S.V. RANADE Ph.D.(Civil Engineering)
Indian Institute of Technology, Kanpur July, 1976

Adoption of dual media filters has gained wide acceptability in the current design practice for water treatment plants in developed countries due to their high filtration capacities and comparatively low installation costs. Notwithstanding the greater need of such Ugh rate filters, in India, no development in this direction could take place due to non-availability of anthracite, a commonly used filter media for dual media filters. Besides field engineers have a general resistance to adopt something on field, the economics and performance of which has not been demonstrated on field scale. Possibility of using high grade bituminous coal as a filter media, in place of imported anthracite, was investigated from engineering as well as theoretical aspects. Detailed laboratory filter column studies and field scale trials were conducted for the purpose.

For studying the functioning  and the overall performance of coal—sand dual media filter on prototype scale,  one full scale rapid sand filter unit with 8 mld capacity was converted to dual media filter. After studying the hydraulics of various components of the filter, necessary changes were made in under drainage and filter piping to increase the filtration rate to about 4 time the designed rate without excessive headless in the system. Number of coal samples from various collieries were tested for all the relevant filter media properties and some selected varieties of coal obtained from Raniganj coal  fields were used for preparing the filter media.  As no large scale crushing and grading facilities existed for processing t}: coal to desired specifications, extensive trials and modifications were carried out on crushing and classifying equipment. After proper modificatlons, the plant could yield a recovery of 37% of  graded coal with desired size and shape from the raw slack coal against ion than 10% at the start. Accordingly about 55 Tonne of coal ma processed to yield tore than. 20 Tonne of graded coal which was then used as a filter media.

Conversion  to dual media filter increased the capacity of the filter from 8 to 30 mld at a coat of its. 45000 and the project was completed in about years. The converted filter was commissioned on October 4, 1975 and detailed studies on its performance as well as removal mechanisms operating in different layers of the filter under variable conditions were carried out for a period of 6 months. Field studies indicated that even under the existing, rather inadequate pretreatment conditions, the coal-sand filter operated at an average rate of 287 1pm/sq.m. with a minimum filter run of 12 hours and the filtrate quality and overall performance at this rate was quite comparable to the unconverted rapid sand filter operating at 98 1pm/sq.m.  Operation of the dual media filter at 332 fpm/ sq.m, reduced the filter run to 9 hours, with breakthrough of turbidity terminating the filter run. No leaching of phenolic  and other compounds was detected and the loss of coal due to attrition was within 9% per year. Proper pretreatment and better backwashing were found to be essential for continued better operation of the filter. 

From theoretical considerations, laboratory and field scale studies. showed that duel media filter acts essentially as two stage filter where top coal layer removes major turbidity and. sand layer does the work of polishing the filtrate quality. Two surface mats were observed to get formed in the two layers of the bed and the overall mechanisms of removal were found to be mechanical straining, interception, gravity and electro-kinetic  phenomena. The results also showed very significant influence of floc characteristics on the filter coefficient of different layers. On the basis of the observations, minor modifications in Ive’s relationships have been proposed which are expected to give a better understanding of the parameters controlling turbidity removals, to give more practical predictions of efficiencies of filter and to serve as guide to more effective operation and controls.

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