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Technique Trace Organic Compounds: What Goes In Must Come Out Chemically enhanced high rate settling (CEHRS) shows promise in removing harmful compounds such as pharmaceuticals and consumer products from wastewater. by Daniel Austria Jr. recent research project involved the pilot test- ing of a chemically enhanced high rate settling (CEHRS) process for removal of selected trace organic compounds (TOrCs). This article is a summary of those piloting efforts. A Increasing amounts of TOrCs at wastewater treatment plants (WWTPs) around the world is a phenomenon that has been occurring for some time now. Some examples of these compounds are consumer products (surfactants, caffeine), pharmaceuticals (drugs, hormones), pesticides (DEET), and volatile organics (flame retardants, household cleaning products). Many of these compounds are the result of every day human activities; this is especially true in developed consumer-based societies where environmentally conscious living has become secondary to keeping up with social sta- tus standards. These compounds can be harmful not only to humans but to the entire ecosystem. It is still not completely known how the compounds' presence will affect the envi- ronment long term, as current data is limited, but they are generally labeled a health risk by many experts. Research is continually being done to gauge the ability of water treatment technologies to remove such compounds from municipal water treatment plants (WTPs) and WWTPs. Current literature demonstrates that generally the major- ity of TOrC removal at WWTPs can be attributed to the typical biological secondary treat- ment of an acti- vated sludge pro- cess (ASP). With activated sludge, the WWTP can fine tune treatment by controlling the balance of new treatment organ- isms that grow to replace those that die and are wasted. A WWTP can oper- ate a CEHRS pro- Figure 1: A CEHRS (ACTIFLO® 28 Water Online The Magazine, Wastewater Edition ■ wateronline.com cess that utilizes "fresh" powdered activated carbon (PAC) and "used" PAC in a similar fashion. With proper PAC addi- tion/recirculation/wasting, a WWTP can control the amount of time PAC is used/recycled to ensure full utilization of its adsorptive capacity. A system that can take full advantage of the inherent efficient adsorption capacity of PAC would potentially see significant removals of TOrCs. Typical CEHRS Process Review ) process schematic. (Image provided by Kruger Inc.) A general process overview of a CEHRS process (see Figure 1) is: raw water enters the CEHRS system in the first flash mix tank (coagulation tank). Here, chemical coagulant is added and thoroughly mixed to destabilize suspended solids and colloidal matter. The flow then enters the second tank (matu- ration tank) where flocculant aid polymer and microsand are added. In the maturation tank, relatively milder mixing pro- vides ideal conditions for the formation of polymer bridges between the microsand and the destabilized suspended solids. The turbomix draft tube produces effective dynamic mixing to ensure that a very dense floc is formed; the fully formed ballasted flocs leave the maturation tank and enter the settling tank. It is here where ballasted flocs rapidly settle out and are collected to a center sludge pit. The sludge/mic- rosand slurry is withdrawn from the collection pit using a cen- trifugal slurry pump where the slurry is then pumped to hydrocyclones for separation. The pumping energy is transferred from pump to hydrocy- clone, which acts as a centrifuge caus- ing chemical sludge to be separated from the higher density microsand. Once separated, the microsand is concentrated and discharged from the bottom of the