Vízügyi Közlemények, 1969 (51. évfolyam)

4. füzet - Rövidebb közlemények és beszámolók

(70) The realization of this ambitious plan would be impossible without expanding existing, and constructing new sewer systems. According to data from 1960 the percentage of population connected to a public sewer system was 22, which increased to 24 by 1965, when sewer systems of 4900 km total length served altogether 2.5 million inhabitants. The average for the economically developed countries was about 60% in 1960. This level was set as the objective of the 20 year development plan from 1965 to 1985 by the National Committee for Technical Development. New sewers of altogether 31 000 km length must be built during the coming 20 years. Most of the networks will be designed as separate systems. The development programme presents a serious challenge also for the designers and forces them to revise many a "classical designing principle". Owing to the flat character of the country, the poor, alluvial soil conditions (sand, fine sand), the high and usually aggressive groundwater, economic operation (few lifting stations) had to be abandoned for reasons of investment policy. By a number of lilting stations and using the minimum required slopes, sewers deeper than 3.5 to 4.0 m could be avoided. The considerable tasks incorporated in the 20 year development programme raised the problem of sewer systems as well. Under the given circumstances preference was given to a separation system for the following reasons: 1. Development can be started on the more urgent sewer system, 2. initial investment costs are considerably lower than with the combined system, 3. by the gradual utilization of investment funds, their higher efficiency can be secured, 4. a separation system is capable of following more flexibly occasional departures from the original settlement development plan, 5. the danger of pollution to the recipients is reduced, 6. possibility is offered for constructing the storm sewer system in several stages (e.g., initially by building a modest network of open ditches), so that the storm sewer system can be more readily adapted to changes in settlement pattern, 7. lower wastes volumes must be handled at intermediate lifting stations, obviating the need for constructing deep sewers, and 8. a basic concept can be developed for regional wastes disposal and treatment. THE ROLE OF COAGULATION AND ADDITIVES IN WATER TREATMENT By L. Bulkai and dr. Z. Körös (For the Hungarian text see pp. 339) The reactions enabling us to treat water by settling occur at the surface of substances carried in suspension. The suspended particles carry negative charges and thus repel each other. These negative charges are neutralized by the positively charged complexes of metal hydroxides freshly formed from the salts added. These metal hydroxides are further modified by natural processes until neutral floes including already the polluting substances are formed. The former, so-called perikinetic process is then replaced by an orthokinetic one during which the negative floes approached by a mixing action to permit the development of mass attraction forces under the action of which larger floes of greater weight are formed. For these processes to take a normal course chemical and physical criteria must be satisfied, such as pH-value, hydrocarbonate content, anions, temperature, concentration of suspended matter. Whenever necessary and possible, these must be modified artificially to improve the efficiency of water treatment. Under certain circumstances, especially in winter, the addition of metal salts fails to secure the desired cleannes and then auxiliary additives are used. These are long-chain polyelectrolytes, the most frequently applied types of which are anionic, i.e., of negative reaction. When these are introduced shortly after the addition о metal salts into the water to be treated, then they bind the still positive metal hyd