Use of calcium carbonate as a wet contact filter

The groundwaters in Finland are usually acidic and soft. In fact, the most common quality factor necessitating water treatment procedures in Finland is the acidity of water. Following this come excessive levels of iron or manganese. Biological methods, such as a combination of dry filtration and slow sand filtration, are commonly used to remove iron and manganese and raise pH. Several advantages can be gained by using limestone filtration to replace the dry filter at the beginning of the process, and as alkalising wet filtration before the slow sand filter. Limestone wet filtration can be combined with aeration, if necessary. This research investigated the suitability of combined alkalising wet filtration and slow sand filtration for the treatment of water from the Monttu water supply plant in Liminka, Finland. It was studied through a series of both short-term filtration tests and long-term pilot tests. Alkalising wet filtration combined with slow filtration proved to work well with this water. The treatment reduced the iron content from 3·5 mg/l to 0·05 mg/l and the manganese content from 0·09 mg/l to < 0·01 mg/l. The pH of the raw water was 6·2, and that of the treated water was 7·5.