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Article

Current Trends in Sustainable Sewage Sludge Management—A Case Study for Poznań County, Poland

by
Monika Jakubus
Department of Soil Science and Microbiology, Poznań University of Life Sciences, 60-637 Poznań, Poland
Sustainability 2024, 16(12), 5056; https://doi.org/10.3390/su16125056
Submission received: 9 May 2024 / Revised: 7 June 2024 / Accepted: 10 June 2024 / Published: 13 June 2024
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Abstract

:
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.

1. Introduction

Nowadays, humans generate many different types of waste, which are classified and catalogued depending on their properties and sources [1,2]. Among the numerous groups of waste, the most noteworthy is stabilized sewage sludge classified to group 19 under the number 19/08/05 [1,2]. This waste, like other pollutants in the above-mentioned group, is the result of municipal wastewater treatment; it is generated in the largest quantities and is the most environmentally harmful. According to the Waste Act [3], SS is sludge from WWTPs from digesters and other installations used for the treatment of municipal wastewaters and other wastewaters with a composition similar to that of municipal wastewaters. Most of the pollutants and substances removed from wastewaters during treatment through physical, biological, and chemical transformations pass into sewage sludge; therefore, the method of its processing is very important to ensure that the resultant sludge can be used outside the wastewater treatment plant. Sewage sludge is semi-liquid in consistency with high contents of organic matter, nitrogen, and phosphorous, which are valuable components. Unfortunately, heavy metals and pathogens may also be present in SS, which poses a risk to its further use. According to Jakubus [4], Rorat et al. [5], Hudcová et al. [6], and Izydorczyk et al. [7], SS may also be loaded with pollutants such as polycyclic aromatic hydrocarbons (PAH), polychlorinated biphenyls (PCB), adsorbable organic halides (AOX), pesticides, hormones, pharmaceuticals, or microplastics. To minimize both its mass and potential negative environmental impacts, SS is subject to conditioning, which changes the physical and chemical composition of SS, improving its quality [3,4]. The basic processes include thickening, stabilization, dewatering, and alternatively drying. After thickening, SS stabilization is performed. Stabilization is understood as the transformation of organic substances into inorganic and poorly decomposable substances through thermal, biological, and chemical processes. After stabilization, SS must be dewatered, which is performed using filter presses or centrifuges. Additionally, conditioners or synthetic organic polymers are used to coagulate colloids in SS and thus accelerate the dewatering process [4,5]. Thanks to these processes, SS increases its dry weight up to 20–30%, while the amount of organic matter is reduced to 55–60%, and at the same time the sludge is sanitized. Additionally, these processes are crucial for future applications by limiting potential risks thanks to the reduction of pathogens and organic matter. Emmanouil et al. [8] state that these treatments, if carried out correctly, may result in sewage sludge being treated as biosolids. According to EPA [9], the terms “biosolids” and “sewage sludge” are often used interchangeably, and biosolids are divided into “Class A” and “Class B” designations based on treatment methods. The different classes have specified treatment requirements for pollutants, pathogens, and vector attraction reduction, as well as general requirements and management practices. Such assumptions do not exist within the European Union; moreover, legal acts of the European Union and many European countries use the term “sewage sludge” [10], and this is a more common term also in practice. Collivignarell et al. [10] reviewed legal acts related to the management of biosolids in agricultural land in Europe, presenting very precise data. However, referring to the situation in Poland, some fundamental changes have occurred, the aim of which is to enhance environmental safety resulting from the presence of SS and its further proper processing. Polish legislation has not only been adapted to the binding EU directives, but has also been extended to include new aspects regulating SS management in Poland, as further specified in the acts listed in Table 1. The documents included in Table 1 indicate regulations regarding the promotion of SS reuse, limits of heavy metal contents in SS, as well as those in soil in the case of SS agricultural application, reduction of biodegradable waste disposal in landfills, as well as waste recycling and reduction of its generation. Of particular importance is the Regulation of the Minister of the Environment on Municipal Sewage Sludge [11] that indicates necessary processes (alternatively: anaerobic, aerobic, chemical, or humification) that must be carried out under specific humidity, temperature, and duration so SS may become a fully stabilized product, which can be further managed in a safe manner. This act also specifies requirements concerning SS and the soil on which it is used. Particular attention is paid to the values of heavy metals in soil and SS, which are strictly specified. The above-mentioned Regulation also indicates the SS form in which it should be applied (liquid, semi-liquid, or solid), treatment time (outside the growing season of plants, assuming that the soil pH is min. 5.6), as well as permitted SS doses to be used within one year depending on the purpose (in agriculture and for land reclamation for agricultural purposes—3 t/year/ha, for land reclamation for non-agricultural purposes—15 t/year/ha).
Depending on the policy of a given country and applicable legal regulations [10], the manner of SS management varies in different countries. According to Shaddel et al. [18], four commonly used strategies for SS management can generally be distinguished: 1. no recycling (landfill, storage, dumping at sea); 2. reuse (land use); 3. conversion (composting, anaerobic digestion); and 4. energy recovery (incineration). Rorat et al. [5] and Izydorczyk et al. [7] divided possible forms of SS management into two main groups: 1. agricultural = matter recovery (SS–to–matter): use in agriculture as a fertilizer or in reclamation of degraded land as a soil amendment, substrate for co-composting, and 2. non-agricultural = energy recovery (SS–to–energy): incineration and alternate thermal methods such as pyrolysis, gasification, or co-incineration in cement plants.
The choice of a specific management method depends on the level of technological and economic development of the country, as well as the public approach to this issue. Underdeveloped and developing countries do not have any procedures in this respect and thus use the simplest and cheapest solutions, i.e., landfill or dumping at sea. This form of management is completely prohibited in most Western European countries, where SS is mainly thermally processed. The EU countries allow the application of SS to soil primarily for soil rehabilitation purposes. It is a cheaper method compared to incineration. Poland is one of very few EU countries allowing various methods of SS management. Of course, each of the appropriate methods has its advantages and disadvantages. Therefore, the choice of a given SS processing method should take into account such factors as acceptance on the part of the local community, possibilities of financing the investment, environmental impact, and potential conformity with the concept of circular economy, including the bioeconomy strategy. However, reconciling all these aspects in practice is unfortunately difficult and often even impossible. This is due to the fact that a number of elements combining aspects of environmental protection, circular economy, and social awareness should be taken into account. Very often, this latter element is the weakest link in the entire decision-making process. The public knowledge concerning appropriate and sustainable principles of SS management is often incomplete. Therefore, the level of local residents’ knowledge concerning this waste is crucial, because SS is generated by humans; moreover, the community participation in decision-making processes promotes public acceptance of individual measures, while enhancing shared responsibility for the environment. This aspect of the study is innovative, as no such connection and interpretation has been presented in the literature to date. It is assumed that the chemical composition will mainly determine the form of SS disposal, with residents being adequately aware of the SS generation and its use. The public knowledge concerning SS is also crucial, because local residents most often express their dissatisfaction with the location of WWTP or the manner of SS management. Many concerns expressed in public protests mainly result from insufficient knowledge or even ignorance of the topic.
Many authors [7,19,20,21] have stated that SS as a waste is perfectly in line with the assumptions of circular economy. Is this position actually justified and confirmed in practice? This study attempts to answer these issues based on the current state of handling and management of the generated SS both on the national scale in Poland and specifically in Poznań County. Focusing on data from Poznań County, the following objectives were adopted: 1. evaluation of the SS chemical composition in relation to its sustainable and proper management; 2. analysis of public knowledge concerning the presence of SS in the environment taking into account the principles and trends of sustainable development.

2. Materials and Methods

2.1. Study Area

The data presented in this study are average values for 10 different municipal wastewater treatment plants (WWTP) over 7 consecutive years. The installations selected for the study are located in Poznań County, as presented in Figure 1. Sewage sludge samples came both from large facilities, serving a large urban agglomeration, and from small, local installations, serving smaller towns. Consequently, individual wastewater treatment plants differ in their designed size expressed as the equivalent number of inhabitants (ENI: from 9100 to 1,200,000) and average capacity Q (m3/d) (from 820 to 200,000). Each of the WWTP works in the same modernized Bardenpho system with the removal of carbon, nitrogen, and phosphorous compounds in a low load active sludge process. This system consists of five distinct reactors: an anaerobic reactor, the first anoxic reactor, the first aerobic reactor, the second anoxic reactor, and the second aerobic reactor. All the selected WWTPs meet the required parameters of wastewater purification and its efficiency, expressed as a percentage reduction of individual parameters exceeding 96%. Sewage sludge from individual WWTPs was subjected to the same conditioning processes (degassing, equalization, dewatering, and compaction) and represented stabilized material according to the standards adopted for a given installation. The approximate mass of generated sewage sludge (t/year) for individual WWTPs ranged between 450 and 20,000 t of fresh matter per year.

2.2. Methods

The data on the chemical composition of SS (SS1 represents WWTP1, SS2 represents WWTP2, and so on) were directly obtained from individual installations based on legal regulations on access to public information. The methods used to determine individual parameters are mandatory standards (PN-EN 12879:2004 for the determination of organic matter, PB-21/2012 for N; PB-22/2012 for P; PB-04/2007 for Ca and Mg, PN-ISO 11047:2001. Methods A for metals), routinely applied in accredited laboratories. Thanks to this approach, the results are standardized and comparable for each SS, regardless of its origin.

2.3. Survey Research

Additionally, in this paper the public knowledge regarding SS issues was investigated in the local community based on the conducted surveys. The survey concerning social awareness in the local community was carried out using a survey form on the Google platform, and Table 2 presents the questions included there. Data collection took place between June and October 2023 via online community Facebook groups. Based on the information contained in the survey, this study includes the responses of people who declared that they live in Poznań County and use municipal sewage treatment plants. An element of the survey was a form covering the respondents’ age (18–25; 26–35; 36–45; 46–60; 60+) and education (vocational, secondary, higher). The assumption of the study was to obtain 50 fully completed questionnaires for each age group. The author of this paper prepared the concept and developed the survey. When developing questions for the survey, it was guided by Polish and EU legislation, where the term sewage sludge is widely used and functions in common terminology and is therefore known to the general public. Participation in the survey was voluntary, and the survey was anonymous and contained only necessary information concerning the respondents, such as age, education, place of residence and connection to the collective sewage system. Survey forms that indicated no use of the local WWTP, i.e., no connection to the sewage network or/and place of residence other than Poznań County, were excluded from analyses.

2.4. Statistical Analysis

The obtained data of SS chemical composition were subjected to statistical analysis. The data were compiled applying one-way ANOVA. Each of twelve parameters was tested independently using the F-test at the significance level α = 0.95. The null hypothesis assumed that mean values of the examined parameter are equal for each of the analyzed SS, and it was tested against the alternative hypothesis that not all the means are equal. As a result of the rejection of the null hypothesis, the least significant differences were calculated using Tukey’s test at the significance level α = 0.05. Tukey’s analysis was performed to distinguish homogeneous groups among the analyzed SS. Homogeneous groups are indicated by the same letters. Also, Box–Whiskers plots were constructed to represent the macronutrient and heavy metal contents in SS from individual WWPTs across the years of study. In the boxplot figures, the distribution of data is given by the minimum value, maximum value, and the median, with the first and third quartiles shown for each parameter.
The survey results were prepared in terms of percentages calculated in relation to the total number of responses in a given group.

3. Results and Discussion

3.1. SS Mass and Chemical Composition

Currently, there are 3258 municipal WWTPs in Poland, with a majority working in the mechanical–biological system in terms of the biogenic compound removal [20]. The introduction of more advanced technologies based on multi-phase biological reactors for the integrated removal of carbon, nitrogen, and phosphorus compounds promotes more effective wastewater treatment, but inevitably it has led to the generation of greater SS mass. Another factor that clearly leads to the production of more SS is the expansion of the water supply network and connecting a larger number of inhabitants to it. The importance of these factors in the production of SS is evidenced by data from the Local Data Bank [22]. According to the cited data comparing the last 12 years (2010–2022), the amounts of SS increased by 10% in that period for Poland and by 15% for Poznań County (Figure 2). In both cases, there is a trend of increasing SS mass, although in the case of Poznań County it was more pronounced. This is due to the fact that in this area, a significant increase in the population was recorded in that period. This is closely related to better opportunities for education and employment in many sectors of the economy. Moreover, local authorities have intensified work on sewage treatment in the entire area.
The composition of SS is highly variable and may depend on many diverse factors such as the season of the year, technology applied at a given WWTP, and unique characteristics of the source area of the influent [4,5]. Important SS parameters include significant amounts of organic matter (OM), N, or P, varying within wide ranges of values from 500 to 700 g∙kg−1 (OM), 34–40 g∙kg−1 (N), or 5–25 g∙kg−1 (P) [4]. In turn, Jakubus [4] provided much wider ranges of macro- and micronutrient contents in SS representing both national and Wielkopolska province WWTPs (Table 3). Regardless of the above, the cited author emphasized a high abundance of organic matter, N, P, Ca, and Fe in SS, as evidenced by the data quoted in Table 3. Figure 3, Figure 4 and Figure 5 show organic matter amounts, and macronutrient and heavy metal contents, averaged over the years of the study, for SS from individual WWTPs located in Poznań County. Referring to the data presented by Jakubus [4] for SS from WWTPs located in the Wielkopolska region, it should be noted that the amounts of OM, N, and Zn were comparable to those found for SS from installations representing Poznań County. However, the levels of P, Mg, and Cu were lower, while that of Ca were higher in SS from Poznań County compared to those presented for SS from the Wielkopolska province (Figure 3, Figure 4 and Figure 5, Table 3).
The organic matter amounts in SS were on average 610 g∙kg−1, ranging from 380 (SS9) to 730.3 (SS10) g∙kg−1 (Figure 4). The amounts of OM averaged over the years of the study for SS1, SS2, SS3, SS4, SS6, and SS10 did not differ, similarly to the values found for SS5, SS7, SS8, and SS9. It should be emphasized that the size of the installation did not influence the OM level (no differences between SS representing different installation sizes), which was primarily related to the degree of effectiveness of the SS stabilization processes. Referring to the German guidelines given in the strategy for dealing with municipal SS [23] (OM content above 650 g∙kg−1 indicates unstabilized sewage sludge; the content of 650–600 g∙kg−1 organic matter—sludge is poorly stabilized; 600–550 g∙kg−1 organic matter—the sludge is considered stabilized; below 550 g∙kg−1 organic matter—sludge is very well stabilized), the analyzed SS5, SS8, and SS9 were characterized by stabilized organic matter, while the other analyzed SS unfortunately indicated a lack of proper stabilization because of their relatively high OM contents (Figure 3).
The macronutrient amounts found in SS usually decreased in the following order: N > Ca > P > Mg. As can be seen from the data in Figure 4, the values for individual SS fell within wide ranges; nevertheless, the average values over the years of the study for individual SS indicate that this waste is a valuable source of nutrients. As in the case of organic matter, also with regard to the amount of macronutrients, the size of the installation had no significant impact on the variation in their levels in the SS. This means that SS representing different sizes of WWTPs were not statistically different in each case. Mean content of total nitrogen amounted to 44.3 g∙kg−1, ranging from 13.3 to 78 g∙kg−1. Calcium content averaged 62.3 g∙kg−1, in the range from 9.5 to 350 g∙kg−1. Mean content of phosphorus amounted to 20.7 g∙kg−1, ranging from 8.7 to 34.0 g∙kg−1. Magnesium averaged 5.8 g∙kg−1, with values within the range from 3.0 to 9.3 g∙kg−1 (Figure 4).
Significant amounts of OM, N, P, or Ca in SS allows them to be recommended for agricultural purposes as an alternative to organic fertilizers. The use of SS in soil fertilization has been well documented [24,25]. The cited studies demonstrate a positive impact of SS on soil chemistry and plant yields. Referring to research on qualitative characteristics of SS, it is worth emphasizing its unique composition of humic compounds [26,27], which significantly emphasizes SS suitability for composting or direct use in fertilization, especially on light, degraded soils with low fertility, i.e., in conditions found for soils requiring reclamation.
As previously mentioned, the Regulation of the Minister of Environment on the Municipal Sewage Sludge [11] specifies the conditions under which SS can be used for environmental purposes. Annex 1 of that Regulation indicates limit values for heavy metals in SS depending on their use. Also, Hudcová et al. [6] and Collivignarelli et al. [10] in their study emphasized that in most European countries the quantitative level of heavy metals is the most important parameter, next to sanitary pollution, when deciding to use SS for agricultural purposes. As can be seen from the data presented in Figure 5, the amounts of heavy metals in the examined SS varied within wide limits. Nevertheless, the maximum values did not exceed the permissible contents of heavy metals specified in the above-mentioned Regulation, which is advantageous in the context of the agricultural use of SS. Jakubus [28] also analyzed the distribution of metals in sewage sludge fractions from Wielkopolska and proved the environmental safety of SS when applied to the soil, which resulted from the presence of metals in less mobile complexes, consequently being less available for plants.
As confirmed by statistical analysis, the amounts of heavy metals in the SS did not always differ, with the level of a given metal comparable for SS from large and small WWTPs. On average, total zinc was 1098 mg∙kg−1, ranging from 311 to 2300 g∙kg−1. Mean content of total copper amounted to 226 mg∙kg−1, with values in the range from 75 to 463.9 mg∙kg−1. The average amount of total lead ranged from 5.2 to 326 mg∙kg−1, with a mean value of 104.1 mg∙kg−1. The total amount of chromium was 97.9 mg∙kg−1, in the range from 20 to 388.5 mg∙kg−1. Total nickel ranged from 12 to 131.6 mg∙kg−1, with an average of 69.9 mg∙kg−1. Total cadmium was on average 1.75 mg∙kg−1, ranging from 0.3 to 3.9 mg∙kg−1 (Figure 5).
The SS chemical composition is also beneficial, and this waste can constitute a substrate for co-composting with other biodegradable wastes. Moreover, thanks to composting, unwanted waste is converted into compost, which is an organic fertilizer and is subject to another legal act—Regulation of the Minister of Agriculture and Rural Development [29]. Thus, according to this Regulation, such composts must meet certain parameters regarding the content of organic matter, as well as the amount of N, P, K, and heavy metals. Composts prepared from SS constitute an attractive source of nutrients and organic matter along with high environmental safety, because they do not exceed the permissible amounts of heavy metals [26]. Thanks to the above, such composts exhibit favorable fertilizing parameters and can be used for agricultural purposes [30,31,32].

3.2. Currently Used Methods of SS Management

Despite many advantages, untreated sewage sludge may pose a threat to the environment due to the release of hazardous substances, both organic and inorganic, pathogenic organisms or CO2 emission. The generated SS mass obligatorily has to be subject to rational management, particularly in view of the fact that landfilling of SS has been practically impossible since 2016. The Regulation of the Minister of Economy on the Criteria of Waste Admission for Landfill [16] clearly prohibits landfilling of SS with the gross caloric value exceeding 6 MJ/kg of dry matter. Therefore, other methods must be used, while at the same time they must optimally fit the assumptions of circular economy, including bioeconomy. In addition, the optimal method combining economic and ecological aspects of SS management in a given area should include life cycle assessment (LCA), possible emissions, as well as conditions and factors related to SS generation and management. LCA is a tool quantifying the environmental impact/cost of particular options for SS management in order to choose the best suitable option for each stakeholder. From the practical point of view, the choice of an SS management method depends on the following factors: reducing the load of organic compounds, reduction of SS mass, limitations on the development of pathogens, climate, topography, availability of financial resources, and possibilities to limit process costs. Such theoretical, even idealistic assumptions regarding the choice of an SS management method are verified by actual conditions and possibilities. Unfortunately, the most important decisions considering SS treatment are still being made based on economic and political criteria.
The current situation regarding various development methods is provided by the Local Data Bank [22], and for the purposes of this study it was presented both on the national scale of Poland and the local scale for Poznań County. This comparison was made in relation to the previous year, i.e., 2022, for which the total SS mass generated was 580,659 tons, whereas for Poznań County it was 18,242 tons [22]. Figure 6 presents the percentage share of management methods applied for the SS mass generated. According to the data contained therein, the treatment of SS in Poznań County is significantly different from what was presented for the entire country. In Poznań County, a very high (35.3% compared to 3.4% for Poland) use of SS for land reclamation purposes should be noted, with a relatively small use of SS for agricultural purposes (4.4% compared to 27% for Poland), along with a small share of thermal methods (4.6% compared to 18% for Poland). The low popularity of thermal SS disposal methods results from the small number of installations intended for this purpose. Currently there are only 11 sewage sludge mono-incineration plants operating in Poland, the locations of which are shown in Figure 7. The lack of such installations in the Wielkopolskie province explains the small share of these methods in SS processing in Poznań County and simultaneously explains the popularity of agricultural methods of SS utilization. It should also be remembered that thermal methods in Poland are about six times more expensive than biological methods [20]. Due to the high costs of SS utilization by thermal methods, this solution is acceptable for large agglomerations [33] and large installations equipped with SS drying stations. It should be taken into account that dried sludge is easy to store and transport. This process also increases its caloric value, and thus contributes to the increased attractiveness of this waste as an alternative fuel. However, the drying process also involves large financial outlays, which makes it less popular and, consequently, its thermal transformation is limited in Poland.
It is important to emphasize the fact that SS generated in Poznań County is not stored, which is still reported for the country (1.4%). Also noteworthy is the relatively large share of temporarily stored SS given for Poland (9.4%) with a small (2%) share reported for Poznań County. A feature common both for Poznań County and Poland is a very high percentage of the so-called “other” methods of SS management, amounting to 52.3% and 36.9%, respectively. There is no specific information on what disposal methods are included here, but it can be assumed that SS is used as a substrate for co-composting together with other biodegradable wastes. It should be emphasized here that biomass composting is perceived as fulfilling the SS-to-matter assumptions. In such a process, it is proposed to mix SS with a structure-forming material, e.g., straw, sawdust, bark, biowaste, etc. According to Goldan et al. [34], in most Western European countries, SS is either thermally processed or composted, and together with compost it may be used for soil rehabilitation. Although composting can be considered a highly beneficial, low-cost option, it still causes some important problems such as nitrogen losses and greenhouse gas emissions. It can also be assumed that this includes the use of SS for the production of mineral organic fertilizers. Kacprzyk and Kupich [21] stated that such a potential use of SS exists while at the same time using calcium, magnesium and potassium compounds, fly ash of combustion of coal, or brown coal.
The popularity of methods related to the natural use of SS (for land reclamation, for growing plants intended for compost production, and for agriculture) is not surprising and is justified from the point of view of circular economy and bioeconomy. When we add up the percentage shares for the three methods of SS management (Figure 6), it turns out that on the national scale they account for 34.3%, while in Poznań County it is 41.1%. These approaches to SS utilization are rational and result from the local conditions and the beneficial properties of this type of waste. Moreover, by acting in accordance with the provisions contained in the Regulation of the Minister of Environment on the Municipal Sewage Sludge [11] or the Regulation of the Minister of Environment on the R10 recovery process [14], we can effectively utilize the fertilizing value of SS. A review by Hudcová et al. [6] also indicated that in most EU countries, SS is used for agricultural purposes.

3.3. Opportunities for Alternative Methods of SS Utilization

However, widespread use of SS for fertilizer purposes may encounter problems in the future due to regulations of the European Parliament and the Council (UE) Regulations (2019/1009) [35] prohibiting the use of SS for the production of fertilizer products. As indicated above, the agricultural use of SS has a number of advantages, but their disadvantages cannot be ignored, related to the presence of dangerous organic compounds (PAH, AOX, PCB), pharmaceuticals, as well as some pathogens that can survive various processes the SS is subjected to. Moreover, not all European countries require the evaluation of SS in terms of the content of hazardous organic compounds, and there are no appropriately developed values limiting the use of SS for agricultural purposes [6]. For this reason, the literature emphasizes the importance of the SS-to-energy concept [19,36], where pyrolysis, gasification, and incineration are the most popular conventional paths for thermo-chemical treatment of SS. As reported by Chen et al. [36], hydrothermal processing (HTP) of SS is a promising method that fits the waste-to-energy (WtE) assumptions, especially as the cited authors show that HTP exhibits an 11-fold higher energy recovery than landfilling. Regardless of the process used, their overarching idea should be emphasized, which is a significant reduction in the SS mass and the odors emitted by this waste, elimination of sanitary hazards, and avoiding landfilling, thus reducing climate change and simultaneously recovering renewable energy [5]. At the same time, we cannot ignore the fact that we can effectively recover P and/or N from solid residues after processes, mainly gasification, HTP, or incineration [18,19,32,36].
In the last decade, the technology for recovering nitrogen and phosphorus from SS has been developing much more rapidly. According to Shaddel et al. [18], N recovery has received less attention than P recovery due to the lower operational needs and economic motivation. Moreover, P is one of the critical raw materials and since its natural resources are shrinking, intensive work is currently underway to find alternative sources of this macronutrient, which is essential for plants. Hence, there is considerable interest in the P recovery method from SS, its ashes, or post-treatment water. Shaddel et al. [18] stated that theoretically P recovery is possible from the liquid phase, SS, and SS ashes; however, only technologies from SS and SS ashes are highly efficient.
Moreover, recovery from SS ashes requires incineration of this waste, which also in relation to Polish conditions is limited and difficult. Also, despite many advantages, the process of pyrolytic transformation of SS is not applied on a large scale in our country, which is evidenced by the lack of specific data on this subject. However, as reported by Singh et al. [37], the pyrolytic conversion of SS in an oxygen-free environment to produce biochar is a promising method and shows environmental benefits. Due to its unique properties (high porosity and high stability), biochar contributes to the improvement of soil properties (increase in CEC, water capacity) and reduces leaching of nutrients. Thanks to this, biochar is recommended for use in agriculture and horticulture [38,39]. Regardless of the benefits, data concerning SS processing into biochar show that it is not a common method, especially in Poland, and at this stage it is limited to experimental research rather than wide-scale use.

3.4. Public Awareness and Knowledge in the Local Community Regarding the Presence of SS in the Environment

As noted earlier, SS as a burdensome waste requires not only specific treatment but should also be managed appropriately by people who are responsible for it. However, not only decision-makers are obliged to have knowledge of the SS economy. Indirect producers, i.e., residents, should also be aware of the presence of SS in the environment and the whole range of consequences that accompany it. In this context, it is important to know how to properly manage SS so as to minimize possible negative impacts while maximizing benefits. The conducted survey of the local community gives some idea of the knowledge and perception of SS by the inhabitants of Poznań County. As shown by the data in Table 4, regardless of the age of the respondents a majority declared their knowledge on SS issues.
The highest percentage (88%) was recorded among people aged 36–45, and the smallest (52%) among people aged 18–25. Unfortunately, regardless of age, a majority of respondents (48–66%) had no opinion concerning the problems of SS in our environment. Sewage sludge as a problematic waste was recognized only by 10–18% of respondents. Despite this statement, respondents aged 26–60+ mostly declared knowledge on SS utilization in agriculture and land reclamation (28–56%), composting (10–16%), or landfilling (5–28%) (Table 4). A study by Nicholas et al. [40] confirmed that the oldest age group of respondents (65+) had greater knowledge and acceptance of the agricultural use of sewage sludge compared to young ones. Mela et al. [41] also found greater knowledge of wastewater management among older people (over 65 years of age) than young people. According to the studies of the cited authors, respondents generally had little knowledge, and higher education did not translate into greater environmental awareness. In this context, the results of this research present a different picture of society. Interpreting the survey results depending on people’s education, it was confirmed that the level of education provided a higher level of knowledge on SS (Table 5). Regardless of education, a considerable majority of respondents confirmed their knowledge concerning SS, although the highest percentage (31%) was found for the group of people with higher education. The highest share (27%) of the mentioned group of respondents declared knowledge concerning the method of SS management. At the same time, in this group of respondents, there was a significant percentage (18%) of responses regarding a lack of opinions on problems of SS in our environment. Irrespective of inhabitants’ education, the largest share (7–18%) of the respondents indicated agricultural and reclamation SS utilization, and 2–7% of respondents considered it appropriate to process SS in the composting process (Table 5).
Unfortunately, the survey results presented above are not satisfactory. Although the inhabitants of Poznań County declare knowledge on the origin of SS and its development, they are not interested in its presence in the environment, downplaying possible problems related to it. At the same time, an overwhelming number of respondents pointed to the natural possibilities of using SS. Public acceptance of the currently dominant agricultural and/or reclamation use of SS is a direct reference to the methods currently used and preferred in SS management, which are consistent with the idea of circular economy.
The interpretation of survey data depending on the respondents’ age and education allowed us to notice certain social dependencies that do not present a very positive image of the local community. The youngest group of respondents, aged 18–25, showed the least interest in the issues discussed and, consequently, the least involvement in matters that directly concern them. The older the inhabitants were, the more awareness and knowledge in a given area, which was especially visible for the age group 26–60. Higher education is conducive to understanding and properly perceiving the presence of the SS in our environment, which was confirmed by the data obtained from the surveys. To sum up, it should be strongly emphasized that the inhabitants of Poznań County should be more informed, and educational campaigns are necessary and should be carried out systematically starting from an early age of the target groups. Transferring knowledge is not only about raising awareness of the current problem, but also about preparing ways to solve it, including those that may arouse fear and aversion, such as thermal transformation of SS, a method that may become necessary in the near future. Similar opinions and positions can be found in the papers of Mela et al., Cagno et al., or Ekane et al. [41,42,43], where the importance of educating the local community is strongly emphasized in shaping environmental awareness. According to Ekane et al. [43], a complex combination of technical, environmental, socio-economical, psychological, and political factors plays an important role in judgment and decision-making processes regarding sewage sludge and its safe use as fertilizer in agriculture. Furthermore, Cagno et al. [42] expressed an opinion that communication and involvement of the public appear to be an important asset to mitigate possible negative relationships in SS management and the decision-making system related to it.
According to the author of the study, in order to increase local residents’ awareness and knowledge concerning SS, a comprehensive educational system should be implemented starting from primary school, which, in addition to theoretical knowledge, would include study visits to WWTP or workshops conducted by qualified staff. Another activity presenting SS management, addressed to a wide audience, could be systematic educational campaigns conducted by appropriate institutions using various types of modern communication media (television, the Internet). This would enhance public awareness of the need for environmental protection and sustainable development, combined with co-decision and co-responsibility for specific activities in this area. At this point, the importance of education in promoting best practices and reducing the negative environmental impact of human activity should be strongly emphasized.

4. Conclusions

As this case study shows, the problem of SS presence in our environment is complex and largely depends on local conditions. This is evidenced by the data presented in this paper relating to SS management throughout Poland and locally in relation to Poznań County. Generally, agricultural methods and non-agricultural methods are used in practice. The former group of the above-mentioned methods is dominant in Poznań County, while the latter prevails on the national scale. The proven popularity of agricultural methods is due to the valuable chemical composition of SS, which was confirmed by multi-year data concerning macro- and microelements and organic matter amounts determined for SS from various WWTPs located in Poznań County. As indicated by the surveys conducted, such a utilization of the SS was accepted by the local community. Despite such a rational approach to the use of SS, residents downplayed the presence of SS in the environment. The youngest and least educated people had the least knowledge and awareness of this topic. This shows the need for continuous education of the general public in dealing with SS.
Regardless of the adopted SS management methods (SS-to-matter and SS-to–energy), they are consistent with the assumptions of circular economy, which should be considered appropriate in the light of the applicable legal and environmental conditions, as well as the waste management hierarchy recorded in the national and EU acts. Each of these methods is characterized by advantages and disadvantages that should be carefully analyzed in relation to current natural, economic, and infrastructure possibilities. The development of new technologies—alternatives to agricultural methods—is a fact and will certainly progress, for example in the field of P recovery from solid materials remaining after thermal processing of SS. One can only assume that over time this method will become widely used, although it is difficult to indicate specific dates in this regard. For now, referring to local conditions, decision makers responsible for SS management are guided by applicable legal regulations, as well as economic calculations related to the proper and sustainable utilization of this waste. The research results indicate the necessity and appropriateness of publishing works on sustainable SS management, which is necessary to support environmental protection, public health, sustainable socio-economic development, and shaping policy and legal regulations.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The author declares no conflicts of interest.

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Figure 1. Location of WWTPs in Poznań County.
Figure 1. Location of WWTPs in Poznań County.
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Figure 2. Sewage sludge amounts (dry matter) generated in Poland and in Poznań County in individual years [22].
Figure 2. Sewage sludge amounts (dry matter) generated in Poland and in Poznań County in individual years [22].
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Figure 3. Organic matter contents in dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
Figure 3. Organic matter contents in dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
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Figure 4. Macronutrient contents in the dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
Figure 4. Macronutrient contents in the dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
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Figure 5. Heavy metal contents in the dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
Figure 5. Heavy metal contents in the dry matter of SS from individual WWTPs over the years of the study (Homogeneous groups are indicated by the same letters).
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Figure 6. Percentage shares of methods used in SS management in 2022 (the author’s elaboration on the basis of [22]).
Figure 6. Percentage shares of methods used in SS management in 2022 (the author’s elaboration on the basis of [22]).
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Figure 7. Location of mono-incineration installations in Poland.
Figure 7. Location of mono-incineration installations in Poland.
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Table 1. Legal acts concerning sewage sludge management in the EU and Poland.
Table 1. Legal acts concerning sewage sludge management in the EU and Poland.
European Legal ActsPolish Legal Acts
2000/532/EC: Commission Decision of 3 May 2000 replacing Decision 94/3/EC establishing a list of wastes pursuant to Article 1(a) of Council Directive 75/442/EEC on waste and Council Decision 94/904/EC establishing a list of hazardous waste pursuant to Article 1(4) of Council Directive 91/689/EEC on hazardous waste (notified under document number C(2000) 1147) [2]Regulation of the Minister of the Environment regarding the waste catalog [1]
Council Directive 86/278/EEC of 12 June 1986 on the protection of the environment, and in particular of the soil, when sewage sludge is used in agriculture [12]Regulation of the Minister of Environment on the Municipal Sewage Sludge [11]
Council Directive 91/271/EEC of 21 May 1991 concerning urban waste-water treatment [13]Regulation of the Minister of Environment on the R10 recovery process [14]
Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste [15]Regulation of the Minister of Economy on the Criteria of Waste Admission for Landfill [16]
Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives [17]Waste Act [3]
Table 2. Questions included in the survey regarding public knowledge in the local community concerning sewage sludge and methods of its management.
Table 2. Questions included in the survey regarding public knowledge in the local community concerning sewage sludge and methods of its management.
Question 1Do you know what sewage sludge is?
￭
Yes
￭
No
Question 2Do you think sewage sludge is an environmental problem?
￭
Yes
￭
No
￭
I have no opinion
Question 3Do you have knowledge about sewage sludge management?
￭
Yes
￭
No
If yes, select the correct answer:
￭
agricultural and reclamation purposes
￭
landfilling
￭
composting
￭
incineration
Table 3. Ranges of macro- (g·kg−1) and microelements (mg·kg−1) in the dry matter of sewage sludge compared for Poland and the Wielkopolska province (based on Jakubus [4]).
Table 3. Ranges of macro- (g·kg−1) and microelements (mg·kg−1) in the dry matter of sewage sludge compared for Poland and the Wielkopolska province (based on Jakubus [4]).
NutrientPolandWielkopolska Province
OM287.0–850.0301.0–788.0 (615.0) *
N12.5–83.512.3–71.6 (39.1)
P0.4–68.04.9–67.5 (23.3)
Ca0.8–89.311.6–63.4 (33.0)
Mg0.2–21.11.3–20.7 (8.3)
Cu17.3–868.098.0–838.0 (259.0)
Zn30.5–4200220–3000 (1416.1)
Mn63.0–479.091.0–794.0 (237)
Fe1205–17,5008419–89,109 (31,066.35)
*—mean value.
Table 4. Responses obtained from surveys depending on the age of respondents (values given in % share).
Table 4. Responses obtained from surveys depending on the age of respondents (values given in % share).
Age of Respondents
Response 18–2526–3536–4546–6060+
Question 1
No4830123241
Yes5270886859
Question 2
No3642313422
Yes010141812
I have no opinion 6448554866
Question 3
No5629123454
Yes4471886646
If yes, select the appropriate answer:
Agricultural and reclamation purposes 3056422830
Landfilling0528125
Composting71014168
Incineration700103
Table 5. Responses obtained from surveys depending on the education of respondents (values given in % share).
Table 5. Responses obtained from surveys depending on the education of respondents (values given in % share).
Education
Response vocationalsecondaryhigher
Question 1
No7127
Yes152831
Question 2
No5715
Yes145
I have no opinion162918
Question 3
No111411
Yes112627
If yes, select the appropriate answer:
Agricultural and reclamation purposes71518
Landfilling122
Composting275
Incineration122
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Jakubus, M. Current Trends in Sustainable Sewage Sludge Management—A Case Study for Poznań County, Poland. Sustainability 2024, 16, 5056. https://doi.org/10.3390/su16125056

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Jakubus M. Current Trends in Sustainable Sewage Sludge Management—A Case Study for Poznań County, Poland. Sustainability. 2024; 16(12):5056. https://doi.org/10.3390/su16125056

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Jakubus, Monika. 2024. "Current Trends in Sustainable Sewage Sludge Management—A Case Study for Poznań County, Poland" Sustainability 16, no. 12: 5056. https://doi.org/10.3390/su16125056

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