Optimization of filtration protocols in hypolimnetic withdrawal and treatment systems

Hypolimnetic withdrawal and treatment systems offer a promising approach to restore eutrophic lakes by removing excess phosphorus (P). However, optimization is needed, particularly regarding filter material performance. In this study, two field column experiments were conducted using a pilot-scale HWTS in Lake Kymijärvi (southern Finland) to evaluate three filter materials: a locally sourced construction sand (“Kymisand”), and two commercial products (Rådasand and Polonite). The effect of adding roughing filters as pretreatment was also tested. Results show that physical retention of solid phases was the dominant removal mechanism for P, followed by phosphate sorption. Kymisand outperformed the other materials, retaining 131.3 mg P compared to 32.3 mg (Rådasand) and 16.7 mg (Polonite). Sequential extraction indicated that most P accumulated as citrate-dithionite-bicarbonate (CDB)-soluble Fe-bound forms. All columns experienced clogging due to surface accumulation of ferrihydrite-rich precipitates, although porosity reduction never exceeded 11% in any depth interval of the columns. This highlights the importance of maintaining permeability through periodic removal of the uppermost filter layer. Our findings offer practical guidance for improving HWTS design and operation, especially under high Fe loading, and contribute to a better understanding of P removal mechanisms in engineered lake restoration systems.