Density and distribution of nitrifying guilds in rapid sand filters for drinking water production: Dominance of Nitrospira spp.

Abstract

We investigated the density and distribution of total bacteria, canonical Ammonia Oxidizing Bacteria (AOB) (Nitrosomonas plus Nitrosospira), Ammonia Oxidizing Archaea (AOA), as well as Nitrobacter and Nitrospira in rapid sand filters used for groundwater treatment. To investigate the spatial distribution of these guilds, filter material was sampled at four drinking water treatment plants (DWTPs) in parallel filters of the pre- and after-filtration stages at different locations and depths. The target guilds were quantified by qPCR targeting 16S rRNA and amoA genes. Total bacterial densities (ignoring 16S rRNA gene copy number variation) were high and ranged from 10 to 10 per gram (10 to 10 per m) of filter material. All examined guilds, except AOA, were stratified at only one of the four DWTPs. Densities varied spatially within filter (intra-filter variation) at two of the DWTPs and in parallel filters (inter-filter variation) at one of the DWTPs. Variation analysis revealed random sampling as the most efficient strategy to yield accurate mean density estimates, with collection of at least 7 samples suggested to obtain an acceptable (below half order of magnitude) density precision. Nitrospira was consistently the most dominant guild (5-10% of total community), and was generally up to 4 orders of magnitude more abundant than Nitrobacter and up to 2 orders of magnitude more abundant than canonical AOBs. These results, supplemented with further analysis of the previously reported diversity of Nitrospira in the studied DWTPs based on 16S rRNA and nxrB gene phylogeny (Gülay et al., 2016; Palomo et al., 2016), indicate that the high Nitrospira abundance is due to their comammox (complete ammonia oxidation) physiology. AOA densities were lower than AOB densities, except in the highly stratified filters, where they were of similar abundance. In conclusion, rapid sand filters are microbially dense, with varying degrees of spatial heterogeneity, which requires replicate sampling for a sufficiently precise determination of total microbial community and specific population densities. A consistently high Nitrospira to bacterial and archaeal AOB density ratio suggests that non-canonical pathways for nitrification may dominate the examined RSFs.

Metagenomic profiling of ammonia- and methane-oxidizing microorganisms in a Dutch drinking water treatment plant

“…However, further studies are needed to analyze the potential nitrite-oxidizing capacity of these bacteria. Although in a very low abundances, Nitrobacter species are also detected in some RSFs 680 (Tatari et al, 2017), which we however did not observe in DWTP Breehei.…”

Comammox Nitrospira are abundant ammonia oxidizers in diverse groundwater‐fed rapid sand filter communities

“…1B). Prefilters are commonly believed to be mainly the site of abiotic iron oxidation and precipitation (Tatari et al, 2017), and this appears to be corroborated based on the relatively lower density of nitrifiers in DWTP-11. Nitrospira spp.…”

“…Several different filter types were investigated in the present study including prefilters (PF) (DWTP‐7‐PF and DWTP‐11), single filters (SF) (DWTP‐1–3 and 12) and after‐filters (AF) (DWTP‐4–10). In groundwater‐fed RSFs, PFs are generally presumed to be the major site of abiotic reactions such as iron oxidation and precipitation, and generally contain a larger grain size, while AFs are considered the main site of nitrification, and contain smaller grain sizes (Tatari et al ., ). The low abundance of nitrifiers at DWTP‐11 agrees with a dominant abiotic role for PFs, but the large nitrifying community at DWTP‐7‐PF does not support this.…”

Abundance and community composition of comammox bacteria in different ecosystems by a universal primer set

“…The high abundance of CAOB was also detected in rapid sand filtration (RSF) system for groundwater treatment (Palomo et al, 2016). It was tempting to speculate that the frequently found numerical dominance of Nitrospira over AOB and AOA in drinking water distribution systems was probably ascribed from the abundant CAOB benefiting from the attached growth on the sand surface provided by the special configuration of RSF system (Gülay et al, 2016; Tatari et al, 2017).…”