Evaluation of the effect of using sewage sludge as a fertilizer on the concentration of heavy metals in soil and the economic implications of its application

The costs of fertilising the soil with sewage sludge were reduced to the operating time of the equipment and the working time of the labourers operating the equipment in the two main operations (manure spreading and ploughing), for three sewage sludge application doses, namely 50, 100 and 200 Mg·ha-1. The costs were calculated using the Katalog Nakładów Rzeczowych nr 2-21: Tereny zielone/Ministerstwo Gospodarki Przestrzennej i Budownictwa (2009) and the current prices from Sekocendbud (2023). The effectiveness of fertilisation was assessed by studying the level and change in heavy metal content after the soil was fertilised with sewage sludge at three proportional doses, namely 50, 100 and 200 Mg·kg-1. The sewage sludge used for fertilisation complied with the sanitary requirements for sludge to be utilised for natural purposes (Regulation, 2015). The estimated total cost of sludge application ranged from PLN 12646.19 to PLN 20456.73 per 1 ha for doses from 50 to 200 Mg per 1 ha. The results of the estimation confirmed the hypothesis that the unit cost of fertilisation with stabilised sewage sludge increases with the dose of sludge in relation to the area of fertilised soil and decreases with the increase of the mass of sludge deposited in the soil. Optimising fertiliser costs, therefore, requires selection - increasing the sludge dose per unit area. No contamination of the soil with copper, cadmium, lead and zinc was found despite an obvious increase in the content of these metals when mixed into the soil. The application of sewage sludge, even in multiple doses, did not result in exceeding the permissible limit for the content of these elements in the soil, as defined in the Minister of the Environment Regulation of 2015 (Regulation, 2015).