Effect of alkaline treatment on pathogens, bacterial community and antibiotic resistance genes in different sewage sludges for potential agriculture use

ABSTRACT

Alkaline treatment is widely used to reduce pathogens in sewage sludge in developing countries and guarantee that it is safe for use in agriculture. The aim of this study was to investigate the effect of alkaline treatment applied to waste-activated (WAS) and Upflow Anaerobic Sludge Blanket (UASB)-sludge on the bacterial community, pathogens (viable helminths eggs and Salmonella spp), and antibiotic resistance genes (ARG). The bacterial community structure was examined through denaturing gel gradient electrophoresis (DGGE), targeting 16S rRNA genes. Polymerase chain reaction (PCR) was applied to evaluate the presence of several ARGs. The conducted alkaline experiment consisted of adding hydrated lime (Ca(OH)₂) to sewage sludges. Samples were taken before and after 2, 24, 48, and 72 hours of treatment. Alkaline treatment changed considerably the bacterial community structure and after 24 hours, shifts in bacterial profiles were more pronounced in the UASB sludge sample than in WAS. Some bacteria remained under extreme pH conditions (pH > 12), such as Azospira oryzae and Dechloromonas denitrificans in the WAS samples, and Geothrix and Geobacter in the UASB sludge samples. The values of pathogens and indicators in the sludge after 24 hours of alkaline treatment meet sanitary law regulations and thus the sludges could have the potential to agricultural distribution. It is important to highlight that ARG, which are not currently present in sanitary regulations, were detected in the sludge samples after the alkaline treatment, which could be a concern for human health.

GRAPHICAL ABSTRACT

KEYWORDS:

• Alkaline treatment
• antibiotic resistance genes
• bacterial community
• pathogens
• sewage sludge

Disclosure statement

No potential conflict of interest was reported by the authors.

ORCID

Bruna Coelho Lopes http://orcid.org/0000-0002-6673-0191

Juliana Calábria de Araújo http://orcid.org/0000-0002-5497-8675

Additional information

Funding

We thank Minas Gerais State Research Foundation (FAPEMIG), and Coordination for the Improvement of Higher Education Personnel (CAPES) for supporting this work. The authors received a research fellowship from the National Council for Technological and Scientific Development (CNPq).