Effects of baffles on the performance of model waste stabilization ponds

Water Res. Vol. 18, No. 8. pp. 941-944. 1984 Printed in Great Britain. All rights reserved

0043-1354 8453.00---0.00 Copyright ~" 1984 Pergamon Press Ltd

E F F E C T S OF B A F F L E S ON T H E P E R F O R M A N C E OF M O D E L W A S T E S T A B I L I Z A T I O N PONDS J. S. KILANI~ and J. A. O G U N R O M B I 2 tDepartment of Civil Engineering. University of Birmingham. Birmingham BI5 2TT. England and ZDepartment of Water Resources and Environmental Engineering, Ahmadu Bello University, Zaria. Nigeria (Receit'ed July 1982)

Abstract--The performance of three baffled laboratory-scale facultative stabilization ponds were compared with that of an unbaffled control pond. The hydraulic characteristics of the ponds were estimated from the results of tracer tests. The biochemical oxygen demand (BOD5) removals achieved with the control pond and with the ponds having 3, 6 and 9 baffles were 79, 81, 86 and 890o respectively and the chemical oxygen demand (COD) removals were 81, 84, 84.2 and 84.2%~.The reductions in total solids (TS) were respectively43, 46, 51 and 64~o. Dispersion indices of 0.161, 0.126, 0.112 and 0.096 were obtained for the control, 3, 6 and 9 baffle ponds respectively, which indicated a trend of decreasing dispersion index with increasing number of baffles. Key words--baffle, pond, performance, plug flow, complete mixing, dispersion index

INTRODUCTION The advantages of waste stabilization ponds in small communities and areas where land is not very expensive and climatic conditions are favourable have been well recognized. Even the most developed countries are resorting to the use of ponds wherever feasible. Barson reported an increase in the number of designed ponds in use in the United States from 45 in 1945 to 4476 in 1971 (Barson, 1973). The average BOD removal for waste stabilization ponds is reported to be 60-90~ (Rebhun and Argaman, 1965) compared with 70-75~o for trickling filters and 80-90~ for activated sludge (Raft Ahmed, 1980; Mara, 1978). Since the hydraulic flow pattern in stabilization ponds is one of the major factors influencing pond performance, a thorough knowledge of the hydraulic characteristics in ponds is required for efficient and accurate pond design. MIXING CHARACTERISTICSIN

The acceptability of the completely mixed flow formula is based on simplicity rather than what actually takes place in stabilization ponds. Investigations have shown that stabilization ponds exhibit non-ideal flow patterns (Fritz et al., 1979; Thirumurthi, 1969, 1974; Uhlman, 1979). Thirumurthi in an attempt to incorporate the existence of imperfect mixing in ponds into pond design procedures suggested the use of the Wehner-Wilhelm equation (Thirumurthi, 1969, 1974):

Li - ( 1 +a)'-e ~!'-a- (1 - a ) ' - e -~/~a

where Le = Ll = K= t=

1

Li

l + Kt

(2)

or its simplified form:

PONDS

The most common practice in the design of waste stabilization ponds is to assume that the pond contents undergo complete mixing and that BOD5 removal in the pond follows first order reaction kinetics. The BOD 5 removal formula is thus expressed as: Le

4ae t'2a

Le

(1)

effluent BODs, mg Iinfluent BOD 5, mg 1- L first order BOD5 removal coefficient, d a y - ' mean detention time, days.

4a

Ze

e I -a'2d

(1 + a)'-

Li

(3)

for design purposes where d a D U L

= = = = =

dimensionless dispersion index = D / U L x / l + 4 Ktd coefficient of longitudinal dispersion, m-" h -~ mean velocity of travel, m h -~ and mean path length of a typical particle in the reactor, m.

The index d is equal to zero for an ideal plug flow and infinity for an ideal completely mixed flow pattern and its value can be estimated by dye tracer tests. In the absence of an accurate value of d, Thirumurthi recommended the use of an ideal plug flow equation: Le --

Li

941

=

e -A'

(4)

942

d S. KILASI and J. A. OGLNRO:",IBI

and rejected the use o f the completely mixed flow equation (1) (Thirumurthi, 1974). a EXPERIMENTAL APPARATUS AND PROCEDURES

9,. =-c ,*.-,

',o

,(

s~ ~o

Four laboratory model ponds, each t00 x 50 × I0cm deep were constructed from galvanized steel sheets. Pond A was a control without baffles, Pond B had three equally spaced baffles across the length. Pond C had six. and Pond D nine equally spaced baffles. Series flow was adopted in the baffled tanks. An Indigo Blue dye was used for the tracer tests and a diluted milk waste made from liquid tinned milk was used as the raw liquor throughout the experiments. Light was provided by four 20W fluorescent bulbs located at a distance of 20cm above the liquid surface. A 12 h-on and 12 h-off light cycle was maintained throughout the period of the study.

Tracer tests The ponds were filled with tap water and a flow rate of about 5.0t day -~ was maintained by pumping tap water from a reservoir using peristatic pumps. After steady state conditions were achieved, a slug of 10 ml Indigo Blue dye solution of known concentration was added near the inlet of each pond. Samples were collected from the outlet of the ponds at regular intervals of 24 h and the concentrations of the dye in the effluent samples were determined using a precalibrated Spectronic-20. The results are described later. Treatment studies The facultative ponds were filled with equal volumes of tap water and the sewage treated at the Ahmadu Be[to University sewage treatment pond. They were then left for about 3 weeks for the algae to develop. Within this period. about l01. of raw sewage from the same source was added daily to each pond after an equal volume had been withdrawn. Milk waste prepared from liquid tinned milk was double diluted in order to give a feed liquor having a BOD5 of about 300 mg l -t. This synthetic feed was continuously pumped from the bottom of a 75 I. storage tank at average flow rates of 4.6, 5.0, 5.1 and 5.2 I day -~ for the control, 3, 6 and 9-baffle ponds respectively. These rates of flow corresponded to detention times of between 10.8-9.65 days. Sampling of the effluents for analysis started after the calculated detention time had elapsed to allow for the flushing-out of the ponds initial contents. The results obtained at steady state conditions were included in the analysis. The influent milk waste and the corresponding effluent samples were analysed at 2 day intervals for BODy, COD, total solids and alkalinity in accordance with Standard Methods for the Examination qf Water and Wastewater

g_ 5

o q2

0

5

70

~v, "5

20

Fig, i. Concentration of dye in eMuent from ponds foIIo~ing the momentary addition of a slug of concentrated d}e to the influent. O--Control pond; ~--3-baffle pond: ~--6-baffle pond; × --9-baMe pond (APHA, 1965). The pH and dissolved oxygen of the effluent and influent samples were determined using a pH meter manufactured by W. G. Pye & Co. Ltd and YSI model 57 DO meter respectively. The experiments were terminated after about 3 weeks of operation at the steady stale conditions. RESULTS AND DISCUSSION

Effects o f baffles on hydratdic characterist~c.Y Tracer recovery (flow-through) curves tot all the p o n d s are shown in Fig. 1. The hydraulic characteristics o f each pond, such as the average detention time, T,, index o f dead spaces, :q, index o f shortcircuiting, %, and dispersion index, d, were c o m p u t e d and the results given in Table 1. The results agree with those obtained by previous workers in showing that the average detention time, T,, o b t a i n e d from results o f tracer tests is generally smaller than the theoretical detention time, T, obtained by dividing p o n d volume by the influent flow rate. This is probably due to the degree o f mixing which is characterized by the effects of shortcircuiting and dead spaces within the ponds. However an interesting trend o f increase in T, with increasing n u m b e r o f baffles was observed. The percentage deviation o f T, from T were c o m p u t e d and the results

"Fable 1. Summary of results of tracer tests for all the ponds Type of pond Control pond, no baffles

t,, (days)

25

Sor~plmq :.me, r(dOySi

T. T tp (days) (days) (days)

:q t~IT

T~-t___zp T~

d

7~, deviation of Tj from T

4.20

4.22

8.93

2.0

0.47

0.53

0.161

52,7

3-Baffle pond

5.98

5.94

8.63

4.0

0.69

0,33

0.126

312

6-Baffle pond

7.30

7.49

8.36

6,5

0,87

0.13

0.112

V,).4

9-Baffle pond 9. t0 9.29 8.38 9.5 1.02 -0.02 0.096 - 10.8 t~--Time to reach centroid of the curve; T--theoretical or calculated detention time, volume divided by flow rate; /p--time to reach peak or maximum concentration.

Effects of baffles on the performance of model waste stabilization ponds show that T~ deviated from T by about 53°0 for the control pond, 31°.o for the 3-baffle pond, 10.4% for the 6-baffle pond and - 1 1 % for the 9-baffle pond. The negative sign indicating that 7"= was higher than T for the 9-baffle pond. It is unusual for T~ to be higher than T but the pattern obtained suggests the existence of an optimum spacing of baffles in waste stabilization ponds. The index of dead space, ~ta, increases progressively with increase in the number of baffles, with ~q having an unusual value greater than unity for the 9-baffle pond. According to Rebhun and Argaman, :q greater than unity may indicate the existence of some dead spaces into which the tracer diffuses during the early stages of the flow and diffuses out in the latter stages, thus, causing the flow curve to be unduly elongated, and leading to a shift in the centre of gravity (Rebhun and Argaman, 1965). The index of short-circuiting, :q, is observed to decrease with increase in the number of baffles as will be expected, although the negative value of - 0 . 0 2 obtained for the 9-baffle pond cannot be explained. A value of ~q greater than unity and a negative value of ~tSobtained for the 9-baffle pond suggest that care must be taken in interpreting the values of~a and ~, and confirm the view expressed by some investigators that they are statistically not very good indicators of the degree of mixing in the ponds. Thirumurthi suggested that the dispersion index was the best indicator of the hydraulic flow within a reactor (Thirumurthi, 1969), since it is normally calculated from the variance of the flow-through curve and therefore involves all points in the curve, as compared with ~s and ~ta which are calculated using only one or two points in the curve. The d values obtained for the ponds in this study were 0.161, 0.126, 0.112 and 0.096 for the control, 3-, (a) T-o,

70

60

0 m

5O

puod ~, ;I~eH'6 ~ ~ ~ i

puod _ ,-~

~_

puod

_

puod

o. - -

.~ .~

u!~id ~- ~

i

--

c-i

....

N

4+

=i

N

~ "~e oo ¢.4

-~,~

,--- N = ~

~

7

N

i i

!

___~

•

u,5~x

.

i i

4O

x/X----x--~x----~x

I

3O

~

120

<

c~ ~ 0 0 0 80

(c) -'~ 3 0 0

~

250

,3_ -- 2 0 0

_o

150 0

10 Time

15 (doys)

20

F i g . 2. ( a ) , ( b ) , ( c ) Unfiltered effluent B O D 5, C O D and total solids for all the ponds. O--Control pond; A--3-baffle pond; F-I--6-baffle pond; x--9-baffle pond.

W.R. I8/~--B

_

=_

e-i

80

--

943

944

J. S+ KILANtand J. A. OGUNROMBI

6- and 9-baffle ponds respectivel~. This trend of decrease in the values of d with increasing number of baffles is expected, since by definition (d = D UL ), d is inversely proportional to both the mean velocity, U and the flow path length, L. The more the number of baffles provided the higher the value of U (since flow rate, Q was almost the same for all the ponds, and the cross sectional area A decreases with decreasing spacing of baffles) and the longer the flow path, L. Thus. if the coefficient of longitudinal dispersion, D. is assumed to be constant, d shows a decreasing trend. Effect o f baffles on treatment

The results of analysis of the effluents for BOD+. COD and total solids are shown in Fig. 2(a-c). Table 2 gives the analyses of the milk waste feed and the effluents from the ponds together with their performances. The influent fed to the ponds varied in BODs from 292 to 356 n g l - ' , in COD from 454 to 596 mg 1-~ and in total solids from 460 to 550 mg l -~, The average BOD 5 of the effluents were 89, 79, 60 and 45 mg t ~and the BODs removals 79, 81, 86 and 89°,; for the control, 3-, 6- and 9- baffle ponds respectively: the corresponding figures for COD were 126, 119, 115 and lOOmgl -t and 8l, 83.5, 84.2 and 84.27{,: the corresponding figures for total solids were: 315, 300, 277 and 227mgi -t and 43, 46, 5l and 64!~o. The removal of BOD, COD and TS for the different ponds showed that the longer detention period obtained as a result of using baffles corresponded with an improvement in the efficiency of removing organic and solid matter. The values of d obtained from tracer tests also showed that the more the number of baffles, the closer is the system to the ideal plug flow pattern giving the best BOD removal efficiency. CONCLUSIONS The performance of laboratory scale facultative stabilization ponds was found to be considerably improved by decreasing the dispersion index of the pond flow pattern by using baffles. Near-plug-flow conditions were obtained and these are desirable for efficient operation of facultative ponds. However,

sufficient operating data of baIfled futl scale ;~ast¢ stabilization pond is needed to be able to assess the effect of baffles on the performance of large scale ponds. Since the installation of baffles in ponds ~)it involve additional construction cost. the expected improvement in the performance of large scale ponds has to be closely related to the additional cost involved. In order to compare these favourable laboratory results with actual field tests, it is recommended that when ponds are constructed in the future, provision should be made for large scale operating data to be obtained by providing at least one cell with facilities for studying the effect of inserting battles, REFERENCES APHA (1965) Standard Methods jbr the F.vamimmon q/ Water and Wastewater. American Public Health Association, New York. Barson G. (1973) Lagoon performance and State of Lagoon Technology. Publication EPA-R2-73-144, ILS. EPA, Washington, DC. Fritz J. J., Middleton A. C. and Meredith D. D. (1979) Dynamic process modeling of wastewater stabilization ponds. J. Wat. Pollut. Control Fed. 51, 2724-2743. Levenspiel O. (1962) Chemical Reaction Engineering. Wile3, New York. Mara D. (1978) Sewage Treatment in Hot Climates. Wile>, Chichester. Raft Ahmed S. M. (1980) Choice and layout of various types of stabilization ponds. WHO/EMRO Technical Publication No. 3, pp. t33-154. Report of a Seminar on Waste Stabilization Ponds Design and Operation, Lahore. Rebhun M. and Argaman Y. (1965) Evaluation of hydraulic efficiency of sedimentation basins. J. sanit. Engng Die.. Proc. Am. Soc. cic. Engrs 91. Reynolds J. H. (1977) performance evaluation of an existing seven cell lagoon system. Technical Report, EPA-600.277-086, U.S. EPA, Cincinnati, OH. Thirumurthy D. (1969) A break-through in the trace~ studies of sedimentation tanks. J. ~l~t. Polhz;~ Contrv+; Fed. 41, No. 11. Thirumurthy D. (1974) Design criteria t2~r waste stabilization ponds. J. War. Pollut. Control. Fed. 46, 2094-2106. Uhlman D. (1979) BOD removal rates of waste stabilization ponds as a function of loading, retention time, ternperature and hydraulic flow pattern. Water Res 13, 193-200.