Treating Microfiltration Backwash

Abstract

A recent project focused on the recycling challenges posed by the potential presence of the pathogen Cryptosporidium in microfiltration (MF) residuals. After reviewing the various methods available for treating MF backwash streams, clarification was selected because it (1) is successful in treating spent filter backwash water from conventional plants, (2) is economical, and (3) is familiar to water plant operators and US regulators. Successful results from bench‐scale treatability tests guided the pilot‐ and full‐scale studies. Microbial performance indicators for the pilot test included inactivated Cryptosporidium oocysts and aerobic endospores. At a sedimentation overflow rate of 0.25 to 0.5 gpm/sq ft (0.6 to 1.2 m3/m2/h) and with the appropriate coagulant type and dosage, coagulation/clarification of MF backwash streams produced a recycle quality similar to the original raw water. Unlike spent filter backwash water from conventional water treatment plants, polymer alone was not sufficient to treat MF backwash because the feedwater had not been previously coagulated with a metal salt coagulant.

Recycle of waste backwash water in ultrafiltration drinking water treatment processes

“…The remaining 5 to 10% of the total feed water is converted to a concentrated waste residual stream, which is generated from backwashing and chemical cleaning operations, with backwash flows generally representing 95 to 99% of the total residuals volume (AWWA 2008). The characteristics of MF and UF backwash waste residual streams have been shown to be consistent with the raw water quality with respect to dissolved inorganic and organic constituents, and with elevated concentrations of particulate matter that is rejected by the membrane, including pathogens and suspended solids (Adham et al 1996;MacPhee et al 2002;LeGouellec et al 2004). A study that investigated treatment requirements for MF residuals (MacPhee et al 2002) found that bulk backwash samples collected from four membrane water utilities exhibited very similar qualities to conventional waste filter backwash water (FBWW) samples collected in Comwell & MacPhee (2001).…”

“…Previous studies have demonstrated that settling of spent filter backwash water was enhanced with addition of only polymer (Comwell & MacPhee 2001), and improved sedimentation of MF and UF backwash water required addition of a metal salt coagulant when the source water was not coagulated (LeGouellec et al 2004). Zetapotential (ZP) measurements showed that the particles present in the FBWW samples were predominately destabilized (i.e.…”

“…Low-pressure membranes (i.e. microfiltration (MF) and Ultrafiltration (UF)) are capable of achieving complete removal of pathogens and particulate matter through physical sieving based on size exclusion (Jacangelo et al, 1989(Jacangelo et al, , 1995Dwyer et al, 1995;Hagen 1998;MacPhee et al, 2002;LeGouellec et al 2004). Low-pressure membranes are classified by the size of their pores.…”

Assessing the feasibility of wastewater recycling and treatment efficiency of wastewater treatment units

“…Therefore, water recycling and reuse become an urgent issue in Taiwan. Wastewater accounts for 2-10% of the total water production in water treatment plants (Cornwell and MacPhee 2001;Dotremont et al 1999;Le Gouellec et al 2004;Nasser et al 2002;Vigneswaran et al 1996). Wastewater contains the backwash water from the rapid sand filter and the drained muddy water from the flocculation/sedimentation basins and the clarifier.…”

Effect of coagulation mechanism on membrane permeability in coagulation-assisted microfiltration for spent filter backwash water recycling

“…Studies have indicated that pathogens, DBPs and particulate contaminants in SFBW can be effectively removed by processes such as microfiltration (MF) or ultrafiltration (UF) [12,13]. In addition, SFBW recycling by dead-end membrane has been proved economically feasible [14].…”