Municipal Sewage Sludge Disposal in the Republic of Poland

Abstract

This paper reviews methods of municipal sewage sludge (MSS) disposal in the Republic of Poland. The MSS amount produced in 2022 in sewage treatment plants is 580.7 thousand tons of total solids. This is related to the increase in the amount of treated sewage and the use of the co-digestion of sewage sludge with waste. MSS generated in municipal sewage treatment plants constitutes waste with code 19 08 05—stabilized MSS. It is subjected to the rules of conduct specified in the Act on Waste. According to data from the Central Statistical Office, in 2022, the most popular disposal method was its use in agriculture (27.1%). Approximately 18% of the MSS from treatment plants was thermally treated. A significant part of MSS is also used for crops, compost production, and reclamation. MSS management requires an individual approach from sewage treatment plant operators and the use of effective disposal methods. Considering the assumption of the circular economy, it is necessary to consider the possibility of recovering valuable raw materials from MSS and producing products for reuse. One of the possibilities of reusing MSS is to generate porous materials. Moreover, MSS can be transformed into multiple types of soil improvers and fertilizers.

1. Introduction

In the case of municipal sewage treatment, biological methods are most often used, which always results in the production of waste sewage sludge. The amount of municipal sewage sludge produced depends on the reduced load of pollutants flowing into the treatment plant, including the content of suspensions and organic matter in the sewage. Depending on the sewage treatment technology adopted, 0.5 to 1.2 kg of total solids (TS) of excess sludge per 1 kg of removed biochemical oxygen demand (BOD₅) is obtained during sludge production. Sludge resulting from the sedimentation of suspensions and sludge from the chemical precipitation of phosphorus should also be considered. In such a case, the amount of sludge may exceed 1.5 kg of TS of sludge per 1 kg of removed BOD₅ [1,2]. With the increasing requirements for the quality of sewage discharged into the environment, the increased degree of biogenic compounds’ removal, and the expansion of the sewage network, the amount of municipal sewage sludge produced has increased. This sludge must be subjected to processing and disposal processes. Sludge processing processes are currently known and commonly used in sewage treatment plants. The main purpose of sludge treatment is to reduce the volume of sludge and to stabilize and hygienize it. The management and disposal of municipal sewage sludge remain a problem. The final disposal method depends mainly on the quality and composition of the sludge [1,2,3,4].

The quality of sewage sludge is determined by the type and the composition of wastewater flowing into the treatment plant, the concentration of pollutants in the raw sewage, the quantity and quality of industrial wastewater discharged into the sewer system, and the wastewater treatment technology and technological parameters, including the age of the sludge, the reactants used and the method of sludge treatment, especially the stabilization method [1,2,5,6,7]. The characteristics of municipal sewage sludge are shown in

Table 1. Characteristics of municipal sewage sludge.

Parameter	Total Solids TS	Volatile Solids VS	Fat	Phosphorus	Protein	Cellulose	Nitrogen	Potassium	pH	Reference
Unit	%	% TS	% TS	% TS	% TS	% TS	% TS	% TS	-
Primary sludge	2–8	60–80	5–8	0.8–2.8	20–30	8–15	1.5–4.0	nd	5.0–8.0	[1]
Excess activated sludge	0.4–1.2	60–85	5–12	1.5–3.0	32–41	nd	2.4–7.0	nd	6.5–8.0	[1]
Primary sludge	4–9	65–80	nd	0.6–2.9	18–30	8–16	1.4–4.2	nd	5.5–8.0	[8]
Excess activated sludge	0.6–1.2	60–85	nd	3–10	30–40	nd	2.5–5.0	nd	6.6–8.0	[8]
Mixed sludge	14.6–67.7	nd	nd	2.1–2.6	nd	nd	1.1–1.66	0.29–0.58	6.7–7.6	[9]

nd—no data.

. The physicochemical parameters characterizing the resulting sludge include total solids (TS), volatile solids (VS), pH, fertilizing compounds (nitrogen, phosphorus, potassium), organic carbon, and toxic substances (heavy metals, polycyclic aromatic hydrocarbons, adsorbable organically bound halogens, polychlorinated bisphenols, furans, dioxins, pesticides, and others). A common feature of municipal sewage sludge is high hydration. Depending on the place of sludge generation in the technological system of sewage treatment, its hydration degree changes. Hydration can be over 99% for raw sludge and 96–97.5% for mixed sludge. On the one hand, fermented, dewatered sludge is characterized by hydration of about 80%. On the other hand, stabilized and dried municipal sewage sludge is characterized by an average hydration of 5% [1,5,6,7,8,9,10]. Preliminary sludge is characterized by the content of organic substances (VS) in the range of 60 to 80% of total solids. The content of organic substances in the excess sludge is at a similar level of 60–85% of total solids. On the other hand, after the drying process, the stabilized sludge has a content of organic substances of 50.6–71.8% of total solids [1,8,9,10]. The maximum nitrogen content in sludge reaches 7%TS for excess sludge. In the case of stabilized sludge, the nitrogen content is about 2.5%TS. At the same time, phosphorus in stabilized sludge occurs at a level of 1.4–4.1%TS [1,8,9,10].

Table 2. Content of heavy metals in municipal sewage sludge.

Heavy Metals	Iron	Nickel	Chrome	Zinc	Lead	Copper	Cadmium	Mercury	References
Mixed fermented sludge [g/kg TS] typical value	20	0.07	1.6	1.7	0.6	nd	nd	nd	[1]
Mixed fermented sludge [g/kg TS] maximum value	153	3.5	99	28	26	99	nd	nd	[1]
Mixed stabilized sludge [g/kg TS] minimum value	nd	0.02	0.03	0,91	0.01	0.04	0.002	0.002	[11]
Mixed stabilized sludge [g/kg TS] maximum value	nd	0.14	0.3	4.5	0.17	0.5	0.016	0.007	[11]
Mixed stabilized sludge [g/kg TS] minimum value	0.0002	0.11	0.32	0.37	0.004	0.06	0.00	0.0003	[10]
Mixed stabilized sludge [g/kg TS] maximum value	0.003	0.12	0.25	0.77	0.15	0.15	0.005	0.004	[10]
Mixed stabilized sludge [g/kg TS] minimum value	12.7	0.03	0.02	0.89	0.12	0.1	0.03	nd	[9]
Mixed stabilized sludge [g/kg TS] maximum value	14.5	0.56	0.33	1.57	0.52	0.45	0.06	nd	[9]
Mixed fermented sludge [g/kg TS] typical value	69	0.014	0.03	0.35	0.01	0.27	0.0005	0.0014	[12]

nd—no data.

presents the indicative content of some heavy metals. The content of heavy metals in municipal sewage sludge may vary depending on the nature of the sewage treatment plant catchment area and the quantity and quality of industrial sewage flowing into the treatment plant [1,8,9,10,11,12].

In addition, sewage sludge may contain numerous pollutants such as pathogens [10,13], microplastics [14,15], pharmaceuticals [16], hormones [17], PFASs [18] and PAHs [19,20].

The main objectives of sludge processing are as follows [2,3,4,21]:

-

To reduce the volume of sludge (removing water from it)—thickening, dewatering and drying;

-

To stabilize sludge (limiting its putrefaction, eliminating unpleasant odors, and reducing the content of organic substances);

-

Hygienization of sludge (covering processes that remove pathogenic organisms from the sludge and obtain a sanitary-safe product);

-

Preparation of sludge for its final form of management or disposal.

The processes of treatment of municipal sewage sludge are presented in diagram Figure 1.

According to the Polish Central Statistical Office data [22], the amount of sewage sludge produced in municipal sewage treatment plants in 2022 was 580.7 thousand Mg of total solids. Sewage sludge produced in municipal sewage treatment plants constitutes waste with code 19 08 05—stabilized municipal sewage sludge. It is subject to the rules of procedures specified in the Act on Waste of 14 December 2012 [23] and requires disposal processes. In 2022, the most popular method of sewage sludge management was its use in agriculture (27.1%). On the other hand, approximately 18% of the sludge from municipal sewage treatment plants (105.2 thousand Mg of total solids) was thermally transformed. A significant part of sewage sludge is also used for crops, compost production, or land reclamation [22].

The choice of the method of recovery or disposal of municipal sewage sludge is difficult. It should be based on a detailed technical and economic analysis taking into account the following factors [2]:

-

The amount of waste and the size of the municipal sewage treatment plant;

-

Sewage treatment technology;

-

Type of catchment area (city, commune, village);

-

The size of the catchment area;

-

The characteristics of the catchment area—presence of industrial plants and type of sewage system in the catchment area;

-

Quantitative and qualitative balance of treated sewage and generated sewage sludge;

-

Fuel properties of sewage sludge;

-

Flexibility of solutions, degree of automation;

-

Soil quality, and the height of groundwater level;

-

Availability of agricultural land and crop structure;

-

The presence of degraded areas;

-

The presence of protected areas;

-

The presence of flood areas;

-

Methods of disposal and utilization of municipal waste in the catchment area (composting plant, incinerator);

-

The presence of plants in the catchment area where the sewage sludge co-incineration process can take place (cement plant, CHP plant);

-

Investment costs;

-

Operating costs.

This article presents current solutions of various methods of MSS management in the Republic of Poland, which was the main aim of the development of this review manuscript. For this purpose, a detailed and comprehensive analysis of the methods used and applied on a technical scale was carried out, indicating specific examples of installations and, based on static data, the amounts of MSS managed in a given way. Moreover, in order to provide a comprehensive discussion of sewage sludge management methods, reference was made to the legal requirements in force in Poland, which specify the possibilities of implementing the selected method. This article also presents new alternatives and possibilities of solutions for MSS management through the production of biochar from it. The possibilities of recovering biogenic compounds, mainly phosphorus, from MSS were also analyzed, which, like biochar production, also fits into the implementation of the circular economy.

2. Methodology

The literature review was conducted using the Elsevier (Science Direct), Scopus and Google Scholar databases for 6 months using the query “sewage sludge” OR “municipal sewage sludge” AND “disposal” OR “treatment” OR “sludge management” OR “methods of disposal”. Additionally, articles containing the queries “composting” AND “sewage sludge” OR “municipal sewage sludge” and “fertilizer” and “sewage sludge” OR “municipal sewage sludge”, as well as “phosphorus recovery” and “sewage sludge” OR “municipal sewage sludge”, were searched using the same databases. Additionally, the search was performed using the above databases for “biochar” or “sludge-based activated carbon” or “waste-adsorbent” and “sewage sludge” OR “municipal sewage sludge”. The query was set to “all fields”, and no additional filters, such as language or publication date, were applied. A total of 755,000 articles were found, of which 226,550 duplicates were excluded. Additionally, articles that were older than 15 years were excluded (354051). Further articles were excluded after reading the abstracts or full texts, which did not concern the MSS disposal methods used in the Polish Republic or did not concern the topics discussed in this review. Detailed full-text analysis was performed for 83 articles.

3. Legal Aspects of Sludge Management in the Republic of Poland

The essential legal act regulating waste management issues is the Act of 14 December 2012 on waste—Journal of Laws 2013, item 21, as amended, and regulations that specifically restrict the handling of municipal sewage sludge [23].

The Act on Waste is consistent with the European Union’s regulations on the disposal of municipal sewage sludge contained in the Directives of the European Parliament:

-

Council Directive of 12 June 1986 on the protection of the environment, particularly the soil, when sewage sludge is used in agriculture—86/278/EEC [24].

-

Council Directive of 21 May 1991 concerning urban wastewater treatment—requiring monitoring and reporting of urban wastewater treatment and final disposal of urban sewage sludge for agglomerations. Article 14 of this Directive states that sewage sludge must be reused [25].

-

Directive (EU) 2024/3019 of the European Parliament and of the Council of 27 November 2024 concerning urban wastewater treatment [26]—Article 20 “Sludge and resource recovery” refers to encouraging the recovery of valuable resources and introducing the necessary measures that sludge management conforms to the waste hierarchy provided for in Article 4 of Directive 2008/98/EC [27]. Such sludge management shall maximize prevention, prepare for reuse, recycling, and other recovery of resources, in particular, phosphorus and nitrogen, and minimize the adverse effects on the environment and human health.

-

Council Directive 1999/31/EC of 26 April 1999 on the landfill of waste [28].

-

Directive 2008/98/EC of The European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives which regulate the recycling of waste, including sewage sludge. According to this Directive, sewage sludge, defined as waste, is subject to the procedure assigned to waste treatment [27].

By the applicable legal regulations, the term “municipal sewage sludge”, according to the definition contained in Art. 3, section 1, point 4 of the Act on Waste, is understood as “sludge from fermentation chambers and other installations used to treat municipal sewage and other sewage with a composition similar to that of municipal sewage” originating from sewage treatment plants [23]. The Act on Waste clearly defines the hierarchy of handling municipal sewage sludge. By the “principle of proximity” described in Chapter 3, waste (including municipal sewage sludge) is first processed at the place of its generation. If it cannot be processed at the place of its creation, it is transferred, taking into account the hierarchy of waste management methods and the best available technology. In general, however, using municipal sewage sludge outside the province where it is produced is prohibited. In exceptional cases, it is possible to use municipal sewage sludge outside the province in which it was made if the distance from the place of waste generation to the place of use located in another province is shorter than the distance to the place of use located in the same province. The provisions of the Act prohibit the processing of waste (sludge) outside installations or devices, the exception being recovery in the R10 process, referred to in Annex No. 1 of the Act (Chapter 10, Article 30). According to the Act, the use of municipal sewage sludge is understood as the spread of municipal sewage sludge on the ground or its introduction into the soil. The limitations and possibilities of using municipal sewage sludge are included in Chapter 4, Article 96 of the Act on Waste. “Recovery involves the use of municipal sewage sludge:

(1)

in agriculture, which is understood as the cultivation of all agricultural products introduced into trade, including crops intended for the production of feed,

(2)

for the cultivation of plants intended for the production of compost,

(3)

for the cultivation of plants not intended for consumption and for the production of feed,

(4)

for the recultivation of areas, including land for agricultural purposes,

(5)

when adapting land to specific needs resulting from waste management plans, local spatial development plans, or decisions on development conditions and land development”.

In addition, municipal sewage sludge may be transferred to the landowner only by the producer of such sludge. The responsibility for correctly using municipal sewage sludge for this purpose rests with the producer of such sludge. The use of municipal sewage sludge is possible if it is stabilized and prepared appropriately for the purpose and method of its use, particularly by subjecting it to biological, chemical, thermal, or other processes that reduce the susceptibility of municipal sewage sludge to putrefaction and eliminate the threat to the environment or human life and health. Irrigating municipal sewage sludge previously subjected to the drying process is prohibited. Before use, municipal sewage sludge and the land on which such sludge is to be used shall be subjected to tests referred to in the regulations issued under paragraph 13 by the producer of municipal sewage sludge. The producer of municipal sewage sludge is obliged to transfer, together with the sludge, to the owner of the land surface on which the municipal sewage sludge is to be applied information on the doses of this sludge that may be applied on individual lands, as well as test results. The producer of municipal sewage sludge used for purposes should notify the provincial environmental protection inspector of the intention to transfer this sludge to the owner of the land surface on which this sludge is to be applied at least 7 days before the transfer.

The act also indicates areas where the use of municipal sewage sludge is not permitted [23]. The use of municipal sewage sludge is prohibited, among others, in national parks and nature reserves, in areas of indirect protection of water intake protection zones, and on land designated for the cultivation of berries and vegetables, the edible parts of which are in direct contact with the ground and are consumed raw—within 18 months preceding the harvest and during the harvest—or on land used for pastures and meadows. The Act on Waste contains rules on the thermal treatment of waste. The term “thermal treatment of waste” refers to the incineration of waste by oxidation and other thermal waste treatment processes, including pyrolysis, gasification, and the plasma process. According to the provisions of Art. 155, the thermal treatment of waste is carried out only in waste incineration plants or waste co-incineration plants. The operator of a waste incineration plant or waste co-incineration plant is the entity that performs the thermal treatment of waste in the waste incineration plant or waste plant, respectively.

Other legal regulations concerning municipal sewage sludge include:

-

Regulation of the Minister of the Environment of 11 May 2015 on the recovery of waste outside installations and facilities—Journal of Laws 2015, item 796 [29]. The regulation specifies the types of waste and the conditions for their recovery in the recovery processes R3—Recycling or reclamation of organic substances which are not used as solvents (including composting and other biological transformation processes)—in Annex No. 1 to the Act of 14 December 2012 on Waste, outside installations, or facilities.

-

Regulation of the Minister of the Environment of 20 January 2015 on the recovery process R10—Surface treatment with agricultural or environmental benefits—Journal of Laws 2015, item 132 [30]. The regulation specifies the conditions that must be met for the R10 process.

-

Regulation of the Minister of the Environment of 6 February 2015 on municipal sewage sludge—Journal of Laws 2015, item 257 [31]. The implementation of regulations related to the R10 process regulates in detail the conditions for the use of municipal sewage sludge. It specifies the conditions that must be met for the reclamation or agricultural use of municipal sewage sludge. The provisions limit the loads on sewage sludge and soils fertilized with sludge by specifying permissible concentrations of heavy metals. It also specifies the doses, scope, frequency, and reference methods of testing municipal sewage sludge and the land on which it is to be used.

-

Regulation of the Minister of Agriculture and Rural Development of 9 August 2024, on the implementation of certain provisions of the Act on Fertilizers and Fertilization—Journal of Laws 2024, item 1261 [32].

-

Act of 10 July 2007 on fertilizers and fertilization—Journal of Laws 2007, item 1033, as amended [33].

-

Regulation of the Minister of Development of 21 January 2016 on the requirements for conducting the process of thermal treatment of waste and methods of handling waste generated as a result of this process—Journal of Laws 2016, item 108 [34]. The regulation specifies the requirements for conducting the process of thermal treatment of waste and methods of handling waste generated as a result of the process.

-

Regulation of the Minister of Climate of 24 September 2020 on emission standards for certain types of installations, fuel incineration sources, and waste incineration or facilities—Journal of Laws of 2020, item 1860 [35].

-

Regulation of the Minister of the Environment of 6 June 2016 on the technical conditions for qualifying part of the energy recovered from the thermal treatment of waste—Journal of Laws 2016, item 847 [36].

-

Act of 20 February 2015 on renewable energy sources—Journal of Laws U. 2015, item 478, as amended [37].

-

Act of 27 April 2001—Environmental Protection Law [38].

-

Act of 20 July 2017 on Water Law—Journal of Laws, 2017, item 1566 [39].

4. Assumptions of the National Waste Management Plan

Based on legal regulations, the assumptions of the National Waste Management Plan were created, including the handling of municipal sewage sludge. The National Waste Management Plan was introduced by Resolution No. 96 of the Council of Ministers on 12 June 2023 on the National Waste Management Plan 2028 [40]. Modern waste management striving to implement an economic model based on the assumptions of the circular economy requires a change in the current approach and the perception of waste as a source of resources. According to the plan, the objectives and actions necessary to achieve municipal sewage sludge management include the following [40]:

(1)

“Complete cessation of municipal sewage sludge storage;

(2)

Increasing the amount of municipal sewage sludge processed before being released into the environment;

(3)

Increasing the amount of municipal sewage sludge subjected to thermal processing;

(4)

Striving to maximize the degree of utilization of nutrients contained in sludge while meeting all requirements regarding sanitary, chemical, and environmental safety, with particular emphasis on the organic carbon contained in sludge and the ability of sludge to increase carbon dioxide sequestration in soils;

(5)

Reducing the amount of sewage sludge constituting waste generated in municipal sewage treatment plants, taking into account the waste management hierarchy;

(6)

Striving to limit the production of municipal sewage sludge constituting waste, which, owing to its quality, creates problems with its management via regulations”.

Following the hierarchy of methods for handling municipal sewage sludge, depending on the form in which it occurs and its quality, the following is necessary [40]:

(1)

Prevent the formation of municipal sewage sludge;

(2)

Recycling municipal sewage sludge—organic recycling, including composting municipal sewage sludge with other waste to obtain material after the composting process is used for fertilizer purposes and mineral recycling with phosphorus recovery or in cement plants;

(3)

Use methods for the recovery of municipal sewage sludge (directly on the ground after meeting the conditions specified in the regulations; recovery, including recovery in composting plants, biogas plants, or cement plants), including energy recovery—for example, the use of sludge as biomass, which means incineration or recovery outside installations;

(4)

Disposal of municipal sewage sludge—in this process, sludge may be thermally transformed in waste incineration plants or co-incineration plants without energy recovery or stored after processing if it meets the requirements specified in legal regulations.

5. Methods of Disposal of Sewage Sludge

5.1. Agriculture

As mentioned earlier, in 2022, agriculture was the most popular method of sewage sludge management in Poland (27.1%), accounting for 157.4 thousand Mg of total solids in the sludge [22]. Biological and chemical safety are important when using municipal sewage sludge to grow plants intended for human consumption and feed production. It is impossible to allow large amounts of harmful chemicals to enter the soil or groundwater [4,41,42].

In the Republic of Poland, according to the Act on Waste, the R10 recovery process involves processing on the ground surface, which benefits agriculture or improves the condition of the environment. According to the implementation regulations, municipal sewage sludge may be used on land if the following conditions are met [31]:

(1)

“The content of heavy metals in the sludge does not exceed the amounts specified in Annex No. 1;

(2)

In the case of using this sludge in agriculture and for land reclamation for agricultural purposes, no bacteria of the Salmonella genus were isolated from a representative sample of sludge weighing 100 g obtained in § 5;

(3)

The total number of live eggs of the intestinal parasites Ascaris sp., Trichuris sp., and Toxocara sp. in 1 kg of total solids, hereafter referred to as “TS.” of sludge intended for research use:

• In agriculture and for land reclamation for agricultural purposes is 0;
• For land reclamation is no more than 300;
• For adapting land to specific needs resulting from waste management plans; spatial development plans, or decisions on the conditions of development and land development is no more than 300;
• For growing plants intended for compost production is no more than 300;
• For growing plants not intended for consumption and the production of feed is no more than 300”.

In addition, the amount of heavy metals in the top layer of soil at a depth of 0–25 cm, on which these sludges are to be applied, does not exceed the permissible values specified in Annexes 2 and 3, and the pH value of the soil is not lower than 5.6. The use of sludge must not cause deterioration of the quality of soil, earth, and surface and underground waters. The use of sludge is possible outside the period of growth and development of plants intended for direct human consumption. The regulation specifies the permissible dose of municipal sewage sludge depending on the type of land, its use, the quality of municipal sewage sludge and soil, and the demand of plants for phosphorus and nitrogen. The permissible doses of municipal sewage sludge may be used during the year per unit of land area provided that the allowable content of heavy metals in municipal sewage sludge specified in Annex 1 to the regulation is observed [31].

The conditions that municipal sewage sludge must meet and the land on which it can be used have been strictly defined in the regulation. This recovery method can be implemented by sewage treatment plants that produce sludge that meets the required quality standards, especially regarding heavy metal content. In addition, there are areas within the province where municipal sewage sludge can be used. The requirements set in the regulation create restrictions, especially for large sewage treatment plants located in agglomerations that produce sludge containing heavy metals. In such cases, treatment plant operators are forced to use other methods of processing municipal sewage sludge.

Municipal sewage sludge is permitted on agricultural land in both Czechia and Japan. In the field of sewage sludge processing, the legislation of Czechia, as a Member State of the European Union (EU), is based on Directive 86/278/EEC [24], which sets limits for heavy metals and other pollutants in sludge. Japan also has limits for heavy metals for the agricultural use of sewage sludge. However, organic pollutants and pathogens do not have direct limits. In Czechia, as shown in the literature, two-thirds of the produced sludge is disposed of by direct application in agriculture or composting [43]. Furthermore, in 2010, more than 50% of the produced municipal sewage sludge was used in agriculture in countries such as Belgium, Denmark, Spain, France, Ireland, and Great Britain [21,44]. However, in other countries, such as Finland and Belgium, <5% of municipal sewage sludge is used for agricultural purposes. In Greece, the Netherlands, Romania, Slovenia, and Slovakia, sludge is not used in agriculture. In turn, Switzerland has banned using municipal sewage sludge on agricultural land since 2005 [21,44].

5.2. Composting

One of the methods of processing municipal sewage sludge is composting. According to the provisions of the Act on Waste Pole [23], this is a recovery process using the R3 method. In the process of composting municipal sewage sludge, the aim is to destroy pathogenic organisms, stabilize organic matter (maturing), dry it, and produce a material that can be used for agriculture or sold. For this purpose, the sewage sludge must be mixed with structure-forming materials in the appropriate proportion to obtain a C:N ratio of approximately 30:1. In practice, construction materials containing cellulose are most often wood chips, sawdust, bark, straw, and leaf litter [45]. The process produces a final product, compost. Compost improves the soil’s physical, chemical, and biological properties, such as organic matter, water and nutrient retention capacity, infiltration, aeration, compaction and erosion resistance, and soil-borne diseases [46,47,48]. Compost promotes the development of soil macrofauna, which plays an important role in improving soil quality. In addition, compost slowly releases nutrients that plants can take up, thus contributing to enhanced crop productivity. Compost contains macronutrients (mainly nitrogen, phosphorus, and potassium) and micronutrients essential for plant growth; so, its use improves soil fertility. Compost stimulates microbial activity, thereby increasing the availability of nutrients to plants and producing hormone-like substances that can contribute to crop growth. It is widely used in agriculture, horticulture, viticulture, forestry, urban landscapes, and greenhouses [46,47,48,49]. It can be used for shaping green areas, municipal and forest management, construction and maintenance of embankments of communication routes, construction of acoustic screens, and reclamation and covering of landfills.

Composting, as a method of processing municipal sewage sludge, is a beneficial technology for sewage treatment plants in rural areas. Municipal sewage sludge contains low concentrations of heavy metals caused by the lack of industrial sewage flowing into the treatment plant through the sewage system. The compost obtained can be used for agricultural purposes to obtain a fertilizer certificate, as is the case at the sewage treatment plants in Słupsk, Nowa Wieś Ełcka, Piła, and Sokółka. Since 1996, a composting plant for biodegradable waste, including stabilized municipal sewage sludge with green waste, has been operating at the Sewage Treatment Plant in Słupsk. The composting plant covers an area of 1.5 ha, of which 0.91 ha is roofed. It has a hardened surface as a reinforced concrete slab placed on the ground with a compost pile aeration system, a leachate drainage system, and a deodorization system. This facility is adapted for the biological processing of approximately 20,000 tons/year of waste subject to aerobic decomposition with the participation of microorganisms, including approximately 13,000 Mg/year of sewage sludge operating using the technology of a shifted heap. The remaining part consists mainly of structural waste obtained from other producers in the form of straw, branches, and bark. As a result of aerobic decomposition processes with the participation of microorganisms, a full-value product is created—a compost called “BIOTOP”, which is admitted to circulation and has the status of organic fertilizer. In composting plants, approximately 7000 Mg/year of compost are produced from 20,000 Mg/year of waste raw materials [50]. Another example of municipal sewage sludge processing into full-value compost is the Sewage Treatment Plant in Nowa Wieś Ełcka. Compost production is based on turning a pile, the so-called additional method involving recirculating the hot phase. Fermented and mechanically dewatered municipal sewage sludge mixed with straw, wood chips, and inoculum from the pile in the hot phase is used for composting. The product obtained this way is a fertilizer called “Kompelk”, which used to grow cereals and establish lawns [51]. The sewage treatment plant in Piła also has a composting plant, which processes not only municipal sewage sludge from its own and nearby sewage treatment plants but also biodegradable waste from the food, paper, and wood industries, agricultural waste, green waste, and other waste whose composition allows it to be used in the composting process. The composting plant has a reinforced concrete slab surface, from which the leachates are discharged to the sewage treatment plant. The technology used is shifting piles, which are temporarily or permanently covered with specialist fabrics, creating a closed reactor. Before composting, the waste is crushed, if necessary, with a mobile crushing device or a chipper. When waste from parks and gardens is crushed, it is fractionated on a drum sieve. The finished compost is a homogeneous material that does not contain plastics, metals, or hard materials, including pieces of glass, and should not emit odors. Batches of the finished product are marked with an information plate, stored, and sent for sale. Compost recipients are often farms [52]. In Sokółka, a mechanical–biological sewage treatment plant treats municipal and industrial sewage (mainly dairy). Sewage sludge dewatered on a filter-belt press is mixed with sawdust from deciduous trees. The entire composting process takes 60–70 days. The prepared mixture is initially formed into a heap, where anaerobic processes occur. After three weeks, the mixture is formed into heaps, on which grids covered with straw are placed. A fan is mounted to them, which ensures the extraction of odors and the ability to conduct the process under aerobic conditions. The duration of aeration ranges from 14 to 20 days. The compost is subsequently allowed to mature for three to six weeks. During the maturation phase, the heaps are poured several times, which speeds up the process and results in a lumpy product structure. As shown by the tests, the content of individual heavy metals in the compost is significantly lower than the permissible values for sludge intended for agricultural use. This is due to the low concentrations of heavy metals in sewage treated in the Sokółka sewage treatment plant. The compost is also free from Salmonella bacteria and parasite eggs. The obtained product is rich in nutrients (nitrogen and phosphorus) and is a good source of magnesium and calcium. The produced compost has a permit for sale as an organic fertilizer under the trade name “Kompost Sokólski” and turf lawns [53].

5.3. Thermal Treatment

Thermal conversion methods are used to process municipal sewage sludge. These methods include incineration and co-incineration processes and so-called alternative methods. Alternative methods include gasification, pyrolysis, and wet oxidation. The possibility of disposal of municipal sewage sludge in incineration processes depends on the content of mineral substances, the content of volatile substances, hydration, ash, and its composition. However, if the dried sewage sludge is high in calories, it can be used as fuel in various processes [4,54,55]. These parameters also affect the selection of the thermal process method that allows for maintaining minimum emissions and avoiding operational hazards [56]. Owing to the high content of nitrogen and sulfur, when considering the possibility of incineration and co-incineration of sewage sludge, the possibility of probable emissions of sulfur and nitrogen oxides, as well as heavy metals, dioxins, and furans, should be analyzed. Notably, the thermal processing process is not neutral to the environment, causing the emission of these compounds [56,57]. In Poland, municipal sewage sludge is disposed of by incineration in large sewage treatment plants, as shown in

Table 3. List of existing installations for incinerating municipal sewage sludge in Poland [22].

No	Location	Nominal Capacity in Thousand Mg TS/Year
1	Warsaw—Sewage Treatment Plant “Czajka”	62.2
2	Krakow—Sewage Treatment Plant “Płaszów”	23.0
3	Łódź—Group Sewage Treatment Plant	21.0
4	Gdańsk—Sewage Treatment Plant “East”	15.24
5	Gdynia—Sewage treatment plant “Dębogórze”	9.5
6	Bydgoszcz—Sewage treatment plant “Fordon”	7.8
7	Zielona Góra—Sewage treatment plant “Łącza”	6.4
8	Kielce—Sewage treatment plant “Sitkówka”	6.2
9	Szczecin—Sewage treatment plant “Pomorzany”	6.0
10	Olsztyn—Sewage Treatment Plant “Łyna”	3.2
11	Łomża—Łomża Sewage Treatment Plant	1.5
	TOTAL	162.04

.

An example of municipal sewage sludge co-incineration in rotary kilns is the Rudniki and Chełm cement plants. In both cement plants, the sludge combines with coal dust, and the addition of sewage sludge does not exceed the emission standards. The amount of municipal sewage sludge co-incineration in both cement plants in 2011 was 6251 thousand Mg of total solids in the Chełm cement plant and 1406 thousand Mg of total solids in the Rudniki cement plant [56,57].

Technical and economic analysis should precede the selection of thermal conversion technology. First, it should be assessed whether it is possible to thermally process the sludge without additional energy, the necessity of using it, and the degree of drying of the sludge before incineration. Incineration significantly reduces the volume of stored sewage sludge. This is important in densely populated countries such as Japan. Large amounts of sludge are produced there, and it cannot be used for agricultural purposes. The amount of sludge incinerated is 55% [4,58]. The incineration process must be preceded by drying the sewage sludge to 18–35% TS (usually about 25%). The waste products of all incineration processes are ash, which must be recycled or used in another method [4,42,59].

In the case of the low calorific value of the municipal sewage sludge, a possible solution is the process of co-incineration with coal, fuel oil, or natural gas. An alternative is also to transfer the sewage sludge to a combined heat and power plant, which can be used as an admixture (approximately 10%) [4,59]. One of the methods of thermal conversion of municipal sewage sludge is pyrolysis. This process involves burning sewage sludge in conditions of limited access or lack of air. Materials processed this way can be further used to produce absorbents [4,60]. Another product of this process is pyrolytic liquids, so-called bio-oil or pyrolytic oil, which can be used as fuel [4,58].

In Czechia, municipal sewage sludge is co-incinerated and used to produce electricity, heat, or cement. One of the Bohuslavice–Trutnov sewage treatment plants uses the pyrolysis process. However, there is no municipal sewage sludge incinerator [43]. In Japan, the situation is quite the opposite, where almost two-thirds of the municipal sewage sludge produced is incinerated. Most of the ash generated after incineration is used in the construction industry (47%) or stored in landfills (36%)—building artificial islands at sea (28%) or directed to landfills (8.4%) [43].

5.4. Phosphorus Recover

One of the trends that fits into the assumptions of the National Waste Management Plan is the recovery of valuable chemical elements from municipal sewage sludge. Different technologies are known for recovering phosphorus from municipal sewage sludge, both after the thickening and dewatering process, as well as from ashes after the incineration process.

There are several examples of phosphorus recovery from municipal sewage sludge. One of them is the AQUA RECI technology, which consists of the oxidation of municipal sewage sludge in the presence of pure oxygen to gaseous products. Phosphorus is precipitated in the form of calcium hydrogen phosphate. The efficiency of this technology is estimated at 70% phosphorus recovery [61,62,63,64]. Another technology called PHOXNAN is carried out in two stages. In the first stage, at a pressure of 12–28 bar and a temperature of 160–220 °C, the pH value of the municipal sewage sludge decreases to less than 1.5 when sulfuric acid is added. A significant degree of removal of organic compounds, polycyclic aromatic hydrocarbons (PAHs), pharmaceuticals, and organic micropollutants occurs. The second stage of PHOXNAN technology involves the following membrane processes: ultrafiltration (separation of liquid from solids) and nanofiltration. Struvite is precipitated from permeate rich in phosphates, whereas heavy metals are precipitated from the concentrate obtained after nanofiltration. The potential for phosphorus recovery is estimated at 50% [61,63,64,65]. The KREPRO process was developed by the Kemira Company, Helsinki, Finland, (Kemwater REcycling PROcess). After the pH value decreases to 1–3 (using sulfuric acid), municipal sewage sludge is thickened to 5–7% TS and heated in a reactor to 140 °C for approximately 30–40 min. During this process, more than 40% of the organic matter is decomposed, and most of the inorganic compounds dissolve. The remaining part of the suspension was separated via centrifugation. The supernatant from centrifugation is directed to a reactor, where orthophosphates precipitate [63,64,66]. Another technology for phosphorus recovery from municipal sewage sludge is the KEMICOND process. This is a technology based on KREPRO technology. The decomposition of organic compounds occurs at pH values ranging from 3 to 4 and a temperature of 20 °C in the presence of hydrogen peroxide [63,64]. In turn, Seaborne (Gifhorner) technology is used in the Gifhorn sewage treatment plant. It consists of the following stages: acid leaching, extraction, removal of heavy metals, and two-phase recovery of nutrients, together with struvite precipitation. The digested sewage sludge is mixed with ash generated during the thermal treatment of sewage sludge and subjected to sulfuric acid action to decrease the pH value to 4.5. Biologically bound phosphorus is subsequently released from the solid phase of the sludge. Heavy metals precipitate in the reactor as metal sulfides because of the addition of sodium sulfide. This process takes place at an increased pH (up to 5.6). Struvite precipitation is possible at a pH of 9.0, and sodium and magnesium hydroxides are added for this purpose. The efficiency of this technology is estimated at 40% phosphorus recovery from municipal sewage sludge. The final product contains 28% P₂O₅, significant amounts of hydroxyapatite, and no heavy metals. Gifhorn technology is expensive because of difficulties with pH adjustment [63,64,66].

Another method used is the recovery of phosphorus from the ashes after incineration the municipal sewage sludge. This kind of method can be used in the case of sewage treatment plants via the mono-incineration process. It is applied in municipal sewage treatment plants where sewage sludge is incinerated in a fluidized bed, in plate furnaces, in furnaces with a mechanical grate, or in rotary kilns. Ashes resulting from the incineration of municipal sewage sludge contain approximately 20% P₂O₅. The use of ash for fertilizer purposes is possible after the contents of heavy metals, mainly Zn, Pb, and Cu, are reduced [64,67]. One of the technologies for recovering phosphorus from municipal sewage sludge is BIOCON. The BIOCON process consists of recovering phosphorus compounds from ash resulting from sewage sludge incineration at a temperature of 850 °C. Ash, as in the case of sewage sludge, is mixed with sulfuric acid to obtain a pH value of approximately 1. In this process, municipal sewage sludge is combusted in a grate furnace for approximately 2 s at a temperature of 850 °C. After the sludge is combusted, the ash is treated with sulfuric acid to a pH of approximately 1. The solution then flows through a series of ion exchangers. The first is a cation exchanger separating iron ions; the next is an anion exchanger separating potassium ions; after regeneration with sulfuric acid, potassium hydrogen sulfate is created. The third anion exchanger separates phosphorus ions, creating a stream of phosphoric acid. Heavy metal chlorides are removed in the cation exchanger in the last stage. This technology has been tested on a pilot scale in the Bronderslev treatment plant in Denmark. Due to the high ion concentration in the exchangers and the necessity of frequent regeneration, BIOCON technology is economically unprofitable [63,64]. Another example is an ASH DEC process. This process is carried out in two stages. The first stage involves the mono-incineration of sludge. As a result, incineration residues, which are ash with high phosphorus contents containing heavy metal compounds, are obtained. In the second stage, the sewage sludge ash is mixed with solid chlorine donors (MgCl₂ and CaCl₂) and exposed to a temperature of 1000 °C for 20–30 min. At this temperature, heavy metals, usually mercury, cadmium, lead, copper, and zinc, react with salts, become gaseous, and evaporate. The amount of chlorine donors added depends on the concentration of heavy metals in the ash and the target removal rates required by national fertilizer regulations in European countries. In addition to removing heavy metals, the process produces phosphorus mineral compounds, which improve the bioavailability of phosphorus. These compounds include calcium phosphate chloroapatite (Ca₅(PO₄)₃Cl_1-x(OH)_x), magnesium phosphate farringtonite (Mg₃(PO₄)₂) and calcium magnesium phosphate stanfieldite (Ca₄Mg₅(PO₄)₆). The ash is then mixed with other nutrients (NH₄NO₃, K₂SO₄, KCl), which is pelletized via special mixers. This way, the ASH DEC NPK fertilizer is produced and sold under the brand name “PhosKraft”. Research institutes in Germany, Switzerland, the Netherlands, and Austria have tested and confirmed the quality of the product in numerous pot and field tests. As a result, the Austrian and German governments approved “PhosKraft” fertilizers for use on pastures and arable land [68]. Another example of phosphorus recovery from sludge is a solution used in Germany called RecoPhos. In this case, the ash after incinerating sewage sludge, which is relatively rich in phosphorus content reaching up to 11%, is transformed into fertilizer. However, phosphorus in the ash after burning sewage sludge usually occurs in difficult forms for plants to access. Therefore, in this solution, it was decided to transform phosphorus into calcium and magnesium dihydrogen phosphate, the main nutrients of the fertilizer called RecoPhos P 38. The processing process consists of preliminary mixing of the ash after burning sewage sludge with phosphoric acid. The reaction of transforming phosphorus forms into the compounds, as mentioned earlier, takes place; then, the whole is subjected to the drying and granulation process [69]. A similar method of producing phosphate fertilizers from ashes after sewage sludge incineration was used in the PolFerAsh Pole [70], invented by the Cracow University of Technology in Poland. It consists of producing solid or suspension fertilizers through a two-stage process [71]. In the first step, the ashes are extracted after sewage sludge incineration by using nitric (V) or phosphoric (V) acid. This step allows for the recovery of up to 70–99% of phosphorus from the ashes. In the second step, the whole is neutralized using a calcium compound or ammonia.

5.5. Biochar/Waste Adsorbent Generation/Preparation

Currently, great emphasis is placed on technological solutions aimed at activities related to the so-called circular economy. These activities should consider the minimization of waste generated, as well as the reduction in the amount of raw materials used. Concerning the management of sewage sludge, research has been carried out for some time on the transformation of this waste into biochar. These activities can contribute to a reduction in the amount of raw materials used for the production of classic commercial adsorbents. In addition, the transformation of sewage sludge (which is waste in the meaning of the law) into porous materials is part of the recovery of raw materials and the production of new products through waste processing. It can be said that waste is given “new life” in this way.

The transformation of sewage sludge, as well as excess activated sludge (which has not been subjected to stabilization processes) into biochar and waste adsorbents, has not yet been introduced within the framework of technologies used on an industrial scale. They are still in the phase of action; however, they are an exciting alternative for the management and disposal of these types of waste.

Biochar is a porous material produced by partial oxidation (mainly by pyrolysis at temperatures above 700 °C in the absence of or with limited access to oxygen) of charcoal and organic carbon-rich material [72]. Therefore, various types of biomass are used to produce biochar, such as products from the industrial sector, agricultural and forestry biomaterials, crops, leaves, wood and other waste from human activities, including sewage sludge [72,73]. A biochar derived from sewage sludge or activated sludge, due to its porous structure with a highly developed surface area, can be a type of adsorbent which is also called sludge-based adsorbent or waste adsorbent.

Various procedures are used to transform sludge into porous materials. However, in general, they most often include common steps [74]:

-

Pre-dewatering;

-

Drying the material;

-

Carried out the carbonization process, usually in an inert gas atmosphere;

-

Chemical or physical activation process;

-

Washing the material after the activation processes;

-

Final drying.

The pre-dewatering sludge aims to reduce the amount of water as much as possible to carry out the next stage of sludge transformation into adsorbents more efficiently, faster, and at the lowest possible operating costs. The drying process includes both the stage of removing water from sludge and often consists of a step of appropriately crushing the dried material to the required grain size (usually in a powdery form). Properly effective drying of sludge before the next step of the preparation of sludge-based activated carbons is crucial to avoid significant energy losses. On the other hand, crushing the material increases the speed and efficiency of the material transformation. Dried and crushed municipal sewage sludge is then subjected to thermal processing, i.e., carbonization. Carbonization is carried out at temperatures ranging from 400 °C to 1200 °C. It aims to transform materials rich in organic matter into carbon-rich biochar. This process is usually carried out in oxygen-free conditions, where a neutral gas, e.g., nitrogen or argon, is most often supplied in the flow. These activities aim to avoid the occurrence of classic material incineration. The authors Pieczykolan and Płonka [75] in their studies produced a porous material from excess sludge subjected to dehydration, drying and grinding for the adsorption of two synthetic dyes, Acid Red 18 and Acid Green 16. In addition, Pieczykolan et al. [76] also produced an adsorbent from excess sludge by preliminary dehydration, drying, fragmentation, and activation using Fenton’s reagent.

Biochar can also be produced in the process of hydrothermal carbonization. This process involves exposing hydrated sewage sludge or excess sludge to a relatively low temperature (usually 180–300 °C) under pressure (2–10 MPa) for several hours and is carried out in autoclaves where the medium in which the entire process takes place is water [73,77]. In this method, there is no need to pre-dry the hydrated sludge, which is a major advantage of this carbonization method.

The next stage of converting sludge into biochar is to carry out the activation process. Its purpose is to increase the specific surface area of porous materials as well as to obtain a porous structure of biochar. It consists of subjecting the converted materials to the action of activating agents at high temperatures. After carbonization, the material is heated (usually in the temperature range of 700 °C to 1000 °C) in the presence of water vapor or carbon dioxide flow. The carbonization process and subsequent physical activation were carried out by Rio et al. [78] using steam at high temperatures as the activation agent. The adsorbent/biochar prepared in this way was tested for its adsorption efficiency towards Cu²⁺, an organic pollutant, phenol, and two organic dyes, acid red 18 (AR18) and basic violet 4 (BV4).

In the case of chemical activation, chemical compounds such as strong acids (H₂SO₄, H₃PO₄, and HCl), strong bases (NaOH and KOH), and zinc chloride are used. The contact of dried sludge with the reagents can occur before or after the carbonization process. In the first case, the sludge is placed in an aqueous solution of a given activating reagent and subjected to its action for a specified time. Then, the sludge is subjected to the carbonization process. An example of such a solution is the research conducted by Wang et al. [79], where excess activated sludge taken from the secondary settling tank was transformed into a sludge-based adsorbent (a kind of biochar). In this case, after dewatering and drying, the sludge was first subjected to 3 M KOH and then to carbonization at 600 °C. The adsorbent (biochar) prepared in this way was tested for its ability to adsorb the Acid Brilliant Scarlet GR dye. In the research of Björklund et al. [80], activated sludge taken from the aerobic zone of biological sewage treatment was dewatered, dried, treated with 5 M ZnCl₂ for 24 h, dried again, and then carbonized at 500 °C. It was then used for organic stormwater contaminant removal. A similar procedure for preparing biochar from sewage sludge was used by Montoya-Bautista et al. [81], who used ZnCl₂ for activation and subsequent carbonization at 500 °C in an N₂ atmosphere. They used this waste adsorbent to remove pharmaceuticals and personal care products from the final discharged effluent after municipal secondary wastewater treatment. Another example of using chemical activation before pyrolysis is the study conducted by Fan et al. [82], where a mixture of sewage sludge and rice straw was first treated with ZnCl₂ solution for 12 h, dried, and then pyrolyzed in a muffle furnace at 600 °C for 2 h. The adsorbent (biochar) prepared this way was used to remove Cr (VI) in the studies conducted by Guo et al. [83]; sewage sludge was initially dried, and then exposed to an activation–impregnation mixture consisting of ZnCl₂ and H₂SO₄ for 24 h, after which the sludge was pyrolyzed in a nitrogen atmosphere at a temperature of 550 °C. This adsorbent was used in studies on removing Cu (II) ions and Methylene Blue dye. However, Rozada et al. [84] used H₂SO₄ for chemical activation before carbonization.

Chemical activation to which the sludge is subjected after the carbonization process also consists of placing it in solutions of activating reagents and subjecting it to this action for an appropriate time. In the studies conducted by Stefanelli et al. [85], hydrothermal carbonization was carried out at 200–210 °C, followed by chemical activation using KOH at a temperature between 550 and 750 °C. The adsorbent (biochar) prepared in this way was used to remove pollutants in gaseous and aqueous environments, and the carbon dioxide (CO₂) and ciprofloxacin (CIP) adsorption capacities were investigated. The last step in preparing adsorbents from municipal sewage sludge is washing the sludge to wash out the remaining reagents after activation and then redrying the material.

Biochar produced from sewage sludge can be used as an adsorbent of various types of pollutants including heavy metals (e.g., Cu, Ni, Zn, Cd, Cr, Hg [73]), antibiotics, synthetic dyes, carbon dioxide, phenolic compounds, emerging contaminants, nitrates, phosphates, sulfates, herbicides [73,77,86,87,88,89]. Moreover, biochar derived from sewage sludge can be used as a soil additive to improve the cultivation of various plants, grasses, rice, etc. [72,90].

5.6. Fertilizer Production

Municipal sewage sludge may also be processed into fertilizers if it meets the requirements specified in the Regulation of the Minister of Agriculture and Rural Development on implementing specific provisions of the Act on Fertilizers and Fertilization [32]. According to the regulation, the permissible contents of pollutants in organic and organic–mineral fertilizers, organic and organic–mineral agents supporting plant cultivation, and post-fermentation products may not exceed 100 mg of chromium (Cr), 5 mg of cadmium (Cd), 60 mg of nickel (Ni), 140 mg of lead (Pb), and 2 mg of mercury (Hg) per kilogram of total solids of this fertilizer, which support plant cultivation of this post-fermentation product. In addition, these products may not contain live eggs of the intestinal parasites Ascaris sp., Trichuris sp., Toxocara sp., and bacteria of the Salmonella genus. Fertilizers should also meet minimum quality requirements regarding organic matter content in total solids, i.e., at least 20%. The contents of individual components cannot be lower than 1% of total nitrogen (N), 0.5% of phosphorus in terms of phosphorus pentoxide (P₂O₅), or 1% of potassium in terms of potassium oxide (K₂O) [32].

The Silesian Mining Institute developed the technological process of processing municipal sewage sludge into organic fertilizer [91,92]. The fertilizer produced meets the procedure’s requirements, enabling its introduction to sale in terms of parameters and form. This innovative approach to processing municipal sewage sludge is protected by a patent (PL 233754B1) [91].

Table 4. Parameters of organic fertilizer [91].

Parameter	Unit	Value
Nitrogen	% by weight TS	1–1.3
Phosphorus	% by weight TS	1–1.92
Potassium	% by weight TS	0.2–0.5
Calcium	% by weight TS	15–19
Magnesium	% by weight TS	6–9
Organic substances	% by weight TS	35
pH	-	6.0–9.0

presents the basic parameters of the produced fertilizer.

In addition, the produced fertilizer is characterized by the ability to retain water, a low content of carbonates and salts, and a low concentration of harmful substances. The technological process of producing fertilizer granules occurs in two stages. The first stage involves mixing and granulation, and the second requires drying and packaging. The technological chain of the first stage includes the following processes: dewatering in a centrifuge to a total solids content of approximately 22%; homogenization with calcium oxide, dolomite, gypsum flour, and cellulose fibers and redirection to the granulator. In the second stage, granulates with a dry matter content of approximately 40–45% are dried at an 80–110 °C temperature to a total solid content of approximately 75%. The dried granulate is then temporarily collected in a silo for cooling and directed to packaging in foil bags or large bags [91,92].

Another proposal is to produce an organic–mineral fertilizer using sewage sludge from two sewage treatment plants in Żywiec and Kędzierzyn Koźle. In this case, research was conducted using dried and ground sewage sludge mixed with mineral fertilizers, namely, potassium chloride, diammonium phosphate, and poultry litter ash, to increase the potassium and phosphorus content. Mineral acids, namely, phosphoric acid (69–73% H₃PO₄) and nitric acid (55% HNO₃), were used as binding agents necessary for mechanical granulation of the entire mixture [11,93].

A different example of producing an organic–mineral fertilizer from sewage sludge is a method also patented by the Central Mining Institute under the patent number PL 234676B1 from 2020 [94]. In this case, stabilized municipal sewage sludge is mixed with an organic hydroabsorbing polymer in a ratio of 100 parts by weight of sludge to 1 part by weight of polymer. This way, a product with a grain size of at least 0.5 mm and a dry matter content of 40–50% is obtained, which is dried at a temperature of 20–105 °C for 2–3 h.

An alternative solution developed by the Central Mining Institute in Poland is the production of fertilizer. For this, stabilized sewage sludge with a hydration of 55–90% is mixed with sodium alginate at a concentration of 1–4% and a calcium chloride solution at a concentration of 1–2%. The mixture is then left to harden and dried at a temperature of 20–105 °C for 2–48 h (patent No. PL 233663B1, Poland [95]).

Rzeszów University of Technology in Poland together with Podkarpackie Centrum Innowacji Sp. z o.o. (Poland) developed a method for producing an organic–mineral fertilizer from stabilized sewage sludge mixed with dusty diatomite (patent No. PL 242576B1, 2023, Poland [96]). The procedure consists of preheating the sewage sludge at 55–65 °C Celsius for 20 to 30 min at a pressure of 0.1 MPa. Then, diatomite is added to the heated sludge, and the whole mixture is mixed. A granulate is formed and dried at a temperature of 60 to 85 °C.

Scientists from the Tadeusz Kościuszko Cracow University of Technology in Poland developed several different multi-component organic-mineral fertilizers with specific applications for rapeseed (patent No. PL 243440B1, 2023 [97]), corn (patent no PL 243439B1, 2023 [98]), sunflower (patent No PL 243441B1, 2023 [99]) and for general use without specifying the type of crop (patent No. PL 243442B1, 2023 [100]). In the case of fertilizers intended for the cultivation of rapeseed and corn, the production process consists of mixing in appropriate proportions dried sewage sludge with a grain size below 0.2 mm, ash from chicken droppings, also with a grain size below 0.2 mm, ammonium nitrate, potassium sulfate, and nitric acid in a bound form in the form of calcium nitrate. Different mixing proportions of the ingredients are used depending on the purpose of the fertilizer.

In the case of fertilizers intended for the cultivation of sunflowers (patent No. PL 243441B1, 2023 [99]), the production process uses a mixture (inappropriate weight proportions) of dried sewage sludge with a grain size below 0.2 mm, chicken manure ash also with a grain size below 0.2 mm, ammonium nitrate, potassium sulfate and sulfuric acid in a bound form in the form of calcium sulfate.

The production of granulated multi-component organic–mineral fertilizer [100] involves mixing dried sewage sludge with chicken manure ash, ammonium nitrate, potassium chloride, and potassium sulfate. The mixture is then granulated with an aqueous solution of sulfuric or nitric acid, and the whole mixture is dried to a moisture content of 2–5% by weight.

A fertilizer designed to improve soil structure in agricultural applications was also developed by the University of Agriculture in Cracow and Paper and Texture Factory Beskidy S.A., and Michalak Piotr Heliochem (patent No. PL 228796B1, 2018 [101]). In this solution, sewage sludge is mixed with flour in a binder, and the whole mixture is mixed. Additionally, quicklime and bentonite are added to this mixture, and after solidification, it is subjected to mechanical granulation and drying with hot gases at a temperature above 50 °C.

Another solution for producing fertilizers from sewage sludge (application No. PL 421521A1, 2018 [102]) is a method developed by the West Pomeranian University of Technology in Szczecin (Poland). It consists of mixing sewage sludge from a mechanical–biological sewage treatment plant with incineration by-products with inappropriate weight ratios. Ashes formed after burning straw in fluidized bed boilers, ashes after burning wood in fluidized bed boilers, and slag after burning sewage sludge are added here. In addition, potassium salt or potassium sulfate is added to this mixture.

6. Conclusions

The solution to the municipal sewage sludge economy is becoming an increasingly important issue. Directives are being implemented in the European Union countries. According to European regulations, management methods involving storage are currently being replaced by methods leading to the stabilization of sludge and safe recycling. Depending on the quality of the municipal sewage sludge produced and the technology adopted by its producer, it can be processed via recovery or disposal processes. Municipal sewage sludge can be subjected to processing to change its properties and obtain products for reuse or waste with other codes.

Based on legal regulations directly influencing the decision regarding the method of MSS management in Poland, it should be emphasized that a very important aspect of the solution for municipal sewage sludge management is the analysis of the possibilities of their disposal at the province/region/country level. An important component of the solution is the physicochemical composition of the sludge itself, particularly the content of heavy metals. The presence of heavy metals in quantities higher than those specified in the Regulation [31] excludes the possibility of using sewage sludge in agriculture or for compost production. In such cases, it is reasonable to use thermal disposal methods (incineration, co-incineration). In such a case, the solution would be a standard installation for thermal processing of sewage sludge for several sewage treatment plants in the province/region. Moreover, in locating sewage treatment plants in areas with no agricultural areas or areas covered by legal protection, the solution may be composting or incineration/co-incineration, and the production of fertilizers or generation sorbents.

Currently, in Poland, the most popular method of disposal of municipal sewage sludge is its use in agriculture for crops (27.1%) and thermal processing (~18%). Much of municipal sewage sludge is also used for compost production or land reclamation.

Sludge disposal according to the R10 method (i.e., surface treatment with agricultural or environmental benefits) is the relatively cheapest method, where the fertilizing potential of the stabilized and dewatered sewage sludge itself, rich in biogenic compounds such as N or P, is used. The main disadvantage of this process is the need to have access to an appropriate amount of land on which the sewage sludge can be applied (the application of which is subject to very rigorous requirements). In the case of converting sludge into compost under the R3 method, a product is produced that can be used as a fertilizer. Compost improves the physical, chemical, and biological properties of the soil. It improves soil fertility by slowly releasing nutrients such as nitrogen, phosphorus, and potassium. This solution is also widely used in Poland.

The method aimed at significantly reducing the flow and mass of sewage sludge produced, and the process of incineration or co-incineration of this waste is also used in Poland. This method is, however, energy-intensive, but, in the case of its use, there are no rigorous requirements regarding the physicochemical characteristics of the sludge. Additionally, in the case of mono-incineration of sewage sludge, it is possible to recover phosphorus from the ashes after incineration, which is in line with the assumptions of the circular economy.

Owing to the circular economy (CE), great emphasis is placed on technological solutions enabling recovery or reuse. One recovery method that fits into the circular economy is phosphorus recovery from municipal sewage sludge. Several technologies are known for recovering phosphorus from sewage sludge, both after the thickening and dewatering process, as well as from ashes after the incineration process. The circular economy trend also includes the technology of transforming municipal sewage sludge into waste sorbents (biochar). In addition, this technology can contribute to reducing the amount of raw materials used to produce classic commercial adsorbents. Owing to the contents of nitrogen, phosphorus, and potassium, municipal sewage sludge can also be successfully used to produce fertilizers. According to the assumptions of the National Waste Management Plan 2028, to maintain the hierarchy of waste management, including municipal sewage sludge, the use of appropriate technologies to reduce their quantity and ensure proper quality should be considered at the stage of planning the construction or modernization of a sewage treatment plant. However, in the case of Poland, the mentioned solutions are either used very rarely or are only used in the research phase. Their implementation will contribute to the recovery of valuable raw materials and a significant reduction in the amount of sludge requiring management.