For a number of years, sulfuric acid was used in water treatment facilities to control
alkalinity. It’s a product that works, but it also has may potential problems. Sulfuric acid
can be difficult to apply and control. It is potentially dangerous to store and handle. Safety
showers have to be installed and readily available to operating personnel who must wear
special clothing for their protection.
The extremely corrosive acid
requires special material for
equipment and piping.
Maintenance of the system
demands frequent component
repairs and replacement. Carbon
dioxide on the other hand is safe
to handle, easy to apply, efficient
and ecologically safe.
When pH control is critical to your process or effluent quality, CO2 can be a most viable
alternative.
The Chemistry
Carbon dioxide is a gas, which once dissolved into water produces as weak acid:
carbonic acid. This substance reacts immediately with alkalis such as caustic soda,
sodium carbonate and dissolved lime, turning them into neutral carbonates and
bicarbonate salts.
CO2 + H2O H2CO3
H2CO3 + 2NaOH Na2CO3 + 2H2O
H2CO3 + Na2CO3 2NaHCO3
Carbonic acid is a mild acid present in water as ions H+
and HCO3
-
, which are highly
reactive. They will immediately react with ions responsible for alkalinity of water.
PH Control System
CO2 is introduced into the high pH water by means of a diffuser, which is typically
installed in an existing pressurized pipe or at the bottom of a basin. Small CO2 bubbles
are then released by the diffusers into the high pH water. A pH probe is installed
downstream of the gas injection point, thus measuring pH in the water after absorption
and CO2 reaction. An in-line CO2 pH control installation is depicted in this document.
Porous Diffuser
pH Controller
CO2 Manifold CO2 Storage Tank
pH Probe