Conventional quality parameters such as Chemical Oxygen Demand (COD) or Biochemical Oxygen Demand (BOD) give information about the quantity of organic matter present in wastewater, but do not give a clear indication of the biodegradability of the pollutants flowing in the WWTP. Detailed knowledge can be obtained by dividing the total COD into fractions. Fractionation and balancing of COD can be determined in various ways and with varying accuracy. Good wastewater characteristics are obtained on the basis of COD fractionation in accordance with ATV-A 131 guidelines, especially when the wastewater characteristics are in high compliance with the assumptions of the method. The article proposes a modification of the ATV-A131 method that increases the accuracy of determining the COD fraction. In order to reduce errors in the calculation of COD fractions, the value of fraction X_S was calculated on the basis of the biochemical degradation rate determined in studies (k) for raw wastewater, whereas the S_I fraction was calculated from the difference between SCOD and BOD_Tot of filtered treated wastewater. BOD_Tot of the treated wastewater was calculated taking into account the rate of biochemical degradation determined in the studies (k) for treated wastewater. The shares of individual COD fractions in raw wastewater calculated on the basis of the standard and modified procedure differed by approx. 10% in the case of suspension fractions. Modification of the methodology to determine the COD of the treated wastewater S_S fraction significantly influenced the contents of all fractions in treated wastewater.

Wastewater treatment plants designed to meet the requirements of discharging wastewater to a receiving water body are often not energy optimised. Energy requirements for conventional activated sludge wastewater treatment plants are estimated to range from 0.30 to 1.2 kWh/m³, with the highest values achieved using the nitrification process. This article describes the energy optimisation process of the wastewater treatment plant in Gubin (Poland) designed for 90,000 PE (population equivalent) using renewable energy sources: solar, biogas, and geothermal. At the analysed wastewater treatment plant electricity consumption for treating 1 m³ of wastewater was 0.679 kWh in 2020. The combined production of electricity and heat from biogas, the production of electricity in a photovoltaic system, and heat recovery in a geothermal process make it possible to obtain a surplus of heat in relation to its demand in the wastewater treatment plant, and to cover the demand for electricity, with the possibility of also selling it to the power grid.

Operating and research experiments indicate that the potential benefits of thermal treatment of sewage sludge before methane fermentation include increasing the biodegradability of substrates, reducing the amount and improving the dewaterability of the fermentate and its hygienization, reducing the emission of odours during stabilization, higher production of biogas, and improving the energy balance of the process. The process of disintegration (liquefaction) can be carried out, for example, through the use of mechanical homogenization, microwaves and ultrasonic waves, chemical agents, thermal methods, and biological processes. The article reviews the literature data on thermal hydrolysis research, from the first source information to the present. The thermal hydrolysis achieved enhanced hydrolysis, biogas potential, and faster sludge degradation during anaerobic digestion without compromising the quality of the end products.

The European Parliament’s climate and energy policy, which aims to cut greenhouse gas emissions by 2030 considerably, imposes on Member States the duty to develop National Action Plans (NAPs), which determine the percentage of renewable energy sources (RES) in gross energy consumption. Poland assumed a percentage of renewable energy sources in gross energy consumption of 21–23% in the NREAP. In 2020, Poland’s utilisation of renewable energy sources was 16.2% (22nd among EU nations), which was lower than the European average. The municipal sector, particularly sewage treatment facilities, can undoubtedly contribute to an increase in renewable energy generation and utilisation. Wastewater and sewage sludge are excellent sources of heat and energy in the methane fermentation process, and the sewage treatment plant area is perfect for solar panel installation. The article is a case study that presents the possibility of decreasing CO₂ emissions from wastewater treatment facilities by 45,000 PE due to using renewable energy sources such as photovoltaics and cogeneration. The average monthly CO₂ emission reduced from 68,905 kg CO₂/ month to 37,385 kg CO₂/month with the adoption of renewable energy sources.

The paper presents results of studies concerning the designation of COD fraction in the raw, mechanically treated and biologically treated wastewater. The test object was a wastewater treatment plant with the output of over 20,000 PE. The results were compared with data received in the ASM models. During investigation following fractions of COD were determined: dissolved non-biodegradable SI, dissolved easily biodegradable SS, in organic suspension slowly degradable XS and in organic suspension non-biodegradable XI. Methodology for determining the COD fraction was based on the guidelines ATV-A 131. The real percentage of each fraction in total COD in raw wastewater are different from data received in ASM models.

The article presents the results of research on changes in selected water parameters during distribution in the water supply network of the city of 22 thousand inhabitants. Water parameters were controlled at 8 points in the network, distant from Water Treatment Plant from 0.4 to 3.1 km. The location of control points was selected in order to assess the water quality depending on the pipe material and at unfavourable points - at the ends of the network. It was shown that as a result of secondary water contamination an increase in turbidity, colour and total iron occurred. The distance from the Water Treatment Plant and the type of material of the network or connection, affect the quality of the water. It was confirmed that at the ends of the network the water parameters were increased and exceeded the admissible values.