Rhode Island fact sheet on ultrafiltration for wastewater.
Pollution Prevention in Rhode Island
Ultrafiltration is a lower pressure membrane separation technology used to remove suspended solids, oils and other impurities from wastewater. Membrane filtration can be used in pollution prevention to recover/recycle process water and valuable process chemicals. This technology can also be used as a polishing step without the addition of treatment chemicals.
Membrane separation technology is based upon molecular size. The semi permeable membrane in an ultrafiltration system has pore sizes in the range of 0.0025 to 0.01 microns. Pressure is applied to one side of the membrane so that water and low molecular weight compounds in the waste stream flow through the pores as permeate, while the larger molecules and suspended solids flow across the membrane and become part of the concentrate. This process is illustrated in Figure 1.
Figure 1. Flow Across a Membrane
See Data Source
In an ultrafiltration system, wastewater flows parallel to the membrane surface, as compared to the perpendicular flow of ordinary filtration. The cross flow motion of the water within an ultrafiltration system allows high filtration rates to be maintained continuously, whereas the constant build up of solids along the filter surface can cause blockage in an ordinary perpendicular filtration system. The difference in filtration techniques is illustrated in Figure 2. below.
Figure 2. Dead End Filtration vs. Cross-Flow Filtration Systems
See Data Source
The layout of a typical ultrafiltration recycling system is depicted in the flow diagram below. As shown in Figure 3, spent process water is pumped from a process tank to a holding/settling tank. Depending upon the nature of the solution, it may first have to pass through a pre-filtration step (e.g. bag filter) before being pumped to the holding tank. (This step is necessary if the spend process solution has a high solids content; e.g., spent vibratory solution.) From the holding tank, the UF unit recirculates and concentrates the process solution while providing a steady stream of clean fluid for reuse. The stream of cleanfluid is directed to the clean holding tank where the process operation can draw on its as necessary.
Figure 3. Solution Recycle Using Membrane Technology
See Data Source
There are three primary configurations of ultrafiltration membranes: tubular, hollow fiber, and spiral wound. The tubular membrane is generally used in small flow, high solids loading applications. The construction of this membrane allows easy cleaning, therefore it is the membrane of choice when severe fouling is expected. The hollow fiber design consists of a membrane would into a hollow cylinder. Cylinder diameters vary; the expected solids loading governs the size of cylinder necessary for a specific application. The third configuration of ultrafiltration membranes is the spiral-wound, and it is usually used for high volume applications. The spiral membrane is constructed by rolling a flat membrane that is netted together with specially-designed spacer material. This type of membrane cannot be mechanically cleaned, and is usually reserved for applications where TSS loading is low or has been reduced by pre-filtration.
The feature of ultrafiltration that is most conducive to pollution prevention is its capacity to recycle soap solutions, aqueous cleaners, synthetic coolants and other water-based process solutions. A properly sized UF membrane will remove oils, dirt, and other large micron sized contaminants from the spent process solutions while allowing clean fluid containing much of the original constituents (e.g., soap or coolant) to pass through the membrane as permeate. With this type of technology, most of the basic components of the solution are recycled; only small amounts of the original material need to be added to the cleaning fluid to bring its concentration level back up to full strength. This means substantial reduction in the consumption of process input materials which can translate into considerable savings for the company. Additional potential savings included reduced water costs, sewer fees and sludge disposal costs.
RI DEM Pollution Prevention Program currently has 13 documented case studies on file that demonstrate the successful application of ultrafiltration technology in Rhode Island companies. Each cast study provides a brief description of the company's process and impact that ultrafiltration technology has had on their operation. Case studies also include financial information including capital costs, operating costs, savings incurred any payback period. Copies of these case studies, and additional information on membrane separation technology can be obtained from DEM's Pollution Prevention Program.
State of Rhode Island and Providence Plantations
Department of Environmental Management
Office of Environmental Coordination
Pollution Prevention Program
83 Park Street
Providence, RI 02903-1037
Phone: (401) 277-3434
Fax: (401) 277-2591
Last Updated: February 7, 1996