## 3 Analysis of Kaunas WWTP Case
In addition, in each N/DN reactor, the activated sludge concentration (at the end of the device), the oxygen concentration and wastewater temperature are measured at two points. The air required to ensure the biological treatment of wastewater is supplied by air blowers. Five air blowers are installed, with a total capacity of 55,800 m\({}^{3}\) of air per hour, and a total electrical power of 1380 kW. Wastewater from the aerotanks is fed to secondary settling tanks. There are four secondary settling tanks; the volume of the settling zone of one settling tank is about 10,000 m\({}^{3}\). During biological wastewater treatment, excess activated sludge is generated. Before it is fed to the digesesters, the sludge is thickened, i.e., its concentration is increased, and its volume is reduced by about 4.5-6.0 times. For sludge digestion, two anaerobic digesters with a volume of 9000 m\({}^{3}\) each and two sludge storage tanks with a capacity of 4500 m\({}^{3}\) each are used. An anaerobic mesophilic digestion process takes place in these anaerobic digesters, maintaining a temperature of 34-35 \({}^{\circ}\)C. After digestion, the sludge is dewatered with centrifuges to 73-77% humidity. After dewatering, about 19-20 tons of sludge based on dry matter are obtained, or 79-83 tons of 76% humidity sludge mass per day. About 50% of the dewatered sludge is dried in belt dryers to 10% humidity. The dried sludge is used in the cement industry for heat production. Some of the dewatered sludge is used in agriculture for fertilization and composting, while the rest is used in the Kaunas municipal waste incineration plant for energy production. The main data of Kaunas WWTP are presented in Table 3.  
**Oxygen demand. ** In Kaunas WWTP aeration consumes 4382 MWh of electricity per year, or 46.9% of the wastewater treatment plant's total electricity demand. At the beginning of 2023, a new biological treatment control system was installed. Ammonium nitrogen and nitrate concentration meters were installed in each aeration tank section, and the work of blowers and mixers was controlled using smart algorithms. This fundamentally changed the management of the biological treatment part of the plant. Since the construction of the biological treatment plant in 2008, aeration has been continuous--a larger or smaller amount of air was supplied according to the dissolved oxygen setpoints, i.e., the blowers and mixers in the aeration tanks worked continuously. After installing the new control system, aeration became periodic--air is supplied to each section separately according to demand. This ensures better efficiency of wastewater treatment in terms of nitrogen and saves electricity. After the modernization of the system, the amount of electricity consumed for biological aeration decreased by 11.4%, the amount of electricity consumed for mixing decreased by 42.1%, and the average total nitrogen concentration decreased by 7.1 mg/L. Based on Equation (2) and data provided by Kaunas WWTP (Q of 2911 m\({}^{3}\)/h, \(BOD_{i}\) of 337 mg O\({}_{2}\)/L, \(BOD_{e}\) of 4.6 mg O\({}_{2}\)/L, \(V_{A}\) of 60,660 m\({}^{3}\), \(MLSS\) of 5.8 g/L, \(Amm_{e}\) of 50 g/m\({}^{3}\), \(Amm_{e}\) of 2.9 g/m\({}^{3}\), \(N_{e}\) of 2.67 g/m\({}^{3}\)), the calculated total oxygen demand for aeration is 5,823,558 m\({}^{3}\)/a.