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Study of the internal hydrodynamics in three facultative ponds of two municipal WSPS in Spain
PII: S0043-1354(98)00297-8
Wat. Res. Vol. 33, No. 5, pp. 1133±1140, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
STUDY OF THE INTERNAL HYDRODYNAMICS IN THREE FACULTATIVE PONDS OF TWO MUNICIPAL WSPS IN SPAIN J. J. TORRES, A. SOLER*, J. SAÂEZ, L. M. LEAL and M. I. AGUILAR Department of Chemical Engineering Ð Murcia, University of Murcia, Campus Espinardo, 30071 Murcia, Spain (First received March 1998; accepted in revised form July 1998) AbstractÐThe hydraulic performances of three facultative ponds, two of the Cieza and one of the LorquõÂ ±CeutõÂ joint municipal wastewater stabilization ponds systems, were studied. The ponds of Cieza have a maximum capacity of 16,500 m3 (F.1) and 32,000 m3 (F.2), and both have a maximum depth of 1.6 m. The pond of LorquõÂ ±CeutõÂ have a maximum capacity of 27,000 m3 and a maximum depth of 1.8 m. A dye tracer, sulforhodamine B, was injected with the feed ¯ow of each one facultative ponds. In the course of time, samples from within and at the outlet of every system were taken. The data obtained prove that the hydrodynamic behavior of the ponds is similar to a perfectly mixed tank reactor. The average real residence times were also determined. When these were compared to ponds, considered ideal perfectly mixed tank reactors, a high level of coincidence was noted. It was also observed that the hydrodynamics of the ponds presented virtually no ¯ow anomalies. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐhydrodynamic, sulforhodamine B, waste stabilization pond, wastewater tracer
NOMENCLATURE
t: c(t): n: r: T: t: t: t*:
Spacial time or theoretical time. Concentration of sulforhodamine B (mg/l). Sample size. Product±moment correlation coecient. Time of the end of experiment (days). Time (days). Mean real hydraulic residence time (days). Mean hydraulic residence time (days) if the reactor has a behavior as CSTR. Qbp: Flow rate or the short-circuit.
1972; Marecos do Monte and Mara, 1987; Moreno, 1990; Agunwamba et al., 1992; Torres, 1995; Frederick and Lloyd, 1996; Dorego and Leduc, 1996; Torres et al., 1997a). In these conditions, the internal ¯ow behavior in the ponds can be considered, working in continuous ¯ow mode as ``closed reactors'' in agreement with the de®nition suggested by Levenspiel (1972), can be studied experimentally by stimulus±response techniques, using a tracer to identify the kind of mixture and to estimate the average real residence time as a consequence of the possible anomalies in the ¯ow (dead areas, short circuits) and their relation with the phenomena which in¯uence the hydrodynamics of the ponds.
INTRODUCTION
The natural origin of the phenomena which cause treatment in the wastewater stabilization ponds does not make them easier to study or to understand, especially when it is a case of applying them to the design of the installations. Among the most complex phenomena, is that of the internal hydrodynamic behavior of the pond. The behavior hydrodynamic is in¯uenced by the shapes and depth of the ponds, the situation of inlet and outlet, the climatology and the direction of the dominant winds in the zone which are situated (Kenneth and Gary, *Author to whom all correspondence should be addressed.
MATERIAL AND METHODS
Description of each system The three facultative ponds studied were situated in the municipal wastewater stabilization ponds systems (WSPS) of Cieza and LorquõÂ ±CeutõÂ (Region of Murcia in the southeast of Spain). The Cieza systems had two anaerobic lagoons in parallel, two facultative lagoons in parallel and a third facultative one in series (Fig. 1). The LorquõÂ ±CeutõÂ joint treatment plant had three anaerobic lagoons in parallel, two facultative lagoons in parallel and a third facultative one in series (Fig. 2). The two facultative ponds of Cieza studied had a maximum capacity of 16,500 m3 (F.1) and 32,000 m3 (F.2). In F.1 the wastewater entered at 1 m depth and in F.2 at 0.5 m depth (points 1, Fig. 1). Both had the outlet at the surface (points O, Fig. 1) and a maximum depth of 1.6 m,
1133
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J. J. Torres et al.
Fig. 1. Waste stabilization ponds of Cieza and sampling points. while the facultative pond of LorquõÂ -CeutõÂ had a capacity of 27,000 m3 and the wastewater entered at 0.5 m depth (point I, Fig. 2). It had the outlet at surface (point O, Fig. 2) and a maximum depth of 1.8 m. The whole construction in all the lagoons was made impermeable by a coat of polyethylene. During the period of experimental studies, the average ¯ows rates measured were, 1450 m3/ day (RSD = 12%, n = 24)(F.1) and 1308 m3/day (RSD = 14%, n = 32)(F.2) for the Cieza ponds and 2614 m3/day (RSD = 18%, n = 36) for the LorquõÂ ±CeutõÂ pond.
Selection and introduction of tracer The tracer was selected in accordance with the criteria for an ideal tracer (Plata, 1972) and, moreover, can be used in wastewater. Sulforhodamine B was also chosen since it has been used in aquatic systems with organic matter (Smart and Laidlaw, 1977) and by other authors in similar studies (Marecos do Monte, 1985; Marecos do Monte and Mara, 1987; Robert, 1991; Torres, 1995; Torres et al., 1997a). Tracer concentrations were measured, as will be described in sampling and analysis, using spectro¯uorimetric techniques (Skoog, 1985; Olsen, 1986). In each of the facultative ponds of Cieza, F.1 and F.2, was added 80 and 160 g of sulforhodamine B, respectively. In the lagoon of LorquõÂ ±CeutõÂ 160 g of tracer was added. The idea (in all cases) was that homogenization would produce a tracer concentration of around 5±6 mg/l water in the systems. These concentrations were judged sucient for variations to be easily observed and such that the cost should not be excessive. The tracer was dissolved in approximately 8 l of water before being poured quickly into the pond.
Sampling and analysis The experiment in the WSPS of Cieza began in November (1995) and in the LorquõÂ ±CeutõÂ in April (1996). In both systems the samplings were carried out in the outlet ¯ow and inside the lagoons. In the ®rst case, the samples were taken every 30 min during the ®rst two days and then every h for one week. During the following week samples were taken every 2 h and 3, 4 and 6 h in subsequent weeks. Samples were taken by an ISCO automatic sampler, installed at the outlet. The internal samplings were carried out intensively during the ®rst few days following the introduction of the tracer. Once it was clear that distribution was uniform throughout the water column, the samplings were made at wider intervals. So four strategically placed points in WSPS of Cieza (Fig. 1) and two LorquõÂ ±CeutõÂ (Fig. 2) were established to take samples at the surface and at dierent depths (1 and 1.5 m) during the investigation in every pond. A device designed by the research team was placed in each pond. This device consisted of a ¯oater ®xed by ropes to opposite shores of the ponds. Three ¯exible polyethylene tubes were attached to the ¯oater and carried a lead weight, such that each penetrated to the desired depth. The other end of each tube was left free on the shore so that it could be ®tted at any time to a centrifugal pump connected to a 12 V battery and through which the samples were taken. In order to establish the possible role of thermal strati®cation in the results, some temperature pro®les were measured in the ponds during the sampling period. The measurements showed that there was no thermal strati®cation in any lagoon (Table 1). After sampling, the samples were carried back to the laboratory, where the concentration of sulforhodamine B was measured, as described in our previous publication (Torres et al., 1997b). The amount of sample that was used depended on the concentration factor required.
Internal hydrodynamics of facultative ponds
1135
Fig. 2. Waste stabilization ponds of LorquõÂ ±CeutõÂ and sampling points. RESULTS AND DISCUSSION
2. Facultative pond F.2. From Table 2, it can be observed how the tracer initially spreads out over the surface of the pond in rapid, erratic movements giving high concentrations after 6 h (the sulforhodamine B was introduced at 12.00 h) at point 4 (the most distant), while lower concentrations persisted at point 3 (intermediate). Homogeneity was not reached in all the water column of the pond, until 29 h after the introduction.
Tables 2±4 show the concentrations of the tracer at each depth, together with the time corresponding to the internal sampling of the three ponds. In each table the following can be observed. 1. Cieza WSPS. 1. Facultative pond F.1. From Table 2, it can be observed that 1 h after its injection (the sulforhodamine was introduced at 16.00 h) the tracer was still concentrated near to the point of entry (Fig. 1, points 1 and 2) and in a layer of water similar in depth to that at which it had been introduced. As time passed the dispersal was rapid since 24 h after the introduction the concentration was practically uniform throughout the pond.
2. LorquõÂ ±CeutõÂ WSPS. Facultative pond F.1. From Table 4, it can be appreciated how during the ®rst 10 h (the sulforhodamine was introduced at 11.00 h) the tracer basically spread out over the surface of the pond. It then slowly dispersed throughout
Table 1. Pro®les of temperatures (8C) Cieza WSPS (at 13 h) Depth
day (11/1)
Surface 1m 1.5 m LorquõÂ ±CeutõÂ WSPS
12.2 12.0 12.0 (at 12 h)
day(11/25)
day(12/1)
day(12/5)
day(12/20)
day(1/15)
11.8 11.6 11.6
13.0 12.7 12.6
12.4 12.1 12.0
12.5 12.3 12.0
12.3 12.0 12.0
Depth
day (4/20)
day (4/28)
day (5/3)
day (5/20)
day (6/3)
day (6/15)
Surface 1m 1.5 m
13.6 12.8 12.8
13.2 13.5 13.5
14.6 13.8 13.7
15.2 14.8 14.7
15.6 15.3 15.0
15.5 15.3 15.2
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J. J. Torres et al. Table 2. Tracer concentration in points 1, 2, 3 and 4 at Cieza F.1 pond (mg/l) Point 1
Point 2
Point 3
Point 4
Time
surface
1m
1.5 m
surface
1m
1.5 m
surface
1m
1.5 m
surface
1m
1.5 m
17 h 19 h 26 h 43 h 50 h 68 h 73 h 91 h 140 h 116 h
2.5 3.9 3.9 4.1 3.5 3.5 2.7 2.5 2.2 2.3
13.1 4.5 4.2 4.2 3.3 3.4 2.9 2.9 1.7 2.5
11.0 3.9 4.3 4.0 3.7 3.5 3.5 3.5 2.1 2.5
0.6 4.0 3.8 4.3 3.5 3.4 3.1 2.7 2.1 2.7
14.2 4.1 4.3 4.0 3.7 4.0 3.5 3.0 2.2 2.7
4.0 3.3 4.8 3.9 3.9 3.6 3.3 3.2 2.1 2.7
0.2 4.4 4.0 4.2 3.9 3.7 3.5 3.1 2.2 2.5
3.5 4.1 4.4 3.8 3.8 3.4 3.7 3.2 2.2 2.5
0.2 4.3 4.2 4.1 3.7 3.5 3.3 3.4 2.0 2.3
0.2 4.0 4.4 3.9 3.8 3.6 3.2 3.2 2.3 2.5
0.2 4.2 4.1 4.2 3.9 3.7 3.3 3.2 2.1 2.6
0.1 4.2 4.1 4.3 3.7 3.6 3.4 3.1 2.0 2.6
Table 3. Tracer concentration in points 1, 2, 3 and 4 at Cieza F.2 pond (mg/l) Point 1
Point 2
Point 3
Point 4
Time
Surface
1m
1.5 m
Surface
1m
1.5 m
Surface
1m
1.5 m
Surface
1m
1.5 m
1h 6h 23 h 29 h 48 h 53 h 72 h 77 h 96 h 120 h
38.1 17.4 3.9 3.5 3.2 3.5 2.9 2.9 2.4 1.8
17.5 11.4 4.6 3.2 3.0 3.7 3.5 3.5 2.3 1.6
12.9 9.0 3.5 4.4 3.3 3.4 3.1 3.6 1.8 1.8
45.4 12.8 4.8 3.8 3.0 4.0 3.3 2.7 2.3 1.9
0.5 10.5 3.6 3.9 4.3 3.6 3.1 3.7 2.5 2.0
0.2 6.7 4.4 4.1 4.1 4.3 3.0 3.6 2.4 1.8
0.3 1.7 4.8 4.4 5.0 4.3 3.4 3.5 2.4 2.1
0.2 1.2 1.6 4.5 4.7 4.3 4.1 3.4 2.5 2.2
0.3 0.3 0.6 3.8 4.3 4.2 3.9 3.5 2.6 2.2
0.3 10.1 5.0 4.9 4.8 4.4 3.8 3.7 2.5 2.2
0.1 10.7 5.2 4.6 4.6 4.5 3.8 4.0 2.8 2.1
0.1 10.8 4.7 4.6 4.5 4.6 4.2 3.8 2.4 2.1
Table 4. Tracer concentration in points 1 and 2 at LorquõÂ ±CeutõÂ F.1 pond (mg/l) Point 1
Point 2
Time
Surface
1m
1.5 m
Surface
1m
1.5 m
1h 2h 6h 10 h 24 h 30 h 48 h 53 h 72 h 80 h 96 h 105 h 121 h 145 h 168 h 193 h 223 h 246 h
0.4 7.5 7.5 7.3 8.3 5.8 5.3 4.7 3.5 3.6 3.5 3.0 3.4 3.2 2.6 2.3 1.6 1.6
0.4 0.0 23.2 11.8 7.7 6.5 5.9 4.8 3.6 3.4 3.3 3.4 3.5 3.3 2.7 1.9 1.6 1.7
0.0 0.1 0.0 0.0 8.5 6.5 5.1 4.6 2.8 3.2 2.7 3.4 3.4 3.2 3.0 2.7 1.7 1.4
0.0 6.4 7.5 6.6 7.4 5.6 5.0 4.6 3.4 3.4 3.9 3.0 3.1 2.8 2.6 2.2 1.7 1.7
0.0 0.9 0.9 0.0 7.4 7.5 5.6 4.4 4.1 3.4 3.3 3.2 3.6 2.9 2.5 1.8 1.6 1.5
0.0 0.0 0.0 0.0 0.2 1.4 4.9 4.9 3.6 3.1 3.4 3.5 3.5 3.0 3.0 2.3 2.1 1.4
Table 5. Predominant winds in both towns during the period of experiment
LorquõÂ ±CeutõÂ Cieza
Range of speeds (Km/h)
Maximum gust (Km/h)
NE (%)
SE (%)
SW (%)
NW (%)
Calm (%)
5±18 2±10
34 22
13.2 5.1
20.7 7.2
15.9 3.9
21.8 83.1
28.4 0.7
Internal hydrodynamics of facultative ponds
1137
Fig. 3. Mean daily tracer concentration at the outlet vs time (Lagoon F.1 of Cieza) and theoretical behavior as a CSTR.
the water, and even 48 h after its introduction, the system was still not totally homogeneous. Even though the three ponds can be considered to have the behavior of a perfect mixed tank reactor and all attain homogeneity in under three days, there are, nevertheless, some dierences regarding the ease with which this situation is reached. In the Cieza F.1 pond, the process, from input to output, is rapid and progressive, slower yet still progressive in the case of the LorquõÂ ±CeutõÂ F.1 pond and rapid and irregular in the Cieza F.2 pond. Constructional and functional dierences arising during the experiments may be related to these dierent behaviors. Thus, the greater input depth in the wastewater of the Cieza F.1 pond (1 m.) could lead to a more uniform distribution than in the other two ponds (depth 0.5 m). Wind speeds and directions are also in¯uencing factors in the dispersion process. Table 5 shows the data corresponding to the ®rst three days of the experiments and it will be appreciated that in the case of the LorquõÂ ±CeutõÂ experiment there was a long period of calm (28.4%) and varying wind directions with no clear predominance. For the Cieza ponds however, there were hardly any periods of calm (0.7%) and there was a clear predominance of northwesterly winds (83.1%), which blew over the pond lengthwise in the case of F.1 and breadthwise in the case of F.2, thus favoring the overall ¯ow.
In order to carry out the quantitative analysis of the hydrodynamic study the concentration curves were determined at the tracer output, Curve C, of the three systems from the average daily concentrations (Figs 3±5). The curves respond to the classic RTD curve for a perfect mixed tank reactor (CSTR): a maximum near to the ordinates axis of asymmetric form and with a pronounced tail (Levenspiel, 1972). This fact supports the above comments on the hydrodynamic behavior of the three ponds. From the average daily sulforhodamine B concentrations at the output, the c0t product was obtained (Hymmelblau and Bischo, 1968; Levenspiel, 1972, 1979): T c0 t c t dt, 1 0
where c0 is the average concentration of the tracer if this had been of uniform distribution throughout the pond at the moment of introduction and t is the average real residence time. Once this product has been found, it is possible (equation 2) to obtain the age distribution function at the output, E(t), and from this (equation 3) the average real residence time (Hymmelblau and Bischo, 1968; Levenspiel, 1972, 1979). The values are given in Table 6. E t
c t c0t
2
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J. J. Torres et al.
Fig. 4. Mean daily tracer concentration at the outlet vs time (Lagoon F.2 of Cieza) and theoretical behavior as a CSTR.
t
T 0
E t t dt
3
If the hydrodynamics of the ponds are similar to that of CSTR, the C curves (Figs 3±5) should correspond to those obtained from the expression below
in logarithmic form (equation 4) (Levenspiel, 1979), in which A and B are constant ln c t ln A ÿ B t:
4
By adjusting the parameters of equation 4, to the data of the average daily sulforhodamine B concen-
Fig. 5. Mean daily tracer concentration at the outlet vs time (Lagoon F.2 of LorquõÂ ±CeutõÂ ) and theoretical behavior as a CSTR.
Internal hydrodynamics of facultative ponds Table 6. Residence times (days) Residence Time
Cieza F.1
Cieza F.2
LorquõÂ ±CeutõÂ F.1
t t* t
10.3 11.0 20.1 11.8
22.1 23.02 0.4 24.6
10.0 10.9 20.1 10.4
trations at the output to equation 4, we obtain for the three ponds: 1. Cieza WSPS. 1. Facultative pond F.1 (for a con®dence interval of 95% with n = 33 and r = 0.998). ln c t 1:6320:008 ÿ 0:09224 10ÿ4 t 5 2. Facultative pond F.2 (for a con®dence interval of 95% with n = 80 and r = 0.994). ln c t 1:5220:003 ÿ 0:04323 10ÿ4 t 6 2. LorquõÂ ±CeutõÂ WSPS. 3. Facultative pond F.1 (for a con®dence interval of 95% with n = 36 and r = 0.993). ln c t 1:8820:011 ÿ 0:092210ÿ4 t
1139
tank reactor, and likewise from the data obtained from the interior of each system, it has been shown that the ponds present a hydrodynamic behavior similar to that of an ideal perfectly mixed tank reactor. This has been con®rmed by observing the analogy that exists between the average real residence times and those determined by supposing that the reactor has this type of hydrodynamic behavior. This hydraulic behavior may be considered satisfactory, since it avoids an anaerobic zone at the entrance to the in¯ow and the accumulation of exclusively anaerobic micro-organisms in the zone (Juanico, 1991). On comparing the average real residence times (t) with the theoretical ones (t), a good correspondence has been appreciated between them, which is indicative of the absence of stagnant zones and the fact that the ponds were adequately designed from this hydraulic point of view. Furthermore, by comparing the C curves at the output, with that of an ideal perfectly mixed tank reactor, it has been con®rmed that, with the exception of the small short-circuit observed that could decrease the treatment performances, there were practically no ¯ow anomalies in the Cieza F.2 and the LorquõÂ ±CeutõÂ ponds.
7
The validity of the adjustments for each of the ponds may be observed in Figs 3±5. According to Levenspiel (1979), t* = 1/B, thus giving the values shown in Table 6, together with those of the theoretical residence time or spacial time (t = V/Q). If the average real residence times (t) and the theoretical times (t) are observed, a good correspondence between them is con®rmed and this indicates that, there are no stagnant zones and the design of the ponds is adequate. Reference should be made to the presence in Figs 4 and 5 of a sharp peak at the beginning of the experiment above curve C, which is representative of the CSTR. This indicates the presence of a small short-circuit in both ponds (Levenspiel, 1979; Torres, 1995; Dorego and Leduc, 1996; Torres et al., 1997a). From the relationship existing between the area of curve C, obtained with the real data, and the curve found by supposing the reactor to behave like a perfect mixed tank reactor, it is possible to determine the percentage of the short-circuit that occurred in both case: 1.30% of the total ¯ow rate (Qbp=17 m3/day) and 1.45% of the total ¯ow rate (38 m3/day) for the Cieza facultative pond F.2 and the LorquõÂ ±CeutõÂ F.1 pond, respectively.
CONCLUSIONS
From the C curves at the output and from the comparison of these with those of a perfect mixed
AcknowledgementsÐThis work was ®nancially supported by the CICYT of Spain (Proyect AMB92-D619) and a grant from the Instituto de Fomento de la RegioÂn de Murcia. The authors gratefully acknowledge these institutions for their economic support. REFERENCES
Agunwamba J. C., Egbuniwe N. and Ademiluy J. O. (1992) Dispersion number in waste stabilization ponds. Water Res. 26(1), 85±89. Dorego N. C. and Leduc R. (1996) Characterization of hydraulic ¯ow patterns in facultative aerated lagoons. Water Sci. Tech. 34(11), 99±106. Frederick G. L. and Lloyd B. J. (1996) An evaluation of retention time and short-circuiting in waste stabilization ponds using serratia marcescens bacteriophage as a tracer. Water Sci. Tech. 33(7), 49±56. Hymmelblau D. M. and Bischo K. B. (1968) Process Analysis and Simulation. Deterministic System. John Wiley and Sons, New York. Juanico M. (1991) Should waste stabilization ponds be designed for perfect-mixing or plug-¯ow?. Water Sci. Tech. 23, 1495±1502. Kenneth A. M. and Gary Z. W. (1972) Treatment eciency of waste stabilization ponds. J. Sanit. Eng. Div. ASCE. 98, 407±425. Levenspiel O. (1972) Chemical Reaction Engineering. John Wiley and Sons, New York. Levenspiel O. (1979) The Chemical Reactor Omnibook. OSU Book Stores, Corvallis, OregoÂn. Marecos do Monte M. H. F. (1985) Hydraulic dispersion in waste stabilization ponds in Portugal. Ph.D. thesis, University of Leeds. Marecos do Monte M. H. F. and Mara D. D. (1987) The hydraulic performance of waste stabilization ponds in Portugal. Water Sci. Tech. 19(12), 219.
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Moreno M. D. (1990) A tracer study of the hydraulics of facultative stabilization ponds. Water Res. 24(8), 1025± 1030. Olsen E. D. (1986) MeÂtodos OÂpticos de AnaÂlisis. Ed. ReverteÂ, Barcelona. Plata A. (1972) IsoÂtopos en Hidrologia. Ed. Alhambra, Madrid. Robert S. P. (1991) The in¯uence of the sulphur cycle on the performance of prymary waste stabilization ponds treating domestic wastewater. Ph.D. thesis, University of Leeds. Skoog D. A. (1985) Principles of Instrumental Analysis, Saunders College Publ., Philadelphia.
Smart P. L. and Laidlaw I. M. S. (1977) An evaluation of some ¯uorescent dyes for water tracing. Water Resour. Res. 13(1), 15±33. Torres J. J. (1995) Estudio hidrodinaÂmico de distintos sistemas de lagunaje para el tratamiento de aguas residuales. Ph.D. thesis, University of Murcia. Torres J. J., Soler A., SaÂez J. and OrtunÄo J. F. (1997a) Hydraulic performance of a deep wasteswater stabilization pond. Water Res. 31(4), 679±688. Torres J. J., Soler A., SaÂez J. and OrtunÄo J. F. (1997b) Determination of low concentration of sulforhodamine B in wastewaters and stabilization ponds. Water Res. 31(12), 3183±3186.
Wat. Res. Vol. 33, No. 5, pp. 1133±1140, 1999 # 1999 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/99/$ - see front matter
STUDY OF THE INTERNAL HYDRODYNAMICS IN THREE FACULTATIVE PONDS OF TWO MUNICIPAL WSPS IN SPAIN J. J. TORRES, A. SOLER*, J. SAÂEZ, L. M. LEAL and M. I. AGUILAR Department of Chemical Engineering Ð Murcia, University of Murcia, Campus Espinardo, 30071 Murcia, Spain (First received March 1998; accepted in revised form July 1998) AbstractÐThe hydraulic performances of three facultative ponds, two of the Cieza and one of the LorquõÂ ±CeutõÂ joint municipal wastewater stabilization ponds systems, were studied. The ponds of Cieza have a maximum capacity of 16,500 m3 (F.1) and 32,000 m3 (F.2), and both have a maximum depth of 1.6 m. The pond of LorquõÂ ±CeutõÂ have a maximum capacity of 27,000 m3 and a maximum depth of 1.8 m. A dye tracer, sulforhodamine B, was injected with the feed ¯ow of each one facultative ponds. In the course of time, samples from within and at the outlet of every system were taken. The data obtained prove that the hydrodynamic behavior of the ponds is similar to a perfectly mixed tank reactor. The average real residence times were also determined. When these were compared to ponds, considered ideal perfectly mixed tank reactors, a high level of coincidence was noted. It was also observed that the hydrodynamics of the ponds presented virtually no ¯ow anomalies. # 1999 Elsevier Science Ltd. All rights reserved Key wordsÐhydrodynamic, sulforhodamine B, waste stabilization pond, wastewater tracer
NOMENCLATURE
t: c(t): n: r: T: t: t: t*:
Spacial time or theoretical time. Concentration of sulforhodamine B (mg/l). Sample size. Product±moment correlation coecient. Time of the end of experiment (days). Time (days). Mean real hydraulic residence time (days). Mean hydraulic residence time (days) if the reactor has a behavior as CSTR. Qbp: Flow rate or the short-circuit.
1972; Marecos do Monte and Mara, 1987; Moreno, 1990; Agunwamba et al., 1992; Torres, 1995; Frederick and Lloyd, 1996; Dorego and Leduc, 1996; Torres et al., 1997a). In these conditions, the internal ¯ow behavior in the ponds can be considered, working in continuous ¯ow mode as ``closed reactors'' in agreement with the de®nition suggested by Levenspiel (1972), can be studied experimentally by stimulus±response techniques, using a tracer to identify the kind of mixture and to estimate the average real residence time as a consequence of the possible anomalies in the ¯ow (dead areas, short circuits) and their relation with the phenomena which in¯uence the hydrodynamics of the ponds.
INTRODUCTION
The natural origin of the phenomena which cause treatment in the wastewater stabilization ponds does not make them easier to study or to understand, especially when it is a case of applying them to the design of the installations. Among the most complex phenomena, is that of the internal hydrodynamic behavior of the pond. The behavior hydrodynamic is in¯uenced by the shapes and depth of the ponds, the situation of inlet and outlet, the climatology and the direction of the dominant winds in the zone which are situated (Kenneth and Gary, *Author to whom all correspondence should be addressed.
MATERIAL AND METHODS
Description of each system The three facultative ponds studied were situated in the municipal wastewater stabilization ponds systems (WSPS) of Cieza and LorquõÂ ±CeutõÂ (Region of Murcia in the southeast of Spain). The Cieza systems had two anaerobic lagoons in parallel, two facultative lagoons in parallel and a third facultative one in series (Fig. 1). The LorquõÂ ±CeutõÂ joint treatment plant had three anaerobic lagoons in parallel, two facultative lagoons in parallel and a third facultative one in series (Fig. 2). The two facultative ponds of Cieza studied had a maximum capacity of 16,500 m3 (F.1) and 32,000 m3 (F.2). In F.1 the wastewater entered at 1 m depth and in F.2 at 0.5 m depth (points 1, Fig. 1). Both had the outlet at the surface (points O, Fig. 1) and a maximum depth of 1.6 m,
1133
1134
J. J. Torres et al.
Fig. 1. Waste stabilization ponds of Cieza and sampling points. while the facultative pond of LorquõÂ -CeutõÂ had a capacity of 27,000 m3 and the wastewater entered at 0.5 m depth (point I, Fig. 2). It had the outlet at surface (point O, Fig. 2) and a maximum depth of 1.8 m. The whole construction in all the lagoons was made impermeable by a coat of polyethylene. During the period of experimental studies, the average ¯ows rates measured were, 1450 m3/ day (RSD = 12%, n = 24)(F.1) and 1308 m3/day (RSD = 14%, n = 32)(F.2) for the Cieza ponds and 2614 m3/day (RSD = 18%, n = 36) for the LorquõÂ ±CeutõÂ pond.
Selection and introduction of tracer The tracer was selected in accordance with the criteria for an ideal tracer (Plata, 1972) and, moreover, can be used in wastewater. Sulforhodamine B was also chosen since it has been used in aquatic systems with organic matter (Smart and Laidlaw, 1977) and by other authors in similar studies (Marecos do Monte, 1985; Marecos do Monte and Mara, 1987; Robert, 1991; Torres, 1995; Torres et al., 1997a). Tracer concentrations were measured, as will be described in sampling and analysis, using spectro¯uorimetric techniques (Skoog, 1985; Olsen, 1986). In each of the facultative ponds of Cieza, F.1 and F.2, was added 80 and 160 g of sulforhodamine B, respectively. In the lagoon of LorquõÂ ±CeutõÂ 160 g of tracer was added. The idea (in all cases) was that homogenization would produce a tracer concentration of around 5±6 mg/l water in the systems. These concentrations were judged sucient for variations to be easily observed and such that the cost should not be excessive. The tracer was dissolved in approximately 8 l of water before being poured quickly into the pond.
Sampling and analysis The experiment in the WSPS of Cieza began in November (1995) and in the LorquõÂ ±CeutõÂ in April (1996). In both systems the samplings were carried out in the outlet ¯ow and inside the lagoons. In the ®rst case, the samples were taken every 30 min during the ®rst two days and then every h for one week. During the following week samples were taken every 2 h and 3, 4 and 6 h in subsequent weeks. Samples were taken by an ISCO automatic sampler, installed at the outlet. The internal samplings were carried out intensively during the ®rst few days following the introduction of the tracer. Once it was clear that distribution was uniform throughout the water column, the samplings were made at wider intervals. So four strategically placed points in WSPS of Cieza (Fig. 1) and two LorquõÂ ±CeutõÂ (Fig. 2) were established to take samples at the surface and at dierent depths (1 and 1.5 m) during the investigation in every pond. A device designed by the research team was placed in each pond. This device consisted of a ¯oater ®xed by ropes to opposite shores of the ponds. Three ¯exible polyethylene tubes were attached to the ¯oater and carried a lead weight, such that each penetrated to the desired depth. The other end of each tube was left free on the shore so that it could be ®tted at any time to a centrifugal pump connected to a 12 V battery and through which the samples were taken. In order to establish the possible role of thermal strati®cation in the results, some temperature pro®les were measured in the ponds during the sampling period. The measurements showed that there was no thermal strati®cation in any lagoon (Table 1). After sampling, the samples were carried back to the laboratory, where the concentration of sulforhodamine B was measured, as described in our previous publication (Torres et al., 1997b). The amount of sample that was used depended on the concentration factor required.
Internal hydrodynamics of facultative ponds
1135
Fig. 2. Waste stabilization ponds of LorquõÂ ±CeutõÂ and sampling points. RESULTS AND DISCUSSION
2. Facultative pond F.2. From Table 2, it can be observed how the tracer initially spreads out over the surface of the pond in rapid, erratic movements giving high concentrations after 6 h (the sulforhodamine B was introduced at 12.00 h) at point 4 (the most distant), while lower concentrations persisted at point 3 (intermediate). Homogeneity was not reached in all the water column of the pond, until 29 h after the introduction.
Tables 2±4 show the concentrations of the tracer at each depth, together with the time corresponding to the internal sampling of the three ponds. In each table the following can be observed. 1. Cieza WSPS. 1. Facultative pond F.1. From Table 2, it can be observed that 1 h after its injection (the sulforhodamine was introduced at 16.00 h) the tracer was still concentrated near to the point of entry (Fig. 1, points 1 and 2) and in a layer of water similar in depth to that at which it had been introduced. As time passed the dispersal was rapid since 24 h after the introduction the concentration was practically uniform throughout the pond.
2. LorquõÂ ±CeutõÂ WSPS. Facultative pond F.1. From Table 4, it can be appreciated how during the ®rst 10 h (the sulforhodamine was introduced at 11.00 h) the tracer basically spread out over the surface of the pond. It then slowly dispersed throughout
Table 1. Pro®les of temperatures (8C) Cieza WSPS (at 13 h) Depth
day (11/1)
Surface 1m 1.5 m LorquõÂ ±CeutõÂ WSPS
12.2 12.0 12.0 (at 12 h)
day(11/25)
day(12/1)
day(12/5)
day(12/20)
day(1/15)
11.8 11.6 11.6
13.0 12.7 12.6
12.4 12.1 12.0
12.5 12.3 12.0
12.3 12.0 12.0
Depth
day (4/20)
day (4/28)
day (5/3)
day (5/20)
day (6/3)
day (6/15)
Surface 1m 1.5 m
13.6 12.8 12.8
13.2 13.5 13.5
14.6 13.8 13.7
15.2 14.8 14.7
15.6 15.3 15.0
15.5 15.3 15.2
1136
J. J. Torres et al. Table 2. Tracer concentration in points 1, 2, 3 and 4 at Cieza F.1 pond (mg/l) Point 1
Point 2
Point 3
Point 4
Time
surface
1m
1.5 m
surface
1m
1.5 m
surface
1m
1.5 m
surface
1m
1.5 m
17 h 19 h 26 h 43 h 50 h 68 h 73 h 91 h 140 h 116 h
2.5 3.9 3.9 4.1 3.5 3.5 2.7 2.5 2.2 2.3
13.1 4.5 4.2 4.2 3.3 3.4 2.9 2.9 1.7 2.5
11.0 3.9 4.3 4.0 3.7 3.5 3.5 3.5 2.1 2.5
0.6 4.0 3.8 4.3 3.5 3.4 3.1 2.7 2.1 2.7
14.2 4.1 4.3 4.0 3.7 4.0 3.5 3.0 2.2 2.7
4.0 3.3 4.8 3.9 3.9 3.6 3.3 3.2 2.1 2.7
0.2 4.4 4.0 4.2 3.9 3.7 3.5 3.1 2.2 2.5
3.5 4.1 4.4 3.8 3.8 3.4 3.7 3.2 2.2 2.5
0.2 4.3 4.2 4.1 3.7 3.5 3.3 3.4 2.0 2.3
0.2 4.0 4.4 3.9 3.8 3.6 3.2 3.2 2.3 2.5
0.2 4.2 4.1 4.2 3.9 3.7 3.3 3.2 2.1 2.6
0.1 4.2 4.1 4.3 3.7 3.6 3.4 3.1 2.0 2.6
Table 3. Tracer concentration in points 1, 2, 3 and 4 at Cieza F.2 pond (mg/l) Point 1
Point 2
Point 3
Point 4
Time
Surface
1m
1.5 m
Surface
1m
1.5 m
Surface
1m
1.5 m
Surface
1m
1.5 m
1h 6h 23 h 29 h 48 h 53 h 72 h 77 h 96 h 120 h
38.1 17.4 3.9 3.5 3.2 3.5 2.9 2.9 2.4 1.8
17.5 11.4 4.6 3.2 3.0 3.7 3.5 3.5 2.3 1.6
12.9 9.0 3.5 4.4 3.3 3.4 3.1 3.6 1.8 1.8
45.4 12.8 4.8 3.8 3.0 4.0 3.3 2.7 2.3 1.9
0.5 10.5 3.6 3.9 4.3 3.6 3.1 3.7 2.5 2.0
0.2 6.7 4.4 4.1 4.1 4.3 3.0 3.6 2.4 1.8
0.3 1.7 4.8 4.4 5.0 4.3 3.4 3.5 2.4 2.1
0.2 1.2 1.6 4.5 4.7 4.3 4.1 3.4 2.5 2.2
0.3 0.3 0.6 3.8 4.3 4.2 3.9 3.5 2.6 2.2
0.3 10.1 5.0 4.9 4.8 4.4 3.8 3.7 2.5 2.2
0.1 10.7 5.2 4.6 4.6 4.5 3.8 4.0 2.8 2.1
0.1 10.8 4.7 4.6 4.5 4.6 4.2 3.8 2.4 2.1
Table 4. Tracer concentration in points 1 and 2 at LorquõÂ ±CeutõÂ F.1 pond (mg/l) Point 1
Point 2
Time
Surface
1m
1.5 m
Surface
1m
1.5 m
1h 2h 6h 10 h 24 h 30 h 48 h 53 h 72 h 80 h 96 h 105 h 121 h 145 h 168 h 193 h 223 h 246 h
0.4 7.5 7.5 7.3 8.3 5.8 5.3 4.7 3.5 3.6 3.5 3.0 3.4 3.2 2.6 2.3 1.6 1.6
0.4 0.0 23.2 11.8 7.7 6.5 5.9 4.8 3.6 3.4 3.3 3.4 3.5 3.3 2.7 1.9 1.6 1.7
0.0 0.1 0.0 0.0 8.5 6.5 5.1 4.6 2.8 3.2 2.7 3.4 3.4 3.2 3.0 2.7 1.7 1.4
0.0 6.4 7.5 6.6 7.4 5.6 5.0 4.6 3.4 3.4 3.9 3.0 3.1 2.8 2.6 2.2 1.7 1.7
0.0 0.9 0.9 0.0 7.4 7.5 5.6 4.4 4.1 3.4 3.3 3.2 3.6 2.9 2.5 1.8 1.6 1.5
0.0 0.0 0.0 0.0 0.2 1.4 4.9 4.9 3.6 3.1 3.4 3.5 3.5 3.0 3.0 2.3 2.1 1.4
Table 5. Predominant winds in both towns during the period of experiment
LorquõÂ ±CeutõÂ Cieza
Range of speeds (Km/h)
Maximum gust (Km/h)
NE (%)
SE (%)
SW (%)
NW (%)
Calm (%)
5±18 2±10
34 22
13.2 5.1
20.7 7.2
15.9 3.9
21.8 83.1
28.4 0.7
Internal hydrodynamics of facultative ponds
1137
Fig. 3. Mean daily tracer concentration at the outlet vs time (Lagoon F.1 of Cieza) and theoretical behavior as a CSTR.
the water, and even 48 h after its introduction, the system was still not totally homogeneous. Even though the three ponds can be considered to have the behavior of a perfect mixed tank reactor and all attain homogeneity in under three days, there are, nevertheless, some dierences regarding the ease with which this situation is reached. In the Cieza F.1 pond, the process, from input to output, is rapid and progressive, slower yet still progressive in the case of the LorquõÂ ±CeutõÂ F.1 pond and rapid and irregular in the Cieza F.2 pond. Constructional and functional dierences arising during the experiments may be related to these dierent behaviors. Thus, the greater input depth in the wastewater of the Cieza F.1 pond (1 m.) could lead to a more uniform distribution than in the other two ponds (depth 0.5 m). Wind speeds and directions are also in¯uencing factors in the dispersion process. Table 5 shows the data corresponding to the ®rst three days of the experiments and it will be appreciated that in the case of the LorquõÂ ±CeutõÂ experiment there was a long period of calm (28.4%) and varying wind directions with no clear predominance. For the Cieza ponds however, there were hardly any periods of calm (0.7%) and there was a clear predominance of northwesterly winds (83.1%), which blew over the pond lengthwise in the case of F.1 and breadthwise in the case of F.2, thus favoring the overall ¯ow.
In order to carry out the quantitative analysis of the hydrodynamic study the concentration curves were determined at the tracer output, Curve C, of the three systems from the average daily concentrations (Figs 3±5). The curves respond to the classic RTD curve for a perfect mixed tank reactor (CSTR): a maximum near to the ordinates axis of asymmetric form and with a pronounced tail (Levenspiel, 1972). This fact supports the above comments on the hydrodynamic behavior of the three ponds. From the average daily sulforhodamine B concentrations at the output, the c0t product was obtained (Hymmelblau and Bischo, 1968; Levenspiel, 1972, 1979): T c0 t c t dt, 1 0
where c0 is the average concentration of the tracer if this had been of uniform distribution throughout the pond at the moment of introduction and t is the average real residence time. Once this product has been found, it is possible (equation 2) to obtain the age distribution function at the output, E(t), and from this (equation 3) the average real residence time (Hymmelblau and Bischo, 1968; Levenspiel, 1972, 1979). The values are given in Table 6. E t
c t c0t
2
1138
J. J. Torres et al.
Fig. 4. Mean daily tracer concentration at the outlet vs time (Lagoon F.2 of Cieza) and theoretical behavior as a CSTR.
t
T 0
E t t dt
3
If the hydrodynamics of the ponds are similar to that of CSTR, the C curves (Figs 3±5) should correspond to those obtained from the expression below
in logarithmic form (equation 4) (Levenspiel, 1979), in which A and B are constant ln c t ln A ÿ B t:
4
By adjusting the parameters of equation 4, to the data of the average daily sulforhodamine B concen-
Fig. 5. Mean daily tracer concentration at the outlet vs time (Lagoon F.2 of LorquõÂ ±CeutõÂ ) and theoretical behavior as a CSTR.
Internal hydrodynamics of facultative ponds Table 6. Residence times (days) Residence Time
Cieza F.1
Cieza F.2
LorquõÂ ±CeutõÂ F.1
t t* t
10.3 11.0 20.1 11.8
22.1 23.02 0.4 24.6
10.0 10.9 20.1 10.4
trations at the output to equation 4, we obtain for the three ponds: 1. Cieza WSPS. 1. Facultative pond F.1 (for a con®dence interval of 95% with n = 33 and r = 0.998). ln c t 1:6320:008 ÿ 0:09224 10ÿ4 t 5 2. Facultative pond F.2 (for a con®dence interval of 95% with n = 80 and r = 0.994). ln c t 1:5220:003 ÿ 0:04323 10ÿ4 t 6 2. LorquõÂ ±CeutõÂ WSPS. 3. Facultative pond F.1 (for a con®dence interval of 95% with n = 36 and r = 0.993). ln c t 1:8820:011 ÿ 0:092210ÿ4 t
1139
tank reactor, and likewise from the data obtained from the interior of each system, it has been shown that the ponds present a hydrodynamic behavior similar to that of an ideal perfectly mixed tank reactor. This has been con®rmed by observing the analogy that exists between the average real residence times and those determined by supposing that the reactor has this type of hydrodynamic behavior. This hydraulic behavior may be considered satisfactory, since it avoids an anaerobic zone at the entrance to the in¯ow and the accumulation of exclusively anaerobic micro-organisms in the zone (Juanico, 1991). On comparing the average real residence times (t) with the theoretical ones (t), a good correspondence has been appreciated between them, which is indicative of the absence of stagnant zones and the fact that the ponds were adequately designed from this hydraulic point of view. Furthermore, by comparing the C curves at the output, with that of an ideal perfectly mixed tank reactor, it has been con®rmed that, with the exception of the small short-circuit observed that could decrease the treatment performances, there were practically no ¯ow anomalies in the Cieza F.2 and the LorquõÂ ±CeutõÂ ponds.
7
The validity of the adjustments for each of the ponds may be observed in Figs 3±5. According to Levenspiel (1979), t* = 1/B, thus giving the values shown in Table 6, together with those of the theoretical residence time or spacial time (t = V/Q). If the average real residence times (t) and the theoretical times (t) are observed, a good correspondence between them is con®rmed and this indicates that, there are no stagnant zones and the design of the ponds is adequate. Reference should be made to the presence in Figs 4 and 5 of a sharp peak at the beginning of the experiment above curve C, which is representative of the CSTR. This indicates the presence of a small short-circuit in both ponds (Levenspiel, 1979; Torres, 1995; Dorego and Leduc, 1996; Torres et al., 1997a). From the relationship existing between the area of curve C, obtained with the real data, and the curve found by supposing the reactor to behave like a perfect mixed tank reactor, it is possible to determine the percentage of the short-circuit that occurred in both case: 1.30% of the total ¯ow rate (Qbp=17 m3/day) and 1.45% of the total ¯ow rate (38 m3/day) for the Cieza facultative pond F.2 and the LorquõÂ ±CeutõÂ F.1 pond, respectively.
CONCLUSIONS
From the C curves at the output and from the comparison of these with those of a perfect mixed
AcknowledgementsÐThis work was ®nancially supported by the CICYT of Spain (Proyect AMB92-D619) and a grant from the Instituto de Fomento de la RegioÂn de Murcia. The authors gratefully acknowledge these institutions for their economic support. REFERENCES
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Moreno M. D. (1990) A tracer study of the hydraulics of facultative stabilization ponds. Water Res. 24(8), 1025± 1030. Olsen E. D. (1986) MeÂtodos OÂpticos de AnaÂlisis. Ed. ReverteÂ, Barcelona. Plata A. (1972) IsoÂtopos en Hidrologia. Ed. Alhambra, Madrid. Robert S. P. (1991) The in¯uence of the sulphur cycle on the performance of prymary waste stabilization ponds treating domestic wastewater. Ph.D. thesis, University of Leeds. Skoog D. A. (1985) Principles of Instrumental Analysis, Saunders College Publ., Philadelphia.
Smart P. L. and Laidlaw I. M. S. (1977) An evaluation of some ¯uorescent dyes for water tracing. Water Resour. Res. 13(1), 15±33. Torres J. J. (1995) Estudio hidrodinaÂmico de distintos sistemas de lagunaje para el tratamiento de aguas residuales. Ph.D. thesis, University of Murcia. Torres J. J., Soler A., SaÂez J. and OrtunÄo J. F. (1997a) Hydraulic performance of a deep wasteswater stabilization pond. Water Res. 31(4), 679±688. Torres J. J., Soler A., SaÂez J. and OrtunÄo J. F. (1997b) Determination of low concentration of sulforhodamine B in wastewaters and stabilization ponds. Water Res. 31(12), 3183±3186.