ATOXIC-ASSET Passive Water Treatment SystemTennessee Valley Authority, U. S. Department of Energy, EPRITreatment 2--ASSET•Trickling Filter•ZVI Extraction Trench•Settling/Oxidation Basin•Constructed WetlandsTreatment 1--ATOXIC•Trickling Filter•Constructed Wetlands125K GPD250K GPD125K GPDProject ObjectivesThe objectives of the ATOXIC-ASSET project are:•Install and monitor the performance of apassive treatment systemdesigned toremove Selective Catalytic Reduction (SCR) derived nitrogen from fossil powerplant Flue Gas Desulfurization (FGD) wastewater;•Integratethe Zero Valent Iron (ZVI) extraction trench concept into ATOXICdesign for enhanced removal of arsenic, selenium, andmercury from FGDwastewater;•Develop a better understanding of the transformations that species of arsenic,selenium, and mercury undergo as they are treated inapassive integratedtreatment systemcontaining Zero Valent Iron (ZVI).System ComponentsThe components of the integrated system consist oftrickling filtersfor ammonia oxidation, azero-valent iron (ZVI) extraction trenchfor removal of trace elements, asettling/oxidation basinforstorage of iron hydroxide floc, andconstructed wetlandsforbiological denitrification. The passive integrated treatment systemoptimizes natural processes to treat up to 0.25 million gallons perday (MGD) of FGD pond effluent, with a configurationrequiring onlygravity flow to obviate the need for pumps.Trickling FiltersIn a passive integrated system for treating power plantwastewater, the first step in the treatment process is removal ofammonium by the process of biological nitrification.Ammonium in FGD pond effluent will be converted to nitrate intrickling filters. In a trickling filter, water is delivered onto thesurface of a gravel bed, trickles downward through the gravel,and exits through an underdrain pipe. Bacteria living on thegravel convert ammonium to nitrate nitrogen.Constructed WetlandsThe ATOXIC/ASSET system has twoconstructed wetlandsto convert nitrate nitrogen to nitrogen gas. Halfof the effluentfrom thetrickling filtersflows into one of the constructedwetlands. The other half of the water from the trickling filtersflows to the ASSET components of the project before enteringthe second constructed wetlands. The vertical-flow wetlandsconsist of a limestone bed under a layer of organic substrate.The organic substrate offers a source of dissolved organiccarbon necessary for denitrifying bacteria to thrive. Becausethe vertical-flow wetland is anaerobic, additional metalsremoval occurs due to the formation of insoluble metalsulfides.The vertical-flow wetland may be planted with plant species toaide in denitrification. Water exiting the system flows to theash pond.Flow diagram for the ATOXIC/ASSET demonstration. Total flow through thesystem is 250,000 gallons per day. Two treatment trains each treat 125,000gallons per day. The first treatment is trickling filters followed by constructedwetlands. The second treatment is trickling filters,followed by ZVI extractiontrenches, followed by settling/oxidation basin, followed by constructed wetlands.Tennessee Valley Authority in collaboration with EPRI and U. S. Department of Energy is demonstratinga passive treatment system for removing ammonium andtraceelements from a coal fired power plant wastewater stream. The demonstration is supported by two projects, ATOXIC and ASSET.ATOXIC-AquaticToxicityImprovement andControlTVA, EPRI, and EPRI member utilitiesASSET-Arsenic andSeleniumExtractionTrench—Fate of Arsenic, Selenium, and Mercury in a Passive Treatment System for Fossil Power Plant WastewaterU.S. Department of Energy-National Energy Technology Laboratory, Tennessee Valley Authority, and EPRIMonitoring the SystemThe ATOXIC/ASSET passive treatment system will be monitored for twoyears. Flow monitoring equipment located throughout the system willenable scientists to measure the water flow through each component of thesystem. Flow monitoring will yield information about evaporation rates andeffects of rain events. ATOXIC water quality monitoring includes samplingand laboratory analysis for nutrients and trace metals. ASSET waterquality monitoring includes laboratory speciation of arsenic, selenium, andmercury. Water quality data along with flow data canyield information onthe mass balance of pollutants in each component of the treatment systemZVI Extraction TrenchesEffluent from thetrickling filtersis split into two streams of equalvolume. One stream gravity flows to the twoZVI extraction trenchcells containing a mixture of zero-valent iron and limestone whilethe other stream flows via gravity to one of twoconstructedwetlands. The purpose of the ZVI extraction trenches is two-fold.First, the reaction cell allows the wastewater to come in contactwith iron surfaces where adsorption reactions take place. Also, thereaction cell introduces a slow release form of iron tothewastewater stream. Zero-valent iron readily oxidizes in thepresence of water and oxygen forming iron oxide particles whichtrap pollutants such as arsenic, selenium, and mercury.Settling/Oxidation BasinAs the zero-valent iron rusts, iron oxides and hydroxides form andslowly settle out of solution. Asettling/oxidation basinallows forstorage of bulky iron hydroxides. The design of the system allowsfor slowly released iron to precipitate from solutionand settle in astorage area to minimize plugging in the downstreamcomponents.Because the iron oxyhydroxide floc particles have adsorptiveproperties, the floc generated is allowed to remain in the settlingpond for long term storage. The collection of iron oxyhydroxide flocin the settling pond is vital to lasting treatment efficiency of theintegrated system.