A note on the forest soil as a biological filter in the sanitary purification of municipal waste water evaluated on the basis of Escherichia coli titre.

The paper presents the results of a 10-year study on waste water purification in forest soil, under conditions of lysimeters and in the field, using pine-, larch- and osier cultures. The investigation has shown that, under test conditions, treatment with waste water does not endanger the sanitary state of the soil and ground water. In weakly podzolised sandy soil of loose texture, at the soil profile depth of at least 150 cm, the watering dose of 25 mm, i.e. 25 l/m2, was found to be safe, when applied 4 times monthly during the vegetation season, from May to September, for the first 4 years of the experiment (annual dose of 500 mm). Initially the waste water had pH equal to 6.8-7.2, BOD5 of about 220 mg O2/l and an Escherichia coli cells number mostly 10(5) per ml, but after filtering through the soil, the E. coli cells number was 10 or less per 1 ml. After a lapse of 4 years the single waste water dose could be increased up to 50 mm (annual dose 1000 mm) without any detriment to the degree of sanitary purification of waste water.