No hard-and-fast rules can be laid down for the construction of bacterial filters, this method of treating sewage being of comparatively recent date. Any kind of tank that will hold water may be used. In some towns shallow pits with sloping sides have been excavated in the earth, the bottom and sides of the pits being lined with clay puddle. But such an arrangement can only be considered as a temporary makeshift; for permanent work the tanks are generally lined with concrete or blue bricks. Many bacterial filters have been made by utilising existing precipitating tanks at sewage treatment works. Experience tends to show that the depth of a filter tank should not exceed 4 ft. The size of the filters should be so proportioned to the amount of sewage to be treated that not more than 200 gal. or 250 gal. per square yard of filter are dealt with; and at least three sets of filters should be available, in order that each filter may be worked in an eight-hour cycle-that is to say, approximately, three hours for filling, two hours for standing quiescent while the bacteria are doing their work, one hour for drawing off the effluent, and two hours standing empty for aeration. Each set of filters should consist of a coarse filter and one or (preferably) two fine filters.

A site should be chosen that will allow of the effluents being discharged from the bottom of the first filter on to the top of the second, and from the bottom of the second filter on to the top of the third. The materials generally used for filling the tanks are - for the coarse filters on to which the sewage is first discharged, coke or clinkers of, say, 2-in. gauge; and for the fine filters, coke-breeze or screened cinders, of not larger gauge than 3/4in. and not finer than 1/4 in. Coal slack, burnt clay ballast, and other materials have been used with success for the body of the filter. It must not be forgotten that the tanks, when supplied with filtering material, will only hold about 40 per cent, of their original capacity. The raw sewage, before it is turned on to the filters, should be passed through a screen of some kind, otherwise rags, corks, cotton-waste, and other matters that are not properly sewage, and therefore not amenable to treatment, will be deposited on, and clog up, the surface of the filters. It is a great advantage to have a large tank, of a capacity sufficient to hold, say, half a day's sewage, in which a preliminary sedimentation and putrefaction may take place; the effluent from such a tank is in a much better condition for filter treatment than fresh sewage.

It is a usual though not an invariable practice to lay at the bottom of the filter-beds a central line of drain-pipes with open joints, and radiating lines of smaller pipes, also open-jointed, arranged herringbone fashion. Various contrivances are used for keeping the bottoms of the filters as open and accessible to air as possible. One device is to have the bottoms lined with two courses of bricks, the lower courses having open spaces of about 2 in. around each brick, and the upper course being close-jointed to keep the filtering material from being washed out. Unless a free supply of air can be made to circulate through the whole body of the filter after each emptying, there is not a chance of success.