TREATMENT PERFORMANCE
Many reports are available on the actual performance of MFMicrofiltration (MF) membranes: Microfiltration membranes are typically hollow-fibers with a pore size range of approximately 0.1 – 0.2 μm (nominally 0.1 μm)./UFUltrafiltration (UF) membranes: Ultrafiltration membranes are typically hollow-fibers with a pore size range of approximately 0.01 – 0.05 μm (nominally 0.01 μm). systems. Braker (2010) described the performance of a 10 MGDMGD: MGD stands for Million Gallons per Day and is a unit of flow measure. UF facility operated by the Duck River utility which serves a population of about 60,000 in Tennessee. The UF system is a GE ZeeWeed® 1000 v2 which is an immersed vacuum systemVacuum systems: Vacuum membrane systems are microfiltration or ultrafiltration systems in which the membranes are immersed in a tank of water and a vacuum applied to pull the water through the membrane. with a nominal 0.02 µmµm: A µm stands for one micrometer (or micron) which is a unit of length equal to one millionth of a meter. pore size. The membrane system has five trains with three casettes per train and 72 membrane modules per casette but only contained 60 at the time. The raw water turbidityTurbidity: Turbidity is a measure of the cloudiness of a water as well as a gross measure of the amount of suspended solids in a water. ranged 2–100 NTUNTU: NTU stands for nephelometric turbidity unit and is a measure of turbidity, i.e., the cloudiness of a water, and is a gross measure of the amount of suspended solids in a water. with settled water 0.5–3.5 NTU, and the TOCTOC: TOC stands for total organic carbon in a water. of raw, settled, and permeatePermeate: Permeate is synonymous with filtrate but is typically used with reverse osmosis and nanofiltration systems. water averaged 4.5 mg/L, 2.2 mg/L and 1.0 mg/L respectively. Other plant operating conditions in 2010 were:
• Design Capacity: 6.5 MGD at 21.1 gfd (gallons per square foot per day) net fluxFlux: Flux is the flow rate of water applied per unit area of the membrane and has units of volume/unit area/time..
• Percent Recovery: 95%
• Maintenance Cleans: daily 25 mg/L NaOCl rinse with daily tank drain > 10 hours of operations
• Clean-in-placeClean-in-place (CIP) or in-situ chemical cleaning: Clean-in-place is a procedure performed periodically to clean a membrane more thoroughly than backwashing can achieve in order to restore the permeability of the membrane towards baseline levels. The process uses chemicals such as citric acid and chlorine or others to remove accumulated foulants on the membrane.: flux recovery cleans after over five years of operation were done five times a year on each train (about 11 week interval) with sodium hypochlorite soak (300 mg/L residualsResiduals: Residuals refers to all waste streams from a treatment process. For membrane systems it could include the backwash wastewater, clean-in-place wastewaters, rinse waters, and concentrates. target, 6-8 hours, not heated) and citric acid soak ( 6-8 hours with a target pH of 2.2, not heated)
• Permeate Turbidity: < 0.009 – 0.025 NTU
Additionally, the plant is expecting the membranes to last more than 10 years. The directDirect integrity test: A direct integrity test refers to the process usually performed daily to detect breaches in the membrane system. Integrity tests are pressure-based or marker-based. integrity testsDirect integrity test: A direct integrity test refers to the process usually performed daily to detect breaches in the membrane system. Integrity tests are pressure-based or marker-based. are pressure-based and aim to hold the pressure at 10 psi for five minutes. The average decay is 0.5 psi over five minutes and the control limitControl limit (CL): A control limit in membrane systems refers to a response from an integrity test, for example exceeding a maximum allowable rate of pressure loss, that triggers a response by the operators such as searching for breaches. is set at a decay of 1.5 psi. The plant repairs about 40 fibers every four months by injecting silicone into a fiber to isolate it. The plant has an LRVLog removal value (LRV) or log reduction credit: The log removal value or log removal credit is the filtration removal efficiency for a target organism, particulate, or surrogate expressed as log10(feed concentration) – log10(filtrate concentration). of 4.5-5.5. Braker did recommend considering pretreatmentPretreatment: In membrane treatment systems, pretreatment encompasses all treatment processes prior to the membrane, e.g., strainers, flocculation, and sedimentation. options.
In contrast, Bernard (2010) describes the performance of a Pall pressure membrane system with lime softening pretreatment installed at Murfreesboro, TN. The design criteria were:
• Design Flux: 87.5 gfd
• Percent Recovery: 95%
• Design Temperature: 4°C
• Clean-In-Place Interval: Once/month
o Irreversible Loss of Permeability <15%
o Fiber Breakage < 0.2%
In comparison with the Duck River utility plant that saw about 40 fiber breakages every four months, the Murfreesboro plant saw a total of five fiber breaks in one year out of a total of 3,125,000 fibers all of which occurred in one rack in one day. The percentage of broken fibers was 0.00016%.
Antrim and Allen (2005) describe a Koch UF membrane plant in Littleton, Massachusetts commissioned in 1997 to remove iron (0.3 mg/L) and manganese (1.0 mg/L) as well as remove microbials. The iron and manganese are oxidized with ozone and the water is then passed through a 200 μm prefilter before being sent to the UF membrane. Some of the operational characteristics are:
• Design Capacity: 1.4 MGD at 100-110 gfd
• Percent Recovery: 95% (Note that a secondary filtration system is used to treat the backpulse which allows an over 99.8% recovery)
• Backpulse Interval and Duration: 60-120 min. at 85 gfd and 15 psi for 1 min.
• Clean-In-Place: every 5-10 weeks depending on demand using 0.6% citric acid by weight at 120°F and every third cleaning also using a pH 10.5-11 soak with 200 mg/L sodium hypochlorite
