Sewage sludge treated with 15% bentonite, vermiculite or biochar was evaluated as a soil amendment in comparison to limed and untreated sludge. Seven treatments were established to two soils, an acid and an alkaline, in three replications, i.e. 2% addition of sludge treated with bentonite, vermiculite, biochar and lime and application of 2% untreated sludge, inorganic fertilization and no sludge or inorganic fertilizers (control). Then, the soil treatments were used in a pot experiment with perennial ryegrass ( Lolium perenne L.) as a test plant. Sludge treated with the clay minerals or biochar improved pH of the acid soil and significantly increased organic matter and available nutrients of both soils compared to control. Although no salinity or sodicity hazard was evidenced, the initial salinity of acid and alkaline soil increased by four‐eight and two‐three times, respectively, upon addition of all sludge treatments, especially that of untreated sludge. Moreover, soil available zinc (Zn) increased by four‐eight times. Soil application of sludge treated with the clay minerals or biochar increased the total aboveground biomass yield of ryegrass in the acid and alkaline soil by 133%–171% and 72%–88%, respectively, compared to control and enhanced nutrient uptake by plants. Furthermore the microbial metabolic quotient indicated lack of low pH and heavy metal stress with addition of sludge to the acid soil. After three harvests of ryegrass, the residual effect of sludge on pH of acid soil and salinity, available phosphorus (P), Zn and boron (B) of both soils still persisted. Thus sewage sludge treated with 15% bentonite, vermiculite or biochar could be applied to soils at a rate of 2% (≈80 Mg ha ⁻¹ ) to serve as soil amendment and fertilizer for grasses and pasture species; however, caution is needed regarding possible P build‐up, Zn phytotoxicity and salinization risks.

From the perspective of finding new, more environmental-friendly methods for the stabilization of sewage sludge to be used in agriculture, sludge treated with 15% bentonite, vermiculite or biochar was investigated as a soil amendment for white clover (Trifolium repens L.) growth, by means of a pot experiment. The sludge treatments, which were applied to two soils (an acid and an alkaline soil), in three replications, were the addition of 2% (≈80 Mg ha⁻¹) treated sludge with the clay minerals or biochar, as well as limed or untreated (air-dried) sludge (for comparison reasons). Additional treatments with inorganic fertilization or neither organic nor inorganic fertilization (control) were also included. The application of 2% sludge to both soils significantly increased salinity compared to the control, which remained below harmful levels for sensitive crops, except for the case of untreated sludge. Furthermore, it significantly increased the soil-available macronutrients N, P and K, and micronutrients Cu, Zn (several times, especially in the case of untreated sludge) and B (up to three times) compared with the control. Moreover, the pH of the acid soil was improved, except for the case of limed sludge, where an undesirable pH increase close to 8.5 was observed. Addition of the treated sludge with the clay minerals or biochar and untreated sludge to the acid and alkaline soil significantly increased the aboveground biomass yield of white clover by 117–233% and 114–153%, respectively, compared to the control, whereas limed sludge had no effect. Plant nutrient uptake increased as well. In general, the effect of sludge on soil microbiological properties and arbuscular mycorrhizal fungal root colonization was ambiguous. It was concluded that 2% soil addition of sewage sludge treated with 15% bentonite, vermiculite or biochar could improve soil fertility and enhance plant growth; however, caution is needed with respect to potential risks of soil salinization or Zn and B phytotoxicities.

The objectives of this study were to evaluate sewage sludge’s stabilization with untested until now materials, such as selective clay minerals or biochar in comparison with liming, for enhancing sludge’s fertilization capacity. Dewatered sewage sludge was mixed with bentonite, vermiculite, zeolite, biochar or lime at rates of 0, 15 and 30%, air-dried and analyzed for pathogens and chemical properties. Almost all fecal indicators of treated sludge with 15% bentonite, vermiculite or biochar were reduced by at least one-logarithmic unit (log₁₀) (indicative value of sludge’s stabilization), whereas those of limed sludge were undetectable. Electrical conductivity of all treatments significantly increased, and the highest values were obtained for untreated (6.1 dS m⁻¹) and limed sludge (above 7.0 dS m⁻¹ for both addition rates). The untreated sludge had the significantly highest water-soluble ammonium-nitrogen (2817 mg kg⁻¹) and phosphorus (263 mg kg⁻¹) concentrations followed by sludge treated with bentonite, vermiculite or biochar, whereas limed sludge had the lowest content. Boron concentration of the untreated sludge was similar to the treated sludge. Total concentrations of heavy metals were far below the legislative permissible levels for sludge’s agronomic use. Nutrients’ total content of treated sludge ranged at levels of similar magnitude to the untreated sludge, except for certain cases where they were increased because of the materials’ composition. Consequently, sewage sludge treated with 15% bentonite, vermiculite or biochar seems to be stabilized, retain bioavailable nitrogen and serve as a fertilizer of macro- and micronutrients. However, potential risks of agronomic use, i.e., soil salinization and boron phytotoxicity, should be considered.