# Risk Assessment of Soil Contamination with Heavy Metals from Municipal Sewage Sludge
## 1 Introduction  
Municipal sewage sludge is an unavoidable by-product of wastewater treatment. An increase in the amount of sewage sludge produced has been observed for many years. The amount of generated sewage sludge is only 1-3% of the volume of flowing sewage [1]. Nevertheless, in the case of improper management, sludge may pose a potential threat to the environment. Sewage sludge contains, among others, heavy metals and pathogenic organisms [2, 3]. On the other hand, sewage sludge is of practical importance as it contains organic substances and biogenic elements [4]. Sewage sludge can be used in agriculture as a valuable source of nitrogen and phosphorus, for the production of compost, and for the reclamation of degraded areas [5, 6]. The choice of the method of sewage sludge management is particularly dictated by its quantity and properties [7]. Moreover, it is subject to legal regulations. In Poland, the Act on Waste [8], the regulation of the Minister of the Environment on municipal sewage sludge [9] and the regulation of the Minister of the Economy on the criteria and procedures for allowing waste to be deposited in a given type of landfill [10] are in force in this respect. In Europe, the limits of heavy metals in terms of natural use are regulated by the Council Directive of 12 June 1986 on the protection of the environment, in particular of the soil, when sewage sludge is used in agriculture [11]; in the United States, it is the Code of Federal Regulations [12]; for China, the 2002 \"Standard for Discharging Pollutants into Urban Wastewater Treatment Plants\" [13]; and for areas in southern Africa, \"Guidelines for the Utilisation and Disposal of Wastewater Sludge\" [14]. The limits for the heavy metal content of sludge for natural use are shown in Table 1.  
The source of heavy metals in sludge is wastewater, which is mainly generated by plants using galvanic processes, steel pickling, and the recycling of lead batteries. Table 2 shows the industries that are the source of heavy metal emissions to the environment. Moreover, heavy metals come from domestic sewage, surface run-off, and corrosion of sewage pipes [15]. Heavy metals entering the environment affect all links in the food chain, from microorganisms, through plants and animals, to humans [18; 19]. Therefore, the accumulation of heavy metals in the ground is particularly dangerous [20]. Heavy metals are divided into two groups. The first one, including cadmium, lead, and mercury, is characterised by high toxicity to humans and animals, but lower toxicity for the growth and development of plants. In excess, the metals of the second group, i.e., copper, zinc, and nickel, are more toxic to plants than to animal and human organisms. The increased content of heavy metals can adversely affect the biological properties of soil, cause changes in the food chain, have a toxic effect on plants, and contaminate groundwater. When the permissible content level is exceeded, heavy metals reduce soil fertility, inhibit soil enzymatic activity, and change soil acidity [21]. Heavy metals can be removed from sewage sludge by chemical or biological methods. Chemical methods include ion exchange, adsorption, electrochemical treatment, and membrane filtration. Biological methods include activated sludge processes and biosorption. Although biological methods are low-cost and environmentally friendly techniques, they need large areas and proper maintenance and operation [22; 23; 24]. The aim of the study was to analyse sewage sludge from three different wastewater treatment plants located in central Poland in terms of the content of heavy metals, their mobility, and the risk of heavy metals accumulation in soil. The geoaccumulation index