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Chemical and economical evaluation of groundwater treatment plants in Riyadh
PII: S0043-1354(99)00040-8
Wat. Res. Vol. 33, No. 15, pp. 3291±3302, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
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CHEMICAL AND ECONOMICAL EVALUATION OF GROUNDWATER TREATMENT PLANTS IN RIYADH ABDULLAH M. AL-REHAILI1* and ABDULRAHMAN I. ALABDULA'ALY2{ 1
College of Engineering, King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia and 2King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh, 11442, Saudi Arabia (First received 1 April 1998; accepted in revised form 1 January 1999)
AbstractÐThe capital of Saudi Arabia, Riyadh is supplied with drinking water from both treated groundwater and desalinated sea water. Groundwater contributes about 35% to the total water supply and is treated in six treatment plants containing cooling, softening, ®ltration, reverse osmosis and posttreatment processes. An extensive two and a half years study aimed at the evaluation of Riyadh water treatment plants performance and the investigation of chemical treatment alternatives has been undertaken. The work involved an evaluation of the plants chemical treatment in which the eectiveness of softening chemicals was investigated by conducting jar test, pilot plant and full-scale implementation experiments. The results indicate that by optimizing the currently used chemicals it is possible to save 19% in chemicals cost or approximately million SR 3.71 a year. Caustic soda can be a competitive alternative in four plants replacing lime, soda ash and sodium aluminate. Moreover, there is a great potential of saving million SR 7.04 a year at the largest plant by simple pH-adjustment using sulfuric acid for the purpose of water stabilization. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐalternative chemicals, RO pretreatment, lime±soda softening, caustic soda, polymers, pilot to full-scale implementation
INTRODUCTION
The capital of Saudi Arabia, Riyadh, with a population of nearly 3.0 million had consumed a daily average of 1.135 106 m3 of water in 1994. Prior to 1983, water for the city was supplied from groundwater sources. Since then, desalinated seawater from Al-Jubail on the Arabian Gulf became the major source of drinking water for the city. At the present, desalinated seawater constitutes approximately 70% of Riyadh water supply. The remainder is provided by treated groundwater mainly from deep wells in the vicinity of the city. There are a total of 161 (25 shallow and 136 deep) wells that supply water to the six treatment plants (Fig. 1). The shallow wells are located in Nesah, Nemar and Al Haair valleys. Nesah well water is considered of high quality with TDS content of about 400 mg/l. The deep wells are tapping the Minjur and Wasia aquifers. Water in the former aquifer is characterized by high TDS content (1000±1500 mg/l), high temperature (50±558C), high in total hardness, iron and hydrogen sul®de. Wasia aquifer water is low in temperature (about 348C) and relatively high in TDS content. The shallow *Author to whom correspondence should be addressed. [Tel.: +966-1-467-6927; fax: 966-1-467-7008]. {Tel.: +966-1-481-3300; fax: +966-1-481-3878.
wells are drilled to a depth of 60±270 m, whereas the deep wells are drilled to a depth of 1200± 2000 m (Ghulaigah and Ericsson, 1979; Wojcik, 1983; Albraithen, 1993). In 1989, the shallow wells produced a total of 19.3 Mm3 of water compared to 101 Mm3 for the deep wells. Five out of the six treatment plants process groundwater through cooling, chemical precipitation, ®ltration, reverse osmosis, chlorination and water stabilization. The sixth treatment plant (Wasia) contains all of the above processes except reverse osmosis due to the blending of its product water with the desalinated sea water. The chemical precipitation is directed mainly to hardness removal through lime±soda softening and silica reduction using sodium aluminate with the addition of polymer or ferric chloride. Silica reduction is considered an important pretreatment process in these plants for prevention of silica scale formation within the reverse osmosis modules. Studies have indicated that the solubility limit for silica in natural waters at 258C is only 120 mg/l (Sanks, 1978; Stumm and Morgan, 1981; Montgomery, 1985). It is also indicated that silica concentration of 10 mg/l (Si) in reverse osmosis feed water is not harmful at a water recovery of less than 98% (Potts et al., 1981). Reduction of silica concentration in Riyadh plants to below 18 mg/l is recommended for membrane protection and process eciency.
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Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Fig. 1. Locations of Riyadh water treatment plants and well ®elds.
In 1993, King Abdulaziz City for Science and Technology (KACST) funded a research project aimed at the evaluation of Riyadh water treatment plants performance and the investigation of chemical treatment alternatives. The work involved an evaluation of chemical treatment in the six drinking water treatment plants located in Riyadh (AlRehaili and Alabdula'aly, 1996). Jar tests and pilot plant experimentation were designed in order to
evaluate the eectiveness of calcium hydroxide (lime), soda ash, sodium hydroxide (caustic soda), aluminum sulfate (alum), sodium aluminate, ferric chloride and polymer for the purpose of reducing hardness and silica from groundwater. Caustic soda, with and without polymer, was investigated in the study as an alternative to lime±soda softening for its eectiveness in hardness and silica removal. Alum, sodium aluminate, ferric chloride and poly-
Chemical and economical evaluation of groundwater treatment plants in Riyadh
mer were tested as precipitation aids in the lime± soda process. Based on the experimental work, an evaluation of the eectiveness of the chemicals on an actual full-size plant was implemented followed by an economic analysis for all of the six plants. The objective of this paper is to present information on the Riyadh groundwater treatment plants, major experimental work (jar and pilot) ®ndings, results of full-scale implementation and economic study evaluation. RIYADH WATER TREATMENT PLANTS
Groundwater is treated in six water treatment plants located within and around the city. Plants located within the city, Manfouha I and II, Malaz and Shemessy were commissioned in 1969 with the original chemical treatment±®ltration schemes. In 1979, reverse osmosis desalination steps were operated in these plants. Treated waters from Manfouha I and II and Shemessy plants is blended with waters from shallow aquifers before discharge to the water network. Salbukh plant is located 40 km north of the city, it started operation in 1978. Buwaib plant is located 70 km north±east, it started operation in 1969. Wasia plant, commissioned in 1982, is the largest of all and located 110 km east of Riyadh. This plant does not have reverse osmosis desalination. After chemical pretreatment, water from this plant is blended with desalinated sea water coming from Al-Jubail desalination plant located 400 km to the east on the Arabian Gulf. Before 1983, Riyadh was totally dependent on groundwater for all its water
3293
needs. Currently, however, desalinated sea water constitutes approximately 70% of water supply to the city. Fig. 2 shows the progressive increase in Riyadh water supply since 1980 and the contribution of the dierent sources. The main focus of chemical treatment at Riyadh plants is reduction of hardness and silica from groundwater using lime±soda ash and precipitation aids such as sodium aluminate, ferric chloride and polymer. Treatment schemes of all plants are similar to a great extent, with dierences in the number of units in each process (Fig. 3). Treatment starts by passing feed water to aeration cooling towers to reduce water temperature to 30±358C, oxidize iron and manganese, and remove CO2 and H2S. Part of the cooled water (23±27%) goes directly to ®lters, the remainder passes through contact clari®ers where chemical addition with rapid mix, slow mix and settling occurs, followed by ®ltration and blending with the ®rst stream. Filtration takes place in a number of ®lter cells, each consists of two ®lters in series (all plants except Wasia). The ®rst is an up-¯ow coarse media ®lter, the second is a down-¯ow ®ne media ®lter. Filters of Wasia plant are dual media with sand and anthracite. Part of the ®ltered water goes to reverse osmosis desalination for TDS reduction, followed by stabilization, blending and chlorination (all plants except Wasia). Table 1 illustrates design capacities, productivities and chemical dosages, consumption and costs for Riyadh treatment plants. Table 2 shows yearly average raw and treated water characteristics at Riyadh treatment plants in 1993.
Fig. 2. Development of Riyadh daily average water consumption.
Fig. 3. Typical water treatment processes at Riyadh water treatment plants.
3294 Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Chemical and economical evaluation of groundwater treatment plants in Riyadh
3295
Table 1. Summary of Riyadh treatment plants productivities, chemical dosages, consumption and costs (1993). US$1=SR 3.75 Plant
Wasia Manfouha I and II Buwaib Salboukh Shemessy Malaz
Average Maximum productivity production in 1993 (m3/day) (m3/day)
221000 86400 66000 66720 57600 28800
191240 53704 27452 36676 40788 14427
Average dosing rate (mg/l)
lime
soda ash
100 110 110 125 110 130
70 325 350 210 220 240
526520 364287 Total (m3/day) Chemical consumption (ton/year) 14234 22114 Chemical cost S.R. (millions/year) 6.53 16.10 Total chemical cost, all plants S.R. (million/year) 32.69
STUDY METHODOLOGY
Two treatment plants were selected for conducting the experimental work on their raw water. Selection was based on raw water quality in terms of total hardness and silica content. Shemessy plant raw water contains the lowest concentration of both total hardness and silica among all plants (535 mg/l as CaCO3 and 23.3 mg/l, respectively), whereas Buwaib plant raw water contains the highest concentration of the two parameters (851 mg/l as CaCO3 and 34.8 mg/l). A total of 52 jar test experiments were carried out in the laboratory using cooled water from both Shemaisy and Buwaib plants. One or more chemicals were tested that represent the chemicals used in the plants in addition to alternative chemicals. Evaluation of chemicals eect on hardness and silica was carried out starting with one chemical and progressing to more chemicals with variation in dosages. The chemicals tested include lime, soda ash, caustic soda, sodium aluminate, alum, ferric chloride and four polymers (anionic super¯oc A100, polycationic Nalcolyte 8100, anionic POL-E-Z 692 and nonionic super¯oc N 100). Based on the jar test results, a number of chemicals were selected for the pilot plant experimentation. Two pilot plants, representing actual full-size operations, were designed, constructed and installed at Shemaisy and Buwaib treatment plants. Each pilot plant consists of chemical precipitation process, ®ltration and required pumps, chemical tanks and backwashing unit (Fig. 4). The precipitation unit consists of a precipitator, chemical tanks and dosing pumps. The precipitator is cylindrical in shape, tapered at the bottom, 1 m in diameter and 2.5 m in height. It provides a detention time of 1.5 h when using 11.4 lpm water ¯ow rate. Rapid mixing chamber is housed in the upper part of the precipitator which is made of a pipe 10 cm in diameter and 1 m long closed at the bottom and equipped
sodium polymer ferric chlorine sulfuric sodium aluminate chloride acid hexametaphosphate ÿ 10 25 25 9 12
978 4.73
0.25 0.06 ÿ 0.20 0.10 0.20
ÿ ÿ 15 ÿ ÿ ÿ
24 0.35
150 0.5
1.70 0.94 0.50 0.85 0.80 1.00
171 0.43
25 50 30 21 35 30
ÿ 5 6 5 4 7
3986 2.67
322 1.38
with a mixer. The slow mixing chamber is located around the rapid mixing chamber and is made of a pipe 30 cm in diameter and 1.70 m long, open at both ends. The clear softened water is collected in a channel around the upper part of the precipitator. The precipitator is equipped with an automatic sludge withdrawal system installed at its lower end. There are two types of ®lters that are used in the pilot plant which simulate the actual ®lters in the Riyadh water treatment plants. The ®rst type consists of up-¯ow and down-¯ow sections similar to Shemaisy, Manfouha, Salbukh, Buwaib and Malaz ®lters. The up-¯ow section contains coarse sand with sizes 3±5 mm and a depth of 2.38 m, whereas the down-¯ow section contains a 15 cm layer of coarse sand (3±5 mm) and a 90 cm layer of ®ne sand (0.8±1.2 mm). Both sections has a 10 cm support layer of gravel with sizes 10±18 mm. The other type of ®lters consist only of a down-¯ow section which is similar to that of Wasia water treatment plant. The ®lter media consists of a 10-cm support layer of gravel, 35 cm of coarse sand and gravel (1.2±12.7 mm), 50 cm of ®ne sand (0.4 mm) and 30 cm of anthracite (1.0 mm). The ®ltering media is placed in a clear plastic column 15 cm in diameter and 3 m in height. The ®lters are equipped with ¯ow meters and piezometers located at dierent depths for the purpose of measuring head loss. The backwash system consists of two pumps, one for air and the other for water. Backwashing is accomplished by pumping air and water (separately or combined) through the bottom of the ®lters. The product water of the pilot plant is collected in a 3m3 tank. Sampling valves are located at dierent locations within the plant. The two pilot plants are supplied with cooled raw water and the ¯ow rate through the ®lters was set at values similar to those used in actual plants, i.e. 256 and 69 lpm/m2 for the up- and down-¯ow sections of the ®lters, respectively and 104 lpm/m2 for ®lters with one section only.
7.30 7.30 a 0.91 169 220 264 70 125 108 99 95 156 42 96 34 704 550 1134 a a 856 a a 14.38 152 88 253 258 178 474 0.032 0.058 0.036 a
Not available.
a
7.22
pH 8.24 7.96 7.93 8.10 8.03 7.96 8.00 7.76 7.08 6.87 6.60 6.65 7.00 7.30 7.86 7.40 7.33 7.80 a a a a a a a a a a a a a a a a 1.03 1.0 Turbidity (NTU) Total hardness 547 733 714 851 672 535 601 473 284 276 420 343 212 265 506 116 130 337 Calcium hardness 373 486 488 514 425 362 404 305 132 91 194 146 85 109 349 58 68 133 Magn. hardness 174 247 226 337 247 173 197 168 152 185 226 197 177 156 157 58 62 204 Alkalinity 118 159 160 140 146 156 156 58 36 34 21 26 31 36 84 60 61 43 a a a a a a TDS 1120 1509 1468 1583 1369 1063 1184 1113 1093 483 508 1507 a a a a a a a a a a a a a a 3379 1628 1725 2267 Conductivity (mS/cm) a a a a a a Silica 16.8 19.7 21 34.8 28.5 23.3 24 10.3 13.7 15.8 14.6 9.9 a a a a a a Chlorides 241 300 295 378 295 191 256 191 245 100 76 489 a a a a a a a a a a a 486 407 457 437 373 450 Sulfates 360 a a a a a a a a a a a Total iron 0.16 0.48 0.25 0.04 0.09 0.016 0.004
4 4 4 1
2
3
Raw water
5
6
7
1
2
3
After ®ltration
5
6
7
1
2
3
Product water
5
6
7
Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Constituents
Table 2. Average yearly characteristics of raw and treated water for Riyadh treatment plants (1993). Units are in mg/l except if otherwise stated, hardness and alkalinity in mg/l as CaCO3. 1 Al wasia; 2 Manfouha-I; 3 Manfouha-II; 4 Buwaib; 5 Salbukh; 6 Shemessy; 7 Malaz
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A total of 45 experimental runs were conducted in groups based on the used chemicals. Each experimental run lasted 48 h and samples were collected every 6 h from six locations within the pilot plant. Parameters such as temperature, pH, turbidity and conductivity were measured in the ®eld during sample collection. Total hardness, calcium, magnesium, alkalinity and silica were determined in the laboratory according to Standard Methods (APHA, AWWA, WPCF, 1985). Detailed results of jat test and the two pilot plant experiments have been published elsewhere (Al-Rehaili et al., 1995; Alabdula'aly et al., 1995; Alabdula'aly and AlRehaili, 1995; Al-Rehaili and Alabdula'aly, 1996). The following sections summarize the major results. Jar test results indicate that lime and soda ash cause noticeable reduction in hardness with no signi®cant removal of silica. With lime dosage of 110 mg/l, hardness was removed by 25.9 and 13.8% for Shemessy and Buwaib cooled water, respectively. The use of 400 mg/l of soda ash with a ®xed dosage of lime (120, 110 mg/l) resulted in hardness removal of 63.7 and 52.7% for the two plants cooled water, respectively. The maximum silica removal was 15.6% when using 120 mg/l of lime and 400 mg/l of soda ash for Shemessy cooled water and 14.6% when using 110 mg/l of lime and 375 mg/l of soda ash for Buwaib cooled water. The dosing of precipitation aids (aluminum sulfate, ferric chloride and sodium aluminate) with ®xed dosages of lime and soda ash did not cause an increase in hardness removal but a signi®cant silica reduction. Sodium aluminate was the most eective among the three chemicals in silica reduction, resulting in 42.4 and 34.8% removal with a dosage of 25 mg/l for Shemessy and Buwaib cooled water, respectively. The addition of aluminum sulfate or sodium aluminate gave the same silica removal based on aluminum concentration in both chemicals. All tested polymers when added to ®xed dosages of lime and soda ash did not cause an improvement in hardness removal. With A-100 polymer, a slight increase in silica removal was observed. A dosage of 0.20 mg/l of the polymer to Shemessy cooled water with 110 mg/l of lime and 220 mg/l of soda ash reduced the silica content to 19.2 mg/l compared to 22.5 mg/l with lime and soda ash only. When the other three polymers were added to ®xed dosages of lime and soda ash, there was insigni®cant reduction in silica removal. Comparison of the eectiveness of polymers with other precipitation aids revealed that sodium aluminate and aluminum sulfate give better silica removal when added to ®xed dosages of lime and soda ash. Dosing of 0.2 mg/l of A-100 polymer and ®xed dosages of lime, soda ash and sodium aluminate resulted in an improvement of silica removal for Shemessy cooled water. Dosing of other polymers resulted in negative impact on silica removal. When polymers are
Chemical and economical evaluation of groundwater treatment plants in Riyadh
added with ®xed dosages of lime, soda ash and aluminum sulfate to Shemessy cooled water, both 692 and N-100 polymers caused negative impact on hardness removal, whereas the 8100 polymer showed slight improvement in hardness removal. For Silica removal, only 8100 polymer showed slight improvement, with 692 and N-100 polymers showing negative impact. Substituting ferric chloride for polymer as the case in Buwaib plant caused negative impact on both hardness and silica removal. The use of caustic soda alone resulted in both hardness and silica removal. With 180 mg/l of caustic soda, it was possible to reduce the hardness and silica content of Shemessy cooled water to 260 mg/l as CaCO3 and 10.8 mg/l, respectively. For Buwaib cooled water, a dosage of 200 mg/l of caustic soda resulted in 40 and 65% hardness and silica removal, respectively, bringing their concentration to 534 mg/ l as CaCO3 and 10 mg/l. The pilot plant experiments results agreed to a great extent with those obtained in the jar tests. The use of lime and soda ash resulted in noticeable hardness reduction and insigni®cant silica reduction. The best combination of lime/soda ash dosages were 110/220 mg/l and 90/300 mg/l for Shemessy and Buwaib cooled water, respectively. Addition of sodium aluminate resulted in a slight
3297
improvement in hardness removal and a noticeable reduction in silica level. Silica removal of 17% was obtained for Shemessy plant using 10 mg/l sodium aluminate and 44.8% for Buwaib plant using 25 mg/l. Dosing of super¯oc A-100 anion polymer in the range of 0.05±0.2 mg/l in addition to lime, soda ash and sodium aluminate did not improve hardness and silica removals but an improvement of softened water turbidity was noticed. The use of ferric chloride in place of polymer in the range of 10±25 mg/l has resulted in an improvement of the hardness and silica removal in Buwaib cooled water. However, it was noticed that the use of ferric chloride increased ®ltered water turbidity. The use of aluminum sulfate in place of sodium aluminate with consideration that one unit of the latter contains twice the amount of aluminum as in the former, it was observed that better removal was obtained when using sodium aluminate than when using alum. The dosing of caustic soda alone has resulted in a noticeable improvement in the hardness and silica removals. It was possible to reduce the Shemessy cooled water hardness and silica by 50 and 34%, respectively, when using 175 mg/l of caustic soda. For Buwaib cooled water, the use of 200 mg/l caustic soda has resulted in 48.7 and 83.6% reduction of hardness and silica, respectively.
Fig. 4. Pilot plant details.
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Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Comparing the performance of the two types of ®lters, it was observed that insigni®cant dierences in water quality exist. The only dierence was in terms of turbidity level, in which the ®lters with two sections (up- and down-¯ow) gave waters with lower turbidity levels as compared to ®lters with down-¯ow section only (sand and anthracite). FULL-SCALE IMPLEMENTATION
Results obtained from the pilot plant experiments were used as a basis for full-scale veri®cation on Malaz water treatment plant. This plant was selected because it is the smallest of all and its raw water characteristics lies between the two raw water extremes (Shemessy and Buwaib plants). An experimental program was designed carefully to cover both normal plant operation (with chemicals and dosages as currently used in the plant) and operation with speci®c modi®cations of chemicals and dosages. A total of 14 experiments, each lasting 72 h, were conducted at Malaz plant. During the experimental period, the plant was working with only one precipitator at approximately 400 m3/hr ¯ow rate. Water samples were collected every 2 h (8 am to 6 pm) from three points, namely cooled raw water, after precipitator and after ®lters. Measurements of pH, turbidity and temperature were conducted on site at the time of collection;
other parameters (total hardness, Ca, Mg, silica, conductivity, iron, alkalinity and sodium) were conducted in the laboratory. Dosages of chemicals were calculated by noting water ¯ow rates, and dosing pump details. Sludge leaving the precipitator was sampled regularly and tested for total solids and a 30-min settling test was conducted using 1litre Imho Cone for each sample to determine sludge volume. Despite the fact that some obstacles were encountered during actual testing, the eects of chemical combinations and dosages on hardness and silica reduction at Malaz plant were reasonably established. The most important obstacles were the instability of the lime and soda ash dosing systems and some diculties in the sludge discharge system. Fig. 5 presents a comparison of the eects of dierent chemical treatment combinations on reduction of total hardness, calcium and magnesium at Malaz plant. Experiment No. 2 shows that omitting polymer addition resulted in slight improvement in hardness removal compared to experiment No. 1 which applies chemicals and dosages as normally practiced at Malaz plant. Decreasing sodium aluminate dosage by 50% (experiment No. 3) resulted also in slight improvement of hardness removal. Experiments No. 4±7 indicate that there is a possibility to reduce dosages of all or some of the chemicals used in the plant while keeping total
Fig. 5. Average total hardness, calcium and magnesium levels in Malaz ®ltered water with dierent chemical treatment combinations at Malaz plant.
Chemical and economical evaluation of groundwater treatment plants in Riyadh
3299
Fig. 6. Average ®ltered water silica concentrations with dierent chemical treatment combinations at Malaz plant.
hardness in ®ltered water within normal performance of the plant. Using caustic soda alone in chemical precipitation (experiments No. 8±11) in the dosages 130, 190 and 250 mg/l resulted in hardness removals as good as or better than obtained by chemicals and dosages normally used at the plant. A dosage of 250 mg/l caustic soda reduced total hardness after ®lters to 302 mg/l, compared to 431 mg/l, using normal plant dosing (98 mg/l lime, 200 mg/l soda ash, 13 mg/l sodium aluminate, and 0.4 mg/l polymer). Adding increasing dosages of polymer with a ®xed dosage of caustic soda (experiments No. 12±14) reduced ®ltered water hardness with all used polymer dosages to approximately 284 mg/l, compared to 400 mg/l when polymer was not added. Figure 5 illustrates that reducing dosages of chemicals currently used at the plant has to some extent similar eects on calcium and magnesium hardness as noticed for total hardness. Using caustic soda alone reduced calcium hardness to the same level obtained with normal plant dosing. Magnesium hardness, however, was reduced to 102 mg/l with the highest caustic soda dosage applied (250 mg/l), compared to 217 mg/l in ®ltered water using normal plant dosing. Adding polymer with caustic soda was more eective for improving magnesium removal than for calcium. It was
noticed that the lowest dosage of polymer used (0.09 mg/l) was more eective than other dosages. Fig. 6 shows that the eects of reducing lime and soda ash dosages to the limits practiced in the experiments had little eect on ®ltered water silica concentration. However, reducing sodium aluminate dosage by 50% (experiment No. 3) resulted in increased ®ltered water silica to 17.6 mg/l, compared to 14.3 mg/l with normal plant dosing. Using 190 mg/l of caustic soda alone reduced ®ltered water silica level to 11.5 mg/l. Adding polymer with caustic soda resulted in excellent reduction of silica in ®ltered water, reaching 7 mg/l when using 195 mg/l caustic soda and 0.09 mg/l polymer. Table 3 illustrates the eects of dierent chemical treatment combinations on pH, alkalinity, turbidity, and sodium levels at Malaz plant. High pH levels are extremely important for chemical precipitation of hardness ions. Table 3 shows that addition of lime and soda ash at normal dosing increased pH to about 8.2 during precipitation, while using caustic soda at 250 mg/l increased pH to 10.2 which gives it higher eectiveness for hardness and silica removal, most notably for magnesium removal (as seen in Fig. 5). It should be made clear that increasing lime dosages to higher values than used here can improve hardness removal by raising pH, but the objective of this comparative study is to treat
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Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Table 3. Average pH, alkalinity, turbidity and sodium levels using dierent chemical treatment combinations at Malaz plant. C is after cooling, P after precipitation and F after ®ltration No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Chemical dosage (mg/l)
pH
Alkalinity (mg/l)
Turbidity (NTU)
Sodium (mg/l)
Lime
soda ash
sodium aluminate
polymer
caustic soda
C
P
F
C
P
F
C
P
F
C
P
F
98 100 69 63 79 70 80 55 ÿ ÿ ÿ ÿ ÿ ÿ
201 185 202 175 179 167 131 144 ÿ ÿ ÿ ÿ ÿ ÿ
12.8 12.2 6.4 13.2 12.8 12.6 13.0 9.8 ÿ ÿ ÿ ÿ ÿ ÿ
0.41 ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ 0.09 0.20 0.39
ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ 130 190 250 195 198 190
8.06 7.76 8.00 7.97 8.07 8.02 8.04 8.06 8.08 8.04 8.04 7.89 7.89 7.99
8.22 8.83 8.26 8.15 8.16 8.13 8.29 8.11 8.85 9.54 10.22 9.82 9.82 9.88
6.81 6.97 6.79 6.69 6.90 8.89 6.84 7.01 6.86 7.32 7.11 6.89 6.50 6.44
140 156 144 147 148 147 145 152 144 144 147 159 156 156
97 21 70 75 87 81 71 91 31 22 33 31 30 29
68 27 54 53 66 64 52 68 27 26 21 16 15 14
2.1 2.6 1.6 4.4 3.5 4.2 2.6 2.7 2.9 2.3 2.1 2.6 3.1 3.6
1.8 35 40 31 25 23 34 38 17 32 52 10 5 2.4
0.35 0.76 0.15 0.28 0.31 0.77 0.54 0.34 0.46 0.46 0.45 0.47 0.53 1.02
208 198 210 206 211 209 207 210 199 196 190 213 209 193
293 285 304 299 298 280 298 289 279 300 347 355 249 321
291 283 310 305 297 286 307 294 291 304 346 358 349 318
water to the levels currently practiced and being acceptable at the plants. Alkalinity levels after precipitation dictate the amount of acid required for pH-adjustment before ®ltration. It is clear that treatment with caustic soda resulted in lower alkalinity after precipitation, but at the same time higher pH levels. Experiments have shown that almost the same amount of sulfuric acid was required to adjust pH before ®lters to about 6.8 in case of lime±soda ash treatment and in case of caustic soda treatment. All experiments conducted without polymer addition resulted in high turbidity after precipitation (Table 3). This was also obvious by the formation of a clear white layer of precipitate on the ®lter top during testing. It is important to note that polymer addition is important for proper operation with all of the tested chemical combinations. Table 3 shows that using lime±soda ash treatment increased sodium level in ®ltered water to an average of 280 mg/l, compared to the cooled water sodium content of about 200 mg/l. Sodium sources are sodium aluminate and soda ash in this case. The use of caustic soda, which also contains sodium, increased average sodium concentration in ®ltered water to 346 mg/l at the highest dosage used (250 mg/l). Addition of polymer with caustic soda may slightly increase sodium release in ®ltered water due to improved hardness removal and for-
mation of more of the soluble chlorides and sulfates of sodium. Table 4 shows a comparison of sludge characteristics with dierent treatment combinations. Generally speaking, sludge volume in case of lime± soda ash treatment is higher than that for caustic soda treatment. In addition, use of polymer resulted in noticeable increase of sludge volume, indicating improved precipitation and treatment eciency. This eect was more pronounced with caustic soda. Increased total solids concentration of sludge was also realized, indicating increased sludge volume and density with the use of polymer. Based on the results of actual testing at Malaz plant, two chemical treatment combinations were selected as optimum, namely: lime (63 mg/l), soda ash (175 mg/l), sodium aluminate (12 mg/l) and polymer (0.2 mg/l); or caustic soda (190 mg/l) and polymer (0.2 mg/l). ECONOMICS
Based on the experimental (pilot and full-scale) results, optimum chemical dosages were proposed to be used in Riyadh water treatment plants (except Wasia). The use of such dosages is expected to produce acceptable water quality at the lowest cost either using the presently used chemicals or using caustic soda as an alternative. The proposed dosages and the expected costs are presented in
Table 4. Comparison of sludge volumes with dierent chemical treatment combinations at Malaz plant Chemical dosage (mg/l) lime 98 98 ÿ ÿ ÿ ÿ
Sludge volume (ml/l)
soda ash
sodium aluminate
polymer
caustic soda
201 201 ÿ ÿ ÿ ÿ
12.8 12.8 ÿ ÿ ÿ ÿ
ÿ 0.2 ÿ 0.09 0.2 0.39
ÿ ÿ 190 195 198 190
325 350 170 323 328 337
Chemical and economical evaluation of groundwater treatment plants in Riyadh
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Table 5. Cost of chemicals in Riyadh water treatment plants using optimum dosages of chemicals currently used and alternatives Chemical
Chemical cost (SR/ton) (1996)
Malaz, Shemessy, Salbukh and Manfouha currently used chemicals
alternative chemicals
Buwaib currently used chemicals
proposed cost proposed cost proposed cost dosage (mg/l) (SR/1000 m3) dosage (mg/l) (SR/1000 m3) dosage (mg/l) (SR/1000 m3) Lime Soda ash Sodium aluminate Polymer Caustic soda Unit cost (SR/1000 m3) Plants production (106 m3/yr) Total cost (SR M/yr)
459 727 1950 14418 730
70 175 12 0.2 ÿ
Table 5. For Buwaib plant, caustic soda was not recommended to be used due to the fact that high dosages are required in order to reduce the calcium hardness to levels below the RO membrane requirements. Wasia plant was not included in Table 5, since the present chemical treatment is not intended for hardness and silica removal, but rather for water stabilization to protect water transmission line. It is clear from Table 5 that the cost of soda ash is the highest among chemicals currently used, followed by sodium aluminate. It is also clear that using the proposed optimum dosages will lead to an annual chemicals cost of SR 15.73 million, compared to the present cost of SR 19.44 million, i.e. 19% cost reduction. The use of caustic soda and polymer in Malaz, Shemessy, Salbukh and Manfouha plants is expected to result in a chemical cost of SR 15.35 million, compared to SR. 11.83 million in the case of using the optimum proposed dosages of the chemicals currently used at the plants. This means that there is no economic incentive to use caustic soda based only on chemical costs. However, when considering the possible savings on operation and maintenance, there might be some justi®cation to use caustic soda and polymer in place of the currently applied chemicals. This is especially true if caustic soda can be obtained at a lower cost (560 SR/ton) as the Riyadh Water Authority had indicated in their correspondence with the authors.
32.10 127.20 58.00 2.90 ÿ 220.20 53.7 11.83
.
.
.
CONCLUSIONS
This extensive study on Riyadh water treatment plants led to many conclusions and recommendations which covered optimization of chemical types, dosages and costs in addition to operation and maintenance capabilities and requirements. Following are major conclusions regarding chemical treatment at the plants based on jar testing, pilot plant testing and full-scale implementation. . Jar tests and pilot plant evaluations were very helpful in comparing chemicals eciencies in
.
ÿ ÿ ÿ 0.20 190
ÿ ÿ ÿ 2.90 283.00 285.90 53.7 15.35
90 300 25 0.1 ÿ
41.30 218.10 120.90 1.40 ÿ 381.70 10.2 3.90
many experimental combinations which resulted in proper selection of chemicals and to some extent narrowed down the range of eective dosages and treatment conditions. The pilot plant used in this study was not totally representative of full-scale performance in the precipitation step, especially for hardness removal by lime± soda softening. The pilot plant was less ecient than the full-scale plant in this regard. This probably re¯ects the eect of the shorter depth of settling in the pilot precipitator compared to the very high depth in full-scale precipitators. Results of jar and pilot plant testing revealed that the use of polymers as precipitation aids with lime and soda ash produced little improvement in hardness and silica removal, they even had negative eects at some dosages. The anionic polymer used at the plants had positive eects on the precipitation process and sludge characteristics which reduces solids loading to the sand ®lters. Optimized chemical dosages obtained indicate that there is a possibility of reducing chemical dosing rates at the plants using the chemicals currently applied at Malaz, Shemessy, Manfouha, Salbukh and Buwaib (especially lime, soda ash and polymer dosages). This will lead to a total saving in chemical cost of 19%, or approximately SR 3.71 million annually. It was realized through this study that caustic soda can be a competitive alternative for all plants except Buwaib. It can replace lime, soda ash, and sodium aluminate, and can economically (if supplied at SR 560/ton) produce ®ltered waters of the required quality or better. The caustic soda dosing system will be much simpler and less maintenance demanding compared to the currently used lime±soda ash system which is too involved and plagued by the instability of dosing due to frequent plugging of feed lines. There is a great potential of saving at Wasia plant. The plant is currently operated with moderate chemical treatment just to stabilize water before mixing with desalinated sea water from Jubail. Annual chemical costs at this plant alone
3302
Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
is SR 7.04 million. Stability calculations made on raw water to this plant indicate that simple pHadjustment using sulfuric acid could produce a stable water. It may not be necessary to fully operate this plant in the mean time. . The work presented in this paper can be further expanded to include other chemical combinations which might lead to more economical gains. Combining lime and caustic soda may lead to such goals.
AcknowledgementsÐThe work reported has been ®nancially supported by King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia under grant No. AR-13-70 which the authors gratefully appreciate. The authors also appreciate the cooperation and assistance of the Riyadh Water and Sewage Authority personnel. REFERENCES
Albraithen I. I. (1993) Municipal water demand management for Riyadh city, Saudi Arabia. Ph.D dissertation, Colorado State University. Alabdula'aly A., Al-Rehaili A. and Al-Mutaz I. (1995) Performance evaluation of water treatment plants in Riyadh using pilot plant studies. In Proceedings, Fourth Saudi Engineering Conference, Jeddah, Saudi Arabia (in Arabic).
Alabdula'aly A. and Al-Rehaili A. (1995) Pilot plant evaluation of chemical and physical water treatment processes in Riyadh, Saudi Arabia. In Proceedings, Second Regional Conference on Save the Environment. American Society of Civil Engineers, Beirut, Lebanon. Al-Rehaili A. and Alabdula'aly A. (1996) Evaluation of chemical treatment at Riyadh water treatment plants. Final Report, King Abdulaziz City for Science and Technology Grant No. AR-13-70, Riyadh, Saudi Arabia. Al-Rehaili A., Alabdula'aly A. and Al-Mutaz I. (1995) Evaluation of the chemical treatment of Riyadh drinking water treatment plants. In Proceedings, Fourth Saudi Engineering Conference, Jeddah, Saudi Arabia (in Arabic). APHA, AWWA, WPCF (1985) Standard Methods for the Examination of Water and Wastewater, 16th edn. Ghulaigah A. and Ericsson B. (1979) Riyadh reverse osmosis water treatment plants: the largest demineralization complex in the world. Desalination 30, 301±314. Montgomery J. M. Consulting Engineers (1985) Water Treatment Principles and Design. John Wiley & Sons, New York. Potts D. E., Alhert R. C. and Wang S. S. (1981) A critical review of fouling of reverse osmosis membranes. Desalination 36, 235±264. Sanks R. L. (1978) Water Treatment Plant Design. Ann Arbor Science, An Arbor, MI. Stumm W. and Morgan J. J. (1981) Aquatic Chemistry, 2nd edn. John Wiley & Sons, New York. Wojcik C. K. (1983) Desalination of water in Saudi Arabia by reverse osmosis, performance study. Desalination 46, 17±34.
Wat. Res. Vol. 33, No. 15, pp. 3291±3302, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
www.elsevier.com/locate/watres
CHEMICAL AND ECONOMICAL EVALUATION OF GROUNDWATER TREATMENT PLANTS IN RIYADH ABDULLAH M. AL-REHAILI1* and ABDULRAHMAN I. ALABDULA'ALY2{ 1
College of Engineering, King Saud University, P.O. Box 800, Riyadh, 11421, Saudi Arabia and 2King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh, 11442, Saudi Arabia (First received 1 April 1998; accepted in revised form 1 January 1999)
AbstractÐThe capital of Saudi Arabia, Riyadh is supplied with drinking water from both treated groundwater and desalinated sea water. Groundwater contributes about 35% to the total water supply and is treated in six treatment plants containing cooling, softening, ®ltration, reverse osmosis and posttreatment processes. An extensive two and a half years study aimed at the evaluation of Riyadh water treatment plants performance and the investigation of chemical treatment alternatives has been undertaken. The work involved an evaluation of the plants chemical treatment in which the eectiveness of softening chemicals was investigated by conducting jar test, pilot plant and full-scale implementation experiments. The results indicate that by optimizing the currently used chemicals it is possible to save 19% in chemicals cost or approximately million SR 3.71 a year. Caustic soda can be a competitive alternative in four plants replacing lime, soda ash and sodium aluminate. Moreover, there is a great potential of saving million SR 7.04 a year at the largest plant by simple pH-adjustment using sulfuric acid for the purpose of water stabilization. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐalternative chemicals, RO pretreatment, lime±soda softening, caustic soda, polymers, pilot to full-scale implementation
INTRODUCTION
The capital of Saudi Arabia, Riyadh, with a population of nearly 3.0 million had consumed a daily average of 1.135 106 m3 of water in 1994. Prior to 1983, water for the city was supplied from groundwater sources. Since then, desalinated seawater from Al-Jubail on the Arabian Gulf became the major source of drinking water for the city. At the present, desalinated seawater constitutes approximately 70% of Riyadh water supply. The remainder is provided by treated groundwater mainly from deep wells in the vicinity of the city. There are a total of 161 (25 shallow and 136 deep) wells that supply water to the six treatment plants (Fig. 1). The shallow wells are located in Nesah, Nemar and Al Haair valleys. Nesah well water is considered of high quality with TDS content of about 400 mg/l. The deep wells are tapping the Minjur and Wasia aquifers. Water in the former aquifer is characterized by high TDS content (1000±1500 mg/l), high temperature (50±558C), high in total hardness, iron and hydrogen sul®de. Wasia aquifer water is low in temperature (about 348C) and relatively high in TDS content. The shallow *Author to whom correspondence should be addressed. [Tel.: +966-1-467-6927; fax: 966-1-467-7008]. {Tel.: +966-1-481-3300; fax: +966-1-481-3878.
wells are drilled to a depth of 60±270 m, whereas the deep wells are drilled to a depth of 1200± 2000 m (Ghulaigah and Ericsson, 1979; Wojcik, 1983; Albraithen, 1993). In 1989, the shallow wells produced a total of 19.3 Mm3 of water compared to 101 Mm3 for the deep wells. Five out of the six treatment plants process groundwater through cooling, chemical precipitation, ®ltration, reverse osmosis, chlorination and water stabilization. The sixth treatment plant (Wasia) contains all of the above processes except reverse osmosis due to the blending of its product water with the desalinated sea water. The chemical precipitation is directed mainly to hardness removal through lime±soda softening and silica reduction using sodium aluminate with the addition of polymer or ferric chloride. Silica reduction is considered an important pretreatment process in these plants for prevention of silica scale formation within the reverse osmosis modules. Studies have indicated that the solubility limit for silica in natural waters at 258C is only 120 mg/l (Sanks, 1978; Stumm and Morgan, 1981; Montgomery, 1985). It is also indicated that silica concentration of 10 mg/l (Si) in reverse osmosis feed water is not harmful at a water recovery of less than 98% (Potts et al., 1981). Reduction of silica concentration in Riyadh plants to below 18 mg/l is recommended for membrane protection and process eciency.
3291
3292
Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Fig. 1. Locations of Riyadh water treatment plants and well ®elds.
In 1993, King Abdulaziz City for Science and Technology (KACST) funded a research project aimed at the evaluation of Riyadh water treatment plants performance and the investigation of chemical treatment alternatives. The work involved an evaluation of chemical treatment in the six drinking water treatment plants located in Riyadh (AlRehaili and Alabdula'aly, 1996). Jar tests and pilot plant experimentation were designed in order to
evaluate the eectiveness of calcium hydroxide (lime), soda ash, sodium hydroxide (caustic soda), aluminum sulfate (alum), sodium aluminate, ferric chloride and polymer for the purpose of reducing hardness and silica from groundwater. Caustic soda, with and without polymer, was investigated in the study as an alternative to lime±soda softening for its eectiveness in hardness and silica removal. Alum, sodium aluminate, ferric chloride and poly-
Chemical and economical evaluation of groundwater treatment plants in Riyadh
mer were tested as precipitation aids in the lime± soda process. Based on the experimental work, an evaluation of the eectiveness of the chemicals on an actual full-size plant was implemented followed by an economic analysis for all of the six plants. The objective of this paper is to present information on the Riyadh groundwater treatment plants, major experimental work (jar and pilot) ®ndings, results of full-scale implementation and economic study evaluation. RIYADH WATER TREATMENT PLANTS
Groundwater is treated in six water treatment plants located within and around the city. Plants located within the city, Manfouha I and II, Malaz and Shemessy were commissioned in 1969 with the original chemical treatment±®ltration schemes. In 1979, reverse osmosis desalination steps were operated in these plants. Treated waters from Manfouha I and II and Shemessy plants is blended with waters from shallow aquifers before discharge to the water network. Salbukh plant is located 40 km north of the city, it started operation in 1978. Buwaib plant is located 70 km north±east, it started operation in 1969. Wasia plant, commissioned in 1982, is the largest of all and located 110 km east of Riyadh. This plant does not have reverse osmosis desalination. After chemical pretreatment, water from this plant is blended with desalinated sea water coming from Al-Jubail desalination plant located 400 km to the east on the Arabian Gulf. Before 1983, Riyadh was totally dependent on groundwater for all its water
3293
needs. Currently, however, desalinated sea water constitutes approximately 70% of water supply to the city. Fig. 2 shows the progressive increase in Riyadh water supply since 1980 and the contribution of the dierent sources. The main focus of chemical treatment at Riyadh plants is reduction of hardness and silica from groundwater using lime±soda ash and precipitation aids such as sodium aluminate, ferric chloride and polymer. Treatment schemes of all plants are similar to a great extent, with dierences in the number of units in each process (Fig. 3). Treatment starts by passing feed water to aeration cooling towers to reduce water temperature to 30±358C, oxidize iron and manganese, and remove CO2 and H2S. Part of the cooled water (23±27%) goes directly to ®lters, the remainder passes through contact clari®ers where chemical addition with rapid mix, slow mix and settling occurs, followed by ®ltration and blending with the ®rst stream. Filtration takes place in a number of ®lter cells, each consists of two ®lters in series (all plants except Wasia). The ®rst is an up-¯ow coarse media ®lter, the second is a down-¯ow ®ne media ®lter. Filters of Wasia plant are dual media with sand and anthracite. Part of the ®ltered water goes to reverse osmosis desalination for TDS reduction, followed by stabilization, blending and chlorination (all plants except Wasia). Table 1 illustrates design capacities, productivities and chemical dosages, consumption and costs for Riyadh treatment plants. Table 2 shows yearly average raw and treated water characteristics at Riyadh treatment plants in 1993.
Fig. 2. Development of Riyadh daily average water consumption.
Fig. 3. Typical water treatment processes at Riyadh water treatment plants.
3294 Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Chemical and economical evaluation of groundwater treatment plants in Riyadh
3295
Table 1. Summary of Riyadh treatment plants productivities, chemical dosages, consumption and costs (1993). US$1=SR 3.75 Plant
Wasia Manfouha I and II Buwaib Salboukh Shemessy Malaz
Average Maximum productivity production in 1993 (m3/day) (m3/day)
221000 86400 66000 66720 57600 28800
191240 53704 27452 36676 40788 14427
Average dosing rate (mg/l)
lime
soda ash
100 110 110 125 110 130
70 325 350 210 220 240
526520 364287 Total (m3/day) Chemical consumption (ton/year) 14234 22114 Chemical cost S.R. (millions/year) 6.53 16.10 Total chemical cost, all plants S.R. (million/year) 32.69
STUDY METHODOLOGY
Two treatment plants were selected for conducting the experimental work on their raw water. Selection was based on raw water quality in terms of total hardness and silica content. Shemessy plant raw water contains the lowest concentration of both total hardness and silica among all plants (535 mg/l as CaCO3 and 23.3 mg/l, respectively), whereas Buwaib plant raw water contains the highest concentration of the two parameters (851 mg/l as CaCO3 and 34.8 mg/l). A total of 52 jar test experiments were carried out in the laboratory using cooled water from both Shemaisy and Buwaib plants. One or more chemicals were tested that represent the chemicals used in the plants in addition to alternative chemicals. Evaluation of chemicals eect on hardness and silica was carried out starting with one chemical and progressing to more chemicals with variation in dosages. The chemicals tested include lime, soda ash, caustic soda, sodium aluminate, alum, ferric chloride and four polymers (anionic super¯oc A100, polycationic Nalcolyte 8100, anionic POL-E-Z 692 and nonionic super¯oc N 100). Based on the jar test results, a number of chemicals were selected for the pilot plant experimentation. Two pilot plants, representing actual full-size operations, were designed, constructed and installed at Shemaisy and Buwaib treatment plants. Each pilot plant consists of chemical precipitation process, ®ltration and required pumps, chemical tanks and backwashing unit (Fig. 4). The precipitation unit consists of a precipitator, chemical tanks and dosing pumps. The precipitator is cylindrical in shape, tapered at the bottom, 1 m in diameter and 2.5 m in height. It provides a detention time of 1.5 h when using 11.4 lpm water ¯ow rate. Rapid mixing chamber is housed in the upper part of the precipitator which is made of a pipe 10 cm in diameter and 1 m long closed at the bottom and equipped
sodium polymer ferric chlorine sulfuric sodium aluminate chloride acid hexametaphosphate ÿ 10 25 25 9 12
978 4.73
0.25 0.06 ÿ 0.20 0.10 0.20
ÿ ÿ 15 ÿ ÿ ÿ
24 0.35
150 0.5
1.70 0.94 0.50 0.85 0.80 1.00
171 0.43
25 50 30 21 35 30
ÿ 5 6 5 4 7
3986 2.67
322 1.38
with a mixer. The slow mixing chamber is located around the rapid mixing chamber and is made of a pipe 30 cm in diameter and 1.70 m long, open at both ends. The clear softened water is collected in a channel around the upper part of the precipitator. The precipitator is equipped with an automatic sludge withdrawal system installed at its lower end. There are two types of ®lters that are used in the pilot plant which simulate the actual ®lters in the Riyadh water treatment plants. The ®rst type consists of up-¯ow and down-¯ow sections similar to Shemaisy, Manfouha, Salbukh, Buwaib and Malaz ®lters. The up-¯ow section contains coarse sand with sizes 3±5 mm and a depth of 2.38 m, whereas the down-¯ow section contains a 15 cm layer of coarse sand (3±5 mm) and a 90 cm layer of ®ne sand (0.8±1.2 mm). Both sections has a 10 cm support layer of gravel with sizes 10±18 mm. The other type of ®lters consist only of a down-¯ow section which is similar to that of Wasia water treatment plant. The ®lter media consists of a 10-cm support layer of gravel, 35 cm of coarse sand and gravel (1.2±12.7 mm), 50 cm of ®ne sand (0.4 mm) and 30 cm of anthracite (1.0 mm). The ®ltering media is placed in a clear plastic column 15 cm in diameter and 3 m in height. The ®lters are equipped with ¯ow meters and piezometers located at dierent depths for the purpose of measuring head loss. The backwash system consists of two pumps, one for air and the other for water. Backwashing is accomplished by pumping air and water (separately or combined) through the bottom of the ®lters. The product water of the pilot plant is collected in a 3m3 tank. Sampling valves are located at dierent locations within the plant. The two pilot plants are supplied with cooled raw water and the ¯ow rate through the ®lters was set at values similar to those used in actual plants, i.e. 256 and 69 lpm/m2 for the up- and down-¯ow sections of the ®lters, respectively and 104 lpm/m2 for ®lters with one section only.
7.30 7.30 a 0.91 169 220 264 70 125 108 99 95 156 42 96 34 704 550 1134 a a 856 a a 14.38 152 88 253 258 178 474 0.032 0.058 0.036 a
Not available.
a
7.22
pH 8.24 7.96 7.93 8.10 8.03 7.96 8.00 7.76 7.08 6.87 6.60 6.65 7.00 7.30 7.86 7.40 7.33 7.80 a a a a a a a a a a a a a a a a 1.03 1.0 Turbidity (NTU) Total hardness 547 733 714 851 672 535 601 473 284 276 420 343 212 265 506 116 130 337 Calcium hardness 373 486 488 514 425 362 404 305 132 91 194 146 85 109 349 58 68 133 Magn. hardness 174 247 226 337 247 173 197 168 152 185 226 197 177 156 157 58 62 204 Alkalinity 118 159 160 140 146 156 156 58 36 34 21 26 31 36 84 60 61 43 a a a a a a TDS 1120 1509 1468 1583 1369 1063 1184 1113 1093 483 508 1507 a a a a a a a a a a a a a a 3379 1628 1725 2267 Conductivity (mS/cm) a a a a a a Silica 16.8 19.7 21 34.8 28.5 23.3 24 10.3 13.7 15.8 14.6 9.9 a a a a a a Chlorides 241 300 295 378 295 191 256 191 245 100 76 489 a a a a a a a a a a a 486 407 457 437 373 450 Sulfates 360 a a a a a a a a a a a Total iron 0.16 0.48 0.25 0.04 0.09 0.016 0.004
4 4 4 1
2
3
Raw water
5
6
7
1
2
3
After ®ltration
5
6
7
1
2
3
Product water
5
6
7
Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Constituents
Table 2. Average yearly characteristics of raw and treated water for Riyadh treatment plants (1993). Units are in mg/l except if otherwise stated, hardness and alkalinity in mg/l as CaCO3. 1 Al wasia; 2 Manfouha-I; 3 Manfouha-II; 4 Buwaib; 5 Salbukh; 6 Shemessy; 7 Malaz
3296
A total of 45 experimental runs were conducted in groups based on the used chemicals. Each experimental run lasted 48 h and samples were collected every 6 h from six locations within the pilot plant. Parameters such as temperature, pH, turbidity and conductivity were measured in the ®eld during sample collection. Total hardness, calcium, magnesium, alkalinity and silica were determined in the laboratory according to Standard Methods (APHA, AWWA, WPCF, 1985). Detailed results of jat test and the two pilot plant experiments have been published elsewhere (Al-Rehaili et al., 1995; Alabdula'aly et al., 1995; Alabdula'aly and AlRehaili, 1995; Al-Rehaili and Alabdula'aly, 1996). The following sections summarize the major results. Jar test results indicate that lime and soda ash cause noticeable reduction in hardness with no signi®cant removal of silica. With lime dosage of 110 mg/l, hardness was removed by 25.9 and 13.8% for Shemessy and Buwaib cooled water, respectively. The use of 400 mg/l of soda ash with a ®xed dosage of lime (120, 110 mg/l) resulted in hardness removal of 63.7 and 52.7% for the two plants cooled water, respectively. The maximum silica removal was 15.6% when using 120 mg/l of lime and 400 mg/l of soda ash for Shemessy cooled water and 14.6% when using 110 mg/l of lime and 375 mg/l of soda ash for Buwaib cooled water. The dosing of precipitation aids (aluminum sulfate, ferric chloride and sodium aluminate) with ®xed dosages of lime and soda ash did not cause an increase in hardness removal but a signi®cant silica reduction. Sodium aluminate was the most eective among the three chemicals in silica reduction, resulting in 42.4 and 34.8% removal with a dosage of 25 mg/l for Shemessy and Buwaib cooled water, respectively. The addition of aluminum sulfate or sodium aluminate gave the same silica removal based on aluminum concentration in both chemicals. All tested polymers when added to ®xed dosages of lime and soda ash did not cause an improvement in hardness removal. With A-100 polymer, a slight increase in silica removal was observed. A dosage of 0.20 mg/l of the polymer to Shemessy cooled water with 110 mg/l of lime and 220 mg/l of soda ash reduced the silica content to 19.2 mg/l compared to 22.5 mg/l with lime and soda ash only. When the other three polymers were added to ®xed dosages of lime and soda ash, there was insigni®cant reduction in silica removal. Comparison of the eectiveness of polymers with other precipitation aids revealed that sodium aluminate and aluminum sulfate give better silica removal when added to ®xed dosages of lime and soda ash. Dosing of 0.2 mg/l of A-100 polymer and ®xed dosages of lime, soda ash and sodium aluminate resulted in an improvement of silica removal for Shemessy cooled water. Dosing of other polymers resulted in negative impact on silica removal. When polymers are
Chemical and economical evaluation of groundwater treatment plants in Riyadh
added with ®xed dosages of lime, soda ash and aluminum sulfate to Shemessy cooled water, both 692 and N-100 polymers caused negative impact on hardness removal, whereas the 8100 polymer showed slight improvement in hardness removal. For Silica removal, only 8100 polymer showed slight improvement, with 692 and N-100 polymers showing negative impact. Substituting ferric chloride for polymer as the case in Buwaib plant caused negative impact on both hardness and silica removal. The use of caustic soda alone resulted in both hardness and silica removal. With 180 mg/l of caustic soda, it was possible to reduce the hardness and silica content of Shemessy cooled water to 260 mg/l as CaCO3 and 10.8 mg/l, respectively. For Buwaib cooled water, a dosage of 200 mg/l of caustic soda resulted in 40 and 65% hardness and silica removal, respectively, bringing their concentration to 534 mg/ l as CaCO3 and 10 mg/l. The pilot plant experiments results agreed to a great extent with those obtained in the jar tests. The use of lime and soda ash resulted in noticeable hardness reduction and insigni®cant silica reduction. The best combination of lime/soda ash dosages were 110/220 mg/l and 90/300 mg/l for Shemessy and Buwaib cooled water, respectively. Addition of sodium aluminate resulted in a slight
3297
improvement in hardness removal and a noticeable reduction in silica level. Silica removal of 17% was obtained for Shemessy plant using 10 mg/l sodium aluminate and 44.8% for Buwaib plant using 25 mg/l. Dosing of super¯oc A-100 anion polymer in the range of 0.05±0.2 mg/l in addition to lime, soda ash and sodium aluminate did not improve hardness and silica removals but an improvement of softened water turbidity was noticed. The use of ferric chloride in place of polymer in the range of 10±25 mg/l has resulted in an improvement of the hardness and silica removal in Buwaib cooled water. However, it was noticed that the use of ferric chloride increased ®ltered water turbidity. The use of aluminum sulfate in place of sodium aluminate with consideration that one unit of the latter contains twice the amount of aluminum as in the former, it was observed that better removal was obtained when using sodium aluminate than when using alum. The dosing of caustic soda alone has resulted in a noticeable improvement in the hardness and silica removals. It was possible to reduce the Shemessy cooled water hardness and silica by 50 and 34%, respectively, when using 175 mg/l of caustic soda. For Buwaib cooled water, the use of 200 mg/l caustic soda has resulted in 48.7 and 83.6% reduction of hardness and silica, respectively.
Fig. 4. Pilot plant details.
3298
Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Comparing the performance of the two types of ®lters, it was observed that insigni®cant dierences in water quality exist. The only dierence was in terms of turbidity level, in which the ®lters with two sections (up- and down-¯ow) gave waters with lower turbidity levels as compared to ®lters with down-¯ow section only (sand and anthracite). FULL-SCALE IMPLEMENTATION
Results obtained from the pilot plant experiments were used as a basis for full-scale veri®cation on Malaz water treatment plant. This plant was selected because it is the smallest of all and its raw water characteristics lies between the two raw water extremes (Shemessy and Buwaib plants). An experimental program was designed carefully to cover both normal plant operation (with chemicals and dosages as currently used in the plant) and operation with speci®c modi®cations of chemicals and dosages. A total of 14 experiments, each lasting 72 h, were conducted at Malaz plant. During the experimental period, the plant was working with only one precipitator at approximately 400 m3/hr ¯ow rate. Water samples were collected every 2 h (8 am to 6 pm) from three points, namely cooled raw water, after precipitator and after ®lters. Measurements of pH, turbidity and temperature were conducted on site at the time of collection;
other parameters (total hardness, Ca, Mg, silica, conductivity, iron, alkalinity and sodium) were conducted in the laboratory. Dosages of chemicals were calculated by noting water ¯ow rates, and dosing pump details. Sludge leaving the precipitator was sampled regularly and tested for total solids and a 30-min settling test was conducted using 1litre Imho Cone for each sample to determine sludge volume. Despite the fact that some obstacles were encountered during actual testing, the eects of chemical combinations and dosages on hardness and silica reduction at Malaz plant were reasonably established. The most important obstacles were the instability of the lime and soda ash dosing systems and some diculties in the sludge discharge system. Fig. 5 presents a comparison of the eects of dierent chemical treatment combinations on reduction of total hardness, calcium and magnesium at Malaz plant. Experiment No. 2 shows that omitting polymer addition resulted in slight improvement in hardness removal compared to experiment No. 1 which applies chemicals and dosages as normally practiced at Malaz plant. Decreasing sodium aluminate dosage by 50% (experiment No. 3) resulted also in slight improvement of hardness removal. Experiments No. 4±7 indicate that there is a possibility to reduce dosages of all or some of the chemicals used in the plant while keeping total
Fig. 5. Average total hardness, calcium and magnesium levels in Malaz ®ltered water with dierent chemical treatment combinations at Malaz plant.
Chemical and economical evaluation of groundwater treatment plants in Riyadh
3299
Fig. 6. Average ®ltered water silica concentrations with dierent chemical treatment combinations at Malaz plant.
hardness in ®ltered water within normal performance of the plant. Using caustic soda alone in chemical precipitation (experiments No. 8±11) in the dosages 130, 190 and 250 mg/l resulted in hardness removals as good as or better than obtained by chemicals and dosages normally used at the plant. A dosage of 250 mg/l caustic soda reduced total hardness after ®lters to 302 mg/l, compared to 431 mg/l, using normal plant dosing (98 mg/l lime, 200 mg/l soda ash, 13 mg/l sodium aluminate, and 0.4 mg/l polymer). Adding increasing dosages of polymer with a ®xed dosage of caustic soda (experiments No. 12±14) reduced ®ltered water hardness with all used polymer dosages to approximately 284 mg/l, compared to 400 mg/l when polymer was not added. Figure 5 illustrates that reducing dosages of chemicals currently used at the plant has to some extent similar eects on calcium and magnesium hardness as noticed for total hardness. Using caustic soda alone reduced calcium hardness to the same level obtained with normal plant dosing. Magnesium hardness, however, was reduced to 102 mg/l with the highest caustic soda dosage applied (250 mg/l), compared to 217 mg/l in ®ltered water using normal plant dosing. Adding polymer with caustic soda was more eective for improving magnesium removal than for calcium. It was
noticed that the lowest dosage of polymer used (0.09 mg/l) was more eective than other dosages. Fig. 6 shows that the eects of reducing lime and soda ash dosages to the limits practiced in the experiments had little eect on ®ltered water silica concentration. However, reducing sodium aluminate dosage by 50% (experiment No. 3) resulted in increased ®ltered water silica to 17.6 mg/l, compared to 14.3 mg/l with normal plant dosing. Using 190 mg/l of caustic soda alone reduced ®ltered water silica level to 11.5 mg/l. Adding polymer with caustic soda resulted in excellent reduction of silica in ®ltered water, reaching 7 mg/l when using 195 mg/l caustic soda and 0.09 mg/l polymer. Table 3 illustrates the eects of dierent chemical treatment combinations on pH, alkalinity, turbidity, and sodium levels at Malaz plant. High pH levels are extremely important for chemical precipitation of hardness ions. Table 3 shows that addition of lime and soda ash at normal dosing increased pH to about 8.2 during precipitation, while using caustic soda at 250 mg/l increased pH to 10.2 which gives it higher eectiveness for hardness and silica removal, most notably for magnesium removal (as seen in Fig. 5). It should be made clear that increasing lime dosages to higher values than used here can improve hardness removal by raising pH, but the objective of this comparative study is to treat
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Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
Table 3. Average pH, alkalinity, turbidity and sodium levels using dierent chemical treatment combinations at Malaz plant. C is after cooling, P after precipitation and F after ®ltration No.
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Chemical dosage (mg/l)
pH
Alkalinity (mg/l)
Turbidity (NTU)
Sodium (mg/l)
Lime
soda ash
sodium aluminate
polymer
caustic soda
C
P
F
C
P
F
C
P
F
C
P
F
98 100 69 63 79 70 80 55 ÿ ÿ ÿ ÿ ÿ ÿ
201 185 202 175 179 167 131 144 ÿ ÿ ÿ ÿ ÿ ÿ
12.8 12.2 6.4 13.2 12.8 12.6 13.0 9.8 ÿ ÿ ÿ ÿ ÿ ÿ
0.41 ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ 0.09 0.20 0.39
ÿ ÿ ÿ ÿ ÿ ÿ ÿ ÿ 130 190 250 195 198 190
8.06 7.76 8.00 7.97 8.07 8.02 8.04 8.06 8.08 8.04 8.04 7.89 7.89 7.99
8.22 8.83 8.26 8.15 8.16 8.13 8.29 8.11 8.85 9.54 10.22 9.82 9.82 9.88
6.81 6.97 6.79 6.69 6.90 8.89 6.84 7.01 6.86 7.32 7.11 6.89 6.50 6.44
140 156 144 147 148 147 145 152 144 144 147 159 156 156
97 21 70 75 87 81 71 91 31 22 33 31 30 29
68 27 54 53 66 64 52 68 27 26 21 16 15 14
2.1 2.6 1.6 4.4 3.5 4.2 2.6 2.7 2.9 2.3 2.1 2.6 3.1 3.6
1.8 35 40 31 25 23 34 38 17 32 52 10 5 2.4
0.35 0.76 0.15 0.28 0.31 0.77 0.54 0.34 0.46 0.46 0.45 0.47 0.53 1.02
208 198 210 206 211 209 207 210 199 196 190 213 209 193
293 285 304 299 298 280 298 289 279 300 347 355 249 321
291 283 310 305 297 286 307 294 291 304 346 358 349 318
water to the levels currently practiced and being acceptable at the plants. Alkalinity levels after precipitation dictate the amount of acid required for pH-adjustment before ®ltration. It is clear that treatment with caustic soda resulted in lower alkalinity after precipitation, but at the same time higher pH levels. Experiments have shown that almost the same amount of sulfuric acid was required to adjust pH before ®lters to about 6.8 in case of lime±soda ash treatment and in case of caustic soda treatment. All experiments conducted without polymer addition resulted in high turbidity after precipitation (Table 3). This was also obvious by the formation of a clear white layer of precipitate on the ®lter top during testing. It is important to note that polymer addition is important for proper operation with all of the tested chemical combinations. Table 3 shows that using lime±soda ash treatment increased sodium level in ®ltered water to an average of 280 mg/l, compared to the cooled water sodium content of about 200 mg/l. Sodium sources are sodium aluminate and soda ash in this case. The use of caustic soda, which also contains sodium, increased average sodium concentration in ®ltered water to 346 mg/l at the highest dosage used (250 mg/l). Addition of polymer with caustic soda may slightly increase sodium release in ®ltered water due to improved hardness removal and for-
mation of more of the soluble chlorides and sulfates of sodium. Table 4 shows a comparison of sludge characteristics with dierent treatment combinations. Generally speaking, sludge volume in case of lime± soda ash treatment is higher than that for caustic soda treatment. In addition, use of polymer resulted in noticeable increase of sludge volume, indicating improved precipitation and treatment eciency. This eect was more pronounced with caustic soda. Increased total solids concentration of sludge was also realized, indicating increased sludge volume and density with the use of polymer. Based on the results of actual testing at Malaz plant, two chemical treatment combinations were selected as optimum, namely: lime (63 mg/l), soda ash (175 mg/l), sodium aluminate (12 mg/l) and polymer (0.2 mg/l); or caustic soda (190 mg/l) and polymer (0.2 mg/l). ECONOMICS
Based on the experimental (pilot and full-scale) results, optimum chemical dosages were proposed to be used in Riyadh water treatment plants (except Wasia). The use of such dosages is expected to produce acceptable water quality at the lowest cost either using the presently used chemicals or using caustic soda as an alternative. The proposed dosages and the expected costs are presented in
Table 4. Comparison of sludge volumes with dierent chemical treatment combinations at Malaz plant Chemical dosage (mg/l) lime 98 98 ÿ ÿ ÿ ÿ
Sludge volume (ml/l)
soda ash
sodium aluminate
polymer
caustic soda
201 201 ÿ ÿ ÿ ÿ
12.8 12.8 ÿ ÿ ÿ ÿ
ÿ 0.2 ÿ 0.09 0.2 0.39
ÿ ÿ 190 195 198 190
325 350 170 323 328 337
Chemical and economical evaluation of groundwater treatment plants in Riyadh
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Table 5. Cost of chemicals in Riyadh water treatment plants using optimum dosages of chemicals currently used and alternatives Chemical
Chemical cost (SR/ton) (1996)
Malaz, Shemessy, Salbukh and Manfouha currently used chemicals
alternative chemicals
Buwaib currently used chemicals
proposed cost proposed cost proposed cost dosage (mg/l) (SR/1000 m3) dosage (mg/l) (SR/1000 m3) dosage (mg/l) (SR/1000 m3) Lime Soda ash Sodium aluminate Polymer Caustic soda Unit cost (SR/1000 m3) Plants production (106 m3/yr) Total cost (SR M/yr)
459 727 1950 14418 730
70 175 12 0.2 ÿ
Table 5. For Buwaib plant, caustic soda was not recommended to be used due to the fact that high dosages are required in order to reduce the calcium hardness to levels below the RO membrane requirements. Wasia plant was not included in Table 5, since the present chemical treatment is not intended for hardness and silica removal, but rather for water stabilization to protect water transmission line. It is clear from Table 5 that the cost of soda ash is the highest among chemicals currently used, followed by sodium aluminate. It is also clear that using the proposed optimum dosages will lead to an annual chemicals cost of SR 15.73 million, compared to the present cost of SR 19.44 million, i.e. 19% cost reduction. The use of caustic soda and polymer in Malaz, Shemessy, Salbukh and Manfouha plants is expected to result in a chemical cost of SR 15.35 million, compared to SR. 11.83 million in the case of using the optimum proposed dosages of the chemicals currently used at the plants. This means that there is no economic incentive to use caustic soda based only on chemical costs. However, when considering the possible savings on operation and maintenance, there might be some justi®cation to use caustic soda and polymer in place of the currently applied chemicals. This is especially true if caustic soda can be obtained at a lower cost (560 SR/ton) as the Riyadh Water Authority had indicated in their correspondence with the authors.
32.10 127.20 58.00 2.90 ÿ 220.20 53.7 11.83
.
.
.
CONCLUSIONS
This extensive study on Riyadh water treatment plants led to many conclusions and recommendations which covered optimization of chemical types, dosages and costs in addition to operation and maintenance capabilities and requirements. Following are major conclusions regarding chemical treatment at the plants based on jar testing, pilot plant testing and full-scale implementation. . Jar tests and pilot plant evaluations were very helpful in comparing chemicals eciencies in
.
ÿ ÿ ÿ 0.20 190
ÿ ÿ ÿ 2.90 283.00 285.90 53.7 15.35
90 300 25 0.1 ÿ
41.30 218.10 120.90 1.40 ÿ 381.70 10.2 3.90
many experimental combinations which resulted in proper selection of chemicals and to some extent narrowed down the range of eective dosages and treatment conditions. The pilot plant used in this study was not totally representative of full-scale performance in the precipitation step, especially for hardness removal by lime± soda softening. The pilot plant was less ecient than the full-scale plant in this regard. This probably re¯ects the eect of the shorter depth of settling in the pilot precipitator compared to the very high depth in full-scale precipitators. Results of jar and pilot plant testing revealed that the use of polymers as precipitation aids with lime and soda ash produced little improvement in hardness and silica removal, they even had negative eects at some dosages. The anionic polymer used at the plants had positive eects on the precipitation process and sludge characteristics which reduces solids loading to the sand ®lters. Optimized chemical dosages obtained indicate that there is a possibility of reducing chemical dosing rates at the plants using the chemicals currently applied at Malaz, Shemessy, Manfouha, Salbukh and Buwaib (especially lime, soda ash and polymer dosages). This will lead to a total saving in chemical cost of 19%, or approximately SR 3.71 million annually. It was realized through this study that caustic soda can be a competitive alternative for all plants except Buwaib. It can replace lime, soda ash, and sodium aluminate, and can economically (if supplied at SR 560/ton) produce ®ltered waters of the required quality or better. The caustic soda dosing system will be much simpler and less maintenance demanding compared to the currently used lime±soda ash system which is too involved and plagued by the instability of dosing due to frequent plugging of feed lines. There is a great potential of saving at Wasia plant. The plant is currently operated with moderate chemical treatment just to stabilize water before mixing with desalinated sea water from Jubail. Annual chemical costs at this plant alone
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Abdullah M. Al-Rehaili and Abdulrahman I. Alabdula'aly
is SR 7.04 million. Stability calculations made on raw water to this plant indicate that simple pHadjustment using sulfuric acid could produce a stable water. It may not be necessary to fully operate this plant in the mean time. . The work presented in this paper can be further expanded to include other chemical combinations which might lead to more economical gains. Combining lime and caustic soda may lead to such goals.
AcknowledgementsÐThe work reported has been ®nancially supported by King Abdulaziz City for Science and Technology (KACST), Riyadh, Saudi Arabia under grant No. AR-13-70 which the authors gratefully appreciate. The authors also appreciate the cooperation and assistance of the Riyadh Water and Sewage Authority personnel. REFERENCES
Albraithen I. I. (1993) Municipal water demand management for Riyadh city, Saudi Arabia. Ph.D dissertation, Colorado State University. Alabdula'aly A., Al-Rehaili A. and Al-Mutaz I. (1995) Performance evaluation of water treatment plants in Riyadh using pilot plant studies. In Proceedings, Fourth Saudi Engineering Conference, Jeddah, Saudi Arabia (in Arabic).
Alabdula'aly A. and Al-Rehaili A. (1995) Pilot plant evaluation of chemical and physical water treatment processes in Riyadh, Saudi Arabia. In Proceedings, Second Regional Conference on Save the Environment. American Society of Civil Engineers, Beirut, Lebanon. Al-Rehaili A. and Alabdula'aly A. (1996) Evaluation of chemical treatment at Riyadh water treatment plants. Final Report, King Abdulaziz City for Science and Technology Grant No. AR-13-70, Riyadh, Saudi Arabia. Al-Rehaili A., Alabdula'aly A. and Al-Mutaz I. (1995) Evaluation of the chemical treatment of Riyadh drinking water treatment plants. In Proceedings, Fourth Saudi Engineering Conference, Jeddah, Saudi Arabia (in Arabic). APHA, AWWA, WPCF (1985) Standard Methods for the Examination of Water and Wastewater, 16th edn. Ghulaigah A. and Ericsson B. (1979) Riyadh reverse osmosis water treatment plants: the largest demineralization complex in the world. Desalination 30, 301±314. Montgomery J. M. Consulting Engineers (1985) Water Treatment Principles and Design. John Wiley & Sons, New York. Potts D. E., Alhert R. C. and Wang S. S. (1981) A critical review of fouling of reverse osmosis membranes. Desalination 36, 235±264. Sanks R. L. (1978) Water Treatment Plant Design. Ann Arbor Science, An Arbor, MI. Stumm W. and Morgan J. J. (1981) Aquatic Chemistry, 2nd edn. John Wiley & Sons, New York. Wojcik C. K. (1983) Desalination of water in Saudi Arabia by reverse osmosis, performance study. Desalination 46, 17±34.