Envlron. Sci. Technol. 1994, 28, 1410-1416
Restoration Concept for Lake Tegel, a Major Drinking and Bathing Water Resource in a Densely Populated Area Bernd Heinzmann' Berliner Wasser-Betriebe, Buddestrasse 33, D-13507 Berlin, Germany
Ingrid Chorus Institut fur Wasser-, Boden-, und Lufthygiene, Corrensplatz 1, 0-14195 Berlin, Germany
Although Berlin is rich in lakes and rivers, discharge rates are low. The water bodies are intensively used in many ways-for gaining drinking water from bank-filtered water, for recreational purposes, as recipients for treated sewage and stormwater, and as waterways. The example of heavily eutrophicated Lake Tegel shows the development of protection and restoration measures for securing surface water quality, especially with respect to drinking water and recreation. Numerous measures were undertaken to improve the water quality of Lake Tegel, such as replacement of oversaturated wastewater irrigation fields by sewage treatment plants, partial diversion of a heavily loaded inlet, treatment of stormwater runoff, further sewerage, aeration, sludge dredging in the harbor area, protection of the remaining reed belt, and construction of a phosphorus elimination plant directly at the major inlet of the lake. By a four-step process, this plant is capable of reducing total phosphorus (P)input concentrations from 5 mg/L P or more down to approximately 0.02 mg/L P and, therefore, proved to be the most important measure. By providing enough water with a low phosphorus content for exchanging the lake volume three to four times a year, this elimination plant caused a decline of phosphorus concentration in the lake from roughly 0.8 mg/L P in 1984 down to the present values of between 0.06 and 0.13 mg/L P (annual means). The algae are beginning to respond to this decrease by a reduction of maxima and an increase in water transparency. The Berlin experience shows that treatment of the inflowwas the correct option in a situation where diffuse sources cannot yet be sufficiently controlled.
Introduction Berlin is located in a lowland region rich in lakes and rivers. However, the rivers have low discharge rates, and treated sewage contributes a high share of the water they carry. During dry summers, the discharge of the River Spree calculates to approximately the same volume as the discharge of the sewage treatment plants which drain into it. Meanwhile, after reunification, the metropolis comprises roughly4 million inhabitants, who use surface waters for recreational purposes and as a reservoir for drinking water. Drinking water is gained through artificial and natural groundwater recharge and especiallythrough bank filtration, a concept widely applied in Germany. Deep wells (in Berlin 70-170 m) situated along the shorelines collect water that has been filtered through the underground for approximately 2 months. Although experience shows that this proceedure eliminates or degrades a number of contaminants quite effectively, surface water of high quality is the best safeguard against the breakthrough of undesirable substances. 1410
Environ. Sci. Technol., Vol. 28, No. 8, 1994
The intensive use of surface waters requires thorough and comprehensive management strategies. During the decades of the Berlin Wall, the achievement and maintenance of self-sufficiency of the drinking water supply for the 2 million inhabitants of West Berlin presented a high management challenge. Also, nearby alternatives to the West Berlin lakes and rivers were not accessible for recreational purposes. Quite a variety of management options were applied during the past decades to counteract increasingly heavy eutrophication. In the following,these will be demonstrated and evaluated using the example of Lake Tegel, which is one of the most important water resources of the city. Lake Tegel, One of Berlin's Important Water Resources Lake Tegel (Figure 1)is situated in the northwest part of Berlin. With its surface area of 4 km2, it is one of the larger lakes within the city. It is chiefly supplied by two heavily polluted brooks, both of which drained wastewater irrigation fields since the beginning of the century until recently: the Tegeler Fliess (catchment area of 124 km2) and the Channel Nordgraben (catchment area of 27 km2). An increasing amount of housing connected to the sewage progressively caused overloading of the wastewater irrigation fields and extremely high nutrient concentrations in their discharge. Thus, during 1972-1983, the Nordgraben carried annual loads of total phosphorus amounting to 130-380 t of P, and the Tegeler Fliess supplied roughly 50 t of P (Heinzmann & Jahn, unpublished). This rapidly lead to hypertrophy. Nonetheless, Lake Tegel is intensively used for recreation. Further, it is surrounded by 130vertically oriented wells of 20-60 m depth, and one horizontal filter well is situated on the island of Scharfenberg. These have a capacity of gaining roughly 370 000 m3/d of a mixture of bank filtered water and groundwater, which is sufficient to provide 20% of the city's drinking water and to supply 700 000 inhabitants. Despite the hypertrophic state of the lake, high drinking water quality could be maintained so far because extremely favorable hydrogeological conditions enable the effective elimination of many biogenic taste and odor substances as well as some pollutants during the passage of the water through the underground (I).No chlorination or other disinfection is necessary before the introduction of this water into the supply network because it is extracted from the wells in a very hygenically satisfactory condition. However, the breakthrough of organic substances produced especially in highly eutrophic surface waters can potentially impair the quality of bank-filtered water seriously. Not all are easily biodegradable (21, and some volatile substances produced by algae, such as geosmin, 0013-936X/94/0928-1410$04.50/0
0 1994 American Chemical Soclety
. .. . . .. .... .. . . . .
Lake Tcgd area: mean depth mar.depth
4 lunl 6.6 m 15m wlume 24.6 mia. ' m retention time 0.3 - 0.15 B
.
wasie
,
. ..:.. .. .;.... . p:
)#-
Flgure 2. Phosphorus input into Lake Tegel (solid curve), phosphorus diversion, and phosphorus elmination by the plant at the lake's inlet.
Figure 1. Lake Tegel with major inlets, connection to the Have1River, the settlement which is now connected to >90% to the sewerage, and water management installations.
are organoleptically detectable at concentrations of 10-20 ng/L (3). Frequently, organic compounds are not eliminated by treatment, and chlorination may transform some into organoleptically very unpleasant substances (e.g., ref 4). In afew cases during the 1970s,musty tastes appeared in the drinking water produced by the Tegel water works, especially after disinfection with a higher dosage of chlorine. Precautionary chlorination regularly carried out earlier was terminated in 1978. This reduced flavor impairment problems, but the problem had called attention to the urgent necessity of restoring Lake Tegel for the protection of this important drinking water resource, and concepts were developed toward the end of the 1970s (5, 6). Measures for Water Quality Improvement in Lake Tege1 The Vollenweider (7) model shows that, for a shallow lake such as Lake Tegel, annual total phosphorus input must be reduced below 0.20-0.35 g of P/m2.a of lake surface total. For Lake Tegel, with a surface area of 4 km2, this limits input to 800-1400 kg/a. P input by precipitation upon the lake surface amounts to about 300 kg/a (a),and to date the reduction of this input is scarcely within reach. A further amount of 200-300 kg/a reaches the lake via surface runoff. The chief input, however, comes from the main inlets Nordgraben and Tegeler Fliess. The concept for counteracting eutrophication attempts an extensive reduction of this phosphorus input. The set of measures undertaken to restore Lake Tegel is described in the following. Some of them were transient or preliminary, and their individual effect on phosphorus retention cannot be quantified because more than one measure went into action simultaneously. (1)From 1973to 1986,364 million m3 of heavily loaded Nordgraben water was diverted from the lake. This amounted to 30-50% of the load carried by this channel. In absolute numbers, this corresponded to a phosphorus load of 70-150 t/year, or in relation to Lake Tegel, 17.537.5 g m-2year1, up to 1981. Through diversion,the annual load decreased to 25 t (or 6.3 g m-2 year-l) in 1985 (Figure 2). Costs amounted to 6 million DM (Deutsche Mark) for
construction and 1.3 million DM for operation of the pumping station. (2) Partial diversion of sewage from the wastewater irrigation fields discharging into the Nordgraben was achieved by 1985 through an expansion of the capacity of the treatment plant at Ruhleben from 75 000 to 240 000 m3/d (costs amounted to 460 million DM). (3) Between 1978 and 1985, more than 90% of the settlements at the southwestern end of the lake were connected to the sewage system. Previous loading by seepage or illegal discharges from septic tanks may be assumed, although this cannot be quantified. Costs amounted to 161 million DM, and 30 million DM are still required to complete the sewage system. (4) In 1980,15 aerators were installed for compensation of oxygen deficiency, which had at times almost reached the surface. Together, they have a nominal capacity of introducing 4.5 t of oxygen/day. Although intended as hypolimnetic aerators, their operation precluded stable thermal stratification in almost every summer. Investment costs were 3 million DM, and full operation required electricity for 0.9 million DM per year. Since 1989, the operation took place only occasionally,when hypolimnetic oxygen concentrations had begun to decline. Thus, stratification was stable except in 1991, where weather conditions caused mixing. (5) For maintenance of the harbor, sludge was dreged from the northeastern bay of the lake (6.4% of the lake area) at the main inflow in 1986 at a cost of 1.8 million DM. (6) In 1987-1989, a total of 8.3 million DM was invested for the installation of stormwater tanks for the sedimentation of stormwater runoff. This removes nearly half of the phosphorus input via stormwater flow from the separate sewer system. (7) An additional sewage treatment plant for the northeast part of the city went into operation in 1985and completelyreplaced the treatment on wastewater irrigation fields. After simultaneous precipitation was begun in 1986, phosphorus concentrations in the discharge of this plant remained below 2 mg/L P. This significantly reduced the load carried by the Nordgraben. Costs for the West German support of this measure (which was undertaken by East Germany) amounted to roughly 70 million DM. (8)Measures for the mechanical protection of the already strongly reduced reed belts were begun in 1970, together with the replanting of several species. To date, roughly 3 million DM were invested in this. (9) The chief measure undertaken directly for the restoration of Lake Tegel was the construction of a Environ. Sci. Technol., Vol. 28, No. 8, 1994 1411
phosphorus elimination plant designed to treat the total discharge of Nordgraben and Tegeler Fliess immediately before their inflow into the lake (Figure 1). Its maximal capacity of 6 m3/sis sufficient to handle medium floodwater amounts. In order to secure a minimal throughflow of 3 m3/seven in times of minimal discharge,when bothTegeler Fliess and Nordgraben together carry only 1m3/s, a double pipeline was installed at the lake bottom. This can be used both to supply the phosphorus elimination plant with water from the Have1 River with up to 2.4 m3/s and to divert floodwaters that excede the plant’s maximal capacity of 6 m3/s. The phosphorus elimination plant is described in more detail below (9).
Engineering Concept f o r Phosphorus Elimination Plant
Table 1. Annual Means and Standard Deviations for Phosphorus and Particulate Matter*
input
output part. PT POd-Pr mat. PT PTr PO4-Pf (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) (mg/L) 1986
mean S
Results Operation of Phosphorus Elimination Plant. Table 1shows that, within 2 years of operation, the phosphorus elimination plant was optimized to a total phosphorus output around 0.02 mg/L with low deviations. Thus, it reduced phosphorus concentrations down to between 1/14th and 1/30th of input concentrations. Also, concentrations of organic matter, chemical oxygen demand, bacterial counts, and heavy metals are reduced by varying percentages (Table 2). The reduction of bacterial numbers is especially significant for bathing water hygiene, as maxima carried by the Nordgraben reached 1.6 105/mLin 1988. Construction of the plant cost 200 million DM, and costs of operation add up to not quite 20 million DM/year. The capital costs for amortization of this sum amount to 15 1412
Environ. Scl. Technol., Vol. 28, No. 8, 1994
0.27 0.11
22.6 8.4
0.080 0.057
0.021 0.006
0.006 0.007
0.55 0.22
0.20 0.06
12.2 7.7
0.040 0.015
0.016 0.006
0.003
0.68 0.89
0.16 0.07
19.3
0.022 0,009
0.011 0.004
0.002
31.3
0.59 0.30
0.22 0.18
13.4 6.2
0.023 0.009
0.012 0.005
0.002 0.003
0.48 0.21
0.16 0.09
11.8 5.5
0.024 0.008
0.011 0.003
0.002 0.001
0.29 0.12
0.09 0.10
11.1 5.0
0.019 0.011
0.009
0.003
0.002 0.001
0.30 0.15
0.12 0.10
11.5 6.3
0.021 0.009
0.008 0.003
0.002 0.001
1987
mean S
0.002
1988
mean S
0.001
1989
mean S
Laboratory and pilot plant experiments showed that a four-step process is capable of reducing the load of even heavily polluted water containing several mg/L P and up to 520 mg/L particulate substances (10): (1)precipitation/ coagulation/flocculation with Al(II1) and/or Fe(II1) salts; (2) sedimentation; (3) postprecipitation/postflocculation with Fe(II1) salt; and (4) filtration. A number of problems specific for the site had to be solved: (1) enormous stormwater runoff maxima; (2) varying water volumes; (3) extremely variable water quality; (4) at times, extremely high content of sludge with high viscosity carried by the Nordgraben; (5) three inputs of different water quality, partially containing sewage treated only mechanically and biologically; (6) the restricted area of the construction site (within the city walls); (7) the immediate neighborhood to a housing area; and (8) the necessity of being flexible and of maintaining possibilities for experimenting and optimizing during operation of the plant, as some problems of this new process could only be solved “on the job” in the course of gathering full-scale experience. Construction began in 1981. At the same time, a very similar but smaller phosphorus elimination plant went into operation at the inflow of asmall lake in the southwest part of the city-Schlachtensee (11). Some of the experience gathered there could be considered for improvements during construction of the large phosphorus elimination plant at Tegel. Construction at Tegel was completed in August 1985. This plant now operates with a discharge of 3 m3/s or more and, thus, flushes the lake roughly three times per year with treated surface water.
0.90 0.23
1990
mean S
1991
mean S
1992
mean S
s, standard deviation; PT,total phosphorus in unfiltered samples after breaking down with acid and oxidizing agent; Ply, dissolved total phosphorus in filtered samples (0.45-pm membrane filters) after breaking down with acid and oxidizing agent; POd-Pf,soluble reactive phosphorus (phosphate) in filtered samples; part. mat., particulate matter.
million DM/year (at an interest rate of 6%). Together with the costs of operation (Table 3) and the amount of surface water treated, this amounts to a price (per m3 treated surface water) of 0.36 DM in 1990 and 0.37 DM in 1991. For comparison, the water price in the western part of Berlin presently is at 5.20 DM /m3. This includes drinking water and wastewater (wastewater treatment with enhanced biological phosphorus removal, at times also with simultaneous P precipitation, but not filtration). Response of Lake. Temperature and oxygen concentrations for 7 years before the installation of the aerators show that stratification was stable with at least 8 “C difference between the surface and 13 m depth by the end of June (unpublished data of the Directorate of Fisheries, Berlin). Operation of the aerators has prevented stable stratification in most years since, but has not been able to prevent oxygen depletion in the water above the sediments (Figure 3). Reduced aeration allowed stable stratification in 1989, 1990, and 1992. In 1991 only, exceptionally rainy and windy weather during April and May prevented stratification almost entirely, even though the aerators were not operated. Data for total phosphorus concentrations in Lake Tegel are available only since 1984. Thus, the effect of measures taken previously cannot be assessed, but total P concentrations in the lake were still extremely high before the phosphorus elimination plant went into operation (Figure 4). As soon as the elimination plant started operating in the autumn of 1985, total phosphorus concentrations in the lake began to decline exponentially. A minimum of 60 pg/L P (annual mean) was reached in 1989. Depth curves indicate that some fluctuation during the following years is probably caused by release from the sediments, as the increase of concentrations in the near-bottom layers
Table 2. Elimination of Further Parameters by Phosphorus Elimination Plant in 1989 input annual mean
parameter spectral absorption coefficient at 254 nm (Urn) chemical oxygen demand (mg/L) dissolved organic carbon (mg/L) bacteria colony counts,
output variation coeff. ( % )
variation coeff. (%)
elimination (%)
20.2
13
14.5
17
28
40.8
22
22.5
18
45
11.8
16
8.9
17
25
8360
104
940
90
89
81 784
94 29
38 124
103 48
53 84
Table 3. Distribution of Costs of Operation from 1990 and 1991 1990
DM
annual mean
1991 %
DM
%
2488426 13 2511397 13 energy 2641776 14 2942822 15 chemicals 2915785 16 2826406 15 sludge 261664 1.5 288022 1.5 costa for screenings, tapwater, insurance,etc. 2696739 15 2644974 14 personnel 1010869 5.5 1422425 7.5 maintenance and repairs 6153177 33 6227983 32 sludge treatment in sewage treatment plant (personnel,maintenance, repairs, repayment and interest) business costa 418753 2.0 439621 2 sum of all costs of operation 18 587 189 100 19 303 650 100 92 503 556 amount of water treated (ma) 94 680 872
always occurred during summer at minimal benthic oxygen concentrations (Figure 5 ) . Probably, the elevation of benthic temperatures caused by aeration enhanced phosporus release. This assumption is demonstrated most clearly by the data for 1991, a year with very short periods of anoxia near the sediments but with high benthic temperatures: Phosphorus concentrations measured in 13 m were equally as high as during 1990, a year with more stable stratification and prolonged hypolimnetic anoxia. For reasons not yet understood, no release was measurable during 1989, in spite of stable stratification and anoxia above the sediments. The example of 1992 shows that, if stratification is prolonged and stable, some increase of hypolimnetic phosphorus concentrations (briefly even up to concentrations above 600 pg/L P) is possible without any significant transport upward to the epilimnion: concentrations in 0-8m depth were lower than in other years. Thus, aeration has shown no benefit for the phosphorus budget of the lake, presumably as it is not sufficient to prevent release from the sediments by maintaining them in an oxidized condition. However, turbulent mixing promoted by aeration transported phosphorus upward into the euphotic zone in which algae can make use of it for biomass production. As virtually untreated sewage no longer reaches the lake, oxygen deficiency is no longer a serious threat, and aeration during summer now appears to do more harm than help. Therefore, it is no longer employed during the summer; however,the aerators remain installed for emergencies. The response of the algae is not yet as dramatic as the range of decline of phosphorus concentrations. A decline in maximum algal densities has occurred: chlorophyll a
concentrations (as a measure of algal biomass) show that the previous maxima between 120 and 160 pg/L were no longer observed since 1987 (Figure 6). Also, whereas before 1990the summer maxima of more than 70 pg/L chlorophyll a were usually caused by blue-green algae, the high maxima observed in 1990 and in 1992 both were dominated by the very large flagellate species Ceratium spp. and not by bluegreens. Thus, the maxima of blue-green algae have not exceeded 70 pg/L chlorophyll a since 1989. For bathing water quality, this means that surface scums of blue-green algae such as Microcystis flos-aquae no longer develop to the noxious extent they had often reached before restoration (12). Annual means of chlorophyll a show some decline from 1984 to 1991 (Figure 4). The increase in 1992 does not reflect nutrient concentrations but rather the unusually fine weather and stable stratification. This caused some change in species composition of the algae by enhancing the growth of chlorophyceans, especially the species Coelastrum, during May and June (instead of the usual clear-water phase) and by allowing Ceratium spp. to become dominant during summer instead of Microcystis spp. Both contributed to augmenting the annual average, although Ceratium is clearly less of a nuisance for bathing than is Microcystis. Transparency, as measured with a secchi disc, shows a clear trend toward higher values since the treatment plant went into operation (Figure 7). It also shows that the somewhat increased chlorophyll a concentrations in 1992 were not relevant for the transparency of the water. This is because this chlorophyll was concentrated in large colonies of Coelastrum or in large cells of Ceratium, both of which allow more light to penetrate between them than would disperse, small-celled species.
Discussion An evaluation of the effect of the various measures undertaken to counteract eutrophication of Lake Tegel (see Table 4) must take into account that they address a variety of problems. Aeration, for instance, was installed primarily to counteract oxygen budget catastrophes. Although theoretically some effect upon the phosphorus budget may be possible via binding of phosphorus upon oxidized iron also during summer stratification, results showed that the oxygen input was too low to achieve this and that increased mixing caused by aeration tended to increase phosphorus release rates by elevating temperatures at the sediment/water interface as well as transport rates to the euphotic zone. Thus, as oxygen budget problems are declining, this measure is now being abanEnvlron. Sci. Technol., Vol. 28, No. 8. 1994
1413
1984
1985
1986
1987
1988
1989
1990
1991
........ 13m
*m
-1m
1991 I-
e24
la5
I_
t a l
,w
(BP
Ism
,Dsl
1-
Figure 4. Concentrations of total phosphorus (P total, ban) and chlorophyll a (Chl.4, curve) In the murw of restoration 01 Lake Tegel (annual m a n s at 1 m depth).
Flgure 5. Concentrationsof total phosphorus at the surface (0 m) and in near-botiom (13 m) water in the deepest basin of the lake.
doned. Similarly,motivations for reed belt protection and replanting did not primarily address the nutrient budget, but rather ecological diversity of habitats, and the primary purpose of sludge dredging was the maintenance of the harbor. Evaluation of the measures directly addressing loading is hampered by the difficulty of assessing the effect of someofthemand bythesimultaneousoccurrenceofothers. The diversion of Nordgraben water was one effort which caused a measurable decrease of loading to the lake, as is showninFigure2. However, itmustbe takenintoaccount
that this was no solution, but rather a temporary diversion of the load to another water body. The expansion of the sewage treatment plant at Ruhlehen and the construction of a further plant to replace the oversaturated wastewater irrigationfields wasdefinitely astep in the proper direction. However, at the present state of technology employed, the runoff of both still contains loads of 0.5-2.0 mg/L P, which is far above the tolerable limit for surface waters in Berlin. Thus, the effect of connection of settlements to the sewage system for Lake Tegel lies in the diversion of this water away from the lake to a treatment plant whose
1414 Envlron. SCI. Tsdnol.. Vol. 28. NO.8. 1994
180
11
I
800
I il
,
,
80
30 0
20
1984
1985
1988
,987
,988
,989
1990
,991
,992
Figun E. Chlomphytl a concentratlon in 1 m depth (monthly sampling from Jan 1984-May 1987 and from Jan 1991 to Dec 1992; mwne frequent sampling, up to weekly. from May 1987 to Nov 1990).
Figure 7. Transparency (measured as secchi depth; annual means together wlth minima and maxima 01 the vegetation period).
Table 4. Measures Taken for Improvement of Water Quality in Lake Teeel: Overview costs (DM) time span 1973-1986 1985
1987-1993 1980-1988 1986 1987-1989 1986
since 1970 autumn 1985
measure taken
for construction; operation
diversion of 3*50% of Nordgrahen expansion of the Ruhleben sewage treatment plant to partially replace sewage irrigation fields connection of settlements to sewage aeration
6 million; 1.3 million/s 460 million
dredging of sludge basins for sedimentation of stormwater runoff sewage treatment plant totally replacing use of wastewater irrigation fields mechanical protection of reed and replanting phosphorus elimination plant for total treatment of the main inflow
1.8 million 8.3 million
161 million 3 million; 0.9 millioda
70 million 3 million
to date 200 million; 19 million/a
effluents go elsewhere. A basic solution of the problem requires more effective phosphorus removal in treatment plants. This can be achieved by filtration after precipitationand flocculation. The phosphorus elimination plants run in Berlin at Lake Tegel and at Schlachtensee show that by the four-step process described above 0.01-0.02 mg/L P can be achieved in the output, whereas biological elimination processes and precipitation without filtration scarcely reach concentrations below 0.5 mg/L P. The reaction of Lake Tegel to the measures taken shows that the exponential decline of phosphorus concentrations in the lake over more than 1order of a magnitude coincides with the operation of the phosphorus elimination plant.
Figure 8. Concentrations of total phosphorus (bars) and chlorophyll a (curve)in the course of restoration 01 Schlachtensee (annual means at 1 m depth).
Quite a similar curve was observed a few years earlier for Schlachtensee,where phosphorus eliminationat the inflow was the decisive measure taken (13,14). In comparison with Schlachtensee,biological reactions of Lake Tegel so far are rather slight. The development of Schlachtensee showed that the reversal of eutrophication may follow a threshold pattern, and this threshold may lie between 0.06 and 0.04 mg/L P (Figure 8). The mechanism for this was a switch in species composition, as phosphorus limitation of blue-green algae became strong enough to significantly reduce their biomass. At lower biomass and correspondingly higher transparency they lost their advantage in competition over other species (14). Thus, three different levels have been achieved through Berlin’s efforts at reducing eutrophication: Level 1: Reduction of phosphorus concentrations,which has occurred in most of Berlin’s water bodies through the introduction of phosphorus precipitation and enhanced biological phosphorus removal in sewage treatment plants. Level 2 Reduction of phosphorus concentrations to the point where the algae no longer leave ‘leftovers” of soluble reactive phosphorus (i.e., SRP concentrations remain below 10 pglL on the average and/or for most of the growing season; ref 15,p 130)means that phosphorus begins to limit algal biomass. The first effect is the reductionofthemaximaofextremelydense phytoplankton blooms especiallyduring the dominance of blue-greensthis is the present state of Lake Tegel at concentrations of total phosphorus between 0.06 and 0.13 mg/L P Level 3 Significant reduction of biomass below levels of stable blue-green dominance-this is the present state of Schlachtensee at concentrations of total phosphorus around 0.03 mg/L P. Lake Tegel appears to be followinga threshold pattern similar to the one observed for Schlachtensee, and it appears to be moving on the verge of the phosphorus threshold for significant reduction of biomass. Whether this will be associated with a shift in species composition as in Schlachtensee remains to be seen, as the species of blue-greenswhich prevailin LakeTegel (Microcystisspp., Aphanizomenon spp., Anabaena spp.) differ from those in Schlachtensee (Planktothrix agardhii, Limnotrix spp.; ref 12). During the summer of 1992,dominance of hluegreens was for the first time supressed by dominance of dinoflagellatesthroughout most of the season. The present perspective for the further development of the lake is a few more years of flushing roughly three times per year with low-phosphorus water to wash out high phosphate concentrations in the upper sediment layers. This may Emiron. Scl. Technol.. VOl. 28. No. 8. 1994
1415
be a prerequisite for reaching total phosphorus concentrations under 0.06 mg/L P. Also, with higher stability of stratification without operation of the aerators, less of the phosphorus released into the hypolimnion will reach the euphotic zone. Production of biomass will then be more strongly P-limited, which in turn decreases sedimentation of phosphorus bound in algae. This phosphorus fraction bound in decomposing algae is readily available for biological processes. If less of it reaches the sediment surface, the cycle of production and internal loading is expected to diminish, as was the case in Schlachtensee (Chorus & Klein, in preparation). After decades of loading with scarcelytreated sewage,anumber of years for recovery of the system are inevitable. The experience at Lake Tegel, especially if compared to that at Schlachtensee, shows to which extent measures are necessary in order to counteract eutrophication to a level at which the algae show any reaction at all. A close causal connection between phosphorus concentrations in a lake and algal density can be expected only beneath a phosphorus threshold, which may vary somewhat depending upon individual characteristics of a lake such as its morphology, but which ranges in the vicinity of 0.040.06 mg/L P. Measures which fail to reach this threshold may be a first step in the right direction, but will have little effect upon the algae. For the effective reduction of point sources, advanced sewage treatment is necessary. This was impressively shown by lakes in Switzerland (e.g., Vierwaldstadter See, Walensee; ref 16). Possible techniques are enhanced biological phosphorus elimination, and for even lower effluent concentrations also filtration after precipitation and flocculation. In situations where diffuse sources are concentrated in the main inlet of a lake, advanced surface water treatment of this inlet may be very effective. Thus, a phosphorus elimination plant for the Wahnbach River substantially improved the quality of the Wahnbach Reservoir (17,18). Equally important is a basic change in the approach to the agricultural use of fertilizers toward well-directed dosage according to the geological and chemical characteristics of each soil. In Berlin, the extent of nonpoint sources in the catchment area of Nordgraben and Tegeler Fliess required directed measures at the inlet of Lake Tegel. Furthermore, this concept provides low-phosphorus water for flushing the lakes several times per year and thus counteracting internal loading. This was especially necessary because
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Environ. Sci. Technol., Vol. 28, No. 8 , 1994
of the low discharge and generally high loading of Berlin’s receiving waters. The experience with Schlachtensee and the present development of Lake Tegel confirm that phosphorus elimination at the main inlet was the correct option. Acknowledgments
Our very hearty thanks go to all those who initiated this restoration project (Dr. Sarfert, Prof. Hasselbarth, and Mr. Kloos), who invested much time and effort into running the phosphorus elimination plant (Mr. Krupop and Mr. Piritsch) and optimizing the treatment process (Prof. Grohmann, Mr. Wascher, and Mr. Stengel), and who contributed to the investigation of the lake before and during restoration (Prof. Klein, Mr. Wassmann, Mr. Wolf, Mrs. Pawlitzky, Mrs. Laskus, and Mrs. Schlag). Literature Cited (1) Chorus, I.; Klein, G.; Rotard, W. Schadst. Grundwasser, in press (2) Schmidt, K. 2. Wasser Abwasser-Forsch. 1982,15(3),143. (3) Persson, P.;Juttner, F. Aqua Fenn. 1983,13,3. Weil, L. Gewaesserschutz,.Wasser Abwasser 1972,8,157. Hasselbarth, U.Hochschulforschung 1974,6, 424. Kloos, R.Massnahmen zur Reinhaltungder Berliner Seen; Senator fur Bau- und Wohnungswesen: Berlin 1980. Vollenweider, R. Schweiz. Z . Hydrol. 1979,37,53. Klein, G.; Wassmann, H. WaBoLu-Hefte 1986,2. Heinzmann, B.; Sarfert, F.; Stengel, A. G WF, Gas Wasserfach: WasserlAbwasser 1991,132,674. Hasselbarth, U.2. Wasser Abwasser Forsch, 1979,12,133. Heinzmann, B.; Sarfert, F. G WF, Gas Wasserfach: Wasser Abwasser 1990,131,262. Chorus, I.; Schlag, G. Hydrobiologia 1993,249, 67. Klein, G. Ecodynumics-Contributions to theoretical ecology; Springer-Verlag: Berlin, Heidelberg, New York, 1988; p 138. Klein, G.; Chorus, I. Mitt. Znt. Ver. Theor. Angew. Limnol. 1991,24, 873. Sas, H., Ed. Lake Restoration by Reduction of Nutrient Loading: Expectations, Experiences, Extrapolations; Academia Verlag Richarz GmbH: St. Augustin, 1989; 497 pp. Ambiihl, H.; Buhrer, H. EAWAG-Mitt. 1992,34,4. Bernhardt, H.2. Wasser Abwasser Forsch. 1982,15,157. Bernhardt, H. Studies on the treatment of eutrophic water; Special Subject No. 12;IWSA Congress: Rio de Janeiro, 1988;pp 12-1. Received for review September 8, 1993.Revised manuscript received March 16,1994.Accepted April 11, 1994.e @
Abstract published in Advance ACS Abstracts, May 15, 1994.
