The improvement of energy efficiency ensuring high nutrients removal is a great concern for many wastewater treatment plants (WWTPs). The energy balance of a WWTP can be improved through the application of highly efficient digestion or its intensification, e.g., through the introduction of the co-substrates with relatively high energy potential to the sewage sludge (SS). In the present study, the overview of the energetic aspect of the Polish WWTPs was presented. The evaluation of energy consumption at individual stages of wastewater treatment along with the possibilities of its increasing was performed. Additionally, the influence of co-digestion process implementation on the energy efficiency of a selected WWTP in Poland was investigated. The evaluation was carried out for a WWTP located in Iława. Both energetic and treatment efficiency were analyzed. The energy balance evaluation of this WWTP was also performed. The obtained results indicated that the WWTP in Iława produced on average 2.54 GWh per year (7.63 GWh of electricity in total) as a result of the co-digestion of sewage sludge with poultry processing waste. A single cubic meter of co-substrates fed to the digesters yielded an average of 25.6 ± 4.3 Nm³ of biogas (between 18.3 and 32.2 Nm³/m³). This enabled covering the energy demand of the plant to a very high degree, ranging from 93.0% to 99.8% (98.2% on average). Importantly, in the presence of the co-substrate, the removal efficiency of organic compounds was enhanced from 64% (mono-digestion) to 69–70%.

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

Currently, one of the main goals is to make municipal wastewater treatment plants (WWTPs) energy-neutral. However, advanced wastewater treatments and sewage sludge processing are still classified as highly energy-intensive. In this study, the energy self-sufficiency potential assessment of the WWTP located in Krosno (Poland) was evaluated. Moreover, the possible paths for improving the energy balance of the analyzed facility are presented in this paper. The performed evaluation indicated that in 2016–2019, the energy consumption at WWTP Krosno varied from 0.25 to 0.71 kWh/m³ of wastewater (average 0.51 kWh/m³), and the highest energy utilization values in each year were recorded around the summer season. An analysis of the data showed that as the pollutant load flowing into the WWTP increased, its energy utilization decreased. Such results indicate that the treatment cost per cubic meter decreases as the load increases due to the capital cost being the same. The estimated self-sufficiency of the facility in the years analyzed was 50.5%. The average energy recovery from 1 m³ of wastewater was 0.27 kWh/m³, and the average energy recovery from 1 m³ of biogas was 1.54 kWh/m³. Since the energy balance of this wastewater treatment plant, determined primarily by the continuously increasing cost of energy purchases, has to be improved, two courses of action were identified that will allow for increasing self-sufficiency. The co-digestion strategy was indicated as the easiest solution to implement, given the on-going anaerobic stabilization of sewage sludge at this WWTP. Moreover, the possible co-substrates that can be obtained from local suppliers were indicated. The second course of action, which requires a thorough economic analysis, is sludge pre-treatment, which might improve sewage sludge properties, resulting in a more favorable biogas yield.

The activated sludge models (ASMs) commonly used by the International Water Association (IWA) task group are based on chemical oxygen demand (COD) fractionations. However, the proper evaluation of COD fractions, which is crucial for modelling and especially oxygen uptake rate (OUR) predictions, is still under debate. The biodegradation of particulate COD is initiated by the hydrolysis process, which is an integral part of an ASM. This concept has remained in use for over 30 years. The aim of this study was to verify an alternative, more complex, modified (Activated Sludge Model No 2d) ASM2d for modelling the OUR variations and novel procedure for the estimation of a particulate COD fraction through the implementation of the GPS-X software (Hydromantis Environmental Software Solutions, Inc., Hamilton, ON, Canada) in advanced computer simulations. In comparison to the original ASM2d, the modified model more accurately predicted the OUR behavior of real settled wastewater (SWW) samples and SWW after coagulation–flocculation (C–F). The mean absolute relative deviations (MARDs) in OUR were 11.3–29.5% and 18.9–45.8% (original ASM2d) vs. 9.7–15.8% and 11.8–30.3% (modified ASM2d) for the SWW and the C–F samples, respectively. Moreover, the impact of the COD fraction forms and molecules size on the hydrolysis process rate was developed by integrated OUR batch tests in activated sludge modelling.

An attempt to explain the phenomenon of the effect of electrode rotation on the ozone generation process is presented. A discharge photography method was applied and computer analysis method was used to find discharge differences between electrode rotational and non-rotational cases. The research presented shows that with electrode rotation the discharge was more uniform and the ozone generation efficiency increased about 15% compared to an ozonizer with a non-rotating electrode. In addition, during the research, the most suitable electrode rotational speed for the ozone generation process was estimated.