Nitrogen-metabolising microorganism analysis in rapid sand filters from drinking water treatment plant

Sand filters (SFs) are common treatment processes for nitrogen pollutant removal in drinking water treatment plants (DWTPs). However, the mechanisms on the nitrogen-cycling role of SFs are still unclear. In this study, 16S rRNA gene amplicon sequencing was used to characterise the diversity and composition of the bacterial community in SFs from DWTPs. Additionally, metagenomics approach was used to determine the functional microorganisms involved in nitrogen cycle in SFs. Our results showed that Pseudomonadota, Acidobacteria, Nitrospirae and Chloroflexi dominated in SFs. Subsequently, 85 high-quality metagenome-assembled genomes (MAGs) were retrieved from metagenome datasets of selected SFs involving nitrification, assimilatory nitrogen reduction, denitrification and anaerobic ammonia oxidation (anammox) processes. Read mapping to reference genomes of Nitrospira and the phylogenetic tree of the ammonia monooxygenase subunit A gene, amoA, suggested that Nitrospira is abundantly found in SFs. Furthermore, according to their genetic content, a nitrogen metabolic model in SFs was proposed using representative MAGs and pure culture isolate. Quantitative real-time polymerase chain reaction (qPCR) showed that ammonia-oxidising bacteria (AOB) and archaea (AOA), and complete ammonia oxidisers (comammox) were ubiquitous in the SFs, with the abundance of comammox being higher than that of AOA and AOB. Moreover, we identified a bacterial strain with a high NO₃-N removal rate as Pseudomonas sp. DW-5, which could be applied in the bioremediation of micro-polluted drinking water sources. Our study provides insights into functional nitrogen-metabolising microbes in SFs of DWTPs.

Fate and distribution of determinants of antimicrobial resistance in lateral flow sand filters used for treatment of domestic wastewater

Residuals of antimicrobial products from anthropogenic uses can create a selective environment in domestic wastewater treatment systems and receiving environments and contribute to the spread of antimicrobial resistance (AMR). On-site wastewater treatment systems are widely used for domestic wastewater management in rural and remote regions, but the fate of determinants of AMR in these types of environments has received little attention. In this study, the mechanisms responsible for the attenuation of determinants of AMR in lateral flow sand filters were explored using a combination of lab, field and modeling investigations. The degradation kinetics and adsorption potential in the sand filter medium of three antibiotic resistance genes (ARGs; sul1, tetO, and ermB) and culturable bacteria resistant to sulfamethoxazole, tetracycline, and erythromycin were measured using lab experiments. The spatial distribution of ARGs and antibiotic resistant bacteria were also assessed in field scale sand filters, and mechanistic modeling was conducted to characterize filtration processes. The results indicated that the primary mechanisms responsible for AMR attenuation within the sand filters were degradation and filtration. The spatial distribution of AMR determinants illustrated that attenuation was occurring along the entire length of each filter. This study provides new insights on primary mechanisms of AMR attenuation in on-site wastewater treatment systems and supports the use of conservative design guidelines and separation distances for reducing AMR transmission.

The impact of biodegradable carbon sources on microbial clogging of vertical up-flow sand filters treating inorganic nitrogen wastewater

The addition of biodegradable carbon sources to sand filters can enhance microbial activity but may lead to substrate clogging, a major operational problem. In laboratory-scale soil columns emulating vertical up-flow filters, the clogging effect of two readily biodegradable organic substrates-sucrose as a sugar source and ethanol as an alcohol source-were examined with coarse sand as the substrate medium. Wastewater without the addition of supplemental organics and a 'control' treated with tap water were monitored as references. Changes in saturated hydraulic conductivity were measured for all treatments over time. Other parameters that can influence the clogging rate, including temperature, dissolved oxygen, chemical oxygen demand, protein, and polysaccharides, were measured in the influent and effluent wastewater on a weekly basis. At the end of the clogging experiment, the main layer of each filter bed was separated into three sections and saturated hydraulic conductivity, organic matter content, and protein and polysaccharide concentrations were measured in each section. The rate of clogging development in the columns depended on treatment, with ethanol-treated cores clogging more quickly than sucrose-treated cores. Wastewater-treated cores took far longer to clog and the tap water control did not clog, but the saturated hydraulic conductivity declined by 60% over a year. Saturated hydraulic conductivity within the treated cores declined far less than the calculated decline in saturated hydraulic conductivities for the entire cores at the end of the experiment, indicating that clogging in the vicinity of the inlet plate by microbial mats was a major factor influencing the reduction in flow through the columns. To reduce bio-clogging in inlet filters, it may be advantageous to inject organic amendments directly into the bed, rather than pass them through the filters, as is usually the case.