ATOXIC-ASSET Passive Water Treatment 

Tennessee Valley Authority, U. S. Department of Energy, EPR

Treatment 2--ASSET

•Trickling Filter•ZVI Extraction Trench•Settling/Oxidation Basin•Constructed Wetlands

Treatment 1--ATOXIC•Trickling Filter•Constructed Wetlands

125K GPD

250K GPD

125K GPD

Project Objectives

The objectives of the ATOXIC-ASSET project are:•Install and monitor the performance of a passive treatment system designed to 

remove Selective Catalytic Reduction (SCR) derived nitrogen from fossil power plant Flue Gas Desulfurization (FGD) wastewater;•Integrate the Zero Valent Iron (ZVI) extraction trench concept into ATOXIC 

design for enhanced removal of arsenic, selenium, and mercury from FGD wastewater;•Develop a better understanding of the transformations that species of arsenic, 

selenium, and mercury undergo as they are treated in a passive integrated 

treatment system containing Zero Valent Iron (ZVI).

System Components

The components of the integrated system consist of trickling filtersfor ammonia oxidation, a zero-valent iron (ZVI) extraction trenchfor removal of trace elements, a settling/oxidation basin for storage of iron hydroxide floc, and constructed wetlands for biological denitrification. The passive integrated treatment system optimizes natural processes to treat up to 0.25 million gallons per day (MGD) of FGD pond effluent, with a configuration requiring only gravity flow to obviate the need for pumps.

Trickling FiltersIn a passive integrated system for treating power plant 

wastewater, the first step in the treatment process is removal of 

ammonium by the process of biological nitrification. Ammonium in FGD pond effluent will be converted to nitrate in 

trickling filters. In a trickling filter, water is delivered onto the 

surface of a gravel bed, trickles downward through the gravel, and exits through an underdrain pipe. Bacteria living on the gravel convert ammonium to nitrate nitrogen.

Constructed WetlandsThe ATOXIC/ASSET system has two constructed wetlandsto convert nitrate nitrogen to nitrogen gas. Half of the effluent from the trickling filters flows into one of the constructed 

wetlands. The other half of the water from the trickling filters flows to the ASSET components of the project before entering the second constructed wetlands. The vertical-flow wetlands consist of a limestone bed under a layer of organic substrate. The organic substrate offers a source of dissolved organic carbon necessary for denitrifying bacteria to thrive. Because the vertical-flow wetland is anaerobic, additional metals removal occurs due to the formation of insoluble metal sulfides.The vertical-flow wetland may be planted with plant species to aide in denitrification. Water exiting the system flows to the ash pond.

Flow diagram for the ATOXIC/ASSET demonstration. Total flow through the system is 250,000 gallons per day. Two treatment trains each treat 125,000 

gallons per day. The first treatment is trickling filters followed by constructed wetlands. The second treatment is trickling filters, followed by ZVI extraction 

trenches, followed by settling/oxidation basin, followed by constructed wetlands.

Tennessee Valley Authority in collaboration with EPRI and U. S. Department of Energy is demonstrating a passive treatment system for rem

elements from a coal fired power plant wastewater stream. The demonstration is supported by two projects, ATOXIC and 

ATOXIC-Aquatic Toxicity Improvement and 

Control

TVA, EPRI, and EPRI member utilities

ASSET- Arsenic and Selenium 

Extraction 

Trench—Fate of Arsenic, Selenium, and Mercury in a Passive Treatment System for Fossil P

U.S. Department of Energy-National Energy Technology Laboratory, Tennessee Valley Authority, and EPRI

Mon

The ATOXIC/ASSET passive years. Flow monitoring equipmenable scientists to measure tsystem. Flow monitoring will yeffects of rain events. ATOXIC and laboratory analysis for nuquality monitoring includes labmercury. Water quality data athe mass balance of pollutants 

ZVI Extraction TrenEffluent from the trickli

volume. One stream g

cells containing a mixtuthe other stream flows vwetlands. The purpos

First, the reaction cell awith iron surfaces wherreaction cell introduces wastewater stream. Zepresence of water and trap pollutants such as 

Settling/Oxidation BAs the zero-valent iron slowly settle out of solu

storage of bulky iron hyfor slowly released iron storage area to minimizBecause the iron oxyhyproperties, the floc genpond for long term storain the settling pond is vintegrated system.