Household sand filters (SFs) are widely applied to remove iron (Fe), manganese (Mn), arsenic (As), and ammonium (NH ) from groundwater in the Red River delta, Vietnam. Processes in the filters probably include a combination of biotic and abiotic reactions. However, there is limited information on the microbial communities treating varied groundwater compositions and on whether biological oxidation of Fe(II), Mn(II), As(III), and NH contributes to the overall performance of SFs. We therefore analyzed the removal efficiencies, as well as the microbial communities and their potential activities, of SFs fed by groundwater with varying compositions from low (3.3 μg L ) to high (600 μg L ) As concentrations. The results revealed that Fe(II)-, Mn(II)-, NH -, and NO -oxidizing microorganisms were prevalent and contributed to the performance of SFs. Additionally, groundwater composition was responsible for the differences among the present microbial communities. We found i) microaerophilic Fe(II) oxidation by Sideroxydans in all SFs, with the highest abundance in SFs fed by low-As and high-Fe groundwater, ii) Hyphomicropbiaceae as the main Mn(II)-oxidizers in all SFs, iii) As sequestration on formed Fe and Mn (oxyhydr)oxide minerals, iv) nitrification by ammonium-oxidizing archaea (AOA) followed by nitrite-oxidizing bacteria (NOB), and v) unexpectedly, the presence of a substantial amount of methane monooxygenase genes (pmoA), suggesting microbial methane oxidation taking place in SFs. Overall, our study revealed diverse microbial communities in SFs used for purifying arsenic-contaminated groundwater and our data indicate an important contribution of microbial activities to the key functional processes in SFs.