The technological evolution of wastewater treatment
Wastewater management has been a challenge since the beginning of human civilization. As long as population density was low, the natural self-purification capacity of the environment was sufficient. Urbanisation, however, forced the development of technical solutions. In antiquity, this meant only the conveyance of wastewater—i.e. the construction of sewers. The evolution of wastewater treatment closely reflects the growth of society’s environmental awareness.
Conveyance and “disposal”
Ancient civilizations, such as the Romans, already used sewer systems, but the objective was not treatment—only rapid removal. Wastewater was discharged directly into surface waters, with little concern for health or environmental impacts.
During the Middle Ages, conditions often deteriorated: wastewater frequently flowed into streets or open ditches, leading to epidemics. The cholera and typhoid outbreaks of the 19th century were the first serious warnings that wastewater management is a public health issue.
The first technological step: mechanical treatment
At the end of the 19th century, the first mechanical treatment solutions appeared:
- screens,
- grit chambers,
- sedimentation tanks.
Their purpose was the removal of larger suspended and settleable solids. Although the water appeared cleaner, most organic pollutants and pathogens still reached the receiving waters unchanged.
The biological revolution: the role of living organisms
At the beginning of the 20th century, it was recognised that natural degradation processes could be controlled under engineered conditions. Activated sludge technology emerged, followed later by trickling filter systems.
During biological treatment, microorganisms degrade organic matter, significantly reducing pollution levels. This resulted in real improvements in water quality and became established as the second stage of wastewater treatment.
Fighting nutrients: the third treatment stage
In the second half of the 20th century, a new problem came to the forefront: excessive nutrient loading. Accumulation of nitrogen and phosphorus caused algal blooms, oxygen depletion, and ecological damage in receiving waters.
In response, the following technologies were developed:
- chemical phosphorus removal,
- biological nitrification–denitrification systems.
This stage constitutes the third treatment level, which is now a basic requirement in developed countries.
New challenges: micropollutants and pharmaceutical residues
At the beginning of the 21st century, it became clear that conventional technologies cannot remove micropollutants such as:
- pharmaceutical active ingredients,
- hormone-active compounds,
- cosmetic components,
- microplastics.
Even at very low concentrations, these substances can exert biological effects and pose long-term risks to aquatic ecosystems.
The present and the future: the fourth treatment stage
In response, the fourth treatment stage has emerged, which the European Union is gradually mandating from 2025 onwards for larger wastewater treatment plants.
This stage includes so-called advanced technologies, such as:
- activated carbon adsorption (PAC, GAC),
- ozonation,
- UV-based advanced oxidation processes,
- membrane technologies.
Their objective is no longer merely the removal of visible pollution, but the treatment of chemically active, difficult-to-detect substances.