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Attrition of granular filter media during backwashing with combined air and water
~
Pergamon
0043-1354(95)00177-8
Wat. Res. Vol. 30, No. 2, pp. 291-294, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/96 $15.00 + 0.00
A T T R I T I O N OF G R A N U L A R F I L T E R M E D I A D U R I N G B A C K W A S H I N G WITH C O M B I N E D A I R A N D W A T E R M. S T E V E H U M B Y a n d C A R O L I N E S. B. F I T Z P A T R I C K * School of Water Sciences, Cranfield University, Cranfield, Bedford MK43 0AL, U.K. (First received January 1995; accepted in revised form June 1995)
Abstract--Attritionresistance of granular filter media is becoming increasingly important as materials such as GAC and anthracite are being more frequently used as filter media. The purpose of this study was to evaluate the attrition experienced by various media during backwashing by performing accelerated backwash tests in a pilot column, using a combined water and air backwash at combinations that gave the condition known as collapse-pulsing. Since the dominant mode of attrition was assumed to be abrasion, the effluent was sampled at a number of intervals to determine the amount of fine material in the effluent. Coal based GAC exhibited the highest weight loss ( ~ 7%) and sand the least ( ~ 2 % ) . Key words--water treatment, granular filter media, attrition, backwashing test, collapse-pulsing, combined water and air scour, GAC, anthracite, sand
INTRODUCTION
3 - 5 % is undesirable, 1 - 3 % is doubtful a n d < 1% is satisfactory. Filters have historically been cleaned with a water only b a c k w a s h at 20--50% expansion. It has been s h o w n t h a t this regime is fairly ineffective in cleaning due to the limited n u m b e r of grain a b r a s i o n s a n d impacts t h a t occur ( A m i r t h a r a j a h , 1978). The m o s t effective cleaning regime has been s h o w n to be c o m b i n e d air a n d sub-fluidisation water b a c k w a s h at rates t h a t give a c o n d i t i o n k n o w n as collapse-pulsing ( A m i r t h a r a j a h , 1993). This was t h o u g h t to be due to m a x i m i s a t i o n o f fluid shear a n d grain abrasions a n d impacts. A study using endoscopes a n d high speed video has confirmed this (Fitzpatrick, 1993). Consequently m a x i m u m attrition of the media will occur at this condition. This p a p e r describes a n investigation ( H u m b y , 1994) into the attrition o f various media d u r i n g b a c k w a s h i n g at collapse-pulsing conditions.
G r a n u l a r filters are used to remove particulate m a t t e r in water t r e a t m e n t processes. They require cleaning at regular intervals a n d this is usually achieved using water or c o m b i n e d air a n d water backwash. T h e m e d i u m c o m m o n l y used in these filters, sand, possesses high attrition resistance a n d therefore there has been little c o n c e r n a b o u t media losses due to attrition d u r i n g backwashing. However the use of g r a n u l a r activated c a r b o n ( G A C ) a n d a n t h r a c i t e as filter m e d i a has been increasing a n d these media do n o t possess the attrition resistance o f sand. Excessive attrition would lead to a gradual loss o f media a n d its size could reduce significantly (Ives, 1990). Therefore the relative abilities o f all these media to w i t h s t a n d attrition need to be established so t h a t the e c o n o m i c a n d o p e r a t i o n a l effects of attrition can be predicted. A t t r i t i o n o f filter media d u r i n g b a c k w a s h i n g has n o t been given m u c h a t t e n t i o n in the literature. M e d i a loss d u r i n g b a c k w a s h i n g is m e n t i o n e d but is generally assumed to be due to w a s h o u t of the media (e.g. Conley, 1961). Cleasby et al. (1975)investigated the abrasive loss o f a n t h r a c i t e d u r i n g 2 weeks o f c o n t i n u o u s air scour. They f o u n d t h a t the media loss was a r o u n d 5 % by weight a n d that the effective size reduced by 2.5%, b o t h of which they considered to be negligible. Ives (1990) used a n accelerated b a c k w a s h i n g test to determine attrition losses in a m e d i a bed t h a t was just fluidised. Guidelines for the acceptability o f losses based o n e q u a t i n g height loss with weight loss were given: > 5 % is unsatisfactory,
EXPERIMENTALAPPARATUSANDPROCEDURE The filter media investigated are shown in Table 1. Selection was based on achieving coverage of media commonly used in practice. The test was based on the BEWA (1993) accelerated backwash abrasion resistance test although there are some important differences, which are described below. Firstly, the system was single pass so that the concentration of material in the effluent could be monitored. The reason for this was that the dominant mode of attrition is assumed to be abrasion, i.e. the removal of corners, edges and surface layers by grains rubbing against each other, the particles produced being much smaller than the parent particles. While collisions between grains may occur it is considered improbable that there is sufficient energy to cause grain fracture. Consequently a good measure of the attrition can be obtained by monitoring the concentration of material in
*Author to whom all correspondence should be addressed. WR3O/2--O
291
292
M. Steve Humby and Caroline S. B. Fitzpatrick Table 1. Filter media investigated Type and size Source Granular coal based GAC 0.f~2.36 mm Chemviron Carbon Ltd Extruded coal based GAC 0.8 mm dia. Norit U.K Ltd Crushed anthracite coal 1.2 2.5 mm Universal Mineral Supplies Ltd Leighton Buzzard 1.(~2.0 mm Garside Industrial Sands Ltd
Media F300 ROW.8 Anthracite Sand
the effluent. Secondly, no measures were taken to prevent media washout which was also monitored but the results are not presented here. Lastly, the bed depth was 0.6 m instead of 0.25 m since this is more representative of the depths used in practice. The apparatus (see Fig. 1) consisted of a perspex column 1.8 m high with an inside diameter of 81 mm. Air and water were supplied via connections at the base, The column outlet was 1.6 m above the base and supplied a trap tank where washed out media was settled out. The effluent from the tank was discharged to drain. The procedure followed for the backwashing attrition experiments was standardised as follows: The media was loaded into the column to a depth of 0.6 m from a height of 1.8 m above the base. Sand was loaded dry while porous media (e.g. GAC) was loaded dry, removed and soaked in water for 24 h before being placed in the column proper. This was necessary to ensure that all the air was removed from the media pores which was difficult to achieve with the media in the column. The media was fluidised with water to a 50% bed expansion (based on dry length) for 30 rain to remove any fine particles present. All the fine particles obtained during the test would then be due to attrition. The water was turned off slowly over 2 rain to enable stratification of the media to occur. The combined water and air flow rates that gave the collapse-pulsing condition were determined using the procedure given by Amirtharajah et al. (1991). At collapsepulsing air cavities form and collapse within the media. The collapse of an air cavity results in rapid downward movement of a section of the bed due to the media moving to fill the space left by the air cavity. The existence of vigorous movements of this kind was taken as the collapse-pulsing condition. The procedure consisted of setting the air flow rate to a particular value and gradually increasing the water flow rate until collapse-pulsing was observed. The water and air flow rates were then recorded. The air flow rate was then set to a new value and the process repeated until a range of values was obtained. The backwashing attrition test followed on directly from the collapse-pulsing observations. The chosen collapsepulsing condition was established in the filter bed and was maintained for a continuous period of 100 h which is equivalent to approx. 3 years of daily backwashing at 6 min per wash (Ives, 1990). At a number of intervals, a 1 1 sample Outlet
~iii~ii!i~
Filter
bed meter ~)
Air supply ~ Valves 9 Water ..~,~ supply
- - ~ - - 1 Mesh Nozzle
in Trap tank
Fig. 1. Schematic of backwashing attrition test apparatus.
of water was obtained from the column outlet, passed through a 106 l~m sieve and then filtered using 1.2 pm filter paper. The paper was then dried for I hour, cooled in a dessicator and weighed. At the end of the 100 h, the test was terminated and the media was removed from the column, dried and weighed.
RESULTS AND DISCUSSION T h e air a n d w a t e r flow rates used to give collapsep u l s i n g are s h o w n in T a b l e 2. A s stated previously, a t t r i t i o n d u r i n g b a c k w a s h i n g is m a i n l y d u e to a b r a s i o n . O n this a s s u m p t i o n Fig. 2 a n d Fig. 3 depict the v a r i a t i o n o f a t t r i t i o n with time. It c a n be seen that for all m e d i a the a t t r i t i o n is very high initially b u t reduces a l m o s t e x p o n e n t i a l l y w i t h time. This high initial b u r s t is p r o b a b l y d u e to the r e m o v a l o f s h a r p c o r n e r s a n d edges. In the case o f s a n d it m a y be d u e to dirt o n the grains t h a t was n o t w a s h e d o f f d u r i n g the initial w a t e r only b a c k w a s h . S o m e o f the r e d u c t i o n is a t t r i b u t a b l e to the fact t h a t the a m o u n t o f m e d i a in the c o l u m n is r e d u c i n g d u e to particles in the size r a n g e being lifted by the air a n d w a s h e d out. The cumulative amount of attrition of each medium as a f u n c t i o n o f time is s h o w n in Fig. 4. It was c a l c u l a t e d by m u l t i p l y i n g the c o n c e n t r a t i o n o f particles at each s a m p l i n g time by the w a t e r flow rate a n d the time e l a p s e d since the p r e c e d i n g s a m p l e was taken, the a m o u n t s t h e n s u m m e d . T h e total a m o u n t o f a t t r i t i o n is given in T a b l e 3. T h e m i n i m u m value is o b t a i n e d f r o m Fig. 4. T h e m a x i m u m value is the difference b e t w e e n the initial a n d final a m o u n t s o f media; the final a m o u n t was o b t a i n e d by a d d i n g the m e d i a w a s h e d o u t d u r i n g the test to the a m o u n t r e m a i n i n g in the c o l u m n at the e n d o f the test. A s s u m i n g that the a m o u n t r e m a i n i n g is all in the size range (a r e a s o n a b l e a s s u m p t i o n ) t h e n the difference is the a m o u n t o f m e d i a lost d u e to undersize particles being w a s h e d out a n d to attrition. T h e difference also includes losses t h a t m i g h t have o c c u r r e d d u r i n g h a n d l i n g , i.e. d u r i n g r e m o v a l , drying, etc. T h e m i n i m u m values u n d e r e s t i m a t e the a m o u n t o f a t t r i t i o n due to the m e t h o d o f calculation. C o n s e q u e n t l y it can be stated t h a t the actual a t t r i t i o n o f the m e d i a lies within the ranges s h o w n in T a b l e 3. T h e r a n g e c a n n o t be t i g h t e n e d f u r t h e r since s o m e m e d i a is Table 2. Air and water rates for collapse-pulsing Experimental Media Water rate (m/h) Air rate (m/h) Vine(m/h) F300 10.3 35.8 16 ROW.8 9.0 35.8 22 Anthracite 20.5 29.8 28 Sand 42.8 23.87 52
Backwashing attrition of filter media 5 --
5k
"~ 4 :=.,= ~-
4 --
3
•
F300
+
ROW.8
am.•- •
• * Anthracite
+ F300 * ROW.8
3 -
/
• Sand
2 -b + ~ + --.-......._.~+ + ~,.,,~ , +. +.._._..___+. +/ ~ + , +
1
•.m
I •
•~•#g
2 .5 •-
293
1
--•
.+~-+'+
-+
-t
10
20
30
40
50
60
70
80
90 100
Time (h) Fig. 2. Variation of media particles in effluent with time for F300 and ROW.8.
inevitably lost d u r i n g handling. However if the m a x i m u m limits are used to specify the attrition loss then the error is o n the safe side. B E W A (1993) specifies t h a t the attrition loss after 100 h is n o t to exceed 3 % for b o t h water only a n d c o m b i n e d air a n d water backwash. F r o m T a b l e 3 it can be seen t h a t only sand met this specification. It is possible t h a t this limit is unrealistic to apply to G A C given its lower a b r a s i o n resistance. However if the m i n i m u m limits are used then only F300 was outside specification. C h a n g e s to the size range a n d distribution before a n d after b a c k w a s h are c o m p a r e d in T a b l e 4. Collapse-pulsing b a c k w a s h generally results in a reduction in the range o f grain sizes, i.e. fines are elutriated, a n d larger grains are b r o k e n d o w n due to attrition for a n t h r a c i t e a n d G A C . Attrition
W = kt ~
0.4
:~ 0.3 .5
• Anthracite + Sand
1
_ % ~0.2
\
"t:l o
0.0
\
"\ I
0
l*-o-÷ 20 30
l *+-o I 40 50 60
~--*1 70 80
I * I 90 100
Time (h) Fig. 4. Cumulative loss of media due to attrition.
where W--- weight fraction a b r a d e d , t = time a n d k, m = empirical constants. Applying this model to Fig. 4 in the form below p = at b
where p = percentage attrition, t = time o f b a c k w a s h ing (h) a n d a, b = empirical constants. T h e c o n s t a n t s a a n d b are f o u n d rewriting this e q u a t i o n in logarithmic f o r m a n d using linear regression. The c o n s t a n t s a a n d b a n d the correlation coefficient r for each media are given in Table 5. The model accurately fits the experimental d a t a a n d consequently it can be used to predict attrition o f these filter media.
CONCLUSIONS
model
A t t r i t i o n in fluidised beds was f o u n d by G w y n (1969) to be represented by a n e q u a t i o n o f the form
.5 ,.~ 0.1
0 [~-*~"0 10
10 20
30
40
50
60
70
80
-.
I
90 100
Time (h) Fig. 3. Variation of media particles in effluent with time for anthracite and sand.
1. The experiments u n d e r t a k e n have s h o w n that the highest attrition occurs in the period following placement of new media in the filter. 2. T h e results for sand a n d a n t h r a c i t e show attrition b e c o m i n g negligible after 30 a n d 50 h respectively. The total loss for sand confirms the traditionally held view t h a t attrition is negligible. F o r a n t h r a c i t e the attrition loss is insignificant w h e n c o m p a r e d to the w a s h o u t loss o f 7 % which was experienced d u r i n g the experiment ( H u m b y , 1994). 3. The two G A C s u n d e r w e n t attrition for the whole 100 h. The total losses were 4.5% for F300 a n d 2.0% for ROW.8. These losses are slightly less t h a n those
Table 3. Attrition losses of the media Attrition Loss Media Minimum ( % ) ~ Maximum(%)~ F300 4.5 7.3 ROW.8 2.1 3.5 Anthracite 0.5 3.2 0.1 2.3 Sand ~As a percentage of initial amount of media.
M. Steve Humby and Caroline S. B. Fitzpatrick
294
Table 4. Size ranges, d~0~,and d~0o,~values, uniformitycoefficientsand hydraulic sizes before and after backwash Media Size range ~o-do~o~o(mm) Hydraulicsize (mm)" d.~o (mm) d~o/o(mm) Uniformitycoefficient F300 before B/W 0.76-2.55 1.40 0.89 2.30 1.97 F300 after B/W 0.85 2.26 1.38 0.94 2.11 1.70 Anthracite before B/W 1.18-2.45 1.73 1.28 2.30 1.50 Anthracite after B/W 1.24-2.36 1.75 1.33 2.23 1.45 Sand before B/W 1.07--1.94 1.42 1.15 1.85 1.36 Sand after B/W 1.07 1.85 1.39 1.15 1.76 1.26 "Calculated in accordancewith BEWA (1993). Table 5. Constants a and b and regressioncoefficients Media a b r F300 ROW.8 Anthracite Sand
0.2010 0.1072 0.0419 0.0210
0.6786 0.6564 0.5828 0.4312
0.9999 0.9984 0.9339 0.8868
typically occurring during regeneration o f 5 - 1 0 % (Metcalf and Eddy, 1991) and are m u c h less than the losses o f 15-20% which occurred as a result o f unrestricted w a s h o u t (Humby, 1994). 4. Attrition during backwashing was f o u n d to be accurately described by a power law relationship which should enable prediction o f media loss due to attrition during practical operation. 5. Since the collapse-pulsing condition maximises grain abrasions and impacts, the limits obtained for attrition are the m a x i m u m expected under any backwashing regime. 6. The attrition behaviour o f the various media correlates well with the results o f a test developed to measure the friability of granular filter media ( H u m b y et al., 1995). Acknowledgements--The authors would like to thank R. Ormesher of Cranfield University for technical support and are also grateful to Chemviron Carbon Ltd and Norit U.K. Ltd for donating bags of granular activated carbon.
REFERENCES
Amirtharajah A. (1978) Optimum backwashing of sand filters. J. environ Engng Div., Proc. ASCE 104, 917-932. Amirtharajah A. (1993) Optimum backwashing of filters with air scour: a review. Wat. Sci. Technol. 27, 10, 195 211. Amirtharajah A., McNeUy N., Page G. and McLeod J. (1991) Optimum backwash of dual media filters and GAC filter-adsorbers with air scour. AWWA Research Foundation Report, Am. Wat. Wks Assoc., Denver, U.S.A. BEWA (1993) Standard for the Specification, Approval and Testing of Granular Filtering Materials. BEWA:P.18.93, British Effluent and Water Association. Cleasby J. L., Amirtharajah A and Baumann E. R. (1975) Backwash of granular filters. In The Scientific Basis of Filtration. (Edited by Ives K. J.). Proc. NATO Advanced Study Institute on the Scientific Basis of Filtration, Cambridge, U.K. Noordhoff, Gr6nigen. Conley W. R. (l 961) Experience with anthracite sand filters. J. Am. Wat. Wks Assoc. 69, 375-378. Fitzpatrick C. S. B. (1993) Observations of particle detachment during filter backwashing. Wat. Sci. Technol. 27, 213-221. Gwyn J. E. (1969) On the particle size distribution function and the attrition of cracking catalysts. A.I.Ch.E.J. 15, 1, 35-39. Humby M. S. (1994) Attrition of granular filter media. M.Sc. thesis, Cranfield Univ., Cranfield, Bedford, U.K. Humby M. S., Fitzpatrick C. S. B. and Stevenson D. G. (1995) Development of a friability test for granular filter media. J. 1WEM. Submitted for publication. Ives K. J. (1990) Testing of filter media. Aqua 39, 144-151. Metcalf and Eddy (1991) Wastewater Engineering-Treatment, Disposal and Reuse, 3rd edn. McGraw-Hill, New York.
Pergamon
0043-1354(95)00177-8
Wat. Res. Vol. 30, No. 2, pp. 291-294, 1996 Copyright © 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0043-1354/96 $15.00 + 0.00
A T T R I T I O N OF G R A N U L A R F I L T E R M E D I A D U R I N G B A C K W A S H I N G WITH C O M B I N E D A I R A N D W A T E R M. S T E V E H U M B Y a n d C A R O L I N E S. B. F I T Z P A T R I C K * School of Water Sciences, Cranfield University, Cranfield, Bedford MK43 0AL, U.K. (First received January 1995; accepted in revised form June 1995)
Abstract--Attritionresistance of granular filter media is becoming increasingly important as materials such as GAC and anthracite are being more frequently used as filter media. The purpose of this study was to evaluate the attrition experienced by various media during backwashing by performing accelerated backwash tests in a pilot column, using a combined water and air backwash at combinations that gave the condition known as collapse-pulsing. Since the dominant mode of attrition was assumed to be abrasion, the effluent was sampled at a number of intervals to determine the amount of fine material in the effluent. Coal based GAC exhibited the highest weight loss ( ~ 7%) and sand the least ( ~ 2 % ) . Key words--water treatment, granular filter media, attrition, backwashing test, collapse-pulsing, combined water and air scour, GAC, anthracite, sand
INTRODUCTION
3 - 5 % is undesirable, 1 - 3 % is doubtful a n d < 1% is satisfactory. Filters have historically been cleaned with a water only b a c k w a s h at 20--50% expansion. It has been s h o w n t h a t this regime is fairly ineffective in cleaning due to the limited n u m b e r of grain a b r a s i o n s a n d impacts t h a t occur ( A m i r t h a r a j a h , 1978). The m o s t effective cleaning regime has been s h o w n to be c o m b i n e d air a n d sub-fluidisation water b a c k w a s h at rates t h a t give a c o n d i t i o n k n o w n as collapse-pulsing ( A m i r t h a r a j a h , 1993). This was t h o u g h t to be due to m a x i m i s a t i o n o f fluid shear a n d grain abrasions a n d impacts. A study using endoscopes a n d high speed video has confirmed this (Fitzpatrick, 1993). Consequently m a x i m u m attrition of the media will occur at this condition. This p a p e r describes a n investigation ( H u m b y , 1994) into the attrition o f various media d u r i n g b a c k w a s h i n g at collapse-pulsing conditions.
G r a n u l a r filters are used to remove particulate m a t t e r in water t r e a t m e n t processes. They require cleaning at regular intervals a n d this is usually achieved using water or c o m b i n e d air a n d water backwash. T h e m e d i u m c o m m o n l y used in these filters, sand, possesses high attrition resistance a n d therefore there has been little c o n c e r n a b o u t media losses due to attrition d u r i n g backwashing. However the use of g r a n u l a r activated c a r b o n ( G A C ) a n d a n t h r a c i t e as filter m e d i a has been increasing a n d these media do n o t possess the attrition resistance o f sand. Excessive attrition would lead to a gradual loss o f media a n d its size could reduce significantly (Ives, 1990). Therefore the relative abilities o f all these media to w i t h s t a n d attrition need to be established so t h a t the e c o n o m i c a n d o p e r a t i o n a l effects of attrition can be predicted. A t t r i t i o n o f filter media d u r i n g b a c k w a s h i n g has n o t been given m u c h a t t e n t i o n in the literature. M e d i a loss d u r i n g b a c k w a s h i n g is m e n t i o n e d but is generally assumed to be due to w a s h o u t of the media (e.g. Conley, 1961). Cleasby et al. (1975)investigated the abrasive loss o f a n t h r a c i t e d u r i n g 2 weeks o f c o n t i n u o u s air scour. They f o u n d t h a t the media loss was a r o u n d 5 % by weight a n d that the effective size reduced by 2.5%, b o t h of which they considered to be negligible. Ives (1990) used a n accelerated b a c k w a s h i n g test to determine attrition losses in a m e d i a bed t h a t was just fluidised. Guidelines for the acceptability o f losses based o n e q u a t i n g height loss with weight loss were given: > 5 % is unsatisfactory,
EXPERIMENTALAPPARATUSANDPROCEDURE The filter media investigated are shown in Table 1. Selection was based on achieving coverage of media commonly used in practice. The test was based on the BEWA (1993) accelerated backwash abrasion resistance test although there are some important differences, which are described below. Firstly, the system was single pass so that the concentration of material in the effluent could be monitored. The reason for this was that the dominant mode of attrition is assumed to be abrasion, i.e. the removal of corners, edges and surface layers by grains rubbing against each other, the particles produced being much smaller than the parent particles. While collisions between grains may occur it is considered improbable that there is sufficient energy to cause grain fracture. Consequently a good measure of the attrition can be obtained by monitoring the concentration of material in
*Author to whom all correspondence should be addressed. WR3O/2--O
291
292
M. Steve Humby and Caroline S. B. Fitzpatrick Table 1. Filter media investigated Type and size Source Granular coal based GAC 0.f~2.36 mm Chemviron Carbon Ltd Extruded coal based GAC 0.8 mm dia. Norit U.K Ltd Crushed anthracite coal 1.2 2.5 mm Universal Mineral Supplies Ltd Leighton Buzzard 1.(~2.0 mm Garside Industrial Sands Ltd
Media F300 ROW.8 Anthracite Sand
the effluent. Secondly, no measures were taken to prevent media washout which was also monitored but the results are not presented here. Lastly, the bed depth was 0.6 m instead of 0.25 m since this is more representative of the depths used in practice. The apparatus (see Fig. 1) consisted of a perspex column 1.8 m high with an inside diameter of 81 mm. Air and water were supplied via connections at the base, The column outlet was 1.6 m above the base and supplied a trap tank where washed out media was settled out. The effluent from the tank was discharged to drain. The procedure followed for the backwashing attrition experiments was standardised as follows: The media was loaded into the column to a depth of 0.6 m from a height of 1.8 m above the base. Sand was loaded dry while porous media (e.g. GAC) was loaded dry, removed and soaked in water for 24 h before being placed in the column proper. This was necessary to ensure that all the air was removed from the media pores which was difficult to achieve with the media in the column. The media was fluidised with water to a 50% bed expansion (based on dry length) for 30 rain to remove any fine particles present. All the fine particles obtained during the test would then be due to attrition. The water was turned off slowly over 2 rain to enable stratification of the media to occur. The combined water and air flow rates that gave the collapse-pulsing condition were determined using the procedure given by Amirtharajah et al. (1991). At collapsepulsing air cavities form and collapse within the media. The collapse of an air cavity results in rapid downward movement of a section of the bed due to the media moving to fill the space left by the air cavity. The existence of vigorous movements of this kind was taken as the collapse-pulsing condition. The procedure consisted of setting the air flow rate to a particular value and gradually increasing the water flow rate until collapse-pulsing was observed. The water and air flow rates were then recorded. The air flow rate was then set to a new value and the process repeated until a range of values was obtained. The backwashing attrition test followed on directly from the collapse-pulsing observations. The chosen collapsepulsing condition was established in the filter bed and was maintained for a continuous period of 100 h which is equivalent to approx. 3 years of daily backwashing at 6 min per wash (Ives, 1990). At a number of intervals, a 1 1 sample Outlet
~iii~ii!i~
Filter
bed meter ~)
Air supply ~ Valves 9 Water ..~,~ supply
- - ~ - - 1 Mesh Nozzle
in Trap tank
Fig. 1. Schematic of backwashing attrition test apparatus.
of water was obtained from the column outlet, passed through a 106 l~m sieve and then filtered using 1.2 pm filter paper. The paper was then dried for I hour, cooled in a dessicator and weighed. At the end of the 100 h, the test was terminated and the media was removed from the column, dried and weighed.
RESULTS AND DISCUSSION T h e air a n d w a t e r flow rates used to give collapsep u l s i n g are s h o w n in T a b l e 2. A s stated previously, a t t r i t i o n d u r i n g b a c k w a s h i n g is m a i n l y d u e to a b r a s i o n . O n this a s s u m p t i o n Fig. 2 a n d Fig. 3 depict the v a r i a t i o n o f a t t r i t i o n with time. It c a n be seen that for all m e d i a the a t t r i t i o n is very high initially b u t reduces a l m o s t e x p o n e n t i a l l y w i t h time. This high initial b u r s t is p r o b a b l y d u e to the r e m o v a l o f s h a r p c o r n e r s a n d edges. In the case o f s a n d it m a y be d u e to dirt o n the grains t h a t was n o t w a s h e d o f f d u r i n g the initial w a t e r only b a c k w a s h . S o m e o f the r e d u c t i o n is a t t r i b u t a b l e to the fact t h a t the a m o u n t o f m e d i a in the c o l u m n is r e d u c i n g d u e to particles in the size r a n g e being lifted by the air a n d w a s h e d out. The cumulative amount of attrition of each medium as a f u n c t i o n o f time is s h o w n in Fig. 4. It was c a l c u l a t e d by m u l t i p l y i n g the c o n c e n t r a t i o n o f particles at each s a m p l i n g time by the w a t e r flow rate a n d the time e l a p s e d since the p r e c e d i n g s a m p l e was taken, the a m o u n t s t h e n s u m m e d . T h e total a m o u n t o f a t t r i t i o n is given in T a b l e 3. T h e m i n i m u m value is o b t a i n e d f r o m Fig. 4. T h e m a x i m u m value is the difference b e t w e e n the initial a n d final a m o u n t s o f media; the final a m o u n t was o b t a i n e d by a d d i n g the m e d i a w a s h e d o u t d u r i n g the test to the a m o u n t r e m a i n i n g in the c o l u m n at the e n d o f the test. A s s u m i n g that the a m o u n t r e m a i n i n g is all in the size range (a r e a s o n a b l e a s s u m p t i o n ) t h e n the difference is the a m o u n t o f m e d i a lost d u e to undersize particles being w a s h e d out a n d to attrition. T h e difference also includes losses t h a t m i g h t have o c c u r r e d d u r i n g h a n d l i n g , i.e. d u r i n g r e m o v a l , drying, etc. T h e m i n i m u m values u n d e r e s t i m a t e the a m o u n t o f a t t r i t i o n due to the m e t h o d o f calculation. C o n s e q u e n t l y it can be stated t h a t the actual a t t r i t i o n o f the m e d i a lies within the ranges s h o w n in T a b l e 3. T h e r a n g e c a n n o t be t i g h t e n e d f u r t h e r since s o m e m e d i a is Table 2. Air and water rates for collapse-pulsing Experimental Media Water rate (m/h) Air rate (m/h) Vine(m/h) F300 10.3 35.8 16 ROW.8 9.0 35.8 22 Anthracite 20.5 29.8 28 Sand 42.8 23.87 52
Backwashing attrition of filter media 5 --
5k
"~ 4 :=.,= ~-
4 --
3
•
F300
+
ROW.8
am.•- •
• * Anthracite
+ F300 * ROW.8
3 -
/
• Sand
2 -b + ~ + --.-......._.~+ + ~,.,,~ , +. +.._._..___+. +/ ~ + , +
1
•.m
I •
•~•#g
2 .5 •-
293
1
--•
.+~-+'+
-+
-t
10
20
30
40
50
60
70
80
90 100
Time (h) Fig. 2. Variation of media particles in effluent with time for F300 and ROW.8.
inevitably lost d u r i n g handling. However if the m a x i m u m limits are used to specify the attrition loss then the error is o n the safe side. B E W A (1993) specifies t h a t the attrition loss after 100 h is n o t to exceed 3 % for b o t h water only a n d c o m b i n e d air a n d water backwash. F r o m T a b l e 3 it can be seen t h a t only sand met this specification. It is possible t h a t this limit is unrealistic to apply to G A C given its lower a b r a s i o n resistance. However if the m i n i m u m limits are used then only F300 was outside specification. C h a n g e s to the size range a n d distribution before a n d after b a c k w a s h are c o m p a r e d in T a b l e 4. Collapse-pulsing b a c k w a s h generally results in a reduction in the range o f grain sizes, i.e. fines are elutriated, a n d larger grains are b r o k e n d o w n due to attrition for a n t h r a c i t e a n d G A C . Attrition
W = kt ~
0.4
:~ 0.3 .5
• Anthracite + Sand
1
_ % ~0.2
\
"t:l o
0.0
\
"\ I
0
l*-o-÷ 20 30
l *+-o I 40 50 60
~--*1 70 80
I * I 90 100
Time (h) Fig. 4. Cumulative loss of media due to attrition.
where W--- weight fraction a b r a d e d , t = time a n d k, m = empirical constants. Applying this model to Fig. 4 in the form below p = at b
where p = percentage attrition, t = time o f b a c k w a s h ing (h) a n d a, b = empirical constants. T h e c o n s t a n t s a a n d b are f o u n d rewriting this e q u a t i o n in logarithmic f o r m a n d using linear regression. The c o n s t a n t s a a n d b a n d the correlation coefficient r for each media are given in Table 5. The model accurately fits the experimental d a t a a n d consequently it can be used to predict attrition o f these filter media.
CONCLUSIONS
model
A t t r i t i o n in fluidised beds was f o u n d by G w y n (1969) to be represented by a n e q u a t i o n o f the form
.5 ,.~ 0.1
0 [~-*~"0 10
10 20
30
40
50
60
70
80
-.
I
90 100
Time (h) Fig. 3. Variation of media particles in effluent with time for anthracite and sand.
1. The experiments u n d e r t a k e n have s h o w n that the highest attrition occurs in the period following placement of new media in the filter. 2. T h e results for sand a n d a n t h r a c i t e show attrition b e c o m i n g negligible after 30 a n d 50 h respectively. The total loss for sand confirms the traditionally held view t h a t attrition is negligible. F o r a n t h r a c i t e the attrition loss is insignificant w h e n c o m p a r e d to the w a s h o u t loss o f 7 % which was experienced d u r i n g the experiment ( H u m b y , 1994). 3. The two G A C s u n d e r w e n t attrition for the whole 100 h. The total losses were 4.5% for F300 a n d 2.0% for ROW.8. These losses are slightly less t h a n those
Table 3. Attrition losses of the media Attrition Loss Media Minimum ( % ) ~ Maximum(%)~ F300 4.5 7.3 ROW.8 2.1 3.5 Anthracite 0.5 3.2 0.1 2.3 Sand ~As a percentage of initial amount of media.
M. Steve Humby and Caroline S. B. Fitzpatrick
294
Table 4. Size ranges, d~0~,and d~0o,~values, uniformitycoefficientsand hydraulic sizes before and after backwash Media Size range ~o-do~o~o(mm) Hydraulicsize (mm)" d.~o (mm) d~o/o(mm) Uniformitycoefficient F300 before B/W 0.76-2.55 1.40 0.89 2.30 1.97 F300 after B/W 0.85 2.26 1.38 0.94 2.11 1.70 Anthracite before B/W 1.18-2.45 1.73 1.28 2.30 1.50 Anthracite after B/W 1.24-2.36 1.75 1.33 2.23 1.45 Sand before B/W 1.07--1.94 1.42 1.15 1.85 1.36 Sand after B/W 1.07 1.85 1.39 1.15 1.76 1.26 "Calculated in accordancewith BEWA (1993). Table 5. Constants a and b and regressioncoefficients Media a b r F300 ROW.8 Anthracite Sand
0.2010 0.1072 0.0419 0.0210
0.6786 0.6564 0.5828 0.4312
0.9999 0.9984 0.9339 0.8868
typically occurring during regeneration o f 5 - 1 0 % (Metcalf and Eddy, 1991) and are m u c h less than the losses o f 15-20% which occurred as a result o f unrestricted w a s h o u t (Humby, 1994). 4. Attrition during backwashing was f o u n d to be accurately described by a power law relationship which should enable prediction o f media loss due to attrition during practical operation. 5. Since the collapse-pulsing condition maximises grain abrasions and impacts, the limits obtained for attrition are the m a x i m u m expected under any backwashing regime. 6. The attrition behaviour o f the various media correlates well with the results o f a test developed to measure the friability of granular filter media ( H u m b y et al., 1995). Acknowledgements--The authors would like to thank R. Ormesher of Cranfield University for technical support and are also grateful to Chemviron Carbon Ltd and Norit U.K. Ltd for donating bags of granular activated carbon.
REFERENCES
Amirtharajah A. (1978) Optimum backwashing of sand filters. J. environ Engng Div., Proc. ASCE 104, 917-932. Amirtharajah A. (1993) Optimum backwashing of filters with air scour: a review. Wat. Sci. Technol. 27, 10, 195 211. Amirtharajah A., McNeUy N., Page G. and McLeod J. (1991) Optimum backwash of dual media filters and GAC filter-adsorbers with air scour. AWWA Research Foundation Report, Am. Wat. Wks Assoc., Denver, U.S.A. BEWA (1993) Standard for the Specification, Approval and Testing of Granular Filtering Materials. BEWA:P.18.93, British Effluent and Water Association. Cleasby J. L., Amirtharajah A and Baumann E. R. (1975) Backwash of granular filters. In The Scientific Basis of Filtration. (Edited by Ives K. J.). Proc. NATO Advanced Study Institute on the Scientific Basis of Filtration, Cambridge, U.K. Noordhoff, Gr6nigen. Conley W. R. (l 961) Experience with anthracite sand filters. J. Am. Wat. Wks Assoc. 69, 375-378. Fitzpatrick C. S. B. (1993) Observations of particle detachment during filter backwashing. Wat. Sci. Technol. 27, 213-221. Gwyn J. E. (1969) On the particle size distribution function and the attrition of cracking catalysts. A.I.Ch.E.J. 15, 1, 35-39. Humby M. S. (1994) Attrition of granular filter media. M.Sc. thesis, Cranfield Univ., Cranfield, Bedford, U.K. Humby M. S., Fitzpatrick C. S. B. and Stevenson D. G. (1995) Development of a friability test for granular filter media. J. 1WEM. Submitted for publication. Ives K. J. (1990) Testing of filter media. Aqua 39, 144-151. Metcalf and Eddy (1991) Wastewater Engineering-Treatment, Disposal and Reuse, 3rd edn. McGraw-Hill, New York.