High fluoride levels in wastewater can be problematic for industries using hydrofluoric acid (HF) as a cleaning agent or etchant, such as the semiconductor, solar cell and metals manufacturing industries. HF is also used as a reactant or catalyst in the plastics, pharmaceutical, petroleum refining and refrigeration industries. Fluoride concentrations from these industries may range from 100 to more than 10,000 ppm in rinse waters, but discharge limits are often below 20 ppm.
Today, recycle and reuse are needed for any viable water source. To reuse or recycle fluoride waters, levels of less than 5 ppm are often required. Onsite treatment is essential to economically support manufacturing facilities. For industrial applications, a Calcium Hydroxide solution (Lime) is frequently used to force the precipitation of fluorides as Calcium Fluoride. This process is good for treatment to approximately 20 ppm fluoride. To achieve levels of 5 ppm or less, a secondary precipitation step using aluminum sulfate or alum is needed. To consistently achieve treatment levels below 5 ppm after precipitation, membrane filtration is required.
The Porex Tubular Membrane Filter (TMF™) modules are specifically designed for this type of high solids treatment system. The exclusive patented materials and processes provide extremely durable and consistent membrane modules that have been in use for more than twenty years. TMF modules will assure consistent, low maintenance performance. Porex Filtration provides system design and operation assistance to OEM partners assuring a reliable, efficient and cost effective treatment system.
The TMF system design (see above) is a cross-flow membrane system that uses the filtrate flow inside the membrane module tubes to continuously scour the membrane surface. This action cleans the membrane surface, greatly increasing flux rates when compared to conventional hollow fiber membranes. The pretreated water with solids flows through a series of membrane modules placed end to end. Solids concentration in the recirculated influent stream typically is kept around 3 - 5%. Solids are drawn off the concentration or recirculation tank to maintain a consistent solids level. Solids are often settled in a small settling tank before other processing such as a filter press.
Filtrate that passes through the membrane typically has an SDI of less than 1 and can be used directly for other processes or fed into a RO for further treatment without any significant additional processing. The TMF system eliminates multiple other treatment steps often used to treat waste water such as clarifier, large settling tanks, multi-media filters, carbon filters and cartridge filtration. The Porex TMF system can usually eliminate all of these treatment steps reducing equipment cost, footprint, maintenance and operating costs. The Porex TMF system can be designed in a number of different ways according to the requirements of the end user. Clean in Place (CIP) processes as well as complete system automation are common designs. Multiple pore size options and multiple module configurations allow extremely diverse treatment options dependent upon requirements.