Page 195 - ro membanes

178 8. GRANULAR MEDIA FILTRATION 8.7.1.2 Pressure FiltersdKey Design Criteria
The main design criteria for single-stage dual-media pressure filters in small- and medium- size desalination plants are very similar to these of gravity filters. Design criteria by which pressure filters differ from gravity filters are presented below:
Number of filter vessels
Filter vessel diameter
Filter vessel length
Depth of filter bed
FILTRATION RATE (AT DESALINATION PLANT INTAKE DESIGN FLOW)
With all filters in service
With two filters out of service
HEADLOSS ACROSS THE FILTER VESSEL Total headloss across the filter
Net headloss available for filtration
8.7.2 Two-Stage Filters
8.7.2.1 Key Design Criteria
6e20
1.2e6 m/4e20 ft (typically 3 m/10 ft) 2.5e15 m/8e50 ft (typically 6 m/20 ft) 0.6e0.9 m (2e3 ft)
12e25 m3/m2 h (5e10 gpm/ft2) 30 m3/m2 h (12 gpm/ft2)
15e30 m/45e90 ft (avg. 20/65 ft) 7.5e15 m (25e50 ft)
Key design criteria for the first, coarse media, stage of two-stage media filtration system are as follows:
Filter type Single- or dual-media, down-flow, airewater backwash Average filter cell run length 24e48 h
FILTRATION RATE (AT DESALINATION PLANT INTAKE DESIGN FLOW)
With all filters in service
With two filters out of service
ANTHRACITE OR SAND FILTER MEDIA Anthracite layerddepth
Anthracitedeffective size Anthraciteduniformity coefficient
Sand layerddepth
Sanddeffective size
Sandduniformity coefficient
AIReWATER FILTER BACKWASH SYSTEM Maximum backwash rate
12e25 m3/m2 h (5e10 gpm/ft2) 30 m3/m2 h (12 gpm/ft2)
0.4e1.0 m (1.3e3.3 ft) 1.0e2.0 mm (typical 1.5 mm) <1.5
0.4e1.0 m (1.3e3.3 ft)
0.4e 0.6 mm
<1.5
60 m3/m2 h (25 gpm/ft2)