Micro- and ultrafiltration of karstic spring water

E L S E V I E R Desalination 117 (1998) 61-71

DESALINATION

Micro- and ultrafiltration ofkarstic spring water

Roberto Pianta a, Markus BolleP, Marie-Laure Janex b, Aurelie Chappaz c, Bernard Birou c, Rafael Ponce d, Jean-Louis Walther d

aSwiss Federal Institute for Water Science and Technology (EAWAG), Ueberlandstrasse 133, CH-8600 Duebendorf Switzerland

Tel. +41 (1) 823-5381; Fax +41 (1) 823-5389; email: [email protected] bCIRSEE, Lyonnaise des Eaux, F-78230 Le Pecq, France CCT Environment Ltd., CH-8404 Winterthur, Switzerland

dRWBSA, CH-2900 Porrentruy, Switzerland

Received 17 June 1998

Abstract

Karstic springs are characterized by their unsteady behaviour of quantitative and qualitative parameters. The irregular and unpredictable increases of turbidity, natural organic matter (NOM), particles and solutes require a flexible and reliable treatment to ensure save drinking water quality at any time. In this study, results of a microfiltration (MF) and an uilrafiltration (UF) pilot plant are presented based on 15 and 12 months on-site experience. The investigations were focused on the optimization of the operation conditions of the membranes (maximum flux, dead-end mode, cross-flow mode) in order to guarantee drinking water production at any raw water quality. The results strongly support that MF performance is mainly influenced by submicron particles contained in the raw water suspension at higher turbidity peaks and that UF membrane permeability is affected by NOM during storm weather events. Additional quality problems in some springs caused by the presence of micropollutants (trichlorethene, telrachlorethene, atrazine) gave rise to experimentally assess the combination of powdered activated carbon (PAC) with membrane filtration. The adsorption characteristics of the solutes on different PACs were tested in laboratory and pilot scale. In the on-site experiments, removal rates of 80-90% could be achieved by adopting the necessary contact time to the previously determined kinetic data. Additionally, NOM adsorption by PAC addition could substantially improve UF performance.

Keywords." Microfiltration; Ultrafiltration; Karst; Activated carbon; Micropollutant; Adsorption isotherm

*Corresponding author.

Presented at the Conference on Membranes in Drinking and Industrial Water Production, Amsterdam, September 21-24, 1998, International Water Services Association, European Desalination Society and American Water Works Association 0011-9164198/$ - See front matter © 1998 Elsevier Science B.V. All rights reserved. PII SO011-9164(98)00067-8

62 R. Pianta et al. / Desalination 117 (1998.) 61-71

I . I n t r o d u c t i o n

In geological regions with karstic rock forma- tions, the supply of drinking water has to rely on restricted water quantities from springs of rather unstable quality. The Jurassic mountains in north- western Switzerland are a typical area where many small communities of some hundred inhabitants are faced with water supply problems arising from natural and manmade contamination of spring waters. The major quality problems are caused by natural turbidity from soil particles, hygienic contaminants, taste and odour, elevated levels of ammonia, nitrates and pesticides from agriculture, chlorinated hydrocarbons and other synthetics from leachates of solid waste deposits, and eventually high sulphate contents. Most of these quality parameters depend strongly on weather conditions and fluctuate in an unpredictable way (Table 1).

Until now, the treatment of these spring waters is performed in conventional multi-stage plants including flocculation, filtration, ozonation, GAC filtration and final disinfection. From 1995 to 1997 intensive lab and pilot experiments were carried

out in order to investigate the performance of different membrane filters and to evaluate membranes as a competing technology for the plant size typically encountered in this region. The case study was carried out in the municipality of Cornol with about 800 inhabitants where micro-, ultra- and nanofiltration (NF) membranes were tested parallel to the existing conventional treatment plant (Fig. 1). This study is focused on results obtained with microfiltration (MF) and ultrafiltration (UF).

I I

C~aguJitlon I

Dlmot FiltnltJon I

O~onatlon I

GAC Flltmtlon I

Flnal Deslnfootlon

CONVENTIONAL

KARSTIC SPRING WATER ] I

i

I Nanoflltration

MEMBRANE TECHNOLOGY

Fig. 1. Water treatment alternatives at the treatment plant of Cornol.

Table 1 Raw water characteristics of Cornol

Parameter Dry weather Storm events up to

Mean Min. - max.

Turbidity, NTU 1.45 TOC, mg/L 0.9 DOC, mg/L 0.8 Abs. UV at 254 nm, m -~ 1.4 Tetrachlorethene a, #g/L 10 Trichlorethene a, ~zg/L 6 Atrazine a, ng/L 60 Cryptosporidium oocysts, per 20 L 3.2

0.1 -4.6 130 0.6- 1.4 6.5 0.6- 1.2 5.1 1.2- 1.7 16.9 1 - 1 8 80 1 - 10 40 <10- 150 1400 <1 - 5

aKarstic water of"Le Betteraz" in Porrentruy in the same region.

R. Pianta et al. / Desalination 117 (1998) 61-71 63

The experiments included different stages aiming at (1) optimal module operation modes according to fluctuating turbidity levels, (2) possible combinations of MF and UF with qualified powdered activated carbon (PAC) for the removal of organic micropollutants. In order to reach satisfactory PAC performance, preliminary isotherm and kinetic laboratory-scale studies were carried out with atrazine, tri- and tetrachlorethene as single and competing adsorbing substances.

2. Materials and methods

The experimental arrangement of the membrane pilot plants parallel to the existing full scale water treatment scheme is shown in Fig. 1. Characteristic data on the used micro- and ultrafilter modules are depicted in Table 2. The ultra- and microfiltration pilots were both operated in dead end and cross-flow mode. Up until now, they were in operation for 12 and 15 months, respectively. For the tests combining membrane filtration with powdered activated carbon, concen- trated PAC suspensions were prepared in separate continuous stirred tanks and dosed to the raw water mixing tanks from where it was fed to the filter units. This set up allowed the necessary contact time of 7-15 min for UF which was previously determined in kinetic laboratory-scale experiments for micropollutant adsorption (trichlorethene, tetrachlorethene, atrazine). The UF experiments with PAC addition were run solely cross-flow at a constant PAC dose whereas the MF plant was operated both cross-flow and dead end including automatic PAC-dosing control according to UV-absorption in the permeate.

Backwash of the UF membrane was performed every 30 to 60 min at low turbidities and automatically regulated in shorter intervals according to feed pressure in case of high turbidity and excessive fouling. Permeate water containing a free chlorine residual of 3.5-5 mgCl2/L was used for regular backwashing. In the first period, the

Table 2 Characteristic data of the micro- and ultrafiltration pilot plants

Parameter Microfiltration Ultrafiltration

Manufacturer Memtec

Prefiltration, #m 500

Membrane type Hollow fibres

Material Polypropylene

Pore size/Cutoff, 0.2 #m/D

Fibres inner dia., 0.5 mm

Fibres length, m 1

Total membrane 45 surface area, m 2

No. of modules 3

Backwash frequency, 1-2 /h

Max. transmembrane 1.5 pressure (20°C), bar

Initial permeability at 300 20°C, L/(m2 h bar)

Aquasource

100

Hollow fibres

Cellulose derivative

0.01/ 100,000

0.93

1.2

7.2

1

1-2

2

240

microfilter membrane was backwashed at fixed time intervals of 30 (-60) min at all operation conditions, whereas in the second phase backwash intervals were controlled according to raw water turbidity within 15-30 min. The backwash procedure of the microfilter module included lumen drain, application ofpressurised air at 6 bar from the permeate side for dislodging accumulated contaminants on the membrane and raw water flush to remove detached particles from the module without addition of chemicals.

All flux and permeability values are normalized to 20°C.

The adsorption capacity of trichlorethene (TRI), tetrachlorethene (PER) and atrazine on various PACs (Fig. 2) was studied in preliminary laboratory experiments in demineralized and Cornol water. In order to limit losses of TRI and

64 R. P i a n t a e t aL / D e s a l i n a t i o n 1 1 7 (1998) 6 1 - 7 1

100.

90. t-

.o 80.

&J 70.

E~ 60.

~ 50.

g40. ~ 30.

• .~ 20.

E 10.

0 0.01

I . Envidi~k M/125

R c a ~ N360

o ~ ~ B.90

• -12.-- Kaain

• Raw w a ~ 75 NTU

0.1 I 10 100

r~aioe Saze [urnl

Fig. 2. Particle size analysis of utilized powdered activated carbon, kaolin and raw water of Cornol.

PER, ethanol solutions with concentrations in the final water samples lower than 60 mg/L were used [ 1 ]. Each experiment was carried out with 14 0.5 L glass bottles containing different concentrations of PAC (0-100 mg/L) and equal amounts of micropollutant (300#g/L). Rotation of the bottles on a horizontal axes at 20 rpm for 48 h allowed reaching adsorption equilibrium in homogeneously stirred PAC suspensions. The results were evaluated with the help of Freundlich isotherms [1]. The kinetic studies were carried out in a similar procedure in which the extent of adsorption of TRI and PER was determined after different time intervals before reaching adsorption equilibrium.

TRI and PER were analyzed by head space gas chromatography with an electron capture detector (GC-ECD). Atrazine was measured by high pressure liquid chromatography with an UV detector (HPLC-UV) after preconcentration by solid phase extraction [2]. PAC granulometric measurements were performed on a Malvern Mastersizer X particle analyzer. Characteristic results of PAC particle size distribution are shown in Fig. 2.

3. Microfiltration experiments

3.1. Operation in dead-end mode

Microfiltration in potable water treatment applications is preferably operated in dead-end mode. This energy saving operation was applied for 9 months in a first experimental phase. After a chemical cleaning and under low turbidity levels (< 1 NTU) a constant membrane flux of 125 L/m2/h and a transmembrane pressure (TMP) of 0.7 bar could be maintained over several weeks without fouling (backwash interval 60 min). However, already during medium turbidity peaks (20- 50NTU) and a reduced membrane flux of 80- 90L/m2/h, the permeability decreased by about 30-40% indicating substantial clogging of the membrane (Figs. 3, 4).

Fig. 3 shows a typical event with a turbidity peak of 25NTU. The time history of the TMP reveals that the ordinary backwash procedure could not achieve full recovery to the previous TMP after 5 days. Subsequently the backwash frequency was increased to 30 min in order to improve performance during higher turbidity levels. Five months later a more critical case was observed where a peak of 25 NTU is followed by a 90NTU peak. Although the module had been cleaned chemically 12 days before, the membrane showed a rapid permeability drop from 180 to 75 L/m2/h/bar and an increase of TMP from 0.5 to 0.7 bar during the first peak and to 1.0 bar during the second peak. Under these conditions, the latter peak was considered to indicate a case of maximum treatable raw water turbidity. In the period after the peak, again a slow and incomplete recovery of the permeability to about 120L/m 2 h/bar was observed.

Fig. 4 shows a typical series of turbidity peaks which finally lead to a blocking of the membrane. Starting with a peak of 50 NTU and a partially clogged membrane due to previous turbidity events (TMP of 0.7 bar, last chemical cleaning one month before), the permeability dropped from 135

R. Pianta et aL /Desalination 117 (1998) 61-71 65

100 - [NrU] 75

50

25

0

300

5 months later

"IMP [bar] . ,,~,_..~____

=d i i~ Turbidity [NTU] i .J)~

~' ', ', ', H', , - - - r - - - ¢ ' , " 3 6 9 12 15 18 21

i J

24 27 30

1.00 [bar) 0.75

0.50

0.25

0.00 2

250

200

150

100

50

0 Flux [I/(r~ h)]

I 3

Permeability [I/(rn 2 h bar)]

i t l # ~ i t t I I 6 9 12 1~ 1)8 ~1 29i 22 $4 1~

days

Fig. 3. Effects o f turbidity peaks on MF permeability and its recovery. First peak of 25 NTU after 4 weeks of operation with new membranes.

lOO

7 5 -

50

2 5 " "

0 i i J i

0 5 10 15 20 200

Permeability [I/(m 2 h bar)]

150 ~ , ~

100 P'I" ' i n ' "" " " v ' ' " - -,"-,

t r

50

0

2 5

TMP [bar]

i

3 0

Flux [I/(rr~ h)]

I I L I t 0 5 10 15 20 25 36

days

chemica l

c l e a r i n g

I 3O

1.00 [bar] 0.75

0.50

0.25

0.00 35

Fig. 4. MF performance after 4 months of operation. The relatively high TMP is due to a previous period with higher turbidity levels without chemical cleaning.

to 80 L/m2/h/bar and TMP increased to 0.9 bar. After this event, the initial TMP was not completely reached when after 8 days a small turbidity of 9 NTU was sufficient to cause another TMP increase to 0.8 bar. A period of low turbidity allowed the membrane to recover for 12 days. Satisfactory performance allowed to increase the flux from 80 to 105 L/m2/h. However, with this flux, the following high turbidity peak of 85 NTU immediately blocked the membrane which had to be regenerated by a chemical cleaning.

The experimental results illustrate that dead- end filtration of turbidity peaks of up to 80- 90NTU is feasible at reduced fluxes of 65- 80 L/m2/h (backwash 30 min) as long as the TMP is below 0.7 bar, indicating a reduced stage of fouling. However, each higher turbidity event is followed by progressive fouling which can lead to final membrane blocking. The run time until chemical cleaning depends on size and frequency of turbidity peaks and may last for a few days only. A typical situation is given in Fig. 5 where the first of two high turbidity peaks with an interval of 7 days could be handled whereas the second peak caused instantaneous clogging.

It has to be mentioned that during this first experimental period the pressurized air volume for backwashing was reduced to 70% because of operational problems. This might have affected backwash efficiency.

In another experimental period the backwash frequency was regulated according to the turbidity levels. Further investigations of the influence of this control device on MF performance are in progress.

3.2. Dead end operation with powdered activated carbon (PAC) dosing

The PAC Envir-Link MV125 and PicactifNC 60 were dosed to the raw water for micropollutant removal in consecutive experiments. The analysis of the karstic spring water with respect to

66 R. Pianta et aL / Desalination 117 (1998) 61-71

100 [NrU] 75

50

25

0

200

150

100

50

0

i i F i

2 4 6 8 10

T M P [bar]

, ~ , . , ~ , ~

i g J

12 14 16 18 20

rn 2 h bar)]

Flux [I/(rn 2 h)] i ~ - chemical cleartng

I I I I I I I I I 2 4 6 8 10 12 14 16 18 20

days

1.00 [mr] 0.75

0.50

0.25

0.00

Fig. 5. Sequence of turbidity peaks in dead-end mode.

turbidity, UV absorption and DOC showed clear correlations. Therefore, the PAC dosing pump was regulated according to UV absorption in the permeate. In the Envir-Link experiment, the flux was set to 150L/m2/h. During the experimental phase, moderate turbidity fluctuations in the range of5-10 NTU were observed. At first, it is not clear whether PAC or turbidity caused a rapid increase of the TMP from 0.6 to 0.9 bar within 6 days leading to complete blocking during the next higher turbidity peak. In a first phase of Picactif dosing, the flux was lowered to 90L/m2/h. A period with low turbidity (<5 NTU) revealed that permeability could be maintained constant at 110- 120 L/m2/h/bar and that no fouling occurred by the PAC. Increasing the flux to 150L/m2/h, however, provoked a steady increase of the TMP from 0.6 up to 0.95 bar in 7 days. Subsequently, a sudden turbidity peak caused final blocking. There is experimental evidence that MF with PAC dosing is feasible at low turbidity levels (<5NTU) and fluxes of 110-120 L/m2/h. Considering the results of MF in dead-end mode without PAC, it can further be concluded that it is not the PAC but the natural turbidity which causes substantial fouling.

3.3. Crossflow operation with PAC dosing

The PAC Chemviron Pulsorb BL90 was used for the cross-flow experiments. The PAC concentrations were again dosed according to permeate UV absorption and ranged between 5 and 20 mg/L depending on water quality conditions. The results during operation with a flux of 85 L/m2/h and constant low turbidity (<2 NTU) confirm that PAC dosing does not cause fouling enabling constant permeability. As soon as turbidity peaks appear, the membrane is rapidly blocked also under cross- flow conditions.

Looking at the respective particle size distributions of the PAC and the natural turbidity, the fouling phenomenon can be explained by the presence of substantial fractions of submicron particles in the natural suspension (Fig. 2). Earlier granulometric analysis ofkarstic spring water with 11.4mgTSS/L with a SediGraph 5100 analyzer revealed that a fraction of 55% of the particle mass was smaller than 0.3 #m which may penetrate the membrane or be trapped in the 0.2/~m pores [3]. On the other hand, particle size analysis of the PAC with mean diameters of 15, 20 and 1.5~m for Envir-Link, Picactif and Pulsorb, respectively, showed only minor submicron fractions (Fig. 2). This fact strongly suggests that the colloidal natural particles in karstic springs are responsible for MF membrane fouling [4-7].

4. Ultrafiltration experiments

4.1. Operation with kaolin~groundwater suspensions

In a first period, fluxes and fouling behaviour of the ultrafilter pilot were studied under extreme solids loading conditions by adding kaolin to a groundwater of excellent quality (TOC< 1 mgC/L). During the first 2 weeks, the pilot was operated with pure groundwater in cross-flow and dead-end mode with fluxes up to 160 L/m2/h and 180 L/m2/h, respectively. Permeability stabilized at 220L/m2/


h/bar. After this initial period, a kaolin suspension of similar particle size distribution as measured in the Cornol karstic water (mean particle diameter 4 ~m, Fig. 2) was dosed. The kaolin concentration was increased stepwise up to a maximum of 1500mg/L. Running in cross-flow mode with a flux of 145 L/m2/h and backwash intervals of 2.5 h, a permeability of 230L/m2/h/bar could be maintained during all experiments independent of kaolin turbidity. Thus, even under extreme solids loading, kaolin particles did not affect membrane performance.

4. 2. Operation in dead-end mode

After transferring the UF pilot to Cornol, first runs in dead-end mode with a flux of 120 L/m2/h at low turbidity (<5 NTU) were carried out. After an initial increase permeability stabilized at 200 L/m2/ h/bar. Further runs under similar conditions with a flux of 140 L/m2/h showed a stable permeability of 180 L/m2/h/bar performing one backwash per hour. Higher fluxes seemed to cause a slow but steady drop of permeability indicating maximum through- put.

Running under the above described operation conditions, a turbidity event of 25 NTU caused a severe permeability drop from 170 to 120L/m2/ h/bar and recovery of the TMP was slow and incomplete. The experience of an event with a high turbidity peak (>60NTU), shown in Fig. 6, revealed that increased fouling has to be expected at higher solids loads although the pilot changes automatically to cross-flow. The results also indicate that permeability cannot be re-established without chemical cleaning.

Analysis of this effect with respect to internal membrane fouling strongly suggests that it is the natural organic matter (NOM) which affects membrane performance. Parallel measurements of turbidity, DOC and UVz54 absorption during turbidity events showed a simultaneous increase of NOM reaching concentrations of up to 6 mgC/L and an increase of the UV/COD ratio from 1.7

100 -

[NTUI 75

25

0 I " -1 r I - ' - I I

0 1 2 3 4 5 6 200

100 Flux

50 Permeabi l i ty [I/(rn 2 h bar)]

0 I I I I

2.00 [barl

1.50

1.00

0.50

0.00

0 1 2 3 4 5 6 days

Fig. 6. UF at high flux in dead-end mode during a turbidity peak. The pilot automatically changed to cross- flow before blocking.

under dry weather conditions to 4 during storm weather. Investigations carried out at CIRSEE by GC/MS analysis showed that the augmentation of the ratio is mainly due to polysaccharide species. A comparison of these results with the kaolin experiments (low NOM water) supports that membrane fouling is caused by the simultaneous presence of NOM substances.

It can be concluded that dead-end UF with fluxes of 140 L/m2/h is appropriate for the treatment ofkarstic waters with turbidity peaks below 20NTU and for a short time. During periods of higher turbidity, cross-flow mode should be applied. For large-scale application automatic adaptation of the operation mode (dead end, cross- flow) according to the prevailing raw water turbidity and/or organic matter content is pro- posed.

4.3. Crossflow operation

Observation of membrane behaviour during succeeding high turbidity peaks (100 and 90 NTU,

68 R. Pianta et aL / Desalination 117 (1998) 61-71

respectively) revealed that cross-flow operation at a reduced flux of 70 L/m2/h was feasible although a considerable permeability drop from 180 to 100 L/m2/h/bar was observed. However, permeability could be re-established within few days to the original value by ordinary backwash (2 per hour frequency). From the analysis of the turbidity peak and the corresponding permeability recovery, it can be hypothesized that it is the peak area, correlating to the foulant load, which determines the recovery pattern.

During cross-flow operation, membrane performance during successions of turbidity peaks was investigated. The results in Fig. 7 represent a demonstration case which shows that rather critical operating conditions with two succeeding peaks of 110 and 75NTU can be managed by careful adaptation of the membrane flux. Operating with a flux of 90 L/m2/h, the first event caused a drastic permeability reduction from 200 to 50L/m2/h/bar, but operation was continued at a reduced flux of 60L/m2/h. When permeability recovered to 130L/m2/h/bar, the second peak occurred and

caused a permeability drop to 30L/m2/h/bar. Again, the flux was reduced to 50L/m2/h, and operation continued and showed a slow but steady permeability recovery under declining solids concentrations.

In a worse case of five consecutive days with a mean turbidity of 40 NTU and peaks of up to 60 NTU, flux reduction from 90 to 70 L/m2/h was sufficient to overcome this critical period, but chemical cleaning was necessary to continue membrane operation (Fig. 8).

From the cross-flow experiments it can be concluded that regulating the flux according to the turbidity level allows continuous ultrafilter operation at any given turbidity levels and that chemical cleaning is able to re-establish the original permeability after periods of critical membrane loading.

4.4. Crossflow operation with P,4C

In the experiment with a constant PAC dose of 15 mg/L of Envir-Link MV 125, a constant flux of

150 -

[NTU]

100

50

0

250

200

150

100

50

0

T u r b i d i t y [NTU]

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

Permeability [I/(m 2 h bar)]

I I I I I I I I I I I I I 1 2 3 4 5 6 7 8 9 10 11 12 13 14

clays

2.00 100 . [bar] [NTU] 1.50 75

1.00 50

0.50 25

0.00 0 I

25O

2OO

150

100

50

0

Turbidity [NTU]

1 2 3 4 6 7 8 9

2.00 [bar] 1.50

1.00

0.50

0.00

10

Permeability [I/(rn 2 h bar)]

I I I I I I I I I 1 2 3 4 5 6 7 8 9 10

days

Fig. 7. Flux regulation during extreme turbidity levels allows continuous operation.

Fig. 8. Period with a series of high turbidity peaks.

R. Pianta et al. /Desalination 117 (1998) 61-71 69

100 - Turbidity [NTU]

2.00 [bar]

50

25

0

200

150

100

50

0

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10 days

1.50

1.00

0.50

0.00

Fig. 9. Addition of 15 mgPAC/L in cross-flow mode allows constant operation during high turbidity levels.

90L/m2/h could be maintained despite of water quality fluctuations (Fig. 9). Although a series of turbidity peaks of up to 70NTU occurred which usually would affect permeability, only minor permeability fluctuations were observed with the addition of PAC [8,9]. The effect of PAC is twofold: (1) part of the NOM is eliminated due to adsorption onto PAC; (2) the PAC particles, being hard and abrasive, contribute to modify the structure of the fouling cake which probably becomes more porous and more easily backwashed. Experiments with lower PAC dosages of 5-8 mg/L showed that permeability drops were similar to the ones without PAC, but permeability recuperation was faster.

5. Micropollutant removal by adsorption on PAC

5.1. Laboratory investigations

Isotherm and kinetic adsorption experiments were carried out with different PACs to select an appropriate PAC for the removal of TRI, PER and

100.

< Q. cm

0 o 121

I--

7

10.

1

@1

f • 315 310 • 200 415

I I I 1 10 100

~ r a t i o n [ppb TRI] ; [ppm DOC] 1000

Fig. 10. Adsorption isotherms of TRI and DOC on PAC (Envir-Link MV 125).

atrazine. The selected PAC (Envir-Link MV125) was also found to be efficient for NOM removal. Subsequent to the experiments with single substances adsorption isotherms under different TRI/PER-ratios in solution were measured in order to determine the extent of competition. The results confirmed findings of other studies [10] that adsorption capacities for PER were 1.5 to 3 times higher than for TRI. The adsorption of PER was not influenced by the presence of TRI, whereas the adsorption of TRI was strongly affected by PER leading to capacity losses of 30% to 60% in the presence of PER concentrations between 300/~g/L to 400~g/L (Fig. 10). Based on the adsorption isotherms, the necessary PAC concentrations to reduce TRI and PER for given initial concentrations were calculated and used to design the pilot experiments.

The adsorption of atrazine was measured in an experiment with demineralized water and an initial atrazine concentration of l#g/L. The results confirmed that with a PAC concentration of 1.5 mg/L removal rates of >99% can be reached. Investigations of Qi et al. [11] with groundwater showed that atrazine adsorption kinetics is very fast and nearly complete in the observed concentration range of 1-5 mg/L and that atrazine is not rate limiting for the design of contact chambers compared to TRI and PER removal.

70 R. Pianta et aL /Desalination 117 (1998) 61-71

5. 2. Pilot p lant investigations

As a result of the laboratory tests, Envir-Link MV125, which showed both satisfactory micropollutant removal and NOM adsorption, was selected for the UF experiments with PAC. The UF/PAC experiments were carried out at 70 L/m2/h and 35 L/m2/h during low turbidity conditions in order to reduce NOM competition as far as possible (TOC <1 mg/L). At 70L/m2/h, different levels of PAC doses of 3, 6 and 12mg/L resulted in TRI removal rates of 45%, 60% and 75% and PER removal rates of 60%, 70% and 80%, respectively. In order to attain higher removal rates of 90% for both substances, a PAC dose of 22 mg/L was necessary.

Reducing the flux to 35L/m2/h doubled the contact time in the mixing tank which caused a significant increase of the removal rates at the lower carbon doses. Removal rates of 75% for TRI and of 90% for PER could be reached with 6 mg PAC/L. With initial concentrations of 170~g TRI/L and 170~g PER/L, permeate concentrations of 40 #g TRI/L and 17 mg PER/L could be achieved at that dose. This pointed out the great impact of kinetics and the need for optimizing flux and PAC concentration.

On-site spiking of 1/.zg/L atrazine combined with a dose of 5 mg PAC/L revealed a permeate concentration of 0.1 ~g/L whereas PAC addition of 11 mg/L led to effluent concentrations of 0.02- 0.04mg/L.

6. Disinfection

With both membranes permeate turbidity reached 0.02-0.48NTU and number counts of E. coli, Enterococcus and aerobic bacteria were under detection limits independent of raw water turbidity. NOM, measured as DOC (0.45/~m membrane-filtered), was removed by UF between 15% and 20% during dry and rain weather, respectively. The NOM removal by the micro filter amounted to 20%. The UF membrane was able to

remove MS2 viruses (size about 20nm) quanti- tatively from 10 7 pfu/100 ml down to the detection limit of 70 pfu/100 ml (i.e., removal greater than 7 log) whereas the MF unit reached a 2.5-3.0 log removal.

7. Conclusions

The results of the on-site pilot experiments clearly show that membrane operation has to be adapted to the prevailing water quality conditions for stable operation. The strong raw water quality fluctuations of the karstic spring in Cornol (Swiss Jura) during storm events with turbidity peaks of up to 130NTU associated with increased contents of natural organic matter (NOM) and UV absorption ask for careful control of the maximum feasible membrane fluxes and the operation mode (e.g., change from dead-end to cross-flow).

The UF investigations indicate that NOM evokes fouling which has to be controlled by relatively frequent chemical cleaning. The combination of UF and PAC revealed improved performance and stabilized membrane fluxes during periods of high turbidity. The results demonstrate that UF in combination with PAC is a feasible process for the treatment of karstic spring water and for the removal of micropollutants (trichlorethene, tetrachlorethene, atrazine).

The treatment by MF showed stable performance at low turbidity levels. However, experimental data indicated that natural submicron particles have an important impact on membrane permeability during turbidity peaks. Tests in dead- end mode (<5NTU) with PAC revealed that a stable permeability can be maintained and that PAC does not affect fouling as long as the turbidity is low.

For both membrane systems, bacterial disinfection was ensured despite raw water quality fluctuations. The removal of MS2 viruses reached 7 logs and 2.5-3 logs in UF and MF, respectively.


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[3] M. Boiler and C. Munz, in: A. Faseur and R. de Bruyne, eds., Particle Technology in Relation to Filtration and Separation, Royal Flemish Society of Engineers (K.VIV), Antwerpen, 1988, pp. 3.1-3.8.

[4] R. Jiramtananon, D. Uttapap and P. Sampranpibon, J. Membr. Sci., 140 (1) (1998) 57.

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Micro- and ultrafiltration of karstic spring water