Process optimisation of a combined system of floating medium and sand filter in prawn farm effluent treatment

A treatment technology known as " a combined system of Boating medium and s and filter (FMSF)" was developed and tested successfully with prawn farm ef fluent. This system has a remarkable technoeconomical advantages. Laborator y and semi pilot-scale studies were carried out to optimise the Biter bed d epth, backwash method and other operating conditions. The dewatering charac teristics of sludge from filter backwash was also analysed. The filter was operated at a high rate. The experimental results indicated that:; (i) in c ase of no in-line chemical addition, the smaller ratio between floating med ium and sand filter depth gave rise better filter performance. At the filtr ation rate of 7.5 - 20 m(3)/m(2).h and with an in-line chemical addition, t he suitable depth of floating medium varied from 400-1000 mm for a sand fil ter 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 d irection and then followed with upflow of water for another 30 seconds. Bac kwash water amount comprised only 1.2-1.8% of the filtered water production . A mechanical backwash system using rotating paddles is a promising altern ative for floating medium filter; and (iii) the filterability of the sludge from filter backwash was low in case of no in-line chemical addition (spec ific resistance, r = 9.34 x 10(10) m/kg) but improved with in-line floccula nt addition (r = 3.07 x 10(9) - 1.29 x 10(10) m/kg). (C) 1998 Elsevier Scie nce Ltd. All rights reserved.