Keywords nanofiltration; capillary membrane; direct treatment; backflush; surface water.

Transcription

1 Direct Capillary Nanofiltration for surface water treatment Harry Futselaar 1*#, Henk Schonewille 1, Idsart Dijkstra 2 (1) NORIT Membrane Technology B.V., P.O. 731, 7500 AS ENSCHEDE (The Netherlands); phone: ; fax: ; info@noritmt.nl authors: h.futselaar@noritmt.nl; h.schonewille@noritmt.nl (2) VITENS, P.O. Box 10005, 8000 GA ZWOLLE (The Netherlands) phone: ; fax: ; idsart.dijkstra@vitens.nl Abstract Capillary nanofiltration (NF) enables surface water to be treated in a single step to produce high-quality permeate. Trial studies show that a stable operation is achievable using capillary NF without pre-treatment (in other words direct NF), that these capillaries are backflushable, that there is a low use of chemicals and that the permeate is of a high quality. The capillary NF membrane module combines the favourable cleaning properties of capillary ultrafiltration membranes with the favourable separation properties of NF membranes in terms of the removal of bacteria, viruses, colour, hardness and pesticides. Keywords nanofiltration; capillary membrane; direct treatment; backflush; surface water. 1. Introduction For many domestic and industrial applications large amounts of water are only used once for cleaning purposes after which these streams are purged into the sewage system. Generally, the source for this washing water is valuable potable water or locally pumped ground water. These means, however, are becoming scarce due to an inspected increase in price of potable water or due to environmental legislation penalising the intake of ground water in order to prevent the dehydration of the soil. The possibilities for new technologies are open to enter the water treatment market enabling the continuation of a sufficient and affordable water supply, especially with lower quality sources as main feed stock, such as: surface water; effluent of waste water treatment plants. Membrane technology is such a new treatment method for producing water from contaminated sources. A nanofiltration (NF) membrane, for instance, removes bacteria and viruses as well as pesticides, organic matter, heavy metals, and to some degree also salts. The permeate is therefore of a high quality, reliable and can be used as process water for industry or as house keeping water for domestic applications. Membranes in current NF installations are usually designed as spiral-wound elements and placed in pressure vessels. In order to use spiral-wound NF membrane elements pre-treatment is necessary before the water can be treated. In practice, membrane filtration is part of a (long) series of treatment steps, such as flocculation/sedimentation + rapid sand-filtration + NF; (double) rapid sand-filtration + NF; (capillary) ultrafiltration (UF) + NF; making membrane filtration not always economically viable to produce water out of lower quality sources.

2 In a previous study [1] a new purification concept was introduced: direct capillary NF and its potential for direct treatment of surface water and effluent of a waste water treatment plant was shown. More recently [2], this capillary NF membrane was compared with commercial flat sheet NF membranes for the treatment of surface water showing an excellent flux behaviour and comparable rejection characteristics. In this paper, the capillary NF concept is developed further for surface water treatment introducing also backflushing as a cleaning option for NF. Moreover, the chemical cleaning frequency is investigated. Figure 1: (a) (b) Capillary 8 inch NF module: (a) front view; (b) mounted in a pilot plant. 2. Direct capillary nanofiltration A capillary NF membrane combines the favourable properties of the capillary UF membranes in terms of ease of cleaning with the favourable properties of the NF membrane in terms of the removal of bacteria, viruses, pesticides and heavy metals. The capillary NF membrane is presently available in the well-known 8 inch modules which are also being used successfully for capillary UF (Fig.1a). The capillary NF module is operated in the same way as semi dead-end UF (Fig.2). During the production run the concentrate valve is closed and all the feed supplied to the system is withdrawn as permeate. In order to stabilise the flux and rejections at an acceptable level a small cross-flow velocity is applied over the module. If the rejection drops too much, the concentrate valve is opened and the system is flushed by means of air-enhanced forward flushing, a so-called AirFlush. During this flushing procedure a backflush can also be carried out. Subsequently, the concentrate valve is closed and the production run starts again. This new process enables the treatment of various water sources in a single step to produce high-quality water. The direct use of the NF process is referred to as "direct capillary NF".

3 recirculation permeate concentrate feed Figure 2: backflush forward air flush air (a) (b) Operation of direct capillary nanofiltration: (a) production; (b) cleaning. backflush 3. Experimental results Synthetic solution In order to show the benefits of operating a NF system in semi dead-end mode a synthetic solution of 2.5 g/l magnesium sulphate was filtrated under various ways of operation: fully dead-end: the purge was closed during the complete experiment; cross-flow: 50% of the feed solution was purged as concentrate; semi dead-end: every 10 min. an AirFlush (AF) of 20 s was carried out. During all the experiments the transmembrane pressure (TMP) was kept at 3.8 bar, while the average velocity at the feed side of the capillary was set at 1.0 m/s R overall (%) retention J (l/m2.h.bar) flux 5 purge 0 + FF 0 s purge 0 + FF 20 s purge 50 + FF 0 s 0 Figure 3: time (min) Rejection and flux as a function of time for various ways of operation; (concentration MgSO 4 = 2.5 g/l; circulation velocity = 1.0 m/s; transmembrane pressure = 3.8 bar). 0

4 Fig.3 shows the flux and the rejection of the feed and the permeate as a function of the processing time. If the system operates in dead-end the flux and rejection decrease in time due to the accumulation of MgSO 4 in the circulation loop. In the final situation the rejection drops until zero, because the permeate concentration equals the feed concentration. By continuous purging 50% of the system content a steady state flux and rejection can be reached with a recovery of 50%. Operating the system in semi dead-end and regularly purging the system content through an AirFlush (in this experiment every 10 min.) comparable, average flux and rejection levels can be reached as for the cross-flow way of operation. The recovery is increased now till around 65% without a lower product quantity and quality. The choice between cross-flow and semi dead-end operation has to be made through an economical evaluation. Surface water At pumping station Elsbeekweg (in the supply area of Water Company VITENS), direct NF was used on surface water from the Twente Canal. A fully-automated pilot installation (Fig.1b) with a capillary NF module was installed which was fed with canal water only strained with 450 µm. The aim was to investigate the production of drinking water from "area-specific" surface water with the focus on the influence of backwashing on the process stability. For a 2-month testing period, Fig. 4 shows the change in TMP for 2 different production rates. Table 1 summarizes the main operation parameters, while Table 2 shows a summary of the main cleaning actions. For the entire testing period, no changes in the AirFlush were made (every 20 min.), while the chemical cleaning always consisted of 250 ppm H 2 O 2, followed by NaOH at ph 11 and HCl at ph 3. The cleaning frequency and duration were varied according to Table ,0 25 Flux (l/m2*hr) ,0 1,0 TMP (bar at 20 C) 5 I. II. III. IV. V. VI. VII. VIII. IX. X. 0 0, time Figure 4: Results of capillary NF used on surface water. Flux TMP The comparison of period I with II shows very clearly the positive effect of a backflush. Over the same experimental time, the increase in TMP with backflushing is much less than without. During period III, the backflush was switched on again resulting again in a much flatter TMP course, although the TMP recovery by a chemical cleaning was not so effective as in period I due to a heavier load in the feed water. After a shut down period because of problems with the feed water supply the pilot was cleaned chemically on 02/05 restoring the original TMP-level.

5 The experiments were continued during 5 days without backflushing (period IV), after which the flow rate was increased without chemical cleaning but turning on the backflushing (period V). The TMP increased strongly, but the module could be cleaned with the standard cleaning procedure after which it behaved again similar (period VI) as in period III. Based on the strong increase in TMP during period V it could be expected that a too long operation at a too high flow rate could lead to an irreversible or difficult to be removed fouling on the membrane. The currently applied chemical cleaning procedure was effective as was shown in the periods I and II and between the periods V and VI. Therefore, the effect of frequency and duration was investigated without changing the overall chemical cleaning time by doubling the cleaning frequency and halving the cleaning duration (Table 2). In period VII, a high flow rate was used and every 2 days the standard chemical cleaning was carried out causing a steady increase in the starting TMP after every cleaning cycle. Next, the module was cleaned with longer soaking times and the original starting TMP was restored. The run at a high flow was repeated (period VIII) with a daily chemical cleaning at half of the soaking time limiting the TMP increase significantly, but not yet restoring completely the starting TMP after every cleaning cycle. Decreasing the flow rate (transition from period VIII to IX) without a chemical cleaning showed that the fouling was too a large extent reversible and could be removed by a long chemical cleaning on 23/05. After a long break caused by maintenance at the pumping station, the pilot was restarted to repeat the frequent cleaning experiment from period VIII (period X). As can be seen in Fig. 4 the daily chemical cleaning at half the duration limited the TMP increase and recovered the starting TMP after every cleaning cycle. The decrease in flow rate after the chemical cleaning showed that the TMP returned to the initial value at the starting of this experimental period. Table 1: Main process parameters for direct capillary NF. Parameter Feed pressure (kpa) Recirculation velocity (m/s) 0.5 Flux (l/(m 2.h)) 12.5 or 20 Hydraulic cleaning: - frequency - type Chemical cleaning: - frequency - order chemicals used see Table 2 AirFlush backflush see Table 2 H 2 O 2 ; NaOH, HCl Table 2: Cleaning parameter for direct capillary NF. period frequency (1/h) AirFlush backflush chemical cleaning dates chemical cleaning duration (min.) I. 3 yes yes & /60/60 II. 3 yes no & /60/ & III. 3 yes yes 25/04 & 28/04 150/60/60 IV. 3 yes no /60/60 V. 3 yes yes none - VI. 3 yes yes /60/60 VII. 3 yes yes & /60/60 VIII. 3 yes yes 19/20/ /30/30 IX. 3 yes yes & /60/60 X. 3 yes yes 03/04/05/ /30/30

6 The removal rates for various parameters are given in Table 3, indicating that this capillary NF membrane has good properties for removal of turbidity, heavy metals and colour, while the demineralisation is limited to partial softening and hardly any removal of monovalent ions. These results on surface water are in agreement with another study performed at a different surface water [2]. Table 3: Removal rates for direct capillary NF. feed Rejection Turbidity ±95 Conductivity mg/l 51 ±15 Ca mg/l 51 ±30 Mg mg/l 7 ±35 Hardness mmol/l 1.55 ±30 Fe mg/l 0.55 ±95 Mn mg/l 0.16 ±60 Al µg/l 97 ±95 DOC mg/l 11 ±95 colour mg/l 32 ±95 4. Conclusions This study has demonstrated clearly that direct capillary NF results in a long-term stable flux behaviour with a minimal of chemical cleanings. AirFlush has proved to be a very efficient, powerful way of operation to stabilise the flux and to reduce the amount of chemical cleanings. Moreover, the capillary NF membrane is backflushable limiting the TMP increase in time significantly. Chemical dosing to the feed side by means of a backflush will still be investigated in order to start the soaking and reaction process at that point where the fouling sticks to the membrane surface. Daily chemical cleaning for a shorter duration time is more effective than less frequent chemical cleaning for a longer time. Optimisation of the frequency and duration of the chemical cleanings will still be subject of investigation, while the temperature level on the cleaning time will also be studied. Direct capillary NF enables raw water to be treated in a single step to produce a high-quality permeate. The permeate can be used as process water for the industry or as house keeping water for domestic applications. 5. References [1] H. Futselaar, H. Schonewille, W. van der Meer, Direct capillary nanofiltration a new high-grade purification concept, Desalination 145 (2002) [2] B. van der Bruggen, I. Hawrijk, E. Cornelissen, V. Vandecasteele, Direct nanofiltration of surface water using capillary membranes: comparison with flat sheet membranes, Separ. Purif. Technol. 31 (2003) Acknowledgements The authors wish to thank Program EET Economy, Ecology and Technology of the Ministry of Economic Affair and NOVEM The Netherlands Agency for Energy, for their financial support.