NOVEMBER 2009
Operations
Eileen McCarthy Feldman and Ian A. Crossley are with
Hazen and Sawyer, P.C. (www.hazenandsawyer.com), New York,
and Richard Ruge is with Westchester Joint Water Works
(www.wjww.com), Mamaroneck, N.Y.
Filtration using membrane fibers is an excellent technique to produce high-quality drinking water. However, as constituents are removed from the water, care must be taken
to treat, recover, and dispose of the waste streams to ensure economical membrane
performance.
BY EILEEN M c CARTHY FELDMAN, IAN A. CROSSLEY, AND RICHARD RUGE
Minimize Microfiltration
Membrane Maintenance
W
estchester Joint Water Works (Mamaroneck, N.Y.) and its consulting engineer designed a 20-mgd water treatment
plant for the Rye Lake supply in Harrison,
N.Y. To determine the best treatment process for Rye Lake water, the design team
conducted multiple evaluations and pilot
studies. Immersed microfiltration (MF) membrane treatment was identified as the best process for several reasons,
including ease of operations and maintenance, treatment
effectiveness, and low waste generation.
In a membrane system, solids accumulate on the
membrane fiber surface, restricting flow. To remove con-
1 Opflow November 2009
taminants from the fibers, filtrate flow is reversed (backpulsed) every 15–30 min to return the membrane to its
design operating flux. In addition, air agitation is used to
reduce deposition of particles on the fiber surface. Consequently, solids accumulate in the membrane tank, and
each membrane tank is drained to a waste holding basin
every 2–3 hr.
After equalization, this waste stream needs to be
treated prior to use, recycle, or disposal. In addition,
the membranes have to be periodically chemically
cleaned. The design team developed several strategies to
minimize these and other chemical waste disposal
operating costs.
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PHOTOGRAPH: GE WATER & PROCESS TECHNOLOGIES
Consider Best Practices for Cleaning and Waste Disposal
The Kamloops Centre for Water Quality (British Columbia,
Canada) is one of the largest membrane facilities in
North America, with a treatment capacity of 160,000
m³/day and the installed hydraulic capacity to expand
to 200,000 m³/day. Membrane cleaning takes place by
a combination of tank aeration at 10-second intervals
and a 30-second backwash occurring every 15 minutes.
Operations
MEMBRANE CLEANING
Several cleaning steps are used to remove contaminants from membrane fibers and keep them
free of fouling. Frequent air agitation, backpulsing, and deconcentration are used to remove
solids from the fibers. More aggressive cleaning
programs—maintenance and recovery cleaning—are used with chemicals to remove fouling.
The wastes produced from each of these steps
must be handled carefully and within the constraints of local utilities, available property, and
capital/operational costs.
Aeration and Backpulsing. Airflow is introduced at the bottom of the membrane elements
to create turbulence, which cleans the outside of
membrane fibers and allows them to operate at a
higher flux. Membrane cleaning by backpulsing
is achieved by reversing the permeate flow (filtered water) and backpulsing the fiber’s lumen
with permeate at low pressure.
The membranes will be backpulsed with
water and air scour at intervals of 15–30 minutes
to reduce membrane fiber fouling. Aeration also
oxidizes iron and organic compounds, resulting
in better filtered water quality than that provided
by direct ultrafiltration alone.
Wastewater Deconcentration. Waste­
water will be generated during deconcentration. Every two to three hours a backpulse clean
(with a full membrane tank) will be conducted.
The membrane tank will be deconcentrated by
draining the wastewater by gravity to the wastewater storage tanks located below the membrane tanks. The deconcentration waste will
contain all of the solids collected in the membrane tank and removed by filtration. The wastewater will account for 5–8 percent of the plant’s
total capacity.
Extended Backpulse Cleaning. Once each
day an extended backpulse maintenance clean
will be conducted automatically with sodium
hypochlorite to control biofouling on the membrane surface and eliminate potential regrowth
in the permeate piping. The process begins by
performing a routine backpulse clean of the
membranes. A deconcentration step is completed by draining the tank’s contents by gravity to wastewater storage tanks located below the
membrane tanks. Next, a backpulse is carried
out with filtered water and sodium hypochlorite
at a concentration of 50 mg/L. A third backpulse
is implemented with only filtered water to dilute
the sodium hypochlorite solution and partially
refill the tank. Any remaining sodium hypochlorite solution is neutralized within the membrane
tank using sodium bisulfite and sodium hydroxide for pH adjustment. The neutralized waste
is drained to the wastewater storage tanks. The
Rye Lake Water Quality
Although Rye Lake’s raw water quality is good, the plant’s design provides flexibility
for changing conditions.
Parameter
Units
Maximum
Raw Water
Minimum
Average
Turbidity
ntu
2.5
0.5
1.1
Temperature
°C
22
3
12
mg/L
3.3
1.4
1.8
TOC
Apparent color
tcu
179
12
30
cm-1
0.100
0.030
0.060
cfu/mL
470
4
116
Alkalinity
mg/L as CaCO 3
20
6
11
Hardness
mg/L as CaCO3
26
8
14
7.6
5.4
6.7
mg/L
0.158
0.030
0.070
UV254
Heterotrophic plate count
pH
Iron
3 Opflow November 2009
estimated volume of neutralized wastewater produced during each maintenance clean is 5,000
gal for one tank and about 30,000 gal/day for
the plant as a whole.
Recovery Cleaning. The most intensive
membrane cleaning method takes place when
membrane permeability falls below its normal
operating range. Recovery clean-in-place (CIP)
is anticipated to occur once each month using
sodium hypochlorite and citric acid. The procedure involves taking a membrane train offline
and draining the membrane tank to wastewater tanks. An initial backpulse with concentrated chemical is applied for 45 seconds. Until
the correct chemical concentration for soaking
is achieved, the membranes are backpulsed multiple times using the CIP pumps. To improve
CIP effectiveness, the chemicals are combined
with heated water at 20–35°C (68–95°F). This
is especially important during winter months
when the source water is cold.
When backpulsing is complete, the membrane tank is refilled with filtered water using the
backpulse pumps until the liquid level covers the
membrane fibers. The resulting target concentrations are 250 mg/L for sodium hypochlorite
and and 1,000 mg/L for citric acid. To enhance
cleaning effectiveness, the membranes are soaked
in the residual cleaning solution for four to six
hours. The recirculation pump and periodic aeration will be used to circulate the cleaning solution in the membrane tanks to ensure adequate
mixing during the soak period.
Following the soak, the cleaning solution is
drained to gravity to separate waste CIP tanks,
one for chlorine solution and one for citric acid
solution. The two streams can’t be allowed to
mix because free chlorine gas will be produced.
The waste cleaning solution is neutralized by
adding sodium bisulfite and sodium hydroxide
to the waste tanks using dedicated recirculation
pumps.
Deconcentration Wastewater Equalization. The waste volume represents 5–8 percent
of plant flow. The wastewater storage tanks store
and equalize the large deconcentration volumes
of wastewater from the membrane tanks. To protect against the buildup of settled solids, the inlet
channel in each tank has perforations at the bottom to provide turbulence and mixing.
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Deconcentration Wastewater Treatment.
Deconcentration wastewater can be treated with
a second stage of membranes; however, the flux
rate would be lower than in the primary membranes because of the wastewater’s high concentration of solids. Although it might be appealing
to use a second stage of membranes and send
the permeate directly to distribution, this process presents potential regulatory challenges
because the second-stage permeate might not
be approved for use as drinking water. Instead,
it may have to be recycled to the head of the
plant, which is costly. Rather than using a second stage of membranes, a less expensive option
of inclined plate settler/thickeners (IPSTs) is proposed to treat the wastewater.
There will be two streams generated by the
IPST units: supernatant (clarified water), which
is to be pumped to the head of the plant at less
than 5 percent of the feed flow, and thickened
solids, which intermittently flow by gravity to
the thickened­-solids storage tanks.
Deconcentration wastewater is pumped from
the wastewater equalization basins to the IPSTs.
Ferric chloride (2 mg/L to 10 mg/L) is added at
a static mixer in the wastewater stream to assist
particle settling in the clarification and thickening processes.
One of the two flocculation trains will
be used to condition the water prior to the
IPSTs. The hydraulic detention time in one
train of flocculators will be 50–120 minutes
for maximum and average wastewater flows,
respectively.
The two 16- by 28-ft tanks are fitted with
inclined plate settler plate packs that are conservatively designed with a 0.3 gpm/ft2 loading
rate and 80 percent plate area effectiveness. Each
tank will have two rows of plate packs located
above a rectangular-to-circular transition zone to
the thickener section.
Two integral 16-ft diameter thickeners are
provided and rated at less than 10 lb/day/ft2 solids loading. The thickener has a conical bottom
and mechanical scraper to help remove thickened solids from the tank. Thickened-solids
concentrations between 2–5 percent by weight
are anticipated.
Wastewater Recycle. Supernatant from the
IPSTs flows by gravity to balance tanks and is
www.awwa.org/opflow pumped to the head of the plant at a rate less
than 5 percent of the plant flow.
Thickened-Solids Handling. Thickened solids will flow by gravity to thickened-solids storage tanks adjacent to the IPSTs. The thickening
units will release thickened solids intermittently
by automatic valves controlled to open and close
at intervals, depending on the influent flow rate
and thickened-solids concentration.
Thickened-solids storage tanks are large
enough for three days’ storage at peak flow and
maximum water quality conditions. Each storage
Process Representation
Raw water can be sent directly to membrane filtration or coagulated and flocculated
prior to membrane filtration.
PHOTOGRAPH: ARTIST
Feature Header
Water treatment using membrane filtration must incorporate
extensive cleaning processes to ensure that membranes
remain effective and unfouled.
November 2009 Opflow 4
Operations
tank will be equipped with a 5-hp submersible
mixer that will operate intermittently when its
respective tank volume is 75 percent or greater.
The thickened-solids tanks will have sloping bottoms for cleaning purposes.
Dewatering and Disposal Options. Several options for thickened-solids handling were
investigated during design, including
n transfer to freeze–thaw lagoons,
n dewatering with centrifuges and cake transfer
to tractor-trailer for offsite disposal,
n direct transfer of thickened solids to tanker
truck for offsite disposal, and
n direct discharge of thickened solids to sanitary sewer.
Membrane Cleaning
Once a day, an extended backpulse
with 50 mg/L sodium hypochlorite
is recommended to control biofouling.
Direct transfer of thickened solids to tanker
trucks was determined to be the most cost-effective alternative for disposing of residuals. While
lagoons have lower operational costs, the location of the plant close to upscale housing and a
golf course precluded their use. The centrifuge
dewatering option, although feasible, resulted in
high capital and operational costs. Westchester
County Department of Environmental Facilities wouldn’t grant a sewer discharge permit for
thickened solids disposal.
MAINTENANCE AND RECOVERY CLEANING
Because it has a much lower chemical concentration than the waste generated from a monthly
cleaning regime, neutralized maintenance cleaning waste is disposed to the wastewater tanks for
recycle to IPSTs.
Following monthly CIP recovery cleaning
of membrane tanks, the cleaning solution will
be drained by gravity to and neutralized in the
waste CIP tanks. Membranes are chemically
cleaned in place. Consequently, all construction
materials in contact with the cleaning solutions,
including tank linings, must be resistant to acids
and strong oxidants.
Wastewater from the CIP will not be recycled to the head of the plant because it would
foul the membranes. WCDEF has agreed that
neutralized cleaning wastes can be discharged to
the sewer, and a neutralization system is included
in the design for this purpose. However, if sewer
facilities hadn’t been available, chemical waste
haulage disposal costs may have rendered the
membrane facility economically unattractive.
CONSTRUCTION COSTS AND SCHEDULE
Treatment plant construction costs—excluding
raw water pumping station upgrade and existing
finished water storage tanks—is $61.5 million.
Included in this cost is $7 million for membrane
equipment. As part of an overall strategic review,
and in parallel with the treatment plant design
effort, WJWW has been pursuing alternative
water sources that involve obtaining drinking
water directly from New York City’s ultraviolet
(UV) light disinfection facilities currently under
construction in Westchester. Consequently, construction of the Rye Lake Water Treatment Facility is being held in abeyance until a decision is
reached on the feasibility and relative capital and
operational costs of this and other alternatives
under consideration.
PLANNING AHEAD
Water treatment using membrane filtration
must incorporate extensive cleaning processes
to ensure that membranes remain effective and
unfouled. Before designing a membrane filtration plant, waste from many streams must be
carefully considered. Wastewater treatment
should take into account regulatory restrictions
and backwash recycle requirements. Wastewater and solids disposal can be handled in various
ways and is controlled by available property and
sewer connections. The Rye Lake WTP incorporates innovative process flows to treat water of
varying water quality. The flexibility included in
the plant’s design allows for flocculation equipment to be used as a prefiltration step or waste
treatment process.
Authors’ Note: The authors would like to
thank Westchester Joint Water Works for permission to present this article and recognize colleagues
who assisted in its preparation.
Reprinted with permission from Opflow, November 2009,©2009 American Water Works Association. By The Reprint Dept. 800-259-0470 (11747-1109)
For web posting only. Bulk printing prohibited.