Kartuzy is a commune town of 15 thousand residents in Kashubia, in the Pomeranian Voivodship. It is picturesquely located among forests and it has 4 cascading, inter-connected lakes within its borders. Lake Mielenko (area 7.8 ha, max depth 1.9 m) is situated at the greatest height. A stream connects it to Lake Karczemne (40 ha, max depth 3.2 m). This, in turn, is connected to Lake Klasztorne Małe (13.7 ha, max depth 20 m). Lake Klasztorne Duże (57.5 ha, max depth 4.8 m) is the last one in this chain.
Combined with the charming landscape, these lakes could be an attraction of the town, if it was not for the fact that they are completely poisoned. Sanitary and storm wastewater have flowed into it from no-one-remembers-when to 2018. Tests have shown that Lake Karczemne, into which wastewater flowed for years through six collectors, was the most polluted.
“The concentration of phosphorus compounds in its water is 20 times higher than the maximum safe level. The situation is compounded by considerable amounts of other substances. The chemical composition of its water corresponds to the composition of diluted wastewater rather than that of lake waters”, says Dr Renata Augustyniak of the Department of Water Conservation Engineering and Environmental Microbiology at the Faculty of Geoengineering.
This makes the lakes a blemish on the town’s image rather than an attraction. People are attracted to water, but they cannot stay there long, and swimming or using waster sport equipment is out of the question – the water stinks.
In 2012, the town authorities approached the Department, asking for help in remediation of the lakes of Kartuzy. Why did they choose scientists from the UWM rather than from nearby Gdańsk? That was because they are the best, but also because UWM graduates work in municipal service institutions in Kartuzy. The research, with breaks, lasted several years. It was conducted by a team of scientists of the Department of Water Conservation Engineering and Environmental Microbiology: Dr Renata Augustyniak, Dr Renata Tandyrak, Dr Michał Łopata, led by Dr Jolanta Grochowska, Professor at UWM, Head of the Department. First, the scientists of Kortowo recommended shutting off the inflow of sanitary and storm wastewater into the lakes, which was done. However, this does not solve the problem.
“We decided that this lake requires two methods of remediation: first, a layer of silt, which contains huge amounts of phosphorus, nitrogen and other chemical substances, should be removed from the bottom. Then, we are going to add unharmful coagulants, i.e. certain chemical substances which permanently bind phosphorus from water and deposit it to the sediments left on the bottom”, explains Prof. Grochowska.
The area of Lake Karczemne is 40 ha. No one in Poland has tried to collect bottoms from such a large area. Various water bodies are deepened in this way, but it is a completely different matter. The scientists of Kortowo collected over 100 deposit samples from the lake bottom and determined how thick the layer collected at various places must be.
“It ranges from 60 to 90 cm. We divided the lake into 28 sections of different area and thickness of deposits to remove”, says Dr Michał Łopata.
The operation started this June. The bottom deposits will be removed by a suction dredge. It is a barge with powerful devices to deposit suction. Its operation resembles that of a vacuum cleaner. It sucks deposits from the lake bottom and pumps them to a pipeline. A suction dredge collects deposits only from one field, which is separated from others with special curtains to prevent clouded water and deposits from flowing around”, explains Dr Renata Tandyrak.
In this way, field by field, the suction dredges will “vacuum” the whole lake. And what will happen to the collected bottom deposits? They will flow to the wastewater treatment plant through a specially constructed 4 km pipeline. Here, they are centrifuged, dried and stored. Some of them will be used as fertiliser in agriculture and the rest, which is unusable, will be disposed of. The water separated from the deposits will be transferred to the treatment plant, where it will go through the whole treatment process.
When the “vacuuming” is completed, Lake Karczemne will receive four coagulant doses over two years and they will bind the remaining phosphorus. This is the treatment that the scientists from Kortowo also prescribed to the other lakes.
“It is a novel method that we decided to apply in Kartuzy. No one in Poland has dared to “vacuum” an entire lake. We are optimistic and we believe that – after Prof. Olszewski’s method and his “Kortowo experiment” – it will become another Kortowo speciality which, like the first one, will be widely applied around the world”, says Prof. Grochowska.
Prof. Olszewski’s method involves cleaning a lake by draining off the most polluted above-bottom waters through a pipeline. It is the first method of remediation applied anywhere in the world on a technical scale. The first pipeline was launched in Lake Kortowskie in 1956, and the system still works.
The lake remediation will cost the commune of Kartuzy 55 million PLN. It received a subsidy of 45 million PLN from the National Fund for Environmental Protection and Water Management. The commune’s own revenue for 2020 amounts to 205 million PLN.