Limestone is a traditional method for
treating acid waste neutralization (AWN). Limestone in chip or rock form
is an alkaline and it will neutralize low pH acid. The effective
ingredient in Limestone is Calcium Carbonate - CaCO3
which reacts with strong acids to form water, carbon dioxide and calcium
salts. The salts usually precipitate into a sludge that will settle into
the bottom of the tank.
In addition, another characteristic
complicates the process. When acid reacts to the CaCO3, it needs a large
surface area and an extensive amount of time for adequate results.
In addition, the limestone will become coated as a result of
precipitation. Once coated, the process degrades and the waste stream
will fail to meet acceptable pH levels. Channeling through the
limestone will also limit effectiveness. This makes it difficult to
predict the outcome and makes sizing continuous neutralization systems
unreliable. In continuous operations, the waste stream will show
varying results from acceptable to unacceptable. Emergency shut off
valves on the waste line are recommended as well as the ability to recirculate low pH waste for additional neutralization. Limestone
therefore demands monitoring and appropriate alarms for safety.
Once coated, the limestone must be treated
or replaced in order to restore its neutralization ability. The
resulting sludge should be removed before it finds itself in the sewer
adding to the salt and TDS levels. Replacing the limestone is labor
intensive and time consuming. Cleaning the limestone is equally costly
and impractical. A 5-gallon tank will use 50 lbs of limestone chips,
while a 100 gallon tank will use 1100 lbs. If not actively managed,
limestone should be used only as a pretreatment in order to raise very
strong acid above 2 pH for final treatment.
Limestone will neutralize strong acid
waste, but it requires maintenance and monitoring. Limestone should not
be relied upon in a passive system to neutralize acid waste. Automated
waste treatment systems such as the Wastech Control's
OMEGA™ Series and
LabDELTA™ Series will actively manage the neutralization process.
Using acid and caustic chemicals that readily dissolve into solution,
with adequate controls and redundant sensing, will produce a safe waste
product. See OMEGA
neutralization system features.
Based upon Wastech Controls extensive
experience, we are unable to recommend the use of Limestone for
neutralization without monitoring controls.
LIMESTONE SHOULD BE USED WITH
MONITORING CONTROLS BECAUSE:
1. Limestone does not guarantee a safe result.
2. Limestone is easily coated and is then ineffective.
3. Limestone must be replaced regularly.
4. Limestone is unpredictable.
