High Surface Area of Small Pumice Heightens Filtration Efficacy: Successful Experimental Applications of Pumice as Column Filter Media for Synthetic Graywater

Abstract

Graywater is wastewater from non-sewage domestic sources like bathing, dishwashing, laundry, and cleaning. Contaminant particles from shampoo, detergent, and other household products can be removed through granular filtration systems using material such as gravel or sand. Many natural materials have been proposed for use as graywater filters; this project tests different sizes of pumice, a porous volcanic rock. After characterizing physical properties of five different size fractions of pumice from the ~7,700 ka Climactic eruption of Mount Mazama, Oregon, we applied the pumice as a filtration medium in a granular column flow experiment and monitored graywater chemistry for seven days. Our experiments suggest that pumice effectively filters contaminants from graywater within 48 hours of column filtration flowing at ~10 liters per hour. We also find that, among the five sample sizes tested, the smallest, 1-2 mm, pumice particles are the most effective. Filtration with this size of rock results in significant reductions of turbidity, total nitrogen, phosphate, and chemical oxygen demand in the graywater over seven days. We believe this size range is the most effective filter because of its higher external surface area over volume ratio compared to larger sizes of pumice. These results suggest that small 1-2 mm pumice would be the most effective if applied to create real-world graywater filtration systems. More work remains to be done to understand the logistical needs and practical lifespans of these pumice systems.