POST-TREATMENT OF RO PERMEATE WITH CALCITE CONTACT TO PROVIDE STABILITY AND CORROSION CONTROL 1. Abstract

Transcription

1 POST-TREATMENT OF RO PERMEATE WITH CALCITE CONTACT TO PROVIDE STABILITY AND CORROSION CONTROL 1 Scott Freeman, P.E., Black & Veatch, 8400 Ward Parkway, Kansas City, MO freemansd@bv.com Phone: Cell: Saqib H Shirazi, P.E., PMP, San Antonio Water System (SAWS) San Antonio, TX, and Jarrett Kinslow, P.E., Tetra Tech, Orlando, Florida Abstract Reverse osmosis (RO) permeate is aggressive, corrosive since it is soft, low alkalinity water with slightly acidic ph. Unless properly post-treated, finished water from RO facilities can cause corrosion to downstream fixtures and piping including the distribution system. Post-treatment includes chemical addition to adjust water quality. Blending a controlled amount of raw water with permeate before disinfection and distribution is typically applied when water quality allows to lower, or in some cases eliminate, consumption of corrosion control chemicals. If corrosion does occur, the problems range from aesthetic (e.g., red water caused by iron) to major public health issues (e.g., elevated lead concentrations). One of the unit processes for post-treatment, contact with calcite (i.e., calcium carbonate, CaCO 3, media), is not widely understood by RO practitioners. This paper applies to both brackish and seawater RO projects and will be of interest to utilities, operators, and engineers who currently own or are considering calcite contactors. The paper reviews the chemistry and theory, including chemical equations, and the impact of key operating parameters (e.g., loading rate, velocity, contact time, bed height). Simple calculation methods are presented to assist engineers and operators in determining addition of alkalinity, consumption rate of calcite media, and the related change in empty bed contact time at those conditions, therefore helping operators and managers plan when new calcite media should be added. Combined, the addition of alkalinity and calcium hardness, as well as increased ph, can adjust corrosion index values of the water, such as Langelier Saturation Index (LSI) and Calcium Carbonate Precipitation Potential (CCPP) to meet corrosion guidelines. Typical guideline values are LSI of 0.2 to 0.4 and CCPP of 4 to 10 mg/l as calcium carbonate, but a site specific determination is advised. One of the key limitations of calcite addition is the risk that it may increase finished water turbidity. Therefore, care must be taken in design and operations of calcite contactors to avoid excessive elevation of turbidity in the finished water. Introduction Reverse osmosis (RO) permeate is aggressive and corrosive since it is soft, low alkalinity water with slightly acidic ph (i.e., <ph7). Unless properly post-treated, finished water from RO facilities can cause corrosion to downstream fixtures and piping including the distribution system. Post-treatment includes chemical addition to adjust water quality. Blending a controlled amount of raw water with permeate before disinfection and distribution is typically applied when water 1 Paper presented at the AMTA/AWWA 2017 Membrane Technology Conf, Feb 13-17, Long Beach, California. 1

2 quality allows to lower, or in some cases eliminate, consumption of corrosion control chemicals. If corrosion does occur, the problems range from aesthetic (e.g., red water caused by iron) to major public health issues (e.g., elevated lead concentrations). One of the unit processes for posttreatment, contact with calcite (i.e., calcium carbonate, CaCO 3, media), is not widely understood by RO practitioners. This paper applies to brackish and seawater RO projects and will be of interest to utilities, operators, and engineers who currently own or are considering calcite. A calcite contactor, which is a vessel containing granules of calcium carbonate media, can reduce the corrosion potential by raising the ph while also adding dissolved calcium and alkalinity as ROP flows through the media. Calcite contactors can apply either upflow or downflow configuration. This paper focuses on the upflow type. There are other methods for adjusting the aggressiveness of RO product water, such as addition of lime or other sources of calcium and/or addition of sodium hydroxide that are not discussed herein. Benefits of calcite contactors include increasing alkalinity and calcium hardness as well as ph without some of the problems that operators experience when practicing lime addition. The presentation will be highlighted by results from the full-scale calcite contactors utilized at San Antonio Water Systems (SAWS) Brackish Groundwater Desalination (BGD) facility. BGD produces up to 10 MGD of RO permeate that can be blended with production well water (i.e., RO bypass) to yield up to 12 MGD of finished water. The BGD plant includes 8 calcite contactors 2 (6 duty and 2 spare) at nominal 12-ft diameter x 9 ft bed and 18 ft height, each. The BGD calcite system includes addition of carbon dioxide and/or sulfuric acid upstream with a nominal hydraulic treatment capacity of up to 30% to treat up to 3 MGD of the 10 MGD total permeate flow. Results from the full scale facility were not available by the submittal date for the paper, but may be included during the oral presentation of the paper and in subsequent publications. Chemical Addition and Reactions In general carbon dioxide (CO 2 ) and/or an acid such as sulfuric (H 2 SO 4 ) is added to the calcite contactor influent to lower the ph and subsequently dissolve calcium from the calcite media. Chemical reactions (Hernandez-Suarez, 2005) that occur during these process steps are summarized below. In the first reaction, CO 2 is added to the water, which forms carbonic acid (H 2 CO 3 ) lowering the ph. In the second equation, calcium carbonate (CaCO 3 ) in the calcite media is dissolved by the carbonic acid (H 2 CO 3 ) which adds calcium (Ca +2 ) and alkalinity, generally in the form of bicarbonate (HCO -3 ), to the water while also increasing the ph. As shown in the third reaction H 2 SO 4 can be applied instead of or in conjunction with CO 2 addition and which would be followed by Reactions 1 and 2. CO 2 + H 2 0 H 2 CO 3 Reaction 1, CO2 Addition: Carbon Dioxide + Water Carbonic Acid CaCO 3 + H 2 C0 3 Ca HCO 3 Reaction 2, Calcite & Carbonic Acid Contact: Calcium Carbonate + Carbonic Acid Calcium ion + Bicarbonate (Alkalinity) 2 Calcite contactor vessels provided by Wigen Water Technologies, 302 Lake Hazeltine Dr, Chaska, MN

3 CaCO 3 + H 2 S0 4 Ca 2+ + S CO 2 + H 2 0 Reaction 3, Calcite + Sulfuric Acid Calcium ion + Sulfate ion + Carbon Dioxide + Water, In an upflow contactor, acidified calcite influent is pumped to the bottom of a calcite contactor vessel and the effluent flows out the top. The upward flow fluidizes the bed of calcite particles. The height of the vessel and the upflow velocity should be selected to provide sufficient contact time while also providing suitable freeboard height above the top of the calcite bed to prevent excessive carryover of particles into the effluent. Carryover is undesirable because that could increase the effluent turbidity in the finished water above the plant s goals. Generally, the calcite contactor effluent then flows to a degasifier step to remove excess unreacted CO 2 and any subsequent post-treatment steps, such as blending with bypass water, ph adjustment, and disinfection. A typical schematic diagram is shown on Figure 1. As the water flows through the contactor vessel the calcite particles dissolve, reducing the size of particles as well as the volume and height of the calcite bed. Consequently, the system must periodically be replenished with new calcite material to maintain sufficient contact time and hence treatment effectiveness to meet finished water quality goals. After loading new calcite material into a vessel the operations staff would generally conduct a flushing step to remove fines that could cause elevated effluent turbidity. Figure 1. Process Schematic Design Parameters Parameters that impact design are described below. Loading Rate (LR) Loading rate (LR) is the flow rate (Q) divided by the cross-sectional area (A) of the calcite contactor vessel, where D is the vessel diameter. In the US system of measurement, as presented herein, Q would be in gal/min (gpm), A in ft 2, D in ft, and LR in gpm/ft 2. LR is sometimes presented in cm/min in metric units. 3

4 LR = Q A A = π D2 4 Eq. 1 Eq. 2 Loading rate, also referred to as superficial velocity, impacts the velocity of the water along the surface of the calcite particles, the amount of contact time available for dissolving the calcite, and the fluidized height of the bed. LR is an important parameter because if the LR is too high, the reaction that dissolves the calcite may not be sufficiently complete, leaving unreacted excess CO 2, which is a drawback because unreacted CO 2 results in unproductive operating cost. Hernandez-Suarez (2005) found successful operations with LR of 7.8 to 9 gpm/ft 2 (32.1 to 36.7 cm/min) with mm limestone particles. Wen, et al (2012) compared effluent turbidity applying 1 mm calcite particles that were 95% CaCO 3 at four LR conditions (1.9, 5.7, 9.5, and 17 gpm/ft 2 ) and found 9.5 gpm/ft 2 to yield the lowest effluent turbidity at their conditions. Hernandez et al (2009) observed a relatively linear relationship with the lowest effluent turbidity (< 0.4 NTU) at about 2 gpm/ft 2, increasing slightly (to < 0.5 NTU) at 4 gpm/ft 2, NTU at 6 gpm/ft 2, and up to slightly greater than 1 NTU at 8.5 gpm/ft 2. Design loading rates of 3.1 up to a maximum of 6.5 gpm/ft 2 were selected for the BGD project to allow for operational flexibility. Empty Bed Contact Time (EBCT) Empty bed contact time (EBCT) in minutes is calculated by the equation shown below where the volume of the calcite bed (Vb) in ft 3 divided by the flow rate (Q) in gpm. EBCT = V b (7.48 gal ft3 ) Q Eq. 3 It is beneficial to allow sufficient contact time for calcite dissolution to approach equilibrium. As shown in Figure 2, Hernandez et al (2009) determined that at 20 o C and greater EBCT of at least 10 minutes results in effluent ph of 8.2, indicating the dissolution reaction approaches equilibrium. Regarding the BGD project a newly loaded 9-ft deep bed operating at the design LR of 3.1 gpm/ft 2 would have an EBCT of about 21.6 minutes. During operations, after half of the calcite material has been consumed, the EBCT would still be greater than 10 min. Pressure Drop (dp) Pressure drop through the Calcite media (dpc) is a function of LR, which is directly related to velocity through the bed, bed depth, and the size and shape of the media. Hernandez-Suarez (2005) determined the dp due to the bed for Calcite media in the 2.0 to 2.5 mm size range. A graph of their findings in metric units is shown in Figure 3. Calculation method in US units is shown in Eq. 4, where dpc is in ft water/ft of bed depth and LR is Loading Rate in gpm/ft 2. Based on 4 gpm/ft 2 the dpc equals Given 9 ft of media with a 20 ft height from base to effluent header would require that the feed pumps provide 1.7 ft of water (9 * 0.188) as well as the elevation change of 20 ft and line losses, indicating that the pressure consumption is more due to the height of the bed than the bed itself. Please note that this equation was developed based on mm media. dpc may be somewhat higher with the approximately 1 mm media that is currently more typically applied. dpc = (0.052 LR) 0.02 Eq. 4 4

5 Figure 2. EBCT to reach ph 8.2 (Hernandez et al, 2009). Figure 3. Pressure drop through 2 to 2.5 mm Calcite media in metric units (Hernandez-Suarez, 2005). 5

6 Influent Water Quality Yi Shih et al (2012) citing Carmical et al (2002) noted that recommended calcium concentration in the influent should be less than 60 mg/l with alkalinity less than 100 mg/l as CaCO3. These parameters are generally not limiting issues when applied to RO facilities since the concentrations in permeate are typically much lower. The percentages of H 2 CO 3, HCO 3 -, and CO 3 2- as a function of ph are shown in the classic graph in Figure 4. Most of the carbonate is in the form of H 2 CO 3 at ph 6 and below. At higher ph values the calcite process tends to be less effective (Yi Shih et al 2012). Figure 4. Carbonate species as a function of ph. Operational Calculations Equations to assist with the operation of calcite contactors are presented below. Calcium Consumption A mass and flow balance is applied to determine the calcium concentration needed in the contactor effluent after first determining the concentration needed in the blended finished water. The calcite consumption rate can be approximated by Eq. 5, which includes a constant to convert units of measurement and also includes the assumption that the calcite being used is 95 percent CaCO 3. The calcite consumption rate would be slightly lower if the calcite purity is higher. The commercially available grades are in the 95 to 98% range. Mcalcite is the calcite consumption rate in lbs./day, Qcc 6

7 is the calcite contactor effluent flow rate in gpm, and [Ca]cc is the calcium concentration in the calcite contractor effluent in mg/l as Ca. The constant, 31.6, is a factor to account for unit conversion where 1/31.6 = (3.785 L/gal)*(1440 min/day)*(1 lb./454,000 mg)*(100 g CaCO 3/mole)*(1 mole/40 g Ca)*(1 lb. calcite/0.95 lb. CaCO 3). A modified version of the equation is presented as Eq. 6 where [CH]cc is the calcium hardness in the calcite contactor effluent in mg/l as CaCO 3. The constant, 79, is a factor to account for unit conversion where 1/79 = (3.785 L/gal)*(1440 min/day)*(1 lb/454,000 mg)*(1 lb calcite/0.95 lb CaCO 3). Mcalcite = Qp [Ca]cc Eq Mcalcite = Qp 79 [CH]cc Eq. 6 The Mcalcite calculation can be used to extrapolate to the decrease in bed depth over time, such as inches of media consumed per day, by accounting for the diameter of the vessel and the bulk density, which is about 95 lbs/ft 3. This calculation can help the staff plan the frequency of adding new media. Alkalinity Addition As shown in the chemical reactions presented above, dissolving calcite in the presence of CO 2 adds alkalinity with the calcium. Even though the alkalinity chemistry is somewhat complex since it is also related to CO 2 concentration, ph, and the associated equilibrium between carbonate and bicarbonate species, the alkalinity value can be approximated by assuming that the alkalinity is present as bicarbonate and that two bicarbonate ions (HCO 3 - ) are added with each calcium ion (Ca +2 ). Eq. 7 can be used to approximate the alkalinity in either the calcite contactor effluent or in the combined effluent including the permeate bypass around the calcite contactors, depending on which calcium concentration is applied. To simplify the calculation, it is assumed the alkalinity in the RO permeate is negligible. In the equation [Alk] is the alkalinity in mg/l as CaCO3 and [Ca] is the increase in calcium concentration in mg/l as Ca. The constant, 2.5, is a factor to account for unit conversion where 2.55 = (2 mmol HCO 3 /1 mmol Ca)*(1 mmol Ca/40 mg Ca)*(61 mg HCO 3 / mmol)*(0.82 to convert Alk as HCO3 to as CaCO 3 ). [Alk] = [Ca] 2.5 Eq. 7 For example if the calcium concentration is increased by 50 mg/l as Ca, then the alkalinity would increased by about 125 mg/l as CaCO3, based on the stoichiometric ratio in Reaction 2. As a simplifying assumption, this equation does not take into account any alkalinity or hardness that is already present in the calcite contactor influent, which is typically RO permeate. The same calculation can be conducted based on calcium hardness, [CH] expressed in mg/l as CaCO 3, by changing the constant to unity as shown in Eq. 8. [Alk] = [CH] Eq. 8 7

8 Conclusions Typically post-treatment of product water is applied at RO facilities to adjust the water quality to prevent corrosion of piping and fixtures downstream, such as the calcite contactor method discussed in this paper. Blending a controlled amount of raw water with permeate before disinfection and distribution is typically applied when water quality allows to lower, or in some cases eliminate, consumption of corrosion control chemicals. If corrosion does occur, the problems range from aesthetic (e.g., red water caused by iron) to major public health issues (e.g., elevated lead concentrations). Equations to assist with the design and operation of one of the unit processes for post-treatment, contact with calcite (i.e., calcium carbonate, CaCO 3, media), are presented herein. With the addition of carbon dioxide and/or an acid such as sulfuric to RO permeate flowing as influent to a calcite contactor, the alkalinity, calcium hardness, and ph increase when reacting with the calcite media. Regarding subsequent work on this topic, the authors intend to present a comparison to full-scale operations from the SAWS BGD facility during the oral presentation of the paper and/or subsequent publications. References Carmical, A.J. et al (2002), R.B.Robinson, R.D. Letterman, and G.S.Mackintosh, Small public water system technology guide volume II, Limestone contactors, UNH Water Treatment Technology Assistance Center, Univ of New Hampshire and Univ of Tennessee- Knoxville. Hernandez, M. J., et al.(2009), G. Cremer, J. Compte, T. Cazurra, J.L. Jurado, H. Orbe, and C. Miguel, "The recarbonation facility of the Barcelona Desalination Plant brings out new standards," IDA World Congress. Dubai. Hernandez-Suarez, M. (2005) "Short guideline for limestone contactor design for large desalination plants (rev 3)," Canary Islands Water Center. Yi Shih,Wen, et al (2012), Justin Sutherland, Bradley Sessions, Erin Mackey, W. Shane Walker, Upflow Calcite Contactor Study, Texas Water Development Board, Contract # , April,