Water treatment plants use alum, Al2(SO4) 3.14H2O, as a coagulant, alum being
converted into insoluble aluminum hydroxide, Al(OH)3 ,during the coagulation process. It is recognized that the disposal of Al3+-laden solids from water
treatment plants will receive a close scrutiny in coming years and therefore, the
prospect of alum recovery from Water Treatment Residuals (WTRs) has merits.
Several processes for alum recovery have been considered in the past, each
having its own limitation. Donnan Dialysis promises to be an appropriate one, as
it can concentrate Al3+ into a small volume of recovered sweep solution and
simultaneously reject Natural Organic Matter (NOMs) or Dissolved Organic
Carbon (DOC) in accordance with the principle of Donnan exclusion. This allows
recovered Al3+ to be free from trihalomethane formation potential and can
therefore be used in a plant. In this study, feed solution of WTRs
exchanged Al3+ ions with H+ ions from a sweep solution of H2SO4, across Nafion
117, a cation exchange membrane. It was possible, using Nafion 117, to
concentrate Al3+ from 74 moles/m3 to 250 moles/m3 in 24 hours with a total
recovery of 75%. The Donnan exclusion of DOC was complete, resulting in a
total DOC concentration of 2-3 mg/l in recovered Al3+ solution, as opposed to
200-225 mg/l in commonly recognized Acid Digestion Process. The fact that Al3+
forms the bulk of the cations in WTRs solution and also because cation
exchange membranes bind more strongly to higher valence cations led to higher
selectivity of trivalent Al3+ ions, as compared to bivalent "heavy metal" ions such
as Zn2+ and Cu2+. Jar tests were performed using recovered solution on Lehigh
River water and the turbidity dropped from 246 NTU to 10 NTU. Settling rate for
post-treatment WTRs feed was high, suggesting that it could be easily
dewatered. The post-treatment WTRs solution at a pH of 1.2-1.5 needed to be
neutralized, before disposal.
Includes 30 references, figures.
| Edition : | Vol. - No. |
| File Size : | 1
file
, 500 KB |
| Note : | This product is unavailable in Ukraine, Russia, Belarus |
| Number of Pages : | 19 |
| Published : | 11/01/2002 |
