ecently, increased attention has been focused on the occurrence, origin, and mobility of arsenic (As) and how to remove it from drinking water, because ingested As can harm health. The current maximum contaminant level (MCL) for total As in drinking water remains at the original value set in 1942 (50 µg/L). 1,2 However, recent studies suggest that this MCL may be too high. Dose-response relationships between the concentration of inorganic As in water supplies and cancer risks were established in an area of Taiwan in which the As concentration of well water was 170-800 µg/L. 3 On the basis of this study, it is estimated that the current As MCL of 50 µg/L adopted by the US Environmental Protection Agency (USEPA) would result in a lifetime risk of dying from cancer of the liver, lung, kidney, or bladder as high as 13 per 1,000 people. Of the approximately 200 conta-On-site ion exchange studies investigated the combined removal of arsenic (V) [As(V)] and nitrate from drinking water in McFarland and Hanford, Calif., and Albuquerque, N.M. Whereas previous ion exchange workers had studied removal of high concentrations of As (> 50 µg/L As) without nitrate present, these studies focused on removing 10-15 µg/L As to achieve a product water with < 2 µg/L As while also maintaining nitrate below its maximum contaminant level . Results of 1-in.-(25-mm-) column experiments showed that conventional sulfate-selective resins were better than special nitrate-selective resins for combined As(V) and nitrate removal. The conventional resins yielded longer run lengths and leaked less As and nitrate into the product water. Decreasing empty bed contact time from 3.0 to 1.5 min did not greatly alter As leakage into the product water. Particulate iron in the ion exchange feed increased As leakage in the product water. Two commercially available computer programs were reasonably accurate in predicting both As and nitrate run lengths for various influent nitrate and sulfate concentrations. Generally, predicted As and nitrate run lengths were within ±35 percent of those observed experimentally.For executive summary, see page 182.
Arsenic (As) removal using ferric hydroxide coagulation followed by direct microfiltration without flocculation was investigated for an application in Albuquerque, N.M. Typically, the influent drinking water (unchlorinated) was contacted with ferric hydroxide for ≤20 s in a rapid mixer and passed through a membrane microfiltration unit with a nominal pore size of 0.2 μm. Variables investigated included pH, iron (Fe) dose, mixing time and energy, filtrate flux, and backwash interval. The pH and ferric dose were found to be the most important variables controlling As removal. As removal to low levels (<2 μg/L) was achieved using either a dose of 7 mg/L Fe without deliberate pH reduction or a smaller dose of 1.9 mg/L Fe after sulfuric acid addition to reduce pH to 6.4. Extended operation (three to five days) showed that consistent As removal was obtained without any membrane fouling. Both the backwash water and the dried sludge passed the toxicity characteristic leaching procedure test as a nonhazardous waste.
The effectiveness of seven oxidants—chlorine (Cl2), permanganate (MnO4−), ozone (O3), monochloramine (NH2Cl), chlorine dioxide (ClO2), a manganese dioxide‐based solid‐oxidizing media, and 254‐nm ultraviolet radiation—for arsenite [As(III)] oxidation to arsenate was studied. The effect of water chemistry variables including pH, temperature, and interfering reductants (manganous and ferrous ions, sulfide, and total organic carbon [TOC]) was investigated. Cl2 and MnO4− provided complete oxidation in less than 1 min under all conditions tested. The effectiveness of O3 was significantly attenuated in the presence of TOC. Both ClO2 (an otherwise powerful oxidant) and NH2Cl were ineffective for As(III) oxidation. When dissolved oxygen (DO) was not limiting, the solid‐oxidizing media provided complete oxidation. However, with low DO and interfering reductants, incomplete oxidation was observed. The adverse effect of interfering reductants was eliminated either by supplying enough DO or reducing the flow rate. UV light alone (254 nm) was not effective, but complete As(III) oxidation was observed when the feedwater was spiked with 1 mg/L of sulfite.
Non‐aseptic production of biosurfactant from molasses by a mixed culture was investigated in stirred batch reactors. Biosurfactant production was quantified by surface tension reduction, critical micelle dilution (CMD), and emulsification capacity (EC). Biosurfactant production was directly correlated with biomass production, and was improved by pH control and addition of yeast extract. Centrifugation of the whole broth increased emulsifying capacity and reduced surface tension. Acidification of the whole broth increased the emulsification capacity but reduced the apparent biosurfactant concentration (CMD), without affecting the surface tension. The emulsification capacity of the cell‐free broth was equivalent to that of a 100 mg/L solution of sodium dodecyl sulfate. The emulsification capacity of the whole broth and cell‐free broth were reduced by about 50% at and above NaCl concentrations of 100mM. Preliminary characterization suggests that the biosurfactant activity is primarily associated with one or more protease‐sensitive species, released from cells in larger quantities after more vigorous centrifugation. © 1994 John Wiley & Sons, Inc.
A conventional sulfate-selective Type-2 polystyrene strong base anion resin was studied for arsenic (As) removal from Albuquerque, N.M., drinking water. Attention was focused on the regeneration aspects of ion exchange and the potential for reusing the spent brine directly without removing As. The major finding was that As-laden spent brine with makeup salt addition to 1 M chloride could be reused up to 20 times with no effect on As leakage and minimal effect on run length. The authors also found that As could be removed from the spent recycle brine using ferric hydroxide coagulation in such a way that the resulting sludge passed the toxicity characteristic leaching procedure test as a nonhazardous waste.on exchange is an important arsenic-(As-) removal process for source waters containing <500 mg/L total dissolved solids and <120 mg/L sulfate (Ghurye et al, 1999;Clifford et al, 1998;Clifford et al, 1997;Clifford & Lin, 1986). Usually, the source water [oxidized, if necessary, to convert As(III) to As(V)] is passed through a bed of chloride-form strong base anion (SBA) exchange resin with the arsenate ion in the source water being exchanged for the chloride ion on the resin. If ferrous iron is present in the raw water above about 0.3 mg/L, the oxidation-filtration pretreatment process, which will remove some As, must precede the ion exchange step to avoid resin fouling. Although chloride-arsenate ion exchange is simple in concept, the following issues need to be addressed before anion exchange becomes a widely accepted treatment choice for As removal: (1) choice of resin, (2) effect of multiple contaminants such as As and nitrate, (3) effect of sulfate concentration, (4) optimum empty bed contact time (EBCT), (5) minimization of As leakage, (6)
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