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.

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.

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