enzymatic cleaning in ultrafiltration of wastewater treatment plant effluent

ELSEVIER Desalination 179 (2005) 73-81

DESALINATION

www.elsevier.com/iocate/desal

Enzymatic cleaning in ultrafiltration of wastewater treatment plant effluent

Sandy te Poele*, Jaap van der Graaf Department of Sanitary Engineering, Faculty of Civil Engineering and Geosciences, Delft University of Technology,

PO Box 5048, NL-2600 GA Delft, The Netherlands Tel. +31 (15) 278-4026; Fax: +31 (15) 278-4918; email: [email protected], [email protected]

Received 30 September 2004; accepted 22 November 2004

Abstract

Membrane fouling is still one of the most important issues in membrane filtration applications. Organic (macro) molecules such as polysaccharides and proteins are the main components ofextracellular polymeric substances (EPS) which contribute to irreversible membrane fouling in ultraflltration of wastewater treatment plant (WWTP) effluent. In this study, irreversible fouling by proteins as constituents of WWTP effluent was investigated by testing a new enzymatic cleaning protocol at pilot scale. Overall the tested enzymatic cleaning protocol showed a better performance in the recovery of clean water flux (CWF) than its basic alkaline cleaning protocol, which implied that proteins may play an important role in the development of irreversible fouling in this research. Application of the new enzymatic cleaning protocol at low temperatures (25-30°C) resulted in recovery of CWF of 100%. If metal complexes were formed during ultrafiltration of conditioned (pre-filtered) WWTP effluent by coagulant dosing, the application of an acid cleaning previous to the enzymatic cleaning is suggested.

Keywords: Enzymatic cleaning; Fouling; Ultrafiltration; WWTP effluent

1. Introduction

Applications of membrane filtration in advanced treatment of wastewater for reuse purposes, or upgrading the effluent quality, are increasing due to water shortages in dry areas and

*Corresponding author.

stricter legislation in the near future. A large number of full-scale installations around the world have been in operation for several years [1].

During operation, membrane fouling problems are becoming more severe. These problems are not only observed in ultrafiltration (UF) of

Presented at the conference on Membranes in Drinking and Industrial Water Production, L 'Aquila, Italy, 15-17 November 2004. Organized by the European Desalination Society.

0011-9164/05/$- See front matter © 2005 Elsevier B.V. All rights reserved

doi: 10.1016/j.desal.2004.11.056

74 S. te Poele, J. van der Graaf / Desalination 179 (2005) 73-81

WWTP effluent, but also in other membrane filtration applications, such as drinking water production. However, the applied hydraulic and chemical cleaning methods are often not efficient enough and are often based on trial-and-error experiences.

Many approaches in applying a cleaning stra- tegy to maximise flux recovery for organic fouled membranes have been studied. The effectiveness of different chemicals, such as NaOH, NaOC1, HC1, citric acid and anionic surfactants, have been studied often by short-term filtration tests [2-4] and occasionally by pilot-scale studies to investigate irreversible fouling [5]. However, more mechanistic studies of cleaning strategies for the removal of irreversible fouling, based on the physicochemical properties of WWTP effluent foulants, are relatively rare.

Specific cleaning agents provide information about the nature and physiochemical properties of the foulants. Interactions between foulants and membrane material are disturbed, or broken, by the use of cleaning agents which provide information about the type of binding between the foulants and membrane material.

Previous research [6] indicated that irreversible fouling during UF of WWTP effluent can probably be accounted for protein adsorption as one of the main constituents of EPS. There- fore, a new enzymatic cleaning protocol based on the enzyme protease has been tested at pilot scale. After membrane cleaning, the influence of organics (macromolecules) on membrane fouling was investigated by UF of pre-filtered WWTP effluent, i.e., microfiltrate.

2. Membrane cleaning

Membranes can either be cleaned hydraulic- ally or chemically. Hydraulic cleaning can be carried out with water or a combination of water and air. Chemical cleaning can be carried out by a variety of chemicals agents.

In general chemical cleaning contains several steps. First a back flush (BF) or forward flush (FF) is provided to remove the reversible fouling. The chemical cleaning solution can be introduced to the membrane surface by a BF or FF. Then the membranes are often soaked for a defined time. To introduce mechanical energy, the cleaning solution can be pumped along the membrane surface. Shear stress is introduced to the bound- ary layer of the membrane surface. The last step is flushing the membranes with permeate, tap water or ultrapure water.

For chemical cleaning four aspects are of major influence: contact time, chemical reaction, temperature and mechanical energy. These para- meters can be changed depending on the fouling present and the cleaning agent. Temperature is an important parameter: the effect of temperature on chemical cleaning can be exponential. Most chemical cleaning is performed between 30 and 50°C, depending on the membrane module's limitations.

3. Methods

In order to investigate the efficacy of enzymatic cleaning with protease on UF membranes used for the filtration of WWTP effluent, pilot investigations were performed at two different locations in the Netherlands: the Nieuwe Water- weg WWTP in Hock van Holland, and the Utrecht WWTP in Utrecht.

3.1. Pilot investigations

The pilot installations were specially designed to perform research at different WWTP's and using WWTP effluent as main feed water. The configuration of the pilot installations used in this research is schematically presented in Fig. 1 by the black arrows. The WWTP effluent was first filtered over a curved screen with a mesh size of 0.45 mm followed by microfiltration and then UF successively.

S. te Poele, J. van der Graaf / Desalination 179 (2005) 73-81

...... ~ D u a l media filter

- i . . . . . . . . . . . . . . . . . . . . . . . . . . !

L . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Curved Sieve Microflltration Oltrafiltration Fig. 1. Schematic overview of the configuration of the pilot installations.

75

The dual-media filter contained one layer of anthracite and one layer of sand and was operated with a fixed water level above the filter bed of 1.73 m. Coagulant could be dosed in-line via the feed water pipe and mixed in a static mixer before the feed water entered the filter. Flocculation took place above and in the filter bed.

The Memcor microfiltration installation contained three modules providing a membrane surface area of 45 m z. The pore size of the membranes was 0.2 #m. The installation was operated at constant flux, which varied between 50 and 105 1/m2xh, and with a production interval of 15 min followed by a backwash. Chemical cleaning was performed once or twice a week depending on the filtration resistance. The chemical cleaning was carried out by a cleaning-in-place procedure with Memclean C at pH 12.

The UF installation contained X-flow membranes with a capillary diameter of 0.8 mm and a pore size of 0.02 #m. The installation capacity is 10 m3/h and was equipped with two 8-inch modules, each with a length of 1.5 m, providing a membrane area of 70 m 2. The installation was operated at constant flux. Coagulant could be dosed in-line to the feed water.

The pilot investigations at Hoek van Holland WWTP were performed for a period of 1 year from October 2002 to November 2003. There-

after, the pilot installations were moved to WWTP Utrecht and further pilot investigations were carried out from February to August 2004. For both pilot investigations the same membrane modules were used.

3.2. Experiments

During the two periods of the pilot investigations at the WWTPs, several cleaning experiments were performed. First an enzymatic cleaning with protease was compared to its basic alkaline cleaning. This was to investigate the effectiveness of the enzyme protease, indicating protein adsorption as a possible fouling mechanism. The applied cleaning solutions and protocol are presented in Table 1. The effect of the cleaning was determined by measuring the clean water flux (CWF) before and after cleaning. These cleaning experiments were performed twice. At the Hock van Holland WWTP, these experiments were performed after 8 months of operation, while at the Utrecht WWTP these experiments were carried out at the beginning of the period of pilot investigations.

After membrane cleaning was done, the UF unit was started with micro filtrate as feed water, at a flux of 28.5 1/m2×h, production interval of 1 h, which was followed by a BF (17 m3/h) for


Table 1 Cleaning protocol of enzymatic and basic alkaline cleaning

Cleaning solution Enzymatic Basic alkaline

Divos l 10 (commercial alkaline cleaning product)

9.3 (adjusted by HC1)

Basic solution Divos 110 (commercial alkaline cleaning product)

pH 9.3 (adjusted by HCI) Enzyme Protease - - Protocol Low-temperature cleaning

Forward flush with permeate Preparing cleaning solution at 25-30°C 1-h circulation Overnight soaking (24 h) 1-h circulation Flushing with tap water

45 s. Micro filtrate was used as feed water in order to investigate the influence of organic macromolecules (i.e., proteins) on UF membrane fouling. The fouling rate was determined by measuring the CWF frequently. During filtration the protein concentration was analysed in the feed water (microfiltrate) and ultrafittrate.

After these first cleaning and fouling experiments, the UF installation was fed by different feed waters: WWTP effluent, dual-media filtrate and microfiltrate, with and without in-line coagulation of polyaluminium chlorine (PAC1) for other research purposes not described in this study.

At the end of the pilot investigations the membranes were cleaned in order to continue the research at a different WWTP. Therefore, different cleaning methods were applied successively. The applied cleaning methods are presented in Table 2.

3.3. Protein analyses

The method of Rosenberger [7] was modified in order to measure proteins in WWTP effluent. This method is based on the method of Lowry [8]. The adsorption of the formed colour is

Table 2 Description of the applied cleaning methods

Cleaning Cleaning pH Protocol method agent(s)

Enzymatic Divos 110 + 9.3 Low-temperature protease cleaning

Basic Divos 110 9.3 Low-temperature alkaline cleaning

Alkaline Divos 110 12.5 Low-temperature cleaning

Acid FF Divos 2 1.5 FFa-30 min soaking-flushing

Acid BF Divos 2 1.5 BFb-30 min soaking-flushing

Acid Divos 2 1.5 Low-temperature cleaning at 22°C

Mild acid Divos 25 2 Low-temperature cleaning

~Forward flush. bBack flush.

measured at 750 nm in a 4 cm glass cuvet by the Milton Roy spectromic 401 photospectrometer The amount of proteins is expressed in mg/1.

S. te Poele, J. van der Graaf / Desalination 179 (2005) 73-81 77

4. Results and discussion

The results of the first enzymatic and basic alkaline cleaning experiments are presented in Table 3 in chronological order. The CWF is normalized to 20°C. The CWF of a new membrane module is between 400 and 500 l/m2xhxbar at 20°C, as stated by the membrane manufacturer.

The results show clearly that after applying the new enzymatic cleaning protocol, the CWF returned to its original CWF for a new membrane module. At the Hoek van Holland WWTP, the effect of the enzymatic cleaning is significantly greater than the basic alkaline cleaning method. In fact, the effect of the basic alkaline cleaning decreases over time. This indicates that protein adsorption occurred as the fouling mechanism. At the Utrecht WWTP, the CWF after the enzymatic cleaning step showed similar results as after the basic alkaline cleaning step. Although the effect of the enzymatic cleaning seems to be larger than for the basic alkaline cleaning, application of both cleaning methods resulted in a 100% recovery of CWF.

In Figs. 2 and 3, the results of these cleaning experiments, as well as the membrane fouling after cleaning at the Hoek van Holland and Utrecht WWTPs, respectively, are presented as the CWF normalized at 20°C against time. The CWF decreases rapidly after starting UF of the microfiltrate, especially when the CWF starts above 400 1/m2xhxbar. After 21 June 2003 the MF unit was fed by floc filtrate. However, this does not seem to influence the results. At the Hoek van Holland WWTP the CWF decreases after enzymatic cleaning from around 440 to 220 l/m2xhxbar in about 1 day and decreases further to approximately 1601/m2xhxbar in about 3 days. The CWF at the Utrecht WWTP decreases from around 430 to 305 1/m 2x hxbar in about 1 day and decreases further to approximately 260 l/mZx hxbar in about 3 days. At the Hoek van Holland WWTP the CWF decline after 1 day of UF of microfiltrate is around 50%

Table 3 Results of enzymatic and basic alkaline cleaning methods

Cleaning method Clean water flux (1/m2xhxbar) at 20°C

Before After Effect

Hoek van Holland WWTP: Enzymatic (1) 200 467 267 Enzymatic (2) 138 411 273 Basic alkaline (1) 153 344 191 Basic alkaline (2) 165 271 106

Utrecht WWTP: Enzymatic (1) 189 421 233 Basic alkaline (1) 237 445 208 Enzymatic (2) 241 446 204 Basic alkaline (2) 231 413 181

compared to around 30% at the Utrecht WWTP, which is 40% less. After about 3 days of filtration the CWF levels off to 160 I/mZxhxbar at Hoek van Holland, whereas at Utrecht this value is around 260 1/m2xhxbar. In other words, the fouling of the UF membrane by organic macromolecules is more severe at the Hoek van Holland WWTP than at Utrecht.

During the periods of UF of the microfiltrate, protein concentrations were analysed twice in one period. The mean protein concentrations in the feed water (micro filtrate) and ultrafiltrate over all periods are presented in Table 4. The results show an overestimation due to the interference of humic compounds in the Lowry procedure [9]. The results show a higher mean protein concentration (17.3 + 4.4 mg/1) in the feed water of the Hoek van Holland WWTP than in the feed water of the Utrecht WWTP (11.4 + 1.9 mg/1). The retention of proteins by the UF membrane is 40% more for Hoek van Holland than for Utrecht. This indicates that the higher CWF decline found at the Hoek van Holland WWTP may be related to the higher retention of proteins by UF of microfiltrate.

At the end of the pilot investigations at the Hoek van Holland WWTP, the enzymatic


500

450

"~" 400 J~ .E 350

300

otJ 250

200

,, 150

U 100

~. Enzymatic (1) • Enzymatic (2) • Basic alkaline (1) • Basic alkaline (2)

• • •

• • a • •

I __~ C l e a n i n g 5 0 ~

0 4-6 8-6 12-6 16-6 20-6 24-6 28-6 2-7 6-7

Date

Fig. 2. CWF measurements of cleaned and fouled membranes by ultrafiltration ofmicrofiltrate at the Hock van Holland WWTP.

o od

M.

L)

50O

450 l 400

350

300

25O

200 q.

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* Enzymatic (1) " Basic alkaline (1) • Enzymatic (2) • Basic alkaline (2) M •

• No feed w~ler g a

• i feed w~ter ~ •

• •

- - ~ C l e a n i n g ~

, i i

17-2 21-2 25-2 29-2 4-3 8-3 12-3 16-3 20-3 24-3 28-3 1-4

Date

Fig. 3. CWF measurements of cleaned and fouled membranes by ultrafiltration of microfiltrate at the Utrecht WWTP.

Table 4 Mean values of protein concentration in feed water (microfiltrate) and ultrafiltrate

WWTP Period Protein concentration (mg/1)

Feed water Ultrafiltrate Retention

Hock van Holland 4-6 till 6-7 2003 17.3 + 4.4 16.3 4- 4,3 1.0 + 0.1 Utrecht 13-2 till 1-4 2004 11.4 ± 1.9 10.8 ± 1.8 0.6 4- 0.1

cleaning protocol was applied because o f its

proven effectiveness compared to the basic alkaline cleaning and 100% recovery o f the CWF. The results o f these cleaning experiments are pre-

sented in Fig. 4 together with the first cleaning experiments in chronological order. After the first cleaning and fouling experiments, the membranes

were successfully cleaned by the enzymatic

S. te Poele, J. van der Graaf / Desalination 179 (2005) 73-81 79

• Enzymatic (1) W Basic alkaline (1) • Enzymatic (3) [] Acid {~ Enzymatic (2) ~ Basic alkaline (2) [] Enzymatic (4) [] Enzymatic (5)

500

450

400 A

a= 350 # ~ 300

=O 250

zoo

t50 U

100

50

After cleaning

Fig. 4. CWF measurements of cleaned membranes by different applied cleaning methods at the Hock van Holland WWTP.

cleaning protocol. Then other filtration experiments were performed, using different types of feed water in combination with coagulant (PAC1) dosing. In order to start the new pilot investi- gation at the Utrecht WWTP, the membranes were cleaned. Again the enzymatic cleaning protocol was applied, resulting in a CWF of 120 1/mZxhxbar.. The membranes were not being cleaned with acid over a long period of time and metal salts were dosed in the previous filtration period. Combining these factors it was suggested that an acid cleaning might work. The applied acid cleaning resulted in an increase in CWF to 200 1/mZxhxbar. A last clean using the enzymatic cleaning protocol was applied again, resulting in an increase of the CWF to almost 4001/mZxhxbar and a recovery of the CWF of 85%. As stated before, the pilot investigations were continued at WWTP Utrecht. The results of the first applied enzymatic cleaning protocol, given in Table 3, shows a CWF of 421 1/mZxhxbar, and therefore it may be concluded that no irreversible fouling remained.

At the end of the pilot investigations at the Utrecht WWTP, similar cleaning experiments were performed as at the Hock van Holland WWTP. In contrast to the results of the first cleaning experiments at Hock van Holland, here

the basic alkaline cleaning protocol was applied because of its proven effectiveness. The results of these cleaning experiments are presented in Fig. 5 together with the first cleaning experiments in chronological order. After a period of filtration tests with different types of feed water, including PACI dosing, first the basic alkaline cleaning protocol was applied, which resulted in a poor CWF of 145 1/m2xhxbar. Second the enzymatic cleaning protocol was applied in order to investigate the presence of protein fouling. As a result, the CWF increased to 210 1/m2xhxbar. The previous results at the Hock van Holland WWTP indicated the usefulness of an acid cleaning. In order to investigate which kind of inorganic fouling was present, an acid forward flush was carried out to remove hardness salts from the feed side of the membrane. Next, an acid back flush was carried out to remove hardness salts at the permeate side of the membrane. Then an acid cleaning was done, to remove metal hydroxides from the feed side of the membrane. Finally, a mild acidic cleaning was done to remove metal oxides at the feed side of the membrane. The CWF increased in small steps of 15, 2, 12 and 7 1/m2xhxbar, respectively, to 240 1/m2xhxbar. Thereafter, an alkaline cleaning protocol was applied similar to the one at the Hock van


500

450

• Enzymatic (1) D Basic alkaline (2) [] Acid FF [] Mild acid N Basic alkaline (1) • Basic alkaline (3) @ Acid BF @ Alkaline El Enzymatic (2) D Enzymatic (3) @ Acid [] Enzymatic (4)

"~ 400 ..Q 2 : 3 5 0 &

300

o eq 200

,, 150

O I00

5O

0 After cleaning

Holland WWTP, which resulted in an increase of CWF to 384 1/m2xhxbar. In order to achieve complete recovery of the CWF, the enzymatic cleaning protocol was applied and the CWF results in an increase of CWF to 431 1/m2xhxbar, indicating the presence of protein fouling.

5. Conclusions

The results show a complete recovery of the clean water flux after applying the enzymatic cleaning protocol. With this protocol it is possible to perform enzymatic cleaning at low temperatures (25-30°C) and is therefore applicable for low-temperature resistance membranes.

Organic macromolecules (<0.2 #m) like proteins contribute to rapid membrane fouling of a clean membrane, which is demonstrated by the rapid CWF decline flux during UF of microfiltrate. The extension of CWF decline may be related to the retention of proteins by UF. The applied enzymatic cleaning does not contribute to maintaining the performance of a clean membrane. Although the CWF is recovered com- pletely, the development of membrane fouling seems even faster.

The approach of applying the enzymatic cleaning protocol compared to its basic alkaline

Fig. 5. CWF measurements of cleaned membranes by different applied cleaning methods at the Utrecht WWTP.

cleaning protocol has been found selective in indicating the presence of protein fouling. The additional value of the enzyme is to cut protein network chains, whereas the basic alkaline cleaning removes isolated proteins from the membrane surface. The different results found at the Utrecht and Hoek van Holland WWTPs suggest that protein fouling is a relatively long-term effect, i.e., building a protein network structure on the membrane surface takes time and depends upon the retained protein concentration.

If metal complexes are formed during filtration of (pre-filtered) WWTP effluent, it is suggested that an acid cleaning be applied before the enzymatic cleaning.

Acknowledgements

This work is financially supported by Witteveen+Bos, Rossmark Water Treatment (Veolia Water) and the Ministry of Economical Affairs, Senter, BTS 99112. In addition, support is given by JohnsonDiversey. The authors would like to thank our project partners, Jeroen Boom of Rossmark Water Treatment, Wilbert Menkveld of Witteveen+Bos, and Marcel Dusamos of JohnsonDiversey.

S. te Poele, J. van der Graaf /Desalination 179 (2005) 73-81 81

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enzymatic cleaning in ultrafiltration of wastewater treatment plant effluent

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