Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Valló Sándor: Összefüggés a felszíni vizek minősége és a hidrológiai viszonyok között

14 Hidrológiai Közlöny 1971. 1. sz. Special Conference Number, Szebellédy, L. tom perpendicular to the direction of flow and compressed air has been introduced through perforations of 0.5 to 0.7 mm diameter, under a pressure of 4.5 kp/sq.cm. Owing to the excessivelv high pressure the power consumption of aeration was similarly high, namely 0.15 kg 0

2 per kWh. The other method involved two vertical-shaft impellers of 2.20 m diameter, mounted on floating pontoons. The rig can be transported to any point within the Baldeney reservoir, or on the lower course of the Ruhr river. The power consumption of aeration is here 0.75 kg 0

2/kWh. The above measures, and first of all aeration by the turbines were found successful in overcoming periodic critical oxygen deficiencies in the Ruhr river. The improved oxygen supply is however perceptible after three days only on the lower Ruhr reaches the aerators whirhng first the bottom sludge and supplying oxygen thereto. A times of sudden oxygen deficiency observed simultaneously along the lower reach of the Ruhr river power production had to be abandoned, discharging the entire flow over the weirs to introduce by gravity a dissolved oxygen content of at least 4 mg/l into the water. Aeration of rivers is most efficient at weirs, where the entire cross section can be aerated. Experience gained with the Baldeney reservoir showed the installation of aeration equipment into existing structures to be justified. The rate of compressed air introduced above the runner can be controlled at the blower. In air-bubble aeration the entire cross-section must be affected, otherwise the bubble curtain may form a barrier deflecting the main current to the part witliout aeration. Experience gained with technology and economics has shown cascades to be warranted up to 3 m head for the sole purpose of aeration, complete saturation being almost attainable at this limit. At higher heads power production becomes predominant and aeration should be combined therewith. The problems encountered in the aeration of reservoir lakes are greater in number and complexity. In fact aeration is required to ensure, besides the introduction of oxygen, alsó a certain mixing of the water, in order to eliminate the adverse consequences of thermal stratification. Considerable attention has been devoted to this problem in the foreign literature, where the blowing in of air is considered superior to surface aeration. The air introduced in the vicinity of the bottom alleviates the oxygen deficiency in the zone of the bottom sludge. At the perforations clusters of bubbles are förmed, which appear at the surface in a circle of larger diameter, resulting in a rising current within this column. Operating experience has shown the advantage of aerating simultaneously the entire layer deficient in oxygen by suitable means, over attaining complete saturation at a particular point and then mixing with water low in oxygen. Besides aerators of the impeller type those operating on the principle of ejector pumps are alsó popular. For instance, on one of the lakes 5 Turbo-Oxidators are operated by the Ruhr Association. Acting as horizontal ejector pumps, the air is admitted in the form of finelv distributed bubbles into the low-oxygen zones and by an ingeniuons device the equipment is selfpropelled. b) Eutrophication control presents one of the most difficult problems in natural and artificial lakes serving the purposes of watersupply. Increasing degrees of trophicity are known to result in the mass prolification of algae in water. Although the role of the natural alga-population is important in the self-purification of surface waters, by photosynthesis dissolved oxygen is introduced into the water, large masses interfere seriouslv with watersupply and primarily with the treatment of water. The most detrimental consequences are taste- and odour complaints, increase of turbidity and discolouration, clogging of filters, algal blooming, deposition of algae in basins and distribution pipelines. The mass decay of algae is moreover accompanied frequently by serious oxygen deficiency. Eutrophication consists essentially of the enrichment of nutrients required by the phytoplancton organisms. Of these nutrients the nitrogén- and phosphorus compounds, trace elements and somé biologically active substances are the most important ones. Natural eutrophication is very difficult to control effectively. The process is accelerated by falling leaves and the excrements of birds. Bottom dredging, the removal of vegetat ion from the surroundings of the laké and the introduction of plánt devouring fish are measures that are diffieult to realize, often uneconomical and entailing other unwanted consequences, so that their justification and extent is highly debated both in the literature and practice. No positive, uniform opinion and policy can be adopted, the optimum control measures depending always on local conditions, the knowledge of particular biological conditions and the results of an economic analysis. The approach to artifical eutrophication is more uniform. The main objective of biological wastes treatment is generallv accepted as the complete removal of organic substances from the wastewaters. This is solved satisfactorily, in that part of the polluting substances is retained within the organisms, the other part being mineralized in the form of inorganic plánt nutrients. Inorganic nutrients are ensured in a similar manner in agriculture by fertilizers and soil erosion promoted by soil cultivation. Most effective eutrophication control would consist of the removal of inorganic nutrients imported from different sources, or the prevention of their access to water. The removal of inorganic plánt nutrients is referred to in wastes treatment as the "third stage". Methods include purely chemical ones, such combining chemical and physical processes and biological methods. A definite trend towards the latter is apparent. However, for the time being the actual inclusion of the third stage treatment is not generally practiced. The disposal of treated effluents by