La précipitation des phosphates comme troisième stade d’épuration des eaux résiduaires

Abstract

The increasing pollution of our lakes and streams gives cause for real apprehension. The modem water purification units work in two steps, using a mechanical and a biological process. The effect of purification, especially the elimination of nitrogen and phosphorus compounds, is inadequate.
Our lakes are especially threatened by overfertilizing. Measures which fight the resulting weed and algae growth are inadequate as a permanent solution. Only the drastic reduction of the amount of incoming fertilizers has a chance of success.
Because of the high costs, the removal of all fertilizers from polluted water is only possible in special instances. Another possibility to consider is the systematic elimination of particular fertilizer compounds.
The denitrifying of sewage water for example, is in principle possible and appears to be attainable in practice. The precipitation of phosphates however, is at this time the most promising measure. From a chemical standpoint and from price considerations ferric chloride and lime must be considered as the best possible precipitants. With lime the phosphate is precipitated as hydroxyapatite. When ferric chloride is applicated, the phosphate is incorporated into the oxyhydrate precipitate. In practice, the elimination of phosphorus compounds can be achieved either by simultaneous precipitation (direct addition of the precipitating agent to the biological purification step) or by subsequent precipitation (separation in flocks in a separate third step).
Tables II and III show the presently available results and cost comparisons for different procedures and precipitants. A direct comparison is of course difficult, since each case of polluted water presents a special situation which must be carefully studied.
Ferric chloride is doubly interesting from the water protection standpoint, since an advantageous removal of corrosive waste liquids from galvanizing works can be simultaneously achieved.
Questions pertaining to the precipitation of phosphates have in no way been completely answered. Especially the elimination of the phosphate slurry and its behavior under reducing conditions needs further investigation.
The precipitation of phosphate as a third purification step is chemically sound, technically logical and economically justifiable.
Thus there is a good chance in the long run to improve the condition of our lakes and streams by systematic application of all available technical possibilities.