Where the pollution is less, so that the dissolved oxygen contained in the raw water is sufficient for the oxidation of the organic matters, continuous filtration will give substantially as good results as intermittent filtration, and in other respects it has important advantages. The application of intermittent filtration for the treatment of public water-supplies is thus somewhat limited, and, as a matter of fact, it has been used in only a few cases. For the treatment of very highly polluted waters double filtration has been used in a number of cases, notably by the Grand Junction Company at London, at Schiedam in Holland, and at Bremen and Altona in Germany. At the two first-mentioned places two separate systems of filters are provided differing somewhat in construction, the first filters being at a higher level than the after filters. The first filters supply water of comparative purity, and very constant composition, to the after filters, which are able to treat it with great efficiency and at very low operating cost. This procedure is probably the most perfect which has been used for the removal of disease-producing qualities from highly polluted waters; and the cost of the process may not be as much greater than that of simple filtration as would at first appear, because the cost of cleaning the after filters is merely nominal, and the attendance, pumping, etc., are practically common to both sets of filters, and are not materially greater than they would be for a single set. For very bad waters the first filters might appropriately be intermittent, while the after filters should be continuous. This was the procedure originally intended for Lawrence, but the intermittent filter first constructed yielded such very good results that it has not been considered necessary to complete the plant as originally projected.