Treatment of Hypolimnion Water on Mineral Aggregates as the Second Step of the Hypolimnetic Withdrawal Method Used for Lake Restoration

Abstract

The study aimed to assess the usefulness of mineral aggregates in orthophosphate (OP) removal from hypolimnetic water withdrawn from eutrophic lakes. Two low-cost and easily available reactive materials were tested: lightweight expanded clay aggregate (LECA) and crushed limestone (LS). Their performance regarding OP removal and the effect on the pH, Ca2+, Mg2+, N-NO3 and N-NH4 concentrations of treated water were investigated in a column experiment with four-filter beds made of LECA and amended with LS (additions of 0, 25, 50 and 75% of the bed volume). The highest OP removal (>50%) was achieved in LECA beds with high (75% by volume) amendments of LS. Neither LECA nor LS distinctly affected the pH (maximum pH increase, from 7.1 or 7.2 to 7.6, occurred in the case of the LECA bed). In real-life conditions, it is not feasible to install a full-scale bed made of these mineral aggregates on the outflow from a lake due to the large required size of such a bed. At the operation time set for 30 d, the size of a bed would need to reach between 6113.2 and 12,226.4 m3. The proposed bed should be just one of the elements of an integrated treatment system. Constructions consisting of sorption beds ought to be coupled with adequately designed zones of aquatic vegetation. Three conceptional solutions were proposed for in situ treatment of the withdrawn water, differing in arrangement and construction of the potential sorption bed. Application of such solutions should be regarded as a substantial improvement of Olszewski’s method, as it can mitigate the pollution of downstream ecosystems.

Removal of Phosphorus from Hypolimnetic Lake Water by Reactive Filter Material in a Recirculating System—Laboratory Trial

“…In Lake Kymijärvi in Finland, experiments are underway where the pumped water is treated with various filter materials and then led via a wetland back to the lake [10]. The use of other RFM with lower initial pH values can be considered, such as Sorbulite ® [31,43] and Leca ® [8]; however, their service life can be much shorter because of their lower P-removal efficiencies. The pumping in our experiment did not affect the DO concentration in the cold bottom water, probably because the redirected water was warmer and slowly mixed with the surface water.…”

“…Nature-based solutions with multiple in-lake techniques can also be applied to suppress eutrophication [6]. A method that addresses the possibility of saving the P resource and applying cycling of P and circular economy is hypolimnetic withdrawal (HW) of water from lakes integrated with reactive media filtration [7][8][9]. Silvonen et al [10] discussed a solution where withdrawn water would flow through a P-capturing purification unit and subsequently return into the same water body.…”

“…Silvonen et al [10] discussed a solution where withdrawn water would flow through a P-capturing purification unit and subsequently return into the same water body. This method involved, in its original version, withdrawing of the lake's hypolimnetic water directly through a pipeline and discharging into a downstream water body [8,11]. HW has several risks, such as the impact on the lake s water budget and impact on downstream waters, including eutrophication, temperature increase, oxygen depletion, and odor development [12].…”

“…The disadvantages that emerged from the experiment were high pH (11.96) of filter effluent and release of vanadium and aluminum from the virgin filter material. Ło ży ńska et al [8,9] used Leca ® and crushed limestone, having a grain size of 10-30 mm, in a column experiment fed by hypolimnetic water with a PO 4 -P concentration of 0.41 to 0.46 mg L −1 . The duration of their experiment was less than 8 h. Best performance showed a mixture of Leca and limestone (>50%), and pH of the effluent water was not higher than 7.6.…”