- Email: [email protected]
Pretreatment of the municipal wastewater feed for reverse osmosis plants
DESALINATION
J(W ELSEVIER
Desalination 109 (1997) 211-223
Pretreatment of the municipal wastewater feed for reverse osmosis plants M. Abdel-Jawad*, S. Ebrahim, F. A1-Atram, S. A1-Shammari Water Resources Division~Water Desalination Department Kuwait Institute for Scientific Research, PO Box 24885, Safat 13109, Kuwait Tel. +965 4879237; Fax +965 4879238
Received November 96; accepted 24 December 1996
Abstract
Potable water can be produced at a reasonable cost if reverse osmosis (RO) technology can be applied to renovate secondary/tertiary wastewater effluent. This implementation would yield many advantages to Kuwait, namely satisfying the increasingly agricultural, industrial and domestic demands for good quality water flee of viruses and bacteria, preserving the natural strategic water resources, reducing environmental pollution resulting from direct discharge of secondary/tertiary municipal effluents to the sea and meeting unexpected emergency cases of shortages in fresh water production for certain applications. Membrane fouling is the most important obstacle that has to be overcome for successful RO. Membrane fouling usually results in reduction of the permeate production rate, an increase in salt passage with time and/or membrane damage. Causes of fouling include scaling, plugging of membrane pores by suspended matter, biological fouling and degradation of the membrane itself. Membrane fouling is very much aggravated with the use of wastewater effluent as a feed for RO systems. The Kuwait Institute for Scientific Research (KISR) is implementing a research project to desalinate tertiary treated wastewater using RO in Kuwait. This paper describes the process of designing a proper pretreatment process capable of producing a substantial reduction in potential membrane foulants. Results obtained from laboratory and pilot studies revealed that fast mixing, coagulation, flocculation and sedimentation using Fe III, cationic polyelectrolyte and a sanitizing agent can produce an acceptable quality of effluent feed for the RO process. This paper discusses the progress made in establishing proper pretreatment of this feed to meet the requirements of RO membrane manufacturers. Keywords: Silt Density Index; Permanganate demand; Conventional treatment; Criteria; Design
*Corresponding author. 0011-9164/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PII S0011-9164 (97)00066-0
212
M. A bdel-Jawad et al. / Desalination 109 (1997) 211-223
1. Introduction
Kuwait, as an arid country, has very limited resources of fresh water. Available resources are: desalinated seawater, brackish ground water and treated wastewater. Almost all its fresh water needs are supplied from seawater distillation. Brackish water resources are limited and nearly unreplenishable. Urban wastewater is collected, treated to a tertiary level and returned back to the sea. Limited quantities are used for landscaping purposes. In Kuwait wastewater effluent is treated to a secondary or tertiary level (a secondary effluent is exposed to chlorination, sand filtration and rechlorination to yield a tertiary effluent). The relatively low salinity of the treated wastewater (1000-1500 mg/l) compared to brackish water of 4000-5000 mg/l) makes it a potentially excellent source for good quality water. Over 100 migd of treated wastewater has a great potential to supplement/replace the brackish water supplies which might be the solution to redress the balance and complement the need for additional water supplies in Kuwait, mainly for greenery. The brackish water network of Kuwait has a large pumping capacity of over 160migd [1]. This capacity is underutilized to transmit brackish water, but it will satisfy the need to transmit the polished treated wastewater effluents for over three to four decades to come. However, the problem remains in the dissolved organics and other contaminants available in the tertiary treated effluents. Different methods can be employed to renovate this effluent and to use it in agricultural, industrial and certain domestic applications. Direct human consumption of the treated effluent cannot be applied due to ethical and physiological reasons. RO is a successful desalination method applied to desalinate seawater, brackish water and industrial and urban wastewater. It relies on a membrane separation technique that requires pressure to force clean water through the membrane, thus rejecting
dissolved salts and harmful contaminants, including bacteria, viruses and chemicals with the reject stream [2-7]. The state-of-the-art of RO, however, is such that the site-specific design specifications of a new plant are still far from routine procedure. This is particularly true in the case of seawater RO. Some of the parameters such as quality of feedwater, recovery of fresh water and problems associated with membrane sensitivity to scaling, fouling, etc. are highly site dependent. Nevertheless, great advances that were made in technology during the last decade resulted in a rapid increase in using RO [8]. In spite of the recent advancements in the application of RO technology to wastewater reclamation, issues related to membrane performance, pretreatment, membrane cleaning and brine disposal must be addressed and solved. Technical and economic assessment must be carried out to obtain an actual unit cost for the renovated wastewater by the RO process. Therefore, the aim of this work is to assess the technical viability of implementing RO membrane technology to renovate Kuwaiti tertiary treated wastewater and to find solutions for the problems that might be encountered in wastewater renovation by the RO process, with special emphasis on the pretreatment of the feed.
2. Research facilities
A suitable location at Ardiya site was designated by the Ministry of Public Works to install the pretreatment system and the RO unit. The allocated site has the features of easy access to the tertiary treated effluents available in a holding tank with a capacity of 17,000 m 3 and to the discharge channel going to the sea. The l136m3/d RO plant was originally supplied to MEW for brackish water desalination. It consists of one 40-foot ISO operation container (12.2×2.4x2.6 m) containing the cartridge filter, two high-pressure pumps, RO modules, a
M. A bdel-Jawad et al. / Desalination 109 (1997) 211-223
Table 1 Technical details of the RO unit Description
Value/type
Capacity High pressure pumps Operating pressure, bar Cartridge filter
1136 m3/d (250,000 migd) 2 (SS 316 L) 15-25 1 (SS 316 L) 5/z (polypropylene) 8 pressure vessels (8") Spiral wound 48 (40" length) Filmtec, BW 30-8040 polyamide, thin-film composite
RO modules Configuration Number of elements Membranes Salt rejection, %
96-98
Table 2 Technical details of the pretreatment system Description
Value/type
Dosing station
Dosing pumps, stirrers and tanks for flocculant aid, disinfectant, acid, etc. 2 (GRP; diameter 1.8 m) 0.7-1.25
Pressure sand filters Sand grain size, mm Sand height, m
1
flushing/cleaning tank, a product transfer pump and electrical switch gear. In addition, the plant has a separate pretreatment container measuring 6.1 x2.4×2.6 m that accommodates two sand filters, a feed pump and air blower. A third container (6x2.4x2.6 m) is also available for chemical and spare parts storage. Technical details of the RO unit are given in Table 1, while the pretreatment is given in Table 2. All infrastructure including two feed transfer pumps from the tertiary treated wastewater holding tank, feed and discharge pipelines, civil work and electrical power supply was completed.
3. M e m b r a n e foulants
Membrane fouling is the most important obstacle to be overcome in the implementation of
213
the RO process. It is ot~en manifested by a reduction of the permeate production rate and/or an increase in salt passage with time. Causes of fouling include concentration polarization, gel formation, scaling, plugging of membrane pores by suspended matter, biological fouling and degradation of the membrane itself. Concentration polarization is caused by preferential rejection of solute and removal of solving creating a concentration gradient from the bulk feed flow to the membrane surface. This phenomenon is likely to exist under conditions of low bulk velocity and high flux and can be controlled by proper turbulence flow. Presence of species such as humic substances, bioslimes, phenols, pesticides and macromolecules may cause formation of a compressible gel layer at the membrane f i l l surface and block the pores. When the solubilities of certain inorganic species exceed the allowable concentration limits, they tend to precipitate on the membrane surface and cause a decrease in membrane permeability. With proper design and operation of the RO system, this phenomenon can be kept under control. Availability of appreciable amounts of finely dispersed or suspended solids and biological foulants also cause plugging of RO membranes. These colloids, therefore, should be reduced to acceptable levels. Finally, if the feed water contains chemicals that react with the RO membrane material, degradation of the membranes results in a loss of their ability to function properly. Generally, all these causes are associated with certain constituents of the feed water. Hence, the design and successful operation of the RO system depends on the feed water characterization and proper treatment. Properties that are often assessed include hardness, turbidity, total suspended solids, total dissolved solids, chemical oxygen demand and total organic carbon. However, the roles of these parameters in the fouling process have not been accurately quantified.
214
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
4. Characteristics o f the feed water Samples of the primary and tertiary treated wastewater effluents from the Ardiya site were analyzed. Table 3 shows fouling characteristics of the primary and tertiary treated wastewater effluents. Chemical characteristics of the tertiary treated effluents are given in Table 4, while the bacteriological characteristics are shown in Table 5. Results shown in Tables 3-5 indicate that the total dissolved solids is low, which means that the RO system can operate at a very low pressure, below 15 bar. Based on 55% recovery, analysis of the performance projection of the RO system, as shown in Table 6, indicates that no scale fouling is anticipated. (This projection is based on tertiary treated water samples collected from Ardiya and analyzed in Austria by the Elin Company, assuming ROGA 8231 LP membranes.) Using the available membrane (Filmtec BW 30-8040) would give much better quality of the permeate as it has excellent salt rejection - between 96-98%. However, the fouling and microbial characteristics indicate that the feed Table 3 Fouling characteristics of the primary and tertiary treated effluents from the Ardiya plant Parameter
TDS, mg/l pH TSS, mg/I COD, mg/l BOD, mg/l Turbidity, NTU Permanganate demand, mg/l Total hardness, mg/l Cd, mg/l Cr, mg/1 Cu, mg/l Pb, mg/I Hg, mg/l Ni, mg/!
Table 4 Chemical characteristics of the tertiary treated effluents from the Ardiya site Parameter
Value
TDS Ca Mg Na Bicarbonate C1 SO4 Total N Ammonia H2S Total phosphate pH
1085 75 20 205 182 290 250 23 22 0.1 11 7.2
Note: All values are in mg/l except pH. Table 5 Bacteriological characteristics of the tertiary treated effluents from the Ardiya site Contaminants (100 ml)
Levels(colonies/100 ml)
Total count Coliform Fecal coliform Salmonella Streptococci Fungi
4.3x 10 3 2.0× 102 50 3.4 1.1 1.9
Value Primary
Tertiary
988 7.3 20 103 28 350 285
983 7.4 12 56 8 4 26 285 6 x 10-4 9x 10-4 3.6x10 -4 4.8x 10 -3 7.9× 10-3 7.5×10-3
Table 6 Performance projection of the RO plant assuming ROGA 8231 LP elements Parameter (mg/l)
Feed
Brine
Permeate
Ca Mg Na SO4 CI TDS
75 20 205 250 220 954
149.8 40.0 294.6 523.9 326.5 1555.0
14.2 3.8 132.3 27.3 133.3 465.5
Conditions:feed flow, 79 m3/h; permeate flow, 43.3 m3/h; recovery, 55%; temperature, 25°C; feed pressure: 11.1 bar.
215
M. Abdel-Jawad et al. /Desalination 109 (1997) 211-223
Table 7 System design guidelines for spiral wound Filmtec FT30, 8040
Feed source
SDI
Recovery per element (%)
Maximumpermeate (m3/d) (GPD)
Maximum feed (m3/h) (GPD)
RO permeate UF permeate Groundwater/softened water Softened surface water Surface water Tertiary effluent Seawater
<1 <1 <3 3-5 3-5 -<5
40 25 19 17 15 10 10
45 41 28 25 22 15 23
16 16 14 13 12 11 13
water should be treated to lower parameters such as total suspended solids, chemical oxygen demand, color, turbidity and permanganate demand. System design guidelines for the Fiimtec membrane elements [9] (Table 7) show that no SDI limit to the treated wastewater feed is given. It also indicates that lower permeate and maximum feed flow for this membrane should be applied.
5. P r e d i c t i o n o f R O m e m b r a n e fouling
Predictive correlations of flux decline with time are very important aspects in the RO process design. However, the complexity of the fouling phenomenon makes mathematical modeling extremely difficult, if not nearly impossible. Only two empirical formulas have become widely accepted, i.e., the silt density index (SDI) and the permanganate demand (PD). The SDI is a very popular model to predict fouling in most membrane processes. It is based on the time required to filter a volume of the feed solution through 0.45 ~zm cellulose acetate membrane at a fixed hydrostatic pressure of 30psig. It requires measurements of two time variables needed to filter 500ml of feed water. Once, immediately and another after additional 5-15 min depending on the feed water quality. Two terms are used to express the two recorded times:
(12,000) (10,900) (7,500) (6,500) (5,900) (4,000) (6,000)
Plugging factor (PF)
=
(
1-
(60) (70) (62) (60) (55) (50) (60)
100
and
1-
SDI
100
T
where t 1 is the time required to filter 500ml of feed water, t2 the time required to filter the second 500ml of the same water, T is the time of continuous filtration. The value T is reduced if [1 -(tl/t2) ] is greater than 0.70 [10]. Although the SDI is a simple and speedy method to predict the quality effect of the feed water on the performance of the RO membranes, it is considered very conservative as some drinking waters fail to pass this test. One mg/ml humic substances is considered unacceptable by the SDI procedure. Moreover, it does not react linearly from foulant to foulant. Perhaps, because of that, none of the wastewater treatment plants using RO reported an SDI value for the feed water [2-5,11-18]. The advantage of the PD test is the readily detectable concentrations of permanganate (MnO4) through photometric analysis at 522 nm and the manganate (MnO4) at 426 nm.
216
M. ,4bdel-Jawad et al. / Desalination 109 (1997) 211-223
The PD procedure is based on the difference in absorbance between the feed and permeate samples containing a known amount of permanganate ions which were allowed to react for 2h in a water bath at 100°C. The absorbance of the mixtures is measured after being cooled in water for 10 min [ I 0]. The reaction pathway is: 2 KMnO
4 ~
K2MnO 4 + MnO 2 + 0 2
Permanganate oxidizes many substances; reaction possibilities could include: electric abstraction, hydroxide ion removal, oxygen donation to organic compounds and formation of manganese ion in acid solutions. Binovi [19] proposed the following correlation for the useful lifetime of a spiral wound membrane before permeate flux becomes unacceptably low: m = -0.01191 - 2.6 x 10p followed by a substitution of the value m into the following relation: y = mx+b
where m is the slope in the linear equation, P is the PD of the feed solution in mg/l, y is the threshold of unacceptable permeate flux in gal/ fl2"d,x the time elapsed for permeate flux to reach flux y in days and b is the clear membrane flux in gal/ft2"d. Knowing that the PD for the tertiary treated effluent of Ardiya-Kuwait is about 26mg/l, as shown in Table 3, and the clear flux for the Filmtec membranes is 14gal/fl2.d [9], the 100% permeate flux decline is expected to occur during half an hour of using this water as a feed for the RO system. Therefore, further treatment of the water to drastically, lower the PD value is inevitable. It is evident that evaluation and assessment of a sewage effluent in terms of its fouling
characteristics when fed to a RO system is potentially an extremely complex process. Components which could cause fouling are numerous and difficult to analyze. However, for wastewater treated with RO, all quoted citations rely on the values of the turbidity in the feed water as a measure of quality. Some rely on other parameters such as color, total organic carbon (TOC), chemical oxygen demand (COD) and suspended solids (SS). Reviewing the water quality objectives in the above citations, the following values can be considered acceptable levels for operating RO plants using treated sewage effluent: Parameter
Value
Turbidity (NTU) Color-Cu (at 408 nm) TOC (mg/I) SS (mg/l) COD (mg/1)
1-5 9-15 10-20 I-5 5-10
Additional tests are useful--but not conclusive indicators--for organics such as COD/TOC, UV absorption at 275nm and turbidity.
6. Chemical treatments
Complete removal of potential foulants from the feed is a very difficult task. However, preventing membrane fouling is addressed through proper treatment of the feed. Based on the quality of the feed, different treatment methods are needed in proper sequence. The characteristics of the tertiary treated wastewater of the Ardiya site indicate that additional treatment to lower the residual concentrations of various solids or dissolved water constituents is required before feeding this water to an RO plant. Coagulation of smaller particles into larger ones prevents colloidal and large organic macromolecules from penetrating into the sand filter
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
bed, and the pores of the RO membranes Fe [], Al-salts, lime and polyelectrolytes are known to enhance the removal of SS/colloidal particulates through floeculation/precipitation and filtration processes. Literature scanning for the most efficient and economical solids separation and adsorption to particulates indicates that FelN salts are the better-suited coagulant for this purpose. Lime, soda ash and other alkaline chemicals were not considered due to the following reasons: •
Capital, operating and maintenance costs of the lime clarification process are higher by 28% than treatment using rapid mixing, flocculation and filtration [ 16]. • Lime treatment requires very high doses to produce an acceptable floe-treating effect: 100-600 mg/l [20]. • The turbidity of the treated water remains higher than the recommended turbidity level for RO (less than 5 NTV) [20]. • Additional costs for pH adjustment are incurred since lime and other alkaline chemicals increase the pH of the feed to an unacceptable level (pH 11). • Large quantities of sludge are produced which create further problems such as dewatering and disposing/cycling of this sludge at additional costs. Activated carbon for filtration or dual-media filtration also was not considered based on previous experience at the Pomona Advanced Wastewater Treatment Research Facility (Los Angeles, CA, USA). Research revealed that irreversible deposition of carbon fines on the RO membranes resulted in flux decline. In addition, biological debris discharged from the carbon columns plugged up the cartridge filters [16]. Nevertheless, further pretreatment of the feed by coagulation may also be needed prior to subsequent treatment, not only with RO membranes but also by activated carbon filters [21]. Otherwise, the bed life and adsorption
217
capacity of the carbon filter can be significantly reduced. Although the F e [ ] and Al-salts showed a comparable effect in lowering turbidity, TOC, SS and metal ions in the treated water [22], F e [ ] consistently outperformed alum. Crozes [23] attributed the better performance of Fe 1II to the following reasons: • F e [ ] has double active positive charges compared to Al-salts. Hence, Fe M is expected to be more effective than alum in destabilizing the colloids in solution (half dosing rate of Fe [ ] compared to A1111). • F e [ ] solution is more acidic than alum solution. • Double concentration of alkalinity is consumed with Fe III compared to A i m . • Lowering the pH of the solution will increase the protonation of the humic substances, increase the positive charge of the coagulant, reduce the coagulant demand and favor the adsorption of organics on to iron hydroxide. • F e [ ] floe has greater surface charges per surface area which will result in a much higher organic matter uptake than for alum salts. • Fe [] has the advantage of reduced sludge. Therefore, the ferric flocculant was preferred over alum for this study. Ferric flocculants are available in three forms: chloride, sulphate and chlorosulphate. The latter was selected due to its being commercially available at a lower dosing cost(percentages of F e [ ] in FeCI3, Fe(SO4)3 and FeCI SOn solutions are 34.4%, 28% and 30%, respectively). Continuous chlorination of the effluent should be applied after removal of a specific percentage of the TOC. Otherwise, halogenated organic compounds (HOC) are formed. Enhanced coagulation with a very high dose of coagulants is used to control the formation of HOC [23]. This type of very high dose of Fell] is not suitable for RO feed since high residual iron in
218
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
the feed will result in permanent membrane damage. Iron concentration in the feed to RO modules should be less than 4 mg/1 when dissolved oxygen is less than 0.1 mg/l. With dissolved oxygen concentration higher than 5 mg/l, iron concentration should not exceed 0.05 mg/l [10]. However, without disinfection of the feed, biological fouling of the RO membrane is inevitable. In this work a new sanitizing agent (a product of FMC) is being tested. Preliminary results are encouraging. An elimination efficiency of 75% for BOD and over 90% for SS may be expected using a polyelectrolyte with Fe IlL Cationic polymers are mainly effective with a high content of colloidal organic matter, while anionic polymers are used with colloidals of a mineral nature [24]. An initial solution is generally 0.5-1%, and the dosing point is usually located almost immediately ahead of the filter. Polyelectrolytes can be used as flocculants without the addition of metal coagulants. However, this use generally results in rather high residual turbidity [25]. When cationic polyelectrolytes are used (0.11.0mg/l) with coagulants, the process offers additional advantages: • reduced Fe III requirements • reduced interference of substances such as phosphates and lignin with coagulation • increased capability of flocculating living organics such as algae and bacteria
7. Experimental work 7.1. Laboratory tests
A jar test apparatus consisting of a variable speed control stirring machine and six paddles was used to determine the appropriate coagulant and dosages needed to achieve an acceptable level of treatment. Lime, ferric chlorosulphate, carbonate and polyelectrolytes were tested using the AWWA jar test procedure [26]. The procedure was modified to 1 min stirring at high
speed (90rpm), followed by 30 min stirring at low speed (30 rpm) and 30 min settling. The main goal of the test is to achieve the most efficient operation of the lowest dosage and cost-effective coagulation with FelII. Dosages varying from 2.5-3.0mg/1 were tested. Visual observation with time indicated that an excellent and well formed speedy floc was formulated after 1 min with the lowest dosage, and time decreased with an increase of iron concentration as shown in Table 8. Most of the formulated floc settled within half an hour. Samples with dosages lower than 10 mg/1 Fe 1]I exhibited clear colorless water, while those with higher Fe III dosages showed a reddish water color, which increased with the increase of dosage. Six samples were repeated starting with 2.5 mg/l up to 15 mg/l FelII. Table 9 shows the iron dosages, the turbidity and the residual iron Table 8 Time required to produce well formed floc with additions of Fe III and 0.5 mg/l cationic polyelectrolyte (pH 7, temperature 25 °C) Fe III (mg/l)
Floc with Fe III (s)
Floc with iron and polyelectrolyte(s)
5 10 15 20 25 30
75 60 45 40 35 30
100 85 75 47 40 33
Table 9 Turbidity and residual iron in samples treated with various dosages of FellI at pH 7 and 25°C Fe III (mg/I) Turbidity (NTU) Residualiron (mg/l) 2.5 5.0 7.5 10 12.5 15
3.2 3.1 3.3 3.0 3.0 2.7
0.72 0.60 0.68 0.59 0.62 0.56
219
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
Table 10 Turbidity and residual iron in samples treated with Fe III and cationic polyelectrolyte Fe III (mg/l)
2.5 5.0 7.5 10 12.5 15
Cationic polyelectrolyte 0.5 mg/I
Cationic polyelectrolyte 1.0 mg/l
Turbidity (NTU)
ResidualFe III (mg/l)
Turbidity (NTU)
ResidualFe III (mg/I)
1.75 2.1 2.06 2.3 2.3 2.11
0.32 0.31 0.41 0.27 0.33 0.32
3.9 3.1 2.9 3.4 3.0 2.5
0.22 0.19 0.17 0.29 0.29 0.12
concentration after filtration. The same experiment was repeated with the same Fe III dosages and with the addition of 0.5 and 1 mg/l cationic polyelectrolyte. Table 10 shows the iron dosages, the turbidity and the residual iron concentration after filtration. Results indicate that the optimum dosage of Fe III is between 5-10 mg/l with 0.5 mg/l of polyelectrolyte. 7.2. Pilot p l a n t tests
Taking into consideration the characteristics of the available feed water and the possible treatments, a pilot pretreatment plant was constructed. It consists of a chemical dosing station, coagulation/flocculation tank, sedimentation tank, transfer pump, two sand filters, product tank and backwash pump. A schematic diagram of the pretreatment pilot plant is shown in Fig. 1. Feed flow to the plant was adjusted to 10001/h.
Based on the bench-scale results, the addition of 7.5 mg/l ferric ion with a contact time of about 30 rain produced an excellent floc inside the first tank. Although the physical appearance of the product water was excellent, the SDI value could not be measured after filtration. An additional dose of 70 mg/l carbonate (NazCO3) did not result in any appreciable improvement as the SDI value was still unobtainable. Further addition of 0.51 mg/l Superfloc 573 also resulted in unobtainable SDI measurement.
Table 11 Characteristics of the feed water before and after the pilot plant Parameter
TDS (mg/l) pH Turbidity (NTU) TSS (mg/I) VSS (mg/l) COD (mg/l) BOD (mg/l) DO (mg/i) Total phosphate Total bacterial count/100 ml Fecal coliform/100 ml Salmonella/100 ml Fecal streptococci/100 ml SDI
Value Before
After
1080 7.1 4.0 12.0 4.0 50 8 2.2 25.0 4300 700 700 5
1070 6.7 2.1 2.0 1.0 22.0 2.0 8.1 13 400 20 40 1 4.7
--
Visual observation of the product water indicated the presence of a very light green color which could be attributed to humic substances. The addition of chlorine with high and moderate doses did not improve the situation (chlorine dosage was between 4-10 mg/l). Changing the pH with sulphuric acid to pH 6 and with alkali to pH 9.5 using sodium carbonate and caustic soda exhibited a very minor improvement to the quality of the water feed (SDI was still higher than 6). The continuous addition of ferric ion (5 mg/1) and polyelectrolyte (0.5 mg/l) produced excellent, well formed floc.
220
M. .4bdel-Jawad et aL ~Desalination 109 (1997) 211-223 Flocculatlon-sedlmenlallOll
bo .~lns
tl
I:eed Wtl©t lallk
Smld/~hcJ+ Feed l,Umlm
II -i~.-
i
I r
,,
,],,
~
Y l'lodu+"lwalct lank
'_~q
I Conduit nd ~inll loop
Tcaitty tr ~l~rd cNluml4
|loldln| tamk ~ 1 ~
Fig. 1. Schematic diagram of the pretreatment pilot plant facilities at Ardiya, Kuwait. Other additives to control the biological growth were tried without success. When a new sanitizing agent (1.0mg/1 peroxyacetic acid with 4 mg/l hydrogen peroxide) was tried along with the addition of Fe III and cationic polyelectrolyte, there was excellent improvement of the physical characteristics of the product water, with an SDI measured at 4.7. Table 11 shows characteristics of the feed water before and after the pilot plant.
8. Design o f the pretreatment process
Based on the characteristics of the tertiary treated wastewater from the Ardiya plant and the results of the bench-scale and pilot plant tests, it
was decided to design the pretreatment facilities to inject the following chemicals: • FelII dosage (5.0-10 mg/l) • Cationic polyelectrolyte (0.5-1 mg/1) • Sanitizing agent (peroxyacetic acid; 3.0mg/1 peroxygen residuals) • Dechlorination agent (1-2 mg/l) • Anti-scalant or other chemicals, if required The treatment facilities include the following: • Static mixer • Conduit mixing loop (80 m), 150 m m in diameter • Flocculation tank (24 m 3) • Sedimentation tank (48 m 3)
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
221
! h
i
,,,d
"3 3:
_
1
1
i I
e-, o
D
f
a ©
i
E
I. r.-;
~ e-,
l~d lo~uo3
5
m
0©© 0
o o
J u
._~
222
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
&
_~
~
~i i
4.
h
I
I"
0 "~__,
)
,
5
l
0 ¢-
[
,'
t
E N e~
I o
E
o~
N
÷
~E e-,
I.
l
I.
,1
li
I
d
P1 r~
,©
°~
M. Abdel-dawad et al. /Desalination 109 (1997) 211-223
• •
Feed water tank (24 m 3) Two sand filters (1.8m in diameter and 1 m sand ligh 0
Fig. 2 shows the layout of the designed pretreatment and RO facilities, and Fig. 3 shows the complete schematic diagram of the pretreatment and the RO plant. Equipment and instruments were purchased and installed at the Ardiya site. Operation of the system was expected to start in March 1996.
Acknowledgments The authors wish to acknowledge the partial support of the Environmental Protection Council, the in-kind contribution of the 250,000migd brackish water reverse osmosis plant from the Ministry of Electricity and Water and the use of the Ardiya site from the Ministry of Public Works.
References [1] MEW Statistical Yearbook, Water. Ministry of Electricity and Water, Kuwait, 1995. [2] P.L. McCarty, D. Argo, M. Reinhard, J. Graydon, N. Goodman and M. Aieta, Proc., Water Reuse Symposium II, Washington, DC, 1981, pp. 23252349. [3] G. Inoue, H. Ogasaware, C. Yanagi and Y. Murayama, Desalination, 39 (1981) 423. [4] W.A.Duvel and T. Helfgott, J WPCF, 47(1) (1975) 57. [5] H.K.Johnston and H.S. Lim, Removal of persistent contaminants from municipal effluents by reverse osmosis. Ministry of the Environment, Ontario, Canada, Report No. 73-3-14, Vol. 3, 1978.
223
[6] Y. Watanabe, Y. Kanemoto, K. Takeda and H. Ohno, Wat. Sci. Tech., 27(11) (1993) 201. [7] M. Boiler, Wat. Sci. Tech., 27(11) (1993) 167. [8] WangnickConsulting, Worldwide Desalting Plants Inventory, No. 13, IDA, 1994. [9] Filmtec, FT30 membrane elements. Technical manual, 1991. [10] J.V. Lepore and R.C. Ahlert, Fouling in membrane processes, in: Reverse Osmosis Technology, Marcel Dekker, New York, 1988. [11] B.J. Marinas, Wat. Sci. Tech., 24(9) (1991) 215. [12] H.F. Ridgway, C.A. Justice, C. Whittaker, D.G. Argo and B.H. Olson, AWWA J, (1994). [13] M.K. Stenstrom, J.R. Davis, J.G. Lopez and J.W. Mecutcha, WPCF J, 54(1) (1982) 43. [14] M. Stenstrom, SIA J, 10(2) (1983) 1. [15] H. Tsuge and K. Mori, Desalination, 23 (1977) 123. [16] R.G. Sudak, W. Dunivin and M.G. Ribgy, Municipal wastewater reclamation and reverse osmosis. Proc., National Desalination and Water Reuse Conference,Vol. 1, Orlando, FL, USA, 1990. [17] B.A. Winfield, Water Res., 13 (1979) 565. [18] D.C. Samon and B. Stringer, Process Biochem., March (1975) 4. [19] R.D. Binovi, A replacement for the silt density index. Permanganate demand to predict reverse osmosis fouling. Ph.D. Thesis, University of Cincinnati, 1984. [20] L. Caceres, War. Sci. Tech., 27(11) (1993) 261. [21] J.G. Jacanglo, J. DeMarco, D.M. Owen and S.J. Randtke, J AWWA, 87(1) (1995) 64. [22] K.N. Scott, J.F. Green, H.D. Do and S.J. Melean, J AWWA, 87(4) (1995) 114. [23] G. Crozes, P. White and M. Marshall, J AWWA, 87(1) (1995) 78. [24] Degremont, Water Treatment Handbook, 1979. [25] AWWA, Water Quality and Treatment Handbook, 1971. [26] AWWA, Principles and Practices of Water Supply Operations, Vol. 4, Introduction to Water Quality Analyses, 1982.
J(W ELSEVIER
Desalination 109 (1997) 211-223
Pretreatment of the municipal wastewater feed for reverse osmosis plants M. Abdel-Jawad*, S. Ebrahim, F. A1-Atram, S. A1-Shammari Water Resources Division~Water Desalination Department Kuwait Institute for Scientific Research, PO Box 24885, Safat 13109, Kuwait Tel. +965 4879237; Fax +965 4879238
Received November 96; accepted 24 December 1996
Abstract
Potable water can be produced at a reasonable cost if reverse osmosis (RO) technology can be applied to renovate secondary/tertiary wastewater effluent. This implementation would yield many advantages to Kuwait, namely satisfying the increasingly agricultural, industrial and domestic demands for good quality water flee of viruses and bacteria, preserving the natural strategic water resources, reducing environmental pollution resulting from direct discharge of secondary/tertiary municipal effluents to the sea and meeting unexpected emergency cases of shortages in fresh water production for certain applications. Membrane fouling is the most important obstacle that has to be overcome for successful RO. Membrane fouling usually results in reduction of the permeate production rate, an increase in salt passage with time and/or membrane damage. Causes of fouling include scaling, plugging of membrane pores by suspended matter, biological fouling and degradation of the membrane itself. Membrane fouling is very much aggravated with the use of wastewater effluent as a feed for RO systems. The Kuwait Institute for Scientific Research (KISR) is implementing a research project to desalinate tertiary treated wastewater using RO in Kuwait. This paper describes the process of designing a proper pretreatment process capable of producing a substantial reduction in potential membrane foulants. Results obtained from laboratory and pilot studies revealed that fast mixing, coagulation, flocculation and sedimentation using Fe III, cationic polyelectrolyte and a sanitizing agent can produce an acceptable quality of effluent feed for the RO process. This paper discusses the progress made in establishing proper pretreatment of this feed to meet the requirements of RO membrane manufacturers. Keywords: Silt Density Index; Permanganate demand; Conventional treatment; Criteria; Design
*Corresponding author. 0011-9164/97/$17.00 © 1997 Published by Elsevier Science B.V. All rights reserved. PII S0011-9164 (97)00066-0
212
M. A bdel-Jawad et al. / Desalination 109 (1997) 211-223
1. Introduction
Kuwait, as an arid country, has very limited resources of fresh water. Available resources are: desalinated seawater, brackish ground water and treated wastewater. Almost all its fresh water needs are supplied from seawater distillation. Brackish water resources are limited and nearly unreplenishable. Urban wastewater is collected, treated to a tertiary level and returned back to the sea. Limited quantities are used for landscaping purposes. In Kuwait wastewater effluent is treated to a secondary or tertiary level (a secondary effluent is exposed to chlorination, sand filtration and rechlorination to yield a tertiary effluent). The relatively low salinity of the treated wastewater (1000-1500 mg/l) compared to brackish water of 4000-5000 mg/l) makes it a potentially excellent source for good quality water. Over 100 migd of treated wastewater has a great potential to supplement/replace the brackish water supplies which might be the solution to redress the balance and complement the need for additional water supplies in Kuwait, mainly for greenery. The brackish water network of Kuwait has a large pumping capacity of over 160migd [1]. This capacity is underutilized to transmit brackish water, but it will satisfy the need to transmit the polished treated wastewater effluents for over three to four decades to come. However, the problem remains in the dissolved organics and other contaminants available in the tertiary treated effluents. Different methods can be employed to renovate this effluent and to use it in agricultural, industrial and certain domestic applications. Direct human consumption of the treated effluent cannot be applied due to ethical and physiological reasons. RO is a successful desalination method applied to desalinate seawater, brackish water and industrial and urban wastewater. It relies on a membrane separation technique that requires pressure to force clean water through the membrane, thus rejecting
dissolved salts and harmful contaminants, including bacteria, viruses and chemicals with the reject stream [2-7]. The state-of-the-art of RO, however, is such that the site-specific design specifications of a new plant are still far from routine procedure. This is particularly true in the case of seawater RO. Some of the parameters such as quality of feedwater, recovery of fresh water and problems associated with membrane sensitivity to scaling, fouling, etc. are highly site dependent. Nevertheless, great advances that were made in technology during the last decade resulted in a rapid increase in using RO [8]. In spite of the recent advancements in the application of RO technology to wastewater reclamation, issues related to membrane performance, pretreatment, membrane cleaning and brine disposal must be addressed and solved. Technical and economic assessment must be carried out to obtain an actual unit cost for the renovated wastewater by the RO process. Therefore, the aim of this work is to assess the technical viability of implementing RO membrane technology to renovate Kuwaiti tertiary treated wastewater and to find solutions for the problems that might be encountered in wastewater renovation by the RO process, with special emphasis on the pretreatment of the feed.
2. Research facilities
A suitable location at Ardiya site was designated by the Ministry of Public Works to install the pretreatment system and the RO unit. The allocated site has the features of easy access to the tertiary treated effluents available in a holding tank with a capacity of 17,000 m 3 and to the discharge channel going to the sea. The l136m3/d RO plant was originally supplied to MEW for brackish water desalination. It consists of one 40-foot ISO operation container (12.2×2.4x2.6 m) containing the cartridge filter, two high-pressure pumps, RO modules, a
M. A bdel-Jawad et al. / Desalination 109 (1997) 211-223
Table 1 Technical details of the RO unit Description
Value/type
Capacity High pressure pumps Operating pressure, bar Cartridge filter
1136 m3/d (250,000 migd) 2 (SS 316 L) 15-25 1 (SS 316 L) 5/z (polypropylene) 8 pressure vessels (8") Spiral wound 48 (40" length) Filmtec, BW 30-8040 polyamide, thin-film composite
RO modules Configuration Number of elements Membranes Salt rejection, %
96-98
Table 2 Technical details of the pretreatment system Description
Value/type
Dosing station
Dosing pumps, stirrers and tanks for flocculant aid, disinfectant, acid, etc. 2 (GRP; diameter 1.8 m) 0.7-1.25
Pressure sand filters Sand grain size, mm Sand height, m
1
flushing/cleaning tank, a product transfer pump and electrical switch gear. In addition, the plant has a separate pretreatment container measuring 6.1 x2.4×2.6 m that accommodates two sand filters, a feed pump and air blower. A third container (6x2.4x2.6 m) is also available for chemical and spare parts storage. Technical details of the RO unit are given in Table 1, while the pretreatment is given in Table 2. All infrastructure including two feed transfer pumps from the tertiary treated wastewater holding tank, feed and discharge pipelines, civil work and electrical power supply was completed.
3. M e m b r a n e foulants
Membrane fouling is the most important obstacle to be overcome in the implementation of
213
the RO process. It is ot~en manifested by a reduction of the permeate production rate and/or an increase in salt passage with time. Causes of fouling include concentration polarization, gel formation, scaling, plugging of membrane pores by suspended matter, biological fouling and degradation of the membrane itself. Concentration polarization is caused by preferential rejection of solute and removal of solving creating a concentration gradient from the bulk feed flow to the membrane surface. This phenomenon is likely to exist under conditions of low bulk velocity and high flux and can be controlled by proper turbulence flow. Presence of species such as humic substances, bioslimes, phenols, pesticides and macromolecules may cause formation of a compressible gel layer at the membrane f i l l surface and block the pores. When the solubilities of certain inorganic species exceed the allowable concentration limits, they tend to precipitate on the membrane surface and cause a decrease in membrane permeability. With proper design and operation of the RO system, this phenomenon can be kept under control. Availability of appreciable amounts of finely dispersed or suspended solids and biological foulants also cause plugging of RO membranes. These colloids, therefore, should be reduced to acceptable levels. Finally, if the feed water contains chemicals that react with the RO membrane material, degradation of the membranes results in a loss of their ability to function properly. Generally, all these causes are associated with certain constituents of the feed water. Hence, the design and successful operation of the RO system depends on the feed water characterization and proper treatment. Properties that are often assessed include hardness, turbidity, total suspended solids, total dissolved solids, chemical oxygen demand and total organic carbon. However, the roles of these parameters in the fouling process have not been accurately quantified.
214
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
4. Characteristics o f the feed water Samples of the primary and tertiary treated wastewater effluents from the Ardiya site were analyzed. Table 3 shows fouling characteristics of the primary and tertiary treated wastewater effluents. Chemical characteristics of the tertiary treated effluents are given in Table 4, while the bacteriological characteristics are shown in Table 5. Results shown in Tables 3-5 indicate that the total dissolved solids is low, which means that the RO system can operate at a very low pressure, below 15 bar. Based on 55% recovery, analysis of the performance projection of the RO system, as shown in Table 6, indicates that no scale fouling is anticipated. (This projection is based on tertiary treated water samples collected from Ardiya and analyzed in Austria by the Elin Company, assuming ROGA 8231 LP membranes.) Using the available membrane (Filmtec BW 30-8040) would give much better quality of the permeate as it has excellent salt rejection - between 96-98%. However, the fouling and microbial characteristics indicate that the feed Table 3 Fouling characteristics of the primary and tertiary treated effluents from the Ardiya plant Parameter
TDS, mg/l pH TSS, mg/I COD, mg/l BOD, mg/l Turbidity, NTU Permanganate demand, mg/l Total hardness, mg/l Cd, mg/l Cr, mg/1 Cu, mg/l Pb, mg/I Hg, mg/l Ni, mg/!
Table 4 Chemical characteristics of the tertiary treated effluents from the Ardiya site Parameter
Value
TDS Ca Mg Na Bicarbonate C1 SO4 Total N Ammonia H2S Total phosphate pH
1085 75 20 205 182 290 250 23 22 0.1 11 7.2
Note: All values are in mg/l except pH. Table 5 Bacteriological characteristics of the tertiary treated effluents from the Ardiya site Contaminants (100 ml)
Levels(colonies/100 ml)
Total count Coliform Fecal coliform Salmonella Streptococci Fungi
4.3x 10 3 2.0× 102 50 3.4 1.1 1.9
Value Primary
Tertiary
988 7.3 20 103 28 350 285
983 7.4 12 56 8 4 26 285 6 x 10-4 9x 10-4 3.6x10 -4 4.8x 10 -3 7.9× 10-3 7.5×10-3
Table 6 Performance projection of the RO plant assuming ROGA 8231 LP elements Parameter (mg/l)
Feed
Brine
Permeate
Ca Mg Na SO4 CI TDS
75 20 205 250 220 954
149.8 40.0 294.6 523.9 326.5 1555.0
14.2 3.8 132.3 27.3 133.3 465.5
Conditions:feed flow, 79 m3/h; permeate flow, 43.3 m3/h; recovery, 55%; temperature, 25°C; feed pressure: 11.1 bar.
215
M. Abdel-Jawad et al. /Desalination 109 (1997) 211-223
Table 7 System design guidelines for spiral wound Filmtec FT30, 8040
Feed source
SDI
Recovery per element (%)
Maximumpermeate (m3/d) (GPD)
Maximum feed (m3/h) (GPD)
RO permeate UF permeate Groundwater/softened water Softened surface water Surface water Tertiary effluent Seawater
<1 <1 <3 3-5 3-5 -<5
40 25 19 17 15 10 10
45 41 28 25 22 15 23
16 16 14 13 12 11 13
water should be treated to lower parameters such as total suspended solids, chemical oxygen demand, color, turbidity and permanganate demand. System design guidelines for the Fiimtec membrane elements [9] (Table 7) show that no SDI limit to the treated wastewater feed is given. It also indicates that lower permeate and maximum feed flow for this membrane should be applied.
5. P r e d i c t i o n o f R O m e m b r a n e fouling
Predictive correlations of flux decline with time are very important aspects in the RO process design. However, the complexity of the fouling phenomenon makes mathematical modeling extremely difficult, if not nearly impossible. Only two empirical formulas have become widely accepted, i.e., the silt density index (SDI) and the permanganate demand (PD). The SDI is a very popular model to predict fouling in most membrane processes. It is based on the time required to filter a volume of the feed solution through 0.45 ~zm cellulose acetate membrane at a fixed hydrostatic pressure of 30psig. It requires measurements of two time variables needed to filter 500ml of feed water. Once, immediately and another after additional 5-15 min depending on the feed water quality. Two terms are used to express the two recorded times:
(12,000) (10,900) (7,500) (6,500) (5,900) (4,000) (6,000)
Plugging factor (PF)
=
(
1-
(60) (70) (62) (60) (55) (50) (60)
100
and
1-
SDI
100
T
where t 1 is the time required to filter 500ml of feed water, t2 the time required to filter the second 500ml of the same water, T is the time of continuous filtration. The value T is reduced if [1 -(tl/t2) ] is greater than 0.70 [10]. Although the SDI is a simple and speedy method to predict the quality effect of the feed water on the performance of the RO membranes, it is considered very conservative as some drinking waters fail to pass this test. One mg/ml humic substances is considered unacceptable by the SDI procedure. Moreover, it does not react linearly from foulant to foulant. Perhaps, because of that, none of the wastewater treatment plants using RO reported an SDI value for the feed water [2-5,11-18]. The advantage of the PD test is the readily detectable concentrations of permanganate (MnO4) through photometric analysis at 522 nm and the manganate (MnO4) at 426 nm.
216
M. ,4bdel-Jawad et al. / Desalination 109 (1997) 211-223
The PD procedure is based on the difference in absorbance between the feed and permeate samples containing a known amount of permanganate ions which were allowed to react for 2h in a water bath at 100°C. The absorbance of the mixtures is measured after being cooled in water for 10 min [ I 0]. The reaction pathway is: 2 KMnO
4 ~
K2MnO 4 + MnO 2 + 0 2
Permanganate oxidizes many substances; reaction possibilities could include: electric abstraction, hydroxide ion removal, oxygen donation to organic compounds and formation of manganese ion in acid solutions. Binovi [19] proposed the following correlation for the useful lifetime of a spiral wound membrane before permeate flux becomes unacceptably low: m = -0.01191 - 2.6 x 10p followed by a substitution of the value m into the following relation: y = mx+b
where m is the slope in the linear equation, P is the PD of the feed solution in mg/l, y is the threshold of unacceptable permeate flux in gal/ fl2"d,x the time elapsed for permeate flux to reach flux y in days and b is the clear membrane flux in gal/ft2"d. Knowing that the PD for the tertiary treated effluent of Ardiya-Kuwait is about 26mg/l, as shown in Table 3, and the clear flux for the Filmtec membranes is 14gal/fl2.d [9], the 100% permeate flux decline is expected to occur during half an hour of using this water as a feed for the RO system. Therefore, further treatment of the water to drastically, lower the PD value is inevitable. It is evident that evaluation and assessment of a sewage effluent in terms of its fouling
characteristics when fed to a RO system is potentially an extremely complex process. Components which could cause fouling are numerous and difficult to analyze. However, for wastewater treated with RO, all quoted citations rely on the values of the turbidity in the feed water as a measure of quality. Some rely on other parameters such as color, total organic carbon (TOC), chemical oxygen demand (COD) and suspended solids (SS). Reviewing the water quality objectives in the above citations, the following values can be considered acceptable levels for operating RO plants using treated sewage effluent: Parameter
Value
Turbidity (NTU) Color-Cu (at 408 nm) TOC (mg/I) SS (mg/l) COD (mg/1)
1-5 9-15 10-20 I-5 5-10
Additional tests are useful--but not conclusive indicators--for organics such as COD/TOC, UV absorption at 275nm and turbidity.
6. Chemical treatments
Complete removal of potential foulants from the feed is a very difficult task. However, preventing membrane fouling is addressed through proper treatment of the feed. Based on the quality of the feed, different treatment methods are needed in proper sequence. The characteristics of the tertiary treated wastewater of the Ardiya site indicate that additional treatment to lower the residual concentrations of various solids or dissolved water constituents is required before feeding this water to an RO plant. Coagulation of smaller particles into larger ones prevents colloidal and large organic macromolecules from penetrating into the sand filter
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
bed, and the pores of the RO membranes Fe [], Al-salts, lime and polyelectrolytes are known to enhance the removal of SS/colloidal particulates through floeculation/precipitation and filtration processes. Literature scanning for the most efficient and economical solids separation and adsorption to particulates indicates that FelN salts are the better-suited coagulant for this purpose. Lime, soda ash and other alkaline chemicals were not considered due to the following reasons: •
Capital, operating and maintenance costs of the lime clarification process are higher by 28% than treatment using rapid mixing, flocculation and filtration [ 16]. • Lime treatment requires very high doses to produce an acceptable floe-treating effect: 100-600 mg/l [20]. • The turbidity of the treated water remains higher than the recommended turbidity level for RO (less than 5 NTV) [20]. • Additional costs for pH adjustment are incurred since lime and other alkaline chemicals increase the pH of the feed to an unacceptable level (pH 11). • Large quantities of sludge are produced which create further problems such as dewatering and disposing/cycling of this sludge at additional costs. Activated carbon for filtration or dual-media filtration also was not considered based on previous experience at the Pomona Advanced Wastewater Treatment Research Facility (Los Angeles, CA, USA). Research revealed that irreversible deposition of carbon fines on the RO membranes resulted in flux decline. In addition, biological debris discharged from the carbon columns plugged up the cartridge filters [16]. Nevertheless, further pretreatment of the feed by coagulation may also be needed prior to subsequent treatment, not only with RO membranes but also by activated carbon filters [21]. Otherwise, the bed life and adsorption
217
capacity of the carbon filter can be significantly reduced. Although the F e [ ] and Al-salts showed a comparable effect in lowering turbidity, TOC, SS and metal ions in the treated water [22], F e [ ] consistently outperformed alum. Crozes [23] attributed the better performance of Fe 1II to the following reasons: • F e [ ] has double active positive charges compared to Al-salts. Hence, Fe M is expected to be more effective than alum in destabilizing the colloids in solution (half dosing rate of Fe [ ] compared to A1111). • F e [ ] solution is more acidic than alum solution. • Double concentration of alkalinity is consumed with Fe III compared to A i m . • Lowering the pH of the solution will increase the protonation of the humic substances, increase the positive charge of the coagulant, reduce the coagulant demand and favor the adsorption of organics on to iron hydroxide. • F e [ ] floe has greater surface charges per surface area which will result in a much higher organic matter uptake than for alum salts. • Fe [] has the advantage of reduced sludge. Therefore, the ferric flocculant was preferred over alum for this study. Ferric flocculants are available in three forms: chloride, sulphate and chlorosulphate. The latter was selected due to its being commercially available at a lower dosing cost(percentages of F e [ ] in FeCI3, Fe(SO4)3 and FeCI SOn solutions are 34.4%, 28% and 30%, respectively). Continuous chlorination of the effluent should be applied after removal of a specific percentage of the TOC. Otherwise, halogenated organic compounds (HOC) are formed. Enhanced coagulation with a very high dose of coagulants is used to control the formation of HOC [23]. This type of very high dose of Fell] is not suitable for RO feed since high residual iron in
218
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
the feed will result in permanent membrane damage. Iron concentration in the feed to RO modules should be less than 4 mg/1 when dissolved oxygen is less than 0.1 mg/l. With dissolved oxygen concentration higher than 5 mg/l, iron concentration should not exceed 0.05 mg/l [10]. However, without disinfection of the feed, biological fouling of the RO membrane is inevitable. In this work a new sanitizing agent (a product of FMC) is being tested. Preliminary results are encouraging. An elimination efficiency of 75% for BOD and over 90% for SS may be expected using a polyelectrolyte with Fe IlL Cationic polymers are mainly effective with a high content of colloidal organic matter, while anionic polymers are used with colloidals of a mineral nature [24]. An initial solution is generally 0.5-1%, and the dosing point is usually located almost immediately ahead of the filter. Polyelectrolytes can be used as flocculants without the addition of metal coagulants. However, this use generally results in rather high residual turbidity [25]. When cationic polyelectrolytes are used (0.11.0mg/l) with coagulants, the process offers additional advantages: • reduced Fe III requirements • reduced interference of substances such as phosphates and lignin with coagulation • increased capability of flocculating living organics such as algae and bacteria
7. Experimental work 7.1. Laboratory tests
A jar test apparatus consisting of a variable speed control stirring machine and six paddles was used to determine the appropriate coagulant and dosages needed to achieve an acceptable level of treatment. Lime, ferric chlorosulphate, carbonate and polyelectrolytes were tested using the AWWA jar test procedure [26]. The procedure was modified to 1 min stirring at high
speed (90rpm), followed by 30 min stirring at low speed (30 rpm) and 30 min settling. The main goal of the test is to achieve the most efficient operation of the lowest dosage and cost-effective coagulation with FelII. Dosages varying from 2.5-3.0mg/1 were tested. Visual observation with time indicated that an excellent and well formed speedy floc was formulated after 1 min with the lowest dosage, and time decreased with an increase of iron concentration as shown in Table 8. Most of the formulated floc settled within half an hour. Samples with dosages lower than 10 mg/1 Fe 1]I exhibited clear colorless water, while those with higher Fe III dosages showed a reddish water color, which increased with the increase of dosage. Six samples were repeated starting with 2.5 mg/l up to 15 mg/l FelII. Table 9 shows the iron dosages, the turbidity and the residual iron Table 8 Time required to produce well formed floc with additions of Fe III and 0.5 mg/l cationic polyelectrolyte (pH 7, temperature 25 °C) Fe III (mg/l)
Floc with Fe III (s)
Floc with iron and polyelectrolyte(s)
5 10 15 20 25 30
75 60 45 40 35 30
100 85 75 47 40 33
Table 9 Turbidity and residual iron in samples treated with various dosages of FellI at pH 7 and 25°C Fe III (mg/I) Turbidity (NTU) Residualiron (mg/l) 2.5 5.0 7.5 10 12.5 15
3.2 3.1 3.3 3.0 3.0 2.7
0.72 0.60 0.68 0.59 0.62 0.56
219
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
Table 10 Turbidity and residual iron in samples treated with Fe III and cationic polyelectrolyte Fe III (mg/l)
2.5 5.0 7.5 10 12.5 15
Cationic polyelectrolyte 0.5 mg/I
Cationic polyelectrolyte 1.0 mg/l
Turbidity (NTU)
ResidualFe III (mg/l)
Turbidity (NTU)
ResidualFe III (mg/I)
1.75 2.1 2.06 2.3 2.3 2.11
0.32 0.31 0.41 0.27 0.33 0.32
3.9 3.1 2.9 3.4 3.0 2.5
0.22 0.19 0.17 0.29 0.29 0.12
concentration after filtration. The same experiment was repeated with the same Fe III dosages and with the addition of 0.5 and 1 mg/l cationic polyelectrolyte. Table 10 shows the iron dosages, the turbidity and the residual iron concentration after filtration. Results indicate that the optimum dosage of Fe III is between 5-10 mg/l with 0.5 mg/l of polyelectrolyte. 7.2. Pilot p l a n t tests
Taking into consideration the characteristics of the available feed water and the possible treatments, a pilot pretreatment plant was constructed. It consists of a chemical dosing station, coagulation/flocculation tank, sedimentation tank, transfer pump, two sand filters, product tank and backwash pump. A schematic diagram of the pretreatment pilot plant is shown in Fig. 1. Feed flow to the plant was adjusted to 10001/h.
Based on the bench-scale results, the addition of 7.5 mg/l ferric ion with a contact time of about 30 rain produced an excellent floc inside the first tank. Although the physical appearance of the product water was excellent, the SDI value could not be measured after filtration. An additional dose of 70 mg/l carbonate (NazCO3) did not result in any appreciable improvement as the SDI value was still unobtainable. Further addition of 0.51 mg/l Superfloc 573 also resulted in unobtainable SDI measurement.
Table 11 Characteristics of the feed water before and after the pilot plant Parameter
TDS (mg/l) pH Turbidity (NTU) TSS (mg/I) VSS (mg/l) COD (mg/l) BOD (mg/l) DO (mg/i) Total phosphate Total bacterial count/100 ml Fecal coliform/100 ml Salmonella/100 ml Fecal streptococci/100 ml SDI
Value Before
After
1080 7.1 4.0 12.0 4.0 50 8 2.2 25.0 4300 700 700 5
1070 6.7 2.1 2.0 1.0 22.0 2.0 8.1 13 400 20 40 1 4.7
--
Visual observation of the product water indicated the presence of a very light green color which could be attributed to humic substances. The addition of chlorine with high and moderate doses did not improve the situation (chlorine dosage was between 4-10 mg/l). Changing the pH with sulphuric acid to pH 6 and with alkali to pH 9.5 using sodium carbonate and caustic soda exhibited a very minor improvement to the quality of the water feed (SDI was still higher than 6). The continuous addition of ferric ion (5 mg/1) and polyelectrolyte (0.5 mg/l) produced excellent, well formed floc.
220
M. .4bdel-Jawad et aL ~Desalination 109 (1997) 211-223 Flocculatlon-sedlmenlallOll
bo .~lns
tl
I:eed Wtl©t lallk
Smld/~hcJ+ Feed l,Umlm
II -i~.-
i
I r
,,
,],,
~
Y l'lodu+"lwalct lank
'_~q
I Conduit nd ~inll loop
Tcaitty tr ~l~rd cNluml4
|loldln| tamk ~ 1 ~
Fig. 1. Schematic diagram of the pretreatment pilot plant facilities at Ardiya, Kuwait. Other additives to control the biological growth were tried without success. When a new sanitizing agent (1.0mg/1 peroxyacetic acid with 4 mg/l hydrogen peroxide) was tried along with the addition of Fe III and cationic polyelectrolyte, there was excellent improvement of the physical characteristics of the product water, with an SDI measured at 4.7. Table 11 shows characteristics of the feed water before and after the pilot plant.
8. Design o f the pretreatment process
Based on the characteristics of the tertiary treated wastewater from the Ardiya plant and the results of the bench-scale and pilot plant tests, it
was decided to design the pretreatment facilities to inject the following chemicals: • FelII dosage (5.0-10 mg/l) • Cationic polyelectrolyte (0.5-1 mg/1) • Sanitizing agent (peroxyacetic acid; 3.0mg/1 peroxygen residuals) • Dechlorination agent (1-2 mg/l) • Anti-scalant or other chemicals, if required The treatment facilities include the following: • Static mixer • Conduit mixing loop (80 m), 150 m m in diameter • Flocculation tank (24 m 3) • Sedimentation tank (48 m 3)
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
221
! h
i
,,,d
"3 3:
_
1
1
i I
e-, o
D
f
a ©
i
E
I. r.-;
~ e-,
l~d lo~uo3
5
m
0©© 0
o o
J u
._~
222
M. Abdel-Jawad et al. / Desalination 109 (1997) 211-223
&
_~
~
~i i
4.
h
I
I"
0 "~__,
)
,
5
l
0 ¢-
[
,'
t
E N e~
I o
E
o~
N
÷
~E e-,
I.
l
I.
,1
li
I
d
P1 r~
,©
°~
M. Abdel-dawad et al. /Desalination 109 (1997) 211-223
• •
Feed water tank (24 m 3) Two sand filters (1.8m in diameter and 1 m sand ligh 0
Fig. 2 shows the layout of the designed pretreatment and RO facilities, and Fig. 3 shows the complete schematic diagram of the pretreatment and the RO plant. Equipment and instruments were purchased and installed at the Ardiya site. Operation of the system was expected to start in March 1996.
Acknowledgments The authors wish to acknowledge the partial support of the Environmental Protection Council, the in-kind contribution of the 250,000migd brackish water reverse osmosis plant from the Ministry of Electricity and Water and the use of the Ardiya site from the Ministry of Public Works.
References [1] MEW Statistical Yearbook, Water. Ministry of Electricity and Water, Kuwait, 1995. [2] P.L. McCarty, D. Argo, M. Reinhard, J. Graydon, N. Goodman and M. Aieta, Proc., Water Reuse Symposium II, Washington, DC, 1981, pp. 23252349. [3] G. Inoue, H. Ogasaware, C. Yanagi and Y. Murayama, Desalination, 39 (1981) 423. [4] W.A.Duvel and T. Helfgott, J WPCF, 47(1) (1975) 57. [5] H.K.Johnston and H.S. Lim, Removal of persistent contaminants from municipal effluents by reverse osmosis. Ministry of the Environment, Ontario, Canada, Report No. 73-3-14, Vol. 3, 1978.
223
[6] Y. Watanabe, Y. Kanemoto, K. Takeda and H. Ohno, Wat. Sci. Tech., 27(11) (1993) 201. [7] M. Boiler, Wat. Sci. Tech., 27(11) (1993) 167. [8] WangnickConsulting, Worldwide Desalting Plants Inventory, No. 13, IDA, 1994. [9] Filmtec, FT30 membrane elements. Technical manual, 1991. [10] J.V. Lepore and R.C. Ahlert, Fouling in membrane processes, in: Reverse Osmosis Technology, Marcel Dekker, New York, 1988. [11] B.J. Marinas, Wat. Sci. Tech., 24(9) (1991) 215. [12] H.F. Ridgway, C.A. Justice, C. Whittaker, D.G. Argo and B.H. Olson, AWWA J, (1994). [13] M.K. Stenstrom, J.R. Davis, J.G. Lopez and J.W. Mecutcha, WPCF J, 54(1) (1982) 43. [14] M. Stenstrom, SIA J, 10(2) (1983) 1. [15] H. Tsuge and K. Mori, Desalination, 23 (1977) 123. [16] R.G. Sudak, W. Dunivin and M.G. Ribgy, Municipal wastewater reclamation and reverse osmosis. Proc., National Desalination and Water Reuse Conference,Vol. 1, Orlando, FL, USA, 1990. [17] B.A. Winfield, Water Res., 13 (1979) 565. [18] D.C. Samon and B. Stringer, Process Biochem., March (1975) 4. [19] R.D. Binovi, A replacement for the silt density index. Permanganate demand to predict reverse osmosis fouling. Ph.D. Thesis, University of Cincinnati, 1984. [20] L. Caceres, War. Sci. Tech., 27(11) (1993) 261. [21] J.G. Jacanglo, J. DeMarco, D.M. Owen and S.J. Randtke, J AWWA, 87(1) (1995) 64. [22] K.N. Scott, J.F. Green, H.D. Do and S.J. Melean, J AWWA, 87(4) (1995) 114. [23] G. Crozes, P. White and M. Marshall, J AWWA, 87(1) (1995) 78. [24] Degremont, Water Treatment Handbook, 1979. [25] AWWA, Water Quality and Treatment Handbook, 1971. [26] AWWA, Principles and Practices of Water Supply Operations, Vol. 4, Introduction to Water Quality Analyses, 1982.