By: Bob Patterson and Sophie Pease
With a little practice, a siphon is a simple way to move water from one container to another. But siphon water through millions of hollow-fiber ultrafiltration (UF) membranes, and you now have an energy-efficient way to provide a community with clean, high quality drinking water.
That’s how nearly 20,000 people in the city of Pendleton, Ore., have been receiving drinking water since June, 2003—a process that not only saves the city thousands of dollars per year in operating costs, but has also dramatically improved the city’s on-demand and reserve water supplies.
For years the city had relied on the nearby Thornhollow Springs and a series of eight wells for water, until the EPA concluded that the aquifer was under the influence of surface water and raised concerns that waterborne pathogens could contaminate the springs. In 1999, the city was notified by the State of Oregon Department of Human Services (DHS) Drinking Water Program that the spring water must be filtered in order to comply with the EPA’s Surface Water Treatment Rule.
As plans for the filtration plant began developing, officials questioned the city’s continued reliance for water on the Thornhollow Springs and the wells. For decades, the city’s water needs had caused the water level of the aquifer to drop by about 2.5 to 3 ft per year.
In response to the concerns, water engineers came up with an innovative plan that would make the Umatilla River the city’s primary water source. The river would not only provide for the daily needs of residents and business, but a portion of the treated water would be fed back into the existing wells in an aquifer storage and recovery (ASR) program. With only about 13 in. of annual rainfall, water becomes scarce in the dry summer months, and the plan would let the city use the wells as a backup water supply to supplement the water treatment plant (WTP) production during lower river flows.
“Selecting the river as a primary water source introduced a new set of challenges for Pendleton, because the new treatment plant would need the capability to remove color and total organic carbon (TOC), and also be able to cope with the river’s varying turbidity,” said Tim Smith, water operations manager for the city of Pendleton. “The headwaters of the Umatilla River are in the Blue Mountains of northeast Oregon and the river can experience turbidity spikes as high as 100 NTU during periods of high rainfall and throughout the spring thaw.”
UF membranes offer a solution
UF were chosen as the best available technology and the best value solution to supply high quality water for the city’s daily water needs and for the ASR program.
After a comprehensive pilot study, ZENON ZeeWeed immersed hollow-fiber ultrafiltration membranes were selected for the new 6 mgd plant.
The filtration process starts by passing the raw water through a 0.069 in. vertical bar screen. Water is then pre-chlorinated and pumped to a rapid mixer where aluminum chlorohydrate coagulant is added prior to flowing into two flocculation tanks. After a brief retention period in the tanks the water overflows to a distribution channel that delivers water to four membrane process trains.
“We currently have eight ZeeWeed cassettes in each process train; however, the trains are sized to accommodate up to 12 cassettes each,” Smith said. “This configuration means that as the community grows and our water treatment needs increase, the Pendleton WTP can quickly increase its capacity from 6 to 9 mgd simply by adding cassettes.”
Permeate is produced by drawing water through the microscopic pores of the hollow fiber membranes using a slight vacuum. By producing permeate directly from the process tanks, clarifiers are not required—a feature that dramatically reduced the footprint and capital costs of the Pendleton WTP.
With a nominal pore size of 0.04 microns, the membranes provide a physical barrier that prevents the passage of suspended particles and pathogens in the raw water including turbidity, bacteria and viruses such as Giardia and Cryptosporidium. Coagulation combined with the UF membranes is also effective in reducing up to 95% of color and up to 75% of TOC from the raw water. The substantial reduction of organic material in the permeate greatly reduces the potential formation of carcinogenic disinfection byproducts such as trihalomethane and haloacetic acids.
Siphon saves city money
Permeate flows from the membranes to a clearwell, which is located about 30 ft below the membrane process tanks. This difference in elevation creates a hydraulic gradient that results in a siphon effect in the membranes; eliminating the need for pumps.
“Without the siphon design, we would have required eight additional pumps along with valves and variable frequency drives for each,” Smith said. “By eliminating this equipment we not only save about 4% annually on our electricity costs, but also benefit from a simpler design, fewer controls, a smaller plant, and lower maintenance costs.”
Reject water from the process trains also flows by gravity to outdoor settling ponds. Once solids have settled out, the clarified water is pumped back to the headworks of the plant and is treated again.
“By recycling the reject water, the plant operates at an overall recovery rate of better than 99%. Our only water loss is due to evaporation,” Smith said. “This means that we can maximize our water usage from the city’s limited supply.”
The operation of the system is highly automated and fibers can be easily cleaned with a clean-in-place backpulsing process that forces permeate water back through the membranes. This dislodges any particles that may adhere to the membranes. Intermittent aeration of the membranes is also used to scour debris from the fibers and provides mixing within the process tank to maintain solids in suspension.
When necessary, in-situ chemical cleaning can be automatically performed if membrane fouling reduces permeability below a specified performance level. During this process, one train can be taken offline for cleaning while the remaining three trains can increase their operating flux to compensate for the other. “Operators are only at the plant for about 20-30 hours per week,” Smith said. “During that time, we just take care of some minor maintenance tasks. For the rest of the time, the plant pretty much just runs itself.”
Monitoring equipment ensures that each train meets turbidity and particle count levels and also indirectly verifies the integrity of the membranes. In the unlikely event that turbidity or particle counts rise, an alarm will notify the operator and the appropriate action can be taken. Membrane integrity is also directly monitored by automatic pressure hold tests. Membrane cassettes are tested on a weekly basis to ensure that the membranes are achieving the required removal rate of microorganisms such as Giardia and Cryptosporidium.
Saving for the future
The current demand for the city of Pendleton is only 2.5 to 2.8 mgd, far below the plant’s current capacity. Despite this relatively low demand, the Pendleton WTP operates at its 6 mgd capacity for most of the winter, when the Umatilla River can provide the most water to the community. The excess treated water is pumped into the aquifer and stored until the dry summer months when it must make up for the reduced flow of the river. During the summer, water rights for the river only permit the city to withdraw 1.6 mgd Historically, Pendleton has used about 60% groundwater and 40% surface water to meet its needs, but since the new WTP came online, groundwater only makes up about 20% of its water usage. The first year of operation alone saved over 385 million gal. of groundwater. The state’s Water Resources Department has designated up to five wells to store the surplus, which when fully recharged, will create over 600 million gal. of fresh water reserves for the area.
About The Author: Bob Patterson, PE, is the public works director for the city of Pendleton, Ore., and Sophie Pease, is a drinking water process specialist for ZENON Environmental, Inc. Patterson can be reached at 541/966-0249 or by e-mail at [email protected]
