Am6_W912PL-12-R-0018.pdf

AMENDMENT OF SOLICITATION/MODIFICATION OF CONTRACT 1. CONTRACT ID CODE PAGE OF PAGES

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2. AMENDMENT/MODIFICATION NO. 3. EFFECTIVE 4. REQUISITION/PURCHASE 5. PROJECT NO. (If applicable) 0006 June 19, 2012

6. ISSUED BY CODE W912PL 7. ADMINISTERED BY(If other than item 6)

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USACE-LOS ANGELES DISTRICT

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P.O. BOX 532711

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NO.

W912PL-12-R-0018

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May 24, 2012

10A. MODIFICATION OF

CONTRACT/ORDER NO.

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CODE FACILITY CODE

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14. DESCRIPTION OF AMENDMENT/MODIFICATION (Organized by UCF section headings, including solicitation/contract subject matter where feasible)

DESIGN AND CONSTRUCTION OF A WATER TREATMENT AND DISTRIBUTION SYSTEM, FORT IRWIN

SAN BERNARDINO COUNTY, CALIFORNIA

The following addendum to Appendix R: BENCH AND PILOT TESTING INVESTIGATIONS, noted as Amendment 6 and as listed below, is to be included with the original specification sections from the Bid Documents dated May 24, 2012.

Supplementary Report of Data Results for Ft. Irwin WTP Project: Development of a Treatment Process an Electrodialysis Reversal Reject Stream Consisting of Lime Softening and Reverse Osmosis to Achieve a Water Recovery

Except as provided herein, all terms and conditions of the document reference in item 9A or 10A, as Heretofore changed, remains unchanged and in full force and effect.

15A. NAME AND TITLE OF SIGNER (Type or print) 16A. NAME AND TITLE OF CONTRACTING OFFICE (Type or print)

15B. CONTRACTOR/OFFEROR

(Signature of person authorized to sign)

15C. DATE SIGNED 16B. UNITED STATES OF AMERICA

BY

(Signature of contracting officer)

16C. DATE SIGNED

NSN 7540-01-152-8070 30-105 STANDARD FORM 30 (REV. 10-83)

PREVIOUS EDITION UNUSABLE Prescribed by GSA

Medina et al. 2012 Page 1

Supplementary Report of Data Results for Ft. Irwin WTP Project

Development of a Treatment Process an Electrodialysis Reversal Reject Stream Consisting of Lime Softening and Reverse Osmosis to Achieve a Water Recovery

Victor F. Medina, Scott A. Waisner, Jared A. Johnson, Roy Wade, Jose Mattei-Sosa, Fredrick C. Brown

U.S. Army Engineer Research & Development Center – Environmental Laboratory, 3909 Halls Ferry Rd. Vicksburg, MS.

Background

ERDC-EL conducted a series of water treatment tests to support the conceptual design of the proposed water treatment plant (WTP) at Ft. Irwin (Figure 1). The results were summarized in Medina, et al. (2012), which also includes a detailed background.

Briefly, the laboratory studies focused on the treatment of a simulated reject stream from a proposed electrodialysis reversal (EDR) system with the goal in increasing water recovery using partial lime softening and reverse osmosis (RO) to a level of 97 to 98% (Figure 1). With the addition of a mechanical vapor compression evaporator, the goal is to achieve 99.6% water recovery. The studies discussed in the report involved jar testing of partial lime softening and bench top reverse osmosis studies of the lime softening effluent. One conclusion of the previous work was that more studies were needed to resolve key, unaddressed issues. With the partial lime softening, it was determined that studies were needed to investigate better silica removal with magnesium chloride (MgCl2). With the reverse osmosis testing, it was determined that performance could be improved with pretreatment of the lime softening effluent, with a better choice of the RO membrane, and with better optimization of flux rates. This short, supplementary report summarizes these additional studies. Some results from the previous study are included here within, so that this report can serve as the primary source of the laboratory study results at this time.

Medina et al. 2012 Page 2

Figure 1. Simplified process flow diagram of the proposed Fort Irwin water treatment plant showing unit processes to achieve 99.6% water recovery. This report focuses on the lime softening/reverse osmosis portion

(circled).

Methods

EDR Reject Simulant

The projected chemical composition of the reject stream from the three-stage EDR treatment system is given in the pilot project report prepared by CH2M HILL (2007, Table 6-5 in the report and Table 1 herein). These concentrations are substantially higher than the constituents found in the groundwater because EDR concentrates these constituents in the reject. This projected water chemistry was to create a simulated waste stream by spiking with the appropriate ions. Table 2 summarizes the chemicals and amounts used to prepare a 20 liter batch of EDR simulant. Strontium (Sr) was not included in the formulation as it was a relatively minor constituent. Although barium (Ba) was also relatively low in concentration, it was included because it could be a factor in reverse osmosis fouling due for formation of barium sulfate (BaSO4). Arsenic (As) was added to the test solution because it is one of the key contaminants found in the groundwater.

Medina et al. 2012 Page 3

Table 1. Expected EDR reject constituents and Effluent from Partial Lime Softening for Ft. Irwin WTP (adapted from CH2M HILL, 2007).

Constituent Unit EDR Reject (Softening influent)

Softening Effluent Post Filtration pH pH units 8.2-8.3 10.3 7.0 Chloride (Cl-) mg/L 1000-1100 1225 1225 Calcium (Ca2+) mg/L 200-300 8.5 8.5 Magnesium (Mg2+) mg/L 30-35 30-35 30-35 Strontium (Sr2+) mg/L 4.5-4.6 2.8 2.8 Barium (Ba2+) mg/L 0.3-0.4 0.2 0.2 Sulfate (SO4

2-) mg/L 1300-1400 1300-1400 1300-1400 Bicarbonate mg/L 1600-1700 38 391 Fluoride (F-) mg/L 63-65 44 44 Arsenic (As) g/L 50 -- 5.77 Silica (Si) mg/L 80-90 10 10 TDS mg/L 6400-6500 6000-6300 5400 Hardness mg as

CaCO3/L 820-850 170 170

*Report gives 10.3. However, modeling was conducted at pH 10.75 (Mavis, 2012, personal communication)

Table 2. EDR Reject Simulant Formulation

Sequence No. Chemical Name Chemical Formula Chemical Weight, g* 1 sodium arsenite NaAsO2 0.0154 2 sodium silicate Na2O3Si 9H2O 17.88 3 magnesium chloride MgCl2 2.54 4 strontium chloride SrCl2 0.1646 5 sodium sulfate Na2SO4 22.18 6 sodium chloride NaCl 31.36 7 sodium fluoride NaF 2.82 8 sodium bicarbonate NaHCO3 41.16 9 barium sulfate BaCl2 0.0106 10 calcium sulfate CaSO4 · 2H2O 21.47

* - Mass required to achieve 20 liters of simulated EDR waste water

Table 1 also shows the expected treatment to be provided by the partial lime softening operation. In order to protect our analytical equipment; we filtered our samples (0.45 m). So, our results are comparable to the post filtration results. The primary purpose of the partial lime softening step is to protect the RO membranes. The key constituents targeted for removal to protect the membranes are calcium, silica, and hardness. As seen in Table 1, removal of these constituents is expected to be substantial. The process is also expected to affect modest removal of TDS, barium, and fluoride, but removal of other water quality constituents is not critical, as the water should be successfully treated by RO.

Medina et al. 2012 Page 4

Lime Softening Studies

Jar testing was conducted using a Phipps and Bird jar test apparatus (Hammer, 1986, Reynolds, 1977, Schroeder, 1983) for automated repeatable flash mixing/settling tests.

Jar testing under this effort was broadly categorized under two sets of experiments. The first experimental set evaluated partial lime softening, focusing on the effect of lime dosage. Initial partial lime softening dosages were calculated following accepted practice (e.g. http://www.mrwa.com/Chapter16LimeSoftening.pdf). The lime used was a standard 98% purity hydrated lime product from Falco Lime (Vicksburg, MS). The second experiment tested lime softening with magnesium chloride as an additive to enhance silica removal. All testing used 1-liter test solution volumes.

For the first set of testing, six 2-L containers with one liter of simulated EDR water were prepared for each test. Next, various dosages of coagulant were flash mixed at 100 RPM for one minute, a flocculation period of 5 minutes at 60 RPM, and a 20 minute period of quiescent settling. After settling, samples were collected at the 750 ml level from each container for physical and chemical analyses. For each set of jar tests, one jar apparatus was set up containing no coagulants as a control. The second set of testing (with MgCl2) was identical, except that the flocculation period was increased to 45 minutes.

In the second set, treatment temperatures were varied from 20.5 to 47C by conducting testing in a temperature controlled environmental chamber.

Reverse Osmosis

RO testing was conducted using an Osmonics benchtop unit, which is designed for laboratory research studies on RO processes (Sterlitech Corporation, Kent, WA). The study used a General Electric (GE) polyamide AD membrane, a standard seawater water high rejection membrane element (fact sheet:

http://www.gewater.com/pdf/Fact%20Sheets_Cust/Americas/English/FSpsADHRSeri es_EN.pdf). The areal dimensions of the membrane were 190 x 140 mm and its thickness was 7 mils. A Sepa CF permeate carrier membrane was used, with dimensions of 145 x 47 mm and a thickness of 14.6 mils. Shims were also from Sepa CF with 5 mil thickness and Sepa CF medium foulant spacer was also used. The spacer had the same dimensions as the carrier membrane. The system was operated in through flow mode.

The influent of the system was derived from the EDR simulant. The simulant was treated by lime softening modified by magnesium chloride (1500 mg/L Ca(OH)2 and 750 mg/L MgCl2), as described in the previous section. This solution then underwent a pretreatment process. This involved filtration with a 0.45 m filter, then pH reduction to minimize carbonate deposition on the membrane. This was accomplished by titration with 1 N hydrogen chloride (HCl) until the pH was reduced to about 5.7. The solution was then sparged with compressed air for 12 hours to simulate carbon dioxide stripping.

Medina et al. 2012 Page 5

This solution was treated by the RO unit. Target fluxes ranging from 4 to 12 gallons/ft2/day (gfd) were tested at target permeate recoveries ranging from 40 to 70%.

Analytical

The analytical regiment for each study included the following:

• Total Dissolved Solids (TDS): determined gravimetrically (EPA method 160.1)

• Fluoride (F-), Nitrate (NO3-), Calcium (Ca2+), Magnesium (Mg2+), and Hardness:

Determined with Ion Chromatography (IC) (EPA method 300.1)

• Arsenic and Silica: Inductively Coupled Plasma – Atomic Emission Spectrophotometry (ICP-AES) (EPA method 200.7).

• Conductivity (EPA method 120.1)

• pH (EPA method 150.1)

Results

Lime Softening Only

As shown in Figure 2, lime dosages ranged from zero to 2000 mg/L. A goal of the lime softening process was to reduce calcium and hardness levels, so as to protect the RO unit from scaling. Lime dosages of 10 mg/L or less did not affect calcium or hardness levels (Figure 2). However, treatments of 200 mg/L began to show modest decreases of these constituents. At 2000 mg/L, removal of hardness and calcium were substantial and would exceed the targets established in Table 1 (post filtration). Silica was a key parameter to remove during the lime softening process, because silica can interfere with RO. Partial lime softening appeared to have a modest removal of silica, reducing levels from about 60 to levels just over 40 at 2000 mg/L. However, this level was higher than the target of 10 mg/L. Although removal of TDS and fluoride were not critical for the lime softening process, some removal was desirable just to lower the load on the RO unit. The partial lime softening treatment did not have any appreciable effect on TDS and fluoride levels for the dosages tested. Fluoride removal would presumably occur by precipitation as calcium (or magnesium) fluoride. Since calcium and hardness decreased continuously as the lime dosage increased, no fluoride removal was achieved.

Medina et al. 2012 Page 6

Figure 2. Effect of partial lime softening on major constituents of a simulated EDR reject stream

Figure 3 summarizes pH and alkalinity increases with increased lime dose. pH increased with lime dosage from about 8.7 to nearly 11.9. Alkalinity also increased as expected with lime dose from 250 to 850 mg CaCO3/L because there was insufficient hardness to precipitate the alkalinity.

Figure3. Effect of partial lime softening on pH and alkalinity of a simulated EDR reject stream

0 500 1000 1500 2000 2500

TD

S, A s

TS

S, F , C a, H ar d n e ss Si

Hydrated Lime Dose (mg/L)

TSS (mg/L) F (mg/L) Ca (mg/L) Total Hardness (mg CaCO3/L) Total Si (mg/L) TDS (mg/L) Total As (µg/L)

0 500 1000 1500 2000 2500

A lk al in it y (m g C aC

O /L p H

Dose (mg/L) pH

Alkalinity to pH 7

Medina et al. 2012 Page 7

Sludge generation was a critical aspect, since the sludge would be a key loss of water in the system. Sludge production ranged from 2 (at a lime dosage of 2 mg/L) to 24 mL/L (lime dosage of 2000 mg/L), making the percentage of sludge generated by the process

0.2 to 2.4% (Figure 4). Solids ranged from 0.22 to 0.63 g/L (NOTE: we remeasured solids - these numbers differ slightly from Medina et al. 2012) at the same lime dosages.

The solids fraction, by mass, of the sludge, ranges from 11 (at a dosage of 2 mg/L) to 2.5% (2000 mg/L). Under the assumption that the solids have a specific gravity of 1.6 (NOTE: this estimated specific gravity was adjusted from Medina et al. 2012), then the percentage of water lost in sludge from the reject stream ranges from 0.19% to 2.4%. To clarify, this is not cake moisture, but rather an estimate of the water in the sludge versus the water treated. Since the reject stream is projected to be 8% of the total flow, then the total water losses would be 0.02% to 0.19% of the design flow. This water here is lost from the recovery and must be considered carefully in any design. The overall water recovery requirement is 99.6%, which leaves 0.4% for system losses.

Figure 4. Sludge results for Partial Lime Softening with no additives. y‐axis units are given in the legend.

Lime Softening with Magnesium Chloride

In order to improve silica removal, we studied the use of magnesium chloride (MgCl2) as an additive. The concept is to promote additional formation of magnesium hydroxide (Mg(OH)2), which can trap and remove silica as it settles. Based on the results of the previous studies, we decided to focus our work on a hydrated lime dosage of 1500 mg/L.

The plan was to test a temperature range from 20 to 47C. Because carbonates are actually less soluble at higher temperature, we conducted the 47C study first screen MgCl2 dosages. Figure 5 summarizes silica, arsenic, and pH results for MgCl2 dosages

0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0 500 1000 1500 2000

D ri ed S lu d g e

M as s, g

/L

S lu d g e V o lu m e, m L

/L

Hydrated Lime Dosage (mg/L)

Medina et al. 2012 Page 8 ranging from 0 to 2000 mg/L at a constant hydrated lime dose of 1500 mg/L. To reiterate, the main goal of the softening process was to remove silica, as it can adversely affect the RO process. Treatment of arsenic is also useful, but not critical, as it would be presumably removed in RO unit. With no magnesium chloride modification, removal of silica and arsenic were modest. With magnesium chloride dosages increasing from 0 to 750, removal of both silica and arsenic improved with increasing dosage. At a magnesium chloride dosage of 500 mg/L, the silica level reached the 10 mg/L goal.

However, at magnesium chloride dosages higher than 750 mg/L, the removal of both silica and arsenic actually decreased slightly.

Figure 5. Assessment of different supplementary magnesium chloride dosages at a hydrated lime dose of 1500 mg/L and at 47C.

Figure 6 shows the effect of the magnesium chloride modified partial lime softening process on TDS conducted at two temperatures, 20.5C and 47C. Focusing on the 47C data indicates that one by-product of the partial lime softening process with magnesium chloride is that it results in higher TDS values. This is not unexpected, as the softening process does add additional dissolved species, chloride, to the water. The RO unit process should remove the excess TDS. However, it is desirable to keep this TDS increase as modest as possible, so as to increase the life of the RO membranes. Dosing the magnesium chloride higher than 750 mg/L could result in unnecessary TDS that will have to be treated with the RO unit.

8.57 8.67

11.23 11.79 11.69 11.60 11.15

10.03 9.66 9.61

0.00

0.10

0.20

0.30

0.40

0.50

0.60

Reject Simulant

0 100 200 400 500 750 1000 1500 2000

A rs en ic

C o n ce n tr at io n , m g /L

S ili ic a

(m g

/L an d p H

C o n ce n tr at io n s

(u n it le ss

Hydrated Lime at 1500 mg/L and Magnesium Chloride Dosage, mg/L at 47 C

Silica pH Arsenic

Medina et al. 2012 Page 9

Figure 6. Total Dissolved Solids at different magnesium chloride (MgCl2) dosages with hydrated lime addition of

1500 mg/L and at 20.5 and 47C. 100 and 200 mg/L MgCl2 were only tested at 47.5C.

Figure 7 shows silica removal results for jar test studies with magnesium chloride levels ranging from 750 to 2000 mg/L, with a hydrated lime concentration of 1500 mg/L.

Since 750 mg/L was optimal at 47C, we chose this as a starting point for the additional studies. At a hydrated lime dosage of 1500 mg/L and a magnesium chloride dose of 750 mg/L at 20.5 C, the silica concentration was about 13 mg/L, compared to over 40 mg/L with 2000 mg/L of hydrated lime alone (Figure 2), slightly higher than the 10 mg/L goal (Table 1). At 25C and higher temperatures, the silica levels at the 750 mg/L magnesium chloride treatment were reduced to well below 10 mg/L.

Figure 7 indicates that the relationship between dosage, temperature, and silica removal is complex. At 20.5C, increasing the magnesium chloride dosage up to 1500 mg/L results in improved silica removal, but at 2000 mg/L, the resultant silica concentration increased slightly. At 25C, the best treatment was at 1000 mg/L, with dosages higher than that resulting in less effective removal. And at 30 and 47C, the best treatment was found at the 750 mg/L magnesium chloride dosage.

Reject Simulant

0 100 200 400 500 750 1000 1500 2000

T o ta l D is so lv ed

S o lid s

C o n ce n tr at io n , m g /L

Hydrated Lime at 1500 mg/L and Magnesium Chloride Dosage, mg/L

20.5 C 47 C

Medina et al. 2012 Page 10

The effect of temperature is also show in Figure 7, which compared TDS values for MgCl2 modified partial lime softening at levels at 20.5 and 40.7C. In all cases, the TDS numbers were either virtually identical, or higher for the 20.5C data.

Figure 7. Removal of silica with lime softening (1500 mg/L dosage) with magnesium chloride as an additive

(ranging from 750 to 2000) at temperatures ranging from 20.5 to 47C.

In reviewing the water chemistry data for the 1500 mg/L hydrated lime dosage with 750 mg/L magnesium chloride at different temperatures, we can see that the process was effective at reducing concentrations of arsenic, fluoride, and calcium. TDS and chloride increased, which was expected as the addition of magnesium chloride results in an increase of chloride in solution. Hardness and magnesium both increased for the treatments at 20.5, 25 and 30C. The increase was greater than 5 fold Mg and greater than 4 fold for hardness at 20.5C. Hardness was reduced nearly 75% in the highest temperature tested, 47C. Alkalinity was reduced with increasing temperature, ranging from over 700 for 20.5C to 200 for 47C.

Table 3. Summarization of water chemistry parameters by partial lime softening (hydrated lime concentration of 1500 mg/L) amended with MgCl2 (750 mg/L) at temperatures ranging from 20.5 to 47C.

Average EDR Rej.

Sim.

Conc.

Temperature (°C)

Analytes Units 20.5 25 30 47

0 500 1000 1500 2000 2500

S ili ca C o n ce n tr at io n

, m g

/L

Hydrated Lime Dosage of 1500 mg/L and Magnesium Chloride Dosage, mg/L

20.5 C 25 C 30 C 47 C

Medina et al. 2012 Page 11

TSS mg/L 654 105 105 223 14 Turbidity NTU 280 44.6 57.0 47.6 8.88

TDS mg/L 4941 5940 5964 5301 5028

As mg/L 0.401 0.206 0.080 0.100 0.061

Si mg/L 51.8 13.9 3.16 6.57 5.01

F mg/L 79.0 43.8 40.8 34.4 26.4

Cl mg/L 1093 1606 1666 1619 1661

SO2 mg/L 1302 1272 1281 1265 1293

NO3 (ion) mg/L 3.51 <0.5 <0.5 2.31 <0.5

Ca mg/L 14.4 8.9 10.0 11.7 8.37

Mg mg/L 14.2 108.9 82.2 54.8 1.02

Total Hardness mg CaCO3/L 94.4 469 363 255 25.1

Total Alkalinity mg CaCO3/L 893 701 701 550 200 pH --- 8.71 10.3 10.1 10.0 11.2

Sludge Data Sludge Volume ml n.a. 24 20 30 35

Percent Wet Sludge % (v/v) n.a. 2.4 2.0 3.0 3.5

Solids per Volume

Treated (dry) g/L n.a 0.69 0.45 0.73 1.18 Sludge Solids % (w/v) n.a. 2.9 2.2 2.4 3.4 n.a. = not applicable

Table 3 also includes sludge generation data for the partial lime treatment (dosage of 1500 mg/L) with supplementary magnesium chloride (750 mg/L). Percent wet sludge ranged from 2.4 to 3.5%, which was higher than the percent wet sludge found by partial lime softening alone (2.2% for 2000 mg/L hydrated lime dosage). The percent solids in the sludges generated from the magnesium chloride modified treatment were close to the value for the 2000 mg/L hydrated lime only value of 2.4. The solids per water treated was higher compared to partial lime softening alone, indicating improved treatment by the addition of the magnesium chloride supplement. The sludge solids were comparable. Using the same assumptions in the section above, the percentage water lost from magnesium chloride (750 mg/L) supplemented partial lime softening (1500 mg/L hydrated lime) ranged from 0.08% (at 25C) to 0.16% (47C) of the initial 6 mgd.

Reverse Osmosis Treatment

As discussed in the Methods, the bench top RO system was used to treat the effluent of the partial lime softening process (20.5C). The partial lime softening effluent underwent pretreatment involving filtration (0.45 m), pH adjustment to about 5.7, and carbon dioxide stripping. The resulting solution was then used for the RO testing, and its

Medina et al. 2012 Page 12 chemical properties were quantified (Table 4). Interestingly, the Mg concentration was greatly reduced in concentration, from 108 mg/L (Table 3) to 18 mg/L and this resulted in a decrease in hardness, from over 400 mg/L to 100 mg/L as CaCO3. The pH of the solution was 7.83. We estimated alkalinity by titration down to a pH = 8.3 endpoint. So, we were not able to quantify the precise alkalinity, but it is reasonable to assume that it would be less than 200 mg/L as CaCO3 based on previous results with the EDR reject simulant (Table 3, 47C data). This indicates that the pretreatment process was successful at reducing alkalinity, which was over 700 mg/L as CaCO3 right after lime softening with magnesium chloride addition (Table 3, 20.5C data).

Table 4. Water Chemistry of RO Influent

RO Influent F (mg/L) 31.2 Cl (mg/L) 1843

SO4 (mg/L) 1277 Ca (mg/L) 10.1 Mg (mg/L) 18.3 As (mg/L) 0.050

SiO2 (mg/L) 7.11 pH 7.84

Hardness (mg/L as CaCO3) 100 Alkalinity (mg/L as CaCO3) <200

LSI -0.36

pH 7.83

The RO studies were conducted with fluxes ranging from just over 3 to just under 12 gfd and with permeate recoveries ranging from just over 40% to nearly 70% for a single stage system. At these conditions, operating pressures ranged from just below 120 to just over 270 psi (Figure 8). At a given flux, operating pressures increased as the system was adjusted to achieve a higher permeate recovery. For example, consider data collected at a flux of about 4 gfd. At a recovery of 48%, the operating pressure was about 130 psi. At a recovery of 66%, the pressure was 160 psi. For a given recovery, pressures increased as fluxes were increased more dramatically. For example, at a 60% recovery and a flux of 3.8 gfd, the operating pressure just below 140 psig, but at a flux of 11.6, the pressure was more than 270 psig.

Medina et al. 2012 Page 13

Figure 8. Reverse Osmosis Results: Relation of operating pressure to recovery and flux.

Table 4 indicated that fluoride concentrations in the RO influent was about 31 mg/L.

RO treatment was success at 90% removal and more at flux rates as low as 3.4, resulting in concentrations around 3 mg/L (Figure 9). As flux was increased, treatment generally improved, and at flux levels greater than 8 gfd, fluoride levels in the permeate were less than 2 mg/L (Figure 9).

Preliminary Treatment of Simulated EDR Waste Stream:

- 1500 mg/L Lime + 750 mg/L MgCl2

- 0.45-m Filtration

- pH neutralization with HCl

- air stripping

1st Stage RO Operating Pressure of Ft. Irwin WTP Simulated EDR Waste Stream

Flux (gfd)

3 4 5 6 7 8 9 10 11 12

R ec o ve ry

Pressure (psig)Data Points

Medina et al. 2012 Page 14

Figure 9. Reverse Osmosis Results: Relation of fluoride concentration in the permeate to recovery and flux.

Table 5 shows water chemistry at flux rate of two flux rates close to 10 gfd with water recoveries of 53 and 56%. Fluoride ranged from 1.65 to 1.90 mg/L, which was below the MCL of 2 mg/L. Chloride was dramatically reduced from over 1800 mg/L to 65 to 75mg/L. Hardness was also greatly reduced, to levels just above 1 mg/L. Arsenic was reduced from 0.050 mg/L to levels on the order of 0.020 mg/L, about a 50% reduction.

However, these arsenic levels were above the MCL of 0.010 mg/L. The lowest arsenic

Preliminary Treatment of Simulated EDR Waste Stream:

- 1500 mg/L Lime + 750 mg/L MgCl2

- 0.45-m Filtration

- pH neutralization with HCl

- air stripping

1st Stage RO Permeate Fluoride Concentration of Ft. Irwin WTP Simulated EDR Waste Stream

1.5

1.5

1.5

1.5

1.5

1.5

2.0

2.0

2.0

2.0

2.0

1.0

1.5

2.5

2.5

2.5

2.0

2.5

2.5

3.0

3.0

3.0

3.0

3.5

Flux (gfd)

3 4 5 6 7 8 9 10 11 12

R ec o ve ry

Fluoride (mg/L)Data Points

Medina et al. 2012 Page 15 concentration we were able to achieve was 0.011 mg/L, at a flux of 8.15 gfd and a recovery of 57.1%.

Table 5. Water Chemistry at two flux/recovery conditions

Influent Permeate Permeate % Rejected % Missing Operating Conditions

Flux (gfd) 9.87 10.77 Recovery (%) 53.0 56.2 Pressure (psig) 241 258

Constituents Fluoride (mg/L) 31.2 1.90 1.65 93.9 – 94.7 Chloride (mg/L) 1843 74 67 96.0 – 96.4 Sulfate (mg/L) 1277 25.1 18.6 98.0 – 98.5 14.3 – 7.9 Calcium (mg/L) 10.1 0.183 0.143 98.2 – 98.6 Magnesium (mg/L) 18.3 0.339 0.197 98.1 – 98.9 Arsenic (mg/L) 0.050 0.020 0.027 60.0 – 46.0 Silica (mg/L) 7.11 0.196 0.172 97.2 – 97.6 Barium (mg/L) 0.012 0.007 Hardness (mg/L) 100.7 1.85 1.17 98.2 – 98.8

Discussion

Partial Lime Softening

Partial lime softening was effective at removing hardness and calcium, particularly at a dosage of 2000 mg/L. Hardness removal was on the order of 59% and calcium removal approached 76%. However, partial lime softening alone was not effective at removing silica or fluoride. Optimized pH for silica removal is 10.8 to 11.3, although effective removal was found at pH up to 11.7 (Kluesner, et al. 1975). At optimal hardness and calcium removal, the pH was around 12. Partial lime softening using only hydrated lime could be an effective pretreatment for the RO system, but would probably require the use of an anti-scalant to address silica fouling.

This study focused on magnesium chloride as a supplement to improve silica removal.

Another option is magnesium oxide (MgO). Both have been used for improving silica removal in lime softening (Betz, et al., 1941; Nalco Chemical Company. 1979; Al-Mutaz & Al-Anezi, 2004). The advantage of magnesium oxide is it does not add to the dissolved solids (Al-Mutaz & Al-Anezi, 2004). Furthermore, magnesium chloride can create calcium non-carbonate hardness thus excess lime will need to be added to precipitate out the calcium non-carbonate hardness (Al-Mutaz & Al-Anezi, 2004). This effect, however, would likely not be important for the Ft. Irwin case because of the high alkalinity in the solution, so that any non-carbonate hardness would be offset by calcium and magnesium precipitation. It was found that magnesium oxide was more effective than magnesium chloride at removing silica (Patrick et al., 2001). Magnesium chloride, on the other hand, has some operational advantages that might be valuable for

Medina et al. 2012 Page 16 application at a small WTP like Ft. Irwin (Mavis, Personal Communication, 2012). It can be applied dry or as a 35% solution, and does not have to be prehydrated. Systems using magnesium chloride tend to be simplier than those using magnesium oxide. Magnesium chloride also has a lower tendency to cause scaling and clogging in associated piping, which are primarily due to dry solids and slurry conveyance needs for magnesium oxide.

Fort Irwin is in a desert area in Southern California that is characterized by hot temperatures. In addition, the EDR process is expected to result in warm the treated and rejected water, although this effect may be modest. In one study found about a 5 C temperature increase (from 20 to 25 C) when testing an Ionic Aquamite I EDR stack (Valderi-Perez, et al. 2001). The effects of higher temperatures, if present, are expected to be beneficial for partial softening. Increased temperature promotes magnesium hydroxide (Mg(OH)2) formation. In addition, calcium carbonate (CaCO3) solubility actually decreases with increasing temperature (Garrels and Christ, 1965, Langmuir, 1997). Heated water can also allow for decreased dissolved carbon dioxide solubility, which will further drive the formation of calcium carbonate. So, increased temperature may actually improve the lime softening process for hardness, calcium, and silica removal. On the other hand, silica solubility increases with increasing temperature. We conducted studies with temperatures ranging from 20.5 to 47C to study the effect of a wide range of potential operating temperatures.

The addition of magnesium chloride resulted in dramatic improvement of removal of silica. It also resulted in improved removal of fluoride, arsenic, and nitrate. Removal of calcium also was effective. However, the processes resulted in higher hardness and magnesium levels at temperatures up to 30C. There parameters are discussed in the next discussion section.

Our results showed a modest effect of increased removal of silica with increasing temperature, although the pattern was complicated. In some dosages, removal initially increased, then leveled off or even got slightly worse. These patterns may reflect the interplay between improved hardness removal and increasing silica solubility.

Magnesium and Hardness

Generally, the partial lime softening with magnesium chloride results were straightforward; the process was effective at lowering the concentrations of the most of the parameters of interest as discussed in the previous section. However, magnesium and hardness had more complicated patterns. At temperatures of 20.5, 25, and 30C, we found that the treatment actually increased the magnesium concentration, and this resulted in higher hardness concentrations (Table 3). At 20.5C, the magnesium concentration was more than 5 times higher the initial concentration and final hardness concentration after treatment more than 4 times greater. At 47C, the Mg and hardness were substantially decreased. Although the temperature at Ft. Irwin certainly can reach

Medina et al. 2012 Page 17

47C and higher, the process also must work effectively at temperatures between 20 and 30C, as well.

In order to prepare the partial lime softening effluent at 20.5C for reverse osmosis, we conducted a pretreatment that consisted of pH adjustment to <5.7 by addition of hydrochloric acid and subsequent stripping of carbon dioxide. After this process, the water chemistry was reanalyzed, and we found that the magnesium and hardness levels (Table 4) were decreased to levels comparable to the levels initially found in the EDR simulant (Table 3).

These patterns appear to result from the high inorganic carbon content (bicarbonate and carbonate, depending on pH) in the EDR simulant. The high alkalinity in the EDR reject simulant indicates that the bicarbonate/carbonate concentration is substantially greater than that of the magnesium, which favors complexation of the carbonate forms with magnesium. There magnesium-carbonate forms are more soluble than magnesium hydroxide. As a result, at the lower temperature, magnesium remained more soluble, resulting in higher dissolved magnesium concentrations and greater hardness.

The purpose of the acidification/carbon dioxide stripping process was to reduce alkalinity. As the alkalinity was reduced, the formation of insoluble magnesium hydroxide was presumably favored. Consequently, we detected lower magnesium and hardness concentrations after this process, even for the lime softening process conducted at 20.5C.

Temperature was also a key factor in the process. As temperature is increased, water disassociation increases. One result of this is that more hydroxide is available. So, not surprisingly, we found that as water temperature increased, the resulting alkalinity decreased and the magnesium and hardness levels also decreased (Table 3).

pH could also be a factor that could be optimized. The measured pH from the magnesium chloride modified partial lime softening ended up being on the order of 10.3, which was within target levels given in the CH2M Hill pilot treatment report (Table 1, CH2M Hill 2007). However, this is slightly lower than the range given by Kluesener et al. (1975), from 10.8 to 11.3. Further pH adjustment could allow for better hardness removal prior to pH adjustment and carbon dioxide stripping.

Sludge Generation from Lime Softening

We compared the sludge generation and solids results to those in the literature (Suthaker et al. 1993, Ratnayaka et al. 2009). Suthaker et al. (1993) was a study on a partial lime softening operation. Ratnayaka et al. (2009) is a reference book summarizing sludge generated from lime softening, which presumably include soda ash addition. Table 6 below summarizes these results.

Medina et al. 2012 Page 18

Table 6. Comparison of Sludge Data from this study to other published studies

Source Percent of sludge

vs. influent flow

(v/v)

Solids per liter treated (g/L, dry)

Solids concentration in sludge (w/v, dry)

Comments

Presented Study – Hydrated Lime Only (2 to 2000 mg/L)

0.2 to 2.4% 0.22 to 0.63 2.5 to 11% The 11% solids value was found at the lowest treatment, which generated the least amount of sludge. This treatment was not effective and floc was likely not formed.

Presented Study – 1500 mg/L Hydrated Lime with 750 mg/L MgCl2 (temperature varying from 20.5 to 47C)

2.0 to 3.4% 0.45 to 1.18 2.3 to 3.4% Mg(OH)2 floc is less dense than CaCO3. So MgCl2 addition results in less dense floc.

Suthaker et al. 1993 0.07 to 0.12% 0.120 to 0.165 10 to 23.5% Solids concentration in sludge was based on solids recovery of 120 to 165 kg/ML treated and sludge generation of 0.07 to 0.12% vs.

influent.

Ratnayaka et al. 2009 1.5 to 2.5% Not given 0.1 to 1.0% Ave. 0.3%

The total percentage of sludge produced in this study (both partial lime softening only and lime softening supplemented with MgCl2) fits within the ranges encompassed with the Suthaker study and the Ratnayaka data. The Suthaker study had a lower range of

0.07 to 0.12%, while Ratnayaka reported 1.5 to 2.5% average sludge generation. This partial lime softening generated 0.2 to 2.4% wet sludge, with the highest percentage coming from the treatment with the highest lime dosage of 2000 mg/L. The wet sludge generated by the MgCl2 modified treatment was higher, 2.0 to 3.4%. As MgCl2 stimulates the formation of Mg(OH)2, which is a more voluminous sludge than CaCO3, this pattern is expected.

Regarding the percent solids in the sludge, Ratnayaka et al. (2009) reported a range of

0.1 to 1.0 % with an average of 0.3%. The Suthaker study did not directly percent solids.

However, the Suthaker study reported the solids recovery in relation to the total water volume treated, and the percentage of wet sludge. This allowed us to calculate the sludge solids in that study to range from 10 to 23.5%.

The solids concentrations in the partial lime softening only study ranged from 2.5 to 11%. This exceeds the range presented in the Ratnayaka study, but is within numbers found in Suthaker. Further, the 11% value was for the lowest lime treatment of 2 mg/L

Medina et al. 2012 Page 19 and did not result in a significant level of floc. In that case, the total volume of sludge generated was relatively small, and the solids concentration was correspondingly high.

The 2 mg/L treatment was not effective overall - so, the sludge percentage (w/v, dry) from the 2000 mg/L treatment is more appropriate. At 2.5%, the solids concentration in the 2000 mg/L treatment is on the upper end of the range given by Ratnayaka. The ERDC study and the Suthaker study are similar in that both are partial lime softening.

Although it is not clearly stated in Ranayaka, it is reasonable to assume that the ranges come largely from lime-soda softening, which can generate less dense sludge.

The solids generated per liter of treated water in by the partial lime softening were 2 to 5 times higher than the Suthaker study. We believe that this results from differences in the water source being treated. As discussed in the introduction, the EDR reject water quality expected for the Ft. Irwin water treatment plant is high in dissolved solids and hardness. The hardness for the EDR reject is estimated to be on the order of 850 mg/L as CaCO3, compared to levels on the order of 180 mg/L as CaCO3 in the Suthaker study.

Some of the EDR reject constituents are predicted to be in the wastestream at supersaturated levels - so, it might not be surprising that the sludges generated are more concentrated.

Another consideration is the scale of the testing. The Suthaker study and the Ranayaka chapter are focused on full scale applications, which would have a water column above the sludge on the order of 1 to 2 m. The testing presented here are benchtop scale, with a water column on the order of 10 cm.

The magnesium chloride supplemented partial lime softening resulted in somewhat more voluminous sludge, and a higher range of solids removed per liter compared to lime softening alone (Table 6). The range of sludge solids content was lower on the upper end compared to the partial lime softening alone, but in this case, the treatments compared all resulted in effective treatment, so this range is probably more realistic.

The partial lime softening process is one of the greatest potential points in the process for water loss in the water treatment system (see Figure 1). Calculations suggest that water losses associated with the sludge ranged from up to 0.19% of the entire water flow treated. Additional water recovery methods, such as filter press, vacuum filtration, or sludge pond decant recovery and recycle could improve water recovery associated with the partial lime softening process.

In conclusion, the amount of wet sludge found in our testing is comparable to literature values. The percent solids in the sludge are higher than some sources be a factor of 10 to 100, but are within some studies. The actual solids per liter water treated are higher than literature studies. The variation in the percent solids and the solids per water treated appear to come from (1) the use of partial softening vs. lime-soda softening, which produces less dense sludges, and (2) the concentrated state of the reject simulant

Medina et al. 2012 Page 20 compared to most other drinking water sources. Some effects may also come from the scale of the lab test compared to a full scale system.

Reverse Osmosis

RO is currently used by Ft. Irwin to treat groundwater to drinking water standards. The unit meets water quality requirements and has a 50% recovery for single stage operation. However, the treatment of the reject stream is more challenging, even with the lime softening step for preparation, because the stream is more concentrated. This study indicated that RO was able to effectively treat the partial lime softening water to meet drinking water standards for the key constituents. Single stage operation achieving 60% recovery was easily achieved, suggesting that 2 stage should be able to meet the 70 to 75% recovery goal.

In the previous study (Medina et al. 2012), we identified rapid pressure buildup. To address this, we used a pretreatment consisting of pH adjustment (with 1N HCl) following by CO2 stripping. This process appeared to address the fouling issus. In the full-scale plant, the carbon dioxide would be left in solution, and stripped from the permeate stream, only. CO2 is not rejected by the RO membranes, whereas bicarbonate is, so permeate pH decreases and reject pH increases. Bicarbonate is retained in the reject stream, where equilibrium between CO2 & HCO3 follows a known mathematical relationship.

Arsenic was not as effectively removed as the other constituents, and removal did not meet MCL levels. However As removal can be improved by oxidizing arsenic with hypochlorite as it enters the lime softener, and adding ferric chloride, which will precipitate arsenic as ferric arsenate, or the mineral scorodite. This would mean dechlorinating with sodium metabisulfite ahead of the RO units to avoid oxidation damage to the membranes.

Implications for Full Scale Operation

Fort Irwin, being in an arid, desert environment, has groundwater resources that are being depleted (static water levels in one aquifer, Bicycle Lake, have reportedly dropped 80 ft since records have been kept). Its mission is growing, meaning that it will have to use its limited water resources to their maximum potential. One means it so maximize water recovery from their water treatment system. The State of California has challenging water resource issues (Schroeder et al., 2012).

Military installations are also facing pressures to develop and use water more efficiently.

Base realignment and closure (BRAC) is resulting in fewer, but larger military installations and the remaining installations must be able to meet water needs for increased mission requirements. Climate change may result in increasing water

Medina et al. 2012 Page 21 pressured throughout military installations (Scholze, 2011). The Army Net Zero Water program is setting a goal of no net water impact within a given watershed. Although Net Zero is only being demonstrated at specific installations (https://eko.usace.army.mil/public/fa/netzero/), it is likely that this program will be adapted Army and even Department of Defense (DoD) wide within the next 20 years.

Enhanced recovery of reject water is a critical method for better use of water resources to address limited water resources in arid environments.

For this report, we made the assumption that the total influent flow rate for the system is 6 mgd. However, in reality, the maximum water flow would be slightly higher, because 6 mgd is actually the production rate. Accounting for losses would give an actual influent flow of 6,013,333 gallons per day. This slight change may result in small differences in actual calculations.

Summary

This study investigates the use of lime-softening and reverse osmosis to increase recovery water from the reject stream of EDR. Although these processes are well established for conventional water treatment, their use for this water stream is unique.

The primary goal of the partial lime softening process was to protect the RO, which involved the removal of calcium, hardness and silica. From the partial lime-softening, the following conclusions were derived:

Partial lime softening using only hydrated lime was effective at removing hardness and calcium, but was not successful at removing silica to the treatment goal levels. This process could still be used, but would probably require an anti-scalant in the RO process to deal with silica fouling.

Additions of magnesium chloride to the partial lime softening resulted in improved silica removal to meet the treatment goal. A hydrated lime dosage of 1500 mg/L and a magnesium chloride dosage of 750 mg/L was effective for removal of the targeted constituents.

Magnesium and hardness actually increased substantially with magnesium chloride supplemented partial lime softening conducted at temperatures ranging from 20.5 to 30C. This appears to be related to excess alkalinity, which created carbonate complexes with the magnesium. However, as the alkalinity was reduced during a RO pretreatment process consisting of acidification and carbon dioxide stripping, the magnesium and hardness numbers declined to acceptable levels. Higher pH levels (>10.8) in the softening process might address this issue prior to the pretreatment.

Wet sludge generation was similar to other partial lime-softening examples in the literature. However, the percent solids in the sludge were higher (10 to 100 times) that what was found in standard design book, although it was similar to

Medina et al. 2012 Page 22 that found in another study (Suthaker et at. 1993). The solids mass generated per unit water treated was higher than the Suthaker study. We attribute these differences to the concentrated nature of the reject water stream.

For the reverse osmosis study, the following conclusions were drawn:

Operating pressures at fluxes ranging from 3 to 12 gfd and with water recoveries ranging from 40 to 70% (for a single stage system) ranged from 140 to 270 psig, which are reasonable pressures. Since water recoveries of 50 to 60% were relatively easily obtained, the goal of 70 to 75% water recovery of the reject stream by these processes are reasonable.

The RO system resulted in >90% removal of fluoride. At flux >8 gfd, treatment was consistently less than the MCL of 2 mg/L.

For two examples with fluxes near 10 gfd and water recovery around 53 to 56%, the RO showed substantial reductions of all constituents measured. Removal of most of the constituents exceeded 95%.

Arsenic was the most problematic of the constituents treated by the RO unit. At the conditions described in the previous bullet, its removal was about 50% and its final concentrations were slightly above the MCL of 0.010 mg/L.

Studying sulfate losses indicated that reducing recovery to less than 50% seemed to limit sulfate precipitation on the membrane. Fluxes around 9 with recoveries around 55% had very modest losses, as did fluxes around 8 with fluxes around 52%.

Disclaimer

The results in this report represent a good faith effort by the ERDC research team to realistically test the partial lime softening and reverse osmosis unit processes in the proposed Ft. Irwin WTP. However, bidders and contractors should be aware of potential study limitations that may result in differences in performance in the full scale system.

The ERDC study used a simulant based on the CH2M Hill pilot study (CH2M Hill, 2007). However, the actual chemistry of the EDR reject could vary substantially depending on the mix of groundwater production wells that are actually used. In addition, the simulant did have some concentration differences for some of the chemical species of interest. Some of these differences resulted from the difficulty of meeting some of the levels that approach supersaturation. Finally, scaling could affect the results. The small scale of the bench top studies could introduce differences compared to a larger scale system. One example could be sludge characteristics due to water column differences. Another key difference is in the geometry of the RO membranes. Most full scale systems use cylindrical membranes, which minimize edge effects compared to the flat membrane used in the laboratory system. Nor does this study address some other key unit processes in the system, such as EDR, MVR, etc.

Medina et al. 2012 Page 23

Acknowledgments

The authors would like acknowledge that funding from the project came from the Army Military Construction (MILCON) program, managed by Debra Ford and LTC Joseph Seybold of the Los Angeles District of the U.S. Army Corps of Engineers. Jorge Martires of the Los Angeles District, Joseph Findley of the Mobile District of the Corps of Engineers, and Jim Mavis of CH2M Hill, Inc., provided technical reviews of the work presented in this paper.

References

Al-Mutaz I., and I. Al-Anezi. 2004. Silica Removal During Lime Softening in Water Treatment Plant. International Conference on Water Resources & Arid Environment.

Riyadh, Saudi Arabia. 5-8 December 2004.

Betz, L.D., C.A. Noll, and J.J. Maguire. 1941. Adsorption of soluble silica from water.

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