A septic system is a highly efficient, self-contained, underground wastewater treatment system. The simplicity of its operation and the low cost of construction and the ease of maintenance raise the depending on the septic tanks in rural areas. But its low efficiency was the main difficulty against the environmental aspects that raise the needs for simple modifications that increase the removal efficiency without complication of its construction or operation. Several trials had been made for this modification but most affects its simplicity & cost. This study used simple modified septic tank divided to three compartments one for settling, second for submerged bio gravel filter & third for upflow bio plastic filter. From three months operation for domestic sewage The study resulted the suitability of the modification to improve the removal efficiency of the septic tank from 50% for BOD & 60% for TSS to become 83.66% for BOD & 86.91% for TSS which are promising removal efficiencies for such simple system that raise the possibility to depend on it as one of the low cost treatment procedures for sewage treatment in rural areas.