Phoslock is a clay to which five percent lanthanum has been added. This is a mineral that binds phosphates and prevents the growth of blue-green algae. By spreading a layer of the clay over the bottom of the Kralingse Plas, the phosphates are neutralized. As a result, it can no longer be food for blue-green algae. "It is not a chemical product, but a natural ore that is sourced from Asia and researched by Australians," says Rotterdam alderman Bert Wijbenga.

The University of Wageningen has recommended the use of Phoslock to the municipality. "This has already been applied in many countries in the world, and also a few lakes in the Netherlands, and it works."

In 2020 and 2021 there have been tests with Phoslock in the puddle. The approach seemed effective. According to the municipality, it has been shown that protection against blue-green algae is achieved faster, more effectively and more sustainably with Phoslock than with the dumping of sand, which was previously tested.

1,000 tons of clay against 500,000 tons of sand

The lake will need maintenance doses of Phoslock in the future. That depends on how much phosphate will later enter the pool. According to the municipality, applying the clay can be done faster and with less burden for local residents than sanding it off. This is because 1,000 tons of clay are needed against 500,000 tons of sand.

The impact on soil life would be smaller and the measure would be cheaper. "A favorable point of Phoslock is that it is several million cheaper. We had 13 million available to really tackle the Kralingse Plas. That amount is 8 million smaller with Phoslock," says Wijbenga.

Blue-green algae consists of bacteria that are important for the underwater food system. But if there is too much of it, they create a green-blue, smelly plaque that floats on the water and creates unhealthy bathing water. Blue-green algae needs light, heat and phosphates to grow. Phosphates arise from human and animal feces, food scraps, plant residues and agriculture and industry.

In addition to Phoslock, the municipality is deploying a number of measures to combat the supply of phosphates. For example, the Kralinger Hout pumping station has been disconnected and the Kralings Verlaat lock has been improved. Adjustments to sewer overflows, the prevention and clearing of leaf greenery and excrement and the renewal of a dephosphatization installation are still on the agenda.

Nuisance from blue-green algae will continue for a while

The municipality will start with Phoslock in the autumn of 2021. The effects of the clay should be noticeable in the summer of 2022. Until then, there is a strong chance that in the summer it is not allowed to swim in the Kralingse Plas for some time. "We are rated unsatisfactory for water quality." In recent years, the lake has been closed for several weeks or months due to blue-green algae.

Even after 2022, Wijbenga does not expect the lake to be free of algae. "Blue-green algae is a natural product and can never completely disappear." Nevertheless, the municipality expects that Phoslock will really be the last chapter in the thick book about Blue-green algae this time. "This is the chapter after which we can swim a lot more and do water sports on the lake."