Enhanced surface water treatment by ultrafiltration - Science Direct

DESALINATION ELSEVIER

Desalination 119 (1998) 113-125

Enhanced surface water treatment by ultrafiltration J.A.M.H.

H o f m a n a*,

M.M. Beumer b, E.T. B a a r s a, J.P. van der Hoek a, H.M.M. Koppers b

aAmsterdam Water Supply, Provincialeweg 21, 1108 AA Amsterdam, The Netherlands Tel. +31 (20) 651-0310; Fax +31 (20) 697-6880; email:[email protected] bwitteveen + Box Consulting Engineers b.v., PO Box 233, 7400 AE Deventer, The Netherlands

Received 15 June 1998

Abstract

For capacity extension of one of the drinking water production plants of Amsterdam Water Supply, four treatment schemes applying ultrafiltration (UF) are considered. The main purposes of the UF process are phosphate removal, removal of suspended solids and colloidal matter; and hygienic water quality improvement. In the pilot plant investigation, water from the Bethunepolder has been treated with UF. The feed water shows very large and rapid water quality variations, making it a difficult source to treat with UF. Hydraulic performance as well as effluent water quality were studied. The results have shown that stable operation of the membranes is feasible and an excellent water quality can be produced, Phosphate was removed from average 60 gg/l P to 20 ~zg/1Pin the ultrafiltrate. Iron and turbidity were almost completely removed, whereas a log-removal of at least 2.5 to 3.5 was reached for indicator organisms. It was concluded that UF is a powerful treatment process for extension of drinking water production at AWS and offers new opportunities for relatively small-scale, on-site production of household or industrial water. Keywords:

Ultrafiltration; Surface water treatment; Phosphate removal; Removal of micro-organisms; Removal of suspended matter

1. Introduction

Amsterdam Water Supply produces drinking water for the City of Amsterdam and neighboring municipalities. The total production capacity is

*Corresponding author.

101 million m3/y (72mgd). Drinking water production takes place in two treatment plants: Production West (70 million m3/y; 50 mgd) and Production East (31 million m3/y; 22 mgd). To fulfill future drinking water demand extension of the treatment facilities of Production East to ultimately 61 million m3/y (44 mgd) is considered.

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 ElsevierScienceB.V. All rights reservecL P// S0011-9164(98)00129-5

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J.A.M.H. Hofman et al. / Desalination 119 (1998) 113-125

The treatment system of Production East consists of two subsequent treatment plants. Raw water is taken in from the Bethunepolder (BP) and the Amsterdam-Rhine Canal (ARC). The water is pretreated at the treatment plant Loenderveen. The pretreatment consists of a double coagulation/ sedimentation step using ferric chloride, storage in a reservoir for 100 days and finally rapid sand filtration. The pretreated water is transported to the treatment plant Weesperkarspel where ozonation, chemical softening in fluidized bed reactors (pellet softening), biological activated carbon filtration and slow sand filtration are applied to treat the water further to drinking water quality. Fig. 1 shows the process scheme of the whole treatment scheme. A very important step in the pretreatment system is the storage basin. Fig. 2 shows an aerial view of the lake. Due to a self-purification effect, the ammonium concentration in the water is reduced to about 1 mg/l N and the hygienic water quality is improved. Moreover, the lake is completely mixed and therefore peaks in raw water quality will be smoothed (e.g., chloride). To prevent eutrophication of the lake, but also to maintain the self-purification effect, a phosphate concentration between 15 and 25 ~g/l P must be available in the influent. Practical experience has proved that with these phosphate concentra-

tions no algae blooms occur, while an ammonium concentration of 1 mg/l N can be reached by biological nitrification. For the extension of the capacity of the treatment system, a second storage lake is planned, and the treatment system will be extended in two steps of 15 million mS/y (11 mgd), by duplicating the existing treatment. However, due to uncertainties in the growth of the drinking water demand, a more flexible - - in small steps - - extendible system can be beneficial. Application of membrane technology can give the desired flexibility. Several alternative treatment systems involving UF are currently evaluated. Fig. 3 shows four different alternative schemes in relation to the existing treatment. In alternative A, UF will be applied for phosphate removal, improvement of hygienic water quality and removal of suspended solids, which are basically the same purposes of the existing coagulation and sedimentation system. This means that a second storage basin must be created for a.o. ammonium removal, which is a difficult and expensive task. In alternative B, the ultrafiltrate is used to feed a nitrifying filter to remove an excess of ammonium. The effluent of this filter will be used mixed with the feed water of the existing storage lake. The ammonium load of the lake is reduced and therefore a shorter residence time in the lake is

1 I Water Treatment Plant ~Loe~derveen"

1 Water T~a~ment P/ant

'W,w.st~rtun.q~'

Fig. 1. Treatmentschemefor ProductionEast.

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Fig. 2. Aerial view of the storage reservoir and the two rapid sand filter buildings.

BP/ARC-water I BP/ARC-water]

I

II

1

1 I

RsF

I

!

D

tll

I NF/RO I :::::::::::::::::::::::::

Alternati~,eA

Alt#rnative B

E.~ing ~amunt

Altemt~h,e C

Alternative D

Fig. 3. Schematic representation ofthe existing treatment and the four extension alternatives.UF, ultrafiltration;RSF, rapid sand filtration; SB2, second storage basin; NF, nanofiltration; RO, reserve osmosis. sufficient to remove ammonium to the desired 1 mg/l N. Creation of a second storage basin is prevented, although the capacity of the reservoir to level off DOC and chloride concentrations with the reduced residence time is a point of concern.

There is experimental evidence, however, that complete nitrification can be realized in the rapid sand filters after UF. This means that the effluent of these filters can be mixed with the effluent of the existing lake. This is shown in alternative C.

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Smoothing of water quality peaks by the existing lake is in this alternative a point of concem as well. In alternative D, the treatment relies completely on membrane technology. In this system nanofiltration (NF) or reverse osmosis (RO) is used for desalination, softening, disinfection and organics removal.

2. Research objectives To investigate the feasibility of the abovementioned treatment alternatives, a pilot plant study has been conducted since April 1997. The objectives of this study were: • to determine if a stable and safe operation of UF on Bethunepolder water is possible; • to determine the produced water quality with respect to removal of suspended solids, iron, phosphates, and microbiological parameters; • to estimate operational costs of the application of UF; • to determine environmental aspects as chemicals and energy consumption and sludge production; • to make an inventory of other applications of UF technology, e.g., production of industrial (process) water or water for domestic use.

fouling layer form the membrane surface. The water together with the fouling is removed. Fig. 4 shows a flow diagram of the pilot plant. The feed water originates from the Bethunepolder. After the pumping-station the water is transported to the treatment plant in an open canal of 6 km. The water for the pilot plant is taken in from the canal and passes a 200~m screen filter. Due to meteorological influences, very large and rapid variations in raw water quality (mainly turbidity, color, iron content, phosphate and oxygen) can occur (see below). The research has been conducted with two different membrane modules. Both membranes applied the same polymer, but with different inner diameters of the capillaries in the module. Moreover, the design of the applied modules was different. In the first part of the research, from April 24th, 1997 until August 28th, 1997, membrane modules with 1.5mm fibers were applied. Starting on August 29th, 1997, the elements were exchanged for elements with 0.8 mm fibers. Table 1 gives a brief overview of the membrane module specifications. Fig. 5 shows a picture of the membrane element with 0.8mm capillaries. These elements contain three feed flow by-pass tubes. The feed water to the first element in the pressure vessel can partly bypass the element and flow directly to the middle element, resulting in a better distribution of the feed flow

3. The pilot plant The pilot plant is based on three 8x40" hollow fiber UF modules. The three modules are placed in series in a pressure vessel. The feed water enters the modules form one side, alternating from "left" to "right" after backwashes. The permeate leaves the central product tube from both sides and is collected in a vessel. To remove the fouling (mainly suspended solids) from the membrane surface, the modules are backwashed periodically at a flux of approximately three times the production flux. Water is fed from the permeate side and removes the

Table 1 Specifications for the X-Flow S-150-X PVC UFC M5 membrane Membrane I Membrane area, m2 15.0 Inner dia. fibers, mm 1.5 Feed bypass present No MWCO a, kD 150-200

Membrane 2 22.9 0.8 Yes 150-200

aMolecular weight cut-off determined with a 1% w/w PVP solution at 100 kPa TMP.

J.A.M.H. Hofman et at/Desalination 119 (1998) 113-125

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~V04

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Fig. 4. Flow diagram of the pilot plant.

Fig. 5. Feed side of hollow fiber membrane element. In the center is the central product collection tube and three feed bypass tubes.

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among the three elements in the pressure vessel and an overall better performance of the elements.

4. Experimental The research was carried out in three phases. In the first phase, a preliminary study was conducted, in which membrane selection for further experiments took place. Membrane selection took place via a so-called "Quick-Scan" protocol, involving the testing of four different membrane elements in parallel. In these experiments the need for addition of ferric chloride prior to the UF treatment was studied. The final membrane selection was based on their fouling tendency. In the second phase of the study the operating conditions of the UF plant were varied in order to determine conditions that would give a stable operation. Stable operation here is defined as operation at a constant level of irreversible fouling or constant water permeability of the membrane. In this part of the research, the two different membrane modules were applied, and flux, backflush and cleaning regime were varied. Finally, the chemical usage and the composition of the backflush water were analyzed. During the third phase, the long-term stability of the [IF process was determined. In this phase a constant flux of 701/m2h was maintained and chemical usage and composition of the backflush water were determined as well. Finally, based on the experimental results, a basic design of a full-scale installation was made. This design was used to estimate investment and operation and maintenance cost of such an installation.

5. Results and discussion 5.1. Hydraulic performance

During operation, a permeate flow of 4.5 m3/h (28,500 gpd) was maintained. Fig. 6 shows the necessary transmembrane pressure (TMP) to

realize the desired flow. Both types of applied membrane modules (i.e., 1.5 and 0.8ram fibers) showed an increasing TMP. The TMP is determined by the resistance of the fouling layer on the membrane surface and the water temperature. In order to separate both effects, mass transfer coefficients (MTC) were calculated and normalized to 10°C using the water viscosity. The MTC at 10°C is defined as follows: Flux rll0 MTC10 - - - - m/s.kPa TMP

r1

(1)

In the period of April 24th to August 28th, 1997 (1.5mm fibers) a flux of 1001/m2h was applied. During the period of application of 0.8mm fibers, the flux was about 701/m2h. Both values were used to calculate the MTCI0 according to Eq. (1). These results as well as the MTC at the actual temperature were plotted against time in Fig. 7. In the first months of the study (1.5mm membrane fibers), most attention was paid to the optimization of the backflush regime in order to realize a stable operation of the pilot plant. Backflush frequencies of 10, 15 and 20min were applied and experiments with forward flushing were conducted. Also the frequency and chemical selection for a chemically enhanced backwash were investigated. Backflushes with hydrogen peroxide, hydrogen peroxide combined with hydrochloric acid, and sodium hypochlorite were carried out. Because the pilot plant is relatively small, it is expected that the optimum conditions found are not representative for a full-scale UF plant. Further optimization will be necessary. The backflush optimization of the pilot plant has resulted in the following operating conditions: • Water backflush every 20 min during 30s • Chemically enhanced backflush with hydrogen peroxide (200ppm) and hydrochloric acid (pH=l.5) every 3 h, with a soaking period of 20 min

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J.A.M.H. Hofman et aL / Desalination 119 (1998) 113-125

2.0 - - B - TMP dght •

1.5

TMP left

Membrane fibers 1.5 iiiii

C" (o .Q

o. 1.0 =Z I--

0.5 Membrane fibers 0.8 mm

0.0 10-04-97

30-05-97

19-07-97

7-09-97

27-1 0-97

16-12-97

4-02-98

Fig. 6. Transmembrane pressure (bar) for the two experimental periods.

1.5 --B- MTC MTC (10"C) Membrane fibers 1.5 mm ~1.0 Membrane fibers 0.8 mm

i

o 0 I-

0.5

0.0

10-04-97

30-05-97

19-07-97

7-09-97

27-10-97

16-12-97

4-02-98

Fig. 7. Actual and normalized mass transfer coefficient.

Membrane disinfection with sodium hypochlorite (100 ppm) every 24h, also with a soaking period of 20 min.

In the second research period (0.8mm membrane fibers), the forward flush was not applied. This was because the membrane elements

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contained three feed bypass tubes. The water of the forward flush would mainly pass these tubes, making the forward flush ineffective. Fig. 7, especially the MTC normalized to 10 oC, shows that a stable operation of the pilot plant could be realized for both types of membrane elements. For both research periods a constant value of the normalized MTC was reached after a period of decreasing MTC. It is expected that the first decrease of the MTC is due to the formation of a irreversible fouling layer on the membranes. After the initial period an equilibrium is reached and stable operation is possible. The last weeks of the second period show again a decrease in normalized MTC, which means that membrane fouling again increases. This effect is also found in the TMP which increased to its maximum acceptable pressure (200 kPa). Because large and rapid variation occur in the feed water, it is necessary to adapt the backflush regime from time to time. Especially in periods of rainfall, when large amounts of water have to be pumped from the polder, a very fast increase in color and iron content of the water is observed. Furthermore, the water becomes anaerobic and turbidity increases. Iron contents above 5mg/l, turbidities between 50 and 100 FTU and color above 100mg/l Pt/Co are found regularly. This means that the foulant load on the membranes increases and that adaptation of the backflush regime or maybe even lowering of the flux is necessary. 5.2. Water quality

The most important aspects of the effluent water quality will be determined by the application of the UF as an altemative for the coagulation process as mentioned in Fig. 3. The guide levels which the process has to fulfill are: • Phosphate removal to a level of 15-25#g/l-P • Removal of suspended solids and colloidal matter • Improvement of hygienic water quality

5. 3. Phosphate removal

Fig. 8 shows the concentrations of total phosphates in the influent and effluent of the UF unit. The results show that almost a complete removal of total-phosphates can be achieved. The iron content present in the water is removed completely as well (Fig. 9). The phosphates are most probably bound to the iron and therefore available in particulate matter. Only a very small portion of the phosphates is available as dissolved orthophosphate. The latter will pass the UF membranes completely. The ortho-phosphate content of the ultrafiltrate has approximately a constant level of 20~g/1 P. This means that the guide level is fulfilled. Due to the presence of 20gg/l P orthophosphate, the water after UF can be nitrified very well in rapid sand filters following the UF as in alternative treatment B and C (see Fig. 3).

5. 4. Suspended matter and colloidal material

Removal of suspended matter and colloidal material is monitored by turbidity. Fig. 10 shows the turbidity of the influent and the ultrafiltrate. Turbidity is removed to values around the detection level (