More on the subject.

Because alum is acidic, it has effects in ad-

dition to coagulation. Alum reacts in water as

follows:

Al�(SO4)� + 6H�O-� 2AI(OH)�

+ 6H I+ + 3SO4 :�-

Some of the aluminum ions react with any col-

loidal clay particles present; the rest precipitate

out as aluminum hydroxide, AI(OH)� (below

pH 6, more aluminum hydroxide is soluble).

Some hydrogen ions also react with colloids, but

most neutralize carbonate and bicarbonate to

reduce total alkalinity and depress pH. The

drop in pH is greater in waters with lower initial

amounts of total alkalinity, as illustrated in Fig.

2. In the absence of other chemicals, 1 rag/liter

of alum theoretically destroys 0.5 rag/liter of

total alkalinity.



In this study, the toxicity of alum to fathead

minnows decreased with increasing total alka-

linity (Table 2). All fish mortalities occurred

during the first 12 hours of the 96-hour tests.

Although pH values were driven as low as 4.41

in these tests, acidity probably was not the cause

of death. Mount (1973) found that fathead

minnows could survive pH 4.5 indefinitely and

Swingle (1961) reported the acid death point

for pond fish as 4.0. Death of fathead minnows

in the present study was likely related to high

concentrations of aluminum ion in waters of

low pH rather than to the direct effect of low

pH. At pH 4.4, the concentration of aluminum

ion in equilibrium with AI(OH)s is approxi-

mately 1.7 mg/liter. Hydrated lime essentially

negated the influence of alum on total alkalin-

ity, pH, and fish mortality .



Data reported by McKee and Wolf (1963)

and findings reported in Tables 2, 3, and 4 in-

dicate that alum treatments of 10 to 30 mg/liter

will not be harmful to fish except in water with

low total alkalinity. For example, treatment of

water with an initial total alkalinity of 12 mg/

liter with 30 mg/liter of alum wouldestroy all

of the alkalinity, reduce the pH, and probably

kill fish.



Sources of positive

electrolytes which are commonly employed for

turbidity removal are aluminum sulfate (alum),

ferric sulfate, calcium hydroxide (hydrated

lime), and calcium sulfate (gypsum). Their po-

tential for removing turbidity from ponds has

not been adequately evaluated (Martin 1978),

so the present study was conducted.





Alum precipitates phosphorus as insoluble

aluminum phosphate (Sawyer and McCarty

1967), so applications of alum have been used

to reduce phosphorus concentrations and phy-

toplankton productivity in natural waters (Pe-

terson et al. 1973; Bandow 1974). However, the

species of aluminum resulting from alum treat-

ment quickly disappear from solution if treated

water contains residual alkalinity (Stumm and

Morgan 1970). Thus, applications of alum to

remove clay turbidity will not adversely affect

the availability of phosphorus added in fertil-

izer later in the growing season. In unfertilized

ponds, alum treatment will lead to lowered

phosphorus concentrations and plankton pro-

duction.



Hydrated lime may be ap-

plied simultaneously at the rate of 0.40 mg/liter

per 1.0 mg/liter of alum to prevent unfavorable

changes in alkalinity and pH. For maximum

coagulation of colloidal particles, water should

contain residual alkalinity after alum treatment

(Sawyer and McCarty 1967), so hydrated lime

application is beneficial in soft water even when

alum toxicity to fish is of no concern.



One of the other articles stated that while Alum precipitates phosphorus it leaves a good amount of phosphorus intact and available for use. Where pH levels are low the acidity makes for a higher degree of the Alum phosphate re-soluble. As per Mike's comments clear water plus residual phosphorus and re-soluble nutrients at the FA level in low alkalinity waters may cause a FA explosion. A better method may be lime (or high alkalinity) and a good plankton bloom to shade out the FA.