Environmental impact of recycling sewage sludge into cementitious matrix: A review

This paper reviews the literature associated with the impact of disposed sewage sludge at the environment and the possibility of recycling sewage sludge into sustainable constructional material. The rapid population growth in India has led to increased demand for drinking water and thereby resulted in the discharge of an enormous quantity of sewage. Globally, the total sewage generation has been higher than the total sewage treatment capacity. Additionally, stringent norms for sewage disposal in surface waters have made it imperative to develop proper infrastructural facilities, and increase the amount of operational sewage treatment plants (STPs). The adoption of physico-chemical treatment processes for the treatment of raw sewage generates higher volumes of sewage sludge. Plant operators face serious challenges in the disposal of the generated sludge, i.e., the requirement of larger disposal areas, higher handling & transportation costs, environmental (groundwater, air, and soil) contamination because of the heavy metals' leaching from the disposed sludge, etc. Hence, the paper includes options for recycling sewage sludge ash (SSA) as a substitute for some of the cement used to make concrete. The present review study aims to understand the impact on the characteristics of the cementitious matrix due to the incorporation of sewage sludge. Also, the study has attempted to identify various mineral admixtures that are expected to enhance the properties of concrete containing sewage sludge. The basic information provided in the paper would help researchers assess the suitability of reusing waste materials containing both organic and inorganic compounds. The decision to reuse waste sewage sludge would depend on whether this waste constituent could compromise the strength of the sustainable cementitious matrix or contribute to releasing toxic constituents from this matrix owing to a high leachability.