Houston, TX 77005
3:45 p.m. Friday, Sept. 27, 2013
On Campus | Alumni
Biological drinking water treatment is undergoing a renaissance in the United States, with increased interest in the systematic study of biofiltration. Bacteria are often cited as requiring a carbon:nitrogen:phosphorus (C:N:P) molar ratio of 100:10:1 for growth, and increased production of extracellular polymeric substances (EPS) has been observed when the nitrogen and/or phosphorus concentrations are limiting. Thus, nitrogen or phosphorus limitation might lead to increased EPS production and headloss in a biofilter, which translates to increased backwash frequency and energy consumption. At the pilot-scale, we previously have observed higher EPS concentrations and terminal headlosses in a phosphorus-limited drinking-water biofilter as compared to a biofilter where phosphorus was not limiting. The goal of our current work is to fundamentally examine EPS production by drinking-water bacteria under a variety of controlled C:N:P conditions in the laboratory. In mixed cultures of drinking-water bacteria, we have observed an increase in EPS polysaccharides per mass of volatile suspended solids under both nitrogen-depleted and phosphorus-depleted conditions. We have interrogated the potential for EPS production at the molecular level by quantifying exoB, a gene involved in EPS polysaccharide production, as a function of the C:N:P ratio. In these studies, we looked particularly at rhizobial bacteria, since these EPS-producing bacteria appear to increase in population size under phosphorus-limited conditions in drinking-water filters. Separately targeting Bradyrhizobium, Sinorhizobium, and Rhizobium, we observed an increase in their exoB gene copies per mass of DNA under nitrogen- and/or phosphorus-depleted conditions. Subsequent experiments will explore the optimal C:N:P ratio in drinking-water biofilters, such that sufficient EPS is produced to sustain the biofilm and its extracellular enzymatic activities but to avoid the overproduction of EPS and associated increases in headloss.