conditions, called here "excursion" conditions, log Rʹs were found to be proportional to alum dose between zero and "optimum" the latter being defined as the smallest dosage that minimized filter effluent turbidity. Mono and dual media did not show performance differences, and conventional filtration showed slightly higher log Rʹs than in-line filtration. The influent concentration did not show a "true" effect on log Rʹs (but an "apparent" effect was seen when effluent values were consistently very low, making log Rʹs proportional to influent concentration). log Rʹs obtained in limited experiments at another site showed significant differences for two algae and two viruses when compared with log Rʹs for reference conditions. The results illustrate the log R variation that may be found among different groups of organisms/particles and indicate the influences of certain independent variables (e.g., influent concentration, dual media versus mono media, conventional filtration versus in-line, alum dose, zero alum dose, and filtration at a different site) on filtration removals. In addition, the importance of sampling repetitions was reinforced. The findings are significant to water treatment practice in that organism removals by filtration are likely to vary, depending on the organism/particle species as well as the filtration conditions. The implication is that a pilot plant study is advisable if confidence in log R is desired for a given organism/particle.