In Rotterdam’s Kralingen reservoir, Dreissena mussels turned a floating solar array into a hanging “shell city,” coating the submerged floats while their constant filtering made the water clearer and created a new headache for engineers wondering what all that growing shell could eventually do to pipes and maintenance

Water treatment engineers working with a sun-tracking floating solar park experiment at Rotterdam’s Kralingen water storage facility for drinking water found out that the submerged pontoons developed an unanticipated biological layer underneath.

In no time, the submerged pontoons experienced massive biofouling as invasive Dreissena mussels colonized the artificial structures in high local densities.

Even though the filtering abilities of the bivalves ensured the purification of the water around by removing particulate matter, it led to heavy biofouling formation.

The engineers now have to ensure renewable energy production alongside maintaining the system.

Colonizing submerged floats in a drinking water reservoir

Evides Waterbedrijf, a water company from Rotterdam, used floating solar panels on the surface of the Kralingen reservoir.

The pilot system relies on rotating floats that rotate all day long to face the sunlight.

The presence of submerged artificial structures served as the perfect attachment sites for the drifting mussel larvae in the freshwater body.

In just a few months’ time, Dreissena mussels colonized the artificial floating structures.

Large numbers of mussels colonized the submerged plastic material below the photovoltaic panels.

Artificial floaters created hundreds of feet of hard substrate within the open water reservoir.

Filtering water while creating dense biofouling layers

The dense mussel populations individually pump water through their systems, filtering out tiny pieces of algae and other organic matter.

Biofiltration resulted in decreased water turbidity, with a clear improvement in water clarity near the floating platforms.

Improved water clarity increases the depth of light penetration, thereby altering microbial behavior in the upper waters.

Nevertheless, mussels with encrustations add more weight to the pontoons that float in the reservoir.

The added mass and encrustation raise concerns about potential drag on the rotating mechanism when turning the panels toward the sun.

Biological filtration of water comes with an added problem of physical fouling of the submerged equipment.

Assessing drinking water safety and reservoir quality

The utility experts carried out thorough water tests to confirm if there were any changes in the quality of drinking water because of the floating solar equipment.

Chemical and biological analysis revealed that the solar farm had not caused any negative effects on drinking water quality.

Although water testing detected elevated concentrations of copper, manganese, and zinc compared to the reference reservoir, all metal levels remained well below public health standards for drinking water production.

All the parameters of water quality were completely within the limits set by European and Dutch public health standards.

Clean water quality is necessary for the reservoir to continue providing drinking water to urban dwellers.

Thorough tests proved that clean energy production is possible at municipal water resources.

Managing long-term maintenance and biofouling risks

Although mussel colonization does not deteriorate water chemistry, heavy biofouling creates potential long-term operational risks for plant infrastructure.

Engineers warn that elevated numbers of drifting larvae could attach to downstream transport mains and reach treatment plants if left unmanaged.

While specific mechanical failures or clogged pumps have not been documented by the study.

Utility operators anticipate that additional cleaning and protective maintenance strategies may be necessary.

Anti-fouling coatings and mechanical cleaning systems are being considered by the engineers as a way to protect the float system without chemicals.

Defining limits of reservoir solar biofouling trials

It has been observed from the Kralingen project that floating solar panels have a safe operation and create artificial biological environments.

The biofouling growth rates from one protected drinking water reservoir can never be generalized for different climatic regions or other types of water bodies.

Engineering professionals do not have any idea about the effects of years of mussels on structural mooring lines and rotatory engines.

Nutrient content and different species compositions of different reservoirs would lead to varied biological impacts.

Biofouling studies have become vital in view of the increasing number of floating solar panel installations on municipal drinking water reservoirs.

The Kralingen case study confirms that water drinking quality is maintained; however, biological growth control needs fresh engineering approaches.

The full study can be found here: Sandrini, G., Wagenvoort, A., van Asperen, R., Hofs, B., Mathijssen, D., & van der Wal, A. (2025). Illuminating the impact of a floating photovoltaic system on a shallow drinking water reservoir: the emergence of benthic cyanobacteria. Water, 17(8), 1178.