Civil Engineering Reference
In-Depth Information
commercial alum. The sludge reduction accompanying aluminum recovery was as
predicted by direct sludge acidification. The economic key was to obtain an aluminum
concentration greater than 1,200 mg / L in the sludge.
Current Use of Alum Recovery
Few full-scale alum coagulant recovery facilities
are currently active.
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It is essential that pilot or laboratory scale operations be per-
formed when considering the feasibility of implementing alum coagulant recovery.
Iron Coagulant Recovery
The recovery of iron coagulants involves acidification of ferric hydroxide and a re-
covery technique very similar to that described for the acidic alum recovery process.
The pH of the iron sludge is lowered by acid addition to a range where the solubility
of ferric iron is significantly increased, and the iron is released back into solution. The
principle of this method is shown in the simplified relationship:
2Fe(OH)
3H SO
→
Fe (SO )
6H O
(23-13)
3
2
4
2
4 3
2
The pH must be reduced to 1.5 to 2.0 to attain 60 to 70 percent recoveries of iron.
23
This method can be improved by adding a reducing agent to the sludge before acid
addition, to convert the form of the iron from the precipitated ferric ion (Fe
3
)tothe
more soluble ferrous ion (Fe
2
). Sodium sulfide (Na
2
S) has been found to be an ef-
fective reducing agent. Recoveries of as much as 60 percent of the sludge iron were
achieved at a pH of 3.0 when the reducing agent was used. The recovery processes
also had a marked effect on the settling characteristics of the residual sludges. For
both residual sludge samples the solids settled to about 20 percent of the original
volume in a period of less than 30 minutes, and the sludge solids concentrations
increased from the original 2 percent total solids before recovery, to 7 to 9 percent
total solids in the settled sludge. In addition, the weight of sludge solids requiring
ultimate disposal was reduced.
The presence of a sulfide residual in the recovered iron solution adversely affects
the coagulation performance of recovered iron. The problem can be remedied by add-
ing potassium permanganate to the recovered solution before reuse. Sulfate buildup
may be a problem with repeated iron recycling. Another alternative for removing
sulfide is to strip it as H
2
S by aerating the recovered iron solution at the low recovery
pH. It is necessary to control the H
2
S release in the off-gas. Iron coagulant recovery
is not commonly practiced.
1
Recalcination of Lime Softening Sludge
Lime recovery by recalcination has been widely used for years. Recalcination has the
potential to recover substantially more lime than used in the original lime-soda water-
softening process, while at the same time producing carbon dioxide for use in recar-
bonation, and greatly decreasing the volume of sludge requiring ultimate disposal. As
with other recovery processes, careful consideration must be given to the expected
quality of the recovered product and to including processes to remove impurities.
Centrifuges can be used to separate some impurities.
Quicklime (CaO) can be produced from lime softening residues, after purification
and dewatering of the calcium carbonate, through use of a drying process. The basic
