Chemical and physical properties of sewage sludge as a substrate for tree seedlings

The disposal of sewage sludge (SS) is an environmental issue in many large cities worldwide. Due to its high nutrients and organic matter contents, the SS can be used as a substrate to produce tree seedlings. However, few studies evaluate the suitability of SS from different wastewater treatment plants (WWTP) as seedlings' substrates. The present study assessed the chemical and physical properties of three SS from different WWTPs and two commercial substrates (CS), aiming to evaluate how the SSs' properties can affect the production of seedlings from two tree species. SS from Alegria, Ilha, and Sarapui WWTP were assessed, and CS was composed mainly of peat and pine bark. As chemical properties, heavy metals, macro, and micronutrients total contents were evaluated. Meanwhile, density and porosity were measured as physical properties. The growth in shoot height was measured for seedlings of two Atlantic Forest species (Colubrina glandulosa and Peltophorum dubium). The SS showed higher nutrients (mainly N and P), and the CS had better physical properties. The Ilha and Sarapui SS are recommended as substrates to produce seedlings of both species. Considering its low macroporosity, the Alegria SS must be mixed or composted with other materials to be used as a substrate for tree species. Although the contents of aluminum, zinc, and other metal were high in the SS, no adverse effects were observed in seedlings. Without chemical fertilizers, SS from Ilha and Sarapui produced seedlings in a shorter period and with higher growth than those grown with CS plus chemical fertilizers.