After replacing underground water for Yellow River water as water source of boiler feedwater for a power plant located in the north of China

the trans-membrane pressure of ultra-filter device increased from 0.025 MPa to 0.05 MPa and the net average permeability coefficient dropped from above 1 870 LMH/MPa to 937 LMH/MPa after 6 hours' unit operation.According to the analysis

the instable backwash gas pressure was one of the important factors causing sharp drop of the net average permeability coefficient of ultra-filter

which had far more influence on unit employing Yellow River than that employing underground water.By adjusting and stabilizing the backwash gas pressure at 0.05 MPa

the trans-membrane pressure of ultra-filter device decreased to 0.032 MPa

and the net average permeability coefficient recovered to about 1 300 LMH/MPa.Moreover

to increase the stability and security of backwashing process

it suggested that independent pressure reducing valve(which has accurate adjustment and good pressure reducing performance at low pressure) and safety valve should be set on each ultra-filter device.