Photocatalytic Treatment of Selenium and Arsenic in SRP Waters

Project: Research project

Project Details

Description

Photocatalytic Treatment of Selenium and Arsenic in SRP Waters Photocatalytic Treatment of Selenium and Arsenic in SRP Waters roundwater in the service area contains naturally occurring levels of chromium, arsenic and selenium. As part of our on-going project funded by SRP (Hexavalent Chromium, Selenium and Arsenic Occurrence, Fate and Treatment in Powerplant Discharge, Canals and Potable Water Treatment Plants, June 2012 July 2013), we have acquired information about the occurrence of these metals in groundwaters and throughout SRP power plants (Santan and Kyrene). Figure 1 shows some recent sample collection results from SanTan. We have also evaluated different treatment strategies for removal of these metals, with focus on hexavalent chromium. The levels of selenium and arsenic measured in our study indicate that treatment methods may be needed in the near future in order for SRP to remain in compliance with regulatory limits. Because SRP powerplants evaporate water to cool/condense steam, metals are concentrated by a factor of 2 to 2.5 at the Santan and Kyrene facilities. SRP has a selenium discharge limit of 2 ppb, and has done limited work in the past regarding water treatment for Se. The drinking water limit for total arsenic decreased from 50 ppb to 10 ppb in 2006, and it is likely that the current NPDES permit levels for SRP could drop from the current 50 ppb to the 10 ppb level for aquifer protection. Figure 1. Sampling results from SanTan Generating Facility (Winter 2012) Selenium is found in the environment in four oxidation states (Fig. 2a): Se(VI), Se(IV), Se(0), Se(-II). It is found as selenate and selenite in oxidized systems, but Se(0) and selenides in anaerobic zones. Both Se(0) and Se(-II) are insoluble. In pH 6-8, only Se(0), selenite, biselenite (HSeO3 -1) and selenate are present. Removal strategies for selenium typically involve reduction of selenate, which is not easily adsorbed onto particulates, to selenite, which can be easily immobilized. Biological reduction of Se with anaerobic bacteria or algae has been shown to be very effective, with>95% selenium removal for influent Se of 0.4 mg/L [1] and a commercially available bioreactor (ABMet) from GE already demonstrated to remove Se< 5 g/L in pilot and full-scale applications [2]. However, such technologies require substantial real estate and capital/operational costs that may not be feasible for SRP. Arsenic is also naturally occurring in Arizona, and can occur as H3AsO4, H2AsO4 -, HAsO4 2-, or AsO4 3- in oxygenated waters (Fig. 2b). In the case of arsenic, trivalent arsenic (arsenite) is more toxic than the pentavalent form (arsenate). The removal of As(III) is also more difficult than As(V). Westerhoffs previous studies on arsenic occurrence and treatment in Arizona5-9 showed that predominately As(V) is present in local groundwaters. SRPs previous speciation studies on arsenic occurrence in well waters also show it is in the form of arsenate. Arsenic can be readily removed by iron3 based packed bed adsorbents or iron-coagulation/membrane filtration separation for well-head treatment, and is already employed by many local cities. Ferric salts may also adsorb arsenic and require< 10 mg/L ferric ion and can achieve residual As < 10 g/L. According to Fred Fuller, arsenic has been detected on ferric sludge at the Santan plant. However, based on our prior sampling results, arsenic levels increase in the blowdown waters, which may be due to concentration of the metals during water evaporation or perhaps even leaching from the arsenical copper chromate treated cooling towers. So, even if arsenic is removed in the clarifier step, it appears there is still a significant presence at a later stage.
StatusFinished
Effective start/end date8/15/138/14/14

Funding

  • INDUSTRY: Domestic Company: $50,247.00

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