Separation performance of a sewage-hydrocyclone anti-blockage device consisting of a hydrocyclone and an ejector-reflux component was studied on sand-water and domestic sewage with various diameters of underflow pipe. Experimental results demonstrated that the hydrocyclone anti-blockage device with continuous underflow had higher separation efficiency but negligibly higher power consumption than conventional hydrocyclones with a closed "grit pot". Contrary to the Kelsall's conversion separation efficiency

a more comprehensive separation efficiency for the hydrocyclone anti-blockage device was proposed to include separation efficiency of the device and amount of available water for heat pump system. Under the optimum diameter of underflow pipe in the range of 5 mm and 10 mm

i.e.

the optimum diameter ratio of the underflow pipe to the vortex finder between 12.5% and 25%

the sewage-hydrocyclone anti-blockage device had the separation efficiency of 92.6%-94.3% and the split ratio of 1.32%-2.54% for foulants of sizes < 4 mm in untreated domestic sewage. The split ratio was proportional to the underflow pipe diameter. Therefore

hydrocyclone separation could isolate foulants temporarily

clean water and recycle heat from sewage.