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Separation of activated sludge from mixed liquor using a continuous centrifuge
Water Research Pergamon Press 1972. Vol. 6, p. 531. Printed in Great Britain
DISCUSSION SEPARATION OF ACTIVATED SLUDGE FROM MIXED LIQUOR USING A CONTINUOUS CENTRIFUGE A. B. WHEATLAND Water Pollution Research Laboratory, Elder Way, Stevenage, Herts. A FEW tests h a d been c a r r i e d o u t b y the a u t h o r a t the W a t e r P o l l u t i o n R e s e a r c h L a b o r a t o r y to d e t e r m i n e w h e t h e r a disc-type centrifuge c o u l d be successfully used to s e p a r a t e a c t i v a t e d sludge f r o m m i x e d l i q u o r f o r recycle to a n a e r a t i o n tank. T h e sludge used in the tests was f r o m the L e t c h w o r t h sewage t r e a t m e n t works. T h e results o f the tests a r e s u m m a r i z e d in TABLE 1. It was f o u n d necessary t o screen the mixed l i q u o r to r e m o v e particles greater t h a n 0.38 ram dia. in o r d e r to a v o i d the nozzles o f the centrifuge b e c o m i n g clogged. U n d e r all the c o n d i t i o n s tested the clarified overflow l i q u o r f r o m the centrifuge h a d a higher t u r b i d i t y a n d B O D t h a n s u p e r n a t a n t liquid o b t a i n e d b y settlement. T h e quality o f the overflow l i q u o r was generally satisfactory when the feed rate was 0.13 m 3 h -1 b u t the c o n d i t i o n o f the s e p a r a t e d activated sludge was very different f r o m t h a t o b t a i n e d b y settlement. I t h a d been subjected to intense sheafing forces a n d a p p e a r e d to have been h o m o g e n i z e d . I f sludge in this c o n d i t i o n were r e t u r n e d to a n a e r a t i o n t a n k it seems likely t h a t there w o u l d be a steady d e t e r i o r a t i o n in the c o n d i t i o n o f the a c t i v a t e d sludge until the p e r f o r m a n c e o f the p l a n t was c o m p l e t e l y unacceptable. T h e centrifuge used was stated b y the supplier to have a t h r o u g h o u t o f 1/38th o f their s t a n d a r d m a c h i n e f o r which a 20-h.p. m o t o r was e m p l o y e d . C a l c u l a t i o n s indic a t e d t h a t the p o w e r r e q u i r e d to s e p a r a t e 4540 m a (1,200,000 U.S. gal) o f mixed l i q u o r p e r d a y w o u l d be a b o u t 350 k W . T h e cost o f this w o u l d p r o b a b l y be prohibitive, TABLE 1.
RESULTSOF TESTSWITHA CONTINUOUSDISC-TYPE CENTRIFUGE ON ACTIVATED SLUDGE MIXED LIQUOR WHICH HAD BEEN SCREENED TO REMOVE PARTICLES GREATER THAN 0"3S m m
Number of test
1
2
3
4
5
6
7
8
Nozzle size (ram) 0.3 0.4 0.5 0"5 0"5 0'5 0"5 0-5 Number of nozzles used 2 4 4 2 2 2 2 1 Number of nozzles clogged 2 3 1 0 0 0 0 0 Rate of flow (m 3 h- 1) 0.31 0.47 0"41 0"36 0'27 0"19 0"13 Sludge volume (per cent) 28 49 -35 34 44 64 50 Overflow volume (per cent) 72 51 -65 66 56 36 50 Suspended solids in feed (mg 1-1). 3440 3570 3570 4700 4700 4700 4700 4700 Suspended solids in overflow (nag 1- x) 18 42 -210 92 24 V. small V. small Suspended solids in sludge (mg 1-1) 8960 7900 -15,300 11,500 10,200 6100 8300 Volume of solids in feed settled 0.5 h (per cent) 19 19 19 25 25 25 25 25 Permanganate value (nag 1-1) Feed settled 1 h ---7 7 7 7 7 Overflow I1 --40 23 11 8 9 5-day BOD (nag 1-t) Feed s e t t l e d 1 h ---17 17 17 17 17 Overflow ---100 63 36 26 24 Clarity of overflow Reason- Reason- V. poor V. poor V. poor Poor Reason- Reasonable able able able *Rounded
values.
531
DISCUSSION SEPARATION OF ACTIVATED SLUDGE FROM MIXED LIQUOR USING A CONTINUOUS CENTRIFUGE A. B. WHEATLAND Water Pollution Research Laboratory, Elder Way, Stevenage, Herts. A FEW tests h a d been c a r r i e d o u t b y the a u t h o r a t the W a t e r P o l l u t i o n R e s e a r c h L a b o r a t o r y to d e t e r m i n e w h e t h e r a disc-type centrifuge c o u l d be successfully used to s e p a r a t e a c t i v a t e d sludge f r o m m i x e d l i q u o r f o r recycle to a n a e r a t i o n tank. T h e sludge used in the tests was f r o m the L e t c h w o r t h sewage t r e a t m e n t works. T h e results o f the tests a r e s u m m a r i z e d in TABLE 1. It was f o u n d necessary t o screen the mixed l i q u o r to r e m o v e particles greater t h a n 0.38 ram dia. in o r d e r to a v o i d the nozzles o f the centrifuge b e c o m i n g clogged. U n d e r all the c o n d i t i o n s tested the clarified overflow l i q u o r f r o m the centrifuge h a d a higher t u r b i d i t y a n d B O D t h a n s u p e r n a t a n t liquid o b t a i n e d b y settlement. T h e quality o f the overflow l i q u o r was generally satisfactory when the feed rate was 0.13 m 3 h -1 b u t the c o n d i t i o n o f the s e p a r a t e d activated sludge was very different f r o m t h a t o b t a i n e d b y settlement. I t h a d been subjected to intense sheafing forces a n d a p p e a r e d to have been h o m o g e n i z e d . I f sludge in this c o n d i t i o n were r e t u r n e d to a n a e r a t i o n t a n k it seems likely t h a t there w o u l d be a steady d e t e r i o r a t i o n in the c o n d i t i o n o f the a c t i v a t e d sludge until the p e r f o r m a n c e o f the p l a n t was c o m p l e t e l y unacceptable. T h e centrifuge used was stated b y the supplier to have a t h r o u g h o u t o f 1/38th o f their s t a n d a r d m a c h i n e f o r which a 20-h.p. m o t o r was e m p l o y e d . C a l c u l a t i o n s indic a t e d t h a t the p o w e r r e q u i r e d to s e p a r a t e 4540 m a (1,200,000 U.S. gal) o f mixed l i q u o r p e r d a y w o u l d be a b o u t 350 k W . T h e cost o f this w o u l d p r o b a b l y be prohibitive, TABLE 1.
RESULTSOF TESTSWITHA CONTINUOUSDISC-TYPE CENTRIFUGE ON ACTIVATED SLUDGE MIXED LIQUOR WHICH HAD BEEN SCREENED TO REMOVE PARTICLES GREATER THAN 0"3S m m
Number of test
1
2
3
4
5
6
7
8
Nozzle size (ram) 0.3 0.4 0.5 0"5 0"5 0'5 0"5 0-5 Number of nozzles used 2 4 4 2 2 2 2 1 Number of nozzles clogged 2 3 1 0 0 0 0 0 Rate of flow (m 3 h- 1) 0.31 0.47 0"41 0"36 0'27 0"19 0"13 Sludge volume (per cent) 28 49 -35 34 44 64 50 Overflow volume (per cent) 72 51 -65 66 56 36 50 Suspended solids in feed (mg 1-1). 3440 3570 3570 4700 4700 4700 4700 4700 Suspended solids in overflow (nag 1- x) 18 42 -210 92 24 V. small V. small Suspended solids in sludge (mg 1-1) 8960 7900 -15,300 11,500 10,200 6100 8300 Volume of solids in feed settled 0.5 h (per cent) 19 19 19 25 25 25 25 25 Permanganate value (nag 1-1) Feed settled 1 h ---7 7 7 7 7 Overflow I1 --40 23 11 8 9 5-day BOD (nag 1-t) Feed s e t t l e d 1 h ---17 17 17 17 17 Overflow ---100 63 36 26 24 Clarity of overflow Reason- Reason- V. poor V. poor V. poor Poor Reason- Reasonable able able able *Rounded
values.
531