Substantia. An International Journal of the History of Chemistry 3(2) Suppl. 3: 65-70, 2019 Firenze University Press www.fupress.com/substantia ISSN 1827-9643 (online) | DOI: 10.13128/Substantia-371 Citation: H.-M. Peter, C. Sutter, W. Schwenk (2019) Study of a Section of a Self-Purifying Stream in Specific Relation to its Water Flow Behaviour. Substantia 3(2) Suppl. 3: 65-70. doi: 10.13128/Substantia-371 Copyright: © 2019 H.-M. Peter, C. Sutter, W. Schwenk. This is an open access, peer-reviewed article pub- lished by Firenze University Press (http://www.fupress.com/substantia) and distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distri- bution, and reproduction in any medi- um, provided the original author and source are credited. Data Availability Statement: All rel- evant data are within the paper and its Supporting Information files. Competing Interests: The Author(s) declare(s) no conflict of interest. Study of a Section of a Self-Purifying Stream in Specific Relation to its Water Flow Behaviour Heinz-Michael Peter, Christine Sutter*, Wolfram Schwenk Institut für Strömungswissenschaften, D-79737 Herrischried E-mail: c.sutter@stroemungsinstitut.de Abstract. The Mettma, a mountain stream in the Black Forest in Germany, had been polluted at a point source by effluent discharge from a brewery, and showed a section of self-purification along 8 km, without further interferences, following the effluent outfall. This section of stream had served as an excellent study model of the self-purifi- cation phenomenon, as much in the physico-chemistry and biology as in the hydrody- namic attributes of the water. The biological evolution along the stretch of self-purify- ing stream showed a succession of species typical of the food chain. To document the hydrodynamics of the stream water the Drop Picture Method, a standardized testing method developed by Theodor Schwenk, was used, based on optically revealing inter- nal flow structures. The watersamples upstream of the pollution source showed diverse and well shaped flow structures, whereas the samples at the effluent outfall appeared with a drastic reduction and inhibition of flow shape diversity and differentiation. After that point the internal flow structures became increasingly intense and diverse the fur- ther downstream. This evolution in movement diversity proceeded in parallel to the development of the biotic community, which showed a similar increase in diversity, differentiated morphology and functional differentiation away from the pollution point to the extent that at a distance of 8 km of the point source downstream a state similar to upstream of the effluent outfall was re-established. Keywords. Self-purification, water quality, flow structure, hydrodynamics, drop pic- ture method. INTRODUCTION The evaluation of water quality in lotic systems relies in principle on the analysis of physical, chemical and biological characteristics. Our proposition here is to study a new descriptor of water quality, not just based on its con- stituent elements but taking into account the general and most outstanding characteristics of water as a liquid: its ability to move and flow, an essential function in its role as a life mediator. The hydrodynamics of water can be shown by using the Drop Picture Method, developed by Theodor Schwenk and published in 1967.1 We looked at this new criterion of hydrodynamic behaviour and applied this methodology along a length of stream polluted at point source by biodegradable organic effluent, and compared the results with customary testing parameters. 66 Heinz-Michael Peter, Christine Sutter, Wolfram Schwenk I. STUDY FRAMEWORK AND SAMPLING This study included ten measurement campaigns from 1972 to 1977 carried out on the Mettma,2 a moun- tain stream in the Black Forest, in collaboration with the Institute of Limnology, University of Freiburg (Ger- many).3,4,5 The Mettma is a trout-inhabited stream, oli- gotrophic and with a low flow rate (150 to 1500 L/s) depending on the season. The ten surveys were done in different situations, including all seasons of the year. In this article however, we used the results collected in a testing survey carried out during a period of low water (192 L/s), the 26th of July 1974. At a certain point a brewery discharged organic pollutant into the stream, in a row effluent quantity of 6000 inhabitants equiva- lent. Subsequently, the stream crossed a wooded moun- tainous zone and had no other interference apart from a dilution factor of 3 due to small tributaries. The study was terminated by the installation of a treatment plant at the brewery in 1977, and was spatially limited by the construction of a weir 9 km downstream of the effluent injection. The study included 11 sampling stations, one upstream and further stations 50, 300, 700, 1450, 1800, 3000, 3900, 5100, 7150 and 8000 m downstream of the effluent injection. Different parameters were measured: temperature, pH, surface tension, dissolved oxygen, ammonium, phosphate and nitrate concentration. In addition, the biological situation of the ecosystem at the stations and the hydrodynamical quality of the water were analysed. II. ANALYSIS AND SEQUENCE OF PHYSICAL- CHEMICAL PARAMETERS II.1 Temperature The introduction of effluent increased the stream temperature from 10 to 13°C. The temperature was only slightly reduced over the total study section of 8000 m (Fig. 1) II.2 pH The Mettma is naturally slightly acidic, with pH val- ues of typically between 6.1 and 7.0. Immediately down- stream of the effluent injection, pH values oscillated between 6.1 and 10.4 because of the neutralization of the effluent. The pH levels stabilized after 3000 m. II.3 Surface Tension The initial surface tension values corresponded to water free of surface-active substances, but decreased dras- tically at the effluent injection from 73 to 57 dyne/cm. The brewery effluent was chiefly composed of organic matter. 3000 m downstream, surface tension values stabilized at levels somewhat lower than the initial values (Fig. 1). II.4 Dissolved Oxygen Close to total saturation upstream of the outfall, the dissolved oxygen values dropped dramatically to 35% at the injection due to the high oxygen demand of bacterial activity and oxidation of organic matter. Oxygen lev- els progressively returned to their initial values 7150 m downstream (Fig. 1). II.5 Ammonium and Phosphates Organic nitrogen and phosphorus were introduced by the effluent and microbially mineralised to ammoni- um and phosphates. Phosphates reached maximum con- centrations 50 m and ammonium 300 m downstream as products of the breakdown of the introduced organic matter. These pollutants were totally metabolised at the downstream checkpoint of 7150 m (Fig. 2). II.6 Nitrate A product of the oxidation of ammonium, nitrate was initially only present at low levels. Its concentra- tion increased progressively along the length of the study section, levels did not totally stabilize at 8000 m down- stream of the effluent injection (Fig. 2). Figure 1. Evolution of temperature, dissolved oxygen and sur- face tension along the length of study section of the self-purifying stream (Peter 1994). 67Study of a Section of a Self-Purifying Stream in Specific Relation to its Water Flow Behaviour III. BIOLOGICAL ANALYSIS III.1 Evolution of the Biotic Community Initially, the Mettma was an oligosaprobic balanced stream ecosystem ty pical to the trout zone. It was inhabited by a wide variety of animal and plant species which constituted its biotic community. 40 different species of benthic fauna were indicative of this diver- sity (Fig. 3). The brewery effluent added an organic load equiva- lent to the raw effluents of 6000 inhabitants. This pro- foundly modified the ecological equilibrium of the eco- system (Fig. 4). The system became polysaprobic. Life conditions favoured the development of filamentous bacteria (Sphaerotilus natans) and ciliates and exclud- ed other species. In this first polysaprobic zone which stretched for approximately 300 m, the number of ben- thic species plummeted to 2. The filamentous bacteria eventually detached themselves and were transported several hundred meters further, where they were depos- ited and became important nutrients for colonies of sludge-worms (Tubifex tubifex) and chironomid larvae (Prodiamesa olivacea). This degradation zone where the breakdown of organic matter dominated, ended with the appearance of blackfly larvae (Odagmia ornata) and monocellular algae 2000 m downstream of the injec- tion point. This marked the beginning of the primary production zone, with a succession of plant species from algae and mosses to higher vegetation. Species of herbi- vores, such as freshwater limpets (Ancylus fluviatilis) and mayfly larvae (Baethis rhodani) as well as carnivores, such as stream amphipods (Gammarus fossarum) and Figure 2. Ammonium (NH4+), nitrate (NO3-) and phosphate (PO43-) concentration in μg/l, as well as the oxygen saturation in % (Peter 1994). Figure 3. Number of species (taxa) of animals on the stone sub- strate during the course of the self-purifying section (after Schreib- er 19756). Figure 4. Typical representatives of the benthic fauna the length of the self-purifying stream section (according to Peter 1994). 68 Heinz-Michael Peter, Christine Sutter, Wolfram Schwenk predatory larvae of various insect species, reinhabited the biotope. The trout-zone had fully regenerated after 7000 to 8000 m, the number of fauna species in the ben- thic zone reattaining its initial value. III.2 Polarities If we take into consideration the evolution, dis- tribution, variety, morphology, mode of nutrition and locomotion of the benthic fauna, the following can be observed: • At the beginning of the self-purifying study section, species diversity was very reduced while popula- tion density was high. The organisms generally had a homogenously segmented morphology with radial symmetry. Sensorial organs were very primitive. Most organisms were sedentary and saprophage. Their rhythm of activity depended only on the sup- ply of nutrients, their life activity being orientated towards metabolism. • At the end of the self-purifying study section - as before the eff luent outfall - there was a greater species diversity while the population numbers remained modest. The morphology of the organ- isms was more complex with heterogeneous seg- mentation, axial symmetry and a greater body sur- face area. Sensorial organs were located at the head, organisms were more mobile and were herbivores or carnivores. They followed day-and-night and season- al rhythms. Their activities were orientated towards sensorial functions and locomotion. IV. HYDRODYNAMIC ANALYSIS IV.1 The Drop Picture Method The Drop Picture Method, developed by Theodor Schwenk, allows the study of water’s aptitude for move- ment, its hydrodynamics. The method is carried out by the systematic and controlled agitation of a water sample through the impact of drops of distilled water released at 5 second intervals. Each impact on the very shallow water sample creates internal movement and flow forms. The addition of a tiny amount of glycerine to the water samples facilitates the photographic optical visualization of the flow movements via a Schlieren optic system. Suc- cessive drops renew the created flow movements, so that a whole series of 30 drop-generated images was docu- mented for each sample (Fig. 5). The Drop Picture Method was examined in the 2000s to optimise and standardize the testing method- ology.7,8 The results are usually interpreted qualitatively, but may additionally be analysed based on quantitative analysis of for example the degree of development of vortex forms. The Drop Picture Method indicates the given movement capacity of a water sample based on the level of complexity and differentiation of its internal flow forms. It is a morphological method, complementary to physical-chemical analysis, revealing more information about the hydrodynamic qualities of a water sample than its chemical composition. It is a system to evaluate water quality based on positive, life-giving criteria, rather than on exclusion of negative criteria. Figure 5. Drop Picture Method procedure and chlieren optic device. 69Study of a Section of a Self-Purifying Stream in Specific Relation to its Water Flow Behaviour IV.2 Application to the Section of Self-Purifying Stream The testing points were identical to those of the pre- vious studies. The water samples were analysed on the same day as they were collected. 30 images were pro- duced for each water sample. Here the 20th is selected to facilitate comparison between samples (Fig. 6). We will now discuss the most distinctive phases along the stream self-purifying section.Upstream of the eff lu- ent outfall, drop-generated image revealed a garlanded composition of vortices where more extended vortices alternate with more stocky ones. Leafy vortices could be observed, as well as radial dendritic structures. The successive 30 images during the analyse were relatively balanced, showing differentiated and varied structures, which were regenerated with each successive drop. Downstream of the effluent injection the drop-gen- erated images were significantly different. They were simply and solely composed of a disc-shape structure centred on the central point of impact. The forms were rudimentary, undifferentiated and monotonous. • 1800 m downstream, the disc-shape structure shrunk while in the centre the beginnings of differ- entiation - the buds of heads of vortices - could be identified. • About 3000 m downstream, the beginnings of leafy vortices and dendritic structures could be observed again. The closed disc-shape form had disappeared. By the end of the studied stretch of stream, the inter- nal flows of the drop-generated image had returned to be the varied, complex and differentiated and polymorphic as in the sample upstream of the effluent injection. V. COMPARISON OF THE DIFFERENT DESCRIPTORS V.1 Physical-Chemical Parameters and Hydrodynamic Evaluation V.1.1 Point of Inflexion The evaluation of the hydrodynamics of a given water sample is not based on any one drop-generated image but rather on the evolution of the ensemble of images. At some point each of the samples reaches a state where after a certain number of drops the gar- landed structure shrinks, disappears and is replaced by a more rigid disc-shaped structure. This is what we call the “point of inflexion”, which occurs sooner or later in the tests, depending on the hydrodynamic qualities of the sample in question. This is a useful parameter. The drop number at which it occurs can be used to compare water samples taken along the length of study stream in question. W hen a disc-shaped structure appears in the first drop-generated image, the point of inflexion has already taken place before the start of the testing. V.1.2 Comparison of Results The graphical comparison of the levels of oxygen saturation, the point of inflexion and surface tension demonstrated a relationship between levels of oxygen and the point of inflexion. This relationship was however not causal but demonstrated that there is a correlation between these two factors and water quality. In most of the measurement surveys, the evolution of physical and chemical parameters stabilised well upstream compared to where the point of inflexion had stabilised (Fig. 7). This in contrast only returned to its pre-effluent levels at the very end of the length of studied stream, parallel to the reestablishment of the biotic community to its initial Figure 6. Drop-generated images after the 20th drop the length of self-purification. Figure 7. Evolution of oxygen saturation, surface tension and point of inflexion (Peter 1994). 1800 m Upstream 3000 m 50 m 8000 m 700 m 70 Heinz-Michael Peter, Christine Sutter, Wolfram Schwenk levels. This point of inflexion thus appeared to be a more sensitive descriptor, indicative of the overall state of the stream ecosystem. V.2 The Biotic Gradient and Stream Hydrodynamics It appeared that along the stream in question: • Where the drop-generated image, in reference to previous images, had a closed, disc- and monoto- nous shape • Where the drop-generated image showed a lack of mobility and a predetermined evolution, • There was the least species diversity, with the pres- ence of sedentary, simple-structured organisms, whose activities were confined to their metabolic activity, that is to say after the effluent outfall. In addition: • Where the drop-generated images revealed a maxi- mum of polymorphic, diverse flow shapes, with the greatest complexity of movement in the water, where the images had a differentiated structure without being predetermined in their evolution • This is where biotic populations were varied and bal- anced, individual organisms having a more differen- tiated anatomy, complex nutrition and locomotion as well as being more sensitive to their environment due to more advanced sensorial organs, that is to say upstream of the effluent outfall and at the end of the stretch of self-purification. CONCLUSIONS This comparative study of the physical-chemical, biological, and hydrodynamic characteristics of a self- purifying section of the Mettma proved that there are parallels between the degree of diversity in the biotic community and the degree of movement diversity in water samples from the same testing stations. In the degradation zone, where metabolism processes deter- mined the physiology and activity of the animal popu- lation, water samples showed monotonous and weakly defined flow shapes. In contrast, in the primary produc- tion zone where, thanks to primary production by veg- etation, anabolic processes dominated, the water sam- ples showed diverse and differentiated water flow shapes. Thus, very different phenomena could carry the same signature of a shared intrinsic quality. Physical-chemical parameters are descriptors of a specific moment of the stream. The biological indicators reveal a more integrat- ed long-term picture of the water, whereas the hydrody- namic analysis revealed the momentary but holistic state of the water. REFERENCES: 1. Schwenk, T. (1967). Bewegungsformen des Wassers. Stuttgart. 2. Peter, H. M. (1994). Das Strömungsverhalten des Wassers in der biologischen Selbstreinigungsstrecke des Schwarzwaldbaches Mettma. Sensibles Wasser 4, 1-160, Herrischried. 3. Franke, U. & Schwoerbel, J. (1972). Hydrographie, Chemie und Nährstoffracht eines mit organischen Abwässern verunreinigten Gebirgsbaches. Arch. Hydrobiol. Suppl. 42, 95-124. 4. Reichardt W. & Simon, M. (1972). Die Mettma - ein Gebirgsbach als Brauereivorfluter. Mikrobiologische Untersuchungen entlang eines Abwasser-Substrat- gradienten. Arch Hydrobiol. Suppl. 42, 125-138. 5. Schwoerbel, J. (1972). Falkauer Fließwasser-Untersu- chungen an der Mettma. Arch Hydrobiol. Suppl. 42, 91-94. 6. Schreiber, I. (1975). Biologische Gewässerbeur- teilung der Mettma anhand des Makrozoobenthos: Methodenvergleich. Arch. Hydrobiol. Suppl. 47, 432- 457. 7. Wilkens, A., M. Jacobi & W. Schwenk (2000). Die Versuchstechnik der Tropfbildmethode. Dokumenta- tion und Anleitung. Sensibles Wasser 5, Herrischried. 8. Wilkens, A., M. Jacobi & W. Schwenk (2005). Understanding Water. Edinburgh.1 1Original title: Suivi du parcours d’autoépuration d’un ruisseau par la dynamique de ses eaux (2004). Société Hydrotechnique de France. SHF-publications 28èmes Journées de l’Hydraulique, 31-38. Paris Substantia An International Journal of the History of Chemistry Vol. 3, n. 1 Suppl. - 2019 Firenze University Press