sand filter systems used in wastewater: 1. Intermittent Sand Filters The treatment involved in the case of intermittent sand filters applies the sewage, that has already undergone preliminar treatment, onto the filter beds of wastewater sand filter at regular intervals. By this, air can enter the interstices of the bed betwee the dose of sewage to supply the required aerobic bacteria. 2. Construction The filter consists of a layer of clean, sharp sand, with an effective size 0.2 - 0.5 mm and of uniform coefficient 2 - 5, 75 to 105 cm deep having under drains, surrounded by gravel to carry off the effluent. The sewage is applied by means of a dosing tank an siphon; it then flows into troughs laid on the filter bed. The troughs have side openings, which allow the sewage to flow on the sand. To prevent any displacement of sand, blocks may also be used underneath the sewage streams. After an interval of 24 hours sewag is now applied over a second bed while the first bed rests. Usually, three to four beds may thus be working in rotation. During th resting period, the dried sludge accumulating on the surface of wastewater sand filter is scraped off. The organic loading of the filter bed is not heavy, only 0.825 to 1.1 million liters per hectare per day. Use It is found that the effluent from an intermittent wastewater sand filter is usually better in quality than that resulting from an other type of treatment and can even be disposed off without dilution. However, because of the large land area required, a wastewater sand filter of this type is now seldom constructed in cities. They are primarily suited for institutions, hospitals and other small installations.