Alum (Aluminium sulfate)
Clay turbidity restricts light penetration in ponds. This reduces
the depth to which photosynthesis can occur and reduces primary
productivity14. Clay turbidity is caused by ultra-fine colloidal
particles, 1-10m in diameter. These particles carry a negative
charge which maintains them in suspension.
Positive ions react to reduce the negative charge of colloids,
causing them to coagulate into larger particles and settle out2
.
Electrolytes with appropriate positive ions can therefore be added
to water to remove colloidal particles. Alum (aluminium sulfate),
hydrated lime (calcium hydroxide) and gypsum (calcium sulfate)
are often used as sources of electrolytes for reducing turbidity14.
Liming materials increase the concentration of calcium and
magnesium ions, which flocculate colloidal particles14. Gypsum
can also be used to flocculate colloids by increasing the
concentration of calcium ions7
. However, in one comparative
study, alum was found to be more effective than lime or gypsum
in the reduction of turbidity. Alum reduced turbidity by 89-
97% within 48 hours at doses of 10-30 mg/litre14. The cost of
treatment with alum is significantly lower than for gypsum7
.
Alum is acid forming and can substantially reduce total
alkalinity and pH as per the following reaction14:
Al2
(SO4
)3
+ 6H2
0 = 2Al(OH)3
+ 6H+
+ 3SO4
2-
This effect makes alum unsuitable for use in ponds with low
total alkalinity as it may lower pH to the point where it is toxic
to fish14. Alum also reduces dissolved inorganic phosphate levels
through the precipitation of insoluble aluminium phosphate,
reducing phytoplankton growth14. The use of alum may therefore
necessitate the addition of lime to correct total alkalinity and pH
if these parameters fall below acceptable levels. Fertilisation to
increase dissolved phosphorus concentrations may also be
necessary to maintain productivity.