Minimizing Backwash Volume From
Coagulation/Filtration for Arsenic Removal
Hsiao-wen Chen, Water Research Foundation research manager
Coagulation/filtration (C/F) with
conventional anthracite–sand media in
pressure vessels is an established, economical
technology for removing contaminants
from groundwater. However, one drawback
of this process is the production of a rather
high volume of spent filter backwash water.
The disposal and/or treatment of this much
water can be burdensome for some water
utilities, especially small systems. Minimizing
Backwash Volume From Coagulation/
Filtration for Arsenic Removal (2010, order/
project #3164) is a revolutionary study that
demonstrated that ultra-light filter media
could potentially replace sand–anthracite
media to significantly decrease the required
volume of filter backwash water while
effectively and consistently removing arsenic
from groundwater in a C/F process.
The required filter backwash water volume
depends on the backwash velocity that is
sufficient to fluidize the media. Thus, it
requires less water to backwash ultra-light
filter media with a specific gravity less than 1. 3
than to backwash sand (specific gravity ~ 2.63)
and anthracite (specific gravity 1.65). The
research team at Water Quality and Treatment
Solutions, Inc. tested polystyrene (specific
gravity 1.05) and PFC100E strong-acid cation
exchange resin (specific gravity 1. 27) ultra-
light media at pilot and demonstration scale.
The key conclusions from the testing are
• The ultra-light media effectively and
consistently removed arsenic from
groundwater to below the maximum
contamination level of 10 μg/L.
• Feedwater recovery with the ultra-light
media was approximately 99%.
• The maximum backwash rate of the
ultra-light media was <20%, as high as
that typically used for conventional sand–
• The backwash regime tested in the study
effectively and consistently cleaned the
ultra-light media. Air scour was critical
for effective backwashing. It provided the
energy necessary to break up floc/media
agglomerates and small mudballs.
• The low unit backwash volume ( 30–35 gal/ft2
or 1. 2–1. 4 m3/m2) made it possible for small
systems to accommodate the spent
backwash water with an on-site septic
system or leach field. The backwash flow
rate of 3. 3 gpm/ft2 ( 8. 1 m/hr) was low
enough to be directly routed to a sewer
with no equalization basins.
• Given that a typical filtration rate is on the
order of 5 gpm/ft2, the backwash flow rate
of 3. 3 gpm/ft2 could be provided by the
production rate of a parallel filter, thereby
eliminating the need for a dedicated
backwash pumping system.