- Email: [email protected]
Application of ZW-1000 membranes for arsenic removal from water sources
DESALINATION ELS~gIER
Desalination 162 (2004) 75-83
~v,,w.alSc~lcr.cc.m/tccal~de~f
Application of ZW-1000 membranes for arsenic removal from water sources Judit Floch a*, Miklos Hideg b ~Faculty of Food Sciences, Department of Food Engineering, University of Economic Sciences and Public Administration, Szent istvan University, 1118 Budapest, Mdnesi fit 44, Hungary Tel. +36 (1) 372-6232; email: [email protected] °Zenon Systems Kfi., Hungary Received 17 July 2003; accepted 18 September 2003
Abstract The quality of the drinking water supply has become more important with Hungary's entry into the European Community. In some regions of the country, similar to many parts of the world, arsenic removal from drinking water is an urgent task to supply water with an arsenic content under 10 gg/1. We have worked out a new technology for arsenic removal using a membrane technique. Measurements on site were performed in a pilot plant with equipped with a pro-treatment system and a ZW-1000 (Zenon) membrane module. Before expanding the technological process, experiments on laboratory scale were done to fred out which kind of pro-treatment and membrane configuration are needed. These experiments were performed with water from deep wells, among them with a high arsenic concentration (200-300 gg/1). The steps of the pre-treatment process were: oxidation with potassium permanganate (KMnO4), coagulation with ferrous(III) sulphate (Fe2(SO4)3),fast mixing of chemicals with a mixer, coagulation with slow mixing and settlement. Following membrane separation, the arsenic concentration was under the permitted value (10 gg/l). Results of experiments an site proved that the new technology was successful and is suitable to produce drinking water at the required quality from raw water with a high arsenic content in a pilot plant.
Keywords: Drinking water treatment; Arsenic removal; ZeeWeed®(Zenon) membranes
1. Introduction The use o f membrane filtration is widespread !n drinking water treatment. A great number o f '~Corresponding author.
industrial reverse osmosis (RO) plants produce drinking water from salty sea water or brackish water [1]. New nanofiltration (NF) membranes are being used mostly for the rejection o f bivalent ions and other contaminants like arsenic [2-7].
Presented at the PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland andSlovakia), September 7-11, 2003, Tatranskd MatIiare, Slovakia. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved pII: S0011-9164(04)00029-3
76
J. Floch, M. Hideg / Desalination 162 (2004) 75-83
The removal of humic substances can be successfully solved by ultrafiltration and NF membranes and in some cases the humic acid content supports arsenic removal [8,9]. In earlier experiments by Hering and Elimelech from the University of Califomia [10,11], arsenic removal was tried using membrane technology combined with coagulation. First they used synthetic water composition which was systematically altered. Later two kinds of ground water and two surface waters were examined. RO and NF membranes were studied. They stated that arsenic(Ill) will be removed less by ferrous chloride than arsenic(V). They could not remove arsenic(Ill) by aluminium salt, so they proposed a pre-oxidation process to be performed to alter arsenic(III) into arsenic(V). The removal of As(V) by ferrous(III) chloride as a coagulation medium proved to be more effective; results in the range of pH 4-8 were acceptable. Aluminium salt was effective only in the range ofpH 6-7. The arsenic concentration of water to be purified was 2-100 ~tg/l. Results of removal were independent of the arsenic concentration in the beginning. Shomey et al. [12] reported that if a microfilter or ultra-filter membrane is used for arsenic removal, a coagulation process should be included. Suggested chemicals were ferrous chloride (FeC13) and ferric sulphate (Fe2(SO4)3). The application of ferric sulphate was more suitable since it caused less corrosion than the ferrous chloride. They examined four kinds of membrane systems and the effects of the abovementioned coagulation media in their experiments including submerged micro-filter and ultrafilter membranes. Other researchers in the USA performed similar measurements. For arsenic removal from drinking water they used microfiltration. They used the same chemicals (FeC13) and Fe2(SO4)3) for coagulation before membranes with different pore sizes. They stated that micro-filtration
combined with coagulation is an effective method for arsenic removal [13]. The aim of our experiments was to develop an effective technological solution for arsenic removal with a ZW-1000 membrane (Zenon) to decrease arsenic belowl0 ~tg/1 and to minimize the quantity of wastes generated from the technology, which has not been solved by traditional technologies. Before the pilot plant experiments, laboratoryscale investigations were performed for arsenic removal to determine the physical and chemical properties of fresh and purified waters and to optimize the operational parameters.
2. Materials and methods
The knowledge of the chemical form of arsenic in water is of basic importance. The arsenic ion in the form of As(V) can be much more easily removed from the water than As(III). Moreover, we had to find out which portion of the arsenic is organic bound. Therefore, well water was analysed and the rate of the different forms of arsenic was determined before the experiments. The first step of the water treatment was the oxidation of raw water by potassium permanganate. During this procedure, arsenic-, ferrousand manganese ions were converted into removable forms. To find the necessary quantity of the oxidant, the value of the chemical oxygen demand (COD) was determined. This COD value shows the upper limit of the quantity of necessary oxidant because, adding this portion to the solution, all materials that will be oxidised will be oxidised during the measurement. One of the characteristics of the raw water used in our experiments was the high humic material content. Humic acids may effect the efficiency of technology in two ways: they may be oxidised, therefore reducing the efficiency of the permanganate; or adsorbed to the surface of
77
J. Floch, lVL Hideg / Desalination 162 (2004) 75-83
Table 1 Arsenic content of the investigated Hungarian well waters Examined wells
E As(ICP) [tig/1]
Y,As [~Ig/1]
As(III) [l~g/1]
As(V) [txg/1]COD [me/i]
Humic acid Humic acid original [rag/l] oxidized [rag/l]
Maroslele 2 0 fOldegk2 Frldefik 1 Mak6 1 Mak6 5 Mak6 6 Mak6 10 Mako 11 Mak6 12
296 94 -40 -57 61 45 52
290 91 32 46 27 59 65 54 57
165 14
125 76 32 18 27 59 65 11 24
43.7 21.1 14.2 11.0 11.4 15.9 29.9 15.6 9.5
6.0 3.2 2.3 1.65 0.90 1.85 2.80 2.4 0.7
1.0 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
DL = detection limit. the generating ferrous hydroxide, therefore reducing the active surface suitable for arsenic removal. Arsenic forms were determined after ion chromatographical separation with the ICP measuring technique. The detecting technique is based on hydride formation after chromatographical separation, which also makes possible the detection o f the arsenic in organic bonds. Humic acids were measured by the fluorescence method where a spectrum of emitted light was recorded in the function o f the wave length of the generating light. This three-dimensional figure gives information from the intensity values for qualitative and quantitative determination of organic materials in the solution. COD was measured according to the standard method; after measuring it we again determined the quantity of the remaining humic acids. Different well waters from southern Hungary with different arsenic contents were studied. The main parameters of the waters are summerised in Table 1. The highest arsenic concentration was measured in the raw water from the Maroslele 2 well. In the pilot experiments this well water was used, and the pilot plant unit was put into operation in Maroslele.
3. Experimental 3.t. Oxidation experiments
The aim of these experiments was to determine the quantity of permanganate necessary for arsenic oxidation of the well water o f Maroslele 2 in order to measure the reaction rate o f the oxidation process and to examine the humic materials. Efficiency of As(III) was determined by analysing the different arsenic forms. During preexperiments half o f the quantity o f KMnO4 (which is enough for the total oxidation o f all ingredients, and was determined by KOI measurements) was added to the Maroslele 2 well water in a form o f 0.04% solution. During the experimental run, samples were taken from the solution after 5-, 10-, 15- and 20-min intervals, and the quantity o f arsenic forms was measured. All the arsenic(III) was transformed into arsenic(V) in the first 5 rain. The experiments were repeated with a ten times more diluted solution where only 5% of the permanganate, measured by KOI method, was used. In these experiments the transformation o f arsenic was also totally finished in 5 min. Repeating the experiment with another ten times more diluted
J. Ftoeh, M. Hideg /Desalination 162 (2004) 75-83
78
solution, the oxidation of arsenic(III) was not complete, but the reaction was again finished in 5 min; during the following 15 rain the rate of different arsenic forms did not change. The reaction rate of oxidation effects the residence time, and through this, the size of equipment. The reaction rate - - under the given circumstances - - is determined basically by the concentration of the compounds. Our aim was to use the minimal quantity of potassium permanganate in the system necessary for oxidation of the whole quantity of arsenic, manganese and ferrous (but not more than that reacting in the given residence period with other oxidable components) and which could be completely transformed into a reduced form. The KMnO4 concentration was measured by the reduction of eolour of permanganate in the reaction (light absorption): 20 ml of KMnO4 solution with 0.04g/l concentrated solution was used with a 100 ml water sample. Then changes in absorbance were measured in function of time. According to the difference of the start value and the value after 30 min, the quantity of the permanganate reaction could be determined. Some permanganate reduction curves are shown in Fig. 1. Extreme values of the water from well Maroslele 2 may be explained by the high arsenic
--¢--Maroslele2 -tll--FNde~kl ---~-(52461d~k2--X-.-Mak6 1 --,~-- Mak6 5
o,,o, ..............................I U I T Z . U Z Z T T T T U I I I I I . Z I 7 Z 7 7 0,@35 ~ ~ 0.085 .
"~ o.o~s i 0.065
........................................... .
.}
.
.
.
.
.
.
.
.
.
.
.
.
\
0,055 0
5
10
15
20
Reaction time [mini
Fig. 1. Permanganate reduction curves.
25
30
35
concentration, humic acid and KOI quantity compared to the other wells.
3.2. Effect of pH on oxidation Dependence of pH on the oxidation rate was measured on three water samples where As(II1) was added. Four different pH values were prepared, and a 0.0004% KMnO4 solution was used. Transformation of As(III) into As(V) was examined in time. Results can be seen in Figs. 2-5. It can be seen that there is no significant difference between results measured at different pHs, so chemical reaction does not considerably influence the oxidation process. Therefore, pH installation in the following experiments was not required since the pH of the water was surely not higher than 8. This situation can be fixed if the carbon dioxide does not desorb from the water, there is no air ventilation or the temperature of the water is too high.
3.3. Coagulation experiments Experiments were performed in a model reactor built from a glass beaker with a volume of 2 dm 3. A magnetic mixer with variable speeds ensured the reactor mixing. Our aim was to find the minimal necessary amounts of chemicals and the simplest reaction steps needed to reach an arsenic concentration level below 10 pg/1. A 2n-type experimental design with three parameters was used to calculate the optimum values of concentrations. The factors were examined on two levels (lower and higher). The varied parameters were the quantity of KMnO4, ferrous(III) sulphate and the period of slow mixing. Experiments were performed with water from the Maroslele 2 well, which was transferred to the laboratory right before experiments started and was chilled till use. The steps of experiments were the following:
79
J. Floch, M. Hideg / Desalination 162 (2004) 75-83 100 90 80 713
100 90
8t3
e0
~6
50
=
413 = O 30 20 t~
o
10 0
5
70 6O 5O 40
30 2o 1o 13
10
0
N/ks(Ill) I~As(V) Time (min)
5
to
EIAs(I!I) QAs(V) Time (rain)
Fig. 2. Effect of pH 7.7 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
Fig. 3. Effect of pH 7.5 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
t00 9O 80 v 70
o°
~5
l
O 3O 2O 10 0
= i
= 1
0
5
6O 5O 40
[
=
10
t@As(tll) ~As(V) Time (rain)
0 ~As(tll) DAs(V)
5 Time (rain)
!0:
Fig. 4. Effect of pH %0 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
Fig. 5. Effect ofpH 6.5 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
1. 1000 ml from the examined water was filled into the reactor and mixing was started. 2. pH o f the water was measured. 3. A calculated quantity of KMnO4 with a concentration of 0.4 g/1 was added to the water, and this mixture was in reaction for 3 min. The quantity o f permanganate was expressed in the percentage of K M n O 4 able to react in the given water in 30 min, which was defined earlier. Intensity o f mixing was proper for fast adding o f ferrous salt.
4. The required quantity o f ferrous(III) sulphate (1% m/m) was added into water and fast mixed. The mixing parameters were: G (speed gradient) = 1000 s <, Ca (Camp number) = 20,000, mixing period (t = Ca~G) = 20 s. The quantity o f ferrous(III) sulphate was determined in relationship to the arsenic content. 5. Slow mixing of the generated precipitated mixture was applied till a given time with G = 50-100 s -1. 6. Atthe end of the mixing process the pH of the mixture was determined.
80
J. Floeh, M. Hideg /Desalination 162 (2004) 75-83
7. A sample was taken from the mixture inthe required periods and the liquid was filtered through a membrane filter with pore size of 0.45 pm. The nominal pore size of membranes in the pilot plant equipment was 0.02 pan, so a "lower level" system was modeled as far as separation is concerned. If the arsenic concentration in the model reached the required concentration, then it could also reached in the pilot plant equipment. According to our expectations, the pilot could remove the precipitate with better efficiency on the surface where the arsenic is adsorbed. 8. Analysis of filtered samples: As, Fe, Mn, KOI measurement were the last steps of experiments. We examined the As concentration of permeate by adding 0.4 ml and 0.4 g/1 K M l l O 4 to 100 ml ofMaroslele 2 well water. Then we added different doses offerrous(III) sulphate compared to the arsenic concentration of the original well water. The mixing period was 20 min. Results are shown in Table 2. We also examined the effects of fast and slow mixing using different mixing times. We measured the As concentration of permeate and observed its pH value before and after mixing. The values o f the experimental water were the same as that of the previous one. The Fe2(504)3 concentration was 40 times more in this case. Results are given in Table 3.
3.4. Pilot plant experiments Industrial measurements were performed in Maroslele. Based on the laboratory experiments, the technological steps for arsenic removal were determined: • oxidation with potassium-permanganate • coagulation with ferrous(HI) sulphate • fast mixing of these chemicals • coagulation with a slow mixing process • sedimentation • membrane filtration (without aeration)
Pilot plant experiments were performed with ZW-1000 surface water treating pilot equipment with two units. According to technological plan, the equipment consisted ofa pre-treating unit and a membrane treating unit (Fig. 6.). Nominal load of the system was 3 m3/h (72 m3/d). Nominal
Table 2 Effect of differentFe2(SO4)3dosesonthe arsenic concentration of permeate F%(SO4)3 dose [rag/l]
As [gg/1]
30x
2.4
40x 50x
3.2 4.0
21.50 24.00 7.91 6.33 3.69
Table 3 Effect of fast and slow mixing on arsenic concentrationof permeate Type of mixing
Mixing period [min]
pH before mixing
pH after mixing
As [gg/1]
Fast mixing
5 10 20
7.81 7.81 7.81
7.10 7.09 7.14
14.90 13.00 12.30
Slow mixing
5 10 20
7.79 7.79 7.79
7.10 7.09 7.14
15.70 11.10 7.91
J. F l o c h , M . H i d e g / D e s a l i n a t i o n
81
162 (2004) 75-83
,.Z..W..A99.9. _ro.e.mbra.n.e._
r......... .Pre~_tr_ea..tjp.g . . . . . . . . i
¢D!I ::
1
i
I ! 1 . . . . . .
.J
1
L. . . . . . . . . . . . . . . . . .
..a
I
-I
Fig. 6. Flow-sheet of the pilot plant equipment. 1 well, 2 fast mixing, 3 flocculator, 4 settler, 5 intermediate tank, 6 membrane, 7 tank for purified water, 8 waste water collector and pump, 9 sludge collector, 10 water pipe.
1~ ~,,',..,, ~-...............-,"~.,~ .......................................................................................... tl~l[r ......
i 2
i
I~
.............. ...................
.......................
............................................
o
2
..............................................................
! ..........................................
~
:1... v
s Days s
; *
..............................
: .....................
10
. . . . i~ ....... ~4
Fig. 7. Arsenic concentration of permeate in fimction of time (well water with arsenic concentration of 250300 gg/1)
experiments showed that in the effective arsenic removal range reduction of the remaining manganese content of the purified water was helped by adding more permanganate. We determined the optimum quality of permanganate dose by means of the redox potential of well water during pilot plant experiments to ensure the required manganese concentration specified for drinking water.
4. Results
4.1. Oxidation experiments pore size of the ZW-1000 membrane fibres was 0.02 gm. The membrane module consists of a branch of membrane fibres which filter from outside to inside (for more information, see: http://www.zenonenv.com). Experiments were performed with Maroslele 2 well water, which contained 200-300 pg/1 arsenic. The pilot equipment strongly decreased the arsenic with high efficiency; the arsenic content of permeate was reduced below the required limit (<10 pg/1) as shown in Fig. 7. Keeping the manganese content of the treated water at a low level caused problems. Laboratory
According to the above experiments and measurements, the following statements can be made: • Arsenic(III) content of water transforms into arsenic(V) from the effect of the soluble oxygen in the air. • The quantity of KMnO4 required for the oxidation of arsenic is less than 5% of the quantity of permanganate calculated according to the COD. • The 50-70% of the quantity of KMnO4 that could react in the purified water is enough for oxidation in 30 min.
82
J. Floeh, M. Hideg /Desalination 162 (2004) 75-83
® Oxidation o f arsenic is completed in a very short period (<5 min) if the required quantity o f KIMnO4 is present. • pH o f the water does not affect the arsenic oxidation considerably. • Content o f humic acid of test samples was smashed during the COD measurements. • Oxidation ofhumic acids is the most effective at a low temperature and in an acidic medium, but in neutral and alkaline media, oxidation takes place as well. • During oxidation ofhumic acid, ameasurable reduction o f permanganate is accomplished, which has to be taken into consideration in the technology. 4.2. Coagulation experiments The chemical used was ferrous(III) sulphate. Related to the arsenic content o f raw water, 50 times more ferrous salt was required using fast mixing. Parameters o f mixing were: G (speed gradient) = 1000 s -1. Slow mixing o f the generated flocculent during 20 min will ensure arsenic absorption. Precipitate must be separated from the purified water (by settling and membrane filtration) as fast as possible. 4. 3. Arsenic removal technology Results o f the pilot plant experiments proved that the basis for proper drinking water treatment technology was developed, which is suitable to produce drinking water at the required quality from raw water with high (200-300 pg/1) arsenic content. The processes o f pre-treatment should be separated into three units. In the first unit there is the fast mixing o f chemicals. In the second unit coagulation is solved using slow mixing. The floating material is sedimented in the third unit. After these chemical and physical steps the arsenic concentration o f t h e water decreases
below the limit of 10 gg/l. The separation o f the treated raw water takes place in the membrane unit, which is the separation with a ZW-1000 hollow-fibre membrane module. The outputs o f the technology are purified drinking water and sludge with arsenic, which later may be dried. A new direction for experiments is the examination o f the sludge and drying, keeping in mind the requirement that the treated liquid flow should be recycled into the technology.
References [1] J. Mallevialle, P.E. Odendaal and M.R. Wiesner, eds., Water Treatment Membrane Processes, McGraw-Hill, New York, 1996. [2] S. Gergely, G. Vatai and E. Bekassy-Molnar, Arsenic, zinc and magnesium ion removal from water by nano-filtration, modelling of rejections, Hung. J. Indus. Chem., 29 (2001) 21-25. [3] S. Gergely, E. Bekassy-Molnar, G. Vatai and P. Biacs, Membrane filtration as a promising technology for arsenicand heavy metalion-removalfrom drink-ingwater sources,Proc. Int. Syrup.Energy and Food Industry, Budapest, 1998, pp. 220-225, [4] E. Bekassy-Molnarand G. Vatai, Metal ion removal from Hungarian drinking water sources, 79th Intemat. Congress of French Association on Water Hygiene and Treatment, Budapest, 1999, pp. 307-321. [5] E. Bekassy-Molnar, G. Vatai and J.-P. Duguet, Membrane filtration application in drinking water treatment: Arsenic removal: Proc. 3rd International Congress l'Eau et sa Reutilisation, Toulouse, 1999, pp. 461--466. [6] E. Bekassy-Molnar,G. Vatai, P. Biacs and F. Godek, Nanofiltration of drinking water: Arsenic and bivalent cation removal, Proe. 8th World Filtration Congress, Brighton, UK, 1 (2000)643--646. [7] J. Floch and E. Bekassy-Molnar,Surfacewater treatment by microfiltration,Proc. 10thMembrane Technical Conference, Tata, Hungary, 2000, pp. 37-38. [8] Z. Domany, I. Galambos, G. Vatai and E. BekassyMolnar, Humic acid substancesremoval from drink-
J. Floch, M. Hideg / Desalination !62 (2004) 75-83 ing water by membrane filtration, Desalination, 145 (2002) 333-337. [9] C.-F. Lm, Y.-J. Huang and O.-J. Hao, Ultrafiltration processes for removing humie acid substances: effect of molecular weight fractions and PAC trea~nent, Water Res., 33 (1999) 1252-1264. [10] J.G. Hering and M. Elimelech, Arsenic removai by enhanced coagulation and membrane process, AWWA Research Foundation, 1996, Order Number: 90706. [11] J.G. Henng and M. Elimelech, International perspectives on arsenic in groundwater: problems and
83
treatment strategies, AWWA Annual Conference, Anaheim, CA, USA, 1995. [12] H.L. Shomey, W.A. Vernon, J. Clune and R.G. Bond, Performance of MI~F,JF membranes with inline ferric-salt coagulation for the removal of arsenic from a wouthwest surface water, AWWAMembrane Conference, 2001. [13] B. Han, T. Runnells, J. Zimbron and R. Wiskramasinghe, Arsenic removal from drinking water by flocculation and microfiltration, Desalinatio~ 145 (2002) 293-298.
Desalination 162 (2004) 75-83
~v,,w.alSc~lcr.cc.m/tccal~de~f
Application of ZW-1000 membranes for arsenic removal from water sources Judit Floch a*, Miklos Hideg b ~Faculty of Food Sciences, Department of Food Engineering, University of Economic Sciences and Public Administration, Szent istvan University, 1118 Budapest, Mdnesi fit 44, Hungary Tel. +36 (1) 372-6232; email: [email protected] °Zenon Systems Kfi., Hungary Received 17 July 2003; accepted 18 September 2003
Abstract The quality of the drinking water supply has become more important with Hungary's entry into the European Community. In some regions of the country, similar to many parts of the world, arsenic removal from drinking water is an urgent task to supply water with an arsenic content under 10 gg/1. We have worked out a new technology for arsenic removal using a membrane technique. Measurements on site were performed in a pilot plant with equipped with a pro-treatment system and a ZW-1000 (Zenon) membrane module. Before expanding the technological process, experiments on laboratory scale were done to fred out which kind of pro-treatment and membrane configuration are needed. These experiments were performed with water from deep wells, among them with a high arsenic concentration (200-300 gg/1). The steps of the pre-treatment process were: oxidation with potassium permanganate (KMnO4), coagulation with ferrous(III) sulphate (Fe2(SO4)3),fast mixing of chemicals with a mixer, coagulation with slow mixing and settlement. Following membrane separation, the arsenic concentration was under the permitted value (10 gg/l). Results of experiments an site proved that the new technology was successful and is suitable to produce drinking water at the required quality from raw water with a high arsenic content in a pilot plant.
Keywords: Drinking water treatment; Arsenic removal; ZeeWeed®(Zenon) membranes
1. Introduction The use o f membrane filtration is widespread !n drinking water treatment. A great number o f '~Corresponding author.
industrial reverse osmosis (RO) plants produce drinking water from salty sea water or brackish water [1]. New nanofiltration (NF) membranes are being used mostly for the rejection o f bivalent ions and other contaminants like arsenic [2-7].
Presented at the PERMEA 2003, Membrane Science and Technology Conference of Visegrad Countries (Czech Republic, Hungary, Poland andSlovakia), September 7-11, 2003, Tatranskd MatIiare, Slovakia. 0011-9164/04/$- See front matter © 2004 Elsevier B.V. All rights reserved pII: S0011-9164(04)00029-3
76
J. Floch, M. Hideg / Desalination 162 (2004) 75-83
The removal of humic substances can be successfully solved by ultrafiltration and NF membranes and in some cases the humic acid content supports arsenic removal [8,9]. In earlier experiments by Hering and Elimelech from the University of Califomia [10,11], arsenic removal was tried using membrane technology combined with coagulation. First they used synthetic water composition which was systematically altered. Later two kinds of ground water and two surface waters were examined. RO and NF membranes were studied. They stated that arsenic(Ill) will be removed less by ferrous chloride than arsenic(V). They could not remove arsenic(Ill) by aluminium salt, so they proposed a pre-oxidation process to be performed to alter arsenic(III) into arsenic(V). The removal of As(V) by ferrous(III) chloride as a coagulation medium proved to be more effective; results in the range of pH 4-8 were acceptable. Aluminium salt was effective only in the range ofpH 6-7. The arsenic concentration of water to be purified was 2-100 ~tg/l. Results of removal were independent of the arsenic concentration in the beginning. Shomey et al. [12] reported that if a microfilter or ultra-filter membrane is used for arsenic removal, a coagulation process should be included. Suggested chemicals were ferrous chloride (FeC13) and ferric sulphate (Fe2(SO4)3). The application of ferric sulphate was more suitable since it caused less corrosion than the ferrous chloride. They examined four kinds of membrane systems and the effects of the abovementioned coagulation media in their experiments including submerged micro-filter and ultrafilter membranes. Other researchers in the USA performed similar measurements. For arsenic removal from drinking water they used microfiltration. They used the same chemicals (FeC13) and Fe2(SO4)3) for coagulation before membranes with different pore sizes. They stated that micro-filtration
combined with coagulation is an effective method for arsenic removal [13]. The aim of our experiments was to develop an effective technological solution for arsenic removal with a ZW-1000 membrane (Zenon) to decrease arsenic belowl0 ~tg/1 and to minimize the quantity of wastes generated from the technology, which has not been solved by traditional technologies. Before the pilot plant experiments, laboratoryscale investigations were performed for arsenic removal to determine the physical and chemical properties of fresh and purified waters and to optimize the operational parameters.
2. Materials and methods
The knowledge of the chemical form of arsenic in water is of basic importance. The arsenic ion in the form of As(V) can be much more easily removed from the water than As(III). Moreover, we had to find out which portion of the arsenic is organic bound. Therefore, well water was analysed and the rate of the different forms of arsenic was determined before the experiments. The first step of the water treatment was the oxidation of raw water by potassium permanganate. During this procedure, arsenic-, ferrousand manganese ions were converted into removable forms. To find the necessary quantity of the oxidant, the value of the chemical oxygen demand (COD) was determined. This COD value shows the upper limit of the quantity of necessary oxidant because, adding this portion to the solution, all materials that will be oxidised will be oxidised during the measurement. One of the characteristics of the raw water used in our experiments was the high humic material content. Humic acids may effect the efficiency of technology in two ways: they may be oxidised, therefore reducing the efficiency of the permanganate; or adsorbed to the surface of
77
J. Floch, lVL Hideg / Desalination 162 (2004) 75-83
Table 1 Arsenic content of the investigated Hungarian well waters Examined wells
E As(ICP) [tig/1]
Y,As [~Ig/1]
As(III) [l~g/1]
As(V) [txg/1]COD [me/i]
Humic acid Humic acid original [rag/l] oxidized [rag/l]
Maroslele 2 0 fOldegk2 Frldefik 1 Mak6 1 Mak6 5 Mak6 6 Mak6 10 Mako 11 Mak6 12
296 94 -40 -57 61 45 52
290 91 32 46 27 59 65 54 57
165 14
125 76 32 18 27 59 65 11 24
43.7 21.1 14.2 11.0 11.4 15.9 29.9 15.6 9.5
6.0 3.2 2.3 1.65 0.90 1.85 2.80 2.4 0.7
1.0 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5 <0.5
DL = detection limit. the generating ferrous hydroxide, therefore reducing the active surface suitable for arsenic removal. Arsenic forms were determined after ion chromatographical separation with the ICP measuring technique. The detecting technique is based on hydride formation after chromatographical separation, which also makes possible the detection o f the arsenic in organic bonds. Humic acids were measured by the fluorescence method where a spectrum of emitted light was recorded in the function o f the wave length of the generating light. This three-dimensional figure gives information from the intensity values for qualitative and quantitative determination of organic materials in the solution. COD was measured according to the standard method; after measuring it we again determined the quantity of the remaining humic acids. Different well waters from southern Hungary with different arsenic contents were studied. The main parameters of the waters are summerised in Table 1. The highest arsenic concentration was measured in the raw water from the Maroslele 2 well. In the pilot experiments this well water was used, and the pilot plant unit was put into operation in Maroslele.
3. Experimental 3.t. Oxidation experiments
The aim of these experiments was to determine the quantity of permanganate necessary for arsenic oxidation of the well water o f Maroslele 2 in order to measure the reaction rate o f the oxidation process and to examine the humic materials. Efficiency of As(III) was determined by analysing the different arsenic forms. During preexperiments half o f the quantity o f KMnO4 (which is enough for the total oxidation o f all ingredients, and was determined by KOI measurements) was added to the Maroslele 2 well water in a form o f 0.04% solution. During the experimental run, samples were taken from the solution after 5-, 10-, 15- and 20-min intervals, and the quantity o f arsenic forms was measured. All the arsenic(III) was transformed into arsenic(V) in the first 5 rain. The experiments were repeated with a ten times more diluted solution where only 5% of the permanganate, measured by KOI method, was used. In these experiments the transformation o f arsenic was also totally finished in 5 min. Repeating the experiment with another ten times more diluted
J. Ftoeh, M. Hideg /Desalination 162 (2004) 75-83
78
solution, the oxidation of arsenic(III) was not complete, but the reaction was again finished in 5 min; during the following 15 rain the rate of different arsenic forms did not change. The reaction rate of oxidation effects the residence time, and through this, the size of equipment. The reaction rate - - under the given circumstances - - is determined basically by the concentration of the compounds. Our aim was to use the minimal quantity of potassium permanganate in the system necessary for oxidation of the whole quantity of arsenic, manganese and ferrous (but not more than that reacting in the given residence period with other oxidable components) and which could be completely transformed into a reduced form. The KMnO4 concentration was measured by the reduction of eolour of permanganate in the reaction (light absorption): 20 ml of KMnO4 solution with 0.04g/l concentrated solution was used with a 100 ml water sample. Then changes in absorbance were measured in function of time. According to the difference of the start value and the value after 30 min, the quantity of the permanganate reaction could be determined. Some permanganate reduction curves are shown in Fig. 1. Extreme values of the water from well Maroslele 2 may be explained by the high arsenic
--¢--Maroslele2 -tll--FNde~kl ---~-(52461d~k2--X-.-Mak6 1 --,~-- Mak6 5
o,,o, ..............................I U I T Z . U Z Z T T T T U I I I I I . Z I 7 Z 7 7 0,@35 ~ ~ 0.085 .
"~ o.o~s i 0.065
........................................... .
.}
.
.
.
.
.
.
.
.
.
.
.
.
\
0,055 0
5
10
15
20
Reaction time [mini
Fig. 1. Permanganate reduction curves.
25
30
35
concentration, humic acid and KOI quantity compared to the other wells.
3.2. Effect of pH on oxidation Dependence of pH on the oxidation rate was measured on three water samples where As(II1) was added. Four different pH values were prepared, and a 0.0004% KMnO4 solution was used. Transformation of As(III) into As(V) was examined in time. Results can be seen in Figs. 2-5. It can be seen that there is no significant difference between results measured at different pHs, so chemical reaction does not considerably influence the oxidation process. Therefore, pH installation in the following experiments was not required since the pH of the water was surely not higher than 8. This situation can be fixed if the carbon dioxide does not desorb from the water, there is no air ventilation or the temperature of the water is too high.
3.3. Coagulation experiments Experiments were performed in a model reactor built from a glass beaker with a volume of 2 dm 3. A magnetic mixer with variable speeds ensured the reactor mixing. Our aim was to find the minimal necessary amounts of chemicals and the simplest reaction steps needed to reach an arsenic concentration level below 10 pg/1. A 2n-type experimental design with three parameters was used to calculate the optimum values of concentrations. The factors were examined on two levels (lower and higher). The varied parameters were the quantity of KMnO4, ferrous(III) sulphate and the period of slow mixing. Experiments were performed with water from the Maroslele 2 well, which was transferred to the laboratory right before experiments started and was chilled till use. The steps of experiments were the following:
79
J. Floch, M. Hideg / Desalination 162 (2004) 75-83 100 90 80 713
100 90
8t3
e0
~6
50
=
413 = O 30 20 t~
o
10 0
5
70 6O 5O 40
30 2o 1o 13
10
0
N/ks(Ill) I~As(V) Time (min)
5
to
EIAs(I!I) QAs(V) Time (rain)
Fig. 2. Effect of pH 7.7 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
Fig. 3. Effect of pH 7.5 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
t00 9O 80 v 70
o°
~5
l
O 3O 2O 10 0
= i
= 1
0
5
6O 5O 40
[
=
10
t@As(tll) ~As(V) Time (rain)
0 ~As(tll) DAs(V)
5 Time (rain)
!0:
Fig. 4. Effect of pH %0 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
Fig. 5. Effect ofpH 6.5 on oxidation. Detected quantity of As(III) and As(V) in pg/1 after 0, 5 and 10 rain.
1. 1000 ml from the examined water was filled into the reactor and mixing was started. 2. pH o f the water was measured. 3. A calculated quantity of KMnO4 with a concentration of 0.4 g/1 was added to the water, and this mixture was in reaction for 3 min. The quantity o f permanganate was expressed in the percentage of K M n O 4 able to react in the given water in 30 min, which was defined earlier. Intensity o f mixing was proper for fast adding o f ferrous salt.
4. The required quantity o f ferrous(III) sulphate (1% m/m) was added into water and fast mixed. The mixing parameters were: G (speed gradient) = 1000 s <, Ca (Camp number) = 20,000, mixing period (t = Ca~G) = 20 s. The quantity o f ferrous(III) sulphate was determined in relationship to the arsenic content. 5. Slow mixing of the generated precipitated mixture was applied till a given time with G = 50-100 s -1. 6. Atthe end of the mixing process the pH of the mixture was determined.
80
J. Floeh, M. Hideg /Desalination 162 (2004) 75-83
7. A sample was taken from the mixture inthe required periods and the liquid was filtered through a membrane filter with pore size of 0.45 pm. The nominal pore size of membranes in the pilot plant equipment was 0.02 pan, so a "lower level" system was modeled as far as separation is concerned. If the arsenic concentration in the model reached the required concentration, then it could also reached in the pilot plant equipment. According to our expectations, the pilot could remove the precipitate with better efficiency on the surface where the arsenic is adsorbed. 8. Analysis of filtered samples: As, Fe, Mn, KOI measurement were the last steps of experiments. We examined the As concentration of permeate by adding 0.4 ml and 0.4 g/1 K M l l O 4 to 100 ml ofMaroslele 2 well water. Then we added different doses offerrous(III) sulphate compared to the arsenic concentration of the original well water. The mixing period was 20 min. Results are shown in Table 2. We also examined the effects of fast and slow mixing using different mixing times. We measured the As concentration of permeate and observed its pH value before and after mixing. The values o f the experimental water were the same as that of the previous one. The Fe2(504)3 concentration was 40 times more in this case. Results are given in Table 3.
3.4. Pilot plant experiments Industrial measurements were performed in Maroslele. Based on the laboratory experiments, the technological steps for arsenic removal were determined: • oxidation with potassium-permanganate • coagulation with ferrous(HI) sulphate • fast mixing of these chemicals • coagulation with a slow mixing process • sedimentation • membrane filtration (without aeration)
Pilot plant experiments were performed with ZW-1000 surface water treating pilot equipment with two units. According to technological plan, the equipment consisted ofa pre-treating unit and a membrane treating unit (Fig. 6.). Nominal load of the system was 3 m3/h (72 m3/d). Nominal
Table 2 Effect of differentFe2(SO4)3dosesonthe arsenic concentration of permeate F%(SO4)3 dose [rag/l]
As [gg/1]
30x
2.4
40x 50x
3.2 4.0
21.50 24.00 7.91 6.33 3.69
Table 3 Effect of fast and slow mixing on arsenic concentrationof permeate Type of mixing
Mixing period [min]
pH before mixing
pH after mixing
As [gg/1]
Fast mixing
5 10 20
7.81 7.81 7.81
7.10 7.09 7.14
14.90 13.00 12.30
Slow mixing
5 10 20
7.79 7.79 7.79
7.10 7.09 7.14
15.70 11.10 7.91
J. F l o c h , M . H i d e g / D e s a l i n a t i o n
81
162 (2004) 75-83
,.Z..W..A99.9. _ro.e.mbra.n.e._
r......... .Pre~_tr_ea..tjp.g . . . . . . . . i
¢D!I ::
1
i
I ! 1 . . . . . .
.J
1
L. . . . . . . . . . . . . . . . . .
..a
I
-I
Fig. 6. Flow-sheet of the pilot plant equipment. 1 well, 2 fast mixing, 3 flocculator, 4 settler, 5 intermediate tank, 6 membrane, 7 tank for purified water, 8 waste water collector and pump, 9 sludge collector, 10 water pipe.
1~ ~,,',..,, ~-...............-,"~.,~ .......................................................................................... tl~l[r ......
i 2
i
I~
.............. ...................
.......................
............................................
o
2
..............................................................
! ..........................................
~
:1... v
s Days s
; *
..............................
: .....................
10
. . . . i~ ....... ~4
Fig. 7. Arsenic concentration of permeate in fimction of time (well water with arsenic concentration of 250300 gg/1)
experiments showed that in the effective arsenic removal range reduction of the remaining manganese content of the purified water was helped by adding more permanganate. We determined the optimum quality of permanganate dose by means of the redox potential of well water during pilot plant experiments to ensure the required manganese concentration specified for drinking water.
4. Results
4.1. Oxidation experiments pore size of the ZW-1000 membrane fibres was 0.02 gm. The membrane module consists of a branch of membrane fibres which filter from outside to inside (for more information, see: http://www.zenonenv.com). Experiments were performed with Maroslele 2 well water, which contained 200-300 pg/1 arsenic. The pilot equipment strongly decreased the arsenic with high efficiency; the arsenic content of permeate was reduced below the required limit (<10 pg/1) as shown in Fig. 7. Keeping the manganese content of the treated water at a low level caused problems. Laboratory
According to the above experiments and measurements, the following statements can be made: • Arsenic(III) content of water transforms into arsenic(V) from the effect of the soluble oxygen in the air. • The quantity of KMnO4 required for the oxidation of arsenic is less than 5% of the quantity of permanganate calculated according to the COD. • The 50-70% of the quantity of KMnO4 that could react in the purified water is enough for oxidation in 30 min.
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J. Floeh, M. Hideg /Desalination 162 (2004) 75-83
® Oxidation o f arsenic is completed in a very short period (<5 min) if the required quantity o f KIMnO4 is present. • pH o f the water does not affect the arsenic oxidation considerably. • Content o f humic acid of test samples was smashed during the COD measurements. • Oxidation ofhumic acids is the most effective at a low temperature and in an acidic medium, but in neutral and alkaline media, oxidation takes place as well. • During oxidation ofhumic acid, ameasurable reduction o f permanganate is accomplished, which has to be taken into consideration in the technology. 4.2. Coagulation experiments The chemical used was ferrous(III) sulphate. Related to the arsenic content o f raw water, 50 times more ferrous salt was required using fast mixing. Parameters o f mixing were: G (speed gradient) = 1000 s -1. Slow mixing o f the generated flocculent during 20 min will ensure arsenic absorption. Precipitate must be separated from the purified water (by settling and membrane filtration) as fast as possible. 4. 3. Arsenic removal technology Results o f the pilot plant experiments proved that the basis for proper drinking water treatment technology was developed, which is suitable to produce drinking water at the required quality from raw water with high (200-300 pg/1) arsenic content. The processes o f pre-treatment should be separated into three units. In the first unit there is the fast mixing o f chemicals. In the second unit coagulation is solved using slow mixing. The floating material is sedimented in the third unit. After these chemical and physical steps the arsenic concentration o f t h e water decreases
below the limit of 10 gg/l. The separation o f the treated raw water takes place in the membrane unit, which is the separation with a ZW-1000 hollow-fibre membrane module. The outputs o f the technology are purified drinking water and sludge with arsenic, which later may be dried. A new direction for experiments is the examination o f the sludge and drying, keeping in mind the requirement that the treated liquid flow should be recycled into the technology.
References [1] J. Mallevialle, P.E. Odendaal and M.R. Wiesner, eds., Water Treatment Membrane Processes, McGraw-Hill, New York, 1996. [2] S. Gergely, G. Vatai and E. Bekassy-Molnar, Arsenic, zinc and magnesium ion removal from water by nano-filtration, modelling of rejections, Hung. J. Indus. Chem., 29 (2001) 21-25. [3] S. Gergely, E. Bekassy-Molnar, G. Vatai and P. Biacs, Membrane filtration as a promising technology for arsenicand heavy metalion-removalfrom drink-ingwater sources,Proc. Int. Syrup.Energy and Food Industry, Budapest, 1998, pp. 220-225, [4] E. Bekassy-Molnarand G. Vatai, Metal ion removal from Hungarian drinking water sources, 79th Intemat. Congress of French Association on Water Hygiene and Treatment, Budapest, 1999, pp. 307-321. [5] E. Bekassy-Molnar, G. Vatai and J.-P. Duguet, Membrane filtration application in drinking water treatment: Arsenic removal: Proc. 3rd International Congress l'Eau et sa Reutilisation, Toulouse, 1999, pp. 461--466. [6] E. Bekassy-Molnar,G. Vatai, P. Biacs and F. Godek, Nanofiltration of drinking water: Arsenic and bivalent cation removal, Proe. 8th World Filtration Congress, Brighton, UK, 1 (2000)643--646. [7] J. Floch and E. Bekassy-Molnar,Surfacewater treatment by microfiltration,Proc. 10thMembrane Technical Conference, Tata, Hungary, 2000, pp. 37-38. [8] Z. Domany, I. Galambos, G. Vatai and E. BekassyMolnar, Humic acid substancesremoval from drink-
J. Floch, M. Hideg / Desalination !62 (2004) 75-83 ing water by membrane filtration, Desalination, 145 (2002) 333-337. [9] C.-F. Lm, Y.-J. Huang and O.-J. Hao, Ultrafiltration processes for removing humie acid substances: effect of molecular weight fractions and PAC trea~nent, Water Res., 33 (1999) 1252-1264. [10] J.G. Hering and M. Elimelech, Arsenic removai by enhanced coagulation and membrane process, AWWA Research Foundation, 1996, Order Number: 90706. [11] J.G. Henng and M. Elimelech, International perspectives on arsenic in groundwater: problems and
83
treatment strategies, AWWA Annual Conference, Anaheim, CA, USA, 1995. [12] H.L. Shomey, W.A. Vernon, J. Clune and R.G. Bond, Performance of MI~F,JF membranes with inline ferric-salt coagulation for the removal of arsenic from a wouthwest surface water, AWWAMembrane Conference, 2001. [13] B. Han, T. Runnells, J. Zimbron and R. Wiskramasinghe, Arsenic removal from drinking water by flocculation and microfiltration, Desalinatio~ 145 (2002) 293-298.