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9.1.2 Acid Selection
Acids to remove inorganic substances should be selected from various points of view such as
cleansing effect, waste chemical solution treatment, toxicity, corrosive nature, and costs.
Classification of acid cleaning solutions
Precautions
According to our experience, the chemical most effective to remove all kinds of inorganic matter
is a mixture of nitric acid and oxalic acid. This mixed acid is used when the membrane is tested
for flux retention by removing the contaminants on the membrane as much as possible.
In many cases, flux is better recovered by cleaning the membrane with alkali to remove organic
substances and then with acid.
Standard chemical cleaning:
(2,500mg/l NaClO +1% NaOH) × 6 to 8 hrs
Rinse
1% Citric acid × 1 to 2 hrs
Rinse
Chemicals
Substance
Difficult to
Remove
Effluent Treatment
Toxicity
Corrosive
Nature
Nitric
Acid
(HNO
3
)
Mn cannot be
removed.
Easy (only neutralization)
Poison
Corrosive
Hydrochloric
Acid (HCl)
Mn cannot be
removed.
Easy (only neutralization)
Poison
Corrosive
Sulfuric Acid
(H
2
SO
4
)
Mn and Ca
cannot be
removed.
Easy (only neutralization)
Poison
Corrosive
Oxalic acid
Ca can hardly be
removed.
Difficult
(Neutralization
+
Organic substance treatment
required)
Poison
None
Citric Acid
Ca can hardly be
removed.
Difficult
(Neutralization
+
Organic substance treatment
required)
None (food additive)
None