Page 501 - Water and wastewater engineering

12-18 WATER AND WASTEWATER ENGINEERING

Comments:

1. On the average day, the number of racks in service may be considerably less than six.
2. Because the maximum day demand generally occurs in the summer, a lower water vis-
cosity coupled with a higher TMP may permit a less conservative design.

Design Criteria
Tables 12-3 and 12-5 provide a summary of range of design values for MF and UF membranes.

Operation and Maintenance
In addition to maintaining the mechanical system and providing chemical cleaning at appropri-
ate times, the system must be carefully monitored to ensure the integrity of the membrane has
not been breached. Routine testing for membrane integrity may be by air pressure tests or sonic
monitoring. In the pressure-hold technique, the rate of decay of applied air pressure is monitored.
In the sonic test, an acoustic sensor is placed against a module and the noise of air bubbling
through a breach is detected.

Hints from the Field. The following hints are from Pressdee et al. (2006):

• Long-term pilot studies rather than short-term evaluations are preferred because the long
testing period allows for identification of source water quality issues that may only occur a
few times per year.

TABLE 12-5
Range of design values for membrane filters

Parameter Range of values Comment
Permeate flux
2
Pressurized 30–170 L/m · h
2
Vacuum 25–75 L/m · h
Transmembrane pressure (TMP)
Pressurized 20–100 kPa
Vacuum 7 to 80 kPa
2
Area of membrane/module 8–70 m /module
Modules/rack 2–300
Module dimensions
Diameter 100–300 mm
Length 1–6 m
Filter run duration 30 90 min
Backwash
Duration 1–5 min
Pressure 35–350 kPa
Flow rate 6 L/min/m 2
Time between chemical cleaning 5–180 d 30 180 d common