Removal of pathogenic bacteria in algal turf scrubbers

Abstract

Algal Turf Scrubbers (ATS) are ecologically engineered systems that utilize the growth of filamentous algae on a submerged surface to remediate polluted water using sunlight as the main energy input into the system. Previous algal turf scrubber research has demonstrated the ability of these ecologically engineered systems to remove or degrade a variety of pollutants from water and wastewater including nutrients, metals and organic chemicals. It is therefore within reason that algal turf scrubbers may also be able to treat polluted water for pathogenic bacteria since the shallow water depth of these systems allow for significant exposure to ultraviolet (UV) light. Additionally, treatment may be a physical function of the algal turf itself, which in natural environments has been shown to trap sediment particles and pathogenic bacteria through the adhesion onto interwoven, mucilaginous algal filaments. Although similar algae-based treatment systems such as High Rate Algal Ponds (HRAPs) have been reported to significantly reduce the level of pathogenic bacteria in treated water, insufficient research has been conducted on pathogen die-off in ATS systems. The objective of this research is to measure the rate of removal of active bacteria in a lab scale ATS and assign significance to two variables identified to affect pathogen removal in algal turf scrubbers: the direct and indirect effects of UV light exposure and the physical sequestration of pathogens by algal turf. The experimental approach of this research utilizes four lab scale ATS reactors that attempt to separate the effect of these variables on the removal of the aquaculture pathogen Flavobacterium columnare and a non-pathogenic strain of Escherichia coli using treatment combinations of reactors with and without algal turf and UV supplemented light. The results suggest that for both F. columnare and E. coli removal from the water column is primarily a physical function of the algal turf itself while UV light does play a role in enhancing the removal of E. coli but not F. columnare. Batch reactor experiments showed an average 3.6 log reduction in F. columnare and 2.6 log reduction with E. coli compared to experimental controls over a 24 hour period. The results suggest that larger ATS may significantly reduce bacteria concentration of treated water although consistency of treatment would likely vary.