It is therefore predictable that regulations and economics will force careful water treatment and stress water re-use. Fortunately, there are water treatment methods that can help you meet the water quality challenges, and related water supply challenges, you face. Among these is crossflow filtration, a technology that has emerged as one of the most effective.
The most familiar type of filtration in water treatment is 'normal' filtration where all influent passes through a filter medium that removes contaminants to produce higher quality water (see illustration). Rough screens, sand filters, multimedia filters, bag filters and cartridge filters are examples of filtration products that operate this way to remove 0.1 micron particles or larger.
Once the medium becomes loaded, it can be backwashed as with multi-media filters or discarded and replaced as with cartridge filters. The method of obtaining clean filtration medium is based on economic and disposal concerns.
Although there are few choices of medium to reject substances smaller than 0.1 micron, the most popular by far is the polymeric membrane, packaged into a membrane element. Millions of small pores per square foot of membrane allow water to pass through it while undesired substances are retained on the influent side.
In this way, crossflow filtration is similar to 'normal' filtration, but mechanical, chemical and economic factors make it impossible for membranes to operate as normal filters do.
First, the polymeric membrane is actually coated onto a support layer. Reversing flow through the membrane may separate the membrane layer from its support, or may separate the membrane’s glued edges.
Secondly, continuous buildup of contaminants on the membrane surface from the precipitation of solids hinders water flux and contaminant rejection. Lastly, the membrane is expensive compared to sand or small cartridge filters, so frequently replacing it isn’t viable.
The solution is to operate membranes in the crossflow mode. By doing so, rejected contaminants are continuously carried away from the membrane surface, thereby minimizing contaminant buildup, leaving it free to reject incoming material and to allow free flow of purified water. Although membrane cleaning is periodically required, the self-cleaning nature of crossflow filtration lengthens membrane life enough to make it economically attractive.
Crossflow operations typically fall into one of three categories: ultrafiltration (UF), nanofiltration (NF) and 'hyperfiltration,' the process more commonly referred to as reverse osmosis (RO). The most common of the three is RO because of its comprehensive capability to reduce levels of dissolved impurities.
In normal filtration (left), all water passes through a filter medium. In crossflow filtration
(right), a portion of feedwater is used to carry away contaminants.
Crossflow filtration is carried out in a machine that includes the membrane element and housings, interconnecting piping, pumps, prefilters and controls and instrumentation necessary for operation.
Because the membrane accounts for 15 to 40 percent of the price of an RO or UF machine and because membranes must be replaced periodically, careful membrane selection is essential. Many types are available and each has unique characteristics. Selection criteria should include chemical tolerance, mechanical suitability, cleanability, separation performance, flow performance and price.
As with other mechanical devices, crossflow filtration machines function best when design and materials enhance one another. Once the correct membrane is chosen, a design that provides for appropriate crossflow rates, pressures and permeate recoveries is critical. Good design should manifest itself in a machine that has consistent performance, needs less frequent membrane cleaning or replacement, consumes reasonable amounts of power and requires little operational attention.
Much of this can also be achieved be pretreating the crossflow unit's feedwater. There is probably no better investment than in equipment that properly prepares feedwater for membrane treatment because it is relatively inexpensive yet provides tremendous benefits.
Cartridges or bag filters that remove residual insoluble material to about .5 microns are often a must. Multi-media filters that can remove turbidity and oxidized metals like iron and manganese efficiently are often desirable, as are chemical pumps that can inject acid or antiscalants to keep salts soluble, or biocontrol agents to prevent biofouling. Depending on the nature of the feedwater, other equipment such as clarifiers or carbon filters may also be appropriate.
With good pretreatment, the membrane in a crossflow machine will bear no burden greater than what's intended for it. The result is optimum performance and the lowest overall cost.
Crossflow filtration provides unique separation and purification capabilities, particularly to reduce levels of small, dissolved impurities normally less than 0.1 micron in size. The crossflow process continually sweeps the membrane surface, minimizing buildup of rejected impurities to make membrane filtration processes cost effective. Successful operation depends on good membrane selection, machine design and pretreatment.