Browsing by Author "Myyryläinen, Jerry"

Hypolimnetic withdrawal and purification is a new method developed to restore eutrophic lakes, which enables the removal and capture of nutrients such as phosphorus from hypolimnetic water. The method is currently being studied in a pilot-scale project at Lake Kymijärvi, Lahti, Finland. The project employs a closed-circuit system, in which the hypolimnetic water is pumped into a filtering system onshore, where the dissolved phosphorus is first precipitated. There are two possible methods for precipitation: aeration of water, in which dissolved phosphorus is precipitated by iron hydroxides as the water aerates, and addition of calcium hydroxide (Ca(OH)2) (hereafter “chemical treatment”), in which dissolved phosphorus is precipitated as hydroxyapatite. The water then flows through a nutrient filter, which traps the precipitate. The nutrient filter consists of two parallel filters, which enables the simultaneous comparison of two different filter materials. Finally, the water is returned to the lake via a wetland. A closed hypolimnetic withdrawal and purification system like this has not been previously studied as a restoration method. The aim of this master's thesis is to study the operation of the hypolimnetic withdrawal and purification system of Lake Kymijärvi by comparing the phosphorus retention capacity with 1) two different filter materials and 2) two different precipitation methods. The compared filter materials were fine gravel (manufactured by Rudus) and calcitic waste rock (manufactured by Nordkalk). Additionally, the thesis investigates the ratio of phosphorus fractions (Ca-P and Fe-P) of the phosphorus trapped in the filters, the distribution of trapped phosphorus between different filter layers and the effect of the filter materials on the concentrations of calcium, iron, manganese and sulfur of the water flowing through the filters. The operation of the hypolimnetic withdrawal and purification system was tested during four hypolimnetic water test pumping periods in summer 2019. The material for this thesis consisted of water and filter material samples collected from the system during the test pumping periods. The water samples were used to determine the capacity of the filter materials to capture phosphorus and other elements. The filter material samples were used to study the ratio of different phosphorus fractions and the distribution of trapped phosphorus between different filter layers. Both filter materials captured phosphorus efficiently via both precipitation methods. At their highest efficiency, both materials captured over 90 % of dissolved phosphorus and over 75 % of total phosphorus. No statistically significant difference was found in the phosphorus retention capacity between the filter materials, nor the precipitation methods. Of other elements, both filter materials captured iron very efficiently during both precipitation methods, and manganese during the chemical treatment. The phosphorus trapped in the filters was not evenly distributed between different filter layers and most of the phosphorus was trapped in the bottom-most layer in both filters. The phosphorus fractions between the two precipitation methods differed less than expected. After water aeration, all the phosphorus trapped in the filters was bound to iron. Even after the chemical treatment, less than half of the trapped phosphorus was bound to calcium, with the remainder bound to iron. According to the results, it is possible to remove phosphorus efficiently from hypolimnetic water with the system, using whichever of the studied precipitation methods or the filter materials.