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The influence of pond geometry and configuration on facultative and maturation waste stabilisation pond performance and efficiency
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Pergamon 0273-1223(95)00500-5
Waz. Sci. Tech. Vol. 31. No. 12. pp. 129-139. 1995. Copyright 0 1995 IAWQ Prioted in Great Britain. All righll reserved. 0273-1223195 $9'50 + Oo()(J
THE INFLUENCE OF POND GEOMETRY AND CONFIGURATION ON FACULTATIVE AND MATURATION WASTE STABILISATION POND PERFORMANCE AND EFFICIENCY H. W. Pearson*, D. D. Mara** and H. A. Arridge* *Department ofGenetics and Microbiology, University ofLiverpool. P.O. Box 147. Liverpool 1.69 3BX, UK **Department of Civil Engineering, University ofLeeds. Leeds LS2 9n, UK
ABSTRACT Differences in length:breadth ratios (in the range of I; I to 6; I) and depths (from I to 2 m) had little effect on the performance and effluent quality (i.e. BOD. SS and Fe) of secondaJy facultative ponds. Shallow maturation ponds (0.4 m) were more efficient at microbiological disinfection than deeper ones and could actually reduce land area requirements. KT values for faecal coliforms. salmonellae and rotavirus differed from one another in the same ponds and for different pond types. The Marais design equation fex predicting faecal coliform numbers ID pond effluents. although not perfect, does provide a reasonable design approach for systems containing more than two ponds ID series.
KEYWORDS K values; lagoons; pathogen removal; pond design; pond geometry; waste stabilisation ponds. INTRODUCfION Design criteria for waste stabilisation ponds are well established and have been the subject of considerable research and publication for more than 30 years. Facultative pond design is based on empirical fonnulae such as that of Gloyna (1976). which detennines pond volume and tends to be conservative by modern standards, and McGarry and Pescod (1970) who derived an equation linking surface or areal BODS loading to temperature. This approach has been refined by Arthur (1983) for Africa. Mara (1987) for global application, Mara and Pearson (1987) for Europe and most recently by Mara et al. (1992) for Eastern Africa. Alternatively kinetic and dispersion models have been used such as Marais and Shaw (1961), which assumes complete mixing and flrst-order reaction rates, and the equation of Thirumurthi (1969) which considers hydraulic dispersion in the pond. Yanez (1982) has modified Thirumurthi's equation for application in South America. The surface loading approach to facultative pond design is currently the most widely accepted (WHO, 1987).
129
130
H. W. PEARSON tl 01.
The design of maturation ponds is based on pathogen removal, usually bacterial decay. Whatever removal or die-off mechanism may be operating (e.g. sedimentation, starvation, predation, high temperature, high light and high pH or a combination), the equation of Marais (1974), which assumes that faecal coliform removal can be modelled by fIrst-order kinetics in a completely mixed reactor, is the most frequently used. Polprasert et al. (1983) have sought to refine the design approach by taking account offactors such as organic loading and algal concentration, and the non-ideal flow equation of Wehner and Wilhelm (1958) has been proposed for predicting bacterial die-off in preference to complete mixing. This attention to process design has not been matched by research into physical design: notably, pond shape and the design and vertical and horizontal location of inlet and outlet structures. Emphasis is put on preventing dead space and the prevention of short-circuiting, but opinions vary on whether the ponds should be square or rectangular and what are the optimum depths for various pond types (Forero. 1993; Mara and Pearson, 1987; Mara et al., 1992; Middlebrooks et 01., 1982; Pearson et aI., 1988; WHO, 1987).
In this paper we consider the impact of variations in pond geometry and depth on the process design and efficiency of operation of secondary facultative and maturation ponds in a large experimental pond complex in Northeast Brazil. The fmdings are also related to physical design and construction. POND SITE DESCRIPTION The experimental pond complex was constructed at the site of the City of Campina Grande's sewage treatment plant at Catingueira, Par:1iba, Northeast Brazil. The pond complex comprises two systems of ponds. A IO-pond linear series and a 17-pond flexible "innovative" system comprising different combinations of pond types in series. Table 1. Characteristics of the experimental pond system Pond
Dimensions (m) length breadth depth
A9 AIO F21 F22 F23 F24 F25 MIS MI6 MI7 Ml8 MI9 M20 M21 M22
4.90 4.90 12.90 12.90 12.90 12.90 4.90 17.35 10.40 10.40 10.40 10.40 10.40 8.45 8.45 8.45 8.45
M23~
M24"
1.65 1.65 2.00 2.00 2.00 200 4.90 8.80 3.75 3.75 3.75 3.75 1.30 3.70 3.70 3.70 3.70
2.50 2.50 1.00 1.33 1.67 2.00 2.00 1.00 0.90 0.64 0.39 0.39 0.39 0.60 0.60 0.60 0.60
Flow-rate (m3/d)
Hydraulic retention time (d)'
20.0 20.0 8.0 8.0 8.0 8.0 8.0 40.0 5.0 5.0 5.0 5.0 5.0 3.75 3.75 3.75 3.75
1.0 1.0 3.0 4.0 5.0 6.0 6.0 3.8 7.0 5.0 3.0 3.0 1.0 5.0 5.0 5.0 4.24
• theoretical values; ~ floating macrophyte pond; "baffled pond; 4 corrected for baffie volume.
The influence of geometry on waste stabilisation ponds
131
In this study the 17-pond system (Fig. I), comprised a combination of two anaerobic ponds (A9 and A 10) in parallel. whose combined effluents fed five secondary facultative ponds (F21-25) in parallel which in tum were followed by ten maturation ponds. The maturation ponds were arranged to give a single primary maturation pond (M 15) receiving the combined effluents of the five secondary facultative ponds, followed by five secondary maturation ponds (MI6-20) in parallel which in tum were followed by four tertiary maturation ponds (M21-24) also in parallel. This arrangement of the complex gave various combinations of five-pond series using ponds of different geometries, depths and retention times. The dimensions, flow rates and retention times of the various ponds are presented in Table I. The two anaerobic ponds had volumetric loadings of l87g BODs/m 3/d and the secondary facultative ponds each had surface organic loadings of 217kg BODsfha/d. MATERIALS AND METHODS The ponds were sampled using the column sampling technique (pearson et al., 1987) to obtain the equivalent of mean 24-h effluent values for the various parameters. This sampling procedure was verified for the system at the start of the sampling programme which extended over 15 months. The techniques for the analysis of the physicochemical parameters are described in Silva et al. (1995) but follow those described in Standard Methods (APHA, 1989). The microbiological assays are described by Oragui et al. (1995). RESULTS Seconda.cy facultatiye ponds The performance of the five secondary facultative ponds in terms of mean effluent quality were studied over a IS-month period which included both wet and dry seasons. The mean mid-depth water temperature remained constant at 25 °e. Although all five secondary facultative ponds received the same surface organic loading their geometries differed. Ponds F21-24 had the same length to breadth ratio of 6: I but differed in depth from I to 2 m. Their retention times also varied from 3 to 6 days, with the increased retention times resulting from the increased pond depths. F25 had the same depth and the same retention time (6 days) as F24 but had a length to breadth ratio of I: I as opposed to 6: I. Mean effluent quality values (based on column sampling) for key physicochemical and microbiological parameters are presented in Table 2. These data suggest that increased depths in the range of I to 2 m and the associated increases in retention times of 3 to 6 days did not significantly alter individual pond performance or effluent quality in terms of BOD, suspended solids and microbiological quality, the latter being individually expressed as numbers of faecal coliforms. salmonellae and rotaviruses.
It has frequently been suggested that pond performance could be improved by increasing length:breadth ratios which favour a plug flow hydraulic regime over complete mixing. A comparison of the results for F24 and F25 (Table 2), however, suggests that a pond length:breadth ratio of either I: I or I:6, which might be considered a realistic range for many pond construction sites, had little effect on pond performance and effluent quality. Maturatjon ponds The tive secondary maturation ponds (MI6-20) each received a flow of 5 m 3/d. The combined effluents of M 16-1 Xfed the four tertiary maturation ponds (M21-24) with each receiving 3.75 m 3/d. Pond M23 was a floating macrophyte pond containing Pistia stratiotes and M24 was baffled. The results of studies on maturation pond performance and effluent quality are summarised in Table 3.
H. W. PEARSON et al.
132
,
~-
-I pumping
box
: house A9
F21
A10
F22 F23
F25 F24
M15
t M16
M21
M22
M23
M24
.-L__, inspection box raw sewage
-
Figure 1. Layout of experimental WSP at Catingueira. nortb-cast Brnzil.
The influence of geometry on waste stabilisation ponds
133
The microbiological data from the secondary maturation ponds. which are the fourth ponds in a combination of 5-pond series. showed that effluent quality varied very little between ponds M16-19 despite differences in depth and retention time. The faecal coliform numbers were. however, up to 0.6 of a log unit higher in M20 than in the effluents of the other secondary maturation ponds. Pond M20 (39 cm deep) received the same flow as the other secondary maturation ponds but was only one third the surface area of ponds MI8 and Ml9 which were also 39cm deep. Consequently it had a shorter retention time of just one day and an increased organic surface loading (Le. 70kg BODihald) It was not surprising. therefore. that its effluent quality was a little poorer in terms of feacal coliform numbers. In contrast salmonellae and rotavirus numbers were comparable to those found in the other secondary maturation pond effluents. However, when pond efficiency is considered in terms of fIrst-order removal kinetics (K r values), then the shallower secondary maturation ponds M18-20 (3g cm oeep) were more efficient at faecal coliform removal than the deeper secondary maturation ponds. Comparison of the Kr values takes account of the differences in retention time between the ponds and M20 with its one-day retention time had the highest removal constant for faecal coliforms of any of the ponds, and also high removal rates for rotaviruses and salmonellae when compared to the other secondary maturation ponds (Table 3). The algal concentration (chlorophyll-a) in M20 although lower than in the other ponds was nevertheless stable and provided high pH values (>9) and supersaturated oxygen concentrations during the day. It is also worth noting that the data for ponds M18 and M19 which are identical in size, geometry and thus retention time were very similar. showing that pond performance within the complex was reproducible. The results for the tertiary maturation ponds showed that the floating macrophyte pond M23 although producing the best BODs effluent quality appeared less efficient at faecal coliform removal than the same• sized algal ponds M21 and M22. These two ponds also provided good evidence of the reproducibility of the data for ponds operating under identical conditions except when comparing the Kr values for rotavirus removal which did diverge. This. however. probably relates to the the low number of virus particles present and the impact small changes in numbers will therefore have on the Kr values. The tertiary maturation ponds (60 cm deep). with the exception of M24, the baffled pond, appeared less efficient at faecal coliform removal than even the deeper secondary maturation ponds, but this may reflect the smaller microbial populations and the fact that those microorganisms still remaining viable might be more resistant to the ambient conditions. The baffled pond M24. which had an effective length:breadth ratio greater than 100: 1, was more efflcient at faecal coliform removal (Kr 7.96) than the other tertiary ponds, although again with such small numbers in the influent the impact on actual effluent FC numbers was small. It was difficult to determine any improvement in the removal efficiencies for salmonellae because of low numbers. Rotavirus removal in the tertiary maturation ponds was good with high removal efficiencies (Table 3) and very few actual virus particles present in the final effluents from any of the parallel 5-pond series. Physicochemical effluent quality was comparable to the other maturation ponds although the special tertiary ponds M23 (the macrophyte pond) and M24 (the baffled pond) both had consistently lower effluent BODS values. Using the equation of Marais (1974) for determining the faecal coliform die-off coefficient (K r ) in maturation ponds, and using a temperature of 25°C, which was the mean mid-depth temperature of the ponds. ~ KT value of 6.20d- 1 was obtained. The primary maturation pond (MI5) and the five secondary maturaoon ponds had KT values which were similar to or greatly exceeded the 6.20 value. whereas the tertiary maturation ponds except M24 (the baffled pond), gave lower values (Table 3). When these values are corrected to 30°C they are higher than those obtained by Ellis and Rodriques (993) at this temperature for ponds on Grand Cayman, and also in contrast to their findings the Kr values were higher in our maturation ponds than in our secondary facultative ponds.
H. W. PEARSON et al.
134
TABLE 2. GEOMETRIC DETAILS AND MEAN EFFLUENT VALUES (BASED ON COLUMN SAMPLES) FOR TIlE ANAEROBIC AND SECONDARY FACULTATIVE PONDS OF TIlE 17-POND SYSTEM. POND UBRATIO
.
RETENTION
DEPTH
1lME(d)
FAECAL COUFORMS eN/IOOml
SALMONELI.4E eN/IOOml
A9
3.1
2:'0
1.0
7.06 x 106 (2.76)'
J61.' (-0.09)
AIO
3:1
2'0
1.0
7." x 106 (2 72)
J6S.6 (,j).10)
F21
6:1
1.00
3.0
1.1 x 106 (1.16)
29.9(4.03)
9.2 x 10' (I 61)
F22
6:1
\.33
4.0
ROTAVIRUS pt\IlL 1.7 x 104 (2.92)
CIIL
TSS
BOD
..wt.
maIL 73(1.70)
121
"(163)
442
1.2 x 101 (OJI)
U(O.44)
II
JO 9 (2 69)
1.0 x 104 (0.11)
29 (OJ9)
77
722
6.9 x 101 (0.30)
I.Ix 104 (2.76)
43'
F23
6:1
1.67
'.0
9.2110' (I.")
19,7 (3.49)
28 (o.m
67
6SS
F24
6:1
2.00
6.0
7.1 x10' (I 36)
14' (401)
, , x 101 (0.36)
24 (OJ')
63
716
23.0 (2.47)
4.6 X 103 (0.46)
2' (0.33)
69
706
F2,
1:1
1.9 xlO' (1.16)
6.0
2.00
• Numben in bracketS represent the Kr valucs for first order removal, cr'. Mean mid depth walcr tempcraIurc 2' DC. VolumcIric loading on the anaerobic ponds was 187& BODslml/d. Arcalloading on the facultative ponds was 217 kgBOD,tha/d.
TABLE 3. GEOMETRIC DETAll..S AND MEAN EFFLUENT QUALITY VALUES (BASED ON COLUMN SAMPLES) FOR THE MATURATION PONDS OF THE 17-POND EXPERIMENTAL SYSTEM. CHLOROPHYlL-oI
POND lIB DEP1H RETENTION AREAL LOAD FAECAL COLIFORMS SALMONElLAE ROTAVIRUS RATIO m TIME (d) kgBOD,/ba/d pfulL cfu 1100 mI cfu /100 mI
BOO,
M15
1.1.1
1.00
3.8
72
2.36 x 104 (9 46)'
3.2 (U6)
',37 x 101 (0.08)
19(0.12)
'3
369
MI6
2,8.1
0.90
7.0
24
"., x 102 (6.04)
1.1 (0.27)
12' x 103 (047) 22 (~02)
74
447
MI7
2.1.1
064
'.0
24
681 x 102 (6,73)
12 (0 33)
1.90 x 103 (0.37)
21
(~.02)
73
392
Mil
2.8:1
0.39
3.0
24
HIx 102(10.04)
I 4 (056)
2 36 x 103 (0.43)
2"~08)
m
m
26(.0.08)
1I8
361
2' (~.24)
103
201
TSS
mall
~
mall
MI9
2....
0.39
3.0
24
6.30 x 102 (12.1$)
1.2(0.$6)
2.34 x 103 (0.0)
M20
"1
039
1.0
70
1.60 x 103 (13 7$)
\4(1.29)
1.79 x \0 3 (2.00)
M21
2.3'1
0.60
$0
29
1.49 x 10 1 (3.'9)
1.0 (0,0$)
15.1 (24)
24(.0.01)
101
39'
M22
2.3:1
0.60
'.0
29
4.17xIO I (297)
10(0.0')
2.6 (141)
26 (-D.03)
89
328
M23
2.3:1
0.60
'.0
29
224 x 102 (0.39)
3.8(oC 14)
0
(.)
15(0.10)
81
338
M24 >100.1
0.60
42
34
1.92 x 10 1 (7.96)
1.1 (0.03)
0
(.)
7(0,0')
107
398
• N\IIIlbcn iIllnckcu repreICIlllbe KT values fer lint order remoVll, dayrl.
Mean TeIIlP"'1Ituro 2'0<:.
11Ie influence of geomeuy on waste stabilisation ponds
13S
Further considerations of first-order rate remoyal constants The actual KT values for faecal colifonn removal for the five secondary facultative ponds (Table 2) were also lower than the theoretical 6.20 value. This was not surprising since facultative ponds are usually found to be less efficient at bacterial pathogen removal than maturation ponds. The theoretical KT values for faecal colifonns calculated from the Marais equation were a poor indicator of the actual K T values for salmonellae in the maturation ponds. since the latter were frequently an order of magnitude lower than the predicted 6.2 value. The actual KT values for salmonell,te in the secondary facultative ponds were also lower than the predicted value but were higher than the actual K T values calculated for faecal colifonns. Table 4. A comparison of actual and predicted numbers of faecal colifonns in pond effluent Actual F.e Pond in Emuent (per 100 mi.)
RS A9 AIO F21 F22 F23 F24
F2S MIS MI6 MI7 MI8 MI9 M20 M21
M22 M23 M24
2.66 x 107 7.06 x 106 7.15 x 106 1.08 x 106 9.22 x 105 9.15 x 105 7.76 x 105 8.90 x 105 2.36 x 104 5.45 x 102 6.81 x 102 7.58 x 102 6.30x 102 1.60 x 103 3.49x 10] 4.17xlO l 2.24 x
102
1.92 x 10 1
Predicted F.e.· (individual ponds (per 100 001.)
3.69 x 106 3.69 x ]06 3.63 x lOS 2.75 x 105 2.22 x 105 1.86 x lOS 1.86 x 105
3.73 x 104 5.32 x 102 7.33 x 102 1.20 x 103 1.20 x 103 3.28 x 103 2.07 x 10 1 2.07 x 10 1 2.07 x 10 1 2.07 x 10 1
Predicted F.e.· (series) (per 100 mi.)
369 x 106 3.6 x 106 1.89 x lOS 1.43 x 105 1.15 x ]05
9.67 x 104 9.67 x 104 4.89 x 103 1.10 x 102
1.53 x 102
2.50 x 102 2.50 x 102
6.79 x 4.8
102
4.8 4.8 4.8
• see explanation in text. Actual KT values calculated for rotaviruses were similar in all the secondary facultative ponds and the primary and secondary maturation ponds, but were lower than the theoretical value for faecal colifonns. and lower than the actual values obtained for faecal colifonns. In contrast very high KT values were obtained for rotavirus removal in the tertiary maturation ponds and few or zero virus particles were present in the final effluents. The results for actual faecal colifonn numbers in individual pond effluents were compared with two theo~tical values for ~ffluent numbers (Table 4). to show the impact. the disparity in KT values has on predicted effluent quality. Both theoretical values were calculated USing the K T value of 6.2 d- I for a temperature of 2S °e. but using either the actual influent Fe values for each pond or the Fe count in the raw
H.VV.PEAJtSONelaL
136
sewage, and detennining the theoretical Ne value for each pond effluent at its position along the 5-pond series. These data show that the theoretical values fur Fe numbers can be nearly an order of magnim.qe lower than the actual effluent values for the early ponds in the series when the fonner are calculated from caw sewage FC numbers. The numbers then converge as the higher than predicted faecal colifonn removal cates come into play in the secondary maturation ponds thus redressing the balance. 8-:
j
•
E :5 5
( ,)
u. 4 (ij
.a
" ~3 .....0 2
O+---r---'["""'"--.-......,.--.,.......-.....---, o
234567
Log Predicted Fe '100 ml.
Figure 2. Comparison of log actual and log predicted faecal coliform numbers in individual pond effluents (N.,) based on the results of column sampling. Log predicted values were derived from actual influent values (N J for eacb pond usmg the Marais equauon.
2
345 Log Predicted
6
Fe '100 mi.
Figure 3. Comparison of log actual and log predicted faecal coliform numbers in individual pond effluents (N.,) based on the results of column sampling. Log predicted values were derived using the Marais equation for detennining N.lo a series of ponds, wbere N j was the faecal coliform numbers in the mw sewage.
Despite these differences in experimentally derived and theoretically detennined KT values. plotting log lICtual effluent FC numbers for each of the ponds against log theoretical effluent FC numbers using either the
The mtluence of geomecry on waste slabilisation ponds
137
actual influent FC numbers for the pond (Fig. 2) or extrapolating from the raw sewage PC values (Fig. 3) gave linear regressions with highly significant positive correlations (r 2) of 0.99. yThis would suggest there is a balancing-(lut effect between the pond types and that predicting effluent quality by using the equation of Marais to predict effluent faecal coliforms numbers is acceptable in pond systems comprising more than three cells but is less reliable where there is only one pond or two ponds in series. DISCUSSION AND CONCLUSIONS The results presented here suggest that increasing the length:breadth ratios does not obviously benefit pond performance or improve the effluent quality of secondary facultative ponds for either physicochemical parameters such as BODs or microbiological parameters such as faecal coliforms. Thus the significance at the physical design stage of building long rectangular ponds or including numerous baffles in facultative ponds to encourage plug or piston flow is probably overstated (Forero, 1993). These findings also lend credence to the idea that the vertical stratification or lamination of the water column is probably as or more important in determining treatment efficiency than the bulk flow along a longitudinal axis (Pearson et ai., 1988). In this respect the positioning and depths of the inlet and outlets may have a greater beneficial impact on treatment efficiency than pond shape. The apparent lack of impact of pond shape on facultative pond performance within a realistic range of length:breadth ratios allows the designer more freedom in shaping the ponds to make best use of available land particularly on small or awkwardly shaped sites.
In these studies increasing facultative pond depth and thus the hydraulic retention time whilst maintaining the same organic surface loading did not significantly improve physicochemical or microbiological effluent quality. This is important in design terms for several reasons. Firstly, there has been a tendency by some designers to increase pond depths (particularly where land is at a premium) as a way of increasing the hydraulic retention time in the pond series, in the belief that this will produce a better microbiological quality effluent. This approach has been used since the increased retention time values can be inserted into the Marais equation resulting in lower predicted faecal coliform numbers in the effluent compared with values for a shallower pond system of comparable area receiving the same flow, and thus having a shorter overall retention time. This design approach can lead to the actual pond effluent qUality falling shon of the design prediction with all the implications this may have on the final destination or use for the effluent. Secondly. on purely practical and economic grounds. building deeper ponds may involve significantly more earth movement and increase construction costs unnecessarily. Alternatively, the site topography may lend itself to the construction of slightly deeper facultative ponds in which case this need not lead to poor pond performance so as long as the implications of depth and retention time on the performance of the various pond types are taken into consideration at the design stage. The data presented here also validate the surface organic loading design approach for facultative ponds. since increasing the depth decreased the volumetric loading rates (whilst keeping the surface loading rates the same), and yet did nothing to improve effluent quality. It would also suggest that more complicated design equations embracing functions taking account of hydraulic dispersion etc.. are probably unneccessarily complicated and offer very little by way of design improvement. In our studies with secondary maturation ponds. reducing the depth also reduced the retention time but this did not adversely affect effluent quality. This implies that the shallower ponds were more efficient at natural disinfection than the deeper maturation ponds. These preliminary data in fact suggest that there is a beneficial trade-off in building shallow maturation ponds lIS they appear to utilise less land for a comparable microbiological effluent quality.
The results of these.flfSt series of experiments support the use of shallow maturation ponds to save land area and reduce excavaUon costs. However, it was also true that the rate of faecal coliform removal tended to decrease through a series of maturation ponds lIS the remaining bacterial numbers decreased despite all other
H. W. PEARSON et al.
138
factors remaining equal. This was particularly noticeable when comparing the faecal colifonn K T values for the tertiary maturation ponds with those for the secondary ponds. This would suggest that the faecal colifonn population is somewhat heterogeneous and that certain species or types may be more resistant than others to the natural disinfection processes in the tertiary maturation ponds, despite the existence of conditions which should be hostile to their survival. In contrast rotavirus removal was more efficient in the tertiary ponds than in the preceding ponds and their removal clearly benefited from the use of 5 ponds in series.
The baffled pond (M24) was the most efficient of the tertiary maturation ponds both in tenns of pathogen removal and organic removal, however, further studies (which are currently underway) are required to evaluate fully the performance of this type of pond particularly when receiving higher influent concentrations of pathogens, ACKNOWLEDGEMENTS We are grateful to the UK Overseas Development Administration for financial suppon (Research Scheme R4569) and to the Universidade Federal da Par.1iba and the Companhia de Aguas e Esgotos da Pacliba for the provision of experimental facilities and the pond site at Catingueira. REFERENCES APHA (1989). Standa,d Methods /0' the Examination 0/ Wate' and Wastewater, 17th edn. American Public Health Association. Washington, DC, USA. Arthur. J. P. (1982). Notes 011 the Design and Operation o/Waste Stabilization Ponds ill Wa171l CliftlQtes ofDevelopillg Coufllries. World Bank Technical Paper Number 7, Washington. DC, USA. Forero. R. S. (1993). Stabilisation ponds in the lrOpics: promotion of the sanitary reuse of water. Built/iII 0/ PAHO. 17(3). 21S•
223.
G1oyna, E. (1976). Facultative waste stabilization pond deSign. In: Ponds as a Wastewater Trealmelll AllemaJrve. Gloyna, E. F~ Malina, J. F. and Davis, E. M. (eds), pp. 143-157. Universily of Texas Press. Austin. USA. McGarry, M. G. and Pescod. M. B. (1970). Stabilization pond design criteria for uopical Asia. In: Proceedings 0/ the 2nd llIlernutional Symposium for Waste Trealmelll LAgoons. McKinney. R. E. (ed), pp. 114-132 Universily of Kansas. Kansas, USA. Mara. D. D. (1976). S~age r,eatmefll ill Hot Climates. John Wiley and Sons. Cbichesler. UK. Mara. D. D.• AJabasler. G. P.. Pearson. H. W. and Mills. S. W. (1992). Waste Stabilization Ponds: A Desigll MlJ1IUal/o, Eastern Africa. Lagoon Technology International Ltd, Leeds. Marais, G. v. R. (I%6). New factors in the design. operation and performance of waste slabilization ponds. Bulletin of the World Health O,gQllisation, 33. 737-763. Marais, G. v. R. (1974). Faecal bacterial kinetics in stabilizauon ponds. Journal of Ihe EIIvirollmefllal Engineering Division, Americall SocieTy ofCivil Engineers. lOO{l), 119-139. Marais. G. v. R. and Shaw, V. A. {I 96 I). A rational theory for the design of sewage stabilization ponds in cenrral and South Africa r,ansaetioflS o/the South A/ricQIIl1ISIitule of Civil Engrneers. 3. 20S-227. Middlebrooks, E. J.. Middlebrooks, C. H~ Reynolds, J. H.• Watters, G. Z.• Reed. S. C. and George, D. B. (1982). Wastewater '" StabilkatiollLAgoolI Design, PerfomuJIICe and Upgrading. Maanilian Publishing Co, New Yark. NY, USA. Oragui, J. I•. Arridge, H., Mara. D. D., Pearson H. W. and Silva, S. (1995). Rotavirus removal in experirnenlal w.as~ stabihsauon ponds wilb diffetent geometries and configurauons. Water Science and Technology. 31( 12),285-290 (thiS Issue). Oswald, W. (I%8). Advances in anaerobic pond systems design. In: AdvafICes in Water QualiTy Improvemelll. G1oyna. E. F. and Eckenfelder. W. W. (cds), pp. 409-426. Universily of Texas. Austin. TX. USA. Pearson. H. W. Mara, D. D., Konig, A., de Oliveita, R.. MiUs. S. W.• Smallman. D. J. and SLIva. S. A. (987). Water column sampling as a rapid and efficient method of determining effiuenl qualily and the performance of waste stabilization ponds. Waler Science and Technology, 19(12), 109-113.
Pearson. H.
w. Mara. D. D. and Mills, S. w. (1988). RationaIising waste stabilisation pond design:
the biological factor. In:
Water Pollution Cofl/rol III Asia, Panswad, T., POlprasen. C. and YamarnOIO, K. (eds). Pergamon Press. Oxford, UK. (Advallced Waltr Pollution COlllrol. no.6, pp. 691~97. Polprasert, C.. Dissanayake, M. G. and Tbanb. N. C. (1983). Bacterial die-olT kinetics in waste slabUization ponds. Journal of the Water Pollution Control Federation, 55. 28S-296. Silva, S. A.. Olive~ R.• Soares, J~ Mara D. D. and Pearson, H. W. (1995). Niuogen temovaJ in pond Systems with different confillurauons and geometries. Water Scie1lCe and Technology, 31(12) 321-330 (this issue)
The influence of geometry on waste stabilisation ponds
139
Thirumurtbi. D. (1969). Design principles of waste stabilization ponds. Journal of Sanitary Engineering Division. American Society of Civil Engineers. 95. 311-329. Wehner. J. F. and Wilhehn, R. H. (1958). Boundary conditions of flow reactor. Chemical Engineering Science. 6, 89-93. Yanez, F. A. (1984). Reduccion de organlsmos patogenos y eliseno de Iagunas de estabilizacion en paises en desarrollo. In: XIX Congresso lnuramericano de lngeneria Sanitaria y Ambiental. Santiago. AlDIS.
Pergamon 0273-1223(95)00500-5
Waz. Sci. Tech. Vol. 31. No. 12. pp. 129-139. 1995. Copyright 0 1995 IAWQ Prioted in Great Britain. All righll reserved. 0273-1223195 $9'50 + Oo()(J
THE INFLUENCE OF POND GEOMETRY AND CONFIGURATION ON FACULTATIVE AND MATURATION WASTE STABILISATION POND PERFORMANCE AND EFFICIENCY H. W. Pearson*, D. D. Mara** and H. A. Arridge* *Department ofGenetics and Microbiology, University ofLiverpool. P.O. Box 147. Liverpool 1.69 3BX, UK **Department of Civil Engineering, University ofLeeds. Leeds LS2 9n, UK
ABSTRACT Differences in length:breadth ratios (in the range of I; I to 6; I) and depths (from I to 2 m) had little effect on the performance and effluent quality (i.e. BOD. SS and Fe) of secondaJy facultative ponds. Shallow maturation ponds (0.4 m) were more efficient at microbiological disinfection than deeper ones and could actually reduce land area requirements. KT values for faecal coliforms. salmonellae and rotavirus differed from one another in the same ponds and for different pond types. The Marais design equation fex predicting faecal coliform numbers ID pond effluents. although not perfect, does provide a reasonable design approach for systems containing more than two ponds ID series.
KEYWORDS K values; lagoons; pathogen removal; pond design; pond geometry; waste stabilisation ponds. INTRODUCfION Design criteria for waste stabilisation ponds are well established and have been the subject of considerable research and publication for more than 30 years. Facultative pond design is based on empirical fonnulae such as that of Gloyna (1976). which detennines pond volume and tends to be conservative by modern standards, and McGarry and Pescod (1970) who derived an equation linking surface or areal BODS loading to temperature. This approach has been refined by Arthur (1983) for Africa. Mara (1987) for global application, Mara and Pearson (1987) for Europe and most recently by Mara et al. (1992) for Eastern Africa. Alternatively kinetic and dispersion models have been used such as Marais and Shaw (1961), which assumes complete mixing and flrst-order reaction rates, and the equation of Thirumurthi (1969) which considers hydraulic dispersion in the pond. Yanez (1982) has modified Thirumurthi's equation for application in South America. The surface loading approach to facultative pond design is currently the most widely accepted (WHO, 1987).
129
130
H. W. PEARSON tl 01.
The design of maturation ponds is based on pathogen removal, usually bacterial decay. Whatever removal or die-off mechanism may be operating (e.g. sedimentation, starvation, predation, high temperature, high light and high pH or a combination), the equation of Marais (1974), which assumes that faecal coliform removal can be modelled by fIrst-order kinetics in a completely mixed reactor, is the most frequently used. Polprasert et al. (1983) have sought to refine the design approach by taking account offactors such as organic loading and algal concentration, and the non-ideal flow equation of Wehner and Wilhelm (1958) has been proposed for predicting bacterial die-off in preference to complete mixing. This attention to process design has not been matched by research into physical design: notably, pond shape and the design and vertical and horizontal location of inlet and outlet structures. Emphasis is put on preventing dead space and the prevention of short-circuiting, but opinions vary on whether the ponds should be square or rectangular and what are the optimum depths for various pond types (Forero. 1993; Mara and Pearson, 1987; Mara et al., 1992; Middlebrooks et 01., 1982; Pearson et aI., 1988; WHO, 1987).
In this paper we consider the impact of variations in pond geometry and depth on the process design and efficiency of operation of secondary facultative and maturation ponds in a large experimental pond complex in Northeast Brazil. The fmdings are also related to physical design and construction. POND SITE DESCRIPTION The experimental pond complex was constructed at the site of the City of Campina Grande's sewage treatment plant at Catingueira, Par:1iba, Northeast Brazil. The pond complex comprises two systems of ponds. A IO-pond linear series and a 17-pond flexible "innovative" system comprising different combinations of pond types in series. Table 1. Characteristics of the experimental pond system Pond
Dimensions (m) length breadth depth
A9 AIO F21 F22 F23 F24 F25 MIS MI6 MI7 Ml8 MI9 M20 M21 M22
4.90 4.90 12.90 12.90 12.90 12.90 4.90 17.35 10.40 10.40 10.40 10.40 10.40 8.45 8.45 8.45 8.45
M23~
M24"
1.65 1.65 2.00 2.00 2.00 200 4.90 8.80 3.75 3.75 3.75 3.75 1.30 3.70 3.70 3.70 3.70
2.50 2.50 1.00 1.33 1.67 2.00 2.00 1.00 0.90 0.64 0.39 0.39 0.39 0.60 0.60 0.60 0.60
Flow-rate (m3/d)
Hydraulic retention time (d)'
20.0 20.0 8.0 8.0 8.0 8.0 8.0 40.0 5.0 5.0 5.0 5.0 5.0 3.75 3.75 3.75 3.75
1.0 1.0 3.0 4.0 5.0 6.0 6.0 3.8 7.0 5.0 3.0 3.0 1.0 5.0 5.0 5.0 4.24
• theoretical values; ~ floating macrophyte pond; "baffled pond; 4 corrected for baffie volume.
The influence of geometry on waste stabilisation ponds
131
In this study the 17-pond system (Fig. I), comprised a combination of two anaerobic ponds (A9 and A 10) in parallel. whose combined effluents fed five secondary facultative ponds (F21-25) in parallel which in tum were followed by ten maturation ponds. The maturation ponds were arranged to give a single primary maturation pond (M 15) receiving the combined effluents of the five secondary facultative ponds, followed by five secondary maturation ponds (MI6-20) in parallel which in tum were followed by four tertiary maturation ponds (M21-24) also in parallel. This arrangement of the complex gave various combinations of five-pond series using ponds of different geometries, depths and retention times. The dimensions, flow rates and retention times of the various ponds are presented in Table I. The two anaerobic ponds had volumetric loadings of l87g BODs/m 3/d and the secondary facultative ponds each had surface organic loadings of 217kg BODsfha/d. MATERIALS AND METHODS The ponds were sampled using the column sampling technique (pearson et al., 1987) to obtain the equivalent of mean 24-h effluent values for the various parameters. This sampling procedure was verified for the system at the start of the sampling programme which extended over 15 months. The techniques for the analysis of the physicochemical parameters are described in Silva et al. (1995) but follow those described in Standard Methods (APHA, 1989). The microbiological assays are described by Oragui et al. (1995). RESULTS Seconda.cy facultatiye ponds The performance of the five secondary facultative ponds in terms of mean effluent quality were studied over a IS-month period which included both wet and dry seasons. The mean mid-depth water temperature remained constant at 25 °e. Although all five secondary facultative ponds received the same surface organic loading their geometries differed. Ponds F21-24 had the same length to breadth ratio of 6: I but differed in depth from I to 2 m. Their retention times also varied from 3 to 6 days, with the increased retention times resulting from the increased pond depths. F25 had the same depth and the same retention time (6 days) as F24 but had a length to breadth ratio of I: I as opposed to 6: I. Mean effluent quality values (based on column sampling) for key physicochemical and microbiological parameters are presented in Table 2. These data suggest that increased depths in the range of I to 2 m and the associated increases in retention times of 3 to 6 days did not significantly alter individual pond performance or effluent quality in terms of BOD, suspended solids and microbiological quality, the latter being individually expressed as numbers of faecal coliforms. salmonellae and rotaviruses.
It has frequently been suggested that pond performance could be improved by increasing length:breadth ratios which favour a plug flow hydraulic regime over complete mixing. A comparison of the results for F24 and F25 (Table 2), however, suggests that a pond length:breadth ratio of either I: I or I:6, which might be considered a realistic range for many pond construction sites, had little effect on pond performance and effluent quality. Maturatjon ponds The tive secondary maturation ponds (MI6-20) each received a flow of 5 m 3/d. The combined effluents of M 16-1 Xfed the four tertiary maturation ponds (M21-24) with each receiving 3.75 m 3/d. Pond M23 was a floating macrophyte pond containing Pistia stratiotes and M24 was baffled. The results of studies on maturation pond performance and effluent quality are summarised in Table 3.
H. W. PEARSON et al.
132
,
~-
-I pumping
box
: house A9
F21
A10
F22 F23
F25 F24
M15
t M16
M21
M22
M23
M24
.-L__, inspection box raw sewage
-
Figure 1. Layout of experimental WSP at Catingueira. nortb-cast Brnzil.
The influence of geometry on waste stabilisation ponds
133
The microbiological data from the secondary maturation ponds. which are the fourth ponds in a combination of 5-pond series. showed that effluent quality varied very little between ponds M16-19 despite differences in depth and retention time. The faecal coliform numbers were. however, up to 0.6 of a log unit higher in M20 than in the effluents of the other secondary maturation ponds. Pond M20 (39 cm deep) received the same flow as the other secondary maturation ponds but was only one third the surface area of ponds MI8 and Ml9 which were also 39cm deep. Consequently it had a shorter retention time of just one day and an increased organic surface loading (Le. 70kg BODihald) It was not surprising. therefore. that its effluent quality was a little poorer in terms of feacal coliform numbers. In contrast salmonellae and rotavirus numbers were comparable to those found in the other secondary maturation pond effluents. However, when pond efficiency is considered in terms of fIrst-order removal kinetics (K r values), then the shallower secondary maturation ponds M18-20 (3g cm oeep) were more efficient at faecal coliform removal than the deeper secondary maturation ponds. Comparison of the Kr values takes account of the differences in retention time between the ponds and M20 with its one-day retention time had the highest removal constant for faecal coliforms of any of the ponds, and also high removal rates for rotaviruses and salmonellae when compared to the other secondary maturation ponds (Table 3). The algal concentration (chlorophyll-a) in M20 although lower than in the other ponds was nevertheless stable and provided high pH values (>9) and supersaturated oxygen concentrations during the day. It is also worth noting that the data for ponds M18 and M19 which are identical in size, geometry and thus retention time were very similar. showing that pond performance within the complex was reproducible. The results for the tertiary maturation ponds showed that the floating macrophyte pond M23 although producing the best BODs effluent quality appeared less efficient at faecal coliform removal than the same• sized algal ponds M21 and M22. These two ponds also provided good evidence of the reproducibility of the data for ponds operating under identical conditions except when comparing the Kr values for rotavirus removal which did diverge. This. however. probably relates to the the low number of virus particles present and the impact small changes in numbers will therefore have on the Kr values. The tertiary maturation ponds (60 cm deep). with the exception of M24, the baffled pond, appeared less efficient at faecal coliform removal than even the deeper secondary maturation ponds, but this may reflect the smaller microbial populations and the fact that those microorganisms still remaining viable might be more resistant to the ambient conditions. The baffled pond M24. which had an effective length:breadth ratio greater than 100: 1, was more efflcient at faecal coliform removal (Kr 7.96) than the other tertiary ponds, although again with such small numbers in the influent the impact on actual effluent FC numbers was small. It was difficult to determine any improvement in the removal efficiencies for salmonellae because of low numbers. Rotavirus removal in the tertiary maturation ponds was good with high removal efficiencies (Table 3) and very few actual virus particles present in the final effluents from any of the parallel 5-pond series. Physicochemical effluent quality was comparable to the other maturation ponds although the special tertiary ponds M23 (the macrophyte pond) and M24 (the baffled pond) both had consistently lower effluent BODS values. Using the equation of Marais (1974) for determining the faecal coliform die-off coefficient (K r ) in maturation ponds, and using a temperature of 25°C, which was the mean mid-depth temperature of the ponds. ~ KT value of 6.20d- 1 was obtained. The primary maturation pond (MI5) and the five secondary maturaoon ponds had KT values which were similar to or greatly exceeded the 6.20 value. whereas the tertiary maturation ponds except M24 (the baffled pond), gave lower values (Table 3). When these values are corrected to 30°C they are higher than those obtained by Ellis and Rodriques (993) at this temperature for ponds on Grand Cayman, and also in contrast to their findings the Kr values were higher in our maturation ponds than in our secondary facultative ponds.
H. W. PEARSON et al.
134
TABLE 2. GEOMETRIC DETAILS AND MEAN EFFLUENT VALUES (BASED ON COLUMN SAMPLES) FOR TIlE ANAEROBIC AND SECONDARY FACULTATIVE PONDS OF TIlE 17-POND SYSTEM. POND UBRATIO
.
RETENTION
DEPTH
1lME(d)
FAECAL COUFORMS eN/IOOml
SALMONELI.4E eN/IOOml
A9
3.1
2:'0
1.0
7.06 x 106 (2.76)'
J61.' (-0.09)
AIO
3:1
2'0
1.0
7." x 106 (2 72)
J6S.6 (,j).10)
F21
6:1
1.00
3.0
1.1 x 106 (1.16)
29.9(4.03)
9.2 x 10' (I 61)
F22
6:1
\.33
4.0
ROTAVIRUS pt\IlL 1.7 x 104 (2.92)
CIIL
TSS
BOD
..wt.
maIL 73(1.70)
121
"(163)
442
1.2 x 101 (OJI)
U(O.44)
II
JO 9 (2 69)
1.0 x 104 (0.11)
29 (OJ9)
77
722
6.9 x 101 (0.30)
I.Ix 104 (2.76)
43'
F23
6:1
1.67
'.0
9.2110' (I.")
19,7 (3.49)
28 (o.m
67
6SS
F24
6:1
2.00
6.0
7.1 x10' (I 36)
14' (401)
, , x 101 (0.36)
24 (OJ')
63
716
23.0 (2.47)
4.6 X 103 (0.46)
2' (0.33)
69
706
F2,
1:1
1.9 xlO' (1.16)
6.0
2.00
• Numben in bracketS represent the Kr valucs for first order removal, cr'. Mean mid depth walcr tempcraIurc 2' DC. VolumcIric loading on the anaerobic ponds was 187& BODslml/d. Arcalloading on the facultative ponds was 217 kgBOD,tha/d.
TABLE 3. GEOMETRIC DETAll..S AND MEAN EFFLUENT QUALITY VALUES (BASED ON COLUMN SAMPLES) FOR THE MATURATION PONDS OF THE 17-POND EXPERIMENTAL SYSTEM. CHLOROPHYlL-oI
POND lIB DEP1H RETENTION AREAL LOAD FAECAL COLIFORMS SALMONElLAE ROTAVIRUS RATIO m TIME (d) kgBOD,/ba/d pfulL cfu 1100 mI cfu /100 mI
BOO,
M15
1.1.1
1.00
3.8
72
2.36 x 104 (9 46)'
3.2 (U6)
',37 x 101 (0.08)
19(0.12)
'3
369
MI6
2,8.1
0.90
7.0
24
"., x 102 (6.04)
1.1 (0.27)
12' x 103 (047) 22 (~02)
74
447
MI7
2.1.1
064
'.0
24
681 x 102 (6,73)
12 (0 33)
1.90 x 103 (0.37)
21
(~.02)
73
392
Mil
2.8:1
0.39
3.0
24
HIx 102(10.04)
I 4 (056)
2 36 x 103 (0.43)
2"~08)
m
m
26(.0.08)
1I8
361
2' (~.24)
103
201
TSS
mall
~
mall
MI9
2....
0.39
3.0
24
6.30 x 102 (12.1$)
1.2(0.$6)
2.34 x 103 (0.0)
M20
"1
039
1.0
70
1.60 x 103 (13 7$)
\4(1.29)
1.79 x \0 3 (2.00)
M21
2.3'1
0.60
$0
29
1.49 x 10 1 (3.'9)
1.0 (0,0$)
15.1 (24)
24(.0.01)
101
39'
M22
2.3:1
0.60
'.0
29
4.17xIO I (297)
10(0.0')
2.6 (141)
26 (-D.03)
89
328
M23
2.3:1
0.60
'.0
29
224 x 102 (0.39)
3.8(oC 14)
0
(.)
15(0.10)
81
338
M24 >100.1
0.60
42
34
1.92 x 10 1 (7.96)
1.1 (0.03)
0
(.)
7(0,0')
107
398
• N\IIIlbcn iIllnckcu repreICIlllbe KT values fer lint order remoVll, dayrl.
Mean TeIIlP"'1Ituro 2'0<:.
11Ie influence of geomeuy on waste stabilisation ponds
13S
Further considerations of first-order rate remoyal constants The actual KT values for faecal colifonn removal for the five secondary facultative ponds (Table 2) were also lower than the theoretical 6.20 value. This was not surprising since facultative ponds are usually found to be less efficient at bacterial pathogen removal than maturation ponds. The theoretical KT values for faecal colifonns calculated from the Marais equation were a poor indicator of the actual K T values for salmonellae in the maturation ponds. since the latter were frequently an order of magnitude lower than the predicted 6.2 value. The actual KT values for salmonell,te in the secondary facultative ponds were also lower than the predicted value but were higher than the actual K T values calculated for faecal colifonns. Table 4. A comparison of actual and predicted numbers of faecal colifonns in pond effluent Actual F.e Pond in Emuent (per 100 mi.)
RS A9 AIO F21 F22 F23 F24
F2S MIS MI6 MI7 MI8 MI9 M20 M21
M22 M23 M24
2.66 x 107 7.06 x 106 7.15 x 106 1.08 x 106 9.22 x 105 9.15 x 105 7.76 x 105 8.90 x 105 2.36 x 104 5.45 x 102 6.81 x 102 7.58 x 102 6.30x 102 1.60 x 103 3.49x 10] 4.17xlO l 2.24 x
102
1.92 x 10 1
Predicted F.e.· (individual ponds (per 100 001.)
3.69 x 106 3.69 x ]06 3.63 x lOS 2.75 x 105 2.22 x 105 1.86 x lOS 1.86 x 105
3.73 x 104 5.32 x 102 7.33 x 102 1.20 x 103 1.20 x 103 3.28 x 103 2.07 x 10 1 2.07 x 10 1 2.07 x 10 1 2.07 x 10 1
Predicted F.e.· (series) (per 100 mi.)
369 x 106 3.6 x 106 1.89 x lOS 1.43 x 105 1.15 x ]05
9.67 x 104 9.67 x 104 4.89 x 103 1.10 x 102
1.53 x 102
2.50 x 102 2.50 x 102
6.79 x 4.8
102
4.8 4.8 4.8
• see explanation in text. Actual KT values calculated for rotaviruses were similar in all the secondary facultative ponds and the primary and secondary maturation ponds, but were lower than the theoretical value for faecal colifonns. and lower than the actual values obtained for faecal colifonns. In contrast very high KT values were obtained for rotavirus removal in the tertiary maturation ponds and few or zero virus particles were present in the final effluents. The results for actual faecal colifonn numbers in individual pond effluents were compared with two theo~tical values for ~ffluent numbers (Table 4). to show the impact. the disparity in KT values has on predicted effluent quality. Both theoretical values were calculated USing the K T value of 6.2 d- I for a temperature of 2S °e. but using either the actual influent Fe values for each pond or the Fe count in the raw
H.VV.PEAJtSONelaL
136
sewage, and detennining the theoretical Ne value for each pond effluent at its position along the 5-pond series. These data show that the theoretical values fur Fe numbers can be nearly an order of magnim.qe lower than the actual effluent values for the early ponds in the series when the fonner are calculated from caw sewage FC numbers. The numbers then converge as the higher than predicted faecal colifonn removal cates come into play in the secondary maturation ponds thus redressing the balance. 8-:
j
•
E :5 5
( ,)
u. 4 (ij
.a
" ~3 .....0 2
O+---r---'["""'"--.-......,.--.,.......-.....---, o
234567
Log Predicted Fe '100 ml.
Figure 2. Comparison of log actual and log predicted faecal coliform numbers in individual pond effluents (N.,) based on the results of column sampling. Log predicted values were derived from actual influent values (N J for eacb pond usmg the Marais equauon.
2
345 Log Predicted
6
Fe '100 mi.
Figure 3. Comparison of log actual and log predicted faecal coliform numbers in individual pond effluents (N.,) based on the results of column sampling. Log predicted values were derived using the Marais equation for detennining N.lo a series of ponds, wbere N j was the faecal coliform numbers in the mw sewage.
Despite these differences in experimentally derived and theoretically detennined KT values. plotting log lICtual effluent FC numbers for each of the ponds against log theoretical effluent FC numbers using either the
The mtluence of geomecry on waste slabilisation ponds
137
actual influent FC numbers for the pond (Fig. 2) or extrapolating from the raw sewage PC values (Fig. 3) gave linear regressions with highly significant positive correlations (r 2) of 0.99. yThis would suggest there is a balancing-(lut effect between the pond types and that predicting effluent quality by using the equation of Marais to predict effluent faecal coliforms numbers is acceptable in pond systems comprising more than three cells but is less reliable where there is only one pond or two ponds in series. DISCUSSION AND CONCLUSIONS The results presented here suggest that increasing the length:breadth ratios does not obviously benefit pond performance or improve the effluent quality of secondary facultative ponds for either physicochemical parameters such as BODs or microbiological parameters such as faecal coliforms. Thus the significance at the physical design stage of building long rectangular ponds or including numerous baffles in facultative ponds to encourage plug or piston flow is probably overstated (Forero, 1993). These findings also lend credence to the idea that the vertical stratification or lamination of the water column is probably as or more important in determining treatment efficiency than the bulk flow along a longitudinal axis (Pearson et ai., 1988). In this respect the positioning and depths of the inlet and outlets may have a greater beneficial impact on treatment efficiency than pond shape. The apparent lack of impact of pond shape on facultative pond performance within a realistic range of length:breadth ratios allows the designer more freedom in shaping the ponds to make best use of available land particularly on small or awkwardly shaped sites.
In these studies increasing facultative pond depth and thus the hydraulic retention time whilst maintaining the same organic surface loading did not significantly improve physicochemical or microbiological effluent quality. This is important in design terms for several reasons. Firstly, there has been a tendency by some designers to increase pond depths (particularly where land is at a premium) as a way of increasing the hydraulic retention time in the pond series, in the belief that this will produce a better microbiological quality effluent. This approach has been used since the increased retention time values can be inserted into the Marais equation resulting in lower predicted faecal coliform numbers in the effluent compared with values for a shallower pond system of comparable area receiving the same flow, and thus having a shorter overall retention time. This design approach can lead to the actual pond effluent qUality falling shon of the design prediction with all the implications this may have on the final destination or use for the effluent. Secondly. on purely practical and economic grounds. building deeper ponds may involve significantly more earth movement and increase construction costs unnecessarily. Alternatively, the site topography may lend itself to the construction of slightly deeper facultative ponds in which case this need not lead to poor pond performance so as long as the implications of depth and retention time on the performance of the various pond types are taken into consideration at the design stage. The data presented here also validate the surface organic loading design approach for facultative ponds. since increasing the depth decreased the volumetric loading rates (whilst keeping the surface loading rates the same), and yet did nothing to improve effluent quality. It would also suggest that more complicated design equations embracing functions taking account of hydraulic dispersion etc.. are probably unneccessarily complicated and offer very little by way of design improvement. In our studies with secondary maturation ponds. reducing the depth also reduced the retention time but this did not adversely affect effluent quality. This implies that the shallower ponds were more efficient at natural disinfection than the deeper maturation ponds. These preliminary data in fact suggest that there is a beneficial trade-off in building shallow maturation ponds lIS they appear to utilise less land for a comparable microbiological effluent quality.
The results of these.flfSt series of experiments support the use of shallow maturation ponds to save land area and reduce excavaUon costs. However, it was also true that the rate of faecal coliform removal tended to decrease through a series of maturation ponds lIS the remaining bacterial numbers decreased despite all other
H. W. PEARSON et al.
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factors remaining equal. This was particularly noticeable when comparing the faecal colifonn K T values for the tertiary maturation ponds with those for the secondary ponds. This would suggest that the faecal colifonn population is somewhat heterogeneous and that certain species or types may be more resistant than others to the natural disinfection processes in the tertiary maturation ponds, despite the existence of conditions which should be hostile to their survival. In contrast rotavirus removal was more efficient in the tertiary ponds than in the preceding ponds and their removal clearly benefited from the use of 5 ponds in series.
The baffled pond (M24) was the most efficient of the tertiary maturation ponds both in tenns of pathogen removal and organic removal, however, further studies (which are currently underway) are required to evaluate fully the performance of this type of pond particularly when receiving higher influent concentrations of pathogens, ACKNOWLEDGEMENTS We are grateful to the UK Overseas Development Administration for financial suppon (Research Scheme R4569) and to the Universidade Federal da Par.1iba and the Companhia de Aguas e Esgotos da Pacliba for the provision of experimental facilities and the pond site at Catingueira. REFERENCES APHA (1989). Standa,d Methods /0' the Examination 0/ Wate' and Wastewater, 17th edn. American Public Health Association. Washington, DC, USA. Arthur. J. P. (1982). Notes 011 the Design and Operation o/Waste Stabilization Ponds ill Wa171l CliftlQtes ofDevelopillg Coufllries. World Bank Technical Paper Number 7, Washington. DC, USA. Forero. R. S. (1993). Stabilisation ponds in the lrOpics: promotion of the sanitary reuse of water. Built/iII 0/ PAHO. 17(3). 21S•
223.
G1oyna, E. (1976). Facultative waste stabilization pond deSign. In: Ponds as a Wastewater Trealmelll AllemaJrve. Gloyna, E. F~ Malina, J. F. and Davis, E. M. (eds), pp. 143-157. Universily of Texas Press. Austin. USA. McGarry, M. G. and Pescod. M. B. (1970). Stabilization pond design criteria for uopical Asia. In: Proceedings 0/ the 2nd llIlernutional Symposium for Waste Trealmelll LAgoons. McKinney. R. E. (ed), pp. 114-132 Universily of Kansas. Kansas, USA. Mara. D. D. (1976). S~age r,eatmefll ill Hot Climates. John Wiley and Sons. Cbichesler. UK. Mara. D. D.• AJabasler. G. P.. Pearson. H. W. and Mills. S. W. (1992). Waste Stabilization Ponds: A Desigll MlJ1IUal/o, Eastern Africa. Lagoon Technology International Ltd, Leeds. Marais, G. v. R. (I%6). New factors in the design. operation and performance of waste slabilization ponds. Bulletin of the World Health O,gQllisation, 33. 737-763. Marais, G. v. R. (1974). Faecal bacterial kinetics in stabilizauon ponds. Journal of Ihe EIIvirollmefllal Engineering Division, Americall SocieTy ofCivil Engineers. lOO{l), 119-139. Marais. G. v. R. and Shaw, V. A. {I 96 I). A rational theory for the design of sewage stabilization ponds in cenrral and South Africa r,ansaetioflS o/the South A/ricQIIl1ISIitule of Civil Engrneers. 3. 20S-227. Middlebrooks, E. J.. Middlebrooks, C. H~ Reynolds, J. H.• Watters, G. Z.• Reed. S. C. and George, D. B. (1982). Wastewater '" StabilkatiollLAgoolI Design, PerfomuJIICe and Upgrading. Maanilian Publishing Co, New Yark. NY, USA. Oragui, J. I•. Arridge, H., Mara. D. D., Pearson H. W. and Silva, S. (1995). Rotavirus removal in experirnenlal w.as~ stabihsauon ponds wilb diffetent geometries and configurauons. Water Science and Technology. 31( 12),285-290 (thiS Issue). Oswald, W. (I%8). Advances in anaerobic pond systems design. In: AdvafICes in Water QualiTy Improvemelll. G1oyna. E. F. and Eckenfelder. W. W. (cds), pp. 409-426. Universily of Texas. Austin. TX. USA. Pearson. H. W. Mara, D. D., Konig, A., de Oliveita, R.. MiUs. S. W.• Smallman. D. J. and SLIva. S. A. (987). Water column sampling as a rapid and efficient method of determining effiuenl qualily and the performance of waste stabilization ponds. Waler Science and Technology, 19(12), 109-113.
Pearson. H.
w. Mara. D. D. and Mills, S. w. (1988). RationaIising waste stabilisation pond design:
the biological factor. In:
Water Pollution Cofl/rol III Asia, Panswad, T., POlprasen. C. and YamarnOIO, K. (eds). Pergamon Press. Oxford, UK. (Advallced Waltr Pollution COlllrol. no.6, pp. 691~97. Polprasert, C.. Dissanayake, M. G. and Tbanb. N. C. (1983). Bacterial die-olT kinetics in waste slabUization ponds. Journal of the Water Pollution Control Federation, 55. 28S-296. Silva, S. A.. Olive~ R.• Soares, J~ Mara D. D. and Pearson, H. W. (1995). Niuogen temovaJ in pond Systems with different confillurauons and geometries. Water Scie1lCe and Technology, 31(12) 321-330 (this issue)
The influence of geometry on waste stabilisation ponds
139
Thirumurtbi. D. (1969). Design principles of waste stabilization ponds. Journal of Sanitary Engineering Division. American Society of Civil Engineers. 95. 311-329. Wehner. J. F. and Wilhehn, R. H. (1958). Boundary conditions of flow reactor. Chemical Engineering Science. 6, 89-93. Yanez, F. A. (1984). Reduccion de organlsmos patogenos y eliseno de Iagunas de estabilizacion en paises en desarrollo. In: XIX Congresso lnuramericano de lngeneria Sanitaria y Ambiental. Santiago. AlDIS.