14 Benefits and Limitations of Utilizing a Raw-Water Blend Stream to Meet

Home , Raw water

Benefits and Limitations of Utilizing A Raw-Water Blend Stream to Meet Production and Quality Goals at a Membrane Facility

James E. Christopher

everse-osmosis membranes were caustic or lime addition to recover alkalin- initially applied to the removal of ity and raise pH to provide a stable water James E. Christopher, P.E., is a vice Rsalts, primarily sodium chloride, relative to calcium carbonate precipitation. president with the Orlando office of from brackish waters to create potable water The designer and operator are often faced Hartman & Associates Inc., a Tetra Tech from raw-water sources that previously could with adding back constituents removed by Company. not be used for public supply. Membrane the membrane process or not found in the facilities were located in coastal areas where raw water, at an increased operating cost to existing fresh groundwater supplies were meet the finished-water quality goals and City of Palm Bay inadequate to meet demands. regulatory requirements. The brackish-water membranes used in Blending raw water or water from South Regional these facilities generally reject 98 percent of another process is a technique practiced in Water Treatment Plant most dissolved constituents in the raw water, many membrane facilities to provide a The city of Palm Bay has an existing lime producing a very pure but often highly corro- mechanism to adjust the quality of the fin- softening plant with a capacity of 10 million sive water. Moreover, the quality of the per- ished water. The quantity of blending is gallons per day (MGD) and a 1.65 MGD meate is a function of the concentration of obviously limited by the constituents in the reverse-osmosis plant which provide potable constituents in the raw water, and the rejec- raw water, the finished-water quality goals, water for the city's utility customers. The tion of individual constituents cannot be and the type of membrane. Blending capa- existing water treatment facilities are located selectively controlled as in the ion-exchange bility offers the operator the flexibility to in the northeast corner of Palm Bay, and process. manipulate the quality of the finished water growth is taking place in the central, south The Safe Drinking Water Act and should be considered for incorporation and western portions of the city. Amendments of 1986 focused attention on into any membrane facility. The raw-water supply for the lime the health effects of disinfectant byprod- The advantages of raw-water blending softening plant comes from the Surficial ucts and the removal of natural organic or sidestream treatment for blending with Aquifer, which is declining in quality and is matter. A new group of “looser” softening permeate may include any or all of the fol- highly maintenance intensive to maintain (nanofiltration) membranes were devel- lowing: capacity; therefore, a new water treatment oped and continue to be developed to pro- • Reducing installed membrane capacity to plant using the Floridan Aquifer as the vide a high rejection of organics and diva- achieve a given capacity raw-water supply is being constructed in lent ions, such as calcium, and a lower • Expanding plant capacity without the southern portion of the city. The plant rejection of nonvalent ions, such as sodium increasing installed membrane capacity will use reverse osmosis (RO) and a raw- and chlorides. The commercial availability • Fine tuning raw-water quality water blend to provide the initial capacity of these membranes, coupled with the • Reducing post-treatment chemical usage of 4 MGD. The raw-water blend will be requirement to meet more stringent water and cost utilized to adjust the quality of the finished quality regulations, has caused many com- • Reducing pHs water to match the water quality from the munities to partially or completely replace • Increasing finished-water buffering existing treatment plant, minimize the their existing lime softening plants with capacity installed membrane capacity, reduce oper- membrane softening facilities. • Reducing raw-water withdrawals for a ating costs, and reduce raw-water with- These applications typically involve given capacity drawals. treating fresh, shallow groundwaters that are Three case studies illustrate the use of The water quality goals for this project moderately to highly colored and hard, con- blending to take advantage of some or all of were established to meet all existing and taining varying concentrations of iron. The these benefits of blending with raw water or a anticipated water quality regulations and nanofiltration (NF) membranes provide a treated sidestream. to be compatible with the existing fin- high level of organic and hardness ished-water quality. The key water removal, but typically produce a soft Table 1 quality parameters from the exist- water that is low in alkalinity and Existing Finished-Water Quality ing treatment plant are summa- buffering capacity. rized in Table 1, based on operat- pH 8.6 The solution to the aggressive ing data from January through membrane permeate produced by Alkalinity 30 November of 2001. most membrane applications Total Hardness 133 Raw water for the new South includes degasification to strip car- Calcium Hardness 106 Regional Water Treatment Plant will bon dioxide to raise pH and lower Color 4.7 be withdrawn from a series of Upper caustic dosage, adding phosphate- Chlorides 153 Floridan Aquifer wells. The water in based corrosion inhibitors and Iron 0.1 the Upper Floridan Aquifer in the

14 • NOVEMBER 2003 • FLORIDA WATER RESOURCES JOURNAL vicinity of the new facility is brackish, color- Table 2 less, and moderately hard, and contains Projected Water Quality and Blending Analysis hydrogen sulfide. Brackish-water mem- Floridan Aquifer Projected branes will be used to reduce chlorides and Finished-Water Maximum Raw- Raw-Water Quality Permeate Quality TDS to meet regulatory limits and water Quality Goal Water Blend Parameter mg/L mg/L mg/L % quality goals. The average raw-water quality, pH 7.6 5.9 pHs+0.1(>8.0) 100 projected permeate quality, finished-water Alkalinity 140 21 30-50 24.4 quality goals, and maximum blend percent- Total Hardness 680 18 120 15.4 ages are presented in Table 2. Calcium Hardness 350 9 100 26.7 The blending analysis shows that no Color <1 <1 <1 100 water can be blended, based upon the con- Sodium 345 79 130 19.2 centration of hydrogen sulfide in the water Chlorides 850 59 160 12.8 and the fact that if raw water blending is to TDS 1840 144 <450 18.0 take place, post treatment for hydrogen sul- Iron <0.1 <0.1 <0.1 100 fide must be provided to meet the water Fluoride 0.4 0.1 0.8 100 Hydrogen Sulfide 0.8 0.7 <0.1 0.0 quality goals. Sulfate 200 30 <200 100 The next constituent that limits raw- water blending is the concentration of chlorides. The finished-water quality goal for chlorides was increased to 160 mg/L from the different water sources. from initial testing. The Floridan Aquifer is existing average of 153 mg/L to allow for a Raw water for the existing lime softening generally considered to be a non-potable higher blend percentage. The permeate was plant is withdrawn from the Biscayne Aquifer water source in the southeast region of blended with 12.8 percent raw water, and the through a group of existing wells, and water Florida. The raw water is typically brackish, quality of the resultant blended water is pre- for the proposed NF process will also be (high total dissolved solids [TDS], chlo- sented in Table 3. withdrawn from the Biscayne Aquifer from a rides and sodium), hard, high in carbonate The results of blending the waters show new group of wells. The quality of the new alkalinity, and containing a significant that most water quality goals have been met, wells is assumed to be similar to that of the quantity of sulfides. The raw water is low in except that post treatment for pH, fluorida- existing raw-water supply; however, over organic carbon and color and contains tion and hydrogen sulfide must be provided. time as the withdrawal rate is increased, a 10- small amounts of naturally occurring Although the blending has reduced the to-20-percent increase in chlorides and color ammonia and fluoride. installed permeate capacity and has helped may occur. The primary treatment required for the produce a quality similar to the existing, the The raw water from the Biscayne Aquifer Floridan Aquifer water includes the removal calcium hardness will be lower than existing. is moderate in hardness, high in carbonate of chlorides, TDS, hardness, sulfides, sodium, Mixing the two waters in the distribution sys- alkalinity, and higher in total organic carbon and sulfates. The highest removal efficiency is tem will have to be evaluated, as well as the (TOC) and color. The moderate hardness generally required for chlorides, total hard- need for additional adjustment of the fin- requires some degree of treatment to reduce ness, and TDS to meet potable-water stan- ished water. the hardness below 100 mg/L as CaCO3. The dards and water quality goals. Design raw- high carbonate alkalinity makes pH adjust- water quality information is presented in City of North Miami Beach ment more expensive, since the water is well Table 4 for the Floridan Aquifer. The city of North Miami Beach is in buffered against changes in pH. The higher FINISHED-WATER QUALITY GOALS the process of expanding its Norwood- TOC of the water mandates that chloramines The primary purposes of the proposed Oeffler Water Treatment Plant to increase be used to reduce disinfection by-products Norwood-Oeffler Water Treatment Plant the capacity from 17 to 32 MGD in order to (DBPs). The higher color requires treatment become independent from the Miami- to produce a finished water that is below the Continued on page 17 Dade Water and Sewer Department water secondary standard supply. The expansion program's other pri- for color, clear, and mary objective is to improve water quality aesthetically pleasing Table 3 for its customers, especially in terms of to the consumer. Blended Water Quality color and organics. Raw-water quality Blended The expansion will consist of use of 15 information is pre- Finished-Water Water Quality MGD of existing lime softening capacity sented in Table 4 for Quality Goal and construction of 6 MGD of RO capacity, the Biscayne Aquifer. Parameter mg/L mg/L 9 MGD of NF capacity, and 2 MGD of raw- Raw water from pH 6.2 pHs+0.1(>8.0) water blend. The source of raw water for the the Floridan Aquifer Alkalinity 36.2 30-50 RO facilities will be the Floridan Aquifer, will be withdrawn Total Hardness 103 120 Calcium Hardness 52.6 100 and the raw-water source for the NF facili- from a group of Color <1 <1 ties will be the Biscayne Aquifer. The quality wells that are under Sodium 113 130 of finished water from the expanded water construction and Chlorides 160 150 treatment plant will vary, depending on the testing. The expect- quality of the source water, the treatment TDS 361 <450 ed raw-water quality Iron <0.1 <0.1 provided, the demand scenario, and the is based on water blending ratio. This situation is due to the Fluoride 0.1 0.8 quality from similar Hydrogen Sulfide 0.7 <0.1 three different processes used to treat two sources and data Sulfate 51.8 <200

FLORIDA WATER RESOURCES JOURNAL • NOVEMBER 2003 • 15 Table 4 City Of North Miami Beach: Norwood-Oeffler WTP Expansion - Phase 1

Design Water Quality Values Biscayne Floridan Lime Softened Low Pressure Nanofiltration Constituent Aquifer Aquifer Water RO Permeate Permeate(1) pH 7.2 7.5 9.2 6.0 6.0 Alkalinity as CaCO3,, mg/l 195 140 65 27 60 Total Hardness as CaCO3, mg/l 215 1030 77 31 67 Calcium, mg/l 83.2 180 29.2 5.5 25.9 Magnesium, mg/l 1.7 140 1.0 4.1 2.2 Sodium, mg/l 19 1000 19 136 19 Chlorides, mg/l 40 2000 45 205 16 Sulfates, mg/l 21 300 20 15 25 Iron, mg/l 0.3 <0.1 .015 <0.1 <0.1 Fluoride, mg/l 0.2 0.1 0.2 <0.1 0.2 Nitrate, mg/l <0.1 <0.1 <0.1 <0.1 <0.1 Color, NTU 35 <1 14 <1 1.0 Hydrogen Sulfide, mg/l <0.1 0.5 <0.1 0.8 <0.1 Sulfide, mg/l <0.1 1.3 <0.1 0.9 <0.1 Total Dissolved Solids, mg/l 405 3,820 270 395 160 Regulated VOC's, mg/l BMCL BDL BDL BDL BDL Regulated SOC's, mg/l BDL BDL BMCL BDL BDL Other Regulated Primary Inorganics BMCL BMCL BMCL BMCL BMCL Other Regulated Secondary Contaminants BMCL BMCL BMCL BMCL BMCL

BDL: Below detection limit Note:1. Finished water quality derived from the use of membrane projection BMCL: Below maximum contaminant level software.

Continued from page 15 in the presence of sulfur oxidizing bacteria, ondary water quality standards will be met. expansion are to provide high-quality it can also create corrosion problems as a Finally, the finished water should have an potable water to all the city's customers and result of the conversion of hydrogen sulfide average apparent color less than six color to provide the capability to meet future to sulfuric acid. units and a maximum less than 10 color water demands while meeting future regula- The finished-water quality goals for the units, a turbidity less than 0.2 NTUs, iron less tions. In order to achieve these purposes, cer- Norwood-Oeffler Water Treatment Plant are than 0.1 mg/L, hydrogen sulfide less than 0.1 tain water quality goals were set for the pro- consistent with and more stringent than the mg/L, haloacetic acids less than 48 micro- posed plant. At a minimum, these goals must projected water quality standards of the grams per liter, and total trihalomethanes less comply with existing and proposed, but soon Florida Department of Environmental than 64 micrograms per liter. The maximum to be implemented, federal and state water Protection and U.S. Environmental residual chlorine of 4 mg/L will not be quality regulations. Protection Agency. The finished-water quali- exceeded in the finished water and a finished- The finished water must be non-corro- ty goals for all synthetic and volatile organic water fluoride concentration of 0.8 mg/L will sive, without a tendency to leach metal ions compounds have been set below the detec- be maintained. in the transmission, distribution, or plumb- tion limits; therefore, the raw water must be ing systems. A slight scale-forming water free from these compounds to allow blending BLENDING SCENARIOS which will have a tendency to coat pipes of raw water. The goals set for the primary The amount of finished water produced with a thin layer of calcium carbonate is inorganic ions will be 80 percent or less than by lime softening, NF, and low-pressure RO desirable to prevent corrosion and inhibit that of the regulatory limit. will determine the quality of the finished growth of bacteria. Softening or removing A total hardness of 40-80 mg/L was water released into the distribution system. the hardness from water is required to selected, based on the fact that this level is Preliminary design finished-water quality for reduce the tendency to scale hot-water pipes consistent with existing treatment results and each of the three treatment processes are and heaters, reduce the tendency to scale will have a lower potential for scaling. A min- summarized in Table 4. The finished-water and form soap scum on plumbing fixtures, imum alkalinity of 40 mg/L as CaCO3 was quality of the NF and low pressure to perme- reduce the amount of soap required to pro- selected to limit post-treatment pH adjust- ate was derived from the use of membrane duce a foam or lather, and increase the effec- ment. The goal for chlorides was set at 100 projection software. tiveness of detergents. mg/L, which is 40 percent of the regulatory To comply with the South Florida Water Hydrogen sulfide must be removed standard. By selecting chlorides at this level, Management District permit conditions, a from the water, since it has a distinctive and the amount of raw-water blend can be maxi- minimum base flow from the low-pressure objectionable odor which it can impart to mized, reducing the post-treatment pH RO treatment process of 5.0 MGD was uti- the finished water. Hydrogen sulfide can also adjustment costs. lized in the blending analysis. The utilization cause turbidity in the distribution system The TDS of the finished water will be was increased up to 6.0 MGD as the demand when oxidized to elemental sulfur in the maintained at less than 350 mg/L, or 70 per- increased up to the initial plant maximum- presence of dissolved oxygen and chlorine; cent of the secondary standard. All other sec- Continued on page 18

FLORIDA WATER RESOURCES JOURNAL • NOVEMBER 2003 • 17 Continued from page 17 Table 5 day design capacity of 32.0 MGD. The City Of North Miami Beach: Norwood-Oeffler WTP Expansion - Phase I assumption was that as the duration of the Preliminary Blending Flows and Quality demand decreased, the downtime for equipment failure, repair, or cleaning would Flows from Each Source, MGD decrease. To minimize finished-water color at Total Reverse Nano- Lime Biscayne Floridan Alkalinity lower demands, the NF process was assumed Demand Osmosis filtration Softened Aquifer Aquifer mg/L as MGD Permeate Permeate Water Raw Water Raw Water CaCO to operate at a minimum capacity of 6.0 3 MGD, or 2 skids. Similar to the low-pressure 15 5 6 3.9 0 0.1 50.4 RO process, the capacity and utilization were 16 5 6 4.9 0 0.1 51.3 increased up to the maximum of 9.0 MGD as 17 5 6 5.9 0 0.1 52.1 the demand increased up to the initial capac- 18 5 6 6.9 0 0.1 52.9 ity of 32.0 MGD. 19 5.5 7 6.4 0 0.1 52.2 The minimum capacity of the lime soft- 20 5.5 7 7.3 0 0.2 53.2 ening process was set at 3.9 MGD and kept 21 5.5 7 8.3 0 0.2 53.8 below 10 MGD up to the average annual 22 5.5 7 9.3 0 0.2 54.3 demand of 22.1 MGD to allow for mainte- 23 5.5 7 10.2 0 0.3 55.1 nance of the hydrotreator units during low- 24 5.5 7 11.2 0 0.3 55.5 demand periods and to maintain color below 25 5.5 7 12.2 0 0.3 55.9 the finished-water quality goals. At the lower 26 5.5 7 13.2 0 0.3 56.2 demands, a small quantity of blend water was 27 6 8 12.7 0 0.3 55.6 utilized from the Floridan Aquifer to mini- 28 6 8 13.6 0 0.4 56.2 29 6 8 14.6 0 0.4 56.5 mize color while recovering hardness and 30 6 8.4 14.6 0.6 0.4 59.3 alkalinity. The use of this source as a blend 31 6 8.5 14.6 1 0.5 61.4 was constrained by meeting the finished- 32 6 9 15 1.5 0.5 63.5 water goal for chlorides. The blend quantities determined in this Note: 1. Blended Water Quality prior to post treatment manner and the resultant finished-water quality are summarized in Table 5,which shows that as demand rises, the color is The membranes were altered to reduce WATER TREATMENT GOALS expected to increase because of the increased the rejection of divalent ions (iron, calcium, The original finished-water goals for use of the lime softening process. The magnesium, etc.) in order to increase the quantity and quality as stated in the specifi- increased color occurs primarily as the hardness of the finished water. After the alter- cations are shown in Table 6. demand approaches the maximum-day ation of the membranes, it was discovered The alkalinity and calcium hardness demand; therefore, the customers will infre- that finished-water iron concentrations were goals are dictated by corrosion control prac- quently be provided this quality of water. All higher than the water quality goals and that tices. The iron is governed by the secondary other water quality goals are met over the calcium hardness concentrations were still MCL. These goals are not attainable with the entire demand range. lower than the specified concentration. existing facilities; therefore, process modifi- It was determined that using a raw-water cations will be necessary to meet these goals City of Sunrise blend would address the water quality issues, and provide for the expansions. Sawgrass Water Treatment Plant increase plant capacity, and reduce the quan- Two options were examined to The city of Sunrise recently completed tity of groundwater withdrawals required to expand the capacity of the water treat- the construction of its new Sawgrass Water produce a given quantity of finished water. ment plant by blending raw water with Treatment Plant that withdraws water from The blending of raw water will decrease the the permeate product water. Each option the Biscayne Aquifer and treats it with NF. proposed withdrawals of raw groundwater requires the treatment of the raw water The plant was designed to treat 100 percent from the Biscayne Aquifer. to remove iron only, so that the finished- of the finished water using NF with a design Raw-water iron concentrations range water iron goals can be met and natural- recovery of 80 percent. The plant has an from 1.45 mg/L to 2.10 mg/L. The current ly occurring calcium and alkalinity in installed membrane treatment capacity of 12 level of iron in the raw water is well above the raw water can be used to meet the MGD but is permitted to produce 9 MGD of the secondary maximum contaminant level stabilization goals. Each option also finished water with one NF train out of serv- (MCL) of 0.3 mg/L. City officials have involves oxidation of the iron to form a ice for reliability purposes. decided to remove iron to <0.1 mg/L for precipitate that is then filtered from the Initial testing of the plant indicated that aesthetic purposes. system. In the first option the filtration iron concentrations in the raw water were In order to remove iron using mem- method involves oxidation and green- higher than anticipated and the removal of brane treatment, other ions such as calcium sand filtration, and the second option calcium hardness was higher than allowed by are also removed, producing a soft, somewhat includes oxidation followed by ultrafil- the specifications. The plant is capable of more corrosive finished water unless addi- tration. providing a high level of treatment through tional chemical facilities are provided at the the use of NF, degasification, ozonation, and end of the treatment process. Thus, the main GREENSAND FILTRATION chlorination. The facility provides iron factors in expanding the plant involve a bal- Greensand filtration is a process used removal and softening by NF. It does not ance between removing iron below 0.1 mg/L to remove manganese and iron. For iron include provisions for blending raw water and retaining enough calcium so the water is removal, typically a continually regenerat- with the permeate or finished water. least corrosive to the distribution system. ing (CR) process is used. The manganese

18 • NOVEMBER 2003 • FLORIDA WATER RESOURCES JOURNAL Table 6 Water Quality Goals Ultrafiltration Parameter Membrane Feed Water Membrane Permeate Water Membrane processes that have applica- Flow (MGD) 11.25 9 tions in drinking-water treatment include Alkalinity, mg/L as CaCO3 65 to 275 40-80 reverse osmosis (RO), nanofiltration (NF), Calcium Hardness, mg/L as CaCO3 250 40-80 ultrafiltration (UF), microfiltration (MF) Iron, mg/L 1.5 <0.3 and electrodialysis reversal (EDR). UF is a pressure-driven filtration process that utilizes hollow-fiber membranes capable of separat- Table 7 ing both insoluble and soluble materials from Oxidation and Green Sand Filtration Raw-Water Blend Treatment the treated water. Hollow-fiber membranes Description Quantity Units Unit Cost ($) Total Cost ($) are either “inside-out” cross-flow membranes Green Sand Filtration Units (3 MGD) 2 EA $1,600,000 $3,200,000 or “outside-in” transverse-flow membranes. Backwash Facilities 1 LS $125,000 $125,000 The applied pressure utilized with UF Piping, Valves, and Fittings 1 LS $150,000 $150,000 processes is much lower than feed pressures Electrical/Instrumentation 1 LS $200,000 $200,000 for NF or RO processes and is referred to as Chemical Feed Improvements 1 LS $150,000 $150,000 the transmembrane pressure. Total $3,835,000 UF has been shown to be very effective Note: Conceptual Cost prepared for comparison basis only. for particle and turbidity removal. Turbidity can be lowered to below 0.05 nephelometric turbidity units on a consistent basis for a greensand CR process is applicable on well erated state. variable feed water quality. Coliforms, bacte- waters where iron removal is the main Conversely, a temporary underfeed of ria, viruses, and cysts can be effectively objective, with or without the presence of oxidant would utilize the oxidizing capacity removed from a water supply by UF. manganese. Waters having iron concentra- of the regenerated manganese greensand to For the purpose of this analysis, Zenon tions in the range of 0.5-3 mg/L would have complete the oxidation of iron and man- was contacted regarding UF performance. run lengths of 18 to 36 hours at a design ganese as required; therefore, in the CR The company’s ZeeWeed® ultrafiltration flow rate of 3-5 gpm/sf. process, the manganese greensand acts as a membrane is oxidant resistant and operates The CR process involves feeding an oxi- redox buffer with capabilities of both oxida- well in the presence of high solids. As a result, dant or combination of oxidants, such as tion and reduction as required by influent the process is efficient in treating water potassium permanganate and chlorine, to water conditions. Second, it is a well-known sources containing high levels of turbidity, raw water prior to contact with the man- fact that in iron and manganese removal by iron and manganese. ganese greensand bed. Chlorine, which is oxidation, the presence of manganese oxide The process for a ZeeWeed® plant treat- recommended, should be fed at a suitable will act as a catalyst, whether the oxidizing ing iron and manganese consists of a simple distance prior to the potassium perman- agent is oxygen, chlorine, ozone, or perman- preoxidation step, followed by ultrafiltration. ganate injection point. The chlorine will ganate, ensuring that the reaction goes rapid- With the zenon process, the ultrafilter cas- oxidize the bulk of the iron and any sulfide. ly to completion. settes are immersed in the water and a pump Potassium permanganate will then complete It was determined from literature is used to draw a vacuum through the mem- the oxidation of trace amounts of iron and review that 98-percent removal of iron can brane to produce permeate. soluble manganese. typically be achieved through the use of Ultrafiltration is not susceptible to oxi- The manganese greensand bed per- greensand filtration. This would reduce a dants and Zenon has successfully used a forms a dual function to complete the raw-water iron concentration from 2 mg/L number of oxidants to aid in the removal of removal of iron and manganese. First, cor- to 0.04 mg/L, which is well below the sec- iron. UF was evaluated for treating the bypass rect operation of a CR filter requires that a ondary MCL, and would reduce the iron blend only; therefore, the only options inves- slight excess of permanganate, indicated by concentration in the blended finished water. tigated for UF are for treating between 3 an influent water having a light orange If only the raw-water blend were treated, the MGD and 6 MGD of raw-water bypass. Table color, will insure that the oxidant demand greensand filters would treat a small portion 8 presents costs for providing a 6-MGD UF has been met, whether using perman- of the overall plant flow—approximately 3 facility provided with pre-oxidation. ganate alone or in combination with chlo- MGD. Table 7 presents costs for the 6 MGD rine. Any slight excess permanganate will installed and 3 MGD firm-capacity green- SUMMARY be reduced to a manganese oxide by the sand filtration option. Figure 1 represents After a review of economic and techni- manganese greensand. The manganese the resultant water quality for treatment of a cal feasibility for the different treatment oxides will then precipitate on the grains, 20-percent blend of raw water with green- techniques, the recommended course of maintaining them in a continually regen- sand filtration. action for the city is to use a raw-water bypass treated with greensand filtration to blend with the nanofiltration permeate TABLE 8 water. Although the ultrafiltration with pre- Oxidation and Ultrafiltration Raw-Water Blend Treatment oxidation provides a high-quality water with very low concentration levels of iron, the Description Quantity Units Unit Cost ($) Total Cost ($) Ultrafiltration and Associated total capital cost for this project using UF is 1EA$4,200,000 $4,200,000 Manufacturer Supplied Equipment greater than that for using greensand filtra- Additional Ancillary Plant Costs 1 LS $3,000,000 $3,000,000 tion. The UF costs did not include concen- Chemical Feed/Oxidation Improvements 1 LS $150,000 $150,000 Total $7,350,000 Continued on page 20

FLORIDA WATER RESOURCES JOURNAL • NOVEMBER 2003 • 19 Figure 1

Continued from page 19 Conclusions from multiple sources to meet finished- trate disposal faculties that could also signif- Successful blending requires a knowl- water quality goals or to reduce membrane icantly impact the cost unless discharge to edge of the expected quality and variations post treatment chemical addition to meet the sewer system was an option. of the raw-water, permeate, and other these goals. This is especially true where the The cost associated with providing two 3- treated-water streams and the develop- objective is to meet the minimum regulato- MGD greensand filtration units and the neces- ment of specific water quality goals. ry requirements and providing the highest sary chemical feed equipment for oxidation for Limiting constituents must then be deter- purity water is not the prime objective. this option is estimated to be $3.835 million. mined to calculate minimum and maxi- Blending in combination with the use of Figure 1 clearly shows that the blending of the mum blending ratios and to adjust fin- reverse-osmosis treatment provides a viable treated-water bypass with the nanofiltration ished-water quality goals to maximize alternative to reduce raw-water withdrawals, permeate allows the facility to be expanded their attainment. chemical usage, and operating and capital without increasing installed membrane capaci- The examples in this article illustrate costs while providing a method to vary fin- ty and meets all the original water quality goals. the ability to blend raw and treated water ished-water quality.

20 • NOVEMBER 2003 • FLORIDA WATER RESOURCES JOURNAL