Date of Award

5-2008

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Hydrogeology

Advisor

Castle, James W

Abstract

This investigation examined the feasibility of using surface-flow constructed wetland treatment systems (CWTSs) to decrease the concentration and bioavailability of targeted constituents of concern (COC) in ash basin water. Ash basin water results from hydraulic transport (sluicing) of coal ash produced during thermoelectric power production. During the sluicing process, potentially toxic trace elements contained within coal ash may be transferred to the aqueous phase and subsequently introduced to aquatic receiving systems. COC in ash basin water were identified by a risk quotient method in order to determine biogeochemical conditions needed within wetland reactors for reducing the aqueous concentration and bioavailability of identified COC. Specific research objectives were: 1) characterize ash basin water from a risk-based perspective and identify COC; 2) evaluate pilot-scale CWTS performance for treating formulated ash basin water by measuring the concentration and bioavailability of COC in CWTS influent and effluent; 3) determine the effectiveness of using CWTSs to reduce reuse limiting parameters (scaling, biofouling, and corrosion); and 4) develop a mathematical model to describe the hydraulics of a pilot-scale reactor in a surface-flow CWTS.
Two pilot-scale CWTSs (i.e. series A and B) were designed to decrease concentrations of arsenic, chromium, mercury, selenium, and zinc through the following removal processes: precipitation as nonbioavailable sulfide minerals, co-precipitation with iron oxyhydroxides and sorption onto iron oxides. Concentrations of identified COC decreased as water moved through the wetland reactor series. In addition, the bioavailability of COC (evaluated by toxicity experiments) was successfully abated through treatment with the CWTSs. Treatment of simulated ash basin water by the CWTSs resulted in effluent concentrations of chromium, zinc, arsenic, selenium and mercury as low as 5.3, 4.8, 7.1, 37.3, 0.1 µg/L, respectively. Effluent concentrations of zinc, arsenic, and mercury were less than 120, 64, and 2 µg/L, respectively in all experiments. Effluent chromium concentrations were less than 11 µg/L in 2 of 9 experiments. The concentration of selenium in CWTS effluent was less than 50 µg/L in 3 of 9 experiments. Performance data suggest that removal of COC occurred in reactors designed to support dissimilatory sulfate reduction. Therefore, it is interpreted that removal of COC in these reactors occurred via precipitation as non-bioavailable sulfide minerals. Additionally, removal of chromium, arsenic, mercury, and zinc occurred in the oxidizing reactors. However, due to lower influent concentrations, less removal occurred in the oxidizing reactors than in the reducing reactors.
Biofouling in hydraulic transportation systems can reduce flow volume, thereby reducing efficiency. However, biofouling in series A and B effluent was 46 and 68%, respectively, less than biofouling in CWTS influent. Although, scale deposits on glass coupons indicate potential scale formation following treatment with CWTS, effluent scale formation was 80 and 40% less than influent scale formation for series A and B, respectively. Corrosion was not decreased in CWTS effluent as compared to influent.
The developed wetland flow and solute transport model simulated transport of a non-reactive tracer (bromide) in a pilot-scale reactor of a surface-flow constructed wetland treatment system. Two zones were identified with the solute transport model. The first zone is a relatively active flow region comprised of the main surface flow channels (i.e. advective solute transport). The second zone is a no-flow (or relatively low flow) 'temporary storage' surface flow zone in which a solute may reside for a portion of time prior to re-entering the actively flowing region of the main surface flow channels. Because a maximum of 10% of CWTS influent entered the hydrosoil and the concentration of trace elements was decreased, the modeling study suggests that removal of trace elements by the surface-flow constructed wetland reactor occurred near the sediment water interface.

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