Some studies on reaeration in cascades

Water Research Pergamon Press 1972. Vol. 6, pp. 297-304. Printed in Great Britain.

SOME

STUDIES

ON

REAERATION

IN

CASCADES

T. H. Y. TEBBUTT Department of Civil Engineering, University of Birmingham, Birmingham, England Abstract--Experimental work on the reaeration characteristics of cascades has shown that they have considerable potential for the addition of dissolved oxygen to sewage efIiuents. There is some evidence to suggest that there is an optimum step height to give maximum rcaeration in a particular situation. IN A RECENTconsideration of the significance o f dissolved oxygen in effluents (TEBBUTT, 1971) it became apparent that, in the absence of special circumstances, the DO level in a sewage works effluent was not likely to exceed about 50 per cent saturation and could be well below this level. Although this oxygen deficit is not significant in situations where there is ample dilution available it could have quite serious consequences in cases where there was little dilution. In such circumstances, which are becoming increasingly common, it would be highly desirable to be able to produce an effluent with a relatively high dissolved oxygen content. Because o f the necessity for most works to be able to discharge to a watercourse even when its level has been raised by flooding there is usually a significant head difference between final treatment units and normal river level. This excess head is normally destroyed in a steeply inclined effluent pipe or channel or, more rarely, by providing a deep channel around the periphery of final settling tanks. It is thus likely that for low flow conditions in the receiving water, many works would have a significant drop available in the effluent line and it is possible that this drop could be utilized to improve the oxygen concentration in the discharge. The fact that any improvement in the D O level would not be so marked at higher river levels is not o f great importance. At eight times dilution, an effluent with zero DO will only depress the DO level downstream by about 1 mg 1-1. It is well known that the existence o f weirs in rivers can have beneficial effects on the oxygen concentration in the water so it was decided to initiate a preliminary investigation into the efficacy of multiple weirs, in the form of cascades, for improving D O levels. E X P E R I M E N T A L APPARATUS AND P R O C E D U R E The initial work was carried out using a small perspex cascade with steps 70 mm wide by 50 mm deep which had previously been used in a CIRIA sponsored investigation into the hydraulic behaviour of stepped spillways (EssERY and HORNER, 1971). The cascade was set up in the laboratory as shown in FIG. I. Tap water was fed via a variable orifice flow meter into a mixing tank where sodium sulphite and cobalt chloride solutions were added from constant head feeders to produce the desired DO level. The mixed flow then entered a second tank in the side of which was a rectangular notch discharging to the cascade. The DO saturation level in the second tank was monitored by an EIL type 94 A meter operating a R u s t r a k recorder. DO levels at various points on the cascade were determined using an EIL type 1520 meter with its electrode in a special sampling container (FIG. 2) which could be used to intercept part of the flow at any step in the cascade. Before starting a test the two DO meters 297 WR. 6 / 3 ~ r

298

T . H . Y . TEBBUTT

were standardized to give the same readings and air and water temperatures and barometric pressure were recorded. Using this apparatus tests were carried out over a flow range of 24-132 m a m -1 d a y - 1 and an initial D O level range of 0-80 per cent saturation. The configuration of the cascade and the limited supply of water available in the laboratory p r o m p t e d the construction of two new cascades both with a slope of 45 ° and transfer of the apparatus to the Public Health Engineering Pilot Plant. The first of the new cascades could be operated without steps to study the reaeration behaviour of a 45 ° slope smooth channel and also with steps 73 by 73 mm. The second cascade was only 150 m m wide, instead of the earlier widths of 300 mm, to permit the use of higher flows and was provided with interchangeable steps of 127 by 127 m m and 254 by 254 ram. DO ~ e r

[~

nnd r ec o r der ] ~ ' ~ J - ' ] ~ J ~ ' ~

ITI~OI,

k'

o,o,°

's

Flow meter

,,,qJ

'y" Sulphite solution

- ~ 7

FiG. 3. General arrangement of apparatus for work on 45° cascades. Water was abstracted from the stream adjacent to the Pilot Plant by a Stuart Turner No. 26 centrifugal p u m p and the deoxygenating chemicals drawn into the flow by tappings on the suction side of the pump. The same tanks as before were used at the top of the cascade and the flow was measured by recording the head on the rectangular notch. Using this set-up which is shown diagrammatically in FiG. 3 a range of flows o f 48-1152 m 3 m - 1 d a y - 1 was covered again with initial D O levels in the range 0-80 per cent saturation. The times of flow down the cascade was measured using a fluorescein tracer. RESULTS

Studies on cascade with 36 ° slope Runs were carried out on the cascade with 50 m m fall steps at flow rates of 24, 48, 84 and 132 m 3 m - ~ d a y - ~ and for each flow rate tests were carried out at initial D O levels of approximately 0, 20, 40, 60 and 80 per cent saturation. The results for samples with approximately zero initial D O at the four rates of flow are shown in FiG. 4. It will be noted that the lowest rate flow appears to offer slightly higher reaeration efficiency than the other rates of flow which produce similar D O levels.

FIG. 1. General arrangement of laboratory apparatus using 36’ slope cascade with 50 r nm steps.

FIG. 2. Detail of sampling cell. Thechute on the right diverts some flow from the step into the ccl I

where a baffle causes it to move past the oxygen probe.

(FUCitlg p. 298)

299

Some Studies on Reaeration in Cascades 70 36 °

A /

50 mm steps

/

9

go

g

50

-~ 4o N o

3o

,o

I

I

o

i

I

500

I

rooo

F o I I.

mm

FIG. 4. Typical results for 36 ° s l o p e c a s c a d e w i t h 50 m m fall steps.

A convenient way of expressing the reaeration characteristics o f a structure is the deficit ratio r (GAMMON, 1957) 100 - - p . r -

-

(I)

-

I00 - - Pd where

p . = D O per cent saturation upstream. Pd = D O per cent saturation d o w n s t r e a m TABLE 1. DEFICIT RATIOS, 5 0 m m STEPS 36 ° SLOPE

M e a n for all rates o f f l o w

Fall (nun)

Deficit ratio, m e a n a n d s t a n d a r d deviation for initial D O p e r c e n t o f 20

0

250 500 750 1000 1250

1"21 1-46 1.75 2-01 2"41

=k 0"05 q- 0"08 ± 0"16 4- 0"30 4- 0"51

1"29 1"53 1"80 2"13 2"58

40

-4- 0"05 4- 0"02 4- 0"15 =1= 0"23 ± 0.40

1-29 1'53 1'81 2"11 2"49

60

-4- 0-07 4- 0"17 4- 0'21 4- 0"30 4- 0"48

1.31 1.52 1-75 2.04 2"34

44444-

80 0"03 0-07 0"23 0"42 0-55

1"43 1-61 1.82 2"08 2"25

4- 0-10 -4- 0"11 4- 0"22 4- 0"25 4- 0"31

M e a n for all initial D O levels Fall Deficit ratio m e a n a n d s t a n d a r d deviation for rate o f flow in m a m - * d a y - 1 o f (ram) 24 48 84 132 250 500 750 1000 1250

1.36 1.62 1.97 2"39 2"94

=1= 0-12 4- 0-08 =1= 0.09 ± 0"I1 4- 0'21

1.25 1-46 1"71 1-90 2"20

=1= 0-07 =1= 0-10 =1= 0"16 4- 0"14 4- 0-30

1.32 1-46 1"65 1'89 2"15

4- 0-12 4- 0-06 4- 0.07 =1= 0"07 =t= 0"16

1-28 1.57 1"81 2"12 2"36

4q4+ 4-

0'09 0"10 0"07 0"15 0"11

300

T.H.Y. TEBBUT1"

TABLE 1 gives the mean deficit ratios and their standard deviations for the tests carried out on the 50 m m step cascade at a slope o f 36 °. There is a tendency for the deficit ratio to increase slightly with increasing initial D O concentration and to decrease slightly with increasing rates o f flow.

Studies on cascades with 45 ° slope The results for the inclined channel alone, i.e. without steps, are shown in FiG. 5. As might be expected the flow rate has a more significant effect on the reaeration 50

45

*

no steps

15 *C

m3rn-td -I

48

/ 40

8

132 3o

ae 0

2o

576

]0-~

0[

~

l

~

I

500

~

E)O0

Fall,

I 1500

l

mm

FIG. 5. Typical results for 45 ° slope channel with no steps. 80

45* 7 3 r n m steps 14 "C -#;.->'~

70

6O

/'

/Z /

°ll/ N

30

~

io

v

,

.

/'

?

a

20

/

.

~71152 m s m "t d "l ~ 576 LX 2 8 8 0 132' × 84. + 24

.

,.~'

5O0

I000 Fall.

Fzo.

6,

150O

mm

Typical results for 45 ° slope cascade with

73 mm

steps.

Some Studies on Reaaretion in Cascades

301

capacity of the channel than would be the case with a step cascade. It appears that due to the lack of turbulence under such conditions the reaeration capacity of the channel becomes quite low at high rates of flow when the water film is comparatively thick. The work with steps of 73, 127 and 254 mm in general produced similar trends to those noted in the initial work with in particular a tendency to reduced efficiency with increased rate of flow. This characteristic is illustrated in FIG. 6 which is for the 73 mm steps. 50

45 ° 576

28

7:3 mm m3m"l& I

/

14 °C

,

~

2

127 m m_.~

24

2 ~z2

_20 "5

Q 1,4 1.2 ).0

im

0

600

1200

Fotl,

1800

mm

Fio. 7. M e a n deficit ratios for various step heights in 45 ° cascade.

.9

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2o4 s ' 5 7 6

~3-,~d"

I. 8

14"C

1'6

1"2 i.o

) 0

I00

Step heighl,

I

I

200

300

mm

Fro. 8. Relationship between deficit ratio and step height in 45 ° cascade for a total fall

o f l m. A point o f some importance is a comparison of the performance of diJTerent cascade configurations at the same rate of flow as shown in FIG. 7 which plots mean deficit ratios for the tests at varying initial DO concentrations. Each of the step sizes examined produces markedly greater reaeration than the inclined channel without steps. By using the data shown in FIG. 7 it is possible to produce FIG. 8 which suggests that there may be an optimum size of step to provide the greatest reaeration in a given situation. Thus it would appear that 127 mm steps are more efficient than either 73 or 254 mm steps. This proposition is supported by the results obtained at a flow rate o f 1152 m s m -1 day -1 where the deficit ratios for steps of 73, 127 and 254 mm were 1.46, 2.00, 1.50 respectively.

302

T.H.Y. TEBBUTr

DISCUSSION Cascade aerators are sometimes used in the water industry for improving DO levels in potable water or to remove excess carbon dioxide. The most popular form of aerator for this purpose is however the spray type and the latest British reference on water engineering practice (INSTITUTIONOF WATER ENGINEERS, 1969) gives no guidance on the design of cascades. The equivalent American publication on water treatment (AMERICANWATER WORKS ASSOCIATION,1969) quotes C O 2 removals of 20-45 per cent and suggests that such aerators should have a fall of 1-3 m with a surface loading of about 200 m 3 m - 2 day-1 A considerable amount of work on aeration by weirs in rivers has been carried out in the U.K. at the Water Pollution Research Laboratory. GAMESON, 1957, analysed the results obtained from river surveys involving 54 weirs of various types and supplemented his study by laboratory investigations. He produced an expression for the deficit ratio r r= 1 4-abh/2 (2) where a = 1-25 in slightly polluted water 1.00 in moderately polluted water 0.85 in sewage effluent. b = 1.00 for a free fall 1-30 for a step weir h ---- fall (m). This work showed that the deficit ratio increased with the height of fall but that a weir coefficient (W) characterizing a particular type of weir could be obtained from the expression (r -- 1) W= ~ h

(3)

For a free fall weir Gameson found that an average value of W was 0.5 m - i . Although unable to study the effect of wide variations in flow on all the structures he found that at one particular weir with a fall of 1.4 m the value of r was 2.0 at the extremes of a flow range of 3.5 : 1. Using equation (2) to predict the deficit ratio for the conditions in the existing work depicted in Fro. 8 the calculated value of r is 1.81 assuming that the stream water used was slightly polluted and that the value of the coefficient b is taken as 1-30. Thus the predicted value of 1.81 is in reasonably good agreement with the values of r plotted in FIG. 8, 1-72, 1.94, and 1.53 for various step heights. Gameson's basis for adopting values for b of 1.00 for free fall and 1.30 for step weirs is that there is considerable splashing on step weirs which improves their aeration efficiency. It does however seem likely that the degree of splashing and hence the value of b will be influenced by the height of the individual steps as the present work indicates. Later work at W P R L (GAMESON,VANDYKE and OGDEN, 1958; BARRETT, GAMESON and OGDEN, 1960) considered free fall weirs in more detail with particular reference to the effects of temperature on reaeration characteristics. It was found that for a free fall weir 0.6 m high the value of the deficit ratio varied from 1.3 to 2.0 with a temperature range of 0-40°C. All the work described in the present paper on the 45 ° slope cascades was at a reasonably constant water temperature of 14-15°C so that temperature effects can be considered negligible.

Some Studies on Reaeration in Cascades

303

Because of the possible effect of flow rate on the reaeration efficiency of a cascade which is indicated by some of the results obtained during the present investigation it is intended to carry out further work in this area using much higher rates of flow than are available with the existing apparatus. Visual observations of the water trajectory at various flow rates showed considerable variation in the degree of splashing and it was noticeable that at a high rate of flow some of the water followed paths which tended to bypass a number of the steps. This could well result in a reduced aeration efficiency under these conditions. The work of ESSERY and HORNER, 1971 classifies the types of flow regime found in cascades into three basic types each of which would appear likely to have characteristic reaeration properties. The present work has been concerned with the degree of reaeration which could be achieved by various cascade configurations with a given total fall and at a given rate of flow. The fact that the flow regime was not necessarily constant for the various configurations may go some way to explain the different reaeration behaviour observed but does not invalidate the conclusion that there may be an optimum step height for a given situation. It is however useful to compare the hydraulic loadings already investigated with those which might be found in a sewage treatment plant. A conventional weir loading on final settling tanks is about 220 m a m-1 d a y - 1 and the highest loading investigated on the cascades was 1152 m a m -1 day -1 so that the results obtained should be applicable to the construction o f cascade-type overflow weirs in place of the normal free fall type. A more economical site for a cascade aerator would be in the effluent outfall channel and even the maximum rate o f flow used in the tests would be equivalent to a channel width of about 1 m for every 5000 population. This is of course an excessive width for most situations and clearly the continuance of the study using higher rates of flow is very necessary. Assuming that some form of cascade aerator were installed in place of inclined channel on the effluent outfall from a works it is of interest to calculate the improvement in D O which would result. Using the data given in FI~. 8 the deficit ratios for a 45 ° channel and a stepped cascade with the same fall of 1 m would be about 1.15 and 1.80 respectively. Thus with a final settling tank effluent DO level of say 20 per cent saturation the inclined channel would raise the DO to 30 per cent saturation and the cascade would raise the DO to 55 per cent saturation which is a significant improvement. Previous work at W P R L does however suggest that effluent may give a rather lower value of deficit ratio than the slightly polluted river water used in these investigations and further study of this factor will be necessary.

CONCLUSIONS A preliminary investigation into the use of cascades for reaeration has led to the following conclusions: (1) The use of cascade weirs offers considerable increases in reaeration efficiency over inclined channels or free fall weirs with the same drop. (2) Experimental work using a 45 ° cascade with individual step heights of 73-254 mm indicates reaeration characteristics similar to those obtained by other workers on larger steps in river weir systems. (3) There is some indication that the reaeration efficiency of a cascade decreases slightly with increased flow.

304

T . H . Y . TeBSUTT

(4) F o r a given set o f conditions it would appear that there is an o p t i m u m height o f step for m a x i m u m reaeration. (5) Further w o r k is necessary at rates o f flow higher than those used in the present investigation (up to 1152 m 3 m - 1 d a y - 1 ) and also into the reaeration characteristics o f waters o f varying pollution contents. Future w o r k should also consider alternative step profiles a n d shallower cascade slopes. It would also be useful to attempt to correlate the flow regime with reaeration performance. REFERENCES AMEgICANWATERWORKSASSOOATXON(1969) Water Treatment Plant Design. AWWA, New York. BARRETTM. J., GAMESONA. L. H. and OGDENC. G. (1960) Aeration studies at four weir systems. Wat. Wat. Engng 64, 407. ESSERYI. T. S. and HORNERM. W. (1971) The Hydraulic Design of Stepped Spillways, Report 33, CIRIA, London. GAME.SONA. L. H. (1957) Weirs and the aeration of rivers. J. Instn. Wat. Engrs 11, 477. GAMMONA. L. H., VAz,a)Yr,Z K. G. and OGDEN,C. G. (1958) The effect of temperature on aeration at weirs. Wat. Wat. Engng 62, 489. INSTITUTIONOF WATER ENGINrF~IS(1969) Manual o f British Water Engineering Practice, 4th Edn. Heifer.

TEnBUTTT. H. Y. (1971) Dissolved oxygen--its significance in effluents. Effl. War. Treat. J. 11, 39.