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- Authority of reservoires of Saxony, Box 94, 01782 Pirna, Germany
- Technical University of Dresden, Laboratory of Hydrobiology, 01062 Dresden, Germany
Pre-reservoirs are small reservoirs, with a water-retention time of a few days,
that reduce the phosphorus input in main reservoirs. The process of phosphorus
removal involves the biochemical conversion from the dissolved to the particulate
form (mainly phytoplankton) and the sedimentation of this particulate matter.
The input variables are light, orthophosphate concentration, temperature of
the inflowing water and discharge. The phytoplankton activity plays the most
important role among the various processes. The maximization of orthophosphate
elimination depends on adequate design, construction and operation of pre-reservoirs.
A simple calculation procedure for the removal rates of orthophosphate-P has
been developed. The efficiency of pre-reservoirs is limited, because the light
intensity and the temperature during the winter period are low; on the other
hand, discharge is often high in March and April (snowmelt). Although pre-reservoirs
are an important tool for reservoir water-quality management, but they cannot
substitute remedial action in the catchment area.
Design of pre-reservoirs; eutrophication; model calculation; nitrogen-elimination;
orthophosphate-elimination; phytoplankton; pre-reservoirs; reservoirs; retention
time; water-quality management.
The dense population of Germany requires a highly efficient management of water
ressources. The available discharge in the several regions shows large seasonal
fluctuations. Therefore, a high water-utilization ratio had to be ensured by
dam construction in the river basins in the German hill regions.
An important field of the uses from reservoirs in Germany is the drinking water
supply. About seventy reservoirs with a total storage capacity of nearly 1100
Mio m3 are used for it. Due to the geography and socio-economic conditions in
Saxony, practically all of the catchment areas for these reservoirs are suffering
from anthropogenic influences. Consequently, there are quite severe water-quality
problems in several reservoirs. The water quality of a reservoir reflects the
physical and geochemical structure of the watershed and is primarily affected
by nutrients from both diffuse and point sources.
The objectives of the water quality management of reservoirs involve:
- protection of water quality against deterioration
- improvement of water quality as required to comply with the standards for
the uses.
As far as eutrophication of these reservoirs is concerned, the objectives can
be achieved by two main strategies:
1 Control of external loading
- sewage treatment plants including P-removal;
- diversion of sewage from the catchment area;
- proclamation of the catchment area as drinking water protective area with
control of agriculture etc.;
- construction of pre-reservoirs.
2 Control of internal mechanisms
- hypolimnion aeration;
- artificial destratification;
- P-precipitation;
- sediment dredging;
- control of the internal ecosystem structure ("Biomanipulation").
The ability of pre-reservoirs to remove nutrients became known during numerous
investigations on various existing pre-reservoirs in Germany (Klapper 1957;
Beuschold 1966; Wilhelmus et al. 1978; Fischer 1980) and was later confirmed
by investigations in Denmark (Nyholm et al. 1978) and in the former Czechoslovakia
(Fiala and Vasata 1982).
Pre-reservoirs are comparatively small reservoirs with an average water retention
time of a few days. They are normally situated immediately above the larger
main reservoirs whose inflowing water quality it is their purpose to improve
(Benndorf and Pütz 1987). This improvement of the water quality is the
result of a number of physicochemical and biochemical processes within the pre-reservoirs
(Fig.1). The first stage in the process of nutrient removal in pre-reservoirs
involves the biochemical conversion from the dissolved to the particulate form
(mainly phytoplankton). The second stage is the sedimentation of phytoplankton
and other particulate matters within the pre-reservoir or in the shallow inlet
sections of the main reservoir. This sedimentation process is enhanced by the
presence of natural precipitants and flocculants. In this context, it must be
stressed that geochemical conditions in the drainage area can affect nutrient
removal by influencing the nature and intensity of the processes involved, which
are listet in Fig.1. Chemical binding or adsorption of the orthophosphate in
solution can take place largely in the inflowing waters, but the uptake of orthophosphate
by the algae is more important in the pre-reservoirs than the competing chemical
or physicochemical processes, particularly in the ph range of 6.0 - 8.0. The
greater the deviation of the pH from this range, the more likely it is that
the orthophosphate will combine with iron, aluminium and manganese (at pH <
6) or calcium (at pH > 8) (Vollenweider 1976).
The sedimentation process is not only enhanced by precipitants and flocculants,
but also by an appropriate plankton structure within the pre-reservoir. It is
favourable, if the phytoplankton consists of algae having a high sedimentation
velocity (above all diatoms), but blue-green algae are highly undesired. Mass
developments of zoo-plankton, especially of effective filter feeders such as
Daphnia, must be avoided because of the high grazing losses of phytoplankton
and the resultant high intensity of the nutrient remineralization. Both the
desired phytoplankton structure and the absence of planktonic crustaceans can
be achieved by an optimal water retention time within the pre-reservoir, which
allows diatoms and other fast-growing algae to grow but flushes away slow-growing
blue-greens and zooplankters. The critical retention time amounts to nearly
2 days in the months of summer, about 4 to 8 days in spring and autumn as well
as more than 20 days in the months of winter. Furthermore, the absence of effective
filter feeders is supported by an appropriate structure of the fish community.
No predatory fish species should be present in pre-reservoirs in order to guarantee
the maximum possible biomass of small zoo-planktonfeeding fish (Nyholm et al.
1978;Benndorf et al. 1983).
Taking into consideration all these processes, some important demands can be
set up for the optimal management of pre-reservoirs:
- the design of the size of a pre-reservoir should aim at an optimum retention
time (and not simply at a long retention time).
- the mean depth of a pre-reservoir should not exceed considerably the depth
of the euphotic zone (zeu, in the most pre-reservoirs about 3 m) because of
the dominance of photoautotrophic mechanisms governing the phosphate removal.
Phosphate elimination decreases exponentially with increasing depth (Uhlmann
and Benndorf 1980).
- if the maximum depth of the pre-reservoir exceeds the mixing depth zmix
as well as zeu - which applies at least at times to almost all
existing pre-reservoirs - a strong vertical orthophosphate stratification
results. This stratification exhibits high concentrations in the deep water
layers and the and the lowest concentrations near the surface. Therefore,
surface release is an urgent need in the management of pre-reservoirs.
- the bottom sediment is to be removed in time intervals of 5 - 10 years (after
emptying the pre-reservoir through bottom outlet).
- optimal management of pre-reservoirs in the sense of this paper primarily
aims at the realization of the maximum possible P-removal. This is in almost
all cases identical with the maximum possible N-removal. The only exception
refers to denitrification, the maximization of which cannot be achieved by
the same management strategy as used for a maximum P-removal.
Fig. 1. Processes governing the phosphorus and nitrogen elimination in
pre-reservoires, see Benndorf and Pütz (1987), slightly altred.
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A procedure for calculating and predicting the removal rates of orthophosphate-P
in pre-reservoirs has been developed (Benndorf et al. 1975) and was further
refined by a subdivision between lightly loaded and heavily loaded pre-reservoirs
(Benndorf and Pütz 1987). This procedure has been successfully as a standard
practice for water quality management of the reservoirs in Germany. It can be
used for calculating the removal rates of P in pre-reservoirs for a given volume
of reaction space, but also to find the optimal volume for a desired removal
rate, that means the procedure can be carried out as predictions for planned
pre-reservoirs as well as for existing pre- reservoirs (Pütz 1995).
There are several input variables as shown below:
Meteorological variables
Light
In connection with the underwater light climate in the pre-reservoir the following
definitions apply: the euphotic zone depth (zeu) is considered to be
3 m, the mean light intensity (MI) in the euphotic layer amounts to 20 % of the
photosynthetically active radiation (400-700 nm) at the surface. Only this euphotic
layer is assumed to be the "reaction space" for the P-elimination process. The
seasonal variation of MI decreases with decreasing geographical latitude and has
to be re-estimated for each geographical location.
Temperature
The mean monthly water temperatures MT in the "reaction space" are calculated
on the basis of multi-year monthly averages of the inflow temperature (MTz)
according to MT = MTz + a (0°C < a < 2°C, depending on season
and retention time).
Hydrological variables
Discharge
The discharge determines the retention time of the inflowing water in the "reaction
space". The probabilistic character of the hydrological processes is considered
by means of the discharge duration curve for the individual months.
Hydrochemical variables
Concentration of orthophosphate-phosphorus
For the calculation, the concentration of orthophosphate-P in the affluent is
employed. This value should be established as a separate monthly average for the
entire 12 months, taking into consideration the discharge duration curve and the
discharge-orthophosphate relationship.
The complete calculation procedure for P-elimination in a pre-reservoir is
described by Benndorf and Pütz (1987).
The subdivision between lightly-loaded and heavily-loaded pre-reservoirs allows
an estimation of the maximum possible areal elimination rate for orthophosphate-P
on the basis of an empirical relationship between areal rate and mean depth.
Heavily-loaded pre-reservoirs require very large areas, and are, therefore,
uneconomic. The majority of the pre-reservoirs of Saxonian drinking-water reservoirs
are lightly loaded.
A simple calculation procedure of the nitrogen elimination similar to that
of the orthophosphate elimination can hardly be developed because of the much
more complicated nature or the nitrogen metabolism in waters. There are two
possibilities to predict the elimination rate of the inorganic nitrogen, namely:
- the construction of a rather sophisticated ecological model involving all essential ocesses which control the nitrogen metabolism within the pre-reservoir;
- the construction of a simple empirical approach which results from the generalization of field observations (Pütz 1995).
The development of a sophisticated ecological model is expensive. Therefore,
the second way showing sufficient results, is demonstrated in Fig. 2. This plot
makes it possible to estimate roughly the removal rate of the inorganic nitrogen
as a function of the mean retention time and of the N:P mass ratio.
Experiments in the Hydraulic Laboratory of the Technical University of Dresden
(Benndorf et al. 1975) have helped to establish some important construction
principles for pre-reservoires that secure a sufficient utilization of the "reaction
space" (Fig. 3).
The sedimentation of suspended solids in the inflowing water near the inlet
prevents the rapid decrease of the "reaction space". A sill (with the crown
about 0,70 m below water surface) separates this "sedimentation space" from
the rest of the pre-reservoir. If the length : width ratio of the pre-reservoir
is less than 2 : 1, the installation of a spillway improves a steady throughflow.
The water is released over the spillway (overflow) and the bottom outlet is
closed (Fig. 3).
Annual mean elimination rates calculated with the pre-reservoir model described
above were checked by means of a comparison between observed and calculated
P-elimination rates in 15 pre-reservoirs two different investigation periods
(1963-1972 (Benndorf et al. 1975) and 1974-1982 (Benndorf and Pütz 1987a)).
The results showed a considerable degree of accuracy. Another comparison between
calculated and observed annual P-removal rates in 11 Saxonian pre-reservoirs
(commissioned after 1972, investigation period 1991-1996) shows a good fit (Table
1). The observed mean elimination rates for total phosphorus are relatively
high in a number of cases, though lower than for orthophosphate-phosphorus.
Table 1 Calculated and observed annual P-elimination of Saxonian pre-reservoirs
in the period 1991-1996
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Main-reservoir
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Pre-reservoir
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Retention- time
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Elimin. o-PO4
calculation
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Elimin. o-PO4
observed
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Elimin. Ptot
observed
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d
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%
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%
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%
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Lichtenberg
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Dittersbach
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2,0
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22
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45
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34
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Eibenstock
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Schönheide
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3,0
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36
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40
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25
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Gottleuba
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Gottleuba
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3,7
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43
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34
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24
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Lichtenberg
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Lichtenberg
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4,5
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48
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44
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30
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Eibenstock
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Rohrbach
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5,0
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50
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42
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22
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Saidenbach
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Forchheim
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6,0
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55
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57
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35
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Eibenstock
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Rähmerbach
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6,3
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56
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56
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40
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Eibenstock
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Weißbach
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7,0
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57
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60
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42
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Eibenstock
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Geidenbach
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7,0
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57
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64
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44
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Dröda
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Ramoldsreuth
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10,0
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60
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53
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41
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Dröda
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Bobenneuk.
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12,0
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61
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62
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46
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The water quality management of the largest (drinking water) reservoir in Saxony,
the Eibenstock-dam (75 million m3, five pre-reservoirs), demonstrates
the importance but also the limits of pre-reservoirs. The objective of water
quality set for the main reservoir was to achieve mesotrophic to oligotrophic
conditions in a recovery process. Productivity is limited by the nutrient phosphorus
and the necessary P-load reduction was to be achieved by the implementation
of a masterplan including:
- sewage diversion out of the watershed;
- conctruction or upgrading of sewage treatment plants;
- additional measures by agriculture
- five pre-reservoirs.
The design of these five pre-reservoirs was based on calculations from the
pre-reservoir model described above Benndorf et al. (1975) and Benndorf and
Pütz (1987 and 1987a). Figure 4 shows their actual P-elimination performance
and its limitations.
Nearly 60 % of the P-input from the tributaries occurs in the period November-April,
nearly 40 % in the period May-October, but the removal rates in the first period
are substantially lower than in the second period. Therefore, the P-input into
the main reservoir during the winter and spring period is relatively high. Consequently
the integral implementation of the entire water-quality masterplan is necessary.
- The phytoplankton activity plays the most
important role among the various processes governing the phosphate elimination
in pre-reservoires;
- The maximization of orthophosphate-P elimination
depends on adequate design, construction and operation of pre-reservoirs:
- a relatively low mean depth;
- a relatively large reaction space, (upper 3 m zone), in relationship to the
total volume with a mean retention time of a few days;
- a constant storage level as a consequence of surface release;
- an optimum size.
- A simple calculation procedure for the monthly
mean removal rates of orthophosphate-P in pre-reservoirs has been developed
on the basis of laboratory experiments, the results of which were combined with
the probabilistic distribution of the water throughflow;
- The efficiency of the pre-reservoirs is limited,
becaus the light-intensity and the temperature in the winter - period are low,
on the other hand first of all in March and April, the discharge is often high
in result of thaw.
- Also pre-reservoirs are an important tool
for reservoir water-quality management, they can be no substitute for remedial
action in the catchment area.
Benndorf J., Pütz K., Krinitz H. and Henke W. (1975). Die Funktion der
Vorsperren zum Schutz der Talsperren vor Eutrophierung. Wasserwirtschaft
Wassertechnik 25, 19-25.
Benndorf J., Kneschke H., Kossatz K. and Penz E. (1983) Manipulation of the
pelagic food web by stocking with predacious fish. Int. Revue ges. Hydrobiol.
68, 407-428.
Benndorf J. and Pütz K. (1987). Control of eutrophication of lakes and
reservoires by means of pre-dams - I. Mode of operation and calculation of
the nutrient elimination capacity. Wat. Res. 21, 829-838.
Benndorf J. and Pütz K. (1987a). Control of eutrophication of lakes
and reservoires by means of pre-dams - II. Validation of the ohosphate removal
model and size optimization. Wat. Res. 21, 839-842.
Beuschold E. (1966). Entwicklungszendenzen der Wasserbeschaffenheit in den
Ostharztalsperren. Wiss. Zeitschr. Karl-Marx-Univ. Leipzig, Math.-Nat.
Reihe 15, 853-869.
Fiala L. and Vasata P.(1982). Phosphorus reduction in a man-made lake by
means of a small reservoir in the inflow. Arch. Hydrobiol. 94, 24-37.
Fischer J.(1980). Nährstoffelimination mittels räumbarer Vorbecken
an Staugewässern. Wasser und Boden 32, 170-173.
Klapper H. (1957). Biologische Untersuchungen an den Einläufen und Vorbecken
der Saidenbachtalsperre (Erzgeb.). Wiss. Zeitschr. Karl-Marx-Univ. Leipzig,
Math.-Nat. Reihe 7, 11-47.
Nyholm N., Sorensen P.E., Olrik K. and Pedersen S.D.(1978). Restoration of
Lake Nakskov Indrefjord Denmark, using algal ponds to remove nutrients from
inflowing river water. Prog. Wat. Technol. 10, 881-892.
Pütz K. (1995). The importance of pre-reservoirs for the water quality
management of reservoirs. J Water SRT-Aqua Vol 44, Suppl. 1, 50-55.
Uhlmann D. and Benndorf J. (1980). The use of primary reservoires to control
eutrophication caused by nutrient inflows from non point sources. In Land
Use Impacts on Lake and Reservoir Ecosystems. 152-188. Proceedings of
a regional workshop on MAB Project 5, Warsaw. Facultas, Wien.
Vollenweider R. A. (1976). Advances in defining critical loading levels for
phosphorus in lake eutrophication. Memorie Ist. ital. Idrobiol. 33,
53-83.
Wilhelmus B., Bernhardt H. and Neumann D. (1978). Vergleichende Untersuchungen
über die Phosphor-Eliminierung von Vorsperren. DVGW-Schriftenreihe
Wasser Nr. 16, 140-176.
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