ELSEVIER Desalination 146(2002)361-366

DESALINATION

www.elsevier.com/locateldesal

Nanofiltration and adsorption on powdered adsorbent as process combination for the treatment

waste water of severely contaminated

Johannes Meier*, Thomas Melin, Ludger H. Eilers

Institutfiir Vegahrenstechnik, Aachen University of Technology, Turmstrasse 46, 52056 Aachen, Germany Tel. +49 (241) 8095428; Fax +49 (241) 8092252; emails: meier@ivt.twth-aachen.de, meEin@ivt.rwth-aachen.de,

eilers.ludger@ bcgcom

Received 7 February 2002; accepted 9 April 2002

Abstract

A new process combination for the treatment of severely contaminated waste water has been developed: powdered adsorbent is injected into the feed of a nanofiltration unit and removed from the concentrate subsequently by a thickener. The powdered adsorbent in the feed has a positive effect on permeate quality, permeate flux and fouling layer in the nanofiltration (NF) unit. Experiments with a semi-continuous pilot plant and biologically pre-treated landfill leachate have shown that a recovery rate of 97% is possible and that the AOX and COD rejection are considerably increased by the powdered adsorbent. Experiments concerning the fouling layer have shown that it can be controlled by a non-chemical flushing procedure consisting of a combination of feed cross flow, air flushing and permeate back flushing. In comparison to reverse osmosis the process combination has higher maximum recovery rate, lower operating pressure and energy consumption which results in lower treatment costs. On the other hand the permeate concentrations are higher than for reverse osmosis but nevertheless still below the legal standards.

Keywords: Nanofiltration; Adsorption; Waste water treatment; Process combination; Landfill leachate; Fouling control

1. Introduction

The major goals of any process for the treat- ment of waste water streams are a low concentrate

volume and a product quality which meets the legal standards. Many waste water streams which can be considered as severely contaminated contain a large number of different contaminants

*Corresponding author. and processes consisting of several unit operations

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France, July 7-12, 2002.

OOl l-9164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 1 1-9 164(02)005 13- 1

362 J. Meier et al. /Desalination 146 (2002) 361-366

are used to treat them. Especially for the treatment of landfill leachate reverse osmosis (RO) is often an important step in the overall process.

The advantages of RO are a very high rejection for almost all contaminants and a high recovery rate. With a two-stage RO plant recovery rates of 90-95% are possible [l]. The disadvantages are comparatively high investment costs and high energy consumption due to high pressure. More- over the membranes need regular chemical cleaning.

NF and adsorption are alternative waste water treatment processes. NF operates at a lower pressure but it has a low rejection for uncharged particles and particles smaller than 200 g/mol. Adsorption is a comparatively cheap process which cannot remove charged or non-polar particles. The basic idea of the presented process combination is to merge NF and adsorption into a hybrid process which similar to RO has a high rejection for many contaminants and a high recovery rate but at the same time causes lower costs.

The process has been developed and tested with landfill leachate which is usually difficult to treat due to the high concentration of many different solutes and because of a high fouling potential. A successful treatment of landfill leachate would indicate that other waste water streams can be successfully treated by the same process combination. The concentration of certain substances in landfill leachate and legal standards in Germany are given in Table 1.

Table 1 Composition of landfill leachate and legal standards in Germany

Pretreated Legal standard, waste water Germany

PH 7.3 - Conductivity, @/cm 11300 COD, mgil 1450 200 BOD, mg/l 30 20 N03-nitrogen, mgil 128 NOz-nitrogen, mgil 0.5 2 AOX, mg/l 1.73 0.5

2. Description of the process combination

Powdered adsorbent is suspended in the waster water in a stirred tank with a residence time which is sufficient for adsorption of the contaminants on the powdered adsorbent to take place. This suspension is then fed to an NF unit. Subsequently to the NF unit, the powdered adsorbent and a part of the concentrate are removed from the process and the remaining clear concentrate is recycled back into the stirred tank. The aims of combining the two processes this way are: l A better permeate quality especially for the

AOX by adsorption before the NF unit l A higher permeate flux due to the powdered

adsorbent acting as a filter aid l A fouling layer which can easily be removed

due to non-polar coal particles within the layer l A high charge of the powdered adsorbent due

to the high concentration of contaminants in the concentrate of the NF stage.

A similar process combining adsorption and ultrafiltration has been developed by Lyonnaise des Eaux and is used for the treatment of surface water in 12 full-scale drinking water treatment plants in Europe with a total capacity of 200,000 m3/d

[XV.

3. Experimental

The process combination has been investigated with a test plant as shown in Fig. 1. The plant was operated in a semi-continuous mode, i.e. the biologically pretreated landfill leachate was supplied from a storage tank with a maximum volume of 2.24 m3 and the concentrate was not removed from the process. The applied pressure was 30 bar and the cross flow velocities were between 0.1 and 0.7 m/s.

The pilot plant was equipped with the DTF module Pall Rochem, Germany, which consists of stacks of membrane cushions and feed spacers (see Fig. 2). The channels between membrane and feed spacer are about 1 mm in height and thus the module can be operated with suspended particles.

J. Meier et al. /Desalination 146 (2002) 361-366 363

fresh adsorbent

sedimentation

landfill leachate

. ..- ,_ -. __ _“_ .” -_.__ __^^“..

Fig. 2. DTF module by Pall Rochem.

As adsorbent either lignite coke dust (LCD) or powdered activated carbon (PAC) was used. The applied powders were similar in particle size distribution but different in specific inner surface (LCD 3.50 m*/g, PAC 1000 m2/g) and price (LCD 300 e /t, PAC 1.500 e /t). For experiments con- cerning the investigation of the fouling layer the adsorbent was made inert prior to the test, i.e. it was charged in landfill leachate of the required concentration.

For the control of the fouling layer a flushing procedure consisting of different combinations of feed flushing without transmembrane pressure, air flushing and back flushing by permeate side pressure were applied.

4. Experimental results

In Fig. 3 the permeate flux over recovery rate is shown for DTF module with LCD and PAC as

I. Flow sheet of the test plant.

DTF module (LCD)

/’ /

,,’ DTF module

Spiral wound module

20

I

80 100

Fig. 3. Permeate flux for different module-adsorbent combinations.

adsorbent and without any adsorbent. Moreover the flux for a spiral wound module is shown which would be the module of choice if no powdered adsorbent was present. For recovery rates below 50% DTP module with LCD and PAC and the spiral wound module show similar permeate fluxes. But for high recovery rates the flux for the DTF module with PAC is almost twice as high. The maximum recovery rate is 97%. The DTF module without adsorption has significantly lower permeate fluxes than the three other combinations.

Besides an increase in permeate flux the rejection for different contaminants is increased as well. The permeate concentrations of COD and AOX (adsorbable organic halogen compounds) for different adsorbent module combinations is shown in Fig. 4. The DTF module in combination

364 Z MNer et aL I Desafinafion 146 ~002) 361-366

400

~ 300

~ 200

$ ~ 100 © O

/ DTF module (LCD) ~,. [ •

~ / ~

. . . . . DTF module (PAC) U/P moeule \ •

//' Spiral wound module \ \ \ / '/ ~ \ \ @ - .

/"' ## • i ~ ~ ~ ~" "~ "O ~ " . . . . . ~;~ • i i i , ~ ~

20 40 60 80 100 Recove~ [%]

1~00

DTF mod~e (LCD) & \\\

• DTF modu~ (PAC)

0 0 20 40 60 80

R e c o v e ~ {%]

~ g . 4. COD - - ( l e ~ ~ d A O X m o ~ e m ~ n O g ~ ~ me ~ e a t e ~ r ~ m m a ~ o ~ e m ~pes.

100

with ~ s o ~ i ~ to ~ C ~ s the ~ w e ~ concen- trations and it is the on~ process wh~h ~ e t s the AOX limit ~ 5 ~ ~ ~ c u ~ .

Four ~ m ~ g W ~ u ~ s w e ~ ~ v e ~ rigged: • ~ s ~ ~ ~ ~ ~

(TMP) and wkh cross flow v d o c i ~ on the same level as during ~ ~ of ~ e m o ~ l e

• ~ c ~ ~ w ~ ~ P ~ ~ ~ c ~ e d cross flow velocky

° ~ ~ ~ ~ ~ c ~ e d cross flow vMoMff + Mr flush~g

• ~ ~ ~ ~ ~ ~ ~ ~ c ~ e d

cross flow ve~c~y + ~r flus~ng + ~ e b ~ k flus~ng.

~ ~ g . 5 ~ e ~ ~ ~ x o v ~ time is shown ~ ~ e ~ ~ m r u i n g p m ~ s ~ ~ e case of no ~ ~ t p~sem ~ ~ d ~ r ~ e case ~ e ~ ~ ~ ~ m ~ . h ~ e ~ a ~ ~1 ~ e flush~g p m c ~ u ~ s ~ y ~ ~ y e ~ . In ~ e ~cond case ~ d ~ y flus~ng pmcedu~ ~ur keeps the ~ e flux on a ~vel wh~h is ~ f f i o ~ y ~ g h ~ ~an ~ r ~ e ~ n w i ~ o ~ f l u ~ g .

~ e ~ e b ~ w ~ n ~ e ~ o c~es ~ c m e s • m ~ e powd~ed ~ e ~ ~ s a ~ s ~ e ~ c t on ~ e ~ g 1 ~

. . . . . . . . .

w~hout flushing ~ ~

1o ~

~'21 | i i ! i i 1,1 t I ................ i ! ........... ! p r o c e d u r e 4 ~ ~ 1,o~

• 0,9 ~ ,8

.~ 0,7

~ 0,6

0,5 ~ u t f ush ng k 2 ~ : ~ L : ..... 0,4 [ i i procedure z i ......

0 1 2 3 4 5 6 0 1 2 3 4 5 6 ~me T ~ e [h] [nj

Eg. 5. Effe~ ~ ~ f f e ~ m flushing pmcedu~s w i g ( l e ~ ~ d w i ~ o m (fig~) a ~ o ~ e m p ~ m .

J. Meier et al. /Desalination 146 (2002) 361-366 365

5. Comparison with an existing plant combination. The treatment costs of the plant in

On a landfill near Aachen the leachate is treated by a combination of biology, ultra-filtration, RO and evaporation (Fig. 6).

In Fig. 7 this plant is compared to a virtual plant

Alsdorf are the actual costs of the process while the costs of the new process combination are estimated. Details of the estimation can be found in [2].

based on the new process combination operated with either LCD or PAC. Although the operating pressure of the RO unit is twice as high as for the NF unit the recovery is only 80% instead of 96% or 97%. The permeate quality regarding the COD and the AOX concentration is much better for the existing plant.

On the other hand the specific treatment costs are significantly higher than for the process

6. Outlook

The investigations have been limited in time due to the low degree of automatisation of the plant and the limited volume of available biolo- gically pretreated landfill leachate.

Questions such as the durability of the mem- branes operated with abrasive feed solution and

Condensed vapour

Fig. 6. Existing plant in Alsdorf Warden.

600

500

Plant Alsdorf

wBio_NF-LCD

oBio_NF-PAC

400

300

200

96 97

80 60 99 72 100

Fig. 7. Comparison of an existing plant and two virtual plants.

366 z Me~r et aL / DesaHna~on 146 ~002) 361-366

the long term success of the membrane flushing could not have been investigated.

Therefore a fully automated pilot plant has been set ~p on the landfill in AlsdorL The plant is in operation since November 2001 and currently detailed investigations concerning the flushing procedure are going on.

References

[1] C. Casper and ~ Mann (Eds.), ATV-Handbuch ~ d ~ e a b w ~ Em~ B e ~ 1999.

[~ I. B a u ~ M.R. Chevalier, ~ Ans~m~ S. Com~ and J.M. L~n6, D~alination, 113 (1997) 273-275.

[3] C. Campo~ I. B a u ~ and J.M. L~n& Proc. Mem- branes m Drinking and ~ d u s ~ Wamr Production, Pari~ O~ober 2000, 1 ~00~ 189-195.

[4] L.H. Eilers, N a n o f i l ~ i o n und Adsorpt~n an Pdve~o~e ~s Veffahrenskombination zur konfi- nuierlichen Abwasse~einigun~ Disseaation, RWTH Aache~ 2000.