AWWA WQTC69276

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 and optimizing treatment performance in conventional drinking water treatment plants. The overall objective of this study was to examine the potential impact of recycling backwash water on natural organic matter (NOM) removal in coagulation-sedimentation processes. Specifically, a series of jar tests were conducted with low turbidity source water from two surface WTPs in Nova Scotia, Canada to investigate the impact of recycling untreated backwash water residual streams on organic removal in aluminum sulphate (e.g., alum) coagulation-sedimentation processes. Backwash water samples from two different main treatment trains were used in the bench-scale study to more closely examine the influence of physical and/or chemical coagulation mechanisms in recycle designs. FBWW from a conventional filtration plant (Windsor, Nova Scotia, Canada) that contained precipitated aluminum hydroxide (Al(OH)₃) solids and membrane backwash water (MBWW) from a microfiltration plant (Hantsport, Nova Scotia, Canada) that does not use a coagulant within the main treatment train were used in the study. Analysis of the settled 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 improved removal of NOM found in this study under FBWW recycle design indicate that enhanced organic removal may be more closely linked to charge neutralization associated with recycling precipitated aluminum hydroxide flocs present in residual streams as opposed to purely physical impact (i.e., number of collision sites or ballasted flocculation) mechanisms as evaluated with the MBWW recycle experiments. Includes 7 references, table, figures.