Microwave drying of sewage sludge: A sustainable additive for improved clay brick manufacturing

Sewage sludge (SS), once considered waste, is now recognized as a valuable raw material with potential for reuse and recycling, aligning with the European Union's push towards a circular economy. This study explores the utilization of dry SS as an organic additive in fired clay bricks, focusing on efficient drying methods like microwaves (MWs). MW drying offers advantages such as significantly reduced drying times compared to conventional methods and the elimination of pathogenic microorganisms. In this research, MW-dried SS (MDSS) was compared with conventionally dried SS (CDSS) in the production of fired clay bricks, with a maximum SS content of 20 %. While an increase in SS mass had a negative impact on brick quality, bricks using MDSS exhibited improved mechanical and durability properties compared to CDSS. Evaluation of the bricks included standard tests like compressive strength, water absorption and initial rate of absorption. The greatest improvements were observed at the lowest MDSS content (5 %), with a 35.35 % improvement in compressive strength, a 11.11 % reduction in water absorption, and a 65.30 % reduction in the initial rate of absorption (IRA). In contrast, at the highest MDSS content (20 %), the improvements were less pronounced, with a 0.96 % improvement in compressive strength, a 6.66 % reduction in water absorption, and a 2.32 % reduction in the IRA. Additionally, assessments of heavy metal content, mineralogical analysis of powdered SS brick samples, soluble salt content determination, leaching tests, ecotoxicity assay using Vibrio fischeri bacteria (a common bioindicator organism), and radioactivity determination were conducted. Surprisingly, ecotoxicity tests indicated toxicity for the sample group of MDSS ZG bricks and control brick. However, additional leaching tests, considering the method's sensitivity, refuted this finding and indicated inert behaviour. Total radioactivity tests generally aligned with typical fired clay concentrations, remaining well below EU maximum limits.