Recycling of sewage sludge incineration ashes as construction material

Abstract

Sewage sludges that are originating from the municipal wastewater treatment plants of Bursa city have been incinerated with a fluidized bed reactor of 400 tons/day capacity since 2017. Each month, on an average, 615 tons of ash is generated as a waste of the incineration process, which needs disposal. The purpose of this study was to investigate if the sludge incinerator ash can be recycled as a construction material. Stabilization/solidification (S/S) and geopolymerization technologies were applied to the sludge ash with this aim. Portland cement, fly ash from a coal-fired power plant, and marble sludge were used as binders. S/S samples were activated with water, while geopolymer samples were activated with 8MNaOH and NaSilNaOH solutions. The prepared paste samples were air-cured for 28 days and analyzed for compressive strength and heavy metals leaching. S/S samples containing sludge ash yielded a compressive strength level of 21.8 MPa, while geopolymer samples resulted in a compressive strength level of 50.0 MPa. Higher compressive strength levels of several samples containing waste sludge ash than that of the control samples without waste were noticed. Leaching tests showed that heavy metals leaching from the samples prepared with sludge ash were much lower than the legal limit values for toxicity. Both leaching tests and compressive strength tests showed that sludge ash has the potential to be considered as a construction material.