Microbial Dynamics of Biosand Filters and Contributions of the Microbial Food Web to Effective Treatment of Wastewater-Impacted Water Sources

Abstract

Biosand filtration systems are widely used for drinking water treatment, from household-level, intermittently operated filters to large-scale continuous municipal systems. While it is well-established that microbial activity within the filter is essential for the removal of potential pathogens and other contaminants, the microbial ecology of these systems and how microbial succession relates to their performance remain poorly resolved. We determined how different source waters influence the composition, temporal dynamics, and performance of microbial communities in intermittently operated biosand filters. We operated lab-scale biosand filters, adding daily inputs from two contrasting water sources with differing nutrient concentrations and found that total coliform removal increased and became less variable after 4 weeks, regardless of water source. Total effluent biomass was also lower than total influent biomass for both water sources. Bacterial community composition, assessed via cultivation-independent DNA sequencing, varied by water source, sample type (influent, effluent, or sand), and time. Despite these differences, we identified specific taxa that were consistently removed, including common aquatic and wastewater bacteria. In contrast, taxa consistently more abundant in the sand and effluent included predatory, intracellular, and symbiotic bacteria.IMPORTANCE Although microbial activities are known to contribute to the effectiveness of biosand filtration for drinking water treatment, we have a limited understanding of what microbial groups are most effectively removed, colonize the sand, or make it through the filter. This study tracked the microbial communities in the influent, sand, and effluent of lab-scale, intermittently operated biosand filters over 8 weeks. These results represent the most detailed and time-resolved investigation of the microbial communities in biosand filters typical of those implemented at the household level in many developing countries. We show the importance of the microbial food web in biosand filtration, and we identified taxa that are preferentially removed from wastewater-impacted water sources. We found consistent patterns in filter effectiveness from source waters with differing nutrient loads and, likewise, identified specific bacterial taxa that were consistently more abundant in effluent waters, taxa that are important targets for further study and posttreatment.

Keywords: biosand filtration; drinking water; microbial ecology.

FIG 1

Average total coliform concentrations over time in the influent and effluent waters (a) and average percent total coliform removal (b) of filters receiving low and high nutrient source waters. Microbial biomass measured by DNA yield over time in the influent and effluent (c) and sand (d) samples. Each point represents the mean, and error bars represent the standard deviation across five replicate samples of source water (influents) or five replicate biosand filters (effluents and sand). Gray lines indicate the detection limits.

FIG 2

Principal-coordinate analyses (PCoA) of microbial communities based on pairwise Bray-Curtis dissimilarity, comparing sample type and water source across all samples (a) and over time for high nutrient effluent (b), high nutrient sand (c), low nutrient effluent (d), and low nutrient sand (e). PCoA of influent microbial communities over time shown in Fig. S4. Note that the low nutrient influent was from a different source during weeks 5 and 7 (see Fig. 4).

FIG 3

Relative abundance of the 19 most abundant classes over time for the low nutrient (left) and high nutrient (right) water sources for samples from the influent (top), sand (middle), and effluent (bottom) over time. Each bar represents combined data from four to five replicates (results from individual replicates are presented in Fig. S2). * indicates that the low nutrient influent was from a different source during weeks 5 and 7.

FIG 4

Boxplot of richness, measured by the number of observed exact sequence variants (ESVs) at a rarefied sequencing depth of 2,122 reads per sample for each water source, sample type, and week. *, indicate that the low nutrient influent was from a different source during weeks 5 and 7.

FIG 5

Differential relative abundances between the influent and effluent of the most abundant microbial classes (>0.01% relative abundance) for each water source across all time points (for time resolved data see Fig. S6), ordered by the average differential relative abundance for both water sources. Bolded text labels indicate classes with the same trend in differential abundance between the two source waters. Stars indicate statistically significant differences (FDR, <1%). Text label colors indicate specific phyla of interest.

FIG 6

Schematic of the experimental setup. Eighty replicate biosand filters (2.54-cm diameter with a 10.5-cm filter bed depth) were constructed from PVC pipes with a standpipe to keep water 4 cm above the sand bed. Daily water inputs were added from either of two water sources (with high or low nutrients), each spiked with diluted wastewater. Samples for DNA extraction, sequencing, and biomass estimation were collected weekly from five replicate filters, including influent, sand, and effluent. Weekly coliform removal was also measured over 8 weeks.