Extracting phosphorus and other elements from lake water: Chemical processes in a hypolimnetic withdrawal and...

Hypolimnetic withdrawal provides a way to remove phosphorus (P) from eutrophic lakes, but the method is still rarely combined with water treatment for capturing this P. Thus, little is known about the chemical interactions of P and other elements upon the treatment of hypolimnetic lake water. We investigated these chemical processes in a hypolimnetic withdrawal and treatment system (HWTS) in which hypolimnetic water is first led into a treatment unit for dissolved P (dP) precipitation and subsequently filtered before being circulated back into the lake. We studied three different field-scale treatment unit setups and water treatments (aeration only, aeration + calcium hydroxide (Ca(OH)₂), aeration + biopolymer) to compare their effectiveness for dP removal and the geochemical properties of the resulting precipitate. In the aeration only treatment, most of the dissolved iron (dFe) (91-95%) and dP (71-91%) were removed when sand filters were used. The addition of Ca(OH)₂ and biopolymer enhanced Fe flocculation, leading to more effective removal of dFe (d99-100%) and dP (88-95%) from the water. Regardless of the water treatment method, dP was always precipitated by amorphous Fe oxides formed in the hypolimnetic water upon aeration. The P content of the resulting precipitates was somewhat lower than expected (2 439-4 145 mg kg^-1), which may be linked to chemical interactions between Fe and other components in the hypolimnetic water, such as organic matter. The precipitates also contained some heavy metals such as copper and zinc. We conclude that all the tested water treatments were effective in removing dP from hypolimnetic water, but the enhanced precipitation by the addition of treatment chemicals is beneficial when a mesh or other rapid filtration method is used, or when there is only negligible accumulation of dFe in the hypolimnion of the treated lake. Depending on the water treatment method and the water chemistry of the treated lake, the precipitate may have potential for nutrient recycling, although it may sometimes require preliminary processing to enhance bioavailability for plants and to reduce the concentration of heavy metals.