EFFECT OF BACKWASHING ON THE FUNCTIONING OF LIMESTONE FILTRATION; RESULTS OF A PILOT TEST

Acidity and excess amount of iron and manganese are the most common problems in Finnish groundwater demanding water treatment measures. Also, groundwater in Finland is usually soft. Biological methods, such as a combination of dry filtration and slow filtration, are commonly used. This method removes iron and manganese from water and increases the pH-level somewhat, as well as reduces CO2-content. However, this treatment method does not raise the hardness of water and the raise in the pH-level may also prove to be modest, which has led to the use of lime filtration at the end of process in some cases.

Limestone filtration in pretreatment as a form of wet filtration prior to slow filtration has many advantages. This pretreatment method can be combined with aeration. Combination of alkalizing wet filtration by calcium carbonate and slow sand filtration raises the pH and increases hardness of water, as well as removes iron and manganese from water.

Regardless of the advantages, the use of limestone filtration in the pretreatment of water with a high iron and manganese content has been little, mainly because of the fear of losing the efficiency of the limestone filter. In this research, the long term functioning of alkalizing wet filtration was tested. The research period was over 19 months, during which groundwater containing iron and manganese was led through wet limestone filters. Different backwashing methods were experimented and their effects on the functioning of the filters were studied. Three parallel limestone columns were used in the study, and each column was backwashed by air/water using varying backwashing velocities. The surface loading used in backwashing varied from 5 to 25 m/h when the backwashing was done by water and from 15 to 60 m/h when done by air. The tests showed no correlation between the efficiency of the backwashing and the reduction of Fe or COD. The reduction of manganese was lowest in the filter with the most efficient backwashing; alkalizing efficiency, however, proved to be slightly better.