Activated sludge ballasting in pilot plant operation

Activated sludge ballasting by calcium carbonate, apatite and a blend of talc and chlorite was investigated in a continuous denitrification–nitrification pilot plant. The sludge–mineral material interaction was studied by analyzing the surface characteristics of the mineral materials, activated sludge and the fate of the mineral material in the treatment process. The blend of talc and chlorite had the highest positive surface charge and hydrophobicity. It was also the only mineral material to import metal ions for the flocculation mechanism. The blend of talc and chlorite improved both settleability and filterability, while calcium carbonate only improved settleability. Assuming that the same amount of mineral material in sludge gave the same sludge volume index (SVI), only about 50% of the added calcium carbonate interacted with sludge. Apatite had no positive effect at all. A one-time dose of 1.2 g of the blend of talc and chlorite per 1 g of mixed liquor volatile suspended solids (MLVSS) immediately improved the SVI from 800 ml/g to less than 200 ml/g. Thereafter, a daily dose of 0.1 g per 1 g of MLVSS maintained the low SVI. This indicates a lower dosage compared to batch tests. Mineral materials did not affect alkalinity, treatment efficiency, floc size distribution or the amount of filamentous microorganisms.