TORAY MBR
Instruction Manual
06-G-MB2-NHPAF-220801
53 of 63
(2) Precautions for Element Chemical Cleaning
a.
Keep the injecting pressure below 10 kPa (100 mbar).
Avoid direct discharge
from the chemical pump to the membrane elements. Too high pressure in the
membrane element will cause damage to the membrane. Note that injecting
pressure may increase after injecting half of the chemical.
b. Make certain the module is completely submerged in the tank prior to injecting any
chemicals. The liquid level above the module should be maintained at 500 mm or
higher.
c. Elevated chemical cleaning solution temperatures increase the membrane cleaning
effectiveness. However, make certain not to allow the cleaning solution
temperatures to exceed 40 deg C. Conversely, lower cleaning solution temperatures
reduce the effectiveness of the chemical cleaning process. Try to maintain the
cleaning solution temperature as high as possible under 40 deg C.
d. When starting up the MBR after a chemical cleaning, some residual cleaning
chemical may be present in the filtrated water for 2 or more filtration cycles. It is
recommended that the filtrate flow be directed back into the raw water tank until no
residual cleaning chemical is detected. Before directing the filtrate from the raw
water tank back to the normal filtrate discharge line, test the filtrate pH and chlorine
concentration.
e.
If a sodium hypochlorite cleaning is immediately followed by acid cleaning or
in reverse order, make certain that these chemicals never come in contact
with one another in the cleaning tank or piping.
Mixing of the two chemicals will
generate toxic chlorine gas.
f. If filtrate is used as makeup water for the cleaning solution, there exist the risk of the
formation of insoluble calcium compounds when sodium hypochlorite is mixed with
the filtrate. If a high calcium concentration is expected in the makeup water, confirm
the generation of precipitation by performing a bench-scale sodium hypochlorite
addition test. If a whitish precipitate of calcium forms, it is recommended that tap
water be used for the makeup water supply instead of filtrate.
g. As a result of contacting with hypo solution, some activated sludge nearby
membrane surface can be destroyed, which leads to a foaming issue. This sludge
damage will be recovered in the course of time after operation restart and thus not a
big trouble.
