Despite these advances, 75% of water bodies remain at ecological risk 1. Furthermore, water chlorination and desalination methods, as well as the use of advanced oxidation processes and nanotechnologies, have reduced the hazards and illnesses related to contaminated drinking-water distribution systems 3.

The concept of ‘water remediation’ dates back to Sanskrit, ancient Greek and Egyptian writings around 2000 bc, from Hippocrates with the discovery of water’s healing properties, the Roman aqueducts and Archimedes’ screw, up to modern-era treatments, including filtration systems with coagulation and sedimentation. Although these biological risks might seem more relevant for developing regions, industrialized regions will also need to adapt to the scarcity of clean water resources, requiring innovative solutions for frequent (re)use of grey water (such as from washing machines and dishwashers) and natural water bodies to ensure drinking water supplies. In this context, bacterial biofilm formation onto industrial pipes and water lines results in a slower water flow and corrosion of the tubes, thus reducing the hydraulic efficiency of power plants and further compromising the safety of drinking-water distribution systems 5, 6, 7. In addition, higher global temperatures increase the concentration of nutrients such as carbon, nitrogen and phosphate in raw water, which favours the regrowth of opportunistic pathogens mainly caused by antimicrobial-resistant pathogens 1, 2. Moreover, aqueous protozoan, bacterial and viral pathogens can cause the spread of typhoid, cholera, salmonella and other diseases. Once within the aquatic ecosystem, contaminants can cause irreversible accumulative, recurrent, carcinogenic, mutagenic and other detrimental effects on the aquatic flora and fauna, while their propagation through the food chain further amplifies the associated risks 4. Contaminants such as plastic waste, heavy metals, persistent organic pollutants (including pharmaceuticals and pesticides) and oil spills are associated with substantial ecological risks. Furthermore, clean water shortage is intensifying owing to the increasing global demand associated with population growth, industrialization and climate change 3. However, of the Earth’s freshwater, less than 1% is estimated to be accessible, and its contamination represents one of the most severe ecological threats 1, 2.

Finally, open challenges and future directions are discussed for the real-world application of smart micro- and nanorobots. After describing removal and degradation strategies, we present the most relevant improvements in water treatment, highlighting the design aspects necessary to improve remediation efficiency for specific contaminants. This Review introduces the evolution of passive micro- and nanomaterials through active micro- and nanomotors and into advanced intelligent micro- and nanorobots in terms of motion ability, multifunctionality, adaptive response, swarming and mutual communication. The use of self-propelled programmable micro- and nanoscale synthetic robots is a promising alternative way to improve water monitoring and remediation by overcoming diffusion-limited reactions and promoting interactions with target pollutants, including nano- and microplastics, persistent organic pollutants, heavy metals, oils and pathogenic microorganisms.

Current water-remediation treatments fail to remove most pollutants completely or release toxic by-products into the environment. Industrialization, population growth and climate change are further exacerbating clean water shortage.

Less than 1% of Earth’s freshwater reserves is accessible.

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