An emergency treatment that quickly
reduces high pH is the application of
alum (aluminum sulfate). This is a safe,
relatively inexpensive chemical that
reacts in water to form an acid. Besides
reducing pH, alum also flocculates and
removes algae by sedimentation, thus
decreasing algal biomass and reducing
photosynthesis. Alum may also help
to reduce pH indirectly by removing
phosphorus—an important nutrient for
plant growth.