Removal of natural toxins in biological sand filters for drinking water

Written by Natasa Skrbic

In Denmark, 99% of water supply is abstracted from groundwater. Due to the good water quality, treatment of groundwater is usually a simple process, comprising aeration and subsequent biological rapid sand filtration with naturally present microbial community. One of the goals of my PhD project is to investigate whether substantial natural toxin removal is possible using rapid sand filters, a cheap and commonly applied water treatment method in Denmark.

This study focusses on two groups of highly mobile and toxic naturally produced compounds: norsesquiterpene glucosides (ptaquiloside and caudatoside) and alkaloids (jacobine N-oxide, senecionine N-oxide, sparteine, gramine and caffeine). These plant toxins have been already found in groundwater and/or surface water in Denmark. They are highly water-soluble toxic compounds with almost no sorption to soil and sediment. In turn, they can potentially contaminate groundwater, which presents a concern for human health if used as drinking water source.

Filter sand is collected from aerobic rapid sand filters at five Danish waterworks and investigated for their natural toxins removal potential. Microcosms were set up with filter sand, groundwater and spiked with natural toxins. During the course of 14 days, degradation potential of the sand filters is investigated by collecting and analyzing water samples. For the toxin analyses, UPLC-MS/MS (alkaloids) and LC-MS (norsesquiterpene glucosides) methods are used. In addition, DNA analyses are included in this study to help understanding which microorganism are involved in toxin degradation processes.

Preliminary results (from one waterwork) showed that concentration of all natural toxins after 14 days is following: ptaquiloside decreased to 8% of the initial concentration, caudatoside to 4%, jacobine N-oxide to 86%, senecionine to 91%, sparteine to 50%, gramine 41-47% and caffeine to 94%.

Alkaloids exhibit recalcitrant character in this study and further investigations of different sand filters are currently performed.

Biological sand filters at the waterwork (left) and batch with sand filters and water in the lab (right)