A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science, School of Environment, Resources and Development

Thesis (M.Sc.) - Asian Institute of Technology, 1985

Bangkhen Water Treatment Plant used dual media filter to remove the suspended solids from the effluent of solid-contact clarifier. The dual media filters used contains 80 cm of anthracite (E.S. = 0.85 mm; U = 1.5) over 40 cm of sand (E.S. + 0.5 mm, U = 1.5) at a filtration rate of 13.75 m3/m2.h. In this study the possibility of using narrowly graded sand as sole filter medium was investigated. The two sand sizes studied (1.00 to 1.19 mm; 1.19 to 1.41 mm and 1.20 m depth) indicated that the narrowly graded coarse medium produces comparable filter quality while leading to less headloss development. In this case the headloss was found to be more uniformly distributed especially when high filtration rates (>15 m3/m2.h) were used. The backwash velocity computation made for single coarse medium and dual media filters indicated that the former required much higher backwash rate than the latter for comparable degree of expansion. The declining rate filter experiments conducted with single medium filters indicated that the headloss development is lesser in the case of declining rate filtration compared to constant rate filtration for the same water production. This indicates the declining rate filtration can be operated for a longer time before backwashing which will reduce the significantly the operating cost of filter. The headloss equation H/Ho = 1+KO is found to be valid for constant rate filters and the value of K decreased with the increase in filtration rate and increase in sand size. The prediction of headloss for declining rate filters was satisfactory when using the K value of the average (both arithmetic and geometric means) velocity.