Recovering P from a eutrophic lake via hypolimnetic withdrawal and purification

Hypolimnetic withdrawal (HW) is a restoration method for eutrophied lakes that aims to remove phosphorus (P) and other nutrients from the system. It is conventionally carried out by pumping or siphoning nutrient-rich bottom water to the discharge of the lake during periods of thermal stratification. However, there is growing interest in developing closed circuit modifications of HW in which nutrients could be captured and the purified water returned to the same lake. This would tackle some problematic aspects of conventional HW, and additionally enable the capture and recycling of P stored in lakes.
A pilot closed circuit HW system has been constructed at a eutrophic dimictic lake located in Southern Finland. This hypolimnetic withdrawal and purification circuit (HWPC) consists of a withdrawal pipe installed at the lake deep, a treatment and filtering unit on shore, and a wetland. In the treatment unit, P is first precipitated and then captured by sand filters, while the purified water flows subsequently through a wetland and finally back into the lake.
In the current study, we investigated the pool of potentially removable P in the study lake, the optimal timing of HW within the annual cycle, and the functioning of the HWPC. The P retention capacity of the purification unit and the composition of the precipitate trapped in the filters were both examined. The results showed that P accumulation in the near-bottom water of the study lake during thermal stratification is substantial, allowing significant amounts of P to be removed from the lake via HW. The concentration of total P of the water entering the HWPC was over 300 µg/L, of which the system captured more than 80%. The P content of the precipitate trapped in the filters varied between 6-12 g P/kg, and consisted of both iron and calcium-bound P phases. These results imply that it is possible to recover P accumulated in eutrophied lakes for potential recycling purposes.