The ballasting performance of various mineral materials was investigated using batch settling tests. These tests revealed the complexity of the ballasting phenomenon; both the sludge and the mineral material affected the ballasting performance. Differences between the materials could be localized using the Tukey test, but no correlation between the ballasting performance and 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 ballasting action. A set of settling tests was done with activated sludges and mineral materials with known surface characteristics. The linear regression analysis produced a model (r2= 0.85) for ballasted settling where the important variables were sludge volatile fraction; mineral material dosage, density, specific surface area, cation exchange capacity; sludge suspended solids, mineral material particle size, and hydrophobicity. A batch pilot study 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 possibility to increase the inflow by 67%, and the limiting solids flux by 155% when calculated as kgSS/m2h and 43% when calculated as kgVSS/m2h. This study gives a clear indication of the possibility of improving the sludge settling by ballasting.
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