Intermittent Sand Filters
The treatment involved in the case of intermittent sand filters applies the sewage, that has already undergone preliminary treatment, onto the filter beds of wastewater sand filter at regular intervals. By this, air can enter the interstices of the bed between the dose of sewage to supply the required aerobic bacteria.
The wastewater sand 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 cms deep having under drains, surrounded by gravel to carry off the effluent. The sewage is applied by means of a dosing tank and siphon; it then flows into troughs laid on the filter bed. The troughs on the wastewater sand filters 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, sewage is now applied over a second bed while the first bed rests. Usually, three to four water treatment sand filters may thus be used in rotation. During the resting period, the dried sludge accumulating on the surface of filter is scraped off. The organic loading is not heavy on the sand filter. Wastewater application is only 0.825 to 1.1 million liters per hectare per day.
It is found that the effluent from an intermittent sand filter is usually better in quality than that resulting from any other type of treatment and can even be disposed off without dilution. However, because of the large land area required, a filter of this type is now seldom constructed in cities. They are primarily suited for institutions, hospitals and other small installations.