## 3 Results and Discussion
### Temperature Value on Outflow from the Septic Tank  
In parallel to the salinity measurements, the temperature of the wastewater flowing into the control well, located downstream of the septic tank, was also measured. Figure 10  
Figure 8: Average NaCl concentration at the septic tank outflow occurring between softener regenerations. Figure 9: Maximum NaCl concentration at the septic tank outflow during water softener regeneration. shows the changes in temperature and NaCl concentration over two days (measurements every 1 min). From the course of the NaCl concentration, it was concluded that the regeneration process of the water softener had started at approx. 8 a.m. (480 min), after which the NaCl concentration in the septic tank outflow reached a value of approx. 4.0 g\(\cdot\)L\({}^{-1}\). Meanwhile, the change in the temperature of the wastewater discharging from the septic tank (and flowing into the control well) showed a difference of approx. 10 \({}^{\circ}\)C, over a period of one day--such a situation generally occurs in winter. The maximum recorded temperature reached about 24 \({}^{\circ}\)C (in the summer) and the minimum reached lower than 2 \({}^{\circ}\)C (in the winter). By comparing the values of the results in Figure 11a,b, which show the temperature pattern from a selected week of the summer and winter period, small temperature variations of around 2 \({}^{\circ}\)C per day in summer (Figure 11a) and up to 10 \({}^{\circ}\)C per day in winter (Figure 11b) were noted. During the winter period, the wastewater cooled down--most often during the night hours, when there was no warm wastewater inflow [37]. However, in the case of the conducted tests, after the first day, as evidenced by the graphical image in Figure 11b, there was no cooling down (in the week considered in the graph) of the wastewater in the control well. Cooling only occurred on subsequent days during the night period, when the temperature decreased to around 5 \({}^{\circ}\)C. The highest wastewater temperatures during the winter period occurred during the evening peak when bathing water (a large volume of wastewater with a significant temperature value) was discharged. The temperature values obtained for the wastewater discharged from the septic tank are consistent with the temperature range (3-19 \({}^{\circ}\)C) reported by [PERSON] et al. (2015), who showed a more significant relationship between the temperature of the wastewater in the tank at the outlet of the septic tank and the air temperature, obtaining a linear determination coefficient for this relationship R\({}^{2}\) = 0.76 [13]. Similar values for the temperature of wastewater draining from septic tanks were reported by [PERSON] (1997) in his work, i.e., ranging from 10 to 23 \({}^{\circ}\)C [41]. On the other hand, [PERSON] et al. (2010) reported monthly average temperatures in a septic tank in San Francisco (USA) that ranged from approximately 17 \({}^{\circ}\)C, between February and May, to 22 \({}^{\circ}\)C, in September, while in Quebec (Canada), temperatures ranged from 9 \({}^{\circ}\)C in January to 22 \({}^{\circ}\)C in July, and in Kansas (USA), from 7 \({}^{\circ}\)C in April to 18 \({}^{\circ}\)C in August, confirming a significant relationship between the air temperature and the wastewater temperature in septic tanks [42]. Figure 10: Changes in the temperature and salinity of treated wastewater, during an example of two days of the winter period. [PERSON] (2003) also carried out an experiment to measure the temperature of the wastewater in a septic tank, located in Australia, using a recorder with a 15 min time interval [43]. In his opinion, temperature fluctuations in the septic tank should be small and brief due to the relatively small volume of hot and cold wastewater compared to the volume of the tank.