Accepted Manuscript

Filterability of surface water from Tbilisi Sea: Preliminary assessment of ultrafiltration as a process alternative

Accepted for publication as a Note in Separation Science and Technology journal on April 15, 2019

Published article DOI: 10.1080/01496395.2019.1614063

Avtandil Kobaladzea; Irakli Lomidzea; Sandro Maludzea; Archil Sakevarashvilia;, Kakha Didebulidzea*; Zaza Metrevelia; Volodymyr V. Tarabarab**; Giorgi Titvinidzec; Tamara Tkeshelashvilic,d a School of Engineering and Technologies, Agricultural University of Georgia, Kakha Bendukidze University Campus, 240 David Aghmashenebeli Alley, Tbilisi, Georgia b Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI 48824, USA c Institute of Chemistry and Molecular Engineering, Agricultural University of Georgia, Kakha Bendukidze University Campus, 240 David Aghmashenebeli Alley, Tbilisi, Georgia d Gardabani Thermal Power Plant, Water Treatment Plant Laboratory, 2b David Aghmashenebeli Ave., Gardabani, Georgia.

* Corresponding author: Phone: +995 (593) 34-44-81; [email protected] ** Corresponding author: Phone: +1 (517) 432-1755; [email protected]

Financial support: V. V. Tarabara was supported by a U.S. Fulbright Scholar fellowship; T. Tkeshelashvili was supported by Volkswagen Foundation grant (Az: 93 331).

Abstract

This preliminary laboratory scale study evaluated the feasibility of ultrafiltration as an alternative technology for treating water from Tbilisi Sea (Republic of Georgia). The analysis was performed with both raw source water and with the source water pretreated by coagulation and flocculation. Coagulation/flocculation pretreatment improved ultrafiltration performance in terms of permeate flux and rejection of dissolved organic carbon (DOC). The pretreatment led to a change in the dominant membrane fouling mechanism from pore blocking to cake filtration. The observed improvement in DOC rejection was attributed to the cake layer’s action as a secondary membrane. Both permeate flux and DOC removal data support retaining coagulation and flocculation as a part of the overall treatment process.

Keywords: ultrafiltration, coagulation, flocculation, membrane fouling, blocking laws

1. Introduction

Tbilisi Sea (თბილისის ზღვა) is a man-made freshwater lake located northeast of Tbilisi at an altitude of 580 m. Also known as Tbilisi Reservoir (თბილისის წყალსაცავი), Tbilisi Sea is ~ 8.75 km long and ~ 2.85 km wide with the average depth of 26.6 m, maximal depth of 45 m and the total surface area of ~ 11.6 km2. Tbilisi Sea has two main tributaries: Aragvi channel (also called Bodorna- Grmagele tunnel), which delivers water of the Aragvi river, and Zemo Samgori channel, which delivers water of the Iori river (Figure 1). The flow rate of another tributary, Kvirikobis Khevi river, is much smaller than that of the first two (flow rate data not available). The outflow is an irrigation channel, which is a part of Samgori irrigation system. Tbilisi Sea is characterized by weak winter and well- expressed summer direct stratification and is classified as oligotrophic; more

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 recently, eutrophic behavior has been observed during summer seasons. The average summer and winter temperatures in the Tbilisi Sea are 23.3 °C and 7.5 °C, respectively [1].

Tbilisi Sea is a source of drinking water for Tbilisi, the capital city of Georgia. The water is treated by the water supply company Georgian Water and Power (GWP) to comply with the national drinking water standards. As of July 2018, Tbilisi Sea was a source of drinking water to ~ 111,300 customers in Tbilisi supplementing other sources including water from the Zhinvali reservoir (also fed by Aragvi river). The treatment train in the GWP-managed water treatment plant includes gravity settling, coagulation/flocculation (C/F), sand filtration and, finally, disinfection by chlorine. The treated water is tested with respect to several water quality parameters to ascertain compliance with government-issued water standards (Drinking Water Technical Regulations).

For high quality feed waters suitable to direct or inline filtration, membranes present a good alternative to conventional surface water treatment processes. With an appropriate choice of the membrane pore size, membranes can ensure very high microbiological quality of the permeate. With the pore size smaller than the size of most viruses (between 2 nm and 100 nm) tight ultrafiltration membranes are especially well suited for application such as drinking water treatment where pathogen removal is of paramount importance. While the coagulation and flocculation are critical for the proper functioning of the granular media filtration process, their effect on downstream membrane filtration is incompletely understood despite being a subject of significant research efforts [2- 4].

The objective of this preliminary laboratory-scale study was to evaluate filterability of Tbilisi sea water by ultrafiltration membranes and to assess the potential of this membrane technology for replacing granular media filtration. The analysis was performed with both raw source water and with water pretreated by

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 coagulation and flocculation. The study tested the hypothesis that retaining coagulation/flocculation as pretreatment would decrease intrapore blocking and improve ultrafiltration performance in terms of both water throughput and rejection of feed organics.

2. Materials and Methods

3.1 Reagents and sampling procedure

Aluminum sulfate Al2(SO4)3 was high purity as specified by the State Standard GOST 12966-85. The water used for membrane compaction was double distilled using an in-house distillation system. Tbilisi Sea water samples were collected from the depth of ~ 0.5 m at two locations in the Tbilisi Sea (Figure 1) in September 2017 ~ 50 meters from the shore at sampling point A and ~ 15 meters from the shore at sampling point B. The summer and early Fall is when the water temperature is evaluated and the load of anthropogenic pollution is expected to be higher [1]. The 20 L samples were transported to the laboratory and stored at 4 °C until further use. The containers were stirred before withdrawing water samples for each treatment test.

3.2 Coagulation and flocculation as an optional pretreatment step

First, 10 g of Al2(SO4)3 was dissolved in 1 L of distilled water to produce 10 mg/L

(as Al2(SO4)3) coagulant solution . 50 mL of the solution was added to 950 mL of sample water for each test to result in 10 mg/L concentration of coagulant in the water sample. This solution was placed in a 1 L cylindrical jar (internal diameter of 10 cm) and rapidly mixed for 1 min at 200 rpm (coagulation stage) and then mixed slowly at 20 rpm for 20 min (flocculation stage). The 76 mm  25 mm  3 mm impeller was positioned ~ 20 mm above the bottom of the jar. The coagulation / flocculation sequence was followed by 30 min of settling in the absence of mixing. After settling, 180 mL of water was carefully withdrawn from

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 the top of the jar by a syringe for further treatment. The coagulation/flocculation tests were performed at room temperature, which was in the 15.5 °C to 17.9 °C range.

3.3 Ultrafiltration experiments

Ultrafiltration tests were performed using a dead-end filtration cell (200 mL,

Amicon 8200, Millipore) at 10 °C. The cell was connected to a pressurized N2 tank to maintain constant transmembrane pressure of 1 bar. The ultrafilters were regenerated cellulose membrane disks (Ultracel® PL-100, Millipore), 63.5 mm in diameter, with the nominal molecular weight cutoff of 100 kDa. Permeate was collected in a reservoir placed on a digital balance (JA 2003B, Chrom Tech) connected to a data acquisition PC. Permeate mass data was recorded at 0.25 s intervals and converted to permeate flux based on the membrane area (28.7 cm2) and water density (0.986 g/mL).

3.4. Water quality analysis

UV254 absorption was measured using M501 Single Beam Scanning UV/VIS spectrophotometer (CAMSPEC, Sawston, UK) with 1 cm optical path length and photometric repeatability of ± 0.0002 Abs. Prior to measurements, all water samples were filtered through a 0.45 µm syringe filter and average values were obtained by triplicate measurements. Zeta-potential values were measured at 25°C using a Malvern Zetasizer Nano Series Nano-ZS (Malvern Instruments Inc.). Dissolved organic carbon (DOC) content in water samples was determined using the persulfate oxidation method (DR 1900, Hach).

3. Results and Discussion

3.1. Water quality

- UV254 absorbance by the feed water was reproducibly very low: 0.202 ± 0.007 m 1 and 0.254 ± 0.003 m-1 for samples A and B respectively. The measured values of DOC were also low - 0.17 ± 0.06 (sample A) and 0.20 ± 0.10 mg/L (sample B), which was at or below the detection limit (0.3 mg/L) of the DR 1900 instrument.

We used the DOC value of 0.2 mg/L to obtain an estimate of the specific UV254 absorbance (SUVA254) for the water samples. SUVA254 is known to positively correlate with the percent aromaticity of dissolved organics in natural water [5, 6]:

SUVA254 values above 4 L/(mgm) are taken to indicate that organic constituents

[7] are predominantly hydrophobic . In the present study, SUVA254 is estimated to be ~2.0 L/(mgm) for sample A and ~0.9 L/(mgm) for sample B pointing to a low fraction of aromatic DOC and predominantly hydrophilic nature of dissolved organics in Tbilisi Sea water. Zeta potential measurements showed that particles in the water were negatively charged (휁 = -14.4 ± 4.0 mV and -13.0 ± 7.0 mV for samples A and B, respectively).

3.2 Process design. Permeate flux behavior in C/F-UF and direct UF tests.

The mean velocity gradient in a mixed volume is given by eq. (1): 퐺̅ = √푃/(휇 ∙ 푉) (1)

3 5 where 푃 is mixing power: 푃 = 퐾푇푛 퐷 휌, 퐾푇 is the impeller constant. 퐷 is the impeller diameter, 휌 is density of the emulsion, 푉 is emulsion’s volume, and 푛 is the rotational speed. The Reynolds number for the impeller is given by 푁푅푒 = 퐷2푛휌/휇, where 휇 is emulsion’s viscosity. Given the impeller diameter of 7.5 cm, the values of the mean velocity gradient, 퐺̅, during coagulation and flocculation were ~ 30.13 s-1 and ~ 0.95 s-1, respectively. These values were calculated using the impeller constant for a turbulent flow (퐾푇=2.25). This choice of 퐾푇 was based on the values of the impeller Reynolds number, 푁푅푒, for the coagulation and flocculation conditions tested (푁푅푒 ~ 2323 and 232, respectively) and the fact that [8] the dependence of 퐾푇 on 푁푅푒 reaches a plateau at 푁푅푒 > 100 ). The values of

퐺̅푡푚푖푥 (where 푡푚푖푥 is duration of mixing) during coagulation and flocculation were 1808 and 1143, respectively.

The C/F-UF tests simulated the scenario wherein the treatment process train employed by Georgia Water and Power currently (coagulation/flocculation followed by sand filtration) media filtration is replaced with UF membranes. The direct UF tests simulated a more radical alteration of the existing practice where UF membranes replace the entire C/F/sand filtration sequence.

The C/F pretreatment helped alleviate membrane fouling and slow down permeate decline (Figure 2). For water sample A, the flux decline after filtering 120 mL was 36.3% ± 2.3% and 24.0% ± 3.0% in direct UF tests and in C/F-UF tests, respectively. The difference in the extent of fouling between direct UF and C/F-UF cases was statistically significant (푝 < 0.01). The results with water sample B indicated the same trend: the flux decline after collecting 120 mL of permeate was 24.7% ± 2.1% and 19.3% ± 1.2% in direct UF tests and in C/F-UF tests, respectively, with a significantly (푝 < 0.02) less fouling observed in C/F-UF tests.

3.3. Membrane fouling: Blocking law analysis

To elucidate operative fouling mechanisms, blocking law analysis [9] was applied to the permeate flux data generated in the constant pressure dead-end filtration experiments. Hermia derived a common characteristic equation for different fouling mechanisms [9]: 푑2푡 푑푡 푛 = 푘 ( ) (2) 푑푉2 푑푉 where 푘 is a constant and 푛 is the blocking index equal to 2, 1.5, 1, or 0 for complete blocking, standard blocking, intermediate blocking, and cake filtration, respectively. A linearized version of Eq. (2) can be used to identify blocking

푑2푡 푑푡 mechanisms by plotting 푙표푔 ( ) against 푙표푔 ( ) and determining the value of 푑푉2 푑푉 the blocking index 푛. The method can be used to determine whether the data can

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 be interpreted in terms of blocking laws and if yes, can potentially identify all operative mechanisms of membrane fouling.

The feed was not stirred to better match the theory’s assumption of no back- transport of foulants. Another assumption - that all membrane pores are of the same size - was not upheld because the phase inversion UF filter has complex pore space morphology and a distribution of pore sizes. Thus, the model’s predictions are only approximations.

푑2푡 푑푡 Figure 3 presents filtration data in the 푙표푔 ( ) vs 푙표푔 ( ) format. The trends 푑푉2 푑푉 could be attributed to either pore blockage (0 < 푛 < 2) or cake filtration ( 푛 = 0). In

푑2푡 푑푡 sample A filtration data, the average slopes of the 푙표푔 ( ) vs 푙표푔 ( ) 푑푉2 푑푉 dependence are 1.06 ± 0.32 and -0.54 ± 0.64 for UF and C/F/UF experiments, respectively. The difference is statistically significant (푝 < 0.02). Filtration of

푑2푡 푑푡 sample B give a more complex 푙표푔 ( ) vs 푙표푔 ( ) dependences especially for 푑푉2 푑푉 the case of the feed water treated with coagulation/flocculation. The negative slope seen in some of the tests can be explained by the combined pore blockage-cake filtration model [10] as resulting from the simultaneous pore blockage and formation of the cake over blocked areas of the membrane. This is consistent with the findings by Yuan et al. [11] who suggested that membrane fouling by humic acid is a combined effect of pore blockage and cake layer formation.

The results of the blocking law analysis support the hypothesis that the main fouling mechanism was pore blockage in direct UF and cake filtration in C/F-UF tests. This is expected given that the C/F pretreatment is designed to decrease the charge on dissolved and suspended species in water and promote formation of larger aggregates. Such larger aggregates are less likely to enter the nanoscale pores of 100 kDa membranes and, instead, are more likely to form a cake on the membrane surface.

3.4 Removal of dissolved organics

Removal by UF alone was not statistically different (푝 > 0.3) between samples A and B. However, C/F alone and the combined C/F+UF treatments showed significantly higher DOC removals than UF alone. The removal of UV254 for sample B was higher than for sample A although with less than 95% confidence (푝 = 0.05 and 푝 = 0.06, respectively). The improved removals can be attributed to the effect of the cake layer formed on the membrane during filtration of C/F- pretreated feeds. The cake layer can act as a secondary membrane and contribute to the removal of organics during filtration.

3.5. Practical implications

The presented data on source water characterization point to high quality of Tbilisi Sea water making membrane filtration a potential alternative to the conventional surface water treatment process that is currently employed. The experiments on ultrafiltration with and without coagulation/flocculation pretreatment suggest that the optimal strategy is to retain coagulation/flocculation as a pretreatment for membrane filtration. While the present study is very preliminary, results indicate that membranes hold promise as a replacement or auxiliary drinking water treatment technology for Tbilisi. Additional studies with membranes of other types (pore sizes, materials) are warranted. Practically, the optimal strategy may be not to replace granular media filters with membranes altogether but, instead, retrofit the existing treatment with membranes using micro- or ultrafiltration as a post-treatment (or polishing) step.

Recent and ongoing developments in the area - proximity to the Tbilisi National Park, planned relocation of Tbilisi Zoo to a nearby site, establishment of the Tbilisi Recreational Zone, and construction of the Hualing Tbilisi Sea New City all lead to a further increase in recreational uses of Tbilisi Sea. These changes will

© 2019. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0 likely have a negative impact the microbiological quality of the water providing additional motivation for replacing or retrofitting the existing drinking water treatment facilities with membrane-based treatment units.

4. Conclusions

The paper presents results of a preliminary laboratory scale study aimed at evaluating filterability of Tbilisi Sea water by ultrafiltration membranes. The analysis was performed with both raw source water and with coagulation/flocculation used as pretreatment. Coagulation/flocculation was found to improve ultrafiltration performance in terms of both water throughput and rejection of feed organics. Permeate flux data analysis showed that the pretreatment resulted in shifting the fouling mechanism from pore blocking to cake filtration. We hypothesize that the cake layer acts as a secondary membrane to contribute to the removal of feed organics. Both permeate flux and DOC removal data support retaining C/F as a part of the overall treatment process. Further studies are warranted to better gauge the potential of the membrane filtration for treating Tbilisi Sea water to drinking water standards. The optimal strategy might be not to replace the currently used granular media filters with membranes but, instead, retrofit the existing treatment with membranes using micro- or ultrafiltration as a post-treatment (or polishing) step.

Acknowledgements

We thank Dr. Tamaz Agladze (Georgian Technical University) for providing access to the Malvern Zetasizer Nano-ZS instrument in his laboratory and Ketevan Petriashvili (Gardabani Thermal Power Plant) for making the DR 1900 TOC analyzer available to the team. We would also like to thank Mrs. Tamar Kiladze (Georgia Water and Power) for providing information on the water treatment processes currently used in the Tbilisi water treatment plant. Volodymyr Tarabara was supported by a U.S. Fulbright Scholar fellowship and

Tamara Tkeshelashvili was supported by Volkswagen Foundation grant (Az: 93 331).

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Figure 1: Tbilisi Sea: Tributaries and sampling locations

(a)

(b)

Figure 2: Transient behavior of permeate flux in experiments with raw feed water and feed water pretreated using coagulation/flocculation. Feed water was sampled at locations A (a) and B (b).

(a)

(b)

Figure 3: Blocking laws applied to filtration of Tbilisi Sea water by 100 kDa UF membranes. Feed water was sampled at locations A (a) and B (b).

Figure 4. Removal of UV254 from Tbilisi Sea samples by ultrafiltration (UF), coagulation/flocculation (C/F) and by the two processes applied sequentially (C/F+UF). The notation of statistical signiﬁcance follows the following convention: “***”:푝 ≤ 0.01; “**”: 0.01 < 푝 ≤ 0.05; “*”:0.05 < 푝 ≤ 0.1; “ns”: 푝 > 0.1.