Direct nanofiltration or ultrafiltration of WWTP effluent?

DESALINATION Desalination 132 (2000) 65-72

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Direct nanofiltration or ultrafiltration of WWTP effluent? Olaf Duin a, Peter Wessels b*, Helle van der Roest b, Cora Uijterlinde a, Henk Schoonewille c aStichtse Rijnlanden District Water Authority, The Netherlands bDHV Water BV, POB 484, Laan 1914, no. 35, 3800 AL Amersfoort, The Netherlands Tel. +31 (33) 4682496; Fax +31 (33) 4682301; e-mail: [email protected] CStork Friesland B V, The Netherlands

Received 11 July 2000; accepted 24 July 2000

Abstract

From August to December 1999, DHV Water BV (DHV) conducted a study at the Dfiebergen sewage Ireatment plant of the technical feasibility of effluent post-treatment to achieve Maximum Acceptable Risk (MAR) quality. The study was commissioned by the Stichtse Rijnlanden District Water Authority and the part of the study concerned with membrane filtration was conducted in co-operation with Stork Friesland. This article provides a brief description of the study results achieved using the two membrane filtration units. Conducting the studies simultaneously enabled proper comparison of product quality, technical operational management and the costs of ultrafiltration and nanofiltrafion. The article attempts to answer the question of how far new developments in membrane technology provide prospects for producing water with a genuine added value.

Keywords: Direct nanofillration/RO of WWTP effluent; Reuse of WWTP effluent; One-step nanofiltration/RO; Pretreatment; Nanofiltration; Ultrafiltration

1. Introduction

From August to December 1999, DHV Water BV (DHV) conducted a study at the Driebergen sewage treatment plant of the technical feasibility of effluent post-treatment to achieve Maximum Acceptable Risk (MAR) quality. The study was commissioned by the Stichtse Rijnlanden District Water Authority and the part of the study concerned with membrane filtration *Corresponding author.

was conducted in co-operation with Stork Friesland. MAR quality is described in the Fourth Water Management Memorandum and is a guideline with which all surface waters must eventually comply in principle. To achieve the required quality, there has to be a significant improvement in the parameters for total nitrogen, total phosphate, copper, zinc, and organic micro's. Disinfection is also required. Various post-treatment scenarios were examined at the laboratory scale and pilot scale

Presented at the Conferenceon Membranesin Drinkingand Industrial Water Production,Paris, France, 3-6 October 2000 InternationalWaterAssociation,EuropeanDesalinationSociety,AmericanWaterWorksAssociation,JapanWaterWorksAssociation 0011-9164/00/$- See front matter© 2000 ElsevierScienceB.V. All rightsreserved PII: So01 |-9164(00)00136-3

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as part of the study. One of the scenarios consisted of four process steps, including biological filtration, chemical UV oxidation and active carbon filtration. Another scenario concerned the use of ultrafiltration, for which a test plant with a capacity of 2-3 m3/h was used. Obviously, MAR quality was not achievable with this scenario. UF membrane filtration only removes suspended material and the compounds attached to it. Consequently, the test plant was also equipped with the possibility of simultaneously adding flocculents. With quality in mind and also the (re-use) value of the product obtained using ultrafiltration, the study also examined the possibilities ofnanofiltration. A membrane was developed in early 1999 for which preliminary tests suggest excellent product quality could be expected combined with low power and chemical consumption levels. This article provides a brief description of the study results achieved using the two membrane filtration units. Conducting the studies simultaneously enabled proper comparison of product quality, technical operational management and the costs of ultrafiltration and nanofiltration. The article attempts to answer the question of how far new developments in membrane technology provide prospects for producing water with a genuine added value.

Fig. 1. Pilot plants at Driebergen sewage treatment plant.

2. Direct ultrafiltration

Various studies using ultrafiltration as an effluent treatment technique have already been conducted. They all showed that membrane fouling is difficult to prevent. In a study at the Kaffeberg sewage treatment plant, in Limburg, microfiltration and ultrafiltration were compared with conventional sand filtration [1]. Although fluxes of 70 to 80 l/(m2.h) were achievable using ultrafiltration and microfiltration, technical operational management experience or the occur-

Fig. 2. The UF pilot with Stork 8 in compact element (5.2 mm tubular, ¢~= 10-30 nm, 30 m2).

rence of membrane fouling meant that sand filtration was the preferred choice. Sand filtration

O. Duin et al. / Desalination 132 (2000) 65-72

as a pretreatment before microfiltration and ultrafiltration is usually recommended to prevent or limit membrane fouling [2,3,4]. Previous experiences that showed pretreatment had a negative influence on the performance of the membrane filtration process led to the decision to study direct ultrafiltration (i.e. without pretreatment) in the trial study at the Driebergen sewage treatment plant (Fig. 1). The UF test plant (see Fig. 2) was in continuous operation for four months. The plant incorporated the Airflush principle, a periodic crossflush with air and water. This method of cleaning was considered extremely effective in other applications [5,6]. With a view to improving the product and the operational management of the membranes, tests were also conducted during the study period to assess the effectiveness of inline flocculent addition and powdered carbon addition.

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m Fig. 3. The NF pilot with Stork 3 in capillary element (1.5 mm tubular, ~ = 0.5-2.0 nm, 2.2 m2).

Likewise for a period of around four months, a test unit using direct nanofiltration was operated in parallel with direct ultrafiltration (see Fig. 3).

3. Direct nanofiltration

Direct nanofiltration of sewage treatment plant effluent is a new technology, which was first extensively tested at the Driebergen sewage treatment plant. The present nanofiltration or RO plants for surface water or effluent are mainly of the spiral wound type. The sealed packing of the membrane surface and the spacers make these systems very prone to fouling by suspended material and biomass. In practice, plants of this kind necessitate extensive pretreatment in the form of capillary microfiltration or ultrafiltration. Capillary or tubular nanofiltration components are far less prone to fouling by suspended material and are also easier to clean (hydraulically but also chemically). This means they can be used directly, i.e. without pretreatment, for surface water or effluent. The much finer pore size means that significantly higher quality can be achieved with nanofiltration than with UF.

4. Direct nanofiltration and ultrafiltration compared

Experiences with both pilot units operating simultaneously at the Driebergen sewage treatment plant and using the same input water make it possible to compare ultrafiltration and nanofiltration in terms of product quality, operational management and costs.

4.1. Comparison of quality

The removal efficiency for various parameters is shown in Table 1. This shows that, unlike with ultrafiltration, dissolved substances are also removed by nanofiltration. Moreover, in comparison with ultrafiltration, significantly better quality is achieved with regard to chemical oxygen consumption (COC), phosphate, heavy metals and electrical conductivity (EC).

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Table 1 Removal efficiency of direct NF and direct UF at Driebergensewage treatment plant Parameter Turbidity, NTU COC, mg O2/1 Phosphate, mg/l Nitrate, mg/l EC, mS/cm Zinc, p.g/l Copper, ~tg/l Diuron, lag/1

Quality of effluent at sewage treatment plant Direct nanofiltration, Direct ultrafiltration, (during trial period) % % 1.5-15 19 0.65 3.7 0.67 73 6 0.13

+95 75-80 50-55 5-10 ±25 75-80 70-75 ±25

±95 +10 < 10 0 0 ±25 ±10 nk

Direct ultrafiitration without addition of flocculent/powderedcarbon. Adding powdered carbon or flocculent before the ultmfiltration membranes enables comparable phosphate removal and COC to be achieved using ultrafiltration. 2 The NF membranes used in the trial study have a lower retention figure for dissolved substances than that of the familiar and frequently used spiral wound NF membranes.

The NF membranes used in the trial study enabled production of high quality permeate. The MAR quality was achieved, with the exception of rather high nitrate and phosphate concentrations (3 and 0.3 rag/l). Although the microcontamination levels measured were below the MAR value, a little caution is called for when translating them for the effluent of other sewage treatment plants. Moreover, micro-contamination levels in effluent may vary according to the season. When nanofiltration was used, the effluent was completely clear and colourless. To further improve quality, the development of capillary nanofiltration should focus on the retention. MAR quality could not be achieved using ultrafiltration with the prior addition of flocculent or powdered carbon. This means that achieving MAR quality requires other treatment in addition to ultrafiltration, in order to attain the required removal of substances.

4. 2. Operational management comparison

The trial study clearly showed that direct nanofiltration enables more robust operational management than direct ultrafiltration. Observations showed that nanofiltration is much less sensitive to sudden quality changes. In particular, positive results were seen in periods with high hydraulic input levels as a result o f rain. During rainy periods, effluent turbidity increased by a factor of about 10. Nanofiltration hardly responded at all to these peaks, whereas with ultrafiltration a clear temporary increase in transmembrane pressure was measured. The number of chemical cleaning processes was also considerably lower for nanofiltration. The nanofiltration membrane was only cleaned once a week, using soap. With ultrafiltmtion, a cleaning process using a chlorine bleaching lye was necessary six times a day. The main operational management parameters are shown in Table 2.

O. Duin et al. / Desalination 132 (2000) 65-72 Table 2 Comparison of process parameters for direct NF and direct UF Parameter

Direct nanofiltration

Direct ultraflltration

Input pressure, kPa

400

30

Stable flux, l/(m2.h)

25

>90

Hydraulic cleaning - frequency - type

2 per hour Airflush

6 per hour Backflush + Airflush

Chemical cleaning - frequency - chemicals used

1 per week soap

- consumption

negligible

Recovery,%

=80

6 per day chlorine bleaching lye 1.6 g/m 3 act. chlorine >85

During the entire trial period, cleaning with hydrogen peroxide was only carried out twice for direct nanofiltration.

It was possible to operate stable ultrafiltration with a gross flux of more than 90 l/(m2.h) and a transmembrane pressure of 0.3 bar (see Fig. 4). Cross flushing with air and water was carried out every 10 min. The stable flux figure for nanofiltration was significantly lower, at around 25 1/(m2.h), with a transmembrane pressure of 4 bar (see Fig. 5). Flushing with air and water was carried out every 30 min for nanofiltration. Airflush was used in both systems to flush contamination from the capillaries. Airflush entails cross flushing with water, to which air is added for 5 to 10 seconds. The Airflush creates a high level of turbulence, thereby enabling optimal hydraulic cleaning of the membrane. The Airflush proved to be very useful in both ultrafiltration and nanofiltration (see Fig. 6). Operational management of nanofiltration is largely dependent on the speed of the (continuous) cross-flow. A very low-speed cross-flow

Week 40 Stable operation at 93 I/m2h and Airflush every 10 minutes

Fig. 4. Stable operation of ultrafiltration.

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high cross-flow

50 45 ~. 40

low cross-flow

low cross-flow dead-end

o 35 O

~. 25

i ~ ~\~

~ 20 15 -,i E 10 5

~ ~ ~

~ ,~

x

r H202 cleaning I

f

Airflush stop H20 ~ cleaning I

15 Sep 22 Sep 29 Sep 6Oct

I

i

r

date

Fig. 5. Results for direct nanofiltration.

Fig. 6. Fault in Airflush ultrafiltration.

,

I

I

I

I

13 Oct 20 Oct 27 Oct 3Nov 10 Nov 18 Nov 25 Nov 2Dec

9Dec

O. Duin et al. / Desalination 132 (2000) 65-72

results in a low flux of around 15 1/(m2.h), and low salt retention (to 0% on EC for dead-end filtration). Initial optimisation indicated it is best to use a longitudinal flow speed of around 0.5 m/s. Flux and retention are reasonably optimised at this rate, and power consumption for the cross-flow is still low with respect to the power required for the filtration process. In November and December 1999, tests involving the addition of flocculent and powdered carbon were also carried out in the UF plant. Stable operational management was achieved here as well. However, the acidity level proved to be an important parameter for the stability of the process and the effectiveness of phosphate removal.

4. 3. Comparison o f costs

Lower flux and higher pressure make direct nanofiltration more expensive than direct ultrafiltration. Cost calculations show that direct nanofiltration is around twice as expensive as direct ultrafiltration (around 0.30--0.50 Euro/m 3, at 85-98% effluent treatment), all depending on the basic design and size of the project. Nanofiltration still works out more expensive, even if sand filtration is used before ultrafiltration. This does not take into account concentrate processing, because it was assumed that the concentrate is fed back to the influent of the sewage treatment plant. Though the latter only applies in the case of partial treatment. In comparison with ultrafiltration followed by spiral wound nanofiltration or low pressure RO, direct nanofiltration using capillary membranes is a cheaper alternative. It should be pointed out that capillary nanofiltration membranes are currently expensive. This is mainly because of low production volumes. However, nanofiltration membranes are produced in the same way as ultrafiltration membranes, so a comparable

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membrane price could be expected in the case of large-scale use. Costs could be further optimised by using a combination of continuous sand filtration and limited parallel, partial flow direct nanofiltration treatment.

4. 4. Direct nanofiltration or ultrafiltration?

Ultrafiltration and nanofiltmtion are physical processes. The removal efficiency for various substances is mainly determined by the properties of the membrane. It is therefore reasonable to expect that using the two techniques for other sewage treatment in the Netherlands will result in a similar removal efficiency rate. However, operational management may display significant differences. It is therefore always advisable to conduct an on-site trial study before starting any actual construction work. Direct nanofiltration offers some advantages vis-a-vis direct ultrafiltration. The quality of the product is significantly better, which may be attractive for specific applications. Stable operational management is another important advantage. Direct nanofiltration proves to be a robust technology for treating effluent. This translates into, amongst other things, lower costs (operational management, cleaning and maintenance) and a lower environmental impact (lower power consumption, fewer chemicals) when compared with spiral wound nanofiltration systems. Owing to the costs, the choice of direct nanofiltration will at present have to be based mainly on quality. Ultrafiltration or another type of filtration can be used, if only suspended material has to be removed. Direct nanofiltmtion already seems an attractive alternative for applications in which post-treatment effluent has to be colourless, hygienic, reliable and, as far as possible, free of organic matter. There is a real possibility that the water produced can be reused

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for all kinds o f applications, such as process water and household water. Within the foreseeable future, high-retention capillary nanofiltration membranes and possibly even capillary low-pressure RO membranes, will be developed. Membranes with a higher retention percentage for dissolved substances make direct nanofiltration or RO even more suitable for effluent refinement. This provides scope for producing water with genuine added value.

References [1]

Ch. Ruiters, F. Kramer, A. de Man and A. Hoeijmakers, H20, 23 (1999) 50, [in Dutch].

[2]

J. Pluim, Pilot onderzoek naar de toepassing van ultrafiltratie op effluent van rioolwaterzuiveringsinstallaties, Afstudeerverslag, TU Delft, 1997, [in Dutch]. [3] M. Weijs, Ultrafiltratie van rwzi effluent, verkennend onderzoek met een pilotplant, Afstudeerverslag,TU Delft, 1997, [in Dutch]. [4] S.C.J.M. van Hoof, C.P.T.M. Duijvesteijn and P.P.R. Vaal, Desalination, 118 (1998) 249. [5] W. van der Meer, R. Termeulen, P. de Moel and H. van Dalfsen, H20, 4 (1999) 20, [in Dutch]. [6] L.P. Wessels, Onderzoek directe nanofiltratie, Post-Academic Course, Delft University of Technology, May 23, 2000, [in Dutch].