Septic Tank Effluent Pretreatment Using Different Filter Materials as a Prevention from Clogging

Septic tanks are common devices for primary wastewater treatment, however, they are prone to periodically raised effluent solids concentration, which can accelerate the clogging process in specific conditions. The aim of the study was to identify the impact of the secondary filter on the reduction of TS load and possibility to increase the operation time. The study was carried out in two series using six filters. In each series six filters (pipes with filtering material installed on the bottom) were used: three filters with filter mesh and the other three with non-woven geo-textile. The filters were being filled with wastewater until clogging was achieved (wastewater table in pipes rising up to 50 cm). For the hydraulic conductivity estimation of falling water table tests were performed. Similar values of accumulated TS to dosed TS ratios were observed during the both series for the both filtering material types. The particle size distributions in the both filtering materials were similar. The fact of accumulation of particles of very small dimension (over 73% smaller than 5 μm), much smaller than the filtering material pores diameter might suggest that the process taking place was that of cake filtration. Despite the higher cumulated dry mass of filter cake and inside the filtering layer, the average hydraulic conductivity of non-woven geo-textile was higher than the average hydraulic conductivity of filter mesh. Probably thanks to the higher porosity of geo-textile higher amount of solids can be trapped inside the filtering material itself. The study showed the relatively low ratio of accumulated solids to supplied solids load; however, the accumulated mass seems to be significant for clogging prevention, especially in the case of several filtering layers usage. This way much more solids can be removed from the septic tank effluent (e.g., using four layers about 10÷20%) and clogging process can be slowed down (two three years longer operation time).