The relationship between the load of pollutants delivered to a vertical-flow sand filter on the load removed by the filter was studied using the data from four household sewage treatment plants consisting of a septic tank and a vertical-flow sand filter, investigated from October 2003 to February 2006. The pollutant load carried by sewage was determined on the basis of the amount of sewage coming to a treatment plant and the values of BOD₅, COD_Cr, suspended solids, total nitrogen, and phosphates in pretreated and in treated sewage. There was a very high correlation between the pollutant load entering the filter bed and the load retained in the bed, especially for BOD₅ (r = 0.99), COD_Cr (r = 0.98), total suspended solids (r = 0.93), and total nitrogen (r = 0.98), whereas for phosphates the relationship between the two factors was weak (r = 0.27).

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