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Current Trends in Sustainable Sewage Sludge Management—A Case Study for Poznań County, Poland
The formation of sewage sludge (SS) is directly related to the number of people served by municipal wastewater treatment plants (WWTPs), while it is also impacted by increasingly upgraded sewage treatment technologies used in such installations. The presence of SS in our environment raises a number of concerns regarding its proper and sustainable management. In practice, the choice of a specific SS disposal method is determined by a number of factors and, as this research has shown, it may vary significantly both on the national and regional scale. This study analyzed this phenomenon in relation to Poznań County, located in the Wielkopolskie province, Poland. As part of this study, the SS chemical composition was assessed based on data obtained directly from local municipal WWTPs over the course of several years (the last 7 years). The currently used SS management methods were analyzed in terms of SS chemical composition. Moreover, in view of the importance of public involvement in decision-making processes related to sustainable management of SS, an original survey was developed to assess local residents’ knowledge concerning SS management in Poznań County. Based on the obtained data, it was found that the generated SS, constituting a form of fertilizer, was primarily used for agricultural and reclamation purposes (over 41% of total SS mass in the case of Poznań County). It is facilitated by the chemical composition of this waste, characterized by high contents of organic matter (380–730.3 g·kg−1), nitrogen (13.3–78 g·kg−1), and calcium (9.5–350 g·kg−1), while the amounts of heavy metals were within the permissible ranges. A survey of the local community revealed that although respondents recognized and could accurately identify SS as a waste, their knowledge concerning harmfulness of this waste and its management was fragmentary. Regardless of the age of the respondents, 48 to 66% of them had no opinion concerning problems related to SS in their area. The youngest people (aged 18–25) showed the lowest level of knowledge on SS and its management. People with elementary education were the least knowledgeable about sewage sludge management. Regardless of the above, a majority of respondents (28–56% for different age groups and 7–18% for various educational backgrounds) were convinced of the validity of agricultural SS use. To sum up, research on sewage sludge is an indispensable element of activities aimed at sustainable development, combining aspects of environmental protection, circular economy, and social awareness and acceptance. Additionally, the study results indicated the need for social education to increase environmental awareness and co-responsibility for SS management.
Rate of Microelement Quantitative Changes during the Composting of Sewage Sludge with Various Bulking Agents
The composting of sewage sludge (SS) with structure-forming additives is a popular and inexpensive method for the management of biodegradable waste. During this process, a number of transformations of organic matter and nutrients occur. This study presents the rates and directions of quantitative changes in Cu, Zn, and Ni during the composting of various mixtures. The following substances were used for preparing compost mixtures: SS, sawdust, straw, and bark. These substances were mixed together in appropriate proportions as follows: C1: 45% SS + 50% sawdust + 5% straw; C2: 45% SS + 50% bark+ 5% straw; and C3: 45% SS + 35% bark + 15% sawdust + 5% straw. Single (DTPA solution) and sequential extraction methods (BCR protocol) were used for microelement mobility assessment. Compost valorization was performed in each individual composting phase. The chain relative increment analysis was used for such assessment. Additionally, the potential metal pollution in the composts was evaluated by applying the following indices: the pollutant accumulation index (PAI), heavy metal enrichment index (HMEI), and heavy metal pollution load index (HMPLI). During composting, generally, the amounts of Ni, Cu, and Zn increased in the various extracted combinations, which was independent of their susceptibility to decomposition. Despite of this, the tested composts should be considered as a source of slowly available microelements for plants. The most intense quantitative changes in metals occurred in the thermophilic phase, and the weakest changes occurred in the cooling phase. At the same time, the calculated indices indicated a lack of contamination of the composts with the analyzed metals, emphasizing their environmental safety and lack of negative impact. The bulking agents used for composting with SS did not significantly influence the intensity of the quantitative changes in the tested metals.