## 3 Results and Discussion  
### Changes in NaCl Concentration within the Septic Tank  
The major type of measurement made during the fieldwork was the recording of changes in the NaCl concentration within the septic tank. Figure 6a presents the salinity measurements obtained over a period of two weeks (13.01-27.01). This was the initial period that the measuring device was installed behind the septic tank. In this graph, a fluctuation in the average NaCl concentration of 1.5 g\(\cdot\)L\({}^{-1}\) can be observed. At minute 1905 (i.e., 14.01), there was an increase to the highest concentration in the period, at 4.4 g\(\cdot\)L\({}^{-1}\), which indicates that regeneration was taking place at this time. Twelve days later (26.01), i.e., at minute 18,797, another increase was observed--one of the highest concentrations of the two weeks in question, at 3.5 g\(\cdot\)L\({}^{-1}\). Due to the time of occurrence, which was 8.23 a.m., the presence of grey waters from the water treatment plant may be inferred. Figure 6b presents the next two weeks of salinity measurements, six months after the beginning of the study (31.05-14.06). The average NaCl concentration in the treated wastewater during this period was 1.6 g\(\cdot\)L\({}^{-1}\). Water softener regeneration occurred twice: at minute 2560, when the maximum NaCl concentration in the septic tank outflow was 4.9 g\(\cdot\)L\({}^{-1}\), and at minute 17818, with a maximum value of 4.3 g\(\cdot\)L\({}^{-1}\). Figure 6a,b shows that the increased NaCl concentration level after regeneration lasted for approximately 1.5 to 2 days, after which it returned to the average concentration. Based on these graphs, an increased concentration of NaCl in the wastewater after the softener regeneration period was also noted. Similar results related to the increase in the NaCl concentration after the softener regeneration and such uncontrolled fluctuations  
Figure 5: Permissible hydraulic loading of soil with treated wastewater q, based on (1) Polish technical guidelines for the design of infiltration drains and sand filters from 1971, calculated in accordance with a correction by [PERSON] (2003) [32], (2) French standard DTU 64.1 [33], (3) [PERSON] (1986) [34]. Figure 6: Changes in the NaCl concentration in the septic tank outflow (**a**) between 13 and 27 January and (**b**) between 31 May and 14 June. in the brine concentration have also been observed by other researchers in pioneering experiments [35; 36]. The destabilisation of the system, caused by the increase in NaCl concentration, inhibited the wastewater treatment processes. The next graph (Figure 7) demonstrates an example of the NaCl concentration over a two-day period from the start of the regeneration on 2 June. The above-discussed changes in the NaCl concentration at the septic tank outflow are in line with laboratory studies carried out by [PERSON] and [PERSON] (2007) [26]. The authors recognised that water backwash brine can be detrimental to onsite wastewater treatment system performance. The researchers concluded that concentrated salt water accumulates at the bottom of a septic tank receiving brine and can be periodically discharged to the advanced treatment unit, with consequent adverse effects on the treatment performance [26]. In the case of the presented studies regarding the grey water from the water softener regeneration, the wastewater most likely did not mix with the liquid in the septic tank and rather accumulated in the clarification zone or moved the accumulated sludge in the tank. The zone of clarification was between the sludge accumulated on the bottom and the floating blanket near the water table. This was followed by leaching of the grey water from the septic tank over a period of up to 2 days, which was associated with a period of an increased NaCl concentration at the septic tank outflow. From the results of the measurements, presented in Figure 7, it may be concluded that the regeneration of the water softener started at around 7:00 a.m. and lasted for more than an hour.