Abstract: A polluted water treatment system 10 operated to reduce impurities in sewerage polluted water to an acceptable level for dumping. The system includes a horizontal subsurface filter (HSF) 12 having an inlet zone 18 through which the inflow of polluted water is permitted to pass, a treatment zone 20 which permits the polluted water to undergo treatment by removal of at least some of the impurities of the polluted water and an outlet zone 22 from which the treated polluted water flows from the HSF 12. The impurities may include, for example, organic materials (BOD and COD) and oxidization processes within the HSF may reduce the quantity of the nitrites that are dumped from the outflow zone 22. The outlet zone 22 also includes a collection well 28 having a pump 30 located therein to recycle the treated effluent back to either the inlet zone 18 of the HSF 12 or to a secondary vertical filter 40.