Grain refinement of aluminium and its alloys is a common industrial practice. Fine equiaxed, grain structure leads to improved castability, strength, machinability, formability, and good surface finish. Filtration is one of the widely used technologies to remove inclusions from the melt. Ceramic Foam Filters (CFFs) are commonly used to clean the aluminium melt before the casting process. However, at a high inclusion load and with grain refiner addition, reduced filtration efficiency is well known to occur. In the current work, the filtration behaviour of CFFs with three different levels of inclusions and grain refiner has been systematically studied in plant scale pilot trials at Hydro’s reference centre in Sunndalsora, Norway. The results show that oxide films capture grain refiner particles. Grain refiners tends to agglomerate heavily with inclusions at higher inclusion content. These heavy and compacted small clusters are more likely to be released from the CFF during the filtration process. A little effect from grain refiner addition on filtration efficiency is observed when the level of grain refiner and chips addition is relatively low or when both are high, but not with high grain refiner addition with middle level of inclusion load. The threshold of the grain refiner addition effect is further discussed in this paper.