Investigating calcined filter backwash solids as supplementary cementitious material for recycling in construction practices Tarique Ahmad ⇑ , Kafeel Ahmad, Mehtab Alam Department of Civil Engineering, Jamia Millia Islamia, New Delhi, India highlights  Filter backwash solids (FBS) are selectively investigated for potential utilization.  Sieve analysis, XRF, SEM, XRD, TG-DTA etc. techniques are used for characterization.  FBS is calcined at 800 °C and investigated as supplementary cementitious material.  Calcined FBS is found suitable for partially replacing cement in construction.  Recycling of FBS in construction would provide sustainable disposal option. article info Article history: Received 9 November 2017 Received in revised form 11 April 2018 Accepted 29 April 2018 Keywords: Filtration Filter reject Recycling Cement Strength Construction abstract Backwashing of filter beds in the water treatment plant produces a large volume of spent filter backwash water containing finer solid particles as waste/residue. In this study, solids present in the spent filter backwash water have been collected, processed, and calcined at 800 °C to prepare calcined filter back- wash solids (CFBS) and investigated for its potential utilization as supplementary cementitious material in construction industry. Up to 20% replacement of 43 grade ordinary Portland cement, the cement-CFBS mixture complies the Indian standard specification required for construction. The study concludes that WTS obtained from filtration units in the form of CFBS is one of the best sustainable and beneficial reuse options. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction Water treatment plants (WTPs) generate a large volume of waste or residue known as Water treatment sludge (WTS) or Water treatment residuals (WTR), during the treatment of raw water for municipal supplies. Raw water often carries contaminants such as clay minerals, sandy and loamy particles, organic matter, microorganisms, and other impurities from the discharge of urban and industrial effluents and other human activities, depending on the source (surface water or groundwater). Contaminant particles are agglomerated during the coagulation-flocculation process using chemical coagulants and further removed from the liquid phase by sedimentation and filtration processes. Larger discrete as well as flocculent particles get removed in the clarifier or sedi- mentation tank and are drawn as clarifier sludge. Suspended finer particles manage to escape from the clarifier/sedimentation tank, are collected on the rapid sand filters. Exhausted filter beds are cleaned through the batch process of backwashing. The residuals generated during the backwashing process are called spent filter backwash water (SFBW) that contain fine solid particles. The clar- ifier sludge plus SFBW form the WTS. The volume of WTS discarded as waste depends on the quality of raw water and the characteris- tics of operational units involved in its treatment. Typically, a WTP produces about 100,000 ton of WTS per year and globally, it is esti- mated that at present daily production of WTS exceeds 10,000 ton [1,2]. Disposal of such a large volume of WTS in an environment- friendly manner remains a challenging task for scientist and engi- neers under strict environmental legislation. In India and other developing countries, WTS from most of the WTPs is discharged directly into nearby river, stream, or drainage basin, causing signif- icant damage to the environment. It leads to the accumulation of aluminium and iron (commonly used as coagulants during https://doi.org/10.1016/j.conbuildmat.2018.04.227 0950-0618/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: tariqueahmadamu@gmail.com (T. Ahmad). Construction and Building Materials 175 (2018) 664–671 Contents lists available at ScienceDirect Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat