Destabilization of biological solids with ferric chloride

Abstract

When ferric chloride is applied to a mixture of sludge pinpoint flocs and dispersed cells, a preferential removal of the flocs is observed at low dose levels. The dispersed cells, which constitute a small percentage of the total suspended solids, require higher dose of coagulant for their complete removal. On a weight basis, the presence of dispersed cells in an activated sludge effluent will cause a significant increase in the stoichiometric quantity of ferric chloride required for complete removal of all suspended biological solids. The coagulation process can be enhanced using a two-step pH control method. Instead of maintaining the solution pH constant at the optimum level during the period of addition of ferric chloride, it was allowed to drop and then adjusted back to the optimum level. The two-step pH control method, which can be implemented by delaying the addition of bases required for pH adjustment, results in more efficient use of the coagulant for destabilizing the biological solids. Results of laboratory studies employing this technique have shown a 25% reduction in the ferric chloride dose requirements.