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MANAGEMENT OF RESIDUALS CONTAINING ARSENIC

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The arsenic (As) MCL is likely to be lowered to about 5 μg/L; the TCLP limit for residuals might also be reduced. Currently, the 5 mg/L TCLP limit is based on a leachate concentration that is 100 times the drinking water MCL—a proportionate decrease would lower that limit to 0.5 mg/L. The problem will be exacerbated by the fact that increasing coagulant doses to remove As from drinking water will both increase the concentration of As in the residual, and generate a greater quantity of that residual. A lowering of allowable As levels for stream discharge or sewering may also occur.

Environmental Engineering & Technology, Inc. (EE&T) completed a comprehensive evaluation of treatment options for removal of As from residuals generated by a wide range of treatment processes designed to remove As from drinking water. That work was conducted for EPA and focused on developing a short-list of As removal options and preliminary costs for liquid, semiliquid, and solid residuals produced by ion exchange, membrane processes (RO and NF), activated alumina, and iron removal processes. Treatment techniques evaluated included precipitation (with and without polymer) using ferric chloride and alum, adsorption using activated alumina and an iron-based media, and ion exchange.

The total As content of liquid residuals samples used in testing ranged widely from 0.5 mg/L in the spiked RO and NF concentrates to more than 30 mg/L for ion exchange brine samples. Other characteristics of the untreated waste samples such as pH, TDS, alkalinity, and total iron also varied widely (e.g., pH 6.6 to 9.7, TDS 340 mg/L in spent filter backwash water to more than 14,000 mg/L in RO concentrate, etc.).

One key finding from this research was the identification of ferric coagulation as a viable treatment alternative for most types of waste streams. With optimal coagulation conditions, As removal from the liquid waste streams ranged from 88 to 99 percent. These removals corresponded to treated waste As levels of 0.005 mg/L to 0.15 mg/L. Existing State As limits for discharges into streams typically are set at 0.05 mg/L. For ion exchange regenerant samples, As removals of nearly 90 percent were achieved, however, As levels remaining in the treated waste were much higher (>1.0 mg/L) because of the very high starting As levels. Overall, precipitation using alum was much less effective. All of the solids evaluated for TCLP would meet the 5.0 mg/L limit.

Both iron-based media and activated alumina media were identified as viable adsorption techniques for removing As from some types of residuals streams. For example, As removals of greater than 95 percent were attained after 120 bed volumes (for 3-min EBCT) for nanofiltration concentrates. Corresponding effluent As contents were less than 0.05 mg/L.

While the regulatory focus is now on drinking water As levels and possible costs associated with meeting those MCLs, minimizing As levels concentrated in residuals could have much more farreaching implications. This project delivers practical information to utilities on the technical feasibility of reducing As levels from a broad array of waste streams and the associated costs.
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Document Type: Research Article

Publication date: 01 January 2001

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