Download on-demand, gravity powered, ceramic filtration facility

ON-DEMAND, GRAVITY POWERED, CERAMIC FILTRATION
FACILITY: FORGET WHAT YOU KNOW ABOUT HOW A WTP IS
SUPPOSED TO OPERATE.
Nathan D. Kutil, HDR, 1715 S. Reserve Street, Suite C, Missoula, MT 59801
[email protected], Ph: 406-532-2239
Introduction
The City and County of Butte-Silver Bow (BSB) serves the public water supply system from
three sources: the Big Hole River, Moulton Reservoir, and Basin Creek Reservoir. The Basin
Creek source has historically operated as a surface water source with disinfection and without
filtration. An administrative order from the Montana Department of Environmental Quality
(DEQ) has resulted in the need to add filtration to the Basin Creek source. Initial efforts focused
on site selection, pilot testing, hydraulic analysis, distribution system modeling, and process
selection. Based on these initial efforts, a pressurized membrane filtration process was selected
and ultimately the membrane equipment was procured by BSB.
The new facility has a firm capacity of 7 million gallons per day. The treatment process includes
fine straining, chemical injection with hydraulic jet rapid mixing, pressurized hydraulic
detention, ceramic membrane filtration, corrosion control capabilities, and finished water
disinfection. Residuals handling processes include inclined plate settling, backwash recovery
ceramic membrane filtration, sludge holding, and residuals pumping. The facility houses office
space for employees, workshop and maintenance areas, a laboratory, and a large conference
room for use as a membrane operators training facility.
Background
One of the primary design objectives was to minimize energy consumption by maximizing the
use of gravity. The site for the new water treatment plant (WTP) was selected based on a
hydraulic analysis which allowed placement of the membrane equipment to be at the perfect
elevation to accomplish that goal. Other water treatment plants have been designed to operate by
gravity, what makes Basin Creek unique is that it operates as an on-demand system. As
complicated as this was to design, it is quite elegant and very simple to operate. Staff does not
make decisions about how much water to treat in order to meet demand, but rather demand itself
sets the flow rate through the WTP at any given moment.
Flow control is accomplished by the interaction between distribution system water demand and
plant hydraulics. If water demand decreases, it causes pressure in the WTP to rise and the
filtration rate decreases. When system demand increases a greater differential pressure between
the reservoir and the distribution system causes the flow rate through the plant to increase and
match the system demand. In this way, the WTP functions as an on demand facility.
Downstream from the filtration process, finished water pumping is not required under most
scenarios. The hydraulics (dependent on degree of filter fouling, water surface elevation in the
reservoir, and flow rate) allow for complete gravity flow through the WTP and into the
distribution system. For scenarios when finished water pumping is required, filter effluent (FE)
pumps pull suction from a standpipe which retains any residual pressure available following
filtration. The FE pumps are sized to meet both minimum and maximum system demand through
the use of variable frequency drives (VFDs) and pump control valves. The FE pumps discharge
to a standpipe to provide the exact pressure and flow rate required in the distribution system.
The project team developed a plan for overcoming the reality of severe winter weather in the
region by procuring under ground piping materials so they would be available to Contractors at
notice of award. This allowed Contractors to begin building immediately in a rush to beat the
winter weather. The buried hydraulic detention pipeline (HDP) is a 600 foot long 48-inch
diameter pipe with internal baffles to promote flocculation and providing 10 minutes of detention
time for chemical activation. The HDP internal baffles represent a novel approach to promoting
chemical pretreatment in a way that has never been done before.
Membrane Filtration
The Basin Creek WTP uses a cutting edge ceramic membrane filtration system which is one of
the first of its kind to be installed in the United States. The ceramic has many benefits over the
polymeric membrane filters that have customarily been used in the US including physical
strength, chemical resistance, and longer life expectancy. Another benefit of this filter is the
higher overall recovery achievable. The Basin Creek WTP includes a recovery skid designed to
filter the backwash waste from the process resulting in an overall 99.95% production capability.
In the US membrane market for drinking water filtration, we have grown accustomed to required
chemical clean in place (CIP) of the filters on a monthly basis. The CIP interval for the ceramic
filters installed at Basin Creek is every 6 months. This means that chemical use is decreased,
operation staff labor is cut, and energy use associated with heating chemicals and pumping is
reduced by 1/6th over conventional systems. The filtration system will operate on a 4 hour
backwash interval with a weekly chemically enhanced backwash (CEB).
Flow Control
From a chemical pretreatment and disinfection stand point, the on-demand aspect is somewhat
problematic because the flow rate through the WTP is constantly in flux and dosing would be
erratic in response. For this reason, designers conceived a plan to effectively set the demand in
the distribution system to result in a stable daily flow rate through the WTP. By placing a pump
station in the distribution system that moves treated water from the Basin Creek WTP to other
pressure zones in the City the operators can regain the ability to choose how much water to treat
on a daily basis similar to how other WTPs operate.
A new pump station in the distribution system is included as part of this project. Since the Basin
Creek zone is the lowest pressure zone in the system there will be times when operators choose
to pump water produced by the new WTP to upper zones. The pump station will be equipped
with a flow meter that constantly monitors the flow into the low zone that the WTP is to serve.
The pumps will also have a flow meter and will run on their VFD’s to maintain an operator
entered daily flow. For instance, if the operators wish to make 7 MGD and the demand in the low
zone is 4 MGD then the pumps will run at 3 MGD. In this way they can effectively set the
demand for a stable operating condition at the new WTP while moving water to the upper zones.
The added benefit of this pump station is that operators can now select the best water source in
various seasons and transfer anywhere within the distribution system. For instance, when the
water in the Big Hole River is bad (in the spring runoff), but the water in Basin Creek Reservoir
is good, operators can turn down the Big Hole WTP and use more water from Basin Creek using
the new pump station to transfer to upper zones historically served by the Big Hole WTP.
Conclusion
Additional features of the Basin Creek WTP will be discussed in the presentation. When it is all
added up, it becomes evident that the Basin Creek WTP is the most reliable, energy efficient,
innovative, and technologically advanced WTP in the United States today.