The reuse of water treatment plant (WTP) waste residual streams such as filter backwash water(FBWW) is viewed as a possible path towards minimizing the generation of waste streams andoptimizing treatment performance in conventional drinking water treatment plants. The overallobjective of this study was to examine the potential impact of recycling backwash water onnatural organic matter (NOM) removal in coagulation-sedimentation processes. Specifically, aseries of jar tests were conducted with low turbidity source water from two surface WTPs inNova Scotia, Canada to investigate the impact of recycling untreated backwash water residualstreams on organic removal in aluminum sulphate (e.g., alum) coagulation-sedimentationprocesses. Backwash water samples from two different main treatment trains were used in thebench-scale study to more closely examine the influence of physical and/or chemical coagulationmechanisms in recycle designs. FBWW from a conventional filtration plant (Windsor, NovaScotia, Canada) that contained precipitated aluminum hydroxide (Al(OH)<sub>3</sub>) solids and membranebackwash water (MBWW) from a microfiltration plant (Hantsport, Nova Scotia, Canada) thatdoes not use a coagulant within the main treatment train were used in the study. Analysis of thesettled water samples from the jar tests included turbidity, color, total organic carbon (TOC),dissolved organic carbon (DOC), UV-254, total aluminum and zeta potential. The improvedremoval of NOM found in this study under FBWW recycle design indicate that enhanced organicremoval may be more closely linked to charge neutralization associated with recyclingprecipitated aluminum hydroxide flocs present in residual streams as opposed to purely physicalimpact (i.e., number of collision sites or ballasted flocculation) mechanisms as evaluated with theMBWW recycle experiments. Includes 7 references, table, figures.
