Biofilms control water quality improvement during soil and aquifer transport

This work is motivated by the search to understand biochemical reactions that remove pollutants in the soil, vadose zone and aquifers. These are essential for the design of Soil Aquifer Treatment (SAT, i.e., recharge the effluents of wastewater treatment plants) or denitrification bioreactors. Contaminant removal is enhanced by the incorporation of woody material that plays two essential roles: (1) release of dissolved organic carbon, which promotes the development of a broad range of redox conditions and thus diverse microbial communities, and (2) a high content of solid organic carbon, which ensures long residence times. We summarize the work we have been doing in recent years. We have found that (1) the dramatic water quality improvement, long known to occur soil and aquifer passage for pathogens and major contaminants, also extends to contaminants of emerging concern and antibiotic resistance; (2) the most toxic species, with significant partition coefficients, tend to accumulate in biofilms, where microorganisms accumulate. The accumulation of degrading microorganisms at the place where they can be degraded suggest natural selection; (3) biofilm growth in porous media, constrained by space and limited microbial mobility, is quite different from growth in open spaces or living tissues. Laboratory experiments are best explained by assuming that a power law of sizes (numerous minuscule and a few large biofilm clusters). High retention and active degradation at biofilms ensure an extremely bioactive environment, which explains the dramatic reduction in toxicity we have observed after relatively modest (some 15 days) residence time in the aquifer. These finding suggest that the strict conditions imposed by the EU on SAT must be relaxed.