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Pulverizing aerator in the process of lake restotation
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ECOENG-4838; No. of Pages 5
Ecological Engineering xxx (2017) xxx–xxx
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Pulverizing aerator in the process of lake restotation Stanisław Podsiadłowski, Ewa Osuch ∗ , Jacek Przybył, Andrzej Osuch, Tatiana Buchwald Pozna´ n University of Life Sciences, Institute of Biosystems Engineering ul. Wojska Polskiego 50, 60-627 Pozna´ n, Poland
a r t i c l e
i n f o
Article history: Received 19 February 2017 Received in revised form 4 June 2017 Accepted 14 June 2017 Available online xxx Keywords: Lake restoration Pulverizing aeration Góreckie lake
a b s t r a c t In 1995, the Institute of Agricultural Engineering of the Agricultural University in Poznan´ launched research on an integrated lake restoration technology, whose core was a wind-powered aerator capable of oxygenating also the bottom layers (the hypolimnion) of deep lakes. The aerator uses energy from the Savonius rotor mainly for water pulverizing, causing the release of gases such as hydrogen sulphide or methane, which usually result in complete water saturation and replace them with oxygen. Even the early studies showed the constructed device to be highly efficient in improving oxygen conditions in the bottom zone. Moreover, the research made it clear that the device should be equipped with an autonomous system designed to inactivate phosphorus, one of the principal factors determining the rate of lake degradation. In 2013 the first wind-driven pulverizing aerator equipped with such a new system was installed at Góreckie Lake. The research describe results of the process of the restoration of Góreckie Lake with using pulverizing aerator system conducted regularly from 2013 to 2015. It was the aim of this work. The results show that the efficience of pulverizing aerator with a phosphorus inactivation system and the efficiency of the coagulant dosage depend on the wind speed from the determined range of the speed. The technical specifications causes that the device uses only energy of the wind, which is the main advantage of the whole system. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The main threat to lakes in temperate zones are factors increasing the rate of eutrophication (Harper, 1992), as they lead to deoxygenation of the deeper layers of water, hypolimnion and metalimnion in particular, which results in mass phytoplankton growth and the general deterioration of water quality (Vollenweider, 1976; Gołdyn et al., 2014). This, in turn, affects the abundance and health of fish communities. The balance between complex (organic) compounds and the oxygen in the body of water is disturbed (Bonsdorff et al., 2002). Natural oxygenation of deep water layers is a relatively slow process, because it relies on the diffusion of oxygen from the epilimnion layer. In summer months, when bottom deposits reveal relatively high biochemical activity, the intensity of the diffusion processes is largely insufficient (Nash and Halliewll, 2000; McGechan and Lewis, 2002; Koschel, 2011). Oxygen depletion leads to a reduction of the redox complex in bottom sediments and release of phosphorus adsorbed on the iron and manganese compounds into the water column (Sondergaard et al., 2002). Such
∗ Corresponding author. E-mail address: [email protected] (E. Osuch).
internal loading can be an important source of phosphorus degrading the lake, especially in summer (Gołdyn et al., 2010). According to Environmental Status Report only about 3.8% of Polish lakes have a very good water quality (Soszka et al., 2003). The majority of Polish lakes have good and moderate status (36.6% and 38.9%, respectively), but still 20% are classified in poor or bad status. The noticeable degradation process of lakes began about thirty years ago due to unsustainable agricultural practices, mass-tourism and communal and industrial sewage discharge. Nowadays, lake restoration is regarded as a feasible but costly alternative which can be effected by (Wi´sniewski, 2000; Wetzel, 2001; Klapper, 2003): – renewal of the entire water mass, – removal of upper layers of bottom sediments, especially in the profundal zone, – extraction of hypolimnetic water (hypolimnetic siphoning), – oxygenation of the near-bottom waters using aeration equipment; presently wind-powered aerators are used. Oxygenating of hypolimnion waters is one of the most frequent methods of lake restoration (Lossow et al., 1998). Aerators were initially used in sewage treatment plants, but the growing problem of lake eutrophication triggered the widespread use of the
http://dx.doi.org/10.1016/j.ecoleng.2017.06.032 0925-8574/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
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Fig. 1. Types of aerators (acc. to Podsiadłowski, 2001, modified).
equipment also in inland waters. Basically, there are two groups of aerators depending on the operation principle, namely pulverizing and pneumatic aerators (Fig. 1). Pneumatic aerators pump air into a water layer. They are becoming obsolete because of high energy consumption and relatively low efficiency, which is due to pumping air into the layer of water already saturated with hydrogen sulphide or methane. Pulverizing aerators propel water into the air. The simplest type of pulverizing aerators do not require an external power source but simulates a waterfall effect based on the kinetic energy of the flowing water. Mechanical splash aerators used in sewage treatment plants and fish ponds are usually electrically-driven, and wind-powered when used in lakes. Importantly, pulverizing aerators spray water in the air, where air is overabundant, making the process of gas diffusion easier. As a result of rising energy prices and intensified human impact on the environment, scientists have been working on new methods of water restoration based on renewable energy sources, especially wind, simultaneously scrutinizing the process of natural self-restoration. A lot of research has been done in order to find technologies that could initiate and stimulate processes that might trigger new food chains (starting also in the hypolimnion), which remove excess of organic matter from the most vulnerable layers of a lake (Bormans et al., 2016; Grochowska and Brzozowska, 2015; Minella et al., 2015; Niemisto et al., 2015; Wesołowski and Brysiewicz, 2015). Consequently, the two main fields of the research are:
which makes it possible to develop the so-called intensive aeration zone (Podsiadłowski, 2005). This zone tends to translocate relatively frequently due to naturally occurring lake tides, which has been proven by our earlier study (Podsiadłowski, 2008a). The aeration process in deep lakes with a well-defined thermal stratification (Fig. 2) is, however, different from the same process occurring in shallow lakes (Fig. 3). In the hypolimnetic waters of deep lakes with a well-defined thermal stratification (Fig. 2), the oxygenated water zone develops, which leads to the oxidation of complex compounds and to surviving of benthic macroinvertebrates. Shallow lakes differ from deep lakes in this respect, because the aeration devices are used to remove the deoxygenated near-bottom layer, with sharp oxycline, which develops during summer months as a result of intensive microbiological processes in the sediment-water interface. Limiting the gradient of the oxycline is crucial for initiating and maintaining the process of lake restoration (Klapper, 2003). Consistent oxygen supply to the over-bottom waters of lakes by means of wind-powered pulverizing aerators make it possible to inactivate phosphorus released from the bottom sediments on oxidised metal compounds (Sondergaard et al., 2002). Innovation of this method consist on the pulverizing aerator was equipped with the phosphorus inactivation system capable of dosing the coagulant rates correlated with pulverization efficiency (Podsiadłowski, 2008a). The general purpose of the study was to determine the relations between coagulant dosage, aeration efficiency and wind speed in
(1) increasing the efficiency of gas exchange during the aeration process (replacing H2 S with O2 ), (2) harnessing wind energy to power aeration (Podsiadłowski, 2005). 2. Description of the device The influence of wind speed on aeration efficiency of the pulverizing device is regarded as the most vital indicator of aeration efficiency and was described earlier (Podsiadłowski, 2008b); nonetheless, precise data regarding pulverizing aeration at different wind speeds had been unavailable. The installation of the phosphorus inactivation system in the aerator was the moment when the data proved to be extremely useful. In this paper, aeration efficiency has been presented as mean water flow capacity in the suction hoses of the aerator. The operating principle of the pulverizing aerator is based on using energy obtained from the Savonius wind turbine for gas diffusion, namely for the pulverizing of hypolimnetic water, during which H2 S and other gases are replaced by oxygen. Thanks to this technology, water flowing through the aerator may absorb up to seven times more oxygen,
Fig. 2. Pulverizing aerator with a phosphorus inactivation system: 1− impeller, 2 − coagulant dispenser, 3 − pressing section, 4 − suction section (acc. to Osuch and Podsiadłowski, 2012).
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
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Fig. 3. Pulverizing aerator.
order to properly adjust the phosphorus inactivation system and to ensure whether the system is stability in the oxygenation zone. 3. Material and methods Between 2013 and 2015 at Góreckie Lake in the Wielkopolski National Park in Jeziory, Poland, the water flow and coagulation efficiency were measured at different wind speeds. Góreckie Lake covers 99.8 ha, with a mean depth of 8.8 m, maximum depth of 16.8 m. The lake displays a good wind exposure facilitating water mixing (Fig. 4). The water flow rate was measured in the pulverizing aerator as water flow speed at a given wind speed. The coagulation efficiency was measured by determining the specific density of the liquid in the coagulant dispenser (iron sulphate). It causing that in the course of examinations there was an available recommendation how effectively depending on the wind speed is running the process. The measurements were taken at wind speeds fluctuating
between 2.2–5.5 m s−1 (Kestrel 3000 Wind Meter) at 3.6 m above the water surface, i.e. at half height of the wind turbine. The research was conducted at the lake every two weeks from 1st of April to 30th of October during 3 years of analysis. One-time analysis on the lake last 2–3 h. In the course of every analysis results were being recorded for the given wind speed. The given wind speed was the wind speed lasting for 10 s. It was recognized as the reliable result. In the entire period of examinations, for every value presented in the form of a graph the number of measurements was different (hesitated from 231 to 423). The influence of the medium wind speed on the productivity of the dosage of the coagulant was determined based on the method of measurement of the density of the coagulant during the aerator work. The dispenser of the coagulant before research started was filled up to the tap hole (10 dm3 ) with Fe2 (SO4 )3 coagulant which density was 1500 kg m−3 . The tap hole of the dispenser was at 5 cm level below the uphill edge of the container of the dispenser. By the tap hole a hose about the diameter of 8 mm was installed. This
´ ´ Fig. 4. Map of Góreckie Lake with sampling station and location of the lake within Poland (Sobczynski et al., 2012; Sobczynski and Joniak, 2013).
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
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Fig. 5. Influence of wind speed V (m s−1) on pulverizing aeration efficiency Ep (dm3 s−1 ), Ep = 10,159 − 46,917 V + 48,144 V2 − 19,439 V3 + 3,4701 V4 − 0.225 V5 R2 = 0.9871.
hose ends with the spray nozzle. The container in the shape of the cuboid consists of the bottom (bases) and of the sides. The upper part of the container is revealed. During the atomization of water by the aerator circle, sprayed water partly found its way to the container inside. Lake water gets to the inside of the container of coagulants causes its overflowing. As a result the excess of the working liquid is finding its way to the upper bottom layer of lake water. Lake water gets to the inside of the dispenser where is mixing with the coagulant causing reducing the density of liquid. The density reducting intensity of the coagulant depends on the wind speed. The greater average wind speed (more sprayed water finds its way to the container), the more quickly density of the working liquid of the dispenser decreases. This situation takes place till the density will be equal with the lake water density. Research consisted on determining the time needed for equaling the density of the coagulant in the container with the density of lake water. During the time of research, the wind speed measurement was also performed in the constant way. It lets determining the average speed of the wind during the dosage of the coagulant. Research was performed ten times every year, at the different wind speeds. Achieved results allowed for calculating the productivity dosage of the coagulant depending on the wind speed. 4. Results and discussion As presented in Fig. 5, pulverizing aeration efficiency increases with the increased speed of wind. The most meaningful increase was noticed at wind speeds (V) between 4.2 m s−1 to 5.2 m s−1 , which results from the self-sealing of the impeller at higher water flow speeds (Konieczny, 2004). Wind speed from Fig. 5 is over 5.2 m s−1 , in turn, lead to a decreased aeration efficiency due to the limited water flow rate of the suction hoses adapted to high frequency wind speed ranges (3–5 m s−1 ). The phosphorus inactivation system in the pulverizing aerator was constructed in a way that allowed adjusting the dosing of the coagulant to the aeration efficiency (Fig. 6). The preparation should be dosed into oxygenated water, but when the process of aeration is inactive and water flow stops, the dosage should be also stopped. Very important is the relative simplicity and high reliability of the system in the lake conditions. As presented in Fig. 6, the study revealed an important relation between the efficiency (Ek) of the preparation dosing (iron sulphate coagulant), and mean wind speed (Vh). At average wind speeds (3–4 m s−1 ). the coagulant was dosed at a rate of approximately 100 g per hour’s work of the aeration device. The research revealed that the amount was sufficient for the process of phosphorus precipitation in the aeration zone (above the bottom layer of lake water).
Fig. 6. The influence of average wind speed Vh (m s−1 ) on the efficiency of dosing the coagulant Ek (g h−1 ), Ek = −3317,2 + 3689,4Vh − 1530,3Vh2 + 284,45Vh3 − 19,526Vh4 R2 = 0.9878.
Methods so far known and described in literature of the restoration of lakes are based, similarly to the method of the pulverization, at a few preliminary stages of identifying factors from which problems of polluting lakes result (Bormans et al., 2016). After recognizing them a choice of the most adequate method is taking place for the given lake. In Polish climatic conditions pulverizing aerators are working very well. What is proving by their increasing number on lakes. The problem of the pulverization with applying these devices requires further monitoring and examinations, because the specificity of every lake is different. The seasons are also playing a role (Dondajewska et al., 2013). On account of climatic conditions in Poland, mainly they are checking the possibility of using the wind speed in combination with the possibility of the dosage of appropriate elements for the lake, aerators are working well. It is important to think the influence of the wind speed on the effectiveness of the pulverization. Proven relations with conducted examinations are pointing, that in determined periods of the wind speed, the process of the pulverization is achieving the optimum. 5. Conclusions 1. Pulverizing aeration efficiency is clearly dependent on wind speed and ranges from 3 to 18 dm3 s−1 . 2. The phosphorus inactivation system, suggested as an element of the pulverizing aerator, has proved to be effective in the dosage of coagulants into previously oxygenated water without additional energy consumption by the wind turbine. 3. Efficiency of the coagulant dosage clearly depends on wind speed (ranging from approximately 20–230 g h−1 ). This finding allows us to undertake the task of creating a model of predicting the process of phosphorus inactivation in the deepest layers of the lakes. References Bonsdorff, E., Rönnber, C., Aarnio, K., 2002. Some ecological properties in relation to eutrophication in the Baltic Sea. Hydrobiologia 475/476, 371–377. Bormans, M., Marsalek, B., Jancula, D., 2016. Controlling internal phosphorus loading in lakes by phisical methods to reduce cyanobacterial blooms: a review. Aquat. Ecol. 50, 407–422. Dondajewska, R., Banaszkiewicz, D., Sczepaniak, S., Tomkowiak, A., 2013. Internal phosphorus loading from sediments of Góreckie Lake (Wielkopolski National Park) – Implications for the aquatic ecosystem end possibilities of its reduction. The Functioning end Protection of Water Ecosystem. Threat, protection and management of water resources. 126–137. Gołdyn, R., Podsiadłowski, S., Kowalczewska-Madura, K., Dondajewska, R., ˛ ´ Szelag-Wasielewska, E., Budzynska, A., Domek, P., Romanowicz-Brzozowska, W., 2010. Functioning of the Lake Rusałka ecosystem in Poznan´ (Western Poland). Oceanological and hydrobiological studies. Int. J. Oceanogr. Hydrobiol. Stud. 39 (3), 65–80, http://dx.doi.org/10.2478/v10009-010-0040-6.
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
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Gołdyn, R., Podsiadłowski, S., Dondajewska, R., Kozak, A., 2014. The sustainable restoration of lakes-towards the challenges of the Water Framework Directive. Ecohydrol. Hydrobiol. 14 (1), 68–74, http://dx.doi.org/10.1016/j.ecohyd.2013. 12.001. Grochowska, J., Brzozowska, R., 2015. Influence of different recultivation methods on durability of nitrogen compounds changes in the waters of an urban lake. Water Environ. J. 29, 228–235. Harper, D., 1992. Eutrophication of Freshwaters: Principles, Problems and Restoration. Chapman and Hall, London. Klapper, H., 2003. Technologies for lake restoration. J. Limnol. 62/1, 3–90. Konieczny, R., 2004. Aeracja pulweryzacyjna w warunkach Jeziora Barlinieckiego. ´ Woda −Srodowisko − Obszary Wiejskie 4 (2b), 291–301. Koschel, R., 2011. Lake restoration by hypolinetic Ca (OH)2 treatment: impact on phosphorus sedimentation and release from sediment. Sci. Total Environ. 409 (8), 1504–1515. ´ Lossow, K., Gawronska, H., Jaszczułt, R., 1998. Attempts to use wind energy for artificial destratification of Lake Starodworskie. Polish J. Environ. Stud. 7/4, 221–227. McGechan, M.B., Lewis, D.R., 2002. Sorption of phosphorus release from soils to surface runoff and subsurface drainage. J. Environ. Qual. 30, 508–520. Minella, M., DeLaurentiis, E., Maurino, V., Minero, C., Vione, D., 2015. Dark production of hydroxyl radicals by aeration of anoxic lake water. Sci. Total Environ. 527–528, 322–327. Nash, D.M., Halliewll, D.J., 2000. Tracing phosphorus transferred from grazing land to water. Water Res. 34/7, 1975–1985. Niemisto, J., Kongas, P., Harkonen, L., Horppila, J., 2015. Hipolimetic aeration intensifies phosphorus recyclong and increases organic material sedimentation in a stratifying lake: effect through increased temperature˛ and turbulence. Boreal Environ. Res. 21, 571–578. Osuch, E., Podsiadłowski, S., 2012. Efficiency of pulverizing aeration on Lake ´ Limnol. Rev. 12 (3), 139–145. Panienskie.
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Podsiadłowski, S., 2001. Aeracja jezior strefy umiarkowanej. Ekopartner 6 (116), 15–16. Podsiadłowski, S., 2005. Beluftung der Seen gemassigter Zone. Konf. Nauk ´ Politechniki Poznanskiej: Intuicja i architektura. Wyd. Politechn. Pozn., 571–580. ˛ Podsiadłowski, S., 2008a. Wstepne badania aeratora pulweryzacyjnego ˙ w system inaktywacji fosforu. Zesz. Probl. Post. N. Roln. 528, wyposazonego 439–447. Podsiadłowski, S., 2008b. Method of precise phosphorus inactivation in lake waters. Limnol. Rev. 8 (1), 3–8. ´ Sobczynski, T., Joniak, T., 2013. The variability and stability of water chemistry in a deep temperate lake: results of long-term study of eutrophication. Pol. J. Environ. Stud. 22 (1), 227–237. ´ Sobczynski, T., Joniak, T., Pronin, E., 2012. Assessment of the multi-directional experiment of restoration of Lake Góreckie (Western Poland) with particular focus on oxygen and light conditions: first results. Pol. J. Environ Stud. 21 (4), 1025–1031. Sondergaard, M., Wolter, K.D., Ripl, E., 2002. Chemical treatment of water and sediments with special references to lakes. In: Perrow, M.R., Davy, A.J. (Eds.), Handbook of Ecological Restoration. Vol. 1. Principles of Restoration. Cambridge University Press. Soszka, H., Cydzik, D., Czajka, J., 2003. Raport stanu s´ rodowiska w Polsce w latach ´ Warszawa, pp. 111–117. 1996–2001. Biblioteka Monitoringu Srodowiska, Vollenweider, R.A., 1976. Advances in defining critical loading level for phosphorus in lake eutrophication. Mam. Inst. Ital. Idrobiol. 33, 53–98. Wesołowski, P., Brysiewicz, A., 2015. The effect of pulverising aeration on changes in the oxygen and nitrogen concentrations in water of Lake Starzyc. J. Water Land Dev. 25, 31–36. Wetzel, R.G., 2001. Limnology. Lake and River Ecosystems. Academic Press, London. Wi´sniewski, R., 2000. Metody rekultywacji zbiorników wodnych − stan obecny i ˛ Konferencja Naukowo-Technicznej, perspektywy. In: IV Miedzynarodowa ˛ Ochrona Jezior Ze Szczególnym Uwzglednieniem Metod Rekultywacji, Przysiek.
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
ARTICLE IN PRESS
ECOENG-4838; No. of Pages 5
Ecological Engineering xxx (2017) xxx–xxx
Contents lists available at ScienceDirect
Ecological Engineering journal homepage: www.elsevier.com/locate/ecoleng
Pulverizing aerator in the process of lake restotation Stanisław Podsiadłowski, Ewa Osuch ∗ , Jacek Przybył, Andrzej Osuch, Tatiana Buchwald Pozna´ n University of Life Sciences, Institute of Biosystems Engineering ul. Wojska Polskiego 50, 60-627 Pozna´ n, Poland
a r t i c l e
i n f o
Article history: Received 19 February 2017 Received in revised form 4 June 2017 Accepted 14 June 2017 Available online xxx Keywords: Lake restoration Pulverizing aeration Góreckie lake
a b s t r a c t In 1995, the Institute of Agricultural Engineering of the Agricultural University in Poznan´ launched research on an integrated lake restoration technology, whose core was a wind-powered aerator capable of oxygenating also the bottom layers (the hypolimnion) of deep lakes. The aerator uses energy from the Savonius rotor mainly for water pulverizing, causing the release of gases such as hydrogen sulphide or methane, which usually result in complete water saturation and replace them with oxygen. Even the early studies showed the constructed device to be highly efficient in improving oxygen conditions in the bottom zone. Moreover, the research made it clear that the device should be equipped with an autonomous system designed to inactivate phosphorus, one of the principal factors determining the rate of lake degradation. In 2013 the first wind-driven pulverizing aerator equipped with such a new system was installed at Góreckie Lake. The research describe results of the process of the restoration of Góreckie Lake with using pulverizing aerator system conducted regularly from 2013 to 2015. It was the aim of this work. The results show that the efficience of pulverizing aerator with a phosphorus inactivation system and the efficiency of the coagulant dosage depend on the wind speed from the determined range of the speed. The technical specifications causes that the device uses only energy of the wind, which is the main advantage of the whole system. © 2017 Elsevier B.V. All rights reserved.
1. Introduction The main threat to lakes in temperate zones are factors increasing the rate of eutrophication (Harper, 1992), as they lead to deoxygenation of the deeper layers of water, hypolimnion and metalimnion in particular, which results in mass phytoplankton growth and the general deterioration of water quality (Vollenweider, 1976; Gołdyn et al., 2014). This, in turn, affects the abundance and health of fish communities. The balance between complex (organic) compounds and the oxygen in the body of water is disturbed (Bonsdorff et al., 2002). Natural oxygenation of deep water layers is a relatively slow process, because it relies on the diffusion of oxygen from the epilimnion layer. In summer months, when bottom deposits reveal relatively high biochemical activity, the intensity of the diffusion processes is largely insufficient (Nash and Halliewll, 2000; McGechan and Lewis, 2002; Koschel, 2011). Oxygen depletion leads to a reduction of the redox complex in bottom sediments and release of phosphorus adsorbed on the iron and manganese compounds into the water column (Sondergaard et al., 2002). Such
∗ Corresponding author. E-mail address: [email protected] (E. Osuch).
internal loading can be an important source of phosphorus degrading the lake, especially in summer (Gołdyn et al., 2010). According to Environmental Status Report only about 3.8% of Polish lakes have a very good water quality (Soszka et al., 2003). The majority of Polish lakes have good and moderate status (36.6% and 38.9%, respectively), but still 20% are classified in poor or bad status. The noticeable degradation process of lakes began about thirty years ago due to unsustainable agricultural practices, mass-tourism and communal and industrial sewage discharge. Nowadays, lake restoration is regarded as a feasible but costly alternative which can be effected by (Wi´sniewski, 2000; Wetzel, 2001; Klapper, 2003): – renewal of the entire water mass, – removal of upper layers of bottom sediments, especially in the profundal zone, – extraction of hypolimnetic water (hypolimnetic siphoning), – oxygenation of the near-bottom waters using aeration equipment; presently wind-powered aerators are used. Oxygenating of hypolimnion waters is one of the most frequent methods of lake restoration (Lossow et al., 1998). Aerators were initially used in sewage treatment plants, but the growing problem of lake eutrophication triggered the widespread use of the
http://dx.doi.org/10.1016/j.ecoleng.2017.06.032 0925-8574/© 2017 Elsevier B.V. All rights reserved.
Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032
G Model ECOENG-4838; No. of Pages 5
ARTICLE IN PRESS
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S. Podsiadłowski et al. / Ecological Engineering xxx (2017) xxx–xxx
Fig. 1. Types of aerators (acc. to Podsiadłowski, 2001, modified).
equipment also in inland waters. Basically, there are two groups of aerators depending on the operation principle, namely pulverizing and pneumatic aerators (Fig. 1). Pneumatic aerators pump air into a water layer. They are becoming obsolete because of high energy consumption and relatively low efficiency, which is due to pumping air into the layer of water already saturated with hydrogen sulphide or methane. Pulverizing aerators propel water into the air. The simplest type of pulverizing aerators do not require an external power source but simulates a waterfall effect based on the kinetic energy of the flowing water. Mechanical splash aerators used in sewage treatment plants and fish ponds are usually electrically-driven, and wind-powered when used in lakes. Importantly, pulverizing aerators spray water in the air, where air is overabundant, making the process of gas diffusion easier. As a result of rising energy prices and intensified human impact on the environment, scientists have been working on new methods of water restoration based on renewable energy sources, especially wind, simultaneously scrutinizing the process of natural self-restoration. A lot of research has been done in order to find technologies that could initiate and stimulate processes that might trigger new food chains (starting also in the hypolimnion), which remove excess of organic matter from the most vulnerable layers of a lake (Bormans et al., 2016; Grochowska and Brzozowska, 2015; Minella et al., 2015; Niemisto et al., 2015; Wesołowski and Brysiewicz, 2015). Consequently, the two main fields of the research are:
which makes it possible to develop the so-called intensive aeration zone (Podsiadłowski, 2005). This zone tends to translocate relatively frequently due to naturally occurring lake tides, which has been proven by our earlier study (Podsiadłowski, 2008a). The aeration process in deep lakes with a well-defined thermal stratification (Fig. 2) is, however, different from the same process occurring in shallow lakes (Fig. 3). In the hypolimnetic waters of deep lakes with a well-defined thermal stratification (Fig. 2), the oxygenated water zone develops, which leads to the oxidation of complex compounds and to surviving of benthic macroinvertebrates. Shallow lakes differ from deep lakes in this respect, because the aeration devices are used to remove the deoxygenated near-bottom layer, with sharp oxycline, which develops during summer months as a result of intensive microbiological processes in the sediment-water interface. Limiting the gradient of the oxycline is crucial for initiating and maintaining the process of lake restoration (Klapper, 2003). Consistent oxygen supply to the over-bottom waters of lakes by means of wind-powered pulverizing aerators make it possible to inactivate phosphorus released from the bottom sediments on oxidised metal compounds (Sondergaard et al., 2002). Innovation of this method consist on the pulverizing aerator was equipped with the phosphorus inactivation system capable of dosing the coagulant rates correlated with pulverization efficiency (Podsiadłowski, 2008a). The general purpose of the study was to determine the relations between coagulant dosage, aeration efficiency and wind speed in
(1) increasing the efficiency of gas exchange during the aeration process (replacing H2 S with O2 ), (2) harnessing wind energy to power aeration (Podsiadłowski, 2005). 2. Description of the device The influence of wind speed on aeration efficiency of the pulverizing device is regarded as the most vital indicator of aeration efficiency and was described earlier (Podsiadłowski, 2008b); nonetheless, precise data regarding pulverizing aeration at different wind speeds had been unavailable. The installation of the phosphorus inactivation system in the aerator was the moment when the data proved to be extremely useful. In this paper, aeration efficiency has been presented as mean water flow capacity in the suction hoses of the aerator. The operating principle of the pulverizing aerator is based on using energy obtained from the Savonius wind turbine for gas diffusion, namely for the pulverizing of hypolimnetic water, during which H2 S and other gases are replaced by oxygen. Thanks to this technology, water flowing through the aerator may absorb up to seven times more oxygen,
Fig. 2. Pulverizing aerator with a phosphorus inactivation system: 1− impeller, 2 − coagulant dispenser, 3 − pressing section, 4 − suction section (acc. to Osuch and Podsiadłowski, 2012).
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Fig. 3. Pulverizing aerator.
order to properly adjust the phosphorus inactivation system and to ensure whether the system is stability in the oxygenation zone. 3. Material and methods Between 2013 and 2015 at Góreckie Lake in the Wielkopolski National Park in Jeziory, Poland, the water flow and coagulation efficiency were measured at different wind speeds. Góreckie Lake covers 99.8 ha, with a mean depth of 8.8 m, maximum depth of 16.8 m. The lake displays a good wind exposure facilitating water mixing (Fig. 4). The water flow rate was measured in the pulverizing aerator as water flow speed at a given wind speed. The coagulation efficiency was measured by determining the specific density of the liquid in the coagulant dispenser (iron sulphate). It causing that in the course of examinations there was an available recommendation how effectively depending on the wind speed is running the process. The measurements were taken at wind speeds fluctuating
between 2.2–5.5 m s−1 (Kestrel 3000 Wind Meter) at 3.6 m above the water surface, i.e. at half height of the wind turbine. The research was conducted at the lake every two weeks from 1st of April to 30th of October during 3 years of analysis. One-time analysis on the lake last 2–3 h. In the course of every analysis results were being recorded for the given wind speed. The given wind speed was the wind speed lasting for 10 s. It was recognized as the reliable result. In the entire period of examinations, for every value presented in the form of a graph the number of measurements was different (hesitated from 231 to 423). The influence of the medium wind speed on the productivity of the dosage of the coagulant was determined based on the method of measurement of the density of the coagulant during the aerator work. The dispenser of the coagulant before research started was filled up to the tap hole (10 dm3 ) with Fe2 (SO4 )3 coagulant which density was 1500 kg m−3 . The tap hole of the dispenser was at 5 cm level below the uphill edge of the container of the dispenser. By the tap hole a hose about the diameter of 8 mm was installed. This
´ ´ Fig. 4. Map of Góreckie Lake with sampling station and location of the lake within Poland (Sobczynski et al., 2012; Sobczynski and Joniak, 2013).
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Fig. 5. Influence of wind speed V (m s−1) on pulverizing aeration efficiency Ep (dm3 s−1 ), Ep = 10,159 − 46,917 V + 48,144 V2 − 19,439 V3 + 3,4701 V4 − 0.225 V5 R2 = 0.9871.
hose ends with the spray nozzle. The container in the shape of the cuboid consists of the bottom (bases) and of the sides. The upper part of the container is revealed. During the atomization of water by the aerator circle, sprayed water partly found its way to the container inside. Lake water gets to the inside of the container of coagulants causes its overflowing. As a result the excess of the working liquid is finding its way to the upper bottom layer of lake water. Lake water gets to the inside of the dispenser where is mixing with the coagulant causing reducing the density of liquid. The density reducting intensity of the coagulant depends on the wind speed. The greater average wind speed (more sprayed water finds its way to the container), the more quickly density of the working liquid of the dispenser decreases. This situation takes place till the density will be equal with the lake water density. Research consisted on determining the time needed for equaling the density of the coagulant in the container with the density of lake water. During the time of research, the wind speed measurement was also performed in the constant way. It lets determining the average speed of the wind during the dosage of the coagulant. Research was performed ten times every year, at the different wind speeds. Achieved results allowed for calculating the productivity dosage of the coagulant depending on the wind speed. 4. Results and discussion As presented in Fig. 5, pulverizing aeration efficiency increases with the increased speed of wind. The most meaningful increase was noticed at wind speeds (V) between 4.2 m s−1 to 5.2 m s−1 , which results from the self-sealing of the impeller at higher water flow speeds (Konieczny, 2004). Wind speed from Fig. 5 is over 5.2 m s−1 , in turn, lead to a decreased aeration efficiency due to the limited water flow rate of the suction hoses adapted to high frequency wind speed ranges (3–5 m s−1 ). The phosphorus inactivation system in the pulverizing aerator was constructed in a way that allowed adjusting the dosing of the coagulant to the aeration efficiency (Fig. 6). The preparation should be dosed into oxygenated water, but when the process of aeration is inactive and water flow stops, the dosage should be also stopped. Very important is the relative simplicity and high reliability of the system in the lake conditions. As presented in Fig. 6, the study revealed an important relation between the efficiency (Ek) of the preparation dosing (iron sulphate coagulant), and mean wind speed (Vh). At average wind speeds (3–4 m s−1 ). the coagulant was dosed at a rate of approximately 100 g per hour’s work of the aeration device. The research revealed that the amount was sufficient for the process of phosphorus precipitation in the aeration zone (above the bottom layer of lake water).
Fig. 6. The influence of average wind speed Vh (m s−1 ) on the efficiency of dosing the coagulant Ek (g h−1 ), Ek = −3317,2 + 3689,4Vh − 1530,3Vh2 + 284,45Vh3 − 19,526Vh4 R2 = 0.9878.
Methods so far known and described in literature of the restoration of lakes are based, similarly to the method of the pulverization, at a few preliminary stages of identifying factors from which problems of polluting lakes result (Bormans et al., 2016). After recognizing them a choice of the most adequate method is taking place for the given lake. In Polish climatic conditions pulverizing aerators are working very well. What is proving by their increasing number on lakes. The problem of the pulverization with applying these devices requires further monitoring and examinations, because the specificity of every lake is different. The seasons are also playing a role (Dondajewska et al., 2013). On account of climatic conditions in Poland, mainly they are checking the possibility of using the wind speed in combination with the possibility of the dosage of appropriate elements for the lake, aerators are working well. It is important to think the influence of the wind speed on the effectiveness of the pulverization. Proven relations with conducted examinations are pointing, that in determined periods of the wind speed, the process of the pulverization is achieving the optimum. 5. Conclusions 1. Pulverizing aeration efficiency is clearly dependent on wind speed and ranges from 3 to 18 dm3 s−1 . 2. The phosphorus inactivation system, suggested as an element of the pulverizing aerator, has proved to be effective in the dosage of coagulants into previously oxygenated water without additional energy consumption by the wind turbine. 3. Efficiency of the coagulant dosage clearly depends on wind speed (ranging from approximately 20–230 g h−1 ). This finding allows us to undertake the task of creating a model of predicting the process of phosphorus inactivation in the deepest layers of the lakes. References Bonsdorff, E., Rönnber, C., Aarnio, K., 2002. Some ecological properties in relation to eutrophication in the Baltic Sea. Hydrobiologia 475/476, 371–377. Bormans, M., Marsalek, B., Jancula, D., 2016. Controlling internal phosphorus loading in lakes by phisical methods to reduce cyanobacterial blooms: a review. Aquat. Ecol. 50, 407–422. Dondajewska, R., Banaszkiewicz, D., Sczepaniak, S., Tomkowiak, A., 2013. Internal phosphorus loading from sediments of Góreckie Lake (Wielkopolski National Park) – Implications for the aquatic ecosystem end possibilities of its reduction. The Functioning end Protection of Water Ecosystem. Threat, protection and management of water resources. 126–137. Gołdyn, R., Podsiadłowski, S., Kowalczewska-Madura, K., Dondajewska, R., ˛ ´ Szelag-Wasielewska, E., Budzynska, A., Domek, P., Romanowicz-Brzozowska, W., 2010. Functioning of the Lake Rusałka ecosystem in Poznan´ (Western Poland). Oceanological and hydrobiological studies. Int. J. Oceanogr. Hydrobiol. Stud. 39 (3), 65–80, http://dx.doi.org/10.2478/v10009-010-0040-6.
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Please cite this article in press as: Podsiadłowski, S., et al., Pulverizing aerator in the process of lake restotation. Ecol. Eng. (2017), http://dx.doi.org/10.1016/j.ecoleng.2017.06.032