|Figure 1: Typical membrane filtration configuration.|
Included Processes: Microfiltration and Ultrafiltration
Membrane filtration processes commonly used in water treatment include microfiltration (MF) and ultrafiltration (UF). MF and UF are typically applied for the removal of particulate and microbial contaminants, and are frequently used as an alternative to rapid sand filtration in conventional treatment and softening applications.
MF and UF can be operated under positive pressure or negative pressure (i.e., vacuum pressure). Positive pressure systems typically operate between 3 and 40 psi, whereas, vacuum systems operate between -3 to -12 psi. There is no significant difference between the range of pressures at which MF and UF systems operate. The pressure differential is used to pass the feed water through the membrane to and produce a filtrate stream. Particulate contaminants are retained by the membrane and removed during backwashing of the membrane filter much the same way particulates are removed from a rapid sand filter.
The primary difference between MF and UF is the pore size (exclusion characteristics) of the membranes. Membrane pore size for MF units typically ranges between 0.1 to 0.5 µm; UF pore sizes range from about 0.01 to 0.1 µm. Both MF and UF membranes are primarily used for particulate and microbiological contaminant removal. Particulates removed include suspended solids, turbidity, some colloids, bacteria, protozoan cysts, and viruses (only UF has been demonstrated to remove viruses to any significant degree). Inorganic chemicals (e.g., phosphorus, hardness and metals) may be removed with suitable pre-treatment. Limited dissolved organics removal may also occur by either of these processes, depending on the molecular weight of the compound; powdered activated carbon is sometimes fed upstream of the membranes to improve organics removal.
MF and UF membrane systems frequently require some type of source water pretreatment to: (1) condition the water for optimum membrane effectiveness, and (2) to modify the feed water to prevent membrane fouling and plugging, and maximize the time between cleanings and prolong membrane life. The type of pretreatment required depends on the source water and membrane type. Generally, surface water requires more extensive pretreatment than groundwater due to higher suspended solids and biological matter content. MF and UF membranes typically can tolerate a pH range from 2 to 13.
Temperature can significantly impact membrane performance. Water temperature has a significant impact on water density and viscosity, which impacts MF and UF membrane flux - the rate of product flow through the membrane, typically expressed in gallons per day per square foot of membrane area (gfd). As the viscosity and density increase, the transmembrane pressure required to pass the water through the membrane also increases, resulting in an increase in the specific flux, which is the flux divided by the transmembrane pressure. Feed water quality conditions can also affect the flux because the rate of plugging Flux rates for MF and UF membranes typically range between 40 and 100 gfd. The design flux is typically determined by bench or pilot testing since it is dependent on specific water quality conditions and temperatures.
The recovery is the ratio of the feed and filtrate flow rates and is primarily impacted by the backwashing frequency for MF and UF systems. Typical recoveries for MF and UF systems range between 85% to over 95%. Higher recoveries can sometimes be achieved by recycling backwash water after solids removal.
Both MF and UF membrane systems include routine backwashing to remove foulants from the membrane. Backwashing systems can use chlorinated or unchlorinated water with or without air scour depending on the membrane system. Backwashes are typically frequent (every 5 minutes to hours) and of short duration (3 to 180 seconds) depending on the membrane system and specific feed water quality conditions and treatment requirements. Chemical clean-in-place (CIP) is used periodically to control fouling and reduce the increase in transmembrane pressure that occurs as a result.
Residuals generated from MF and UF systems include the spent backwash and spent cleaning solutions. Spent backwash may be recycled to the process to increase system recovery, reduce chemical doses, and improve overall treatment performance. Otherwise, disposal of spent backwash is generally accomplished by discharge to a sanitary sewer or receiving stream, much the way spent backwash from a rapid sand filter would be handled. Spent cleaning solutions are generally acidic in nature and require neutralization prior to disposal.