Development of an improved solids-liquid separation reactor for floatable & settleable solids in DAF slurry of a slaughterhouse

Abstract

The dissolved air flotation (DAF) is used widely for wastewater treatment as a primary treatment unit to remove suspended solids (SS) and oil & grease (O & G). However, the less studied part is the management of concentrated slurry from the DAF operation that exhibit both settling and floating nature. It is a common practise so far to further dewater the DAF slurry by decanting centrifuge. This study addresses the problems of dewatering the DAF slurry by developing a simple yet novel solid-liquid separation unit on a pilot scale and its implementation on a full-scale. The DAF slurry (SS:9100 ± 830 mg/L) contained settled and floating or suspended solids (SS) layers of 125 ± 9 and 130 ± 11 mL/L, respectively, and a distinct clear water zone of 744 ± 16 mL/L sandwiched between two. The solid-liquid separation was achieved using two reactors in series. In the first reactor, the settleable solids are separated and the overflow containing floatable suspended solids are taken to the second unit and clear water was recovered from the bottom of the reactor. The SS removal efficiency was ~95 % and the volume reduction of sluryy was 70 % at an overall hydraulic retention time (HRT) of 2 h. The rest of the thickened slurry has SS concentration of 32,174 to 33,866 mg/L indicating effective thickening. As a result of volume reduction, the operation time of decanting centrifuge was also reduced by 70 % and hence a total power saving of 77 to 117 kW/day (for slurry volume of 10 to 15 m3/day).