Water Online

JUL 2017

Water Innovations gives Water and Wastewater Engineers and end-users a venue to find project solutions and source valuable product information. We aim to educate the engineering and operations community on important issues and trends.

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Page 14 of 33

Physical Treatment Treatment technologies will involve physical, chemical, or biological processes. Physical processes include clarification, filtration, and membrane technologies. Except for the most rigorous membrane process (reverse osmosis), physical processes will generally not remove dissolved contaminants. Clarification uses a combination of coagulation, flocculation, and settling to remove suspended particles and typically involves sludge recycle. Filtration methods include bag filters, cartridge filters, sand filters, and multimedia filters (Figure 1). Multimedia filters, which typically utilize anthracite coal, sand, and garnet, are probably the most common filters now in use. These filters are pressure vessels that use downflow operation to remove suspended contaminants and a periodic upflow backwash to transfer these contaminants to a waste stream. The most common membrane technologies are microfiltration, ultrafiltration, nanofiltration, and reverse osmosis (RO). These are listed in order of decreasing pore size, increasing removal efficiency, and increasing pressure requirements. The primary disadvantage of RO is a high-volume waste stream, which often limits its applicability. Chemical Treatment Chemical treatment processes include hydroxide precipitation, sulfide precipitation, oxidation/reduction, ion exchange, and natural zeolites. Hydroxide precipitation typically uses lime to increase the pH. Hydrated lime or pebble lime (which requires a slaker) may be used. Other chemical alternatives include caustic soda (sodium hydroxide), soda ash (sodium carbonate), or magnesium hydroxide. For ease of addition and to avoid make- up of chemical solutions, liquid caustic soda or lime slurry is sometimes purchased. The pH target for hydroxide precipitation depends upon the contaminants of concern. After precipitation and subsequent clarification or filtration, acid is often added to meet discharge requirements for pH. Coprecipitation, a process in which dissolved contaminants are pulled out of solution along with precipitation of high concentrations of contaminants such as iron, manganese, and sulfate, can also help to meet discharge limits. Sulfide precipitation, which can achieve lower levels than hydroxide precipitation, is typically used as a "polishing" step to meet low metals concentrations. Sodium sulfide or sodium hydrosulfide (NaHS) is typically used. This process requires only small quantities of reagent and a short retention time. The process is typically done at neutral-to-high pH to avoid generating dangerous H2S gas. Oxidation/reduction processes are used to transform contaminants into less soluble or more easily removed forms. For arsenic removal, oxidizing agents such as chlorine/sodium hypochlorite, hydrogen peroxide, ozone, or permanganate are commonly added. Conversely, reducing agents such as sodium bisulfite or metabisulfite may be added to remove contaminants such as chromium and selenium. Oxidation and reduction are typically rapid reactions but, since they require chemical addition, will increase the total dissolved solids (TDS) in treated water. Specific ion exchange resins from several manufacturers are available to remove dissolved metals, arsenic, and nitrate (Figure 2). In this process, sodium or chloride ions are exchanged for the target contaminants. Resin is relatively expensive but has a long life and can be chemically regenerated (either on-site or off-site). The waste stream from ion exchange is typically much less than that generated by RO. Biological Treatment Biological treatment processes include attached growth, suspended growth, and membrane bioreactors. Attached growth processes are most common, but membrane bioreactors are a growing application. Biological treatment can be used to remove ammonia, nitrate, selenium, sulfate, and dissolved metals. In an attached-growth system, bacteria are attached to a media surface (Figure 3). Media can range from plastic to activated carbon to rock, with media diameters ranging from microns to centimeters. The attached bacteria (a biofilm) provide a very robust process in that it is very resilient to changes in flow, pH, and contaminant concentration. Attached-growth systems are the best choice for treating high or variable concentrations. Suspended-growth systems are commonly used for municipal wastewater treatment but can also be used for industrial wastewater. Activated sludge is an example of suspended-growth biological treatment. Suspended growth is often used for removal of nutrients (nitrogen and phosphorus). When properly designed, these systems can be used for both nitrification (ammonia removal) and denitrification (nitrate removal). Nitrification is an aerobic 12 wateronline.com n Water Innovations INORGANICCONTAMINANTS Figure 1. Multimedia filter configuration Figure 2. Treatment with ion exchange resins

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