Hidrológiai Közlöny 1971 (51. évfolyam)

1. szám - Abos Bruno: A talajvízdúsítás vízminőségi problémái

Hidrológiai Közlöny 1971. 1. sz. 26 Quality problems in groundwater recharging ABOS, BRUNO Department Head (Budapest Municipal Waterworks) The term slow filtration includes natural, or artificial groundwater recharging and the slow filters developed in closed basins. Filtration in bank layers (bank filtration) can alsó be regarded as slow filtering, this problem, however, will not be dealt with here in any detail. Slow filtration is known írom ancient times, but for community water supply purposes it is used only since the beginning of the 19th century. The water resources available in the soil were increased first by groundwater recharging, and later artificial slow filters developed in closed basins were used to produce potable water. The increasing demand for drinking water of the communities promoted the development of rapid water treatment methods and thus groundwater recharging and artificial slow filtering was neglected for a time. Following World War II, the rapidly increasing demand of towns and industry for pure water of drinking quality, focussed attention on the problem of water treatment. At the same time the growing pollution of surface waters caused deterioration of the quality in drinking water produced by rapid filtration methods. Attention has thus turnéd again to slow filtration, this being able — between certain limits, of course — to eliminate the pollutions causing trouble, which would be very expensive to eliminate with rapid filtration methods. Slow filtration, under a new approach to utilization, is once again gaining ground. The basic aim is unchanged: to produce as much good quality potable water for man and industry, as possible. The new approach, coupled with this basic aim is to guarantee potable water free of pollutants, which although present perhaps in traces only, may still endanger the health of humán beings. We have aquired by now a rather good understanding of the potential technologies and attainable efficiencies which can and may be applied under particular conditions in ground water recharging or artificial slow filtration. It is a well-known fact that the necessary volume of good drinking water — apart from somé fortunate places, as for instance Romé or Munich — becomes more and more difficult, as well as costly, to .supply all over the world. The necessity of transporting water to ever greater distances and the use of rapid water purification technologies containing an increasing number of stages, cause the cost of drinking water production to rise steadily. Slow filtration has again become competitive, especially where conditions are favourable, e. g. suitable filtration areas are available. Two circumstances may limit the application of slow filtration. In the practice of slow filtration the velocity of seepage varies between 0.5—7 m/day. In groundwater recharging this is in generál no more than 0.25—2.0 m/day. The low rate of slow filtration, which, in the interest of suitable biological action must not be increased above 7 m/day, indicates the fact that the surface requirement of slow filters is many times that of the rapid ones. In many cases, the large area is impossible to provide, or at least causes considerable difficulties for waterworks. Owing to the even lower filtering velocity, groundwater recharging plants need even grater areas. In groundwater recharging, as well as in the case of slow filters, the filter surface is regenerated by scraping at intervals depending on the quaüty of raw water. This is carried out with the help of small machines, but often manuallv. The work is slow and labour consuming, and therefore costly. It is expected that under a particular set of conditions and where required, waterworks will surely apply somé method of slow filtration. Several facts already point in this direction. Most waterworks are faced with temporary or regular water shortage. Those drawing on welb attempt to increase the yield of potable water by groundwater recharging. Waterworks, supplying water clarified from surface waters will be forced to apply slow filtration as the last stage of the purification technology, due to the growing pollution of watercourses and lakes, and to the increasing quaüty demands as regards potable water. In developed, industrial countries, the renewed interest in slow filtration is the result of the fight for good potable water. The mechanism of slow filtration, highly complicated by the many factors involved, has been studied intensively. A number of physical, chemical and biological processes affect it in a complex and dynamically changing way. Several researchers developed theories concerning the mechanism of slow filtration. The first explanation was put forward by Piefke in 1880. According to him a thin layer is förmed on the filter sand, the pores of which remain effective until clogged. The validity of this assumption was not substantiated by further investigations. In 1915 Kisshalt published a theory which was supported by several researchers. He attributed the removal of bacteria on slow filters to the protozoa prolificating in the sand layer. He tried to support this theory with experimentál results. The removal of bacteria by protozoa, the Kisshalt theory was rejected later by Viehl, Liebmann and other researcher. They demonstrated other factors to be responsible for the great decrease of bacteria in the filter. In the period between World War I and II the Kisshalt theory was replaced by the absorption hypothesis. It was suggested that it is the absorption effect of sand particles, which retains the bacteria and other pollutants. They supposed the sand grains to have molecular electrostatic boundary surface potentials, and outward