Substitution of the final clarifier by membrane filtration within the activated sludge process with increased pressure; initial findings

Desalination, 68 (1988) 179-189 Elsevier Science Publishers B .V., Amsterdam - Printed in The Netherlands

179

Substitution of the Final Clarifier by Membrane Filtration within the Activated Sludge Process with Increased Pressure ; Initial Findings* KH . KRAUTH and K .F . STAAB InstitutfurSiedtungswassehau, Wassergue-undAbfaUwirtsc . Bandtkle 1, D-7000 Stuttgart 80 (Busnau) (F.R .G .)

U ioersit

SUMMARY

The Institut fdr Siedlungswasserbau, Wassergiite- and Abfallwirtschaft of the University of Stuttgart has performed a series of tests with the activated sludge process at increased pressures over a long period . The advantage of this process is that it increases the activated sludge content by improving the oxygen supply . This advantage is accompanied by a decrease in the production of excess sludge as an additional benefit . A major disadvantage of this process is the fact thatt the separation of the activated sludge from the water has to be done at the same pressure as that applied to the biological reactor as a result of the choice for a continuous operation . This fact leads to technical difficulties. Tests carried out with membranes in another research project suggested that membrane filtration could be effective for solid/liquid separation . Our initial findings clearly confirmed this, even though the testing conditions were not optimal . SYMBOLS AND ABBREVIATIONS"

A BA BOD5 BE, Bis C COD F

- Surface, m2 - Solids loading, kg/ (in' h) - Biochemical oxygen demand, mg/I - Volumetric loading, kg/ (m 3 d) - Sludge loading, kg/ (kg d ) - Control] - Chemical oxygen demand, mg/I -Flow

*Paper presented at the 5th Symposium on Synthetic Membranes in Science and Industry, Tub-

ingen, F.R.G ., September 2-5,1986 . Sponsored by BMFT under 02-WA $556 . **In acccordance with DIN 4045 . 001149164/88/$03 .50

© 1988 Elsevier Science Publishers B .V .

ISO I L MLSS 02 P 4 P qA qsv SS SVI SVIv to d

- Indicator - Level - Mixed-liquor suspended solids, kg/m3 - Oxygen - Pressure - Differential pressure - Surface flow rate, m3/ (m2 h ) - Sludge volume surface loading, m3/ (m2 h ) - Suspended solids, mg/1 - Sludge volume index, ml/g - Index by the dilution method, ml/g - Hydraulic retention time, h -Temperature, °C

INTRODUCTION The Institut fiir Siedlungswasserbau, Wassergiite- and Abfallwirtschaft of the University of Stuttgart has performed test runs with the activated sludge process with increased pressure over a long period [ 1-31 . The pilot plant used is shown in Fig. 1 . The ranges of the test conditions for the aeration reactor as well as for the final clarifier are listed in Table I . Fig . 2 shows that the amount of BODE effluent that can be obtained is the same as in the conventional aeration process, on condition that the suspended solids (SS) are below 30 mg/I . An increase in the suspended solid values in the effluent of the final clarifier beyond 30 mg/l leads to a decrease in the efficiency as shown in Table II . The minimum and the maximum values in the effluent of the final clarifier for GODS, COD and SS listed in Table II are mean values of the test periods at the different pressures . The advantage of this process is the increase in the activated sludge content by improving the oxygen supply . An additional benefit is the decrease of the excess sludge production shown in Figs . 3 and 4 . This fact is valid in relation to the sludge load BTS as well as to the age of the sludge . A major disadvantage of this process is the fact that the separation of the activated sludge from the water has to be done at the same pressure as that applied to the biological reactor as a result of the choice for a continuous operation. This fact leads to technical difficulties because the final clarifier is hydraulically overloaded with regard to a sludge volume surface loading of 0 .4 m3/(m2 h) . Fig . 5 shows clearly the dependency of the surface loading qA on the activated sludge content in connection with the sludge volume index SVI as measured by the dilution method, when the sludge volume surface loading is constant . For instance an increase in the activated sludge content is connected with a decrease in the area charge qA in m3/ (m2 h) = m/h .



18 1

Was Air Current

'I

I

11

---L

.

Fig, 1 . Pilot plant for aeration with increased pressure : flow sheet. (1) Wastewater pump ; (2) aeration reactor ; (3) final clarification ; (4) return sludge pump; (5) discharge tank; (SV 1) safety valve 1; (SV 2) safety valve 2 . TABLE I Test conditions for the aeration reactor and for the final clarifier Pressure (bar)

Aeration reactor B R, kg/(& d) Bw, kg/(kg d) MLSS, kg/m 3 ta , h Final clarifier 4Ar m3/(m3 h) BA, kg/(m' h)

1

3

5

1 .490-10 .308 0.476- 2 .036 3.179- 5 .660 0 .529- 1 .530

0 .818-4.568 0 .222-1 .357 3 .212-5.084 1 .649-7.988

1 .149-5.494 0.196-4.438 1 .184-5.782 1 .231-5.10.5

0 .088- 0 .757 0 .279- 4 .283

0 .091-0 .891 0 .447-2.862

0.280-1 .308 0.667-1 .772

~.uualu~ .u .

....1111 .u ..ull

... .n •UI t..PIIalmii. n531 ;0!~ M.. M

ulu.all'u

02 0.5 1 .0 2 345 Sludge load in kg/(kgd) Fig. 2 . Effluent BOD E in mg/i dependent on BOD E sludge load in kg/ (kg d),



182 TABLE II Major pollutants within the effluent pressure (bar)

BOD5 , mg/l COD, mg/I SS, ntg/l

i

3

5

12 .4- 52.0 51 .0-116.5 87.0-171.0

7 .8- 94.5 60.3-230.7 12 .9-138.3

5.8- 88.6 62 .7-247 .2 38.1-146.8

~ do 1 2

4 5 6 7

BOD5 sludge load in kg/(kgd)

Fig. 3. Excess sludge production dependent on the BOD3 sludge load. ( (----) 5bar, ( ) conventional aeration.

) 1 bar, (---) 3 bar ;

> ON ~

oil

w

11 10

1 20 30 Sludge age in d

40

Fig. 4 . Excess sludge production dependent on the sludge age . (-s-) Excess sludge=1 .003-0.374 log d, l bar: (-+--) excess sludge =0.918-0 .54 7 log d, 3 bar, (- . A- .) excess sludge=0 .742-0.409 Iogd, 5 bar ; (-a-) excess sludge= 1 .180-0 .454 log d, conventional aeration .



0

5

5.0 7.5 10 .0 125 MLSS in g/I Fig. 5. Area charge versus MLSS at different SVI's measured by the dilution method . Sludge volume surface loading=0 .4 m3/( m' h) constant . METHODS AND MATERIAL

'Pests carried out with membranes in another research project [41 showed that membrane filtration could be effective for solid/liquid separation . The first tests with the pilot plant were carried out under normal atmospheric conditions (see Fig. 6) . The pressure necessary to maintain the required test conditions was reached by means of pneumatic valves in the outlet of the membrane modules . Each module was equipped with another membrane tube to allow a reliable comparison . The manufacturers and the types of membranes are given in Table 111 .

Fig . 6 . Pilot plant membrane filter : flow sheet. (1) Wastewater pump; (2) aeration reactor ; (3) aerator; (4) recirculation pump; (5) distribution pipe; (6, 7, B) filter element .

184 TABLE III Types of membranes and manufacturers Type of membrane

Manufacturer

Designation

HFM 251 HFP 276 Microdyn Ell

Abcor Abcor Enka Hoechst AG

B C D E

TABLE IV Characteristic values of the membranes used for the study Type of membrane

Mean Separation boundary (mot wt.)

Max .0 (°C)

pH range

Max. dp (bar)

Max. permeability (1/(m' h)

B C D E

18,000 35,000 4 25,000

25 90 60 90

0.5-13 0.5-13 1 -14

4.2 17.6 3.0 10.0

94 .8 94 .8 500 300

*Pore diameter=0.2 pm. TABLE V Characterization of the sludge used in the test Test period

MISS, 9/1 SV, ml/I SIlL m1/g

1

2

3 .27 -

2 .06 350 171 .4

2 .20 229 104 .1

The characteristic values of the most effective membranes are given in Table IV. The characterization of the activated sludge used in the test runs is given in Table V . Analyses

All analyses were done in accordance with the standards shown below. BOD8 DEV [5] COD DIN 3841)9 Teil 41 SS DEV [5] MLSS DEV [5]

1 85

Total germs Coliforms

DIN 38411 DIN 38411

K 5-2 K6.1

RESULTS AND DISCUSSION

Figs, 7 and 8 clearly show that the permeability is dependent firstly on the flow rate within the membrane tube and secondly on the transmembrane pressure . The transmembrane pressure is defined as the difference between the pressure at the outlet of the membrane tube and the atmospheric pressure . Membranes B and C in particular show a very different behaviour at a transmembrane pressure of 2 bar and 3 bar. From Fig. 7 it can be seen that there is nearly no difference between membrane B and C within the range of flow rate from 1 .5 m/s to 4.0 m/s. On the other hand, Fig. S shows a clear difference between the two membranes at a transmembrane pressure of 3 bar . At both 2 and 3 bar transmembrane pressures membrane E is clearly superior to membrane B or C . Backwashing of the membranes was not necessary during an operation period of 6 months after activation, nor could any decrease in the permeability be observed within this period .

L L rv E 300 t

~

300-

c0

T

00

0 0

4

8

1

Rate of flow vMF in m/s

0

4

8

Rate of flow NF

12

in m/s

Fig. 7. Permeability q versus membrane flow rate u w. Tranamembrane pressure dp=2 bar . Type of membrane: ( •- .. .-) B ; (111111- .-) C ; (A-) E. Fig. 8. Permeability q versus membrane flow rate uyg,. Transmembrane pressure dp=3 bar . Type of membrane: ( •- -.--) B ; (E- .-) C ; (A-) E.

186

TABLE VI

Effluent SS in mg/1 (0.45 }ma) for each membrane Type of membrane

B C D E

Test period 1

2

0 0 0 0

0 0 0 0

0 0 1 .3 0

TABLE VII

Effluent COD in mg/I for each membrane Type of membrane

B C D E

Test period 1

2

16 .5

-

15 .9

9 .8 14.6

3

15 .2 23 .2 14 .6

TABLE VIII

Number of bacteria in the effluent of the membranes Type of membrane

Total germs

B

1

D

800 11200 18200

E

16400

0

C

(KBE/ml)

Coliforms (n/ml)

6 0

Membrane D required backwashing after a short period . After an operation period of 60 d the test runs had to be stopped as a consequence of the fact that the permeability had decreased by clogging of the membrane pores . This clogging was not reversible . The SS, the COD and the number of bacteria in the effluent are of importance if this water is intended to be reused . Table V I shows that SS could only be detected in the effluent of membrane D . The COD values in the effluent are given in Table VII . They are directly connected with the quality of the biological process and the efficiency of the succeeding separation process. Table VII shows values in the range of the an-



187 TABLE IX Membrane filter in comparison with different clarification processes System used

SS (mg/1)

COD (mg/I)

Total germs (KBE/ml)

Colsforms (n/ml)

Membrane filter Final clarifier [6] Final clarifer with additional polishing lagoons [7] Sandfilter [8] Microstrainer [9]

0 18.5 5 .5

14.6-23 .2 29 .4 22 .8

8 X 1OR-1 .64 X 104 4 .9 X 104 1 .58 X 104

0.5 4.35 X 104 2.5 X 109

Curcent

5 .8 7 .5-12 .1

I

38 .5

-----------

------------ I -------

Fig . 9 . Pilott plant : flow sheet

alytical detection limit. With regard to COD, the permeability and the removal of SS, membrane E shows the best results . Microbes in water have an important influence on its use . Therefore the number of bacteria in the effluent of the membranes was counted and the results are given in Table VIII. Table VIII shows that the number of coliforms in the effluent of membranes B and C are negligible in the context of the possible recontamination of the effluent .

IPA

Fig . 10 . Membrane filter pert of the pilot plant shown in Fig . 9.

1 89 A comparison of the membrane process with several other clarification processes is given in Table IX . As for the SS, the COD and the coliforms, significantly better results are achieved by the membranes, FURTHER TESTS The tests described in this paper will be completed in the future when further tests using the equipment shown in Figs . 9 and 10 are carried out. The biological reactor (see Fig . 9) is a loop reactor for pressures up to 5 bar . The main membrane in this test will be membrane E . The tests will be aimed at optimizing the permeability . They will be executed on a semitechnical scale . The microstructure of membrane E is shown in Fig . 11 . Additionally membrane E and two other membranes will be tested for hygienic efficiency in a smaller scale test plant .

REFERENCES 1 K .F . Staab and Kh . Krauth, Verfahren zur biologiachen Abwasserreinigung bei Drucken zwischen I and 5 bar mit Nitrifikation and Denitrifikation, Bundeaministerium tilr Forachung and Technologie, Bonn, Forschungsbericht Nr . 02-WA 621, Nov. 1980 . 2 KY . Staab, Uberschu6achlammproduktion moglichst gering batten, Maschinenmarkt, 89 (1983)2-5, 3 K .F. Steeb, Reinigung von Abwasser mit dam Belebungsverfahren bei erhthtem Druck, Maschinenmarkt, 88 (1982) 1859-1861 . 4 Kh . Krauth and K .F. Staab, Steuerung einer vorgesehalteten Denitrifikatiun clutch Verlinderung des Volumens der Zone ohne gelasten Sauerstoff . In: Kh . Krauth (Ed.) . Anwendung des Belebungsverfahrens zur Nitrifikation and Denitrifikation, Stuttgarter Berichte zur Siedlungswasserwirtschaft, Vol . 93, Kommissionsverlag R . Oldenbourg, Milnchen, 1986 . 5 Deutsehn Einheitsverfahren zur Wasaer-, Abwasser- and Schlammunterauchung, Verlag Chemie, Weinheim, 12th edn ., 1983. 6 H . Resch, Untersuchungen an vertikal durchstromten Nachklarbecken von Belebungsanlagen, Berichte aus Wasserwirtschaft and Gesundheitsingenieurwesen, Vol . 29, Institut fair Bauingenieurwesen V, Technieche Universitat MUnchen, 1981 . 7 Kh. Krauth and K .F. Staab, Ban von Schdnungsteichen als dritte Reinigungsstufe, Schriftenreihe des Bundesministers ftlr Ernahrung, Landwirtachaft and Forsten, Vol . 328, Landwirtschaftsverlag GmbH, 4400 Munater-Hiltrup, 1986. 8 V . Maier, Suspensaentnahme aus biologisch gereinigtem Abwasser im aufwarts durchstrumten Schnellsandfrlter, Stuttgarter Berichte zur Siedlungswasserwirtachaft, Vol . 69, Kommissionsverlag R . Oldenbourg, Miinchen, 1980 . 9 M . Roth, Untereuchungen zur Mikrosiebung nach aerober biologischer Abwasserreinigung, Stuttgarter Berichte zur Siedlungswasserwirtschaft, Vol . 75, Kommissioneverlag R. Oldenbourg, Munchen, 1982 .