providing first free flow space 15 the primary purpose of screening 18 is to prevent filter 9 from extending substantially into grating openings 17 in a more preferred embodiment screening 18 should contain no rough edges that face the interior of sludge receiving cavity 5 in order to avoid tearing screening 18 or filter 9 as filter 9 is removed from tank 1 in another preferred embodiment screening 18 has four to six openings per linear inch and is made of stainless or galvanized steel or plastic or fiberglass if grating openings 17 are sufficiently small screening 18 need not be used in another preferred embodiment grating 16 should be made of a strong corrosion resistant material such as stainless or galvanized steel or plastic or fiberglass in an alternate preferred embodiment first support means 14 comprises a heavy coarse screening not shown mounted over bed 2 and walls 3 filter 9 rests over coarse screening to provide first free flow space 15 if necessary screening 18 may be used in conjunction with coarse screening in a more preferred embodiment coarse screening should be a strong corrosion resistant material such as stainless or galvanized steel or plastic or fiberglass in another alternative preferred embodiment first support means 14 comprises corrugated sheets 41 mounted over bed 2 and walls 3 filter 9 rests over corrugated sheets 41 to provide first free flow space 15 if necessary screening 18 may be used in.