Sewage sludge chemical attributes can limit the use of this organic residue in agricultural soils

What is it about?

Among the organic wastes, sewage sludges are residues generated from wastewater treatment. In Brazil, only ~41% of the sewage was collected by municipal wastewater treatment plants (WWTPs) in 2014, and ~71% of this quantity (3.76 x 10e9 m3) was treated via primary treatment (only involves organic load reduction), generating 1.5 x 10e5 - 3.0 x 10e5 t y-1 of sludge (dry basis). The generation of sludge would reach ~4.3 x 10e6 t y-1 and raise concerns about its proper disposal if treatment rates reach those of the developed nations. The best sludge disposal strategy at Brazilian conditions would be its agricultural use since tropical soils are frequently characterized by low fertility and low levels of organic matter. It would allow the recovery of valuable nutrients and improvement of soil quality, as well as avoiding costs with mineral fertilizers supply substantial amounts of N, P, Ca, and Mg.

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Photo by Roman Synkevych on Unsplash

Photo by Roman Synkevych on Unsplash

Why is it important?

The levels of nutrients and hazardous elements in sewage sludges are often reported, but most works were limited to few sludge samples, which constrain results interpretation. Brazil lacks off studies relating the sources and treatments of the sewage with the chemical attributes of the generated sludges. It would help selecting the most suitable sludges to be used in agriculture, as fertilizers; choosing the best alternative technology to enhance sludge chemical attributes; and improving Brazilian legislation. The present research has sought to evaluate the impact of the source of the sewage, domestic or mixed (domestic + industrial), and the treatments applied to the sewage, such as liming and redox conditions during biological treatments, on the chemical attributes of the resulting sludges. In pursuit of these aims, sludge samples were collected from 19 WWTPs located in the State of São Paulo, Brazil. The levels of plant macronutrients and of toxic elements (mostly heavy metals) were compared against the literature and the threshold values set by the Brazilian legislations to assess the suitability of the sludges for agricultural applications, either “in natura” (as a residue) or as a product (fertilizer).

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