Closed-circuit hypolimnetic withdrawal and treatment: impact of effluent discharge on epilimnetic P and N concentrations

Abstract

Closed-circuit hypolimnetic withdrawal and treatment systems (HWTS) represent a novel lake restoration technique in which nutrient-rich near-bottom water is pumped through a treatment system and returned to the same lake. However, the design of such systems is not yet standardized and routing of effluent waters must be planned carefully to minimize the risk of adverse water quality impacts. Here we assessed the risk of HWTS effluent to elevate epilimnetic nutrient concentrations under a range of withdrawal and effluent discharge scenarios (4.5–45 L/s, sand filtration only and sand filtration combined with wetland) at Lake Kymijärvi, Finland. The filter of the HWTS removed most of the phosphorus (67%), but only a small fraction of nitrogen (14%). For both nutrients, filter effluent concentrations were elevated with respect to the lake epilimnion. However, the results of our calculations suggest only minor increases (0–12%) in epilimnetic phosphorus concentrations in all withdrawal and discharge scenarios. For nitrogen, somewhat higher increases (1–17%) are expected unless the filter effluent is first discharged into a wetland as part of the HWTS circuit. We conclude that the impacts of the filter effluent on the epilimnion do not mask the benefits gained in the treated lake by the closed-circuit HWTS, but use of a buffering system such as a wetland decreases the risks further.