Specific deposit and models of horizontal counter-current filtration

Abstract

Horizontal filtration with counter-current principle has got distinct advantages over conventional rapid sand filters (RSF), in terms of longer duration of filter run and better utilization of the entire media depth and overcomes most of the shortcomings of RSF, such as mud ball formation, development of negative pressure inside the filter media at relatively shallow operating depths of water over the filter media, etc. Even direct filtration may be feasible without secondary sedimentation. The present study was carried out in the laboratory on various models of horizontal filters with counter-current flow under varying discharge conditions with constant head, using sand and gravel as filter media and bentonite clay suspension in water as turbidity. In the coarsest media, incremental head losses were found to increase with time whereas in the finest media, it was found to be reversed. In the intermediate layers, incremental head losses increased at the initial filter run but decreased later on. The increase in specific deposit resulted in incremental head loss whereas decrease in permeability caused a decrease in approach velocity and hence decrease in incremental head loss. Coarsest media layer attracts maximum deposit and leads to more head loss increase as compared to other layers. Exponential decrease in effluent discharge in three distinct stages was observed during the filter run. Such a variation is modelled. Specific deposit is maximum in coarsest media but decreases sharply along the depth of media. Such a variation is also modelled. The depth of the media is increased in order to maintain throughout quality requirements. A model is presented to relate depth of media with duration for a discharge of more than 100 litres per minute per metre square (1 min −1 m −2). It was observed that due to small magnitudes of specific deposit in the sand and gravel media, the exponential constants y and z in the Ives' model of efficiency are not important. The exponential constant x is however found to play a significant role. Methodology of evaluation of the exponential constant x in Ives' model is presented. This constant x is shown to remain around 1.7.