September 3, 2019

Purchase of Water Treatment and Water Reclamation Chemicals on an Annual Contract BL093-19

Addendum #2 Q1. In light of the attached report that was commissioned by Gwinnett County, in conjunction with Brown

and Caldwell, help me understand why the specifications for the magnesium hydroxide where written the way they were?

A1. The specifications were in fact copied from those developed by Brown and Caldwell in that effort. We

use a different format from them so it was a copy and paste job for the most part.

Where we deviated was on the requirement that the company providing the mag did not have to do the testing. They could submit a sample to us for testing with a third party lab to perform the testing.

We also did not require the testing of a product we already are familiar with unless a current product is being changed to an untested one. Assuming the current provider intends to bid the product we are currently using, test for efficacy is not necessary. However, if they are proposing a new one, they’ll need to submit a sample.

We also added in supply requirements for equipment to dose magnesium hydroxide due to the lengthened times to deploy new equipment via IWQ. Maintenance requirements were also added to assist pump stations staff in operating the mag dosing systems.

Q2. The last phosphate bid in 2016 had Sterling Chemical as the low bid at $308.10 per 100 gallons. The bid

tab stated award was to Sterling.

Can you share why Sterling was rejected and an award given to Carus?

A2. The product submitted by Sterling did not meet minimum requirements. Acknowledge receipt of this addendum by signing and returning as part of your bid submittal. Shelley McWhorter Shelley McWhorter, CPPB Purchasing Associate III Attachment:

Brown and Caldwell Mg(OH)2 Report Supplier Name Authorized Rep. Name

Magnesium Hydroxide Addition for Odor and Corrosion Control in Conveyance Systems: Product Selection and Dose Optimization Gayathri Ram Mohan1, J.C. Lan1*, Sean Kilpatrick2, and Houston Flippin3

1Gwinnett County Department of Water Resources, Lawrenceville, Georgia 2Brown and Caldwell, Atlanta, Georgia 3Brown and Caldwell, Nashville, Tennessee *Email: JC.Lan@gwinnettcounty.com Abstract Gwinnett County Department of Water Resources uses magnesium hydroxide (Mg(OH)2) in the collection systems to aid with odor and corrosion control. This study was conducted to establish a common testing protocol to compare various commercial Mg(OH)2 products to help achieve the target pH of 8.5 within a 150-minute testing period. Parameters such as particle size distribution, magnesium hydroxide content, and dosage requirements were analyzed to compare product performance.

Keywords: Collection system, magnesium hydroxide, Mg(OH)2, pH control

Background Based on proactive sewer planning, water conservation, and reduction in infiltration and inflow (I/I), more sewers and force mains are conveying higher strength wastewaters at lower velocities and longer retention times. These conditions are leading to reduced pH (greater fermentation) and greater sulfide generation in the collection systems. To combat the increased potential for corrosion and control hydrogen sulfide emissions from the collection system, many municipalities add alkali, such as caustic or magnesium hydroxide (Mg(OH)2), to raise pH and keep sulfide in solution. Gwinnett County Department of Water Resources (DWR) uses the Ostara Pearl process for phosphorus recovery at their treatment plant—a process that requires magnesium for struvite precipitation. DWR decided to add Mg(OH)2 to selected force mains to increase the pH and provide additional magnesium. A testing protocol was developed to compare the performance of various commercially available Mg(OH)2 products, and verify the required dose to maintain a certain pH in respective collection systems.

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Identifying Type and Dose of Mg(OH)2

Figure 1. Evaluation of Actual Wastewater under Varying Doses of Magnesium Hydroxide DWR tested two types of Mg(OH)2 products at 7 mL/L and 10 mL/L to determine the most effective type and dose of Mg(OH)2 slurry to control pH in its collection system as illustrated in Figure 1. DWR evaluated various commercially available Mg(OH)2 slurries for odor control and established a common testing protocol. The dose of Mg(OH)2 slurry required to maintain target pH was verified, and the effectiveness of alternate products were compared. Two Mg(OH)2 slurries, Product A and Product B, were compared based on respective solids content, product purity (described in Table 1), particle size, and titration curves. Based on the difference in total solids by weight, initial estimates showed approximately 10 percent more of Product B than Product A would be needed to achieve the same alkalinity addition (pH change). Table 1. Product Characteristics Parameter Product A Product B Total Solids, percent by weight 56 51 Total Suspended Solids, g/L 818 757 Purity of Total Solids, percent by weight as Mg(OH)2 >95 >95 Eighteen percent of the particles in Product B were greater than 25 microns in size, while all particles of Product A were smaller. The larger particle sizes of Product B imply that it would be slower in reacting with wastewater than Product A. The larger particles are more likely to fall out of suspension and settle in the bottom of the pipe. Figure 2 shows the particle size distribution of the two different magnesium hydroxide products.

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Figure 2. Particle Size Distribution of Two Different Magnesium Hydroxide Products

DWR’s evaluation of the two Mg(OH)2 products indicate that Product B’s larger particles are more likely to fall out of suspension and settle in the bottom of a pipe. Testing Impact on pH Control To determine the relative impact of both products on pH control, a special titration test was conducted. This test recreated, as best as possible, the conditions of the wastewater flow, including mixing energy and wastewater characteristics. To increase reproducibility of this test, a formula for a synthetic domestic sewage representative of typical Gwinnett County sanitary sewage characteristics was created. This formula deviated from other synthetic sewage recipes (OECD, 2009) by using powdered milk to contribute a portion of the organic content. Powdered milk was added to replicate the dairy industry’s contribution to the wastewater and to add a source of organic nitrogen and oil and grease in a more available form than found in prior recipes (e.g., meat extract). Gwinnett County tap water was used as a source, with the addition of various chemicals to match the characteristics found in representative force mains. Data shown in Table 2 indicated that the synthetic wastewater matched the chemical oxygen demand (COD) concentrations found in the force mains. Variations observed in magnesium and alkalinity were due to magnesium addition being practiced at the time of sampling. Variations observed in other analytes (volatile fatty acids [VFA], orthophosphate, and ammonia) were partly due to variations of septicity found in the force mains.

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Table 2. Synthetic and Source Wastewater Characteristics Used in Titration Test

Parameter Synthetic

Wastewater Alcovy Force

Main Beaver Ruin Force Main

pH 7.0–7.2 7.9 6.7 Ammonia-Nitrogen, mg/L 35.8 38.1 23.8 CBOD, mg/L 542 327 220 Total COD, mg/L 733 724 687 Total Mg, mg/L 3.3 38 4.5 Soluble Mg, mg/L 3.3 29 3.7 Ortho-P, mg/L 5.4 5.5 3.5 Alkalinity, mg/L as CaCO3 345 321 133 VFA, mg/L as Acetic Acid 79 365 40

Titration of Mg(OH)2 Products in Synthetic Wastewater Testing showed that titration from pH 7 to pH 8.5 occurred over a 140-minute period during which the pH could stabilize for at least 10 minutes between each incremental addition of Mg(OH)2 slurry. These results illustrate that approximately 145 percent more slurry addition is required for synthetic wastewater treated with Product B to reach the same pH as that treated with Product A—much greater than the 10 percent increase initially expected. This result indicates that predicting usage strictly based on Mg(OH)2 content of alternative products is not reliable. Figure 3 compares the pH titration curves of two Mg(OH)2 products in synthetic wastewater.

Figure 3. pH Titration Curve of Synthetic Wastewater with Two Magnesium Hydroxide Products

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Titration testing showed that much more slurry than initially expected was needed for synthetic wastewater treated with Product B to reach the same pH as that treated with Product A—indicating that predicting usage solely based on the content of the Mg(OH)2 product is unreliable. Synthetic Wastewater Makeup The following chemicals are needed for the synthetic wastewater makeup: • 99 percent acetic acid • 99 percent propionic acid • Ammonium chloride (NH4Cl) • Sodium sulfide nonahydrate (Na2S.9H20) • Magnesium sulfate (MgSO4) • Potassium phosphate monobasic (KH2PO4) • 1 N sodium hydroxide (NaOH) • Peak whole milk powder • Deionized (DI) water • Gwinnett County tap water With the chemicals listed, the following two stock solutions need to be mixed: • Acetic acid/propionic solution (1 L):

o 959 mL of DI water o 31 mL of acetic acid o 10 mL of propionic acid

• Ammonium chloride/sodium sulfide/magnesium sulfate/potassium phosphate solution (1 L): o 15.4 g NH4Cl o 3.9 g Na2S.9H20 o 0.63 g MgSO4 o 1.43 g KH2PO4 o Make up the remaining volume with deionized water to 1 L

After all components of each stock solution have been added together, vigorously mix to ensure that they are well mixed and that all chemical compounds are dissolved and uniformly distributed. Once the two stock solutions are mixed, the synthetic wastewater can be generated. The following stock solutions and compounds should be added to replicate the characteristics in Table 2. Ensure that the solution is vigorously mixed while the ingredients are added and pH is adjusted. The following is for a 1 L synthetic makeup: • 1 L of Gwinnett County tap water • 1.5 mL/L of acetic acid/propionic solution • 10 mL/L of ammonium chloride/sodium sulfide/magnesium sulfate/potassium phosphate

solution • 0.43 g/L of peak whole milk powder • Adjust pH to between 7.0 to 7.2 by carefully adding 5N and 1N NaOH. Only 2 to 3 drops of

5N NaOH per liter are needed. Use 1N NaOH as needed to finish adjusting the pH.

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Testing Protocol and Further Product Comparison DWR established a testing protocol (shown in Table 3) to mimic conditions in a specific large-diameter Gwinnett County force main with longer retention time. This protocol was used for both raw and synthetic wastewaters.

Table 3. Test Protocol Step Supporting Logic

Add fresh synthetic wastewater to 1-L beakers1

Increases test reproducibility

Mix contents at mean velocity gradient available in force main when flowing2

Provides representative mixing

Measure initial pH, turbidity and alkalinity Establishes baseline Add range of doses of test product using 2% by volume solutions (T=0 minutes)

Establishes reliability of adding alkali

At t=1 minute, collect mid-depth sample for total magnesium and turbidity

Establishes concentration available pre-settling

At t=55 minutes (expected minimum duration of pump run time), collect mid-depth for pH and turbidity

Estimates initial pH impact and turbidity prior to mixer off

From t=60 to 85 minutes (expected duration of pump off time), keep mixers off

Establishes settling experienced during times of no pumping

At t=85 minutes, resume mixer operation at same mean velocity gradient

Models re-initiation of pump cycle

At t=150 minutes, measure mid-depth sample for turbidity, alkalinity above pH 8, total magnesium

Estimates dose to hit target pH, residual buffering capacity above pH 8, product loss, and total magnesium available for downstream use

1. Recipe for synthetic wastewater shown above. 2. Mixing Energy Calculation shown below.

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Testing of the synthetic wastewater at room temperature (recorded in Table 4) showed good reproducibility (among four discrete tests) for pH and alkalinity above a pH of 8 (within ±10 percent). Total magnesium in suspension proved hard to replicate; this parameter was not very reproducible (±35 percent). Table 4 - Reproducibility of Synthetic Wastewater at Room Temperature at End of Test

Synthetic at Room Temperature, 21 mL/L of 2% Product A Solution (Continuous Mixing) Parameter Minimum Average Maximum

pH 8.6 8.6 8.7 Alkalinity to pH = 8 (mg/L CaCO3) 19 23 26 Total Mg in suspension (mg/L) 31 44 51

Synthetic at Room Temperature, 21 mL/L of 2% Product A Solution Parameter Minimum Average Maximum

pH 8.7 8.8 8.9 Alkalinity to pH = 8 (mg/L CaCO3) 27 30 32 Total Mg in suspension (mg/L) 43 57 65

Synthetic at Room Temperature, 32 mL/L of 2% Product A Solution Parameter Minimum Average Maximum

pH 8.8 8.9 9.0 Alkalinity to pH = 8 (mg/L CaCO3) 35 37 40 Total Mg in suspension (mg/L) 31 44 51

Synthetic at Room Temperature, 63 mL/L of 2% Product A Solution Parameter Minimum Average Maximum

pH 9.0 9.1 9.2 Alkalinity to pH = 8 (mg/L CaCO3) 44 47 51 Total Mg in suspension (mg/L) 30 46 65

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Equations to Calculate Mixing Energy Mixing energy for sample during testing protocol was intended to duplicate that available in the modeled Gwinnett County force main while the lift station was pumping. Mixing was stopped to model average times that the pumps were not operating. A four-step process was used to calculate the jar test paddle speed to replicate the mean velocity gradient (G) in the force main. 1. The mean velocity gradient of the force main was calculated.

G value was calculated using G = 52 (f/D)0.5v1.5 where:

G = root-mean square velocity gradient, s-1

f = Darcy-Weisbach friction factor, dimensionless D = pipe diameter, ft V = fluid velocity, fps

2. The power in a jar test that would yield a comparable G value in the force main was calculated.

Total power required was calculated using G = (P/Vμ)0.5

where: P = total power input V = volume in force main μ= dynamic viscosity

3. The relative velocity of the paddle in jar test was calculated. 4. P = CD x A x (ϒ/g) x (v3/2), 5. Where:

CD = dimensionless drag coefficient for plates moving with faces normal to direction of motion (assumed to be 1.9) A = cross-sectional area of paddles, ft2 v= relative velocity between paddles and fluid (assumed to be 0.66 )

6. The required speed of the paddle was calculated. Rotational speed of the paddles was then calculated using VP = 2πrn/60, where:

VP = Velocity of paddles blades, fps n = number of revolutions per minute

Effect of Seasonal Temperature Variations on Dose DWR also evaluated the impact of temperature on Mg(OH)2 solution addition. Results indicate the required addition rates will be lowest in the winter, but relatively consistent in the spring, summer, and fall. Figure 4 shows the impact of temperature on pH titration curve under varying doses of magnesium hydroxide.

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Figure 4. Impact of Temperature on pH Titration Curve under Varying Doses of Magnesium Hydroxide Testing indicated that required Mg(OH)2 doses are lowest in colder winter temperatures, and relatively consistent at warmer temperatures throughout the rest of the year. Results from Product Comparison Testing Table 5 shows the results of the time-based testing comparing the two products. The testing results in Table 5 indicate that Product B required 200 percent greater addition to achieve the target pH 8.5 in 150 minutes. This increase could not be explained by turbidity loss (a surrogate for loss of suspended solids.) Furthermore, the disparity between Product A and Product B additions remained following three additional days of quiescence in the test. Soluble magnesium increased during this quiescent period, indicating that the sediment layer (settled Mg(OH)2) continues to contribute alkalinity.

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Table 5. Product Comparison Results Parameter Product A (2% V/V Solution) Product B (2% V/V Solution)

Dosage Rate (mL/L) 6.6 7.7 11.0 16.6 10.2 14.5 20.3 29.0 43.5

Dosage (gallon/MG) 132 154 220 330 203 290 406 580 870

Total Mg Added to Solution, mg/L 1

45 52 74 112 64 91 127 182 273

T = 0 minutes pH 7.1 7.1 7.1 7.1 7.1 7.1 7.1 7.1 7.1 Turbidity (NTU) 2 17 17 16 16 17 17 17 17 17

T = 5 minutes Turbidity (NTU) 140 128 173 273 55 80 94 130 182

T = 30 minutes Turbidity (NTU) 104 114 157 253 45 64 69 91 140

T = 55 minutes pH 8.3 8.0 8.5 8.7 8.0 8.1 8.4 8.4 8.6 Turbidity (NTU) N/A 111 166 248 N/A N/A 67 89 128

Alkalinity to pH = 8; (mg/L CaCO3)

N/A N/A N/A 35 N/A N/A 10 10 N/A

Total Mg in suspension, mg/L (% of total Mg)

32 (71%)

53 1 (101%)

59 (80%)

141 1 (126%)

26 (40%)

36 (40%)

45 (35%)

75 (41%)

108 (40%)

Soluble Mg, mg/L (% of total Mg)

14 (31%)

17 (31%)

30 (39%)

27 (23%)

8 (13%)

9 (10%)

15 (12%)

12 (6%)

14 (5%)

T = 88 minutes Turbidity (NTU) 81 100 151 211 35 49 59 79 116

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Table 5. Product Comparison Results (continued) Parameter Product A (2% V/V Solution) Product B (2% V/V Solution)

Dosage Rate (mL/L) 6.6 7.7 11.0 16.6 10.2 14.5 20.3 29.0 43.5

Dosage (gallon/MG) 132 154 220 330 203 290 406 580 870

Total Mg Added to Solution, mg/L 1

45 52 74 112 64 91 127 182 273

T = 150 minutes pH 8.5 8.6 8.8 8.9 8.2 8.2 8.5 8.5 8.7 Turbidity (NTU) 73 87 130 198 31 42 52 67 85

Turbidity loss (5-minute–150-minute values) (%)

48 32 25 27 44 48 45 48 53

Alkalinity to pH =8 (mg/L CaCO3)

< 26 26 30 30 N/A 11 10 24 37

Total Mg in suspension, mg/L (% of total Mg)

40 (88%)

43 (83%)

56 (76%)

77 (68%) N/A 26

(28%) 33

(26%) 24

(13%) 85

(31%)

Soluble Mg, mg/L (% of total Mg)

14 (31%)

21 (41%)

21 (29%)

15 (13%) N/A 10

(11%) 11

(8%) 37

(20%) 19

(7%)

T = 3 days pH 8.4 8.3 8.4 N/A 8.2 8.0 8.3 N/A N/A Total Mg in suspension, mg/L (% of total Mg)

45 (99%)

41 (79%)

47 (63%) N/A 19

(30%) 31

(34%) 29

(23%) N/A N/A

Soluble Mg, mg/L (% of total Mg)

40 (88%)

41 (79%)

46 (63%) N/A 18

(28%) 29

(32%) 28

(22%) N/A N/A

1. Total magnesium is calculated by multiplying the total suspended solids, calculated in Gwinnett County (performed in February 2017) in products, by the dilution of the working solution, by the volume of solution added, divided by the test volume. Values rounded to nearest two significant figures.

2. NTU = nephelometric turbidity unit(s).

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Conclusions Both Product A and Product B can achieve target pH conditions in the force main. However, Product B requires approximately 200 percent the dose compared to Product A to achieve the target pH of 8.5 in the 150-minute test period. Even at this three-times increase in dose, Product B was unable to contribute as much total magnesium in suspension and soluble magnesium in suspension. The price of Product B would have to be less than one-third the price of Product A to warrant further consideration. Mg(OH)2 content and particle size distributions are not sufficient parameters to judge required chemical dose for pH control. Not all Mg(OH)2 solids—even of the same particle size—are equally reactive. Where mixing is limited, testing protocols to evaluate alternative Mg(OH)2 products must include these mixing limitations. The outcome of the information contained herein has enabled DWR to more effectively set Mg(OH)2 dosing rates throughout the collection system and objectively evaluate alternate Mg(OH)2 products. References OECD (2009), OECD Guidelines for Testing Chemicals, Proposal for a Revised Guideline 209,

page 5. USEPA (1985), Design Manual Odor and Corrosion Control in Sanitary Sewer Systems and

Treatment Plants, USEPA/625/1-85/018, page 17.

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