Evaluation of the Heavy Metals Content in Sewage Sludge from Selected Rural and Urban Wastewater Treatment Plants in Poland in Terms of Its Suitability for Agricultural Use

Abstract

The amount of sewage sludge produced in Poland is increasing every year. Once stabilised and hygienised, sewage sludge is a valuable product, rich in biogenic elements such as nitrogen and phosphorus, which are essential for plant growth. However, in addition to the compounds that are beneficial to the soil, this sludge can also contain harmful substances such as heavy metals. The aim of this assessment is to analyse the content of seven heavy metals in sewage sludge from urban and rural wastewater treatment plants (WWTPs) located in Poland. This analysis allows for the suitability of sludge from the studied wastewater treatment plants for natural management to be assessed. The concentration ranges of Cd, Cu, Ni, Pb, Zn, Cr, and Hg in sludge samples from rural wastewater treatment plants were 0.6–9.5, 9.3–524, 4.8–90.0, 8.8–275.2, 575–1732, 7.5–170.0, and 0–3.8 mg/kg dry matter, respectively. In sludge samples from urban wastewater treatment plants, the concentration ranges of Cd, Cu, Ni, Pb, Zn, Cr, and Hg were 1.07–16.7, 32–195, 1.3–128.9, 21.2–322.4, 20–5351.1, 12.7–2759.8, and 0.1–1.55 mg/kg dry matter, respectively. Only one of the analysed wastewater treatment plants (Skarżysko-Kamienna) exceeded the limit values.

1. Introduction

According to Alloway and Ayres, heavy metals are defined as a group of metals and semimetals with a density greater than 6 g/cm³. However, this is not a strict definition, and the term is usually used to describe metals associated with toxicity and environmental pollution, such as Cd, Cr, Cu, Hg, Ni, Pb, and Zn [1]. They are widespread in the environment, occurring in various forms, such as oxides, hydroxides, sulphates, sulphides, phosphates, and silicates. A consequence of soil and air pollution is the contamination of plants due to their tendency to absorb and accumulate heavy metals [2]. The main source of heavy metals in sewage sludge is human activity, especially advanced industry, which emits large quantities of these elements into the environment every year.

Sewage sludge is a by-product of the wastewater treatment process [3,4]. It is composed of mineral and/or organic compounds and is separated from the liquid phase through sedimentation. The most cost-effective method for the management of sewage sludge is its natural utilisation [5]. Sewage sludge contains biogenic elements essential for plant growth and development [3,6]. It is also a valuable source of macro- and micro-elements as well as organic matter, but on the other hand, it is a potential threat to humans and the environment due to the presence of organic pollutants, pathogens, and heavy metals [7,8,9,10,11,12,13,14,15]. When added to soil, it enhances soil fertility, improves volumetric density and porosity, and increases water-retention capacity. It also reduces the need for mineral fertilisers, resulting in economic benefits. Municipal sewage sludge can be utilised provided it is stabilised and appropriately prepared to eliminate any potential risks to the environment or human health [15,16,17,18,19]. The heavy metals found in soil are not immediately absorbed by plants; however, over time, they can slowly form hazardous solutions. Heavy metals, such as Fe, Co, Cu, Cr, Mo, Mn, Se, Ni, and Zn, are necessary for the development of some organisms in trace amounts, but at higher concentrations, they become toxic. Elements such as Sb, Pb, Hg, Ag, and As are toxic and unnecessary for living organisms. However, most sewage and waste contain heavy metals in sufficient amounts to cause toxicity to crops [20]. The impact of heavy metals on the soil environment and plant is different. Lead in high concentrations has a toxic effect on soil microorganisms, leading to inhibition of the decomposition of organic matter. It also interferes with the metabolism and normal development of plants. Cadmium is one of the toxic metals. Absorbed from the soil by plants, it mainly accumulates in the roots and interferes with photosynthesis. Nickel, as a trace element, is essential for plants in trace amounts; however, excess amounts are toxic. When taken up by plants, it blocks the access of other elements needed for growth. Zinc is one of the micronutrients essential for plant vegetation; however, both deficiency and excess are undesirable. A deficiency of this element mainly results in stunted plant growth. Excessive uptake of these elements by plants is stored in the roots, where it impairs photosynthesis, calcium metabolism, and the binding of copper, iron, and other essential elements. Copper is an essential plant nutrient, but a deficiency can prevent normal growth, while an excess can inhibit photosynthesis. Chromium is an undesirable element, as its excessive availability for plants causes chlorosis, which leads to a disruption of the water balance and damage to growth cones and the root system [21]. Mercury (Hg) is considered a global pollutant. For plants, mercury is not an essential trace element, and its positive biological functions are not known. Usually, 2–10 mg/kg of Hg in the soil is phytotoxic. The plants accumulate mercury primarily in their roots, and in most of the cases, its transfer to aboveground organs is low [22]. Typically, the content of Zn and Cu in municipal sludge is high, while the content of Cd, Hg, and As is low. Heavy metals in sediments have strong geographical distribution characteristics. Higher contents of heavy metals in municipal sludge will occur in regions with greater industrial development [23]. The content of heavy metals in sediments also varies from country to country due to different national industrial characteristics [24,25,26,27,28]. Also, the permissible upper concentration values of heavy metals in sewage sludge that is intended for natural applications vary from country to country; it is important to note that the use of sludge must comply with all relevant regulations [15].

The objective of this assessment is to analyse the content of heavy metals in sewage sludge from municipal and rural sewage treatment plants and fill the gap associated with the lack of such comparisons. The analysis will assess the suitability of sludge from selected sewage treatment plants for use in agriculture, which is important information because this use is their basic management in Poland, and according to the literature [29,30], it constitutes 49% of the total use. Prior to selecting a sludge management method, it is essential to conduct an in-depth analysis of sludge composition, taking into account the presence of heavy metals and their concentrations.

2. Sludge Regulations in Poland

On 14 December 2012, the Waste Act [31] entered into force. It is a transposition of European Union regulations, including the following:

• 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 [32].
• Council Directive 91/271/EEC of 21 May 1991 concerning urban sewage [33].

The Waste Act [19] identifies methods for managing waste and reducing its environmental impact. It provides measures for reducing the effects of resource use and improving resource efficiency. The Act prohibits the use of municipal sewage sludge outside the province where it was generated. An exception to the prohibition of sludge application in another province is when the distance between the sludge production site and the application site in another province is less than the distance to the application site in the same province (Art. 20 Section 2). The land application of previously dried municipal sewage sludge is also prohibited.

According to Article 96 (1) of the Waste Act [31], sewage sludge can be used as follows:

–

in agriculture, understood as the cultivation of all marketable agricultural crops, including crops intended for the production of feed;

–

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

–

for the cultivation of non-food and non-fodder crops;

–

for land reclamation, including land for agricultural purposes;

–

for the adaptation of land to specific needs resulting from waste management plans, spatial development plans, or decisions on the conditions of development and land use.

To ensure its safety and efficacy, municipal sewage sludge must be stabilised and properly treated before use. This can be achieved through biological, chemical, thermal, or other treatments that eliminate environmental and health risks and reduce the sludge’s susceptibility to compaction.

The limit values for specific heavy metals in municipal sewage sludge intended for natural use are given in

Table 1. The maximum permissible levels of heavy metals in sewage sludge used for natural purposes, expressed in milligrams per kilogram of dry matter (mg/kg d.m.) [34].

Heavy Metal	In Agriculture and for Land Reclamation for Agricultural Purposes	For the Reclamation of Land for Non-Agricultural Purposes	When Adapting Land to Specific Needs Arising from Waste Management Plans, Spatial Development Plans
Cd	20	25	50
Cu	1000	1200	2000
Ni	300	400	500
Pb	750	1000	1500
Zn	2500	3500	5000
Hg	16	20	25
Cr	500	1000	2500

.

3. Materials and Methods

The aim of this study is to analyse the content of seven heavy metals in sewage sludges from urban and rural wastewater treatment plants located in Poland. The sites selected for analysis were chosen primarily on the basis of the availability of literature data, the location of the plants (the least and most industrialised parts of Poland), and the different WWTP capacities (

Table 2. Locations and characteristics of the sewage treatment plants and the areas.

Location	Characteristic of the Object and Area
Rural WWTP
Dobrzeń, Lower Silesian Voivodeship	The capacity is 160 m3/d, it is a mechanical–biological sewage treatment plant, and it receives domestic sewage from the villages of Dobrań and Strzelce. Dobra is fed to the plant, followed by sludge stabilisation and dewatering, and treated wastewater is discharged into the Dobra River [35]. The villages from which the wastewater is discharged to the treatment plant are located in the southern part of Dobroszyce municipality. This part of the municipality is dominated by the agricultural sector, so the share of industrial wastewater in the total amount of wastewater discharged to the treatment plant is practically zero.
Kamieniec Wrocławski, Lower Silesian Voivodeship	The capacity is 1600 m3/d, and sewage flows into it from the Czernica commune. The technological line of the sewage treatment plant includes the following facilities and equipment: sewage pumping station, grating hall, two-chamber horizontal sand trap with a manual grate, horizontal multi-funnel settling tanks, sludge pumping station, and irrigated fields serving as biological treatment plants [36]. There are several industrial centres in the municipality, such as the SYNPEKO Organic Synthesis Plant in Dobrzykowice (production of specialty and common solvents, automotive chemicals, cleaning agents, and car cosmetics), Military Communication Works in Czernica (design, production, renovation, and modernisation of communications equipment, provision of mechanical and electronic services), and PREMAG Sp. z o.o. in Ratowice (production and assembly of equipment for transport of loose food materials—lifts, conveyors, gravitational and pneumatic transport).
Świlcza, Podkarpackie Voivodeship	The capacity is approximately 1093 m3/d, and sewage flows into it from the following towns: Błędowa Zgłobieńska, Bratkowice, Dąbrowa, Mrowla, Rudka Wielka, Świlcza, Trzciana, and Woliczka. The technological line of the sewage treatment plant includes the following facilities and equipment: catchment point for sewage supplied from septic tanks, sewage pumping station with basket grate, stair trellis, vertical sand trap, four bio-block chambers (anaerobic, hypoxic, aerobic chambers, stabilising secondary settling tanks), excess sludge thickeners, belt press, and sludge hygienisation station [25].
Kostomłoty—Laskowa, Świętokrzyskie Voivodeship	The capacity is 450 m3/d, it supports 3333 PE, and sewage flows into it from the following towns: Ciosowa, Miedziana Góra, Kostomłoty Trzecie, Kostomłoty Drugie, and Tumulin-Wykień. The technological line of the sewage treatment plant includes the following facilities and equipment: automatic catchment station for supplied sewage, sewage pumping station, drum screen with a piston press for dewatering screenings, sewage retention tank, SBR biological reactors with aeration blowers, and press for dewatering excess sludge [26]. There are two transport routes in the municipality, national roads 7 and 74, which are the main sources of pollution in the area. In addition, in the commune, there are operations of “Laskowa” opencast mine in Kostomłoty Druga and “Kostomłoty” opencast mine in Kostomłoty Druga.
Strawczyn, Świętokrzyskie Voivodeship	The capacity is 633 m3/d, and it serves 6770 PE. The process line of the wastewater treatment plant includes the following facilities and equipment: automatic reception station for hauled wastewater; raw sewage pumping station; screen with scrubber; sand trap with separator and sand washer; suspended biological bed; screw press for dewatering of excess sludge, and sludge hygienisation station. There are no industrial plants in the municipality that may have a major impact on the environment [26].
Urban WWTP
Białystok, Podlaskie Voivodeship	It is the largest sewage treatment plant in north-eastern Poland and supports over 200,000 PE. The average daily capacity of the treatment plant is 100,000 m3/d, the maximum capacity of the treatment plant is 176,500 m3/d, approximately 80% of the sewage going to the treatment plant is domestic sewage, 20% of the sewage comes from industry, it is a mechanical–biological sewage treatment plant using the activated sludge method in the sewage treatment process and using the produced biogas, and it receives sewage from the city of Białystok and the areas located in the basin of the Narew and Supraśl rivers, where the following communes are located: Choroszcz, Dobrzyniewo Kościelne, Juchnowiec Kościelny, Supraśl, and Wasilków. The technological line of the sewage treatment plant includes the following facilities and devices: automatic station for receiving delivered sewage; grating hall; screen of screenings with a washer; raw sewage pumping station; two two-chamber sand traps with a separator and sand washer; four primary, oblong settling tanks constituting a set with a predenitrification chamber and a dephosphatation chamber; two pools containing biological reactors with activated sludge; six radial secondary settling tanks; activated sludge pumping station; mechanical activated sludge thickener; separate fermentation chambers; screw press for dewatering excess sludge; and dryer–granulator [27,28].
Przemyśl, Podkarpackie Voivodeship	The capacity of the treatment plant is 28,200 m3/d for an average daily flow, and it supports 100,000 PE. Treated sewage is discharged into the San River, and sewage flows into it from the city of Przemyśl and suburban towns. The technological line of the sewage treatment plant includes the following facilities and devices: automatic station for receiving delivered sewage, grating hall, screen of screenings with a washer, raw sewage pumping station, sand traps with a separator and sand washer, primary settling tanks, biological reactors with activated sludge, secondary settling tanks, activated sludge pumping station, mechanical activated sludge thickener, separate fermentation chambers, belt press for dewatering excess sludge, and sludge hygienisation station [15].
Ostrowiec Świętokrzyski, Świętokrzyskie Voivodeship	The maximum capacity of the treatment plant is 20,000 m3/d, it supports 88,060 PE, and purified sewage is discharged into the Kamienna River. The technological line of the sewage treatment plant includes the following facilities and devices: automatic station for receiving delivered sewage, grating hall, screen of screenings with a washer, raw sewage pumping station, two centrifugal sand traps with a sand separator, radial primary settling tank, biological reactors with activated sludge, secondary settling tanks, activated sludge pumping station, mechanical activated sludge thickener, separate fermentation chambers, belt press for dewatering excess sludge, and sludge hygienisation station [35,42]. It is located in a highly industrialised city, characterised by great metallurgical and foundry traditions. The largest industrial plant located in Ostrowiec Świętokrzyski is Huta Ostrowiec, which produces steel products. There is an industrial zone around the steelworks with an area of 500 ha, where other companies from the metallurgical processing sector are also located.
Sędziszów Małopolski, Podkarpackie Voivodeship	It supports 18,000 PE, approximately 450,000 m3 of sewage is treated there each year, and sewage flows into it from the following towns: Sędziszów Małopolski, Góra Ropczycka, Wolica Piaskowa, Wolica Ługowa, Czarna Sędziszowska, Boreczek, Borek Wielki, Krzywa, and Kawęczyn. The areas from which sewage flows are characterised by a low degree of industrialisation, and treated sewage is discharged into the Budzisz River. The technological line of the sewage treatment plant includes the following facilities and equipment: automatic station for receiving delivered sewage; raw sewage pumping station; screen of screenings with a washer; sand trap with a separator and sand washer; bioreactor; gravity thickener; belt press for dewatering excess sludge; and sludge hygienisation station [37]. The areas from which sewage flows to the sewage treatment plant located in Sędziszów Małopolski are characterised by low industrialisation; the treatment plant mainly accepts sewage from households.
Busko-Siesławice, Świętokrzyskie Voivodeship	The average daily capacity is 7400 m3/d, the maximum capacity is 9660 m3/d, and treated sewage is discharged into the Maskalis Reaver. The technological line of the sewage treatment plant includes the following facilities and equipment: automatic station for receiving delivered sewage, raw sewage pumping station, grates with screening washer, sieve-sand trap with a washer and sand separator, three biological reactors with activated sludge combined with secondary settling tanks, three recirculation pumping stations, excess sludge pumping station, excess sludge aerobic stabilisation chamber, gravity thickener, belt press for dewatering excess sludge, and sludge hygienisation station [43]. The spa character of the Busko-Zdrój commune excludes the introduction of heavy industry into its areas. The economic activity of the commune’s inhabitants focuses on services.
Skarżysko-Kamienna, Świętokrzyskie Voivodeship	The maximum capacity is 30,000 m3/d, it supports 59,000 PE, and purified sewage is discharged into the Kamienna River [38]. The technological line of the sewage treatment plant includes the following facilities and equipment: automatic station for receiving delivered sewage, raw sewage pumping station, grates with screening washer, horizontal sand trap with a sand washer, degreaser, four horizontal primary settling tanks, three biological reactors with activated sludge, three horizontal secondary settling tanks, pumping station for recirculated and excess sludge, gravity thickener, separate fermentation chambers, belt press for dewatering excess sludge, and sludge dryer [38]. It is located in a highly industrialised communication hub where National Road No. 7 and No. 42 intersect. The following industrial plants are located in the city: MESKO S.A. (production of ammunition and portable missile systems), MESKO AGD (equipment for catering and households, lighting fixtures for industry), MESKO ROL (agricultural and forestry machines), and also the food, footwear, clothing, chemical, and building materials industries.
Sitkówka-Nowiny, Świętokrzyskie Voivodeship	The capacity is 51,000 m3/d, it supports 172,569 PE, and sewage flows into it from the city of Kielce, the Sitkówka-Nowiny commune, and the western part of the Masłów commune. Approximately 14% of the incoming sewage is industrial sewage (mainly food and metal industries), 86% is domestic sewage, and treated sewage is discharged into the Bobrza River. The technological line of the sewage treatment plant includes the following facilities and equipment: automatic station for receiving delivered sewage, raw sewage pumping station, three stepped screens with hydraulic screening presses, three two-chamber aerated sand traps with degreasing chambers, four radial primary settling tanks, two biological reactors with activated sludge, four horizontal secondary settling tanks, pumping station for recirculated and excess sludge, three rotary sieve-drum thickeners, two separate fermentation chambers, two centrifuges for sludge dewatering, and sludge hygienisation station [38,44]. It is located in one of the most industrialised communes in the Świętokrzyskie Voivodeship. The largest industrial plants located in these areas are the following: Cementownia “Nowiny” Sp. z o. o. (cement production), Dyckerhoff Sopro (concrete production), and ZPW “Trzuskawica” S.A. (production of quicklime, limestone flour, various types of aggregates).
Suwałki, Podlaskie Voivodeship	The average daily capacity is 25,600 m3/d, 29% of the sewage flowing into the treatment plant is industrial sewage with the remainder domestic sewage, it uses the produced biogas to produce electricity and heat, and purified sewage is discharged into the Czarna Hańcza River. The technological line of the sewage treatment plant includes the following facilities and equipment: raw sewage pumping station, grating hall, horizontal sand trap, primary settling tanks, biological reactors with activated sludge, horizontal secondary settling tanks, pumping station for recirculated and excess sludge, excess sludge thickeners, two separate fermentation chambers, and sludge hygienisation station [45]. The dominant industries in Suwałki are the agri-food and wood industries. These are industrial sectors that do not emit heavy metals into the environment.
Włocławek, Kujawsko—Pomorskie Voivodeship	Maximum capacity is 48,000 m3/d, it uses the produced biogas to produce heat energy, it produces approximately 160 m3 of primary sludge and approximately 700 m3 of excess sludge every day, and treated sewage is discharged into the Vistula River. The technological line of the sewage treatment plant includes the following facilities and equipment: raw sewage pumping station, two dense grates, three-chamber sand trap with a sand separator, two rectangular primary settling tanks, two predenitrification chambers equipped with low-speed mixers, two dephosphatation chambers equipped with medium-speed mixers, three biological reactors equipped with aeration grates and mixers, six secondary settling tanks with scrapers, gravity and mechanical thickeners, two closed fermentation chambers, sludge dewatering presses, and belt dryer [46]. Włocławek is a highly industrialised centre with a very developed chemical industry, which explains the high chromium content in the sewage sludge. One of the largest chemical companies in Poland—Anwil S.A.—operates there (production of PVC and nitrogen fertilisers) and the chemical complex of Orlen S.A. (production of paraxylene and terephthalic acid).
Częstochowa, Śląskie Voivodeship	The nominal capacity is 50,700 m3/d, production of thermal energy and devices is generated by biogas, 100% of the demand is for by-products, and purified sewage is discharged into the Warta River. The following products and devices are included in the industrial sewage treatment process: raw sewage pumping station, grating hall, horizontal sand traps, aerated degreaser, primary settling tanks, gravity thickeners, eight activated sludge reactors, six secondary settling tanks, recycled sludge pump, mechanical thickener, separate fermentation chamber, three belt presses for sludge dewatering, and sludge dryer–granulator [47]. It is located in a city with a rich industrial sector with metal industrial equipment. Power sources in Częstochowa include the following: ISD Huta Częstochowa (the largest producer of ship steel sheets in Poland), Koksownia Częstochowa Nowa (the leading coke producer in the country), and “Wulkan” iron foundry (castings for the construction, automotive, and machinery industries).
Strzelce Opolskie, Opolskie Voivodeship	The average daily capacity is 6364 m3/d, the maximum capacity is 15,000 m3/d, and 98% of treated sewage is discharged into the ground (filtration fields). The technological line of the sewage treatment plant includes the following facilities and equipment: raw sewage pumping station, stepped grate, two aerated sand traps combined with a degreaser, gravity thickeners, three activated sludge reactors, recirculated sludge pumping station, screen-belt press, sludge hygienisation station, and infiltration fields [48]. In the area of Strzelce Opolskie, there is mainly a developed industry dealing with wood processing and window production, which does not introduce heavy metals into the environment.

). The heavy metal concentrations collected in this article are from measurements carried out by various authors between 2004 and 2021 [35,36,37,38,39,40,41,42,43,44,45,46,47,48,49]. This analysis allows for the suitability of sludge from the studied wastewater treatment plants for natural management to be assessed in accordance with the Regulation of the Minister of Climate and Environment of 8 November 2022 on municipal sewage sludge [34]. The locations of the analysed rural and municipal sewage treatment plants are shown in Figure 1.

3.1. Determination of Total Heavy Metal Concentrations

The total concentrations of the selected heavy metals (Pb, Cd, Cr, Cu, Ni, and Zn) were determined in the sludge samples by atomic absorption spectrometry AAS-FAAS [35,36,38,39,41,42,43,45] after microwave closed mineralisation in aqua regia or concentrated acids [35,36,39,41,47] or by using inductively coupled plasma—optical emission spectroscopy ICP–OES [31,34] after four-stage sequential extraction (BCR) [38,42,43,45,47]. In the case of Świlcza [37], the authors report that the results presented in their paper are based on data provided by the Wastewater Treatment Plant in Świlcza. For Włoclawek, the samples were determined in the laboratory of the Institute of Environmental Protection, but the determination methodology is not provided [49]. In accordance with [34], there are two reference methods for testing municipal sewage sludge for determining the content of the heavy metals lead, cadmium, mercury, nickel, zinc, copper, and chromium, and these are atomic absorption spectrometry after mineralisation in aqua regia or concentrated acids or inductively coupled plasma—optical emission spectroscopy ICP–OES.

3.2. Statistical Analysis

All calculations were performed using Excel 2013 (Microsoft Corporation, Washington, DC, USA). The occurrence of a linear correlation between analysed variables was evaluated by Pearson’s correlation coefficient (r). A confidence level of 95% was assumed.

4. Results and Discussion

The analysis included in this study was to determine the heavy metal contents of sewage sludge from both rural and urban wastewater treatment plants in terms of its natural use. The concentrations of individual heavy metals were compared with the permissible limits set by current legislation and with the results of the metal content of sludge generated at individual wastewater treatment plants.

Table 3. Heavy metal content in mg/kg d.m. of sludge from rural wastewater treatment plants.

Metals	Dobrzeń [35]	Kamieniec Wrocławski [36]	Świlcza [37]	Kostomłoty-Laskowa [38]	Strawczyn [38]
Cd	1.15	2	0.6	7.3	9.5
Cu	203	524	93.6	9.3	79.4
Ni	22.3	90	6.1	5.7	4.8
Pb	28.6	46	8.8	275.2	68
Zn	575	1732	-	596	1246
Hg	-	3.8	0.3	-	-
Cr	36.9	170	7.5	28.4	28.9

and

Table 4. Heavy metal content in mg/kg d.m. of sludge from urban wastewater treatment plants.

Metals	Białystok [36]	Przemyśl [41]	Ostrowiec Świętokrzyski [42]	Sędziszów Małopolski [40]	Busko-Siesławice [43]	Skarżysko-Kamienna [40]	Sitkówka-Nowiny [45]	Suwałki [45]	Włocławek [49]	Częstochowa [47]	Strzelce Opolskie [48]
Cd	1.3	2.2	2.5	2.7	16.7	12.1	5.6	8.9	1.07	2.8	-
Cu	195	158.5	186.7	32	78.8	21.8	83.5	128	106.8	95.8	47
Ni	23.9	22.5	35.5	33	1.3	28.5	51.9	14	14.8	128.9	45
Pb	21.2	38.9	62.2	30	322.4	31.3	67.7	15.6	37.4	130	6.8
Zn	1054	1104.6	1387	108	840.7	5351.1	1315	1072	662.2	2046	754
Hg	0.8	-	-	0.1	-	-	-	0.7	1.55	-	-
Cr	59.5	43.5	85.6	20	35.2	2759.8	238.5	14.2	367.1	33.8	12.7

The values marked in bold exceed the permissible levels of heavy metals in sewage sludge presented in Table 1.

compare the content of heavy metals in sewage sludge from the sixteen analysed sewage treatment plants. The values marked in Table 3 and Table 4 in bold exceed the permissible levels of heavy metals in sewage sludge presented in Table 1 [34].

According to the data presented in Table 3 and Table 4, no exceedances of the permissible cadmium content in sewage sludge were found in the analysed wastewater treatment plants. The highest concentrations of cadmium were recorded at two municipal wastewater treatment plants—Busko-Siesławice (16.7 mg/kg d.m.) and Skarżysko-Kamienna (12.1 mg/kg d.m.). In the case of the wastewater treatment plant located in Strzelce Opolskie, there is no data on the content of this element in the sludge.

The permissible copper content in sewage sludge was not exceeded in any of the analysed wastewater treatment plants. It is worth noting, however, that the highest concentrations of copper (524 mg/kg d.m.) in sewage sludge were detected at a rural wastewater treatment plant located in Kamieniec Wrocławski.

It can be observed that, in all analysed wastewater treatment plants, nickel concentrations did not exceed the limit values. It can also be observed that there was little overall sludge contamination with this element. In only one of the analysed wastewater treatment plants did the nickel concentration exceed 100 mg/kg d.m.–128.9 mg/kg d.m., which was at the wastewater treatment plant located in Częstochowa. Slightly elevated concentrations may be related to the activities of the metal industry in the city. Częstochowa is a city with a rich industrial tradition and a developed metal industry.

No exceedances of the limit values for lead were recorded at the analysed wastewater treatment plants; the highest concentration of lead was in the sewage sludge coming from the municipal wastewater treatment plant located in Busko-Siesławice, and it contained 322.4 mg/kg d.m. of this element. Slightly increased cadmium and lead concentrations may be caused by the national road No. 73 running through the centre of the city of Busko-Zdrój, which is a road with heavy traffic.

According to the data in Table 4, at the municipal wastewater treatment plant located in Skarżysko-Kamienna, the level of zinc (5351.1 mg/kg d. m.) exceeded the requirements for the use of sewage sludge in agriculture, for land reclamation for agricultural purposes, for land reclamation for non-agricultural purposes, for the adaptation of land to specific needs, for the cultivation of plants intended for the production of compost, and for the cultivation of plants not intended for consumption and the production of fodder. The area of Skarżysko-Kamienna is home to industrial plants, which, combined with the high volume of traffic in the city, is not indifferent to the heavy metal content of the sewage sludge from the city’s wastewater treatment plant. No exceedances of the limit values were found at the other wastewater treatment plants. A slightly increased zinc concentration of 2046 mg/kg dry weight was observed in sewage sludge from the wastewater treatment plant located in Częstochowa, which may be (like the increased nickel concentration) related to the activities of the metal industry in the city. No data are available on zinc concentrations at the rural wastewater treatment plant in Świlcza.

Mercury concentrations in the analysed wastewater treatment plants did not cause exceedances of the limit values, with as many as nine cases of sludge being characterised by the absence of mercury contamination (Dobrzeń, Kostomłoty-Laskowa, Strawczyn, Przemyśl, Ostrowiec Świętokrzyski, Busko-Siesławice, Skarżysko-Kamienna, Sitkówka-Nowiny, Strzelce Opolskie). The highest mercury concentrations were recorded at a rural wastewater treatment plant located in Kamieniec Wrocławski (3.8 mg/kg d.m.). In the case of the wastewater treatment plant in Częstochowa, there are no data on mercury concentrations in the sludge.

As in the case of zinc content in sewage sludge, the only treatment plant where exceedances of the limit values for chromium were observed was the municipal wastewater treatment plant in Skarżysko-Kamienna. The high concentration of chromium (2759.8 mg/kg dry weight) disqualifies the use of the sludge in agriculture, for land reclamation for agricultural purposes, for land reclamation for non-agricultural purposes, for adapting land to specific needs, for growing plants intended for the production of compost, and for growing plants not intended for food and feed production. In other cases, the chromium content of the sewage sludge did not result in exceedances. Close to the limit value for the use of sewage sludge in agriculture and for land reclamation for agricultural purposes was the sludge from the wastewater treatment plant located in Włocławek. The chromium concentration there was 443 mg/kg dry matter. Włocławek is a highly industrialised centre with a very strongly developed chemical industry, which explains the high chromium content in the sewage sludge. It is home to one of Poland’s largest chemical companies, Anwil S.A. (production of PVC and nitrogenous fertilisers) and the chemical complex Orlen S.A. (production of paraxylene and terephthalic acid).

The sewage sludge from the wastewater treatment plant in Przemyśl did not exceed the limit conditions specified in the Regulation [34]. Przemyśl is not one of the highly industrialised areas, dominated by the wood, clothing, and furniture industries. There is one factory producing household chemicals—Pollena Astra—and one cosmetics factory—Inglot. Ostrowiec Świętokrzyski is a heavily industrialised city with strong metallurgical and foundry traditions. The largest industrial plant located on its territory is Huta Ostrowiec, which manufactures steel products. An industrial zone of 500 ha is located around the steelworks, where other companies from the metallurgical processing sector are also located. Despite the industrialised nature of the city, no exceedances of the limit values given in the Regulation [34] were recorded in the sewage sludge coming from the wastewater treatment plant located in Ostrowiec Świętokrzyski.

The areas from which wastewater flows to the treatment plant located in Sędziszów Małopolski are characterised by low industrialisation. The treatment plant mainly receives wastewater from households [38]. This fact explains the very low content of heavy metals in the sewage sludge from this treatment plant.

The dominant industries in the Suwałki area are the agro-food industry and the timber industry. These are industries that do not emit heavy metals into the environment. The absence of heavy industry means low levels of heavy metals in the sewage sludge, which is confirmed in Table 4, and there were no exceedances of the limit values.

As a result of treatment processes, heavy metals in wastewater are removed from it and accumulate in the resulting sludge. Due to the increasing number of wastewater treatment plants being built and modernised as well as an increase in the population using the sewage network, the amount of sewage sludge generated will increase in the coming years. Sewage sludge containing certain quantities of metals can be treated and managed in various ways. Thermal sewage sludge treatment methods are not only environmentally safe but are also an economically viable solution. Until 2016, the predominant method of sludge management had been landfilling, although this is now prohibited. Another common method is the natural use of sludge, although this is linked to compliance with strict standards in terms of heavy metals.

Typically, the contaminant metal concentrations in sewage sludge decrease in the order Zn > Cu > Cr ≈ Pb ≈ Ni > Cd [50]. The analysis showed that the trend of metal concentrations in sludge from rural WWTPs was as follows Zn > Cu > Cr > Pb > Ni > Cd for Dobrzeń, Zn > Cu >Cr > Ni > Pb> Hg > Cd for Kamieniec Wocławski, Cu > Pb > Cr > Ni > Cd > Hg for Świlcza, Zn > Pb > Cr > Cu > Ni > Cd > Hg for Kostomłoty-Laskowa, and Zn > Cu > Pb > Cr > Cd > Ni for Strawczyn.

In the case of sludge from urban WWTPs, the relationships between heavy metal concentrations were as follows: Zn > Cu > Cr > Ni > Pb > Cd > Hg for Białystok, Zn > Cu > Cr > Pb > Ni > Cd for Przemyśl and Ostrowiec Świętokrzyski, Zn > Ni > Cu > Pb > Cr > Cd > Hg for Sędziszów Małopolski, Zn > Pb > Cu > Cr > Cd > Ni for Busko-Siesławice, Zn > Cr > Pb > Ni > Cu > Cd for Skarżysko-Kamienna, Zn > Cr > Cu > Pb > Ni > Cd for Sitkówka-Nowiny, Zn > Cu > Pb > Cr > Ni > Cd > Hg for Suwałki, Zn > Cr > Cu > Pb > Ni > Hg > Cd for Włocławek, Zn > Pb > Ni > Cu > Cr > Cd for Częstochowa, and Zn > Cu > Ni > Cr > Pb for Strzelce Opolskie. As can be observed, the results varied slightly, but in all cases, Zn and Cu showed the highest concentrations and Cd the lowest. Overall, the distribution pattern found in the WWTPs was similar to that found for sludge produced in other WWTPs [44,50,51,52].

In this assessment, the concentration ranges of Cd, Cu, Ni, Pb, Zn, Cr, and Hg in the sludge samples from rural wastewater treatment plants were 0.6–9.5, 9.3–524, 4.8–90.0, 8.8–275.2, 575–1732, 7.5–170.0, and 0–3.8 mg/kg dry weight, respectively. In the sludge samples from urban wastewater treatment plants, the concentration ranges of Cd, Cu, Ni, Pb, Zn, Cr, and Hg were 1.07–16.7, 32–195, 1.3–128.9, 21.2–322.4, 20–5351.1, 12.7–2759.8, and 0.1–1.55, respectively. This assessment suggests that sewage sludge from rural wastewater treatment plants can be used for natural management, but in the case of municipal sludge, some sludge should not be used directly for agricultural purposes unless it is remediated to reduce the heavy metal load to a permissible level. Metal concentrations in sludge can vary from country to country, but typical metal concentrations in all countries are similar to those presented in this assessment [9,51,52,53,54,55,56,57,58,59,60,61]. However, it should be noted that metal concentrations in European and Japanese sludge are generally lower than in sludge from other countries [50].

The Pearson correlation test was used to analyse the possible relationship between heavy metal contents in sewage sludge from rural and urban WWTPs (

Table 5. Correlation matrix (Pearson correlation coefficients (r)) for metal concentration values in sewage sludge from rural WWTPs.

	Cd	Cu	Ni	Pb	Zn	Hg	Cr
Cd
Cu	−0.4817
Ni	−0.3956	0.9812
Pb	0.5915	−0.4409	−0.2690
Zn	0.3023	0.6863	0.7245	0.0314
Hg	−0.3372	0.9366	0.9726	−0.2379	0.7100
Cr	−0.2348	0.9449	0.9852	−0.1569	0.8146	0.9708

Coefficients in bold are statistically significant at p < 0.05.

and

Table 6. Correlation matrix (Pearson correlation coefficients (r)) for metal concentration values in sewage sludge from urban WWTPs.

	Cd	Cu	Ni	Pb	Zn	Hg	Cr
Cd
Cu	−0.3268
Ni	−0.3284	0.3887
Pb	0.6146	−0.2352	−0.1125
Zn	0.3962	−0.0135	0.2758	−0.0782
Hg	−0.2439	0.8324	0.3161	−0.1971	0.0114
Cr	0.3758	−0.1947	−0.0171	−0.1410	0.9010	−0.0504

Coefficients in bold are statistically significant at p < 0.05.

).

The Pearson correlation coefficients (r) shown in Table 5 indicate a strong positive correlation between Cu and Ni, Cu and Cr, Cu and Hg, Ni and Hg, Ni and Cr, Hg and Cr, Zn and Cr, Zn and Hg, and Zn and Ni in sludge from rural WWTPs. In the case of sludge from urban WWTPs (Table 6), a strong positive correlation exists between Zn and Cr, Cu and Hg, and Cd and Pb.

5. Conclusions

• Due to the presence of biogenic elements and organic matter in its composition, sewage sludge can be used to fertilise the soil and can also be used as a substitute for organic fertilisers.
• The natural use of sewage sludge is considered the cheapest method for its management. The use of sewage sludge has ecological benefits as the fertilising substances contained in the sludge are reused.
• The analysis showed that the concentrations of heavy metals in sewage sludge from Polish wastewater treatment plants are very low. Only one of the sixteen analysed sewage treatment plants exceeded the permissible values.
• Failure to meet the required criteria occurred at the municipal sewage treatment plant located in Skarżysko-Kamienna. This is due to the fact that the city is a large industrial centre with numerous production plants as well as a transport hub where two national roads intersect.
• Rural treatment plants tend to have a lower heavy metal content in sludge due to the nature of the wastewater delivered to them, which is mainly domestic wastewater.
• In most of the analysed sewage treatment plants, the concentrations of heavy metals in sewage sludge are several times lower than the most restrictive limits applicable to the use of sewage sludge in agriculture and for agricultural purposes.
• This assessment, therefore, suggests that sewage sludge from rural wastewater treatment plants can be used for natural management, but in the case of urban wastewater treatment plants, some sludge should not be used directly for agricultural purposes unless it is remediated to reduce the heavy metal content to an acceptable level.