Influence of thermal stratification on the behaviour of a deep wastewater stabilization pond

½"at. Res. Vot. 26. No. 5. pp. 569-577. 1992 Pnnted in Great Britain.All rights reserved

0043-1354,92 $5.00+0.00 Copyright ~ 1992 PergamonPresspie

INFLUENCE OF THERMAL STRATIFICATION ON THE BEHAVIOUR OF A DEEP WASTEWATER STABILIZATION POND M. LLORENS,J. SAEZand A. SOLER Department of Chemical Engineering. University of Murcia. 30071 Murcia. Spain (First receired May 1991; accepted in ret,ised form Norember 1991)

Abstract--Deep lagooning is an alternative to conventional lagooning and it implies smaller land requirements as an additional advantage. Treatment in deep wastewater stabilization ponds is influenced by thermal stratification. The object of this paper is the study, over a I-yr period (October 1986-December 1987), of the influence of temperature on the behaviour of a treatment unit (an 8 m deep pond) which receives domestic wastewater. For this study, a spatiotemporal follow-up of the natural treatment process has been conducted. The removal efficiencies achieved for the following parameters varied as follows: COD. 71-92%: BOD, 83-97%; total Kjeldahl nitrogen. 51-98.8%; ammonia nitrogen, 38.8-99.9%: total phosphorus. 42.2-92.9% and orthophosphate, 40-99%. Log reduction of microorganisms varied between 2.0 and 3.0 for total coliforms, I.I-3.0 for faecal coliforms and 1.7-3.3 for faecal streptococcus. Other physicochemical parameters observed were: temperature, dissolved oxygen, pH. nitrates and nitrites. The continuous flow of wastewater did not hinder the appearance of a marked stratification in spring-summer: an aerobic epilimnetic zone and an anaerobic hypolimnetic one remained clearly differentiated. The evolution of all the parameters observed was similar. In the absence of thermal stratification, the system was found to function in approximately complete mixing conditions, and the value of each parameter was almost constant throughout the water column; nevertheless, the presence of stratification caused a vertical distribution with marked gradients in the thermocline zone. Ke.v words--deep pond, stratification, chemical performance, microbiological performance

matter transport between thc different layers defined in the water column as a consequence of thermal stratification.

INTRODUCTION

Wastcwater treatment by stabilization pond is a lowcost but high-effective procedure. Since this treatment is achieved by natural processes, wastewater retention time is long, and a large area of land is normally required. Deep lagooning is an alternative to conventional lagooning and implies smaller land requirements as an additional advantage. The development of this kind of system is relatively recent, and its use is spreading through urban areas where the price of land is high, and through urban areas surrounded by rich agricultural land. The depth of these ponds is > 5 m and data about them are limited in the literature. Some preliminary studies have been carried out in Spain (Moreno et al., 1984a, b, 1988; Bern:~ et al., 1986; Soler et al., 1988, 1991), in Brazil (Silva et al.. 1987), and in Israel (Abeliovich, 1983). The values and distribution of temperature in a mass of water play a fundamental role in the behaviour of these systems, due to the following factors: --physicochemical and biological processes depend greatly on temperature values; - - t h e mixing phenomena are closely related with the thermocline, because it restricts the heat and

As in other water bodies (lakes, reservoirs, etc.), the thermal stratification of a deep pond begins in spring and, starting from a homogeneous profile of temperatures, a stratification starts developing that eventually results in a well-defined thermocline. This thermocline separates two clearly differentiated layers: an aerobic superior one (epilimnion) and an anaerobic inferior one (hypolimnion). This situation remains until the beginning of autumn, when the surface cooling provokes a gradual deepening of the thermocline until isothermicity is re-established. This gradual deepening throughout the cycle causes the nutrient-rich water of the hypolimnion to ascend to the surface layer that has become depleted as a result of the intense biochemical activity favoured by the higher temperature, the presence of solar radiation, and the photosynthetic and surface oxygenation. On the other hand, the biomass generated in the epilimnion settles down and, once its mineralization takes place, it constitutes an additional contribution of nutrients to the hypolimnion (Moreno, 1983; Schertzer et al., 1987; Simons and Schertzer, 1987; Soler et al., 1991).

569

M. LLORENSet aL

570

,85

i

.......

/i B"

B I

I l ÷ ..... ?__

CC"

c I

BB"

/

l __#_

i

i

i

, i C"

i i B"

C"

I

,\ B"

Fig. 1. Layout of the deep pond located in the Campus of the University of Murcia, Spain. (Dimensions in m.) (O): Sampling points, (&): wastewater inlet, ( I ) : outlet.

T h e aim o f this paper is the study of the influence o f t e m p e r a t u r e o n the b e h a v i o u r o f a deep stabilization p o n d (8 m in depth) which receives domestic wastewater. F o r this research, a spatiotemporal follow-up study o f the t r e a t m e n t processes has been conducted. The physicochemical parameters observed were: temperature, dissolved oxygen, pH, C O D , BOD~, total Kjeldahl nitrogen ( T K N ) , a m m o n i a nitrogen, nitrates, nitrites, total p h o s p h o r u s a n d o r t h o p h o s p h a t e . M e a s u r e m e n t s o f total coliforms, faecal coliforms a n d faecal streptococcus were also performed. MATERIALS AND METIIODS Description of the system The deep wastewater stabilization pond where the study has been carried out (Fig. I) measures 85 × 40m, it has a maximum depth of 8 m, and a maximum capacity of approximately 15,000 m ~. It is located in the campus of the University of Murcia, near the Mediterranean coast in the south east of Spain, and was built, under design of the Department of Chemical Engineering, for the treatment of wastewater of the different University Centres existing there. The already treated water is pumped up to tanks from where it is distributed for the irrigation of the park and garden areas of the University Campus.

Table I. Average composition of the incoming wastewater (number of" samplcs: 25)

pH BODs (mg/I) COD (m8/I) Total phosphorus(mg PO,~ l) Orthophosphale (mg I)O41 ,I) TKN (rob N/I) Ammonia nitrogen (rob Nil) Nitrates (/~g N-NOj/I) Nitrites (~g N-NOr/I) Total coliforms/10Oml Faecal coliforms/10Oml Faecal streptococci/10Oml

Average 8.04 350 550 25 20

35 25 0 0 4.3 x I0" 2.1 x 106 4.3 x 10s

Range 6.94-8.98 170--420 210-650 18- 36 10-26 22~15 8-37

During the study period, October 1986--December 1987, the average inflow to the pond was 125 m3/day, which, in reaching approximately I 1,000 m; of water, supposes an average retention time of 88 days. The characteristics of the influent wastewater are described in Table I. Table 2 shows the monthly average of the ambient temperature and solar radiation of the geographical area where the pond is located. Sample collection and measurements The samples were collected every 2 weeks with a sampler specially designed to obtain water samples from discrete layers in stratified water bodies (Jorgensen et al., 1979). Sampling was performed between l0 and 12 a.m. A small boat was used for sampling, from surface to bottom, every metre at three different points (Fig. i). The outlet (Fig. I) is located 20 cm from surface. It has been proved that the effluent and the surface samples have the same physicochemical and microbiological characteristics. Physicochemical parameters. In situ measurements of dissolved oxygen and temperature were performed every 25 cm surface to bottom. The pH was measured every metre by a portable pH meter. The BODs, COD, TKN, ammonia nitrogen, nitrates, nitrites, total phosphorus and orthophosphate determinations were carried out according to the analytical methods described in Standard Methods for the Examination o f Water and Wastewater (APHA et al., 1985) and in Rodier (1981).

Table 2. Monthly average of ambient temperature and solar radiation of the geographical area in which the pond is situated Temperature Solar radiation Months

CC)

(cal/cm 2day)

1.8 x 106-6.5 x 10e 7.1 x 10~-4.1 x 104

January February March April May June July August September October November

10.0 11.5 14.4 17.6 19.5 24.0 26.4 27.2 25. I 18.9 13.2

195 292 397 488 582 623 572 516 460 298 215

7.1 x 104-8.7 × 10~

December

11.5

161

Thermal stratification in wastewater ponds In the Results section, and for clarity of the figures, only the surface, 3 m, and bottom data are represented. Microbiological parameters. The total coliforms, faecal coliforms and faecal streptococci were analysed according to the method of "Most Probable Number" (MPN), following the APHA recommendations (1985). RESULTS AND DISCUSSION

The study period started on 15 October 1986 and continued until 16 December 1987. Initially, the pond was in a steady state condition. Continuous flow was maintained. The inlet and the outlet of water are situated at the surface level and in opposite locations, about 87 m apart. The continuous wastewater inflow did not hinder the appearance of a marked stratification in springsummer: an aerobic epilimnetic zone well differentiated from an anaerobic hypolimnetic zone was observed. The evolution of the most significant physicochemical parameters is described as follows: Temperature. The temperature profile evolved from homogeneous conditions in winter (from October 1986 to February 1987)--with a minimum temperature around 7-9°C in late January--to a marked stratification in spring-summer, with a maximum temperature in surface layers around 24--26°C during July-September, the thermocline being located between 1.5 and 2.5 m deep, and returning again to a thermal homogeneity situation in the autumn-winter period (Fig. 2). Previous studies carried out in a 5m deep wastewater stabilization pond situated in the same

571

geographical area showed no thermal stratification (Moreno, 1983). No significant differences have been found in the temperatures observed at different points at the same depth, that is, the isotherms of the pond can be considered as horizontal. Dissolved oxygen. The general pattern is anaerobiosis in the whole water column in the absence of thermal stratification (this phenomenon was observed at the beginning and at the end of the cycle), and the formation of a highly oxygenated layer above the thermocline during the period of thermal stratification (Fig. 3). This aerobic layer gets deeper along with the thermocline. Under this situation, most oxygen originates from the photosynthetic activity of the phytoplankton. The transition from the hyperoxygenative surface zone to the hypooxygenative zone takes place sharply; this observation coincides with earlier data from hypereutrophic lakes (Seki et al., 1980). This fact is closely related with the thermal stratification and the thermocline location. pH. pH shows marked variations with depth. At the hypolimnion, the anaerobic degradation is the controlling factor of the pH evolution. In this zone, pH values remain almost neutral, indicating an equilibrium between the acid and methanogenic phases of the fermentation. However, photosynthesis is the controlling factor at the surface. As spring begins, this process is so significant that the contribution of CO, due to respiration is not enough to satisfy the demand required by algae, thus, in assimilating CO 2, these algae provoke the disequilibrium of the f. P

0 1

~2 ~3 O

4

Fig. 2. Temperature profiles. Measurements were made between 10.00 and ! 1.00 a.m. WR 26/~--C

572

M. LLog£.~set al.

.o

,>

o Q

o 1

~3 o

4 5

J

6

Fig. 3. Dissolved oxygen concentrations. Measurements were made between I0.00 and I1.00a.m.

carbonates balance. Therefore, the pH of the whole environment increases notably. In the absence of stratification (which occurred at the beginning and at the end of the study period), the pH value throughout the watcr column was almost neutral, but once the thermal barrier appeared, the pH at the surface layer showed significant oscillations due to the photosynthetic action of the phytoplankton developed at that time (Fig. 4). A great increase o f p H at the surface layers was observed from April on, and its values were as high as 9.95 by early July (Fig. 4).

On the other hand, being photosynthesis subject to the cycle of solar radiation, pet values also show variations throughout the daylight, and these are similar to the ones exhibited by dissolved oxygen (Table 3). As the intensity of the solar radiation increases, during the middle hours of the day, photosynthesis activates, and the corresponding disequilibrium of the carbonates balance determines an increase in the pH. This situation remains until the late evening, when the maximum values of pH are reached (Fritz et al., 1979).

10

9

8-

""~ 7

-',,,~

-..,..,-....,-

_.._,/ 0

I N I D I J

"-, i F ]'N

1986

I A i M lj

Ij

I A 1S

•

~"

.-

/Bottom

I 0

i N 1D

I

1987

Nonths

Fig. 4. pH evolution from October 1986 until December 1987, at three depths. Measurements were made between 10.00 and II.00a.m.

Thermal stratification

in w a s t c w a t e r p o n d s

Table 3. pH and dissolved oxygen concentration at different times of the day (date: 16 IX-cember. 1987). The measurements were carried out at the surface level Time

Dissolved oxygen (rag O:/I)

pH

7.0 7.9 13.5 16.7 17.2

7.0 7.98 8.25 8.38 8.56

8.45 a.m. 10.15 a m . 12.30 p.m. 14.30 p.m. 17.00 p.m.

573

characterized by minimum values at the surface and maximum values at the bottom, as well as by marked gradients of concentration that correspond to the therrnocline. The maximum and minimum treatment efficiencies found during this cycle were, respectively, 97 and 86% (the effluent quality was 8.3 and 46.7 rag/I, respectively), corresponding to a removal of 145 and 129 kg BODs/ha. day. Chemical oxygen demand (COD). The evolution of this parameter is similar to that of BOD~. In a situation of thermal stratification an increase of COD occurred at the hypolimnion (Fig. 6); this phenomenon, as commented above, is due to the anaerobic redissolution and digestion of the matter previously sedimented, and to the generation of reducing agents such as sulphides, which in some stages of the cycle were even above 100 ppm. The maximum and minimum treatment efficiencies during this cycle were 92 and 71%, respectively. Total Kjeldahl nitrogen (TKN). During the situation of thermal homogeneity the TKN values were similar throughout the water column. The stratification started gradually, with the consequent generation of a hypolimnion rich in TKN, and a depleted epilimnion; the various degrees of biological activity in the two zones of the pond, and the different temperatures account for the different TKN values observed. As autumn began, the warm surface layers started to cool until homogeneity was again reached (Fig. 7). During this cycle the maximum and minimum removal efliciencies were 98.8 and 51%, respectively. The maximum effectiveness corresponds to the summer months. Ammonia nitrogen. The evolution of this parameter is similar to that described for TKN. The

Biochemical oxygen demand (BO D s). The low temperatures observed at the beginning and at the end of the cycle provoked the slowing down of the biochemical degradation processes under the anoxic conditions prevailing at that time of the year. The thermal homogeneity situation (Fig. 2) leads to similar BOD5 values throughout the water column (Fig. 5). As temperature increases the process of organic matter stabilization activates. The thermal stratification significantly affected the evolution of this parameter at different depths. Once established, the thermocline, an increasing of this parameter at the hypolimnion, and a stabilization at the surface layers occurred (Fig. 5). The maximum BOD~ reductions occurred at surface layers, at higher temperature, and, frequently, under aerobic conditions. The degradation below the thermocline occurred under anaerobic conditions, at temperatures sensibly Iowcr than those of the epilimnion, with the contribution of dctrital biomass by sedimentation, and the redissolution and digestion of the previously sedimented matter; all these factors account for the higher BODs values and for the vertical distribution of this parameter. This vertical distribution is

200

/\ 150

A ¢-4

,

/

I

/

i

loo

50

/

I

/

N

'.'

/'"

D

I

J

I

F

I

N

I

~'~ I!

soo o°

/

l

A

1986

~/ Bottom |

'

3ml

,," '

I

,/--;I

°°*IS

i

J

0

I

..

I

Ii

0

/"

I\ i ~.

:

e

I

i

I

N

I

J

I

J

I

A

I

S

I

0

I

N

1987 Nont.lm

Fig. 5. BOD~ evolution f r o m October 1986 until December 1987, at three depths.

1

D

M. LLORENSet al.

574

/

300

\

j /--"

~..

\

/

\ 200-

'".

/ \

./'~

',,

8r j

/'\

. ..... ~',

i 13o~:tom

_

,

100

Surface

O

t

0

I

N

D

i

J

I

F

I

N

t

i

A

N

i

I

J

1986

I

J

A

I

S

I

O

i

N

I

1987 Nonths

Fig. 6. COD evolution ~omOctober1986 untilDccember1987, atthr~ depths. phytoplankton. On the other hand, under conditions of pH > 8 provoked by the high phytoplanktonic growth, a fraction of the ammonia nitrogen undergoes the following process:

concentrations found at the bottom layers of the pond have been, in all instances, superior to those found at the surface (Fig. 8). The anaerobic digestion liberates as ammonium the nitrogen present in the organic matter which is subject to the degradation process, which determines the high NH3 values found at the bottom layers. At the surface level, having a smaller biodegradable load, and in the presence of dissolved oxygen, the ammonia nitrogen is oxidized to other compounds, and assimilated by

60

_

N H ; + H20

1

NH3 (gas)

J

/!

/'

/' ,~../!

;

/

i

I

! I

./

/./ /~../'

I 3

, NH3(aq) + H30 +

i 40

I

/

iB ° t t ° m

i I

/

i

w

/

2O

0

I

L! I

/

I

I

N

, ....

/

I

D

v

J

I

F

I

N

I

....

f

3m

'

•

1986

!

g

I

I

-/

A

I

~

J,'.

~

I

J

I

J

!

A

I

S

1

0

,,LZ:::

1

N

I

D

1987 Months

Fig. 7. Evolution of T K N content from October 1986 until December 1987, at three depths.

Thermal stratification in wastewater ponds

!i # I

=

r -

e.

575

.I

I

i

I

i

Ixl

./

.

I

i

I

i

".

i

,

,

.•

i;...

i

0

7 O

I

....~-."~

i..

.-

I

N

O

|

I

J

I

N

I

!

tl

..-"

i

F

i

~'°" A

I

N

I

1966

J

I

I

J

A

I

I

S

I

O

N

I

D

1987 Ikmths

Fig.

8.Evolution

of ammonia nitrogen concentration from October 1986 until December 1987, at three depths•

which determines a net loss of nitrogen to the atmosphere. This set of phenomena contributes to the strong reductions in the concentrations of ammonia nitrogen at the epilimnion. The maximum concentrations appeared at the bottom layers of the pond, and the minimum ones at the surface (Fig. 8). The maximum and minimum removal e~iciencies were 99.6 and 38.8%, respectively. The maximum effectiveness occurred in the summer months.

Nitrates. Nitrates were not detected until the end of January, once an aerobic surface layer had developed. The concentration at the surface layers never surpassed I rag/I, this fact seems to be chiefly due to the fast consumption by phytoplankton and to the denitrification process possibly induced by the strong oscillations in the concentration of dissolved oxygen that occurred daily. From late September, nitrates disappeared again as a result of the

40-

4 30-

!

T t

/

//I i

,o-

.i

!

,

Q.

i

/.s~.

i

\

•

lO,

'/~Bottom

/"

',

~,,~/'---

o

O

I

N

I

D

I

J

I

i 1

i

F

I

!,!

I

A

I

N

I

J

I

J

1

A

I

S

m

O

-

I

N

i

D

1987

1986 Months

Fig. 9. Evolution of orthophosphate concentration from October 1986 until December 1987, at three depths.

576

M. LLOglL'qSet al.

disappearance of the aerobic layer caused by the phenomenon of mixing in the water column. The maximum value of the concentration of nitrates at the surface was 580#g N - N O f / I by early July, coinciding with a dissolved oxygen concentration of 19.9 mg/l. Nitrites. They are a labile form which rapidly gives way to nitrates in the next stage of the biooxidation. They were never detected at all. Orthophosphates. Phosphorus removal from wastewater is decisive to prevent the eutrophication of the water bodies where the treated water is to be discharged. Its vertical distribution is similar to that observed for other parameters. The bottom of the pond, quite influenced by a layer of mud rich in P in its various forms, and from which this element recirculates obeying difussional processes, has showed high concentrations of PO~-, while the surface layers have become notably depleted as a result of the strong consumption by algae, and the precipitation and temporary stagnation as insoluble compounds (FePO~, Ca:HPO4(OH):, Ca~(PO4)~(OH)). The values of this parameter at the surface are quite satisfactory, even from the point of view of its pouring into public water bodies. Given its future utilization for irrigation, the orthophosphate concentrations of the effluent impose no problems at all, and its presence, as well as that of nitrogen, gives the effluent an additional fertilizing character. At the beginning of the cycle there existed strong vertical mixing conditions (autumn-winter), which evolved until reaching a marked stratification during the spring-summer months; at the end of the cycle homogeneity conditions were again recovered. During the months when the mixing conditions took place, the orthophosphate concentration was almost the same throughout the water column (Fig. 9), nevertheless, during the months when stratification occurred, a gradual enrichment in the deep layers, and a depletion at the surface layers were observed (Fig. 9). This depletion is chiefly due to the two aforementioned phenomena, namely, the consumption by phytoplankton and the precipitation as insoluble compounds. The maximum and minimum treatment efflciencies during this period were 99.0 and 40.0%, respectively. Total phosphorus. The evolution of this parameter throughout the cycle is similar to that of the orthophosphate, since most total phosphorus is found as orthophosphate. The maximum and minimum treatment efflciencies were 92.9 and 42.4%, respectively.

1.1; and faecal streptococci, 1.7; corresponding to removal efflciencies of 91, 92 and 98%, respectively. During the warmer months (April-September) log reductions were higher: 3.0, 3.0 and 3.3, respectively; corresponding to removal efflciencies of 99.9, 99.9 and 99.95%, respectively. A stronger persistence of these microorganisms was always observed at the bottom layers. CONCLUSIONS The effectiveness of deep ponds has clearly been established. The treatment eliiciencies have been high, even in cold periods, though the higher percentages have been obtained during the summer months. A marked period of thermal stratification with the appearance of a clearly differentiated epilimnion and hypolimnion has been observed. The homogeneity of physicochemical characteristics inside the water body is influenced by stratification. The distribution and variation of such characteristics (dissolved oxygen, pH, B O D , COD, ammonia nitrogen, TKN, nitrates, nitrites, orthophosphate, total phosphorus) have been showed. The effluent water of this system has the following characteristics: --high removal eflicicncy of the organic loading, with maximum and minimum BOD5 efficiencies of 97 and 83% (corresponding to 8.3 and 46.7 mg/l, respectively); and 92 and 71% (42 and 157 rag~I, respectively) for the COD (Figs 5 and 6); - - a n oxygen concentration nearly equal, or superior, to saturation, from April to September (Fig. 3); --the content of nutrients oscillated along the cycle; the maximum and minimum values for orthophosphate were 12.2 and 0.3 mg PO~- (40 and 99% removal efflciencies, respectively), and 15.3 and 0.1 mg N/I for the ammonia nitrogen (38.8 and 99.9%, respectively) (Figs 8 and 9); --log reduction of microorganisms varied between: 2.0-3.0 for total coliforms, I.I-3.0 for faecal coliforms, and 1.7-3.3 for faecal streptococci (removal efficiencies between: 91-99.9, 92-99.9 and 98-99.95%, respectively). Acknowledgement--This paper was supported by project Nos 18/A.G.-82 and PBT87-0014 from the Comisi6n Interministerial de Ciencia y Tecnologia (CICYT) of Spain.

REFERENCES

Microbiological characteristics Two periods can be differentiated during which the efficiency of the pond in removing the microorganisms was different. During the months with low temperatures (December-February) the effluent concentration of these microorganisms was high and log reductions were: total coliforms, 2.0; faecal coliforms,

Abeliovich A. (1983) The effects of unbalanced ammonia and BOD concentrations on oxidation ponds. Wat. Res. 17(3). 299-301. APHA. AWWA, WPCF (1985) Standard Methods for the Examination of Water and Wastewater, 16th edition. AWWA-APHA-WPCF, Washington, D.C. Bern:i L. M., TorreUa F., Soler A.. S:iez L. LIorens M. and Martinez !. (1986) Estudio de la autodepuraci6n

Thermal stratification in wastewater ponds microbiol6gica y fisico-quimica de las aguas residuales por lagunaje proftmdo. Anales Biol. 10~2), 49-59 (Universidad de Murcia). Fritz J. J., Middleton A. C. and Meredith D. D. (1979) Dynamic process modelling of wastewater stabilization ponds. J. War Pollur Control Fed. $1(11). 2724-2743. Jorgensen B. B., Kuenen J. G. and Cohen Y. (1979) Microbial transformations of sulfur compounds in a stratified lake (Solar Lake, Sinai). Limnol. Oceanogr. 24~5), 799-822. Moreno M. D. (1983) Aplicaci6n de estanques de estabilizaci6n profundos a la depuraci6n de aguas residuales urbanas. Tesis Doctoral, Universidad de Murcia. Espafia. Moreno M. D., Soler A,, Sfiez J. and Moreno J. (1984a) Thermal simulation of deep stabilization ponds. Trib. Cebedeau 491(37). 403-410. Moreno M. D., Soler A., Sfiez J. and Romera P. (1984b) Study of the hydrodynamic behaviour of a deep waste stabilization pond. Trib. Cebedeau 489-490(37), 323-328. Moreno M. D., Medina M. A., Moreno J., Soler A. and Saiez J. (1988) Modelling the performance of deep stabilization ponds. War Resour. Bull. 20~2). 377-380. Rodier J. (1981) L'analyse de I'eau, Eaux Naturelh's, Eaux Residuaires, Eau de Mer. Bordas, Paris.

577

Schertzer W. H., Saylor J. H., Boyce F. M., Robertson D. G. and Rosa F. (1987) Seasonal thermal cycle of Lake Erie. J. Great Lakes ICes. 13(4), 468-486. Seki H., Takahashi M., Hara Y. and lchimura S. (1980) Dynamic of dissolved oxygen during algal bloom in lake Kasumigaura, Japan. War Res. 14, 179-183. Silva S. A., Mara D. D. and De Oliveira R. (1987) The performance of a series of five waste stabilization ponds in northeast Brazil. (Proceedings of an IAWPRC International Conference on Waste Stabilization Ponds, Lisbon). Wat. Sci. TechnoL 19(12). Simons T. J. and Schertzer W. M. (1987) Stratification, currents and upwelling in Lake Ontario, Summer 1982. Can. J. Fish. Aquar Sci. 44(12), 2047-2058. Soler A., S~iez J,, Llorens M., Martinez I., Bcrn~ L. M. and TorreUa F. (1988) Evolucibn de los par~imetros fisico-quimicos y microbiol6gicos en la depuracibn de aguas residuales por lagunaje profundo. Tecnol. Agua 48, 52-58. Soler A., Shez J., Llorens M., Martinez 1., Bernh L. M. and Torrella F. (1991) Changes in physicochemical parameters and photosynthetic microorganisms in a deep wastewater self-deputation lagoon. War Res. 25(6), 689-695. WHO (1977) Directh'es Applicables ,i la Surveillance Sanitaire de la Qualite des Eaux Littorales. O.M.S., Copenhagen.