Impact of phosphorus limitation on the hydraulic performance of drinking-water biologically active filters

Drinking-water biologically active filters (biofilters) can transform organic and inorganic contaminants. One concern with biofilters is their potential to develop headloss more rapidly than do conventional, abiotic filters. Phosphorus (P) supplementation is one proposed engineering enhancement strategy for mitigating increased headloss, but experiences with P supplementation in the field have been mixed, with some plants observing lower headloss while others observe no hydraulic benefit. P limitation in drinking water typically is identified by a biodegradable carbon to phosphorus molar ratio (C:P) greater than 100 in the water. This study evaluated the impact of five increasingly restrictive C:P ratios on substrate consumption, biofilm properties, and headloss in bench-scale biofilters. An improved protocol was developed to extract extracellular polymeric substances (EPS) from granular filter media so that EPS concentrations could be related to headloss. The ratio of phosphatase activity to total glycosidase activity (PHO:GLY) was positively correlated to the ΔC:P ratio (calculated using the change in dissolved organic carbon concentration across the biofilter), suggesting its suitability as an indicator of P limitation in biofilters. As the imposed P limitation was increased, substrate consumption decreased, biomass concentration decreased, EPS concentrations increased, the biofilm was more filamentous, and the microbial community contained a higher proportion of Bradyrhizobiaceae and Hyphomicrobiaceae. The biofilm alone did not cause substantial increases in headloss within the timescale of interest for full-scale biofilters; however, in the presence of influent particles, headloss accumulated more rapidly in the P-limited column (average PHO:GLY 205), which had higher EPS concentrations and more filamentous biofilm morphology, than in the balanced column (average PHO:GLY 31). The impact of P supplementation was evaluated at three pilot systems. P supplementation lowered headloss by >35% and resulted in less filamentous biofilm morphology in the one system where headloss was monitored, but substrate consumption, biomass concentration, EPS concentrations, and microbial community composition did not change in response to P supplementation in any of the tested pilot systems. The pilot-scale biofilters had PHO:GLY ratios similar to the balanced (ΔC:P 150) bench-scale column, so the existing P limitation likely was mild. This study demonstrated the utility of PHO:GLY in identifying P limitation.Civil, Architectural, and Environmental Engineerin