Impact of treated wastewater sludge on soil and wheat growth characteristics in a semi-arid climate

Sludge accumulation as a byproduct of wastewater treatment is an increasingly growing concern which can be addressed by utilizing the sludge as a soil amendment. A two-year study was conducted to valorize sludge as an organic amendment on soil cultivated with rainfed wheat (Triticum icaversea) and its impact on soil properties, microbial activity, wheat yield and grain quality. The study was conducted in Lebanon, where the sludge accumulation problem is especially important given the absence of proper treatment and disposal methods. Baseline characterization of sewage sludge collected from secondary (SS) and tertiary (TS) wastewater treatment plants showed that both sludge types can be classified as suitable for restricted agricultural use (Class B), which cannot be used on soils to grow fruits or vegetables that are eaten raw. Post-harvest analysis of the amended soils revealed a significant enhancement in organic matter (OM), soil moisture, wheat yield and grain quality in both seasons in SS and TS treatments compared to the control. All tested heavy metals in the sludge were much lower than the allowable limits for agricultural soils, except for zinc (Zn). Wheat biomass and grain quality improved with a significant increase in grain yield (61–76 %) in both treated soils (SS: 74 g/m², TS: 81 g/m²) compared to the control (46 g/m²). Under the TS treatment the grain had the highest protein content (14.5 %) in the first season. Soil microbial analysis were not consistent in the two seasons, but showed a potential risk of total coliforms contamination with SS application in the second season. This research provides valuable insights into the positive effects of treated sewage sludge application on soil fertility and wheat grain quality emphasizing the potential benefits of this sludge in sustainable agriculture. It also highlights the need for monitoring sludge and soil quality.