The Effects of Aluminum Hydroxide on
Primary Wastewater Treatment Processes
D. Y. HSU, Graduate Student
W. O. PIPES, Professor
Department of Civil Engineering
Northwestern University
Evanston, Illinois
INTRODUCTION
Sludges produced by water treatment plants usually contain large concentrations of
aluminum hydroxide along with the suspended material removed from the water since alum
is used as the flocculating agent. The traditional method of handling the sludge was to
discharge it back to the river or lake from which the raw water was obtained. This method,
however, has recently been regarded as unsuitable because the sludge, which is the
collection of the impurities from the raw water, may cause the formation of sludge banks
(I). The recognition of the pollutional effects of the waste sludge can be seen from the
increasing number of states establishing regulations to restrict the direct discharge of water
treatment plant sludge (2). Methods are being developed for sludge treatment before final
disposal and the number of water treatment plants discharging their wastes back to the
water body has decreased in the past several years.
Among the methods developed sludge lagooning is the most popular, being used by
about 11 percent of all the water treatment plants surveyed in 1968(3). However, it has been
pointed out by both Almquist (4) and Smith (5) that a better procedure is to discharge the
sludge to the sanitary sewer and let the wastewater treatment plant do the job of waste
disposal if 1) the sewers are not damaged by the sludge, 2) the sludge can be removed by the
existing wastewater treatment facilities, 3) the wastewater treatment plant can handle the
increased sludge volumes, 4) the sludge does not interfere with the existing wastewater
treatment processes, and 5) the sludge does not cause a deterioration in final effluent
quality. When the land is not available for construction of the lagoon, the discharge of the
waste to the sewer is obviously the method of choice.
Since the main component of the sludge is aluminum hydroxide with pH values
ranging from 6.0 to 7.1 (1,4,6), damage of the sewer by corrosion is not likely to occur. The
volume of the sludge should not contribute any serious problem in the wastewater
treatment plant because they are in small quantity in comparison with the sewage flow
(about three percent (1)).
The last and also most important problem to be solved is obviously to discover if there
are any adverse effects on the wastewater treatment processes if this method of sludge
disposal is to be used. The purpose of this study is to make some investigation of this
problem on the primary treatment of wastewater. Since aluminum hydroxide is the main
component of water treatment plant sludges, the problem was generalized and simplified by
investigating the effects of high concentrations of aluminum hydroxide floe on the primary
settling of wastewaters. The method used was to add synthetic aluminum hydroxide floe at
various concentrations in each batch process and compare the results with a control test.
EXPERIMENTAL PROCEDURE
The synthetic aluminum hydroxide floe was prepared by adjusting the pH of an alum
solution (2000 mg/1 Al) to 7.0 using 4 normal potassium hydroxide. The solution was then
centrifuged and the precipitate was washed six times with distilled water. The pH of the final
sludge was readjusted to 7.0 with sodium carbonate. The concentration of aluminum in this
sludge was determined by titration with zinc chloride as suggested by Schwarzenbach (7).
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