Optimizing nanofiltration membrane properties to meet
The Biscayne Aquifer provides a source of drinking water for the city of Deerfield Beach, Fla., and many other communities in southeast Florida.
This shallow, highly rechargeable aquifer covers 4,000 sq mi, ranges in depth from 0 to 240 ft and contains a high content of natural organic matter in the range of 8 to 20 mg/L total organic carbon (TOC). Treatment of the aquifer is required to provide drinking water of suitable quality.
The average hardness is 235 mg/L as CaCO3. A level of less than 90 mg/L CaCO3 is desirable in most softened waters. The high organic level in the aquifer imparts color in the range of 30 to 70 color units. A level of less than 15 color units is desirable in treated water.
In Deerfield Beach, an existing lime softening process has been used to meet these finished drinking water goals. However, the high concentration of chlorine required to lower the color to acceptable levels also converts a portion of the natural organic material to unwanted by-products, known as trihalomethanes and haloacetic acids.
The EPA has set legally enforceable standards for these two carcinogens. The limits are 0.080 mg/L and 0.060 mg/L for trihalomethanes and haloacetic acids, respectively. The feed water has an average trihalomethane formation potential of 400 mg/L and an average haloacetic acid formation potential of 300 mg/L.
To meet their growing need for drinking water and to meet current and soon to be implemented regulated limits, Deerfield Beach opted for a nanofiltration membrane process, also know as membrane softening, over expansion of the existing lime softening process. The permeate from the nanofiltration plant is blended with the lime softened effluent; therefore the nanofiltration permeate limits were set lower than those listed above.
The separation objectives of the nanofiltration process are to reduce the feed hardness from 235 mg/L CaCO3 to a range of 20 to 33 mg/L CaCO3, to reduce natural organic matter so the permeate contains less than two color units, and to achieve a total trihalomethane formation potential less than 0.040 mg/L and a total haloacetic acid formation potential less than 0.030 mg/L.
The section of the Biscayne Aquifer underlying Deerfield Beach also contains an average of 1.5 mg/L of iron. An additional objective is to reduce the iron to less than 0.20 mg/L. Unaerated, ferrous iron is rejected at the same rate as the hardness ions of calcium and magnesium.
To be economically viable, the nanofiltration units must operate at a transmembrane pressure of less than 90 psi and at a recovery of 85%. Transmembrane pressure is the feed pressure minus the final permeate pressure, and the recovery is calculated as the percentage of the permeate flow to the feed flow. Also, the membrane must possess a resistance to fouling due to adsorption of natural organic material on to the membrane surface. At 85% recovery, the concentration of organics in the nanofiltration concentrate is approximately 6.5 times the concentration in the feed.
To meet the demands of this separation objective, Hydranautics developed a new class of fouling resistant, selective rejection nanofiltration membranes. These new membranes, labeled the ESNA1-LF series, can be adjusted in the manufacturing process to meet the specific hardness passage requirements of individual water utilities. They have a high rejection rate of natural organic material and exhibit a resistance to organic adsorption fouling.
The 10.5 mgd Deerfield Beach nanofiltration system consists of five units, each with a capacity of 2.6 mgd. Only four units are required to meet the total demand, so the fifth unit is in standby mode. Each unit operates at 85% recovery and is a 48-24 two-stage array of pressure vessels. Each pressure vessel contains seven spiral wound nanofiltration elements for a total of 504 ESNA1-LF1 nanofiltration elements per unit. A total of 2,520 elements were supplied for the project. The units operate at an average flux of 13 gal per ft of membrane per day.
The pretreatment to the nanofiltration system consists of sulfuric acid addition and scale inhibitor injection followed by five micron cartridge filtration. This is the typical pretreatment to nearly all of the reverse osmosis and nanofiltration systems in Florida that operate on unaerated well water supplies.
The permeate is sent to a degassifier for carbon dioxide reduction and later blended with lime softened effluent. The nanofiltration concentrate is directed to a pressurized sanitary main.
All five units were operating by December 2003. They were within the specified ranges at startup and continue to meet specifications after startup. The system hardness rejection averages 95%, and the total trihalomethane and haloacetic formation potentials of the permeate are well below limits.
The average flux decline of all five units is approximately 10% after two years of service. To date, no unit has required a cleaning based on flux decline even though the feed TOC levels have been measured as high as 36 mg/L.
In addition to the successful startup of this plant, the 40 mgd Boca Raton nanofiltration plant, the largest in the world and located just four miles north of Deerfield Beach, was brought online during the period of August 2004 through April 2005.
This plant utilizes ESNA1-LF2 membranes, a looser nanofiltration membrane tailored for higher hardness passage than the ESNA1-LF.
This was possible due to the absence of iron at the Boca Raton location.
In conclusion, the new ESNA1-LF has proven to be a low fouling nanofiltration with stable operating performance and hardness rejection that can be tailored as needed.