SHORT GUIDELINE FOR LIMESTONE CONTACTOR DESIGN FOR LARGE DESALINATION PLANTS (REV. 3) JUNE 11, 2005

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1 SHORT GUIDELINE FOR LIMESTONE CONTACTOR DESIGN FOR LARGE DESALINATION PLANTS (REV. 3) JUNE 11, 2005 Manuel Hernández-Suárez Ph.D., M.Sc., Dipl.Ing. Canary Islands Water Center 1 DESCRIPTION OF UP-FLOW LIMESTONE CONTACTORS WITH CONTINUOUS FEEDING SYSTEM CONDITIONS FOR DESIGN SELECTION OF SUPERFICIAL VELOCITY SELECTION OF EMPTY BED CONTACT TIME (EBCT) WATER QUALITY AFTER TREATMENT DIMENSIONS OF LIMESTONE BED CHARACTERISTICS OF THE LIMESTONE CELLS CHARACTERISTICS OF LARGE TANKS SUMMARY DATA FOR THE DIMENSIONS THE LIMESTONE BEDS HEAD-LOSS OF THE LIMESTONE BED PERFORMANCE OF THE LIMESTONE BED AT VERY LOW FLOWRATES LIMESTONE CONSUMPTION LIMESTONE PARTICLE SIZE QUALITY OF THE LIMESTONE AUTONOMY OF THE SYSTEM LIMESTONE TREATMENT COSTS SUPPLY AND LOAD OF LIMESTONE FILLING OF LIMESTONE CELLS CLEANING OF LIMESTONE BED WITH AIR SPARGING ON THE USE OF SULFURIC ACID INSTEAD OF CO2 TO ENHANCE LIMESTONE DISSOLUTION SUMMARY OF LIMESTONE CONTACTOR CHARACTERISTICS FOR THE EXAMPLE R.O. DESALINATION PLANT REFERENCES...20 Short guideline for limestone contactor design for large desalination plants (rev.3) 1

2 SHORT GUIDELINE FOR LIMESTONE CONTACTOR DESIGN FOR LARGE DESALINATION PLANTS 1 DESCRIPTION OF UP-FLOW LIMESTONE CONTACTORS WITH CONTINUOUS FEEDING SYSTEM Passing corrosive water (i.e. with negative Langelier Saturation Index) thru a bed of crushed limestone produces an increase in ph as well as an increase in alkalinity and hardness. In upflow limestone contactors water moves upwards thru a bed of crushed limestone dissolving the calcium carbonate as it rises. The reaction involves is as follow: CaCO 3 + CO 2 + H 2 0 = Ca HCO 3 - (1) The equilibrium ph of this reaction depends on a series of factors: initial ph, CO 2 content, alkalinity, total dissolved solids, water temperature and upflow velocity, among others. Given enough contact time inside the limestone bed, water can be saturated with calcium carbonate. Consequently Langelier Index rises to values equal or very close to cero. In praxis, the parameter empty bed contact time (EBCT) is used to describe the residence time inside the bed. It is calculated by dividing the volume of the limestone bed by the flowrate, and is given in minutes. Large upflow limestone contactors are difficult to operate because of : ( ) turbidity problems in the effluent during reloading operations, ( ) decreasing performance due to continuously diminishing height of limestone bed between loadings, and ( ) turbidity problems when flushing the filter bed with air and water. A new concept for upflow limestone contactor is proposed for this project that resolves these problems as follow: ( ) Maintaining constant height of limestone bed by means of a continuous feeding system. Limestone feeding is, grain by grain, and driven by water chemical reactions inside the bed. ( ) Turbidity events during re-loadings are eliminated, as re-loading is carried out inside an in-built silo that is separated from the filter bed surface. ( ) A modular design allows sparging of a single module during normal operations. Since each module treats only a fraction of the total effluent, turbidity problem is minimized, as fine limestone particles leaving the treated module are dissolved by the much larger volume of clear water. Short guideline for limestone contactor design for large desalination plants (rev.3) 2

3 The design is based on research and development work that was initiated in 2001 in the Canary Islands (see Section 17: References). The proposed upflow limestone contactor modules built in concrete tanks. They can be divided into six parts, starting from the bottom: 1) the air injection system; 2) the water distribution zone; 3) the limestone bed, 4) the still-water zone, 5) the limestone feeding zone, and 6) the limestone silo. (see Figure 3 for details). Part 1.- The air injection system consists of a network of PVC pipes embedded underneath the contactor concrete base. These pipes are collected into a manifold outside the concrete tank. The air blower is connected directly to this manifold when sparging is required. Part 2.- The water distribution zone is located in the lower part of the tank. It consists of a 300 mm height infiltration platform. This platform consist of 996 x 996 mm 2 platforms that are placed against each other to conform an uniform infiltration surface. Contact surfaces between platforms, as well as platforms and walls, are sealed with a plastic resin. Each platform has three large openings on each sidewall to facilitate water distribution. They also have internal beams to withstand more than 6,000 kg/m2. Each module has 81 nozzles, uniformly distributed on its surface. Part 3.- The limestone bed zone (made of crushed limestone particles mm in diameter) is located above the infiltration platform. The height of the limestone bed may vary according to specifications but it is normally between 1 and 2 meters. Part 4.- The bed surface is approximately 800 mm below perimetral spillway. This area is called the still-water zone. This distance reduces the possibility of particles elutriation even at high superficial velocities. Part 5.- The limestone feeding zone is located above the still zone. It consist of a steel supporting structure and 996 x 996 mm 2 feeding plates. Each plate has 9 equally spaced funnels. These funnels allow distribute crushed limestone particles on to the limestone the bed surface, grain by grain and without creating turbidity. Limestone falls onto bed by gravity and regulated by water chemical demand. Dosing pipes cross the still-water zone to reach bed surface. Part 6.- The feeding structure in itself makes the floor of the limestone in-built silo. The material stored in the in-built silo falls thru the funnels on to the limestone bed below and by gravity. The in-built silo is loaded from above thru strategically located openings. Material supplied in regular bags or big-bags can be transported above the silos with a moving crane and unloaded directly into them. Storage capacity of the silos allow for gives an autonomy of several weeks. Short guideline for limestone contactor design for large desalination plants (rev.3) 3

4 The preliminary design presented here is for a R.O permeate flowrate of 100,000 m 3 /d with a range between 92,400 m 3 /d and 105,600 m 3 /d. 2 CONDITIONS FOR DESIGN For this preliminary study the following water characteristics are assumed: Table 1: Chemical characteristics of the permeate Parameter min. max. Field Water Temperature 18 ºC 39 ºC TDS (mg/l) ph 4, Langelier Index CO Calcium (mg Ca ++ /L) Alkalinity (mg CaCO 3 /L) Table 2: Chemical characteristics required after remineralization treatment TDS (mg/l) 200 ph Langelier +0.2 Turbidity < 0.5 Calcium (mg Ca ++ /L) 20 HCO - 3 (mg HCO - 3 /L) Other parameters such as Fe, F, Mn or B are not considered relevant for the limestone treatment if limestone quality is such as shown on Table 8. Short guideline for limestone contactor design for large desalination plants (rev.3) 4

5 3 SELECTION OF SUPERFICIAL VELOCITY Assuming a nominal flowrate of 100,000 m3/d and a range between 92,400 m 3 /d and 105,600 m 3 /d the following upflow velocities can be obtained for a contactor surface of 200 m 2. Table 3: Range of upflow velocities for a contactor surface of 200 m 2 Flowrate Superficial velocity (m 3 /d) (cm/min) 105, , , These ranges are considered adequate for limestone contactor performance according to experience and available data. 4 SELECTION OF EMPTY BED CONTACT TIME (EBCT) As shown on Table 1 temperature varies between 18ºC and 39ºC, consequently equilibrium ph also varies. Figure 1 and 2 illustrate the gradual increase in ph and Langelier SI with an increase in EBCT for 18 ºC and 39 ºC. Surface velocity was assumed to be between 32.1 and 36.7 cm/min (34.2 cm/min). The differences between the curves are mainly related to the effect of temperature on CO 2 and speed of reaction (1) of Section 1. The curves are obtained using the simulation model of Schott (2003) and Letterman and Kothari (1997), corrected with field data for R.O. desalinated water obtained by M. Hernández et al., (2004). Considering water should comply with the conditions specified on Table 2 the residence time should be between 4 and 5 minutes. For design purpose the EBCT will be set at 4.1 minutes. Short guideline for limestone contactor design for large desalination plants (rev.3) 5

6 ph ºC 39 ºC EBCT (min) Figure 1: Evolution of ph with increasing EBCT for RO permeate (based on preliminary data of Table 1) Langelier SI ºC 39 ºC EBCT (min) Figure 2: Evolution of Langelier Index with increasing EBCT for RO permeate (based on preliminary data of Table 1) 5 WATER QUALITY AFTER TREATMENT Table 4 shows the estimated characteristics of permeate after limestone treatment. Assumptions were: 98-99% rich limestone, mm particle size, and an upflow velocity thru the limestone bed between 32.1 and 36.7 cm/min. Short guideline for limestone contactor design for large desalination plants (rev.3) 6

7 Table 4: Estimated quality of the permeate after limestone treatment Parameter 18ºC 39ºC EBCT (min) ph HCO - 3 (mg/l) Ca ++ (mg/l) CO 2 (mg/l) Langelier Index Turbidity (NTU)* < 0.5 < 0.5 * Assuming a 99% limestone has water treatment quality grade, as given on Table 8 6 DIMENSIONS OF LIMESTONE BED Assuming a nominal flow of 100,000 m 3 /d, the dimension of the limestone contactor is analyzed considering the following basic design: Four large concrete tanks of equal size, for treating 25,000 m 3 /d each, operating in parallel. Five small limestone cells per large tank, for treating 5,000 m 3 /d each, also operating in parallel. Details of this arrangement can be visualized on Figures 3, 4, 5, 6 and Characteristics of the limestone cells As indicated in the previous paragraph, flowrate for each limestone cell was set at 5,000 m 3 /d, equivalent to 3.47 m 3 /min. Considering the selected EBCT is 4.1 minutes, the volume of cell limestone bed is: 4.1 x 3.47 = 14.2 m 3. Thus, considering a surface of the cell of 10 m 2 (2 x 5 m) the limestone bed height should be 1.42 m. However as indicated in Section 2 flowrate can go up to To guaranty an EBCT of 4.1 minutes under those conditions bed height is set at 1.55 m This height is considered adequate as shown in several experiments carried out by Hernández et al., (2004). Assuming a height for the water distribution zone of 0.30 m, adding 1.55 m for the limestone bed, 0.8 m for the feeding structure and 1.35 for the silo, a total internal height of 4.00 m is Short guideline for limestone contactor design for large desalination plants (rev.3) 7

8 obtained. Figure 3 depicts a cross section with the main characteristics of the proposed limestone bed Figure 3: 3D View of a 5,000 m3/day limestone cell. Numbers correspond to the different parts described in Section Characteristics of large tanks As indicated above, each of the large tanks will treat 25,000 m 3 /d and will house five limestone cells of 10 m 2 each, operating in parallel. Therefore total surface of limestone bed for each large tank is 50 m 2. The internal height of the large tank is also 4.00 m. To allow uniform water distribution under the five limestone beds, the water entrances from both sides of the limestone cells. Considering there will be 4 large tanks the total surface of the limestone bed will be 200 m2. Figures 4, 5, 6 and 7 shows a conceptual design of a 25,000 m 3 /d tank. Short guideline for limestone contactor design for large desalination plants (rev.3) 8

9 2.0 m OUTFLOW INFLOW Figure 4: Layout of the inflow level Channel 2.0 m Channel OUTFLOW Lid INFLOW Figure 5: Layout of the outflow level Limestone dosing platform air blower Subterranean air sparging system Infiltration modular platform Figure 6: Cross section thru a limestone contactor cell Short guideline for limestone contactor design for large desalination plants (rev.3) 9

10 OUTFLOW INFLOW Figure 7: Individual cell sparging system 6.3 Summary data for the dimensions the limestone beds Table 5: Summary data for the limestone beds Flowrate to be treated m 3 Nr. of large tanks 4 Nr. of limestone cells per large tank 5 Total number of limestone cells 20 Surface of each limestone cell 10 m 2 Total surface of the limestone beds 200 m 2 Height of the limestone beds 1.55 m Total volume of the limestone beds 310 m 3 Specific weight of mm limestone (dry) 1.5 ton/m 3 Total weight of the limestone bed (dry) ton EBCT min Upflow velocity cm/min Short guideline for limestone contactor design for large desalination plants (rev.3) 10

11 7 HEAD-LOSS OF THE LIMESTONE BED Using the correlation shown on Figure 8 (M. Hernández, unpublished) and for an average upflow velocity of 34.7 cm/min a head-loss of 43 cm/m of bed is obtained. Considering the bed is 1.55 m in height, the estimated head-loss would be 1.55 x 0.43 = 0.66 m (0.066bar). The head-loss of water distribution modules (81 nozzles/m 2 ) located below the bed is considered irrelevant. The total head-loss of the limestone cells is shown on Table 6. Table 6: Head-loss inside the limestone cells Height of the water outflow 2.65 m Head-loss of the limestone bed 0.62 m Total head required 3.27 m Head-loss (cm water column/m bed height) y = x R 2 = Superficial velocity (cm/min) Figure 8: Head-loss of the upflow limestone bed (particle size mm) Short guideline for limestone contactor design for large desalination plants (rev.3) 11

12 8 PERFORMANCE OF THE LIMESTONE BED AT VERY LOW FLOWRATES For the purpose of this evaluation, it is assumed flowrate to the limestone contactor could diminish to 13,600 m 3 /d (570 m 3 /h). Assuming this flowrate is passed thru one large tank, i.e. thru five 2 x 5 m cells (originally design for 25,000 m 3 /d) the flowrate per limestone cell would be: Flowrate per limestone cell = 570 m 3 /h /5 = 114 m 3 /h = 1.9 m 3 /min Thus the EBCT becomes: 14.2 m 3 /1.9 m 3 /min = 7.5 min. On the other hand upflow velocity becomes: Upflow velocity = 1.9 m 3 /min/10 m 2 = m/min = 19.0 cm/min Diminishing upflow velocity has an effect on reaction (1). Decreasing water velocity around particles slows down the reaction and consequently increases required EBCT, for same ph. This effect has been simulated by the model and shown on Figure 9. As before, these curves have been obtained using the simulation model of Schott (2003) and Letterman and Kothari (1997), corrected with field data for R.O. desalinated water obtained by M. Hernández et al., (2004). Increasing EBCT from 4.1 to 7.5 minutes tends to compensate this effect as output of the model suggests, and similar ph and Langelier SI can be expected than those obtained with nominal upflow velocities of 34,7 cm/min and 4.1 EBCT ph cm/min 19.0 cm/min EBCT (min) Figure 9: Effect of upflow velocity on ph for the 18ºC conditions. Short guideline for limestone contactor design for large desalination plants (rev.3) 12

13 ph EBCT (min) 18ºC 39ºC Figure 10: Effect of temperature on ph with increasing EBCT at slow 19 cm/min Langelier SI ºC 39ºC EBCT (min) Figure 11 Effect of temperature on the Langelier SI with increasing EBCT at 19 cm/min. 9 LIMESTONE CONSUMPTION Considering reaction (1) and assuming all additional Calcium present in the permeate after treatment comes from the Calcium Carbonate dissolved by the CO 2, it can be calculated from Tables 2 and 3 the following: Ca ++ = = 25.9 mg/l Ca ++ Assuming all the Calcium comes from the consumption of limestone it be calculated: CaCO 3 consumption = 25.9 mg Ca ++ /L x mg CaCO 3 /Ca ++ = mg/l de CaCO 3 (100%) Short guideline for limestone contactor design for large desalination plants (rev.3) 13

14 Considering the CaCO 3 is 99% pure, real consumption would be: 64.57/0.99= 65.2 mg/ L de CaCO 3 (99% rich) Table 7: Summary of limestone consumption data CaCO 3 consumption per m 3 (99% rich) g/m 3 Daily CaCO 3 /d consumption (99% rich) 6.52 t/d (for 100,000 m 3 /d) 10 LIMESTONE PARTICLE SIZE Limestone particle size affects contact surface and therefore limestone bed performance. Figure 12 shows the relationship between particle size and limestone bed height calculated using Letterman and Kothari (1995) model. Conditions for this example were: ph before treatment: 5.75; saturation ph = 8.16; objective ph = 8.08; EC = 600 µs/cm; 1.23 mg Ca/L; Alkalinity = 5.5 mg CaCO 3 /L, Surface velocity = 650 L/m2 and day. Limestone quality = 98% Ca CaCO height of the limestone bed (m) Particle diameter (mm) Figure 12: Example of relationship between particle diameter and height of the limestone bed (see text above for details) On Figure 13 particle size distribution recommended for this project is given. As can be seen the size of the particle size should be between 1.25 and 2.5 mm, with an average of 2 mm. Short guideline for limestone contactor design for large desalination plants (rev.3) 14

15 90% 60% % weight 30% 0% <0,32 0,32-0,63 1,25-2,5 >2,5 screen size (mm) Figure 13: Recommended limestone particle diameter distribution 11 QUALITY OF THE LIMESTONE Table 8: Recommended quality of limestone to be used for limestone contactor. Purity 99.1 % SiO % Al 2 O % MgO 0.2 % SO % Iron Oxide < 0.1 % Humidity 0.14 % ph 9.1 Hardness (Mohs) 3 Specific weight 2.7 Short guideline for limestone contactor design for large desalination plants (rev.3) 15

16 12 AUTONOMY OF THE SYSTEM Assuming the height of the silo is 1.35 m the autonomy of the system for 100,000 m 3 /d can be calculated as follow. Table 9: Autonomy of the system Height of the in-built silo 1.35 m Volume of the in-built silo 270 m 3 % Volume used 70 % Volume of limestone stored 189 m 3 Specific weight of mm limestone (dry) 1.5 ton/m 3 Weight of limestone stored in silos (dry) ton Daily consumption (99%) 6.52 ton/day Nr. of days of autonomy 43 days This period of autonomy is considered adequate, as it is equivalent to approximately one reloading per month. 13 LIMESTONE TREATMENT COSTS The price of crushed limestone (2.5 mm) varies with location. However, assuming a price of 0.10 /kg including freight, the treatment cost with limestone contactor can be estimated as follow: Table 10: Limestone treatment cost Estimated of limestone 0.10 /kg Daily consumption (100% pure) 6,520 kg/d Yearly cost 237,980 /y Cost per m /m 3 Short guideline for limestone contactor design for large desalination plants (rev.3) 16

17 14 SUPPLY AND LOAD OF LIMESTONE Crushed limestone screened to diameter 1.5-2,5 mm can be obtained in big-bags (1,100 kg) and transported in 20,000 kg container. Big-bags can be elevated above the loading area with bridge-crane. Picture 1: Loading limestone contactor with big-bag in South Africa 15 FILLING OF LIMESTONE CELLS Limestone particles should always fall on water during loading operations. Limestone bed should not be filled when no water is present inside the cell, as limestone might suffer some compaction and consequently might tend to form clusters. When particle fall through water they tend to settle smoothly, leaving a spongy bed. 16 CLEANING OF LIMESTONE BED WITH AIR SPARGING Limestone may, with time, develop preferential pathways. Also some cluster may develop here and there when bed is left to dry. Consequently regular air sparging is recommended at least once or twice a year. Sparging should only last for seconds. Larger sparging may cause elutriation, i.e. particles get washed away with water. Short guideline for limestone contactor design for large desalination plants (rev.3) 17

18 It is also recommended to flush the limestone bed when working with "not so pure" limestone as it helps to wash away impurities in particular silica and iron oxides that tend to deposit on particles surface. The design presented in this proposal offers the possibility of sparging each cell individually (see Figures 6 and 7). 17 ON THE USE OF SULFURIC ACID INSTEAD OF CO2 TO ENHANCE LIMESTONE DISSOLUTION The addition of sulfuric acid as pretreatment to limestone contactor produce to the following reactions: CaCO 3 + H 2 SO 4 = CaSO 4 + CO 2 + H 2 O (2) CaCO 3 + CO 2 + H 2 0 = Ca(HCO 3 ) 2 (3) 2 CaCO 3 + H 2 SO 4 = CaSO 4 + Ca(HCO 3 ) 2 (4)=(2)+(3) As shown by recent work carried out by Hernandez, M. (2004) results agreed well with the stochiometrical analysis. However, data suggest acid pretreatment have some limitations, as Langelier SI does not surpass the value of -0.4 when doses are higher than 20 ppm, even after 8 minutes of EBCT. On the other hand, with the CO 2 treatment Langelier SI can reached -0,1 after 4 minutes EBCT. In addition cost of treatment with the sulfuric acid treatment is higher than with CO 2 particularly because CaCO 3 consumption is double (Hernandez, M., 2004) Consequently, it is recommended that the use of sulfuric acid should be carefully evaluated before implementing it in a large project. Short guideline for limestone contactor design for large desalination plants (rev.3) 18

19 18 SUMMARY OF LIMESTONE CONTACTOR CHARACTERISTICS FOR THE EXAMPLE R.O. DESALINATION PLANT Table 11: Summary of data for the limestone contactor characteristics for a 100,000 m 3 /d desalination plant Flowrate to be treated m 3 Nr. of large tanks 4 Nr. of limestone cells inside per large tank 5 Total number of limestone cells 20 Surface of each limestone cell 10 m 2 Total surface of the limestone beds 200 m 2 Height of the limestone beds 1.55 m Total volume of the limestone beds 310 m 3 Specific weight of mm limestone (dry) 1.5 ton/m 3 Total weight of the limestone bed (dry) ton EBCT min Upflow velocity cm/min Height of the water outflow 2.65 m Head-loss of the limestone bed 0.62 m Total head required by limestone cells 3.27 m Height of the in-built silo 1.35 m Volume of the in-built silo 270 m 3 % Volume used 70 % Volume of limestone stored 189 m 3 Specific weight of mm limestone (dry) 1.5 ton/m 3 Weight of limestone stored in silos (dry) ton Limestone consumption per m 3 (99% pure) 65.2 g CaCO 3 /m 3 Daily limestone consumption (99% pure) 6.52 ton CaCO 3 /d Nr. of days of autonomy 44 days Estimated of limestone 0.1 /kg Daily consumption (100% pure) 6,520 kg/d Daily costs 652 /d Yearly cost 237,980 /y Cost per m /m 3 Short guideline for limestone contactor design for large desalination plants (rev.3) 19

20 19 REFERENCES Letterman, R.D. and Kothari, S (1995). Instruction for using Descon: A computer program for the design of limestone contactor. US-EPA, Cooperative Agreement Nr Letterman, R.D., (1997). Project summary: calcium carbonate dissolution rate in limestone contactors, EPA document EPA/600/SR-95/068. Hernández-Suárez, M. et al., Development of a new limestone contactors with continuous feeding system. Proceedings of the III Congress Spanish Association of Desalination and Water Reuse, AEDyR, Málaga. Hernández-Suárez, M., (2003). Post-treatment of RO water for irrigation. Proceedings of the Regional Meeting of the Euromediterranean Institute of Hydrotechnics, University of Murcia., Spain. Hernández-Suárez, M. et al., (2003). Advances on remineralization of desalinated water with limestone contactors. Proceedings of the IV Congress Spanish Association of Desalination and Water Reuse, AEDyR, Las Palmas. Hernández, M. (2003). Limestone contactors in the Canary Islands, Spain in Small Public Water System Technology Guide, Vol. II. Limestone Contactors by Azarina Jalil et al.,. University of New Hampshire. Water Treatment Technology Assistance Center. p.26. Hernández, M. (2004). On the costs of remineralization (rev). Canary Islands Water Center, available at International Section. Hernández, M. (2004). On the use of acids intead of CO2 to enhance performance of limestone contactors: a comparative analysis. El Manantial, Bulletin of the Canary Islands Water Center, December 2004, page 4, available at Schott, G. Limestone bed contactor corrosion control and treatment analysis. Program V1.02 (2003). in Small Public Water System Technology Guide, Vol. II. Limestone Contactors by Azarina Jalil et al.,. University of New Hampshire. Water Treatment Technology Assistance Center. M. Hernández et. al. (2004). R&D on remineralization of desalinated waters with limestone contactors. Canary Islands Water Center, 171 pp. Short guideline for limestone contactor design for large desalination plants (rev.3) 20