GIRE-097-2-2
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3. Chemical Cleaning of Element
Chemical cleaning of element should be conducted before the trans-membrane pressure
rises to excess. Such a pressure increase can be caused when contaminants clog the pores
of the membrane surface. The timing of chemical cleaning is determined as follows:
(1)
When the trans-membrane pressure rises by 5 kPa (50 mbar) from its initial
operating level at the same permeated water flow rate or every 6 months,
whichever comes first. For example, if an MBR was operated at 7 kPa (70 mbar)
TMP in its initial period, then 12 kPa (120 mbar) is the TMP trigger for chemical
cleaning.
(2)
The earlier chemical cleaning is much effective to recover and keep the
permeability of the membrane. From a preventive maintenance point of view,
please conduct chemical cleaning at the earlier stage of TMP increase; otherwise
the trans-membrane pressure will rise rapidly and become difficult to recover the
permeability by normal chemical cleaning procedure.
(3)
In the case that the trans-membrane pressure rises by 5 kPa (50 mbar) within 6
months, it is recommended to observe how many months it takes for the
trans-membrane pressure to rise by 5 kPa (50 mbar) and conduct chemical
cleaning periodically within safety interval. This measure is a sure way to conduct
effective chemical cleaning and leads to extension of membrane life time.
(4)
When the permeability of the membrane is not recovered by only once chemical
cleaning, there is a chance to recover permeability by carrying it out repeatedly.
4. Chemical Agents Used for Chemical Cleaning
It is quite important to select chemicals for the chemical cleaning in accordance with the type
of adherent contaminant. Cleaning under inappropriate cleaning conditions or using the
wrong chemicals may cause poorer filtration performance or damage the membrane. Please
select chemicals suitable for each contaminant. Table III-1 shows suitable chemicals and
standard cleaning conditions.
Table III-1 Cleaning Chemicals and Standard Cleaning Conditions by Contaminant
Contaminant
Chemical
Solution concentration
Amount used
Hold time
Organic matter
Sodium
hypochlorite
2,000-6,000 mg/L
(Effective chlorine concentration)
(pH is about 12)
3 L/element
1 to 3 hours
Inorganic matter
(either of the two
chemicals)
Oxalic acid*
0.5-1.0wt%
3 L/element
1 to 3 hours
Citric acid
1-3wt%
3 L/element
1 to 3 hours
