Environmental Science & Engineering - www.esemag.com - May 2005
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Greensand filter media gives high quality water
By Harold Aronovitch, Hungerford & Terry
Cape Breton Regional Municipality
(CBRM) located in
Sydney, Nova Scotia, has four
10-foot diameter 18.5 foot
straight shell horizontal manganese
greensand filters to treat 1.8 million
Imperial gallons per day. The water
source is a well containing iron and
manganese. The design water analysis
has 0.2 mg/L iron and
0.35 mg/L manganese.
The filters are designed to
use the catalytic oxidation
process with chlorine
added prior to the filters,
followed by a reaction
tank to allow added contact
time for the chlorine.
Over a five-year period
of operation, the length of
run between backwashes
was gradually getting
shorter. Investigation determined
that the reduced
run length was caused by
a breakdown of the manganese
greensand media.
After testing, it was determined
that the low silica
level in the raw water contributed
to the breakdown of the
media. A feed of sodium aluminate
was instituted to help alleviate this
problem. Sodium aluminate was fed at
a rate of 0.2 mg/L (as Al), and this
managed to stabilize the media and
enable the CBRM to handle the high
summer demand.
New media, GreensandPlus™, was
obtained from the Inversand Company.
This is more rugged than manganese
greensand, but has identical operating
characteristics, and does not require
sodium aluminate to prevent breakdown.
A pilot plant was sent to CBRM
for operation using the same feed as
the full-scale units. After chlorine
addition and reaction time, a 3.1 gpm
flow was sent to a Hungerford & Terry,
Inc. pilot plant. This plant measured 9-
1/2" ID with a 0.5 square foot surface
area. The filter bed consisted of 18" of
0.6 – 0.8 mm effective size anthracite
and 18” of GreensandPlus. After conditioning
with potassium permanganate,
the pilot plant was ready for
operation. The chlorine feed served to
oxidize the iron and manganese in the
feed water so that it would be filtered
out in the media.
Soluble manganese would be oxidized
by the manganese oxide coating
on the GreensandPlus. The free chlorine
in the feed water would then oxidize
the coating, which had temporarily
been reduced. The limiting factor of
the pilot plant was pressure drop
across the bed. Feed water from
CBRM was limited in pressure and
flow since it was fed to the pilot plant
through a garden hose.
The pilot plant operated at flow rates
between 4.4 to 6.2 gpm/square foot, and
run lengths between backwashes of 120
to 200 hours were experienced. Influent
manganese was 0.3 to 0.33 mg/L while
iron varied between 0.045 and 0.09
mg/L. With seven runs complete, an
average loading of 1,200 grains/square
foot of filter area was obtained.
Manganese greensand is expected to
have a capacity of between 500 to 700
grains/square foot of filter area.
The normal pressure drop end point
for the full-scale plant and the pilot
plant was 10 psi. In an attempt to
extend the run, the plant was operated
to a 334 hour run with a pressure drop
of 17.9 psi at the end of the run. The
pilot plant was still producing good
quality water with less than 0.005
mg/L manganese and 0.001 mg/L iron
in the effluent. The only reason for terminating
the run was a drop off in
flowrate due to the high pressure drop
across the filter bed. After backwashing,
there was no sign of damage to the
media, and the pressure drop at the
start of the next run was consistent
with that of a clean bed.
During the pilot testing, the water
treated by the pilot plant consistently
averaged less than 0.01
mg/L manganese and less
than 0.01 mg/L iron. The
effluent free chlorine
averaged between 1.5 and
1.8 mg/L as determined
by the chlorine feed to the
full-scale plant and
required for the distribution
system.
Because of the compact
size of the pilot
plant, a full 40% expansion
of the bed during
backwash was not possible.
Backwash flowrate
was limited to 9 to 10
gpm/square foot in lieu of
the normal 12 gpm/
square foot. No air scour
capability was provided
with the pilot plant. However, it is
being considered for the full-scale
plant due to the extremely long filter
runs between backwashes that can be
achieved with GreensandPlus.
The results of the pilot plant indicated
that the GreensandPlus filter
media could treat the water at the Cape
Breton Regional Municipality filter
plant in Sydney, Nova Scotia, without
breakdown due to the low silica level
in the water. It could also be operated
to higher pressure drops and longer filter
runs without any detrimental
effects. It also demonstrated that while
being operated for longer periods of
time to higher pressure drops, it could
still produce extremely high quality
water.
Harold Aronovitch is Vice President/
Technical Director of Hungerford
& Terry, Clayton, N.J.
He is a graduate
of the University of Toronto with a
degree in Chemical Engineering.
Contact: www.inversand.com.
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