The longevity of immersed membrane is highly affected by the operating and maintaining condition as follow.
· Installation technique/skill – In addition to the membrane’s build quality, how it is installed is another factor affecting membrane longevity. As discussed here, uneven air flow rates among modules and cassettes can cause physical damages on membranes eventually. It is crucial to maintain evenly distributed air flows across the membrane modules during the operation.
· Exposure to chlorine – Exposure of membranes to free chlorine during the periodic membrane cleaning not only gradually reduces membrane permeability, but also makes membrane prone to breakage. Membrane chemistry and manufacturing methods are the two major factors affecting the chlorine tolerance. Polyviny difluorides (PVDF), polyethersulfone (PES) and polyvinylpyrrolidone (PVP) blends are known to be highly chlorine tolerant, e.g. 500,000 ppm.hr for PVDF (Fenu, 2012). Chlorine tolerance of PES/PVP blends are also reported high at >250,000 ppm.hr (Pilutti and Nemeth, 2003). There had been a perception that polyethylene (PE) membranes were vulnerable to free chlorine. Given the fact that most household bleach bottles for 5-10% NaOCl are made of PE, the low chlorine tolerance of some PE membranes might be related with the additives and manufacturing method. In fact, one manufacturer claims its hollow fiber membranes have a chlorine tolerance of 1,000,000 ppm.hr (Econity Inc., 2012).
According to a recent study (Abdullah and Bérubé, 2013), membrane damage is much more related with the contact time rather than chlorine concentration. Fig. 1 shows SEM pictures of a virgin PVDF membrane in the left as a control and three other damaged membranes by chlorine in the order of the extent of damage. Though the second and the fourth membranes were exposed identically to chlorine at 2,000,000 ppm.hr, the second membrane exposed to high strength NaOCl (44,300 ppm) for short time (45 hr) was damaged substantially less than the fourth membrane exposed to low strength NaOCl (3,600 ppm) for long time (556 hr). The second membrane was less damaged than even the third membrane that was exposed to 3,600 ppm NaOCl for 278 hr, which makes total exposure of 1,000,000 ppm.hr.
Fig. 1. SEM images of virgin membrane (from GE?) and membranes exposed to different exposure doses and concentrations of NaOCl (Abdullah and Bérubé, 2013)
· Membrane fouling tendency – All the factors affecting membrane fouling including HRT, SRT, toxic or harmful components in raw wastewater, etc. indirectly affect membrane longevity by affecting membrane cleaning frequency.
· Cassette design – Flat sheet membrane panels must be tightly held in a cassette to prevent them from moving when scouring air vibrates the panels. In addition the spaces among panels must maintain evenly to prevent air flow channeling, which can eventual cause bent panels due to excessive air flow to some channels. As shown in Fig. 2, the top and the bottom corners of membrane panels have abraded after 10 year of service as a result of the abrasion with rubber holders in cassette.
Fig. 2. Abrasion seen at the top and bottom corners of membrane panels (left: top corner, right: bottom corner, scale in mm) (Nishimori, 2010)
· Prescreening of wastewater – Prescreening of wastewater is a crucial step to protect membranes from the persistent abrasion. It is discusses here, but the persistent scratches of membrane by leaves, plastics debris, small branches, etc. can lead to the failure of membrane integrity eventually. Fibrous materials from fabrics, hairs, etc. can clog hollow fibers, which can be only cleaned by hand pickings that may cause physical damages on membrane surface. Therefore, proper pretreatment is crucial for longer membrane service life.
· Organic solvents are unlikely factor affecting membrane longevity – In theory, the organic solvents contained in incoming industrial wastewater can damage membranes especially when the solvent can dissolve any of the membrane components. However, it is highly unlikely that solvents are a threat to the membranes in realistic MBR condition since commonly used solvents in industry, e.g. benzene, acetone, hexane, DMSO, short chain alcohols, ethers, etc., are readily biodegradable especially when SRT is high at 12-30 days. Even the solvents with more complex molecular structure such as methylethy keton (MEK), dimethyl amine (DMA), dimethyl formamide (DMF), p-nitrophenol (PNP), o-chlorophenol (OCP), trichlorophenol (TCP), dichlorodiethyl ether (DCDEE), fluorescent whitening agents (FWA) based on stilbene-cyanuric type are readily biodegradable in activated sludge condition (— USEPA 600/2-79-163, 1979). Therefore, solvent concentrations under steady state should not be high enough to destroy the membrane as long as MBR runs under reasonable condition. There is no credible research reports that prove the notion of negative solvent effect on membrane in the public domain to the author’s best knowledge.
© Seong Hoon Yoon