NIRE-170-1-6
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The color of healthy activated sludge is typically brown, however this color can differ among
plants, depending on wastewater composition, organic load etc. If an unexpected change is
observed in sludge characteristics, check current operating conditions such as BOD load,
MLSS concentration, DO, pH, temperature and/or wastewater composition, and take
appropriate corrective actions.
(4) Water temperature
The desirable water temperature is 15 degree C to 40 degree C. If the temperature is out of
this range and the activated sludge properties are not optimum, it is recommendable to install
a temperature control device to cool or heat the liquid.
(5) DO
The aeration tank and the membrane tank should be kept in an aerobic condition. If DO is
less than 1.0 mg/L, take necessary actions such as increasing the air-flow rate of the
biological aeration unit, or lowering the sludge concentration by increasing the excess sludge
discharge rate. In the case where the air supply for biological aeration requirements and air
piping for membrane scouring comes from the same blower, do not exceed the
recommended range of membrane scouring air flow rate to the diffuser. To increase DO
concentration of the liquid within the membrane tank adjust the separate biological aeration
system.
(6) pH
The recommended pH range is 6 to 8. If pH is out of this range and the activated sludge
properties are not optimum for filtration, adjust the pH by gradually adding acid or alkali.
(7) MLSS
MLSS concentration in the membrane tank is recommended to be between 7,000 to 18,000
mg/L. If MLSS is too low, increase the MLSS concentration in the membrane tank by
reducing the sludge discharge rate. If the MLSS concentration is too high, check the RAS
(return activated sludge) flow rate. If it is determined that the RAS flow rate is too low
increase the RAS flow rate to the proper value. If RAS flow rate is correct, increase excess
sludge discharge rate.
(8) Water levels
Check to confirm that the water levels of the membrane tank are being maintained within the
recommended values. If it is determined that the levels are not as per the design, check the