The Benefits of a Wider TMP and Temperature Operating Ranges
Polymeric hollow fiber Ultrafiltration / Microfiltration (UF/MF) membranes have been used in mainstream water and waste water treatment systems for several decades. The early versions were submerged and operated under vacuum. In recent years, pressurized hollow fiber UF/MF membrane modules have come onto the market and are the preferred choice in most industrial and midsized municipal water and waste water reuse applications.
Pressurized polymeric UF/MF modules are more limited in suspended solids & turbidity tolerance than submerged membranes but are more cost effective in overall system design. With submerged UF/MF membranes, the typical limit of trans-membrane pressure (TMP) is approximately 1 bar or 14.5 psi. With pressurized UF/MF membrane modules, the TMP limits are typically higher at 1.3-3.0 bar (18-45 psi). Though the higher pressure ratings have benefits, the pressurized UF/MF modules are still limited due to the potential for polymeric hollow fiber breakage.
Ceramic membranes are widely recognized to be more robust than polymeric membranes with higher pressure limits but have historically been much more expensive than polymeric. Nanostone Water’s CM-151* Ceramic UF membrane module changes this trend with a price competitive to polymeric pressurized hollow fiber membranes while maintaining the more robust performance inherent with ceramic membranes.
Design and Operating Freedom
The chart below outlines the typical operating TMP and max TMP values of submerged and pressurized polymeric UF/MF membranes compared to Nanostone’s ceramic membrane module. With pressurized polymeric UF/MF modules, the average operating TMP is typically 25-50% of the maximum whereas with ceramic membranes, the average operating TMP is only 10% of the maximum. Overall, the additional TMP ceramic headroom offers operators significantly greater flexibility. For example, with ceramics, extended filtration runs between cleanings become practical if needed and much more aggressive backwashing methods can be applied without risking fiber breakage common with polymeric membranes.
Figure 1: Average TMP vs. Maximum TMP for Various UF/MF Membranes
COLD WATER CASE
With some submerged and pressurized polymeric UF/MF membranes, manufacturers establish flux expectations at a standard temperature of 20°C (68°F) and modify the flux guideline based on viscosities at different temperatures. In cold water applications less than 20°C , the designs have a lower flux expectation.
Table 1: Polymeric UF Design Warm vs. Cold Water
|2 MGD (7,500 m3/day)
Surface Water Treatment
|Design Temperature||20°C (68°F)||4°C (39°F)|
|Polymeric UF System Design||3 trains, 30 modules per train
90 modules Total 55m2 (600ft2) Module
|3 trains, 46 modules per train
138 modules Total 55m2 (600ft2) Module
|Polymeric UF Peak Flux||45 GFD (76 LMH)||28 GFD (48 LMH)|
|Design Change For Colder Water||N/A||53% more area or less flow|
As depicted in Table 1, lowering the operating temperature from a 20°C (68°F) to 4°C (39°F) results in a in permeate flow reduction of over 50%. The remedy for the lower flux is to add a proportional amount of the membrane area in the design which significantly increases both the capital cost and foot print of the system. With a the CM-151* ceramic module, simply sizing the feed pump with more pressure capability for the colder period is all that is required. The foot print of the ceramic system would be the same regardless of the design temperature and the capital cost impact of a larger feed pump is minimal.
HIGHER MODULE PRESSURE RATING
With pressurized UF/MF systems, there are several manufacturers with maximum module pressure ratings between 3-6 Bar (40-90psi).
Figure 2: Maximum Feed Pressures for Common UF/MF Modules
Higher pressure ratings can be a significant benefit by allowing the design to utilize feed pressure available thus avoiding the need for a break tank and re-pressurizing pump required to feed a down stream process. The higher pressure rating afforded by ceramics can also be used to push the UF/MF permeate further and to higher elevations with additional permeate backpressure. Another benefit is the design freedom presented by not operating the membrane near the pressure limits. In a situation where the normal design pressure is near the limit of the module, additional process design controls and devices are needed to avoid pressure increases with the potential to damage the polymeric membrane. A system that operates well below its pressure limits reduces the risk of damaging membranes and simplifies the overall process design.