section 145 as with prior coagulants preferred constructions employ the same coagulant for the fifth coagulant 150 as is employed as the first coagulant 30 the second coagulant 90 the third coagulant 100 and or the fourth coagulant 140 with other coagulants also being possible in the illustrated construction the third section 145 includes a mixed media filter 155 that receives the third flow 120 including the fifth coagulant 150 if added near a top portion 160 of the filter 155 the mixed media filter 155 includes the top portion 160 and a bottom portion 165 that supports an underdrain structure 170 mixed media 175 e g gravel sand fine heavy density material and the like is arranged above the underdrain structure 170 such that the particle size becomes smaller from the top portion 160 to the bottom portion 165 this course to fine arrangement contributes to the filter 39 s ability to capture unwanted contaminate particles to produce a high quality low contaminant effluent 20 e g drinking water as the third flow 120 passes through the filter media 175 additional contaminates are removed the flow eventually reaches the underdrain structure 170 which collects the fluid and discharges it from the multi stage treatment system 10 as the effluent 20 periodically the mixed media filter 155 is backwashed as illustrated in fig2 to remove the contaminates captured by the filter media 175 during the backwash water and or air 178 are introduced into the underdrain.