PRIMARY-TREATMENT OF A SOYBEAN PROTEIN BEARING EFFLUENT BY DISSOLVED AIR FLOTATION AND BY SEDIMENTATION

The present paper reports on work performed on the recovery of proteins suspended in the effluent of a soybean protein plant. The work involved studies of effluent characterization, protein suspension destabilization, dissolved air flotation (DAF) and gravity settling. The flotation and settling tests were performed both at a batch bench scale and at a continuous plant scale in the actual industrial operation. The soybean proteins, present as a colloidal suspension in the effluent, are dispersed by combined electrostatic repulsion and steric stabilization mechanisms. The destabilization and aggregation of the proteins into high quality flocs were achieved by addition of 200-300 mg/l FeCl3, setting the pH at the isoelectric point of the proteins (pH 4.5) and an anionic polyacrylamide polymer (high molecular weight) dosage of 2-3 mg/l. The bench scale tests indicated under these conditions that a solid-liquid separation can be made by both DAF and gravity settling with DAF giving superior results. However, in the industrial plant tests, gravity settling proved generally to give better separation efficiencies. In plant operation the DAF process worked well only if the flocs were sufficiently hydrophobic and resistant to mechanical degradation, which was not always true in the actual operation. The settling process was much less sensitive to feed stock variations.