A biotechnological perspective on groundwater sand filtration

Drinking water production works well, but we can do better.

Anaerobic groundwater is an excellent drinking water source. It presents several advantages over its counterpart, surface water, such as constant quality and temperature, and it is considered to be microbiologically safe. The main contamination sources of anaerobic groundwater are the decomposition of natural organic matter and the dissolution of soil minerals. The first one produces compounds such as ammonia, while the second introduces manganese, iron, and trace metals. Iron, ammonia and manganese must be removed from groundwater to produce drinking water. For this purpose, humans have been using rapid sand filtration - preceded by an aeration step - for over a century.

The purpose of aeration is to strip out undesired gases and to introduce oxygen up to saturation levels. At this oxidation-reduction potential, iron, ammonia and manganese are oxidized by different physical-chemical and biological processes in the subsequent rapid sand filter. As a result, most contaminants precipitate, forming solids that are captured by the filter, and clean water is produced. Although widely used and robust, solid understanding of the intricacies of rapid sand filters is still missing. A high degree of complexity is hidden behind their seemingly simple working principles.

The main reason underneath this extraordinary complexity is the high oxygen load introduced during the aeration step. The saturation of anaerobic groundwater with oxygen onsets a series of simultaneous, interwoven and uncontrolled reactions whose nature and contribution to the overall process remains unknown and generally unpredictable. This process convolution precludes understanding and optimization of rapid sand filtration.

The overarching goal of this thesis is to gain knowledge to advance towards the design of high-flow, resource-efficient sand filters. To do so, we must be able to understand the mechanisms that govern which reactions take place, in which order they occur, and how they affect each other, which will ultimately allow us to predict and control i) microbial community assembly and performance and ii) the interplay between chemical and biological reactions. In the first part of this thesis, we focused on gaining mechanistic understanding of how current sand filters work using laboratory, pilot, and full-scale experiments. In the second one, we used this freshly acquired knowledge to design and test novel systems…