Agricultural valorization of sewage sludge: Impacts on soil bio-physico-chemical quality and Triticale (X Triticosecale wittmack) production

The current investigation aimed to assess the impact of sewage sludge on the physico-chemical and bacteriological characteristics of soils cultivated with the forage Triticale “X Triticosecale Wittmack." The sewage sludge was sourced from the activated sludge treatment plant in the city of Béja, in northern Tunisia. These effects were compared with those of the unaltered control plot and the plot treated with chemically fertilized ammonitrates. The findings indicated an enhancement in soil organic content (TOC and organic matter) and a notable increase (S-N-K for p = 0.05) in fertilizing elements (N, P, and K) and trace elements (Fe, Cu, Zn, and Mn). Some observed this increase depended on the quantity of sludge added. These outcomes were consistent across both soil horizons examined (0–20 and 20–40 cm). Nevertheless, this practice resulted in a marked rise in soil salinity and contamination by Cd and Pb. The accumulation of Pb in the soil was noticeable with increasing sludge doses of 6, 12, and 18 t/ha, leading to Pb levels surpassing the maximum allowable concentration outlined in the NFU 44-041 standard. The findings revealed that the sewage sludge obtained from Béja contained significant fecal indicator bacteria, particularly fecal coliforms and fecal streptococci (values), as well as enteric pathogens like Salmonella spp. and Pseudomonas aeruginosa. The high nutrient content of the sewage sludge also provided an ideal environment for the growth of these microorganisms. Additionally, an increase in the quantity of sewage sludge (6, 12, and 18 t/ha) resulted in the accumulation of Pb in the soil, surpassing the maximum allowable concentration (MAC) specified in the NFU 44-041 standard. Therefore, it is crucial to enhance and restore the bacteriological quality of the sewage sludge from Béja through natural air-drying or thermal drying before its reuse in agriculture and application on agricultural soils.