FEASIBILITY OF ALUM SLUDGE RECLAMATION
Michael Goldman, Project Engineer
Buchart-Horn
Lewisburg, Pennsylvania 17837
M. Mohan Varma, Director
Department of Bio-Environmental Engineering
Howard University
Washington, D.C. 20001
Frederick Watson, Engineer
Washington Technical Institute
Washington, D.C. 20001
INTRODUCTION
Disposal of alum sludge is a traditionally thorny problem for water treatment plants.
With regulatory agencies taking a closer look at disposal practices, midnight dumping of
sludge to the raw water source is becoming less and less viable as a means of solving the
problem.
Water works sludge consists primarily of silt and other suspended matter removed
from raw water early in the treatment process.   Addition of alum as a coagulant aid has
the desirable effect of reducing the time required for settling of suspended solids; the
major drawback is the fact that the product sludge is voluminous, low in solids (about
1%) and difficult to dewater.   Consequently, a great deal of effort has been spent in research on dewatering of alum sludge.   Vacuum filtration has had limited success; pressure
filtration has been demonstrated to be quite useful in this regard.   These techniques invariably lead to questions of capital and operating costs, which are generally high.   Freezing and thawing of alum sludge conditions the material to a point where it almost dewaters
itself.   The sludge is converted from a watery, voluminous mass to a dense, sandlike consistency.   Freezing and thawing is, of course, feasible only in cold climates.   Even there,
large amounts of land and extra storage capacity must be available.
Present-day treatment of sewage frequently requires removal of nutrients such as phosphorus.   Addition of a metal salt such as alum is a common method of achieving such
removals.
2A1+3 + HP04"* + HjO ■ Al(OH)3 • A1P04 + 4H+ (1)
We considered that aluminum present in waterworks sludge could be recycled and used
as a precipitant for phosphate in a sewage treatment plant.   The idea of recycling alum
sludge is not new; in general, however, recycling schemes have concentrated on reuse of
aluminum within a waterworks.   This in turn  generated the problem of impurities building up in recycled coagulant.   Use of recovered aluminum by a sewage treatment plant
would avoid some difficulties associated with reuse by a waterworks.
1. Recovery of alum can reduce disposal costs at the waterworks and/or chemical costs
at the sewage plant.
2. The problem of impurities concentration is not as important in sewage treatment as
in water treatment.
3. Sewering of alum sludge is an environmentally acceptable method of sludge disposal.
4. Treated alum sludge may be useful to a sewage treatment plant, in terms of high-
density solids and available aluminum.
The other side of the coin should also be presented:
1. Costs of alum recovery may exceed the cost of purchasing fresh alum.
2. Transportation costs of untreated alum sludge, as well as recovered aluminum, may
be significant.
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