Efficiency of pollution removal in preliminary settling tanks of household wastewater treatment plants in the Roztocze National Park

Abstract

The paper presents an evaluation of the pollutant removal efficiencies of four primary settling tanks that were components of on-site domestic wastewater treatment plants located in the Roztocze National Park in Poland. We studied two four-chamber settling tanks which were elements of the technological lines of activated sludge treatment plants, and two three-chamber settling tanks which provided primary treatment in hybrid constructed wetlands. The tests were conducted in the years 2017-2019. During this period, wastewater samples for analysis were collected from the first chamber (raw wastewater) and the last chamber (primary settled wastewater) of each settling tank. We tested the following pollution parameters: total suspended solids (TSS), BOD5, COD, total nitrogen (TN), and total phosphorus (TP). The following mean pollutant removal efficiencies were obtained for the four-chamber settling tanks: TSS-68.3%, BOD5-50.4%, and COD-49.5%; the three-chamber settling tanks were much less efficient at reducing those pollution parameters: 50.9%, 17.0%, and 2.3%, respectively. Neither the three-chamber nor the four-chamber settling tanks ensured effective elimination of biogenic compounds. In most cases, the concentrations of TN and TP in primary settled wastewater were higher compared to raw sewage, which means that these elements were being released from sewage sludge during treatment. Despite this finding, the study shows that a primary settling tank should be used as a basic component of any domestic wastewater treatment plant, as it ensures a considerable reduction in TSS and organic matter.