Why Modern Supply Systems Fail

Municipal water treatment was designed for a different era. For clean source water, for few known contaminants, for an industrial society with linear material flows. The reality of the 21st century is different.

The Starting Point: Three Basic Assumptions

Germany's municipal water infrastructure was built between 1950 and 1990. It is based on three basic assumptions:

1. Clean raw water: The shallow groundwater aquifers and dams were considered largely uncontaminated.
2. Known contaminant palette: COD, BOD, nitrogen, phosphorus, pathogenic germs – all controllable with biological and oxidative standard processes.
3. Stable population distribution: Predictable utilization, linear scaling, static design.

All three assumptions no longer hold today.

The Reality: Trace Contaminants Overload Municipal Plants

A wastewater treatment plant at 1990s standards reliably removes:

• 95–99 % of biodegradable organic substance (BOD, COD)
• > 90 % of nitrogen (nitrification/denitrification)
• > 95 % of phosphorus (precipitation)
• > 99.9 % of pathogenic germs

What it does not remove:

• Pharmaceutical residues: e.g. diclofenac, carbamazepine, metformin – hardly or not biologically degradable
• Hormones: Ethinylestradiol from contraceptives, natural estrogens
• PFAS (“forever chemicals”): per- and polyfluorinated alkyl substances with carbon-fluorine bonds that are not biologically degradable
• Microplastics: Particles < 5 mm from textiles, cosmetics, tire wear
• Antibiotic resistances: Resistance genes that are spread via wastewater

Consequences for Environment and Health

The consequences of these contaminant loads have been detectable in water bodies and organisms for years:

• Fish mortality and reproductive disorders caused by hormonally active substances (e.g. detected in German rivers).
• Drinking water contamination: Bank filtration and groundwater recharge transport trace substances into drinking water.
• Bioaccumulation: PFAS accumulate in the food chain and are detectable in human blood and breast milk.
• Antimicrobial resistance: Wastewater plants are hotspots for the spread of antibiotic resistance genes.

Why Classical Treatment is Insufficient

The mechanical-biological stage with secondary clarifiers and flocculation filtration was designed for particulate and easily degradable substances. For dissolved trace contaminants, it is nearly ineffective. Even an additional ozonation in the 4th treatment stage removes only part of the substances and generates transformation products that are sometimes more toxic than the parent compounds.

The Solution: Decentralized Treatment with Advanced Processes

Three building blocks form the state of the art for future-proof water treatment:

1. Oxidative Processes

Ozone and Hydrodynamic Cavitation (HC) generate hydroxyl radicals that oxidatively degrade a broad spectrum of trace substances. The combination HC + Ozone has proven particularly energy-efficient in pilot projects.

2. Membrane Filtration

Nanofiltration and Reverse Osmosis physically retain dissolved substances. Retentions for pharmaceuticals and hormones: > 90 %.

3. Activated Carbon as Polishing Stage

Powdered activated carbon (PAC) or granular activated carbon (GAC) adsorbs residual concentrations that oxidation and membrane do not fully eliminate.

Why Decentralization?

Expanding central large-scale wastewater plants with 4th treatment stages is technically possible, but:

• Investment costs: 30–80 million euros per treatment plant > 100,000 PE.
• Space requirements: Ozone reactors and membrane modules need space that is often not available on existing sites.
• Permitting procedures: Expansions are fully subject to the Federal Immission Control Act.

Decentralized units at the point of discharge into water bodies bypass these bottlenecks. They are modularly scalable, often easier to permit, and their economic viability can be optimized site-specifically.

Conclusion

Municipal water treatment is no longer a matter of course, but a demanding engineering task. Inputs have fundamentally changed in 30 years; plants respond with a combination of oxidative treatment, membrane separation, and intelligent control. Those who want to retain control over their own water quality cannot avoid decentralized treatment units with modern process engineering.