Improving with Age
Aging or underperforming conventional water treatment plants are retrofitting with UF membranes and benefiting from improvements in performance and operational cost savings
- By Tony Kobilnyk
- May 01, 2005
Too much chlorine. That's what the residents of two Tennessee cities said about their drinking water following a recent upgrade to their water treatment plant (WTP). As part of the upgrade, granular filter media from the conventional plant were removed and immersed, hollow-fiber ultrafiltration (UF) membranes were added to the plants treatment processes.
"The water produced by our new ultrafiltration membranes is so clean that there is no significant organic matter entering the distribution system to consume the residual chlorine," says Randal Braker, general manger of the Duck River Utility Commission (DRUC). "We have significantly reduced the chlorine levels and early complaints about the high level of chlorine have changed to compliments about the high quality of the water."
Retro is the Rage
Braker's plant, which provides water to about 48,000 people in Manchester and Tullahoma, Tenn., is among the first in North America to remove granular filter media and retrofit with low-pressure UF membranes. This strategy is being increasingly evaluated and implemented by municipalities with aging or underperforming conventional treatment plants. Rather than build a new plant or expand with conventional technology, water managers are opting to retrofit plants with hollow-fiber UF membranes, and doubling or even tripling plant output while significantly improving treated water quality. This can often be achieved with little or no expansion of the plant footprint.
For many conventional treatment plants, immersed hollow-fiber membrane cassettes can simply replace granular filter media in the existing basins. During the retrofit process, granular filter media are removed, piping is modified to accommodate the membrane system and increased flow parameters, and pumps and blowers are installed for permeate production, backpulse cleaning, and aeration. For other plants, it may make more sense to use existing basins for other necessary water treatment processes and construct new tanks to accommodate the membrane ultrafiltration equipment.
Such was the case for the DRUC, which needed to increase the output from its 7.5 million gallons per day (MGD) plant and also minimize taste and odor problems that were not being adequately controlled by the activated carbon caps on the top of its granular filter media. The DRUC examined several options for the plant expansion including conventional technologies, pressurized membrane systems, and low-pressure, immersed membranes.
"Our twenty-year life cycle evaluation showed that UF membranes would provide the best water quality for our customers at the lowest possible cost," Braker says. "In fact, the evaluation showed that the cost of expanding the plant with membranes was virtually the same as using conventional technology, and would provide much higher quality water with greater plant expandability."
UF Membranes Replace Granular Filter Media
Typically membrane cassettes are immersed into the existing filter media basins once the granular media are removed and the necessary piping is added to accommodate the membrane system. However, the DRUC implemented a slightly modified retrofit strategy to further reduce costs, to ensure that the plant would remain operational during the expansion, and to maximize the benefits of the membrane system. Rather than retrofit the membranes into the existing granular media basins, the DRUC constructed new membrane tanks ahead of the existing basins, and converted the basins into granular activated carbon (GAC) contactors.
The original clarifiers also remained and now provide pretreatment of the raw water prior to membrane filtration. The Normandy Reservoir, which is the DRUC water source, typically provides water with a turbidity ranging from one to 20 nephelometric turbidity units (NTUs); however, the plant can experience turbidity spikes up to 120 NTU. After pre-oxidation, flocculation, and clarification, the membranes receive feed water ranging in turbidity from 0.3 NTUto a maximum of 3.0 NTU.
Clarified water flows to the membrane tanks where 15 cassettes of membranes filter up to 10 MGD of settled water. The ZeeWeed® 1000 is ZENON's unsupported hollow-fiber membrane that is specifically designed to treat water with low concentrations of suspended solids. At the DRUC WTP, each cassette currently holds 60 membrane elements, but can hold up to 72 elements. This extra capacity means that the plant can easily expand treatment capacity from 10 MGD to 12 MGD simply by adding membrane elements. Additional membrane cassettes can also be added to bring the plant's treatment capacity up to 15 MGD without requiring construction or major equipment additions.
Thousands of membrane fibers are loosely suspended in each membrane cassette and a slight vacuum is applied to the end of each membrane fiber to draw water through microscopic pores and into the hollow fibers. With a nominal pore size of 0.02 microns the membranes form a physical barrier to suspended solids and provide greater than 4-log removal of pathogens such as Giardia and Cryptosporidium. Rejected particles remain in the process tank.
The operation of the system is highly automated and fibers can be easily cleaned with a backpulsing process that forces permeate water back through the membranes. This dislodges any particles that may adhere to the outside of the membranes. Simultaneously, aeration of the membranes is also used to scour debris from the outside surface of the fibers prior to overflowing the backwash water to the drain.
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 off line for cleaning while the remaining three trains can increase their operating flux to compensate for the other.
"We believe that the GAC in our carbon contactors will last about 66 percent longer due to the high quality water supplied by the UF membranes," Braker says. "Instead of changing the carbon every three years, we now expect it to last up to five years, which should save us about $50,000 per year in GAC expense."
GAC is necessary to treat taste and odor complaints that typically increase in the fall when algae levels rise in the Normandy Reservoir. State regulations also require the DRUC to maintain a residual level of chlorine in the treated water, but Braker says that chlorine feed levels had to be drastically reduced because the chemical was not being dissipated in the system. With the previous conventional plant, the DRUC was adding about 2.5 milligrams per liter (mg/L) of chlorine to the clearwell, but now adds only about 1.5 mg/L. Trihalomethane and haloacetic acid levels, byproducts of chlorine disinfection, are also dramatically reduced, falling from 35 micrograms per liter (ìg/L) to 10 ìg/L and 24 ìg/L to 5 ìg/L respectively.
Regulations Require Retrofit in Utah
Achieving higher water quality and increasing treatment capacity was also the main reason for a similar retrofit in Draper, Utah. In February, 2004 the city commissioned its newly retrofitted plant where granular filter media was replaced with UF membranes. The plant treats water from several mountain streams, but the aging conventional water treatment system was unable to consistently meet the 0.3 NTU turbidity maximum for treated potable water under the EPA's LT1 Enhanced Surface Water Treatment Rule.
During periods of high rainfall and throughout the spring thaw, this mountain community could only meet the turbidity regulations by cutting the plant's output; at times by as much as 50 percent. Water managers wanted to upgrade the plant to ensure that it could consistently produce sufficient quantities of high quality water and also more than double the plant's treatment capabilities from 3 MGD to 6.6 MGD to meet increasing demand.
Unlike the Duck River WTP which is located in a rural area, the WaterPro WTP is surrounded by upscale residential property. As a result, any retrofits to the plant would have to remain within the existing footprint. The company examined several treatment options including conventional systems, high rate settling, and membranes.
Increased Capacity without Increasing Footprint
"Of all the solutions we looked at, membranes produced the highest quality water during the pilot testing and also gave us the ability to expand our plant within the existing footprint," says David Gardner, Utilities Manager for WaterPro. "We're now ahead of the game when it comes to meeting standards because membranes produce water that exceeds state and federal drinking water requirements. We expect that this will remain true for quite some time and the membranes will enable us to meet these regulations well into the future."
Gardner says that the configuration of the ZENON ZeeWeed® 1000 membranes was also a key factor in selecting the membranes for the retrofit since the modular cassettes provided the best fit into the plant's existing granular filter media basins. Although new piping was required along with permeate pumps, backpulse pumps and other associated equipment, the retrofit was still considerably more cost effective than constructing a new plant. Today, two compact membrane trains each hold five membrane cassettes and more can be easily added to increase capacity.
Capital and Operational Savings
Further savings are realized in plant operations because coagulation chemicals are no longer required. UF membranes provide direct filtration of the raw water, and according to Gardner, chemical costs will be much lower than that of the old conventional WTP, which required the use of large amounts of coagulant to achieve flocculation in the cold mountain water. Automated operation also reduces costs since an operator is required at the plant for only about 20 hours per week. The plant's SCADA (supervisory control and data acquisition) system enables the operator to monitor the plant remotely when he is not on site.
"Our capital costs were also minimized since we did not have to expand the plant footprint," Gardner says. "We have the capability to expand our capacity to 10 MGD, more than three times what our old plant could produce, simply by adding membrane cassettes to our former granular filter media basins. Accommodations for the expansion are already in place so when we're ready to expand, no new construction will be required to increase capacity."
However, during the retrofit one new building was constructed to house a power generating turbine that supplies electricity to the WTP. To drive the turbine, a portion of the water from the mountain creeks is directed into a conduit that channels water to the turbine. Once past the turbine, the water continues into the plant for treatment. Even during low stream flows, the turbine can generate two to three times more power than the WTP requires, and the surplus is sold to the local power grid. Gardner estimates that between the electricity savings at the plant and the electricity sales to the community, the turbine will pay for itself within about eight years.
Cost, along with regulatory compliance and security of supply are among the most critical issues that utilities face when making decisions about water treatment processes and technology. Gardner notes that in 1999, when WaterPro began examining treatment options for its plant upgrade, membranes were still relatively expensive compared to conventional technologies. But by 2002, when company decision makers selected the upgrade strategy, membrane prices had fallen considerably.
"Our cost analysis showed that upgrading our WTP with UF membranes was actually more cost effective than conventional technologies over the life cycle of the project," Gardner says. "Not only was the price a good fit, but the membrane cassettes fit exceptionally well into our existing tanks."
This article originally appeared in the 05/01/2005 issue of Environmental Protection.