A treatment technology known as “a combined system of floating medium and sand filter (FMSF)” was developed and tested successfully with prawn farm effluent. This system has a remarkable techno-economical advantages. Laboratory and semi pilot-scale studies were carried out to optimise the filter bed depth, backwash method and other operating conditions. The dewatering characteristics of sludge from filter backwash was also analysed. The filter was operated at a high rate. The experimental results indicated that: (i) in case of no in-line chemical addition, the smaller ratio between floating medium and sand filter depth gave rise better filter performance. At the filtration rate of 7.5–20 m3/m2.h and with an in-line chemical addition, the suitable depth of floating medium varied from 400–1000 mm for a sand filter depth of 400 mm; (ii) frequent (once in every 90–120 minutes) but short duration of backwash (not more than 60 seconds) was found to be suitable. During the backwash, the water and air were sent for 30 seconds in upward direction and then followed with upflow of water for another 30 seconds. Backwash water amount comprised only 1.2–1.8% of the filtered water production. A mechanical backwash system using rotating paddles is a promising alternative for floating medium filter; and (iii) the filterability of the sludge from filter backwash was low in case of no in-line chemical addition (specific resistance, r = 9.34 × 1010 m/kg) but improved with in-line flocculant addition (r = 3.07 × 109 − 1.29 × 1010 m/kg).

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