Microwave pyrolyzed sewage sludge: influence on soil microbiology, nutrient status, and plant biomass

Abstract Background Sewage sludge (SS) has been considered a potent source of soil nutrients. However, its direct application to agricultural soils have been discouraged owing to its toxic nature. Therefore, conversion and modification of SS to decrease its toxicity has resulted in advanced methods. Co-pyrolysis of SS with other amendments is an ideal treatment resulting in an environmentally safe and nutrient rich final products with additional properties to sequester carbon. In the present study, a novel biochar was produced through the microwave pyrolysis of SS mixed with zeolite and sawdust. The pyrolysis product was thus characterized for elemental composition, polycyclic aromatic hydrocarbons, via Fourier Transform Infrared Spectroscopy (FTIR), and for its effects on soil microbial characteristics, soil health and plant biomass after soil application. Results Results revealed that, the SS modification resulted in stable product with higher nutrients which further depend on the type and ratio of feedstock used. Its application to soil significantly improved soil chemical and microbiological properties and altered lettuce biomass. Conclusions We concluded that sawdust feedstock promoted nutrient availability in the resulting biochar and induced higher activity of nutrient mineralizing enzymes, whereas zeolite slowed down the release of nutrients from soil and putatively immobilized enzymes. This joint effect of sewage sludge biochar, sawdust and zeolite benefited the plant acquisition of nutrients in comparison with the microbial nutrient uptake. We thus conclude that microwave pyrolyzed SS could be used as a soil enhancer. Graphical Abstract