In this type of wastewater filtration, the sewage applied on the contact material is allowed to stand undisturbed for some time before, being emptied and an interval is allowed before recharging the contact beds. During the ‘contact period’, when the filter is standing full, the fine suspended particles of sewage are deposited on the contact material and worked over by the anaerobic organisms. During the ’empty period’ that follows next, the deposited matter is oxidized by the aerobic bacteria. It is then washed off the contact material and carried out with the sludge effluent on the next emptying of the tank.
Contact beds are watertight tanks with masonry walls and very much similar in construction to an intermittent sand filter. The contact material is made of broken stone called ballast and is of 2.5 – 7.5 cm gauge. The tank is filled with the sewage over a period of an hour; allowed to stand full over a period of two hours, then emptied through underdrains. This process takes another hour. The tank is now left empty for 3 to 4 hours before admitting the next charge. (Thus with a total working period in a shift of 8 hours, the contact bed can be worked in three shifts daily). The organic loading in this case is about the same i.e., 1.1 million liters per hectare per day.
The contact beds method is now only of historical interest and not commonly used. This is mainly because of the loss of efficiency brought about by the exclusion of air when the tank is standing full. For an efficient biological action, it is imperative that the aeration should be through the mass of sewage. It has therefore, been superseded by more efficient biological wastewater treatment process, as in the case of trickling filter wastewater treatment method and activated sludge plants. However, the contact beds have some merit when compared to the trickling filters as:
(i) Lesser operating head required
(ii) Freedom from filter (psychoda) flies
(iii) Lesser nuisance due to odor