Activated sludge ballasting in batch tests

The ballasting performance of various mineral materials was investigated us ing batch settling tests. These tests revealed the complexity of the ballas ting phenomenon; both the sludge and the mineral material affected the ball asting performance. Differences between the materials could be localized us ing the Tukey test, but no correlation between the ballasting performance a nd the density or the particle size distribution was found. It was assumed that the interaction mechanism between the activated sludge and the mineral material, and hence the surface characteristics, are crucial for the balla sting action. A set of settling tests was done with activated sludges and m ineral materials with known surface characteristics. The linear regression analysis produced a model (r(2)=0.85) for ballasted settling where the impo rtant variables were sludge volatile fraction; mineral material dosage, den sity, specific surface area, cation exchange capacity; sludge suspended sol ids, mineral material particle size, and hydrophobicity. A batch pilot stud y was performed with calcium carbonate and the simulation program EFOR 2.2 was used to simulate the effect of the ballasting agent on sludge settling in connection with a higher inflow and higher sludge concentration than was possible to achieve in the pilot plant. EFOR 2.2 simulations indicate a po ssibility to increase the inflow by 67%, and the limiting solids flux by 15 5% when calculated as kgSS/m(2)h and 43% when calculated as kgVSS/m(2)h. Th is study gives a clear indication of the possibility of improving the sludg e settling by ballasting. (C) 1999 Elsevier Science Ltd. All rights reserve d.