Heavy Metal Ultrafiltration

Ultrafiltration membrane technology can be an efficient and economical choice for removing heavy metals from industrial wastewater

Metals are commonly used in manufacturing plants and technical facilities. Production processes for the metal finishing, transportation (automotive, aviation, railroad, subway), electronics (including computers and semiconductor devices), telecommunications and mechanical parts fabrication industries consume vast quantities of heavy metals cadmium (Cd), chromium (Cr), copper (Cu), iron (Fe), nickel (Ni), lead (Pb), zinc (Zn), etc.) and metal-containing chemicals every day. Their wastewater streams often contain toxic metals (e.g. Cu, Ni and Pb), grease, oil, suspended solids and other contaminants.

Federal and local environmental protection regulations and cost minimization strategies demand wastewater treatment prior to direct discharge or reuse. Most methods for the removal of metals are based on sedimentation, filtration or chemical treatment. Since the solubility of different metals varies with the pH (acidity and alkalinity) of the waste stream, chemical physical technology traditionally includes pH adjustment systems to precipitate the metals, followed by dosing of additional chemicals, flocculants and coagulants to make a sludge particle large and heavy enough to settle. A large volume of this residual is produced and the volume of chemical additives required changes constantly throughout process cycles. Changes in the feed as a result of variable waste composition and volume, metal concentration spikes, incorrect chemical dosing or operator error will lead to variations in effluent constituents.

Conventional separation methods are extremely labor-intensive because operators must perform periodic analytical tests and adjust operating parameters to maintain consistent effluent quality that will meet stringent discharge limits. Not only are the chemicals expensive, these systems require frequent cleaning. Some chemicals produce hazardous byproducts as well. Recent studies show that the latest cross-flow ultrafiltration (UF) membrane technology offers a cost-effective option for the separation of heavy metals from industrial wastewater.

Ultrafiltration Membrane Technology
Metal solubility varies with the pH of the solution. At a pH of 10, for example, most heavy metals have concentrations of 1.0 milligrams per liter (mg/l) or less < 1 parts per million (ppm). In the insoluble or precipitated state, the metal particle size is 0.1 micron or larger (> 0.1 µm). A UF membrane has submicroscopic pore size of < 0.01 micron and thus will retain the metal precipitate. 

Cross-flow UF is a pressure-driven filtration process in which the process liquid flows parallel to the membrane surface. Under a pressure of 10 pounds per square inch (psi) to 100 psi, the filtrate passes through the membrane and exits as clear permeate. The rejected species are retained and collected for disposal or recycling. The membrane's performance is measured by the permeate flux and the rejection of the constituent metals.

In addition to the pore size, pore construction is critical to the performance of a membrane. Conventional filters have irregularly shaped pores that permit aggregation of particles at bottlenecks and crevices within the cross section of the filter. The UF membrane pores are asymmetrical and shaped like inverted cones, with smaller diameters on the feed side and larger diameters on the permeate side. Since any particle that passes through a pore continues unimpeded without accumulating within the membrane, UF membrane pores do not plug. Cleaning of these filters is thus easy and inexpensive and routine cleaning allows for repeated use over long periods of time. With proper operation and maintenance, UF membranes will operate for several years without replacement.

UF Membrane Materials and Construction
Advancements in polymer science enable membrane scientists to utilize engineering plastics to improve the structure-property relationships of membrane products. The most common membranes are based on durable polymers such as polyvinylidene difluoride (PVDF), polysulphone (PS) and polyacrylonitrile (PAN). These materials are suitable for continuous, reproducible processing cycles and are cleaned with acid, caustic and/or surfactants. Membrane life expectancy (normally three to five years) is dependent on the process conditions and the cleaning frequency.

Membranes are packaged into tubular, spiral and hollow fiber configurations. The most significant difference among the three is the characteristics of the flow channel. Tubular membranes are open channel designs, ranging from 0.25 inch to 1 inch in diameter, which accommodate wastewater with large particles, high viscosity and/or high concentrations of suspended solids. Tubular membranes process such liquids without channel plugging and extensive prefiltration. Some tubular membranes are designed for enhanced mechanical cleaning using spongeballs to assist standard chemical circulation cleaning of membranes. The ability to mechanically wipe foulant from the surface of the membrane means these systems can be designed to treat heavy-duty industrial waste. It can be safely pushed to very high concentrations and maximum wastewater volumetric reduction levels, relative to the thin-channel membrane configurations.

Compared to tubular membranes, spiral modules have a thin, tortuous flow channel, ranging from 0.020 inch to 0.10 inch in height. The flow channel is constructed of porous netting placed between adjacent layers of flat membrane sheets. The materials are combined with permeate carrier and adhesive, then wound into a cylindrical shape. The feed liquid passes over the netting and membrane. Permeate collects on the low-pressure side of adjacent membrane sheets and travels to the central collection tube. Since the flow channel is not completely open, it is easily plugged by fibers, lint or other suspended solids, making the use of spiral modules limited to water that is largely free of such matter, or cases where the ability to reach high concentrations is not important.

Hollow fiber membranes are made by extruding polymers into the shape of a tube. The flow channel diameter ranges from 0.020 inch to 0.10 inch. Compared to spirals, hollow fibers are more resistant to channel plugging. Hollow fiber may be back pulsed or subjected to reverse flow conditions to achieve optimum removal of foulants.

Performance and Cost Benefits
Ultrafiltration membranes used in wastewater treatment of many industrial plants have proved to be highly successful and offer economic advantages over other filtration and gravimetric methods available. Membrane systems are very simple in design and operation, which is especially important in wastewater treatment. In some cases where precious metals are used, the metal precipitate can be processed for reuse. 

A pilot study conducted at an electropolishing company in Oakland, Calif., showed that UF membranes reduced the heavy metal constituents in the acid wash wastewater by 99 percent in the UF permeate. No cleaning was necessary for more than six weeks, compared to the MF technology that required cleaning every one to three days. While UF membranes incurred an initial investment that was higher than MF technology, the lower chemical costs, fewer inline analyses, reduced maintenance and decreased overall sludge volume and disposal fees led to a 66 percent reduction in labor and a total savings of 68 percent in annual operation cost.

In another study carried out at an automobile manufacturing facility, a UF system replaced a less effective, conventional one based on gravity settling and upflow clarification. The UF process consistently reduced the concentration of nickel and zinc, both toxic contaminants from zinc phosphate wastewater. With the cost savings, the plant estimated that the UF system paid for itself in 3.6 years. This is in sharp contrast to the old system that required comparable capital investment but was yielding a diminishing return of only 2.7 percent of the UF system's net annual savings. At that rate, the old system would have had a payback period of 117.6 years!

Conclusion
Ultrafiltration is an efficient and cost-effective replacement for traditional and microfiltration technology for the removal of heavy metals from industrial wastewater. Due to the submicroscopic pore size and unique pore construction, UF membranes yield consistent wastewater quality that meets stringent discharge and recycling regulations, while demanding less frequent cleaning and reduced maintenance. The elimination of flocculants lead to lower chemical costs, reduction in analytical testing and labor and diminished sludge volume and disposal fees. Either used alone or combined with other membrane technologies such as reverse osmosis for purification, ultrafiltration is becoming a popular wastewater application.

This article originally appeared in the 09/01/2003 issue of Environmental Protection.

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