Summary:

The article analyses the efficiency of the treatment process in the small sewage treatment plant with a gravel filter, after its modernization. The basic criterion adopted in modernization was to increase the capacity and efficiency of the existing gravity flow system, assuming the minimal operating costs and no additional power consumption. The sewage treatment plant consists of a septic tank, with a separate filter installed during the modernization that keeps suspended solids in the septic tank and a gravel filter for biological treatment; the filter surface was increased during the modernization. The analysis of the effluent quality confirmed very high treatment efficiency of both organic compounds (average removal of BOD and COD was over 90% and 85%, respectively) and total suspended solids ( average removal above 77%) in the modernized wastewater treatment plant.