The paper shows that methane in water, which is not removed completely at the first treatment step at waterworks, contributes to growth of invertebrates in drinking water systems. The study investigates different degrees of methane removal and presents a methane concentration at which growth of the primary producers, methane oxidizing bacteria, does not occur.

Methane is a potential source of carbon in drinking water. Typically, it is removed at waterworks during an initial treatment step such as aeration or stripping. Remaining methane may be converted by methane oxidizing bacteria to organic carbon, which is then available for heterotrophic growth and may ultimately contribute to invertebrate growth. We investigated presence of invertebrates at a waterworks with incomplete methane removal and at a waterworks without methane. Microscopy and analyses of 16S and 18S ribosomal genes were conducted on filter sand from full-scale biological rapid sand filters. Primary filters with methane, were dominated by methane- and ammonia oxidizing bacteria. Upper layers of secondary filters were dominated by heterotrophic bacteria while the deepest layer contained 92% eukaryote DNA. Rotifers, nematodes, platyhelminths and annelids constituted 22% of the DNA in the secondary filters. Filters with methane contained higher shares of invertebrates (13%) than the filter without methane (7%). Furthermore, pilot studies were conducted to estimate suitable levels of methane when implementing methane removal technologies. Methane concentrations of 0.24 mg/L caused rapid visible growth. Vacuum stripping and nitrogen addition removed methane to 0.018-0.03 mg/L and prevented growth of methane oxidizing bacteria.