Carbon dynamics and functioning of slow sand filters for drinking water production

Abstract

Slow sand filtration (SSF) is a well-established and sustainable drinking water treatment technology used worldwide. Functioning as a biofilter, it produces biologically stable water by removing organic matter and pathogens. However, quantitative measures of biological activity in SSF remain insufficiently defined. This dissertation advances the understanding of SSF by examining carbon dynamics as indicators of biological activity and by assessing the roles of both biological and physicochemical processes in filter performance. Stable isotope approaches were developed and applied to investigate carbon transformations and the contribution of the biological community to carbon retention and mineralization in SSF. The results demonstrate that biological activity can be assessed through combined measurements of carbon concentrations and isotopic signatures in influent and effluent waters. Furthermore, the potential for assimilation and mineralization of organic carbon in SSF can be determined using stable isotope tracers. Stable isotope tracer experiments revealed that bioactivity varies across filters at different locations, depends on operational and environmental conditions, and extends well below the schmutzdecke layer, previously regarded as the primary bioactive zone. The relationship between biological activity and pathogen removal was evaluated using Escherichia coli as an indicator organism, showing that both biological (e.g., biofilm attachment and predation) and physicochemical processes (e.g., adsorption to metal (oxyhydr)oxides) contribute to pathogen retention, with their relative importance determined by filter bioactivity and material characteristics. Pilot-scale studies further demonstrated that doubling the filtration velocity doubled the biological activity and promoted beneficial inorganic matter accumulation, which ultimately improved E. coli removal. The findings emphasize the complexity of microbial carbon processing in SSF and demonstrate that stable isotope methods are a promising approach to elucidate the fate and transformation of carbon through specific microbial pathways. They also highlight that enhancing SSF performance requires careful consideration of operational parameters, sand properties, source water characteristics, and upstream treatment steps. Together, these insights provide a scientific basis for optimizing SSF design and operation for sustainable drinking water treatment.