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%.

Modern technologies designed to treat wastewater containing phosphorus compounds are based on the processes of adsorption and precipitation. In addition, more environmentally friendly and cheaper materials are being sought to ensure greater conformity with overarching assumptions of green chemistry and sustainable development. Against that background, this paper offers a review and analysis of available information on the considered reactive materials that have the capacity to remove phosphorus from wastewater. These materials are categorised as natural (with a sub-division in line with the dominant sorption groups of Al/Fe or Ca/Mg), waste, or man-made. Notably, most studies on sorbents have been carried out in laboratory systems via experimentation under static conditions. Among the natural materials, opoka has the highest sorption capacity of 181.20 g P/kg, while red mud (in the waste material category) is most efficient at binding phosphorus with a level of 345.02 g P/kg. Finally, among the group of commercial materials, Rockfos^® has the highest sorption capacity of 256.40 g P/kg. In addition, this paper recognises the effect of composition, pH, and physical properties on a reactive material’s capacity to absorb phosphorus, as well as the possibility for further potential use in the production of fertilisers.

Using Poland as an example, it was shown that 41.6% of the requests for intervention in 2016–2021 by Environmental Protection Inspections were related to odour nuisance. Further analysis of the statistical data confirmed that approximately 5.4% of wastewater treatment plants in the group of municipal facilities were subject to complaints. Detailed identification of the subject of odour nuisance at wastewater treatment plants identified hydrogen sulphide (H₂S), ammonia (NH₃) and volatile organic compounds (VOCs) as the most common malodorous substances within these facilities. Moreover, the concentrations of hydrogen sulphide and ammonia exceed the reference values for some substances in the air (0.02 mg/m³ for H₂S and 0.4 mg/m³ for NH₃). A thorough assessment of the properties of these substances made it clear that even in small concentrations they have a negative impact on the human body and the environment, and their degree of nuisance is described as high. In the two WWTPs analysed in Poland (WWTP 1 and WWTP 2), hydrogen sulphide concentrations were in the range of 0–41.86 mg/m³ (Long-Term Exposure Limit for H₂S is 7.0 mg/m³), ammonia 0–1.43 mg/m³ and VOCs 0.60–134.79 ppm. The values recognised for H₂S cause lacrimation, coughing, olfactory impairment, psychomotor agitation, and swelling of the cornea with photophobia. Recognition of the methods used in practice at WWTPs to reduce and control malodorous emissions indicates the possibility of protecting the environment and human health, but these solutions are ignored in most facilities due to the lack of requirements specified in legislation.

The topical challenge for the Polish, European, and global fertilizer industry is to produce sufficient nutrients for growing plants using more energy-efficient and environmentally friendly methods. The appropriate course of action, in terms of the challenges posed, could be the production of liquid fertilizers, made from waste materials that exhibit fertilizer properties. This solution makes it possible not only to reduce the exploitation of natural resources but above all, to implement elements of a circular economy and reduce the energy intensity of the fertilizer industry. This study shows that both in Poland and the European Union, there are current regulations aimed at elements of a circular economy and indicating the need to obtain fertilizers containing valuable plant nutrients from organic waste or recycled materials. The recognition carried out for the Polish market clearly indicates that to produce liquid organic fertilizers and soil conditioners, the most used is the digestate from the fermentation process. The preparation of liquid organic–mineral fertilizers is mainly based on algae extracts. Mine minerals are used in the production of mineral–liquid fertilizers. An analysis of data has shown that the above-mentioned waste materials, used as substrates to produce fertilizers, contain chemical substances and elements important for, among other things, stimulation of proper plant development, growth of aboveground and underground parts of plants, increased resistance to diseases and pests, and regulation of plant water management. Referring to the above information, the production of liquid fertilizers from waste materials seems reasonable and is an alternative to mineral–solid fertilizers, whose production process is energy-intensive and produces air emissions. Detailed identification of the properties of the various components made it possible to demonstrate their usefulness in terms of fertilizing plants and soils, but also to emphasize the importance of this line of research and the need to look for other groups of waste for reuse within the framework of a circular economy.

Due to inappropriate agricultural treatments, soils often turn into wastelands. After years, the return to cultivation is very difficult or even impossible. It is a serious problem for agriculture, also in Poland. In order to improve soil quality and to restore its value in use, different fertilizers are commonly used. As an alternative for artificial fertilizers, sewage sludge and biomass ashes might be applied. High cost of sewage sludge conditioning with the use of polyelectrolytes has resulted in the development of research dedicated to finding new and inexpensive conditioners. The article discusses the possibility of sludge conditioning by means of wood biomass ash. In the paper, the findings of sewage sludge dewatering with the use of wood ash on a laboratory and on a technical scale are presented. The analysis of results has shown that the addition of wood biomass ash improved sludge dewatering by 10–23% in a laboratory scale and by 15–27% on a technical scale. The application of wood ash also reduced the total bacteria number by 83–89% in a laboratory scale and by 40–53% on a technical scale. Additionally, the obtained mixture of sewage sludge and wood biomass ash might be applied in agricultural practices, especially in fertilizing perennial plants plantations. Thanks to that it is possible to obtain inexpensive and valuable natural fertilizer while reducing costs of sludge dewatering.