Jurgita Babilienė has spent 25 years watching wastewater in Kaunas, Lithuania, flow through the same pipes, tanks and filters—except the city itself changed around them. The pipes stayed, but the treatment plant learned new tricks. “We didn’t just build a new plant,” she says. “We rebuilt the biology inside it.” The result, documented in a new study published in *Discover Water* (Lietuvos vandens tyrimai), shows that by modernizing the biological stage of treatment, Kaunas has lifted its nutrient removal rates to levels that consistently meet—and often exceed—EU Urban Wastewater Treatment Directive targets.
Over the period from 2000 to 2025, Babilienė and her team at Lietuvos inžinerijos Kolegija (Lithuanian College of Engineering) tracked five key parameters: biochemical oxygen demand (BOD7), chemical oxygen demand (COD), suspended solids, total nitrogen and total phosphorus. The data tell a story of steady improvement. By 2025, BOD7 removal reached 98%, nitrogen removal climbed to 90%, and phosphorus removal stabilized between 90% and 95%. COD and suspended solids removal stayed above 85% every year. “What began as compliance,” Babilienė notes, “became a platform for resilience.”
The commercial implications ripple beyond Lithuania. The Nemunas River, into which Kaunas discharges its treated effluent, is one of the largest freshwater inputs to the Baltic Sea. Reducing nitrogen and phosphorus loads here helps curb eutrophication hundreds of kilometers downstream—an environmental dividend that aligns with international commitments under the Helsinki Commission (HELCOM). For energy-intensive utilities, the modernization also offers a tangible pathway to lower operating costs. Advanced biological nutrient removal systems typically require less chemical dosing and can be integrated with energy recovery processes such as sludge digestion and biogas capture. In Kaunas, the shift reduced reliance on external carbon sources and cut aeration energy by up to 15% in peak years—a figure that resonates in boardrooms where energy budgets are scrutinized as closely as effluent quality.
Babilienė emphasizes that the long-term monitoring was critical. “You can’t manage what you don’t measure,” she says. “We installed continuous online sensors in 2010, which let us fine-tune the biology in real time. That’s when the real gains started.” The study’s findings suggest that utilities in Central and Eastern Europe facing similar regulatory and ecological pressures can replicate Kaunas’s trajectory—not by building entirely new plants, but by upgrading the biological core and coupling it with smart monitoring and adaptive control.
Looking ahead, Babilienė points to emerging challenges: pharmaceutical residues, microplastics and the need for even lower nitrogen limits. “The next frontier isn’t just compliance,” she says. “It’s creating a treatment system that can evolve as fast as the pollutants we’re trying to remove.” For energy managers, that evolution may hinge on integrating AI-driven process optimization with renewable energy inputs—turning wastewater plants from energy consumers into net producers while safeguarding coastal ecosystems. Kaunas shows that with the right biology, the right data, and the right incentives, the future of wastewater treatment can be both cleaner and more cost-effective.