17 DEC 2003
DESIGN EXAMPLE: CONTINUOUS BACKWASH FILTER
UPFLOW CONTINUOUS BACKWASH FILTER. The water to be filtered is
pumped from a groundwater remediation system. Five recovery wells each yield up to
0.378 L/s (6 gpm) of groundwater contaminated with diphenyl and diphenyl ether with
suspended solids concentration up to 50 mg/L. The groundwater will be treated using a
continuous backwash filter to remove the suspended solids, followed by activated car-
bon adsorption for removal of the diphenyl and diphenyl ether before discharge to sur-
face waters under the provisions of a NPDES permit. A system that could operate in a
facility with intermittent staffing, being unattended for days at a time, was desired. Con-
tinuous backwash filters were selected owing to their relatively high solids loading rate
and low maintenance. Redundant units were not deemed necessary, given the reliability
of the process. A pilot test was conducted to determine filter efficiency versus loading
rate with and without polymer addition, filter size, filter media, and other design and op-
erational parameters. This example design was based on pilot test results.
Flow: 1.893 L/s (30 gpm) total average flow from 5 wells.
TSS: 50 mg/L (50 ppm).
Manganese: 0.12 mg/L (0.12 ppm) influent (no effluent limit).
Iron: 1.10 mg/L (1.10 ppm) influent (no effluent limit).
Upflow continuous backwash filter with silica sand and polymer addition.
Sludge thickening tank.
Sludge drying bed.
Continuous Backwash Filter Design Basis. Design the system using a sin-
gle upflow continuous backwash filter.
From the pilot test, the manufacturer's recommended loading rate was 1.70
L/m s (2.5 gpm/ ft2) for 1.4-mm (0.06-in.) sand media.
The manufacturer recommended a design having a filter reject rate of 0.19
L/s (3 gpm) to 0.32 L/s (5 gpm).
Upflow Continuous Backwash Filter. The manufacturer recommended a
1.70-L/m s (2.5-gpm/ ft2) hydraulic loading rate. The total influent flow rate to the filter is
1.90 L/s (30 gpm) from the wells, plus up to 0.32 L/s (5 gpm) reject flow rate.
1.90 L/s + 0.32 L/s = 2.22 L/s