OPERATIONS AND LRV CALCULATIONS AT NORTH DAKOTA’S SOUTHWEST PIPELINE PROJECT OLIVER-MERCER-NORTH DUNN (OMND) DRINKING WATER TREATMENT FACILITY

摘要

This presentation will discuss the operation of a 4 MGD pressurized two-stage Ultrafiltration(UF) plant over a 14 month period at the Oliver-Mercer-North Dunn (OMND) Drinking WaterTreatment Facility. The OMND Treatment Facility utilizes Ultrafiltration followed by lowpressure Reverse Osmosis Softening to produce a blended Finished Water capacity of 3.6 MGD.The 4 MGD capacity of the UF system supplies the feed water for the softening system and asoftening bypass system for a blended Finished Water consisting of 60% softening systemproduct water and 40% UF Product water. The plant is located approximately 12 miles (20kilometers) northwest of Beulah, North Dakota, USA and uses Missouri River water from theLake Sakakawea reservoir as a source of supply. This plant experienced a minor issue duringstartup in that a shipment issue resulted in all the UF membranes being subjected to sub-zeroFahrenheit temperatures (-20 deg. C) during transportation to site. Unlike flat-sheet PVDFmembranes which are shipped dry, pressurized hollow fiber membranes are shipped wet with apreservative solution to ensure the membranes do not dry out. Subjecting the modules tofreezing temperatures can result in freezing of the preservative solution as well as drying out ofthe membrane fibers. Extreme cold sub-zero F temperatures can cause the fibers to becomebrittle and allow them to snap easily with any jarring motion. Solutions to these problemsinclude pinning of the broken fibers and re-wetting of the dried fibers with a 40 wt% ethanolsolution. For the specific plant at OMND only two (2) modules out of one hundred and fifty(150) were significantly damaged by the low temperature and fully replaced under warranty bythe membrane manufacturer. Both of the damaged modules were pinned and returned to themanufacturer. One of the modules was re-wetted back at the factory. The re-wetted modulerecovered back to within the manufacturing Quality Control Release Value (QCRV). Fiveadditional modules had damaged fibers but the damaged fibers were limited in number andrepaired on-site. The five repaired modules, along with the other modules in the plant appearedto "wet-up" during start up and became tighter, passing less air, resulting in lower pressure decayand higher LRV's over a period of a few days.To ensure the integrity of all 150 modules was intact, daily manual logsheet readings along withPLC data was gathered. Data included the start pressure, end pressure and calculated LogRemoval Value (LRV) result from the daily Pressure Decay Test (PDT) also known as DirectIntegrity Test (DIT). To ensure a conservative result the LRV calculations used the Darcy-PipeFlow Model. Based on the USEPA Membrane Filtration Guidance Manual membranemanufacturers have the option of choosing either the Darcy –Pipe flow model which assumesturbulent flow in a breach in the fiber or Hagen–Poiseuille model which assumes laminar flow ora combination of the two. The calculated LRV values are always lower using the Darcy-Pipeflow model. For example the following was written in an independent challenge test reportprepared for the California Department of Public Health for a pressurized UF membrane modulesimilar to the one used at OMND, “The removal efficiency of microspheres was measured at 4.6and 5.3-log when the Darcy model predicted approximately 4.0-log removal and the Hagen-Poiseuille model predicted approximately 4.7-log removal.” By using the Darcy-Pipe model aconservative LRV result is assured.At the OMND Plant calculated LRV values are consistently 4.3 log or greater using the Darcy-Pipe model. Other conservative parameters were also used in the LRV calculations indetermining the minimum test pressure required for detection of a 3 μm or smaller breach in themembrane surface. For the pore-shape correction factor (ĸ) the most conservative value of 1was used. For the liquid – membrane contact angle (Ѳ) zero degrees was used as opposed to ameasured contact angle.Overall integrity performance of the plant for fourteen month period will be discussed in detail.Trans-Membrane Pressure (TMP), flux, temperature corrected permeability (specific flux) andcleaning efficiency for both the first and second UF stages will also be presented.