Phosphorus is Destroying our Lakes – Could LakeBots and Hemp help?
Only very few of Denmark’s lakes are in good ecological condition. Biologist Kasper Reitzel wants to change that. His idea: Use LakeBots to vacuum phosphorus-rich sediment from the lakebed, clean it with hemp, and return it to the fields as fertilizer.
We’re familiar with robot vacuum cleaners — now get ready to meet the LakeBot. That’s the vision of biologist Kasper Reitzel, an expert in lake restoration at the University of Southern Denmark’s Department of Biology.
Denmark’s lakes are in desperate need of help. Over decades, phosphorus has accumulated on the bottom of almost every lake in the country. When this sediment releases phosphorus into the water, it fuels the growth of microscopic algae.
In summer, they bloom so intensely that sunlight can't reach the underwater plants. The result? The plants die, oxygen levels drop, and aquatic life collapses.
Kasper Reitzel
Biologist and Associate Professor of Ecology at the Department of Biology. He’s research focus is on interdisciplinary collaborations that can lead to cleaner lakes in Denmark and protect critical resources such as phosphorus. His research is supported by the Poul Due Jensen Foundation, the Danish Agricultural Levy Fund, EU Horizon Europe, and Innovation Fund Denmark.
"We’ve developed a prototype of a machine - a LakeBot —that gently vacuums phosphorus-rich sediment without disturbing the ecosystem," says Kasper Reitzel.
“We tested it in 2024 at Lake Ormstrup, and it worked very well. This gives us hope that it could benefit many other lakes across Denmark. But it still needs further development to become suitable for a wider range of lakes – and to be cost-effective compared to current lake restoration methods.”
The main target is phosphorus – a key component of the NPK fertilizers used in agriculture. Not all of this phosphorus is absorbed by crops, and the excess can wash into lakes.
Phosphorus and geopolitical tensions
Phosphorus is not just a pollutant – it's also a limited global resource. Only a few large phosphorus mines exist globally, none of which are in the EU.
Most are in geopolitically sensitive countries like China, Morocco, the USA, and Russia. Even if supply is stable now, the mines will eventually run dry.
“That’s why we need to start talking about how to recover and reuse the phosphorus that’s already built up in our lakes,” Reitzel explains.
Using phosphorus where it does good
In the Ormstrup Lake project, sediment collected by the LakeBot was placed in large dewatering bags on the shore, allowing water to drain off naturally.
"The idea is to remove phosphorus from places where it harms and reuse it where it benefits — on agricultural land. But before applying this sediment, we must ensure it doesn’t carry harmful substances”, he says, adding:
“Certain heavy metals and PFAS can be a problem in lake sediments. Currently, we are researching if a solution could be to leave the bags by the lake shore and plant crops in them that are capable of removing the pollutants from the shore when they grow”.
The many benefits of industrial hemp
The use of plants to clean contaminants is called phytoremediation. One particularly promising candidate in this case is industrial hemp.
"Hemp can thrive in nutrient-rich sediment and, it will grow tall and strong and while growing, it will absorb pollutants like PFAS and heavy metals."
Even better, hemp is a high-value crop. Its 4-5 meters tall, fibrous stalks can be turned into rope or biocement. It can be used in pharmaceuticals or pressed into a high-protein green juice for animal feed.
Too much phosphorus in Danish Lakes
Denmark monitors the environmental status of 986 lakes. However, the country actually has around 120,000 lakes larger than 100 square meters. Nearly all of them have accumulated phosphorus over time — mostly from wastewater and nearby agricultural land, where it’s spread as fertilizer. Excess phosphorus not absorbed by crops often ends up in lakes, settling on the bottom. Since phosphorus is a finite resource, scientists are exploring ways to recover it from lakes and reuse it as fertilizer.
"So hemp not only cleans the sediment — it can also become a profitable crop for farmers."
The Ormstrup Lake project is a collaboration between researchers from DTU, AU, AAU, and KU. It’s been funded by the Poul Due Jensen Foundation since 2020.
Ormstrup was chosen due to its severe phosphorus pollution and its suitability as a model lake. The hope is that successful methods developed here can be scaled across the country.