Utilization of sludge from water treatment plant as a filler material in pavements

Abstract

Water treatment plant (WTP) sludge is an inevitable by-product of the water treatment process, such as coagulation, flocculation, clarification, and filtration. It is composed of raw water impurities and primarily hydrolyzed aluminium sulfate since it is a widely used coagulant. The management of WTP sludge is an emerging global problem. Traditional options for the management of WTP sludge, such as landfilling, may no longer be cost-effective and sustainable due to scarcity of land and associated storage issues. The utilization of waste materials removes the need to exploit environmental capital and thereby eliminate any damage to the ecosystem, as well as minimizing any area requirement. Utilizing WTP sludge as a filler in asphalt concrete mix could be a beneficial solution for both environment and society. Mineral filler, an important component of the binder–filler–aggregate composite system, has a major impact on asphalt mixture pavement performance, such as its capacity to withstand the deformation and stress due to the traffic load. When the filler proportions in the mixture change, different characteristics of hot bituminous mixtures are observed, this shows that WTP sludge fillers have a significant influence on the Marshall properties. Optimum bitumen content was estimated to be 5.22%, 5.09%, 5.34%, 5.59%, 5.89% and 6.4% when filler content percentages are set at 0%, 2%, 4%, 6%, 8% and 10% respectively. The corresponding optimum bitumen contents for stone dust filler were 4.51%, 4.3%, 4.7%, 4.12%, and 4.37%. For WTP sludge filler, the respective optimum bitumen content increased with the increase in filler proportions, whereas optimum bitumen values decrease with the increase of stone dust filler. Due to higher bitumen consumption, the design flow, design VMA, and design VFA values for WTP sludge filler at optimum bitumen content are found slightly higher than that of SD. The results have been found to satisfactory with respect to AASTHO standards. All the stability values at optimum bitumen contents meet the Marshall mix design minimum stability requirement of 5.34 KN. The optimum filler content for WTP sludge and stone dust was found to be 4.5% and 9%, respectively. At their optimum filler content, bitumen consumption of WTP sludge mixes was slightly higher than conventional SD mixes. For moisture resistance, WTP sludge filler shows more susceptibility to water action than SD. The leaching concentration outcomes for targeted heavy metals were marginal for all samples relative to the USEPA regulatory limits and will not be likely to pose any significant environmental hazard. The WTP sludge may be productively recycled in asphalt pavement as filler, which will reduce the burden of solid waste management, paving the way for sustainable infrastructure development.