Pyrolysis of Sewage Sludge: Physical, Chemical, Morphological and Mineralogical Transformations

HIGHLIGHTS

• Biochar from sewage sludge (SS) was produced and characterized.

• Pyrolysis increased organic and total carbon.

• Pyrolysis increased micro and macronutrients concentration with exception of K.

• Biochar preserved the mineralogical characteristics of the raw material.

• Pyrolysis is an excellent alternative to transform SS into organic fertilizer.

Abstract

The sewage sludge (SS) use in agriculture has been limited by the Brazilian legislation to a few situations, mainly as a precautionary measure due to inorganic pollutants and pathogens. Thus, a large amount of SS has been accumulated in landfills, with no prospect of use, generating great concern for governments and society. Thermal treatment via pyrolysis has stood out as an option for SS recycling, transforming it into a carbon-rich product known as SS biochar (SSB). Biochar from SS showed good potential to be used for agricultural and environmental purposes. The present study aimed to evaluate the influence of pyrolysis at 300°C on the physical, chemical, morphological and mineralogical characteristics of SSB. In general, pyrolysis increased total carbon, total nitrogen, macro and micronutrient contents, except potassium. Pyrolysis also increased heavy metals (HMs) concentration in SSB. However, HMs values remained below the maximum limits allowed according to the legislation on SS agricultural use. X-ray analysis showed that both SS and SSB present silica (SiO₂) as the main mineral. Pyrolysis also increased the SS surface area (SA) and porosity. In general, results of the present study prove showed that pyrolysis is a technological alternative to enable SS use as a sustainable input in agriculture.

Keywords:
biochar; thermal treatment; elemental composition