By intermittent nitration is understood that filtration in which the filtering material is systematically and adequately ventilated, and where the water during the course of filtration is brought in contact with air in the pores of the sand. In continuous filtration, which alone has been previously considered, the air is driven out of the sand as completely as possible before the commencement of filtration, and the sand is kept continuously covered with water until the sand becomes clogged and a draining, with an incidental aeration, is necessary to allow the filter to be scraped and again put in service.

In intermittent filtration, on the other hand, water is taken over the top of the drained sand and settles into it, coming in contact with the air in the pores of the sand, and passes freely through to the bottom when the water-level is kept well down. After a limited time the application of water is stopped, and the filter is allowed to again drain and become thoroughly aerated preparatory to receiving another dose of water.

This system of treating water was suggested by the unequalled purification of sewage effected by a similar treatment. It has been investigated at the Lawrence Experiment Station, and applied to the construction of a filter for the city of Lawrence, both of which are due to the indefatigable energy of Hiram F. Mills, C.E.

In its operation intermittent differs from continuous filtration in that the straining action is less perfect, because the filters yield no water while being aerated, and must therefore filter at a greater velocity when in use to yield the same quantity of water in a given time, and also on account of the mechanical disturbance which is almost invariably caused by the application of the water; but, on the other hand, the oxidizing powers of the filter, or the tendency to nitrify and destroy the organic matters, are stronger, and in addition, if the rate is not too high, the bacteria die more rapidly in the thoroughly aerated sand than is the case with ordinary filters.

It was found at Lawrence in connection with sewage filters that when nitrification was actively taking place the numbers of bacteria were much lower than under opposite conditions, and it was thought that nitrification in itself might cause the death of the bacteria. Later experiments, however, with pure cultures of bacteria of various kinds applied to intermittent filters with water to which ammonia and salts suitable for nitrification were added, showed that bacteria of all the species tried were able to pass the filter in the presence of nitrification, producing at least one thousand times as much nitrates as could result in any case of water-filtration, as freely as was the case when the ammonia was not added and there was but little nitrification. These results showed conclusively that nitrification in itself is not an important factor in bacterial removal, although nitrification and bacterial purification do to some extent go together; perhaps in part because the nitrification destroys the food of the bacteria and so starves them out, but probably much more because the conditions of aeration, temperature, etc., which favor nitrification also favor equally, and even in its absence, the death of the bacteria.

The rate at which water must pass through an intermittent filter is, on account of the intervals of rest, considerably greater than that required to give a corresponding total yield from a continuous filter, and its straining effect is reduced to an extent comparable to this increase in rate; and if other conditions did not come in, the bacterial efficiency of an intermittent filter would remain below that of a continuous one.

As a matter of fact the bacterial efficiency has usually been found to be less with intermittent filters at the Lawrence Experiment Station, when they have been run at rates such as are commonly used for continuous filters in Europe, say from one and one half to two million gallons and upwards per acre daily. With lower rates, and especially with rather fine materials, the bacterial efficiency is much greater; but it may be doubted whether it would ever be greater than that of a continuous filter with the same filtering material and the same total yield per acre. The number of bacteria coming from the underdrains is apparently generally less, and with very high summer temperatures much less, than in continuous filters, and this often gives an apparent bacterial superiority to the intermittent filters.

The effluents from intermittent often contain less slightly organic matter than those from continuous filters; but, on the other hand, hardly any water proposed for a public water-supply has organic matter enough to be of any sanitary significance whatever, apart from the living bodies which often accompany it; and if the latter are removed by straining or otherwise, we can safely disregard the organic matters. In addition, the water filtered will in a great majority of cases have enough air dissolved in itself to produce whatever oxidation there is time for in the few hours required for it to pass the filter, and it is only at very low rates of filtration that intermittent filters produce effluents of greater chemical purity than by the ordinary process. The yellow-brown coloring matter present in so many waters appears to be quite incapable of rapid nitrification; and where it is to some extent removed by filtration, the action is dependent upon other and but imperfectly understood causes which seem to act equally in continuous and intermittent filters.

The peculiarities of construction involved by this method of filtration will be best illustrated by a discussion of the Lawrence city filter designed by Hiram F. Mills, C.E., which is the only filter in existence upon this plan.[30]