In this research, possibility of reducing excess sludge production in wastewater treatment was investigated using a combined anaerobic and aerobic submerged biological filter in a pilot scale. The physical model designed, erected and operated consisted of two pipes of PVC type with 147mm and 237mm diameter used as aerobic and anaerobic filters, respectively. The effective height of porous media in these filters was 70cm. Two filters were connected to eachother in a series form and the resulted system was loaded using synthetic wastewater based on sucrose in the range of 1.91 to 30.61 kg/m3 for anaerobic filter and 1.133 to 53.017 kg/m3 for aerobic filter. For similar loadings, the aerobic filter showed efficiency of 1.8 times that of anaerobic filter in removal of soluble COD. Return of 100% flow from the aerobic filter to the anaerobic filter for 30kg/m3.d of organic loading increased the efficiencies of the anaerobic filter, the aerobic filter and the combined system as 17%, 14% and 15%, respectively and the effect of the return of the flow was more pronounced in smaller hydraulic retention times and larger loadings.100% return of the flow reduced the yield coefficient for the whole system to 0.037 for 53 kg/m3 loading which is a suitable value with regard to the scheme and no use of chemical materials such as chlorine and ozone. This coefficient reached a value as small as 0.007 in common loadings (7.5kg/m3) for 100% return of the flow which is very close to zero. So, this method could be considered as a complete biological treatment with low excess sludge and could be assessed in full scale.