Use of microwaves to dry sludge from drinking water treatment plants (DWTPs): gas emission and chemical/microbiological/ecotoxicological characteristics of sludge extracts.

Abstract

The quality of sludge from drinking water treatment plants (DWTPs) depends not only on its origin but also on the specific treatment processes applied within the plant. Drying is a crucial step in sludge management, particularly when sludge must be transported or repurposed. Microwave (MW) drying has emerged as a technically and economically viable method for this purpose; however, its environmental impacts (including potential atmospheric emissions and the ecotoxicity of the resulting dried sludge) require thorough evaluation. This study assessed atmospheric emissions generated during microwave drying of sludge from water purification processes and compared the ecotoxicity of wet and dried sludge using various test organisms, including worms, bacteria, microorganisms, and Daphnia magna. No significant emissions of toxic gases such as SO₂, NO₂, or volatile organic compounds (VOCs) were detected during the MW drying process. Moreover, MW treatment effectively sanitized the sludge. Toxicity tests revealed that fresh sludge extracts were more harmful to Daphnia magna and Aliivibrio fischeri than extracts from dried sludge. These findings indicate that microwave drying not only reduces sludge volume but also decreases its ecotoxicity and pathogenic potential, which are critical factors for sustainable sludge management and environmental protection.