Membrane softening versus lime softening in Florida: A cost comparison update

DESALINATION ELSEVIER

Desalination 102 (1995) 11-24

Membrane softening versus lime so~ening in ~o~da: A cost comparison update Robert A. Bergman CH2M Hill, 7201 N W 11th Pit-e, Gainesville, FL 32605-3158, USA

Tel.: 904-3312442, Fax: 904-3315320

Abstract Increasingly, nanofiltration is being used to soften water in Florida - particularly for applications where the feed water has high color and dissolvedorganics (whichare precursors to regulated disinfectantbyproducts). The slightly higher costs for membrane softening than lime softening, are more than justified for many utilities that desire the generally superior product water quality. The+~eutilities anticipate the ability to meet future drinking water standards, even those not yet identified,without the need to modify the process or add additionalprocesses required for lime softening.

1. Introduction In Florida, about 87 percent of public water supplies are derived from groundwater [1]. Commonly, these water supplies are classified as being "hard"; having relatively high concentrations of calcium. Many of these supplies also have substantial dissolved organic color, hydrogen sulfide, and iron. Until about ten years ago, essentially all municipal groundwater treatment plants in Florida practicing softening used the lime softening process and, in many cases, relatively high dosages of chlorine for disinfection and "bleaching-out color." With the onset of more stringent drinking water standards, particularly for disinfectants and disinfectant byproducts, and the development and commercialization of nanofiltration membrane~, new softening plants in the state have favored membrane softening over lime

softening in treating colored groundwater. In fact, Florida water utilities are the world leaders in nanofiltration application in terms of installed capacity. Nanofiltration is a physical barrier treatment process that separates hardness ions, dissolved organics (natural color, trihalomethane formation potential (THMFP), and other disinfectant byproduct precursors), and total dissolved solids (to a lesser degree) from water. This paper explores the costs and other factors that affect the selection of lime softening or membrane softe~:-;ngfor water treatment.

2. Groundwater supplies Fig. 1 shows the principal groundwater aquifers used for public water supplies in Florida [2]. Surface -waters often influence the shallow sand and gravel aquifers, resulting in

0011-9164/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved. SSD1 0011-9164(95)00036-4

12

R.A. Bergman / Desalination 102 (1995) 11-24

highly colored water. Wells in deeper limestone formations yield water containing substantial amounts of dissolved minerals, principally calcium bicarbonate, with lower concentrations of magnesium, sodium, potassium, chloride, sulfate, silica, fluoride, nitrate~ and iron. Table 1 presents the raw water charat;tefistit;~ from flaee iu~.at,u,o' . . . . ;~-~ in Florida where lime softening and membrane softening are used.

GUlfo, MeMcO

:~ k Affont~ ~.r~.,._.__.J~.|' ~ O¢~

3. Treatment processes

3.1. Lime softening Fig. 2 presents a process flow schematic commonly used in lime softening plants in Florida. Groundwater enters an upflow solids contact reactor (softening unit), sometime~ after falling through a tray aerator to remove hydrogen sulfide (if present). Lime and, in many cases, a coagulant aid (polymer) are added. Sludge is pumped from the reactor for disposal, often after dewatering with thickeners and vacuum filters. The softened water pH is sometimes adjusted as it passes from the reactor to gravity media filters. Disinfectant, corrosion control, and fluoride chemical addition typically complete the treatment process. Ammonia and chlorine commonly are used to form chloramines for disinfection, without producing high concentrations of trihalomethanes (THM). Several communities have experienced color and bacterial regrowth problems in the distribution system a/ter switching from free chlorine to chloramines, which is not as strong an oxidant. The finished water is stored and pumped to the distribution system.

3.2. Membrane softening Fig. 1. Principal sources of groundwater for public use in Florida. (Adapted from H-:aly, 1972).

1 Groundwater characteristics at three locations in Florida T~ble

Raw water source (mg/l)

Ft. Myers Shallow wells by river

Hardness, as CaCO 3 230 Alkalinity, as CaCO3 200 Chloride 70 TDS 480 Iron 0.3 Color 75 H2S O.3

Boynton Beach Surficial aquifer 295 265 50 380 0.2 40 1.5

Plantation Biscayne aquifer

315 285 60 420 1.7 65 Trace

Fig. 3 shows a typical membrane softening plant in Florida. Sulfuric acid and, in many cases, antiscalant for pH and scale control are added to raw water. Cartridge filters, usually rated at 5 microns, remove particles that may foul the membrane system. Feed water pumps boost the pretreated water pressure to about 90 to 130 pounds per square inch (psi) before entering the membrane system. The softening membranes typically are spiral wound nanofiltration (NF) membranes. However, if greater removals of dissolved solids are needed than can be activated by NF alone, reverse osmosis (RO) membranes also are included in the system (sometimes referred to as a "hybrid" NF/RO system). The membrane barrier separates the water into

R.A. Bergman I Desalination 102 (1995; 11-24

TIoy A~ater (l~ H2SRrer,enl)

13

IJme and Coagulant AIO m

FMers

m

k~,Joilot

~

,

Son

Utlils

AmmonCo

,

"

To~ ono 6ackwo~l --

w~er $1ofc~¢ Seedco P ~ O

Water

SluOge

Weft FtelO

Fig. 2, Typical lime softening plant schematic.

,.~_..~_~

~ , ~ , . , ~,,,,,,.

~ 1

~ . WQ~'~fef

s~em Carlfldge Rlte~

Feedwaler Boost Pumm

Men~ones and Pressure Vesse~

~,~e Pu.~D Clearw~

ConcQnhate to Dfsposo4

Fig. 3. Typical membrane softening plant schematic. permeate (produc0, which has passed through a membrane, and concentrate (reject) streams. The permeate is sent to a degasifier for carbon dioxide (pH adjustment) and hydrogen sulfide removal. The concentrate discharges to waste, sometimes after treatment, depending on site specific conditions and regulatory requirements (deep injection wells, discharge to saline surf:.ce water (e.g. ocean

ouffall), sanitary sewer, blending with other water for irrigation/reuse). The degasified water usually is fed chemicals for disin£ection (free chlorine or chioramines), pH adjustment (caustic soda), and often corrosion control (corrosion inhibitor) and fluoride. The treated water is then transferred, stored, and pumped to the distribution system.

14

R.A. Bergman I Desalination 102 (1995) 11-24

4. Membrane softening capacity in Florida The first membranes that were developed specifically for water softening were cellulosic RO membranes that were chemically modified (hydrolyzed) to diminish overall dissolved ion rejection in 1977 [3]. Since then, several NF membrane products (nearly all noncellulosic composites) have been developed and commercialized. Table 2 and Fig. 4 present the accumulated membrane softening plant capacity in Florida. Currently, a total of more than 60 mgd of membrane softening capacity is in operation and the cumulative capacity will likely approach 100 mgd within five years. 6C

data. The cost of land and engineering, legal, administrative and other indirect capital costs were not included. Additionally, membrane manufacturers and other sources were used to obtain plant data.

3 Collier

'k,l- :L 6 In,an ltlver co south l, 5,

5C

~?!~.:

1_.. . . .

8 Oolmton Beach (4) 9 Plantollon (12) Undoi Construction ~

3C

j

I

..,,~.1~. *"

A I~ramor (4.5) B Hollywood (14) C Palm BoachCounl¥ (8]

IC

01975

Fig. 5. M e m b r a n e softening/nanofiltration plants with 1 m g d or greater capacity.

_ _

_ _ 1980

1905

1990

1995

Year

Fig. 4. C u m u l a t i v e installed m e m b r a n e treatment plant capacity in Florida.

softening

6. Cost analysis methodology 6.1. Construction costs

5. Membrane softening plant survey In July and August 1994, a cost survey was sent to all nine membrane softening water treatment plants in Florida, 1 mgd and larger, that are currently in operation (Fig. 5). Tables 3 and 4 list plant capacity and design and operating data, respectively, for these facilities, as well as design information for two other plants in the construction phase. Eight of the nine utilities with large membrane softening plants responded with construction and operation and maintenance (O&M) cost

Construction costs and plant product water capacity data were used to develop unit construction costs. Costs were adjusted (from the date of start-up) to January 1994 dollars using Engineering News Record construction cost index values (January 1994 ENR CCI is 5336). Because many of the membrane softening plants were constructed with facilities sized for future membrane treatment capacity expansion, "adjusted" estimated unit construction cost per installed membrane capacity were determiner, For example, in a case where the treatment process building and other general plant facilities were sized for 8

R.A. Bergman / Desalination 102 (1995) 11-24 Tablc 2 Membrane softening water treatment plant capacity in Florida Date WTP name commissioned operational

NF/RO capacity (mgd)

Operational June 76 Estero Woods June 77 Pheasant Walk June 77 Imperial Harbor June 78 Pelican Bay June 78 Ft. Pierce Electric June 80 Camelot Lakes June 87 Public School #~7 Feb 88 St. Lucie West June 88 Palm Beach Bank of Commerce June 88 Charleston Park Jan 91 Indian River Co. South May 91 Plantation {Central WTP) June 91 Gulf Utilities/Corkscrew July 92 Palm Coast Utilities Dec 92 Ft. Myers Dec 93 Collier Co. Jan 94 Dunedin Jan 94 Boynton Beach Jan 94 Indian River Co. South Aug 94 Village of Ruyal Palm Beach Under construction Hollywood Miramar Palm Beach Co. Martin Co. Volusia Co./Spruce Creek In design Plantation East

Accumulated NF/RO capacity (mgd)

WTP copacity (mgd)

Accumulated WTP capacity (mgd)

0.01 0.04 0.096 0.25 0.8 0.1 0.04 1 O. 185 0.025 3 12 0.5 2 12 12 4.8 4 3 1.5

0.01 005 0.146 0.396 1.196 1.296 1.336 2.336 2.521 2.546 5.546 17.546 18.046 20.046 32.046 44.046 48.846 52.846 55.846 57,346

0.01 0.04 ,.096 0.25 0.8 0.1 0.04 1 0.185 0.025 3.2 12 0.5 2 12 12 9.5 4 3.5 1.5

0.01 0.05 0,146 0.396 1.196 1.296 1.336 2,336 2.521 2.546 5.746 17.746 18.246 20.246 32.246 44.246 53.746 57.746 61.246 62.746

14 4.5 8 0.75 0.5 6

71.346 75.846 83.846 84.596 82;.096 91.096

14 3 6 t~.75 0.5 6

76.746 79.746 85.746 86.496 86.996 92.996

Table 3 Membrane softening water treatment plant capacity data Plant name

Plantation Ft. Myers Collier Co. Indian River Co. South Dune,din Boynton Beach Village of Royal Palm Beach St. Lncie West Development Hollywood Miramar

,.Q~rrent capacity (mgd) Membrane Plant

12 12 12 6 4.8 (a) 4 1.5 1 i4 4.5

12 12 12 8.5 9.5 4 1.5 1 18 (b) 4.5

Build, size Buildot~ capacity (mgd~ Number Permeate based on Membrane Plant membrane capacity/ membrane trains train (mgd) capacity (mgd) 12 20 20 6 6 8 3 15 20 8

12 20 20 6 9 16 3 15 20 '. 8

12 20 20 8.5 12 !6 3 15 20 18

4 3 6 4 4 3 2 1 7 (NF) 2

3 4 2 1.5 12 1,33 0,75 1 2 1.5

(a) Membrane capacity supplied equals 6 mgd. (b) Plant includes 4 mgd of RO treating brackish Floridian aquifer water.

Floridan Upper Florid.

Hydranautics

Hydranautics NF&RO & Fluid Systems

Hydranautics

FilmTec

FilmTec

Indian River Co. South

Dunedin

Boynton Beach

Village goyal Palm Beach

NF(b)

Hydranauties

FilmTec

Hells'wood

Miramar

Surficial

Biscayne

85

80

85

83

85

90

g6(c)

90 (NF)(c)

Shallow wells

Surficial

Surficial

Lower Tamiarai

95(c)

130(c)

100

95

105

105

110-120

110

155

130

400

585

500

800

360

450

910

420

480

480

-

210(c)

170

250

90

300

230

140

285

35

265

350

250

350

255

240

310

260

230

310

-

120(c)

70

60

50

90

30

80

130

20

Hardness l.lllg/l as CaCO3] Feed Permeate

<5

<5

<1

<5

-

<4

<5

<5

115

<5(c)

30(c)
70

35

45

30

-

17

75

65

Feed Permeate

Color (CU)

(a) Membrane feed after treatment. (b) Plant includes 4 mgd of RO treting brackish Floridian aquifer water. (c) Projected values.

NF

NF

St. Lucie Hydranautics West Develop.

NF

NF

NF

NF

85

Recovery Feed TDS (mg/l) (a) (%) pressure (psi) Feed Permeate

Shallow wells 90 near river

Collier Co.

NF&RO

Hydranautics

Ft. Myers

Biscayne

Fluid Systems NF&RO

Membrane Membrane Groundwater manufacturer type aquifer

Plantation

Plant name

Table 4 Membrane softcaing water treatment plant design and operating data

Two deep injection wells

Ocean outfall

WWTP& irrigation pond

ToWWTP

Deep injection well

To WWTP

Canal to ocean

Deep injection well

Irrigation pond

Deep injection well

Concentrate disposal

~,

R.A. Bergman / Desalination 102 (1995) 11-24

17

mgd treatment capacity but only 4 mgd of membranes were installed, the total treatment plant unit cost was calculated by adding the membrane system unit cost in dollars per gallon per day ($/gpd) permeate capacity (membrane system cost divided by 4 mgd) to the unit cost of the remainder of the plant's cost (the non-membrane system part of the construction cost divided by 8 mgd). In this manner, more comparable unit construction costs were determined for the various plants. Furthermore, the unit construction costs were based on the membrane permeate flow rate, i.e. not including any feed water that may bypass around the membranes for blending.

production rates at the facilities surveyed, the actual plant operating factor would need to be considered. Under normal conditions, one can assume about 5 percent of the time should be adequate for membrane cleaning and other facility maintenance requiring full or partial plant shutdown. Therefore, a membrane softening plant operating as a "base load" plant could have costs vary similar to the costs presented.

6.2. O&M costs

Fig. 6 presents unit installed membrane process equipment costs as a function of membrane permeate capacity based on the cost survey data. The plants identified by the two-letter initials shown on the graph in Fig. 6 can be identified by using Table 5. Generally, costs per installed gallon per day capacity ranged from about $1.25 to $1.75/gpd for l-mgd plants to about $0.30 to $0.55/gpd for 15-mgd facilities. These costs generally include supply and installation of pretreatment chemical pumps and day tanks, cartridge filters, feed water boost pumps, membranes pressure vessels, and support skids, and control system.

O&M costs provided by the utilities were actual costs incurred while treating a given quantity of water or, in some cases, budgeted amounts. 1~is is because: (1) either the actual cost data was not available or (2) the plant recently had been commissioned and representative long-term costs were not yet available. The ratio of actual production to plant capacity is called plant operating, or utilization factor. Because the plants had varying plant operating factors, O&M costs per 1,000 gallons of membrane permeate were, adjusted assuming 100 percent plant factor. This was done by separating the O&M costs into two categories: (1) "fixed" costs, which generally are not dependent on plant vperating factor (e.g., labor salaries and fringe benefits) and (2) "variable" costs, which vary proportionately with plant production output (e.g., chemicals, power, cartridge filter replacements). Membrane replacement costs were considered fixed costs because the utilities reported annual budgeted costs, rather than membrane actual replacement costs; in some cases the utilities had not replaced any membranes yet or, in other cases, replacements were covered under warranty and the utilities did not incur costs. To use the O&M cost data for estimati..|g the costs at other locations, or for different

7. M e m b r a n e system costs

Z1. Installed membrane equipment costs

2

-

0

I

0

1/ 15 5 Insh311odMembcone P e ¢ m ~ t e CoDoelly (roOd)

Fig. 6. Installedmembraneprocess equipmentunit cost. Note: Assumes no membrane bypass/blend flow. Costs: January 1994 US Dollars. Refer to Table 5 for abbreviations.

18

R.A. Bergman /Desalination 102 (1995) 11-24

Table 5 Construction costs. Membrane softening water treatment plants in Florida Plant name

Year conunissioned

Installed membrane

capacity (mgd) Operational Plantation (PN) 1991 Ft. Myers (FM) 1992 Collier Co. (CC) 1993 Indian River Co. SO (IR) 1991 & 1994 Dunedin (DN) 1993 Boynton Beach (BB) 1994 Village R. Palm Beach (RP) 1994 St. Lucie West Dev. (SL) 1988 In construction Hollywood (HD) Miramar (MR)

Treatment Estimated unit cost per installed membrane plant const. capaci~ in 1994 Dollars ($1eDd~(b.c~ cost Treatment plant Disposal well Total ($ million) (a)

12 12 12 6 4.8 (d) 4 1.5 1

12.2 14.2 16.8 11.4 10.5 12.2 3.8 7

1.13 0.88 0.97 1.9 1.22 1.88 2.3 2.34

0.23 na 0.1 na na 0.13 na na

1.36 0.88 1.07 1.9 1.22 2.01 2.3 2.34

14 (e) 4.5

17.4 12.3

0.97 1.7

na 0.23

0.97 1.93

(a) Does not include raw water supply and concentrate disposal well cost (if applicable). (b) Estimated unit cost does not include raw water supply wells and piping and is expressed per gpd of membrane permeate capacity. Raw water supply costs typically vary from $0.03 to 0.24/gpd for the plants shown. (c) Unit cost adjusted to account for component facilities with capacity greater than the installed membrane capacity. For example, if a building is sized for future membrane trains, the building cost is divided by the full-load membrane capacity. (d) Six mgd of membrane capacity supplied in contract. (e) Additionally, 4 mgd of RO capacity will be installed treating Floridan aquifer brackish groundwater.

7.2.

Total m e m b r a n e construction costs

softening

plant

T a b l e 5 and Fig. 7 s h o w total m e m b r a n e s o f t e n i n g p l a n t c o n s t r u c t i o n costs. These costs g e n e r a l l y i n c l u d e all treatment plant site facilities including installed membrane process equipment, buildings, degasifiers, clearwells, and transfer p u m p s (if applicable), ground storage and high-service pumping, bulk chemical storage, emergency power, y a r d p i p i n g , and site d e v e l o p m e n t . S u p p l y wells and r a w w a t e r p i p i n g are not included. However, where applicable, concentrate treatment and d i s p o s a l facilities are included. C o n s t r u c t i o n costs v a r y from about $2.25 to $ 2 . 7 5 / g p d for l - m g d f a c i l i t i e s to $0.75 to $1.30/gpd for 15-mgd plants.

7.3. Membrane softening plant O & M costs O & M c o s t s a s s u m i n g a 100 p e r c e n t o p e r a t i n g f a c t o r are s h o w n in T a b l e 6 and Fig. 8. As in the case with construction costs, the data p o i n t s a n d c o r r e s p o n d i n g t w o letter location identifications are shown. O&M c o s t s v a r y f r o m a p p r o x i m a t e l y $ 1 . 6 0 to $ 2 . 0 0 / 1 , 0 0 0 g a l l o n s p r o d u c t i o n for 1 - m g d plants to a b o u t $0.45 t o $0.55/1,000 g a l l o n s for 15-mgd facilities. Fig. 9 p r e s e n t s an a v e r a g e b r e a k d o w n o f O & M costs. L a b o r c o s t s ( i n c l u d i n g salaries and fringe benefits) and power are a p p r o x i m a t e l y equal (at 100 p e r c e n t p l a n t factor) at 30 p e r c e n t each. C h e m i c a l s and budgeted membrane replacements are approximately 15 and 10 percent,

R.A. BergmanI Desalination 102 (1995) 11-24

19

mblono i~/

CC

wllhOisposol W~ ~

Fig. 9. Breakdown of membrane softening operation and maintenance cost. *Includes fringe benefits. 0

5 I0 15 InslolledMem~ane PermealeCopaclly(mOO)

Fig. 7. Membrane softening water treatment plant unit construction cost. Note: Assumes no membrane bypass/blend flow. Costs: January 1994 US Dollars. Refer to Table 5 for abbreviations,

~

,

FM

CC

0 0

5 10 15 Membrane $oflenlnO Perme,',tn C o o ~ i l V(moO)

Fig. 8. Membrane softening water treatment plant unit operation and maintenance cost. Note: Assumes 100 percent plant operating factor. Costs: January 1994 US Dollars. Refer to Table 6 for abbreviations. respectively. Finally, "other" O & M costs, such as cartridge filter replacements and repairs and maintenance, account for the final 15 percent of total O&M costs.

7.4. Effects of blending and plant operating factor When estimating the construction, O&M, and total treated water unit costs, membrane bypass/blending rates and plant operating factors must be considered. Treated water unit costs include amortized construction costs and O & M costs. Costs are lowered by the rate water is bypassed around the membrane system. The water flow rate that can bypass the m e m b r a n e s and blend with permeate to produce the overall finished water is controlled by the membrane feed and permeate water quali~ies and the product water goals. In many cases, all o f the water must be treated by the membranes (i.e. no bypass/blend is acceptable). Only three of the 10 plants listed in Table 3 have membrane bypass flow. Plant operating factor also affects the total treated water costs and variable O&M costs but not amortized annual construction costs and fixed O & M costs. Table 7 presents an example o f varying plant factors and bypass/blending rates on total treated water costs. Based on the assumption shown, the total treated water cost is $0.85/1000 gallons at 100 percent plant

R.A. Bergman / Desalination 102 (1995) 11-24

20

Table 6 Operation and maintenance costs. Membrane softening water treatment plants in Florida Membrane O&M cost assuming 100 percent operating plant factor without membrane permeate bvnass blendine ($11.000 ~,allons) (a) capacity Labor (b) Chemicals Power M~mbrane Other Total (mgd) replacement (c)

Plant name

Plantation (PN) Ft. Myers (FM) Collier Co. (CC) Indian River Co. SO OR) Dunedin (DN) Boynton Beach (BB) St. Lucie West Dev. (SL)

12 12 12 6 4.8 4 1

0.12 0.17 O. 15 0.13 0.17 O. 38 0.56

0.1 0.06 0.08 0.13 0.18 0.05 0.32

0.13 0.15 O. 14 0.21 0.19 0.12 0.63

0.08 0.06 0.08 0.04 0.1 0.07 0.11

0.02 0.1 0.09 0.04 0.06 0.05 0.35

0.45 0.54 0.54 0.55 0.7 0.67 1.97

(a) Membrane b3.)ass (percent of finished water) for Indian River County South and Dunedin is approximately 30 percent and 50 percent, respectively. The other plants treat all the finished water with membranes. (b) Labor includes cost of fringe benefits. (c) Budget values.

Table 7 Example of the effects of plant operating factor and membrane bypass/blending on total production cost Assumptions MS permeate capacity, mgd Capital cost, $ million ($/gpd) Fixed O&M cost, $/1,,900 gal Variable O&M cost, $/1,000 gal Amortization Example MS bypass rate, %* Plant operat, factor, % MS permeate, mg/y MS bypass, mg/y Total plant production, mg/y Amortized capital cost O&M cost Fixed Variable Tot',d O&M Total treated water costs

10 12.5 (1.25) 0.15 0.35 8%. 20 years (capital recovery factor = 0.102)

0 100

30 100

3,650

3,650 1,564.3

0 3,650

$1,O00/y $1,O00/y 1,275 0.349

0 75

5,214.3 $1,000/y 1,275

$1,O00/y 0.245

30 75

2,737.5 0

2,737.5 1,173.2

2,737.5

3,910.7

$1,000/y 1,275

$/1,000 gal $/1,000 gal $/1,000 gal 0.466 1,275 0.326

547.5 1,277.5 1,825

0.15 0.35 0.5

547.5 1,277.5 1,825

0.105 0.245 0.35

547.5 958.1 1,505.6

0.2 0.35 0..¢5

547.5 958.1 1,505.6

0.14 0.245 0.385

3,100

0.849

3,100

0.594

2,780

1.016

2,780

0.711

Summary Total treated water production cost, $/1,000 gal No MS bypass (100% MS) 30% bypass (70% MS)

100% plant factor 0.85 0.59

75% plant factor 1.02 0.71

MS = Membrane softening. *MS bypass rate is defined as the ratio of bypass water to finished (blended) water volume.

R.A. Bergman / Desalination 102 (1995) I1--24

factor with no membrane bypass. The unit cost increases to $1.02/1000 gallons if the plant factor drops to 75 percent. With a 30 percent bypass the unit costs drop to $0.59 and $0.71/1000 gallons, respectively, at 100 and 75 percent plant factors. Z5. Effects o f concentrate disposal

Concentrate disposal methods and permitting also affects costs. Table 8 shows the concentrate disposal methods used for membrane softening plants in Florida. Methods include deep injection wells, discharge to salt water bodies, transport to wastewater treatment plants, and blending and use tor irrigation/reuse. Deep injection wells are used only when a favorable injection zone is t~resent and when other less costly 6isposal meu~ods are not available. As shown in Table 5 and Fig. 7, deep injection wells for concentrate disposal cost approximately $0.10 to 0.23 per 1,000 gallons of installed permeate capacity. Table 8 Concentrate disposal methods used for membrane softening plants in Florida Plantation Ft. Myers Collier Co. Indian RiverCo. SO Dunedin Boynton Beach Village RoyalPalmB. St. LucieWestDev. Hollywood(a) Mir:mar

Deep injectionwell Irrigation/reuse Deep injectionwell Surfacedischarge(salt water) ToV ~ Deep injectionwell ToWWTP Irrigation/reuse Surfacedischarge(salt water) Deep injectionwell

(a) Possible deep injection well may be required by regulatory agency in lieu of preliminary surface disposal plans.

8. Lime softening costs Lime softening construction cost data for new plants in Florida is difficult to obtain because essentially all new large public water softening plants built in the last ten years have

21

utilized membrane softening, except where existing lime softening plants were expanded. Three sources of data were used: (I) construction cost of one new lime softening plant built in Florida, (2) The Cost Digest [4] data, and (3) in-house CH2M Hill cost estimate data for Florida lime softening plants. All costs were updated to January 1994 dollars using ENR CCI values. The lime softening plant considered is the Florida Keys Aqueduct Authority's ( F ~ ) 22 mgd plant commissioned in 1988 for approximately $13.5 million. Updating to January 1994 dollars, the unit construction cost for the FKAA plant is $0.74 per gpd. The Cost Digest has calculated relationships between lime softening system capacity and capital investment, net annual operating expenses, and unit annualized cost [4]. Total capital investment covers equipment, facility construction, engineering, loan interest, land, working capital, and startup for initial operation. Net annual operating expenses include operation and maintenance as well as overhead, insurance, taxes, and general expenses. To allow proper comparisons with cost data presented in this paper, only the estimated construction cost part of The Cost Digest's capital investment data was used. The construction costs were estimated to be about 55 percent of the total capital investment. Construction costs varied from about $1.76/gpd for 1 mgd plants to $0.70/gpd for 15 mgd plants. CH2M Hill's in-house cost estimate data for Florida lime softening plants agrees quite well with construction cost data reported in The Cost Digest. Based on CH2M Hill estimates, construction costs ranged from $1.65 to 0.85/gpd for 1 to 10 mgd facilities. The Cost Digest did not report plant operating factors for the net annual operating expenses. Therefore, this reference is not used for the lime versus membrane softening O&M cost comparison. Based on CH2M Hill's estimates for Florida plants, lime softening O&M costs ranged from $0.95 to $0.40/1,000 gallons for 1 to 10 mgd plant capacities.

22

9.

R.A. Bergman / Desalination 102 (1995) 11-24

Lime softening softening

versus

membrane

9.1. Cost comparison Fig. 10 presents a comparison of unit construction costs for lime and membrane softening WTP's from 1 to 15 mgd capacity. Lime softening plant costs were lower than membrane softening throughout the entire range but the relative difference in costs decreased with larger facilities For 15 mgd plants, the lower range of the membrane softening plant costs is only about 10 percent greater than for lime softening plant cost. Therefore, for any specific application both softening technologies should be considered, at least for initial feasibility analysis. Furthermore, if additional treatment processes (such as ozone) are added to lime softening to produce water quality comparable to membrane softening permeate (with lower color and THMFP), membrane softening can have lower construction costs than lime softening. Fig. 11 presents O&M cost c:;,ta for lime and membrane softening plants in Florida. As in the case for construction costs, lime softening (alone) generally is less expensive than membrane softening, assuming no bypass/blending and a 100 percent plant operating factor. However, as plant capacities increase, the relative costs between the two processes decrease. Generally, membrane softening O&M costs are about 15 percent greater than lime softening O&M costs. Again, if additional treatmeat processes are added to lime softening to match the better membrane softening permeate water quality, or if some water can be bypassed around the membranes and blended to produce the finished water (thus taking advantage of the superior water quality from the membrane process), membrane softening can be less expensive than lime softening.

~,

"~

..

-.!..T. I UmeSo.~.g w1~ (TheCostI~gest)

5 10 Production Capacity (mgd)

Fig. 10. Lime and membrane softening water treatment plant unit construction cost. Note: Assumes no bypass/blend flow. Costs: January 1994 US DcUars.

k FlO(IdoLimeSonel~ WIPs(CH2MHILl. ~llmales)

0

5 10 Production Capacity (mgcl)

15

Fig. 11. Lime and membrane softening water treatment plant unit operation and maintenance cost. Note: Assumes no bypass/blend flow. Costs: January 1994 US Dollars. Assumes 100 percent plant operating factor.

R.A. 8ergman / Desalination 102 (1995) 11-24 9.2. Non-economic issues

A number of issues, not directly related to costs, that often favor membrane softening to lime softening are: Site aesthetics: Membrane softening plants have fewer tanks and unsightly structures and can be designed to "blend in" aesthetically with the surrounding neighborhood. - P r o d u c t quality: Membrane permeate produces a superior product quality, not only in terms of meeting health-related maximum contaminant levels for many individual water constituents, but also for color, turbidity and other aesthetic parameters. - Sludge disposal: Even though membrane softening has a liquid waste concentrate stream to dispose, residual sludge is not generated. Sludge is difficult and costly to handle and dispose in many cases. - Land requirements: Generally, membrane softening requires less land area than lime softening. -Ease of phasing: Membrane softening plants typically are constructed using multiple parallel process "trains" and lend themselves well to construction phasing. This allows relatively small capacity trains to be added economically, on an as needed basis, to meet growing water demands, rather than having to install large incremental facility capacities each expansion. - Process flexibility: Membrane softening has better process flexibility in terms of meeting new drinking water regulations. It is a "barrier" process that removes contaminants without creating chemical byproducts. -

Some of the other issues that favor lime softening over membrane softening are: - Regulatory concerns: Lime softening is a well-known, established process relative to membrane softening.

23

risk and uncertainty: Globally, lime softening has been practiced for more than 200 years. Membrane softening, being a relatively new commercial process (less than 20 years old), has greater process risk and uncertainty - particularly in terms of treatment cost with varying site-specific feed waters. - O&M staff: In Florida many municipal drinking water systems have utilized lime softening. From an operator training cost and experience standpoint, lime softening has an advantage over membrane softening. -ConcenUate disposal: Although lime softening has residual sludge to dispose, it does not have a liquid concentrate flow stream to waste. Membrane concentrate disposal can be costly and difficult to permit in many cases.

-Process

Acknowledgements

The author would like to acknowledge and thank the following individuals for their input and providing plant survey information: Mel Entus and Duane Wallace (City of Plantation, FL); Robert Kenyon, Robert David Ailstock, and John Reynolds (City of Boynton Beach, FL); Byron Weightman (City of Fort Myers, FL); George Morgan (St. Lucie West Services District, Port St. Lucie, FL); Randy Garay (Collier County, FL); Thomas Blumberg (Village of Royal Palm Beach, FL); Brad O'Keefe (Indian River County, FL); Lynn Bolin (City of Dunedin, FL); Smart McClellan (FilmTec Corporation); and Paul Laverty (Membrane Systems Corp). References

[1] E.A. Fernald and D.J. Patton, Water Resources Atlas of Florida, Florida State University,Institute of Science and Public Affairs, 1985. [2] H.G. Healy, Public Water Supplies of Selected Municipalities in Florida, 1970. Bureau of Geology Information Circular No. 81; Florida Department of Natural Resources, Division of

24

R.A. Bergman / Desalination 102 (1995) 11-24

Interior Resources, B u r e a u of Geology, Tallahassee, 1972. [3] W.J. Conlon and S.A. McClellan, Membrane softening, a water treatment process that has come of age, Journal AWWA 81 (1989) 11. [4] G. DeWolf, P. Murin, J. Jarvis, and M. Kelly. The

Cost Digest: Cost Summaries of Selected Environmental Control Technologies EPA-600/884-010, Office of Environmental Engineering and Technology, Office of Research and Development, U.S. E n v i r o n m e n t a l P r o t e c t i o n Agency, Washington DC, October 1984.