1. Introduction
The progress of civilization in the last century has contributed significantly to the improvement of human life quality. As a result of these changes, the water demand and pollutant load of discharged wastewaters have increased [
1,
2,
3]. It is, therefore, necessary to continuously expand and modernize the infrastructure for wastewater collection and treatment. However, this contributes to an increase in the amount of sewage sludge generated as a byproduct of sewage treatment plant processes. So far, no waste-free method or effective solution has been developed to completely eliminate sewage sludge from the environment [
4,
5]. However, there are many methods to utilize the sludge generated this way. Agricultural use of sludge fertilizers is particularly beneficial due to its high soil-forming and fertilizing properties [
6,
7,
8]. To this end, the method of sewage sludge management is primarily determined by the amount and properties of the sludge [
9,
10]. Sludge with high reclamation fertilizer values can be used as an organic fertilizer as long as the micropollutant content does not have a negative impact on the soil environment [
11,
12,
13]. The permissible levels of heavy metals in the application of sewage sludge in Poland and the world are listed in
Table 1.
The chemical forms of metals present in sewage sludge can be identified by sequential extraction or speciation based on the fractionation of compounds. The use of this analytical procedure ensures the separation of the test material into fractions characterized by different degrees of mobility [
19].
Soluble metals, which are highly mobile and readily available, pose the greatest threat to soil inhabitants as micronutrients that enter ground and surface waters move up the trophic chain. Heavy metals in the soil are not immediately absorbed by plants; however, they can slowly form hazardous solutions over time [
20]. Some essential elements, such as Fe, Co, Cu, Cr, Mo, Mn, Se, Ni, and Zn, are required for organisms in trace amounts; however, they become toxic at higher levels. Nonessential elements such as Sb, Pb, Hg, Ag, and As are toxic and not needed by living organisms [
21]. However, most wastewaters and wastes contain heavy metals in amounts sufficient to cause toxicity to crop plants [
22].
Sewage sludge, a byproduct of wastewater treatment, can be managed in several ways. However, the most favorable variant from an ecological standpoint and in terms of a circular economy is its use for agricultural purposes.
Municipal sewage sludge can be used as a substrate for the production of organic fertilizers or plant growth aids, but the most important criterion it must meet is the total content of heavy metals. In that regard, the goal of this study is to confirm that a high concentration of heavy metals in the sludge does not always rule out the possibility of sludge agricultural use. The key, therefore, is the content of metals in fractions that tend to migrate deep into the environment and, thus, can easily enter the food chain.
This study investigates the sewage sludge content of four wastewater treatment plants in Poland using different wastewater treatment technologies. The investigation considers heavy metal concentrations, mobility, and the risk of contamination of the environment. Based on the results, the geoaccumulation index (Igeo), potential environmental risk index (PERI), risk assessment code (RAC), and environmental risk determinant (ERD) were calculated. All indicators were then compared to sewage sludge use regulations in Poland and Europe. It was also determined whether or not the treatment technology is critical in terms of the content of heavy metals in mobile forms. The importance of analyzing the form in which heavy metals are present became apparent when considering their use for agricultural purposes.
3. Results
This section is divided into subsections that provide a concise description of the experimental results, their interpretation, and experimental conclusions. The results of chemical speciation of heavy metals in sewage sludge are shown in
Table 7.
The Igeo index is largely dependent on the heavy metal content of the soil at the site of potential use. Because of this fact, a common site for sludge from all four WWTPs was chosen as a potential sludge use point.
Figure 2 shows the Igeo value for all samples analyzed
For all analyzed cases of sewage sludge, the RAC showed no high risk of environmental contamination. This is due to the low content of heavy metals in the FI fraction. Statistically, sediments collected from WWTP4 have the highest percentage of metals in the FI fraction. The highest percentage was recorded for cadmium from WWTP4, which was 25%, but it did not reach the high-risk level. The outcomes can be considered satisfactory; however, it should be noted that the RAC index only considers the FI fraction, ignoring the heavy metals in the FII and FIII fractions.
Figure 3 shows the RAC value for all sewage sludge samples.
Analyzing the PERI indicator for the studied sewage sludge samples, it can be concluded that copper, cadmium, and zinc are the main heavy metals that pose a threat to the environment. Other heavy metals showed low levels of potential environmental contamination.
Figure 4 shows the PERI values.
The ERD index revealed similar risk levels to the RAC, but it was more accurate due to the inclusion of metals from fractions II and III. As with the RAC, sewage sludge collected from Wastewater Treatment Plant 4 proved to be the most hazardous in terms of potential metal migration.
Figure 5 depicts the ERD values.
For the results of potential risk, for all indices and wastewater treatment plants, a noncompliance table was prepared (
Table 8). The heavy metals listed in the table did not meet the criterion that would qualify the sludge for potential environmental use. As can be seen in
Figure 2 and
Figure 4, indicators comparing total sediment content to content at the point of use were far more critical than indicators considering mobility. Although the metal content of sediment is high, it may be in stable fractions that cannot migrate in soil or vegetation. As a result, it appears appropriate to consider the form in which the metal occurs when evaluating the possibility of natural sewage sludge use.
4. Discussion
This paper presents the analysis of sewage sludge taken from four treatment plants that use various water treatment technologies. The heavy metal content in all samples did not exceed the permissible metal content limit for agricultural use. However, the results of the analyzed indicators of contamination risk were not so encouraging. The most stringent proved to be the Igeo index, which compares the metal content of the sludge with the content in the geological substrate at the site of potential use. According to Igeo, all sediments posed a very high risk of ecological contamination, with zinc and cadmium proving particularly toxic. Another indicator considering metal mobility was RAC. According to the indicator value, the sediments showed a medium or low risk of contamination. This is related to their exchangeable fraction content. With respect to RAC, the most toxic sludge sample was the one taken from Treatment Plant 4: Heavy Metal. In all samples, cadmium and copper proved to be the most toxic and risky. The other metals showed medium or low levels of toxicity. The index proposed by the authors (ERD), which is based primarily on the issue of heavy metal mobility, identified WWTP 4 as having the highest risk of ecological contamination. The other three WWTPs, on the other hand, do not pose a high contamination potential risk. Copper is the heavy metal most likely to penetrate deep into the soil from the sludge samples of all analyzed wastewater treatment plants.
5. Conclusions
This paper examines the risk of environmental contamination caused by the agricultural use of sewage sludge from four wastewater treatment plants in Poland. Each of the analyzed facilities uses different wastewater treatment technology. Sewage sludge from all wastewater treatment plants met the applicable heavy metals limits imposed by legal acts, which is the primary criterion for using sewage sludge as a fertilizer. Conventional indices based solely on total heavy metal content, such as Igeo and PERI, were significantly more critical in assessing the feasibility of sludge use than the ERD and RAC, which also look at heavy metal chemical forms. As a result, most of the metals in the sludge were in a completely stable form, and despite their high concentration, there was no possibility of migration and entry into the crop. The study found that wastewater treatment technology has no significant impact on the total content of heavy metals in the sludge; however, sludge from a treatment plant that uses press dewatering has the highest content in the mobile sections. In all sewage sludge samples, copper was the most mobile heavy metal.