## 2 Materials and Methods
### Study of the Effect of Brine on the Hydraulic Conductivity of the Soil  
The second part of the study, carried out at a semi-technical scale, consisted of an analysis of the effect of brine from water softener regeneration on the hydraulic conductivity of the soil. This involved the use of nine pipes with an internal diameter of 105 cm, simulating the action of an infiltration drain into which the grey water from the regeneration of a water softer enters. The pipes, which were given the function of filter columns, were filled with soil extracted at a distance of approximately 4 metres from the infiltration drainage of the DWTPs under study. The excavation of the test material was carried out to a depth of 0.5 m. A layer of stones of about 10 cm was poured onto the bottom of the pipes, and then soil was added to a height of 1 m, as shown in Figure 4.  
The columns were stably placed in a shaded area and secured so that the dosed liquid flowed freely through them. Treated wastewater was poured into the first group (columns: 1a, b, c), treated wastewater and brine into the second group (columns: 2a, b, c), and water into the third group (columns: 3a, b, c). The treated wastewater was collected from the septic tank outflow in the DWTPs under study when the salt concentration did not indicate softener flushing. The brine, on the other hand, was prepared (prior to each injection) using tap water and regeneration salt, with a concentration analogous to that of the brine used in the technical-scale study. In addition, the columns were moistened each time with the  
Figure 3: Annual costs of using a water softener expressed in: (**a**) PLN and (**b**) %. appropriate type of liquid before measuring the time it took for the liquid to soak into the soil (see Figure 4). The hydraulic conductivity \(K_{s}\) was determined by analysing the time it took for the water to soak into the soil, using the Van Hoorn Equation (1):  
\[K_{s}=1.15\cdot r\ \frac{\log\cdot\left(H_{0}+\frac{r}{2}\right)-\log\cdot\left(H_{1 }+\frac{r}{2}\right)}{t} \tag{1}\]  
where:  
\(K_{s}\)--hydraulic conductivity, m\(\cdot\)s\({}^{-1}\);  
\(r\)--opening radius, m;  
\(t\)--water subsidence time, s;  
\(H_{0}\)--initial water level in the opening, m;  
\(H_{1}\)--final water level in the opening, m [31]. Hydraulic conductivity testing was performed after 2-3 cycles of brine dosing to the corresponding filter column group. On the basis of the monthly water meter readings--in terms of water consumed per day at the property in question--it was calculated that the average water consumption was approximately 300 L\(\cdot\)d\({}^{-1}\) (d--day). In accordance with the diagram (see Figure 5), which shows the permissible hydraulic loading of the soil surface (q) with the wastewater treated in the septic tank--it was determined, in accordance with the source literature [32; 33; 34], that for the soil located at the household in question, the maximum hydraulic loading was 17.7 L\(\cdot\)m\({}^{-2}\cdot\)d\({}^{-1}\)[32; 33]. On the basis of the value read from this diagram and the value of the surface area of the filter column (A = 0.00865 m\({}^{2}\)), the theoretical maximum daily dosage of solution to the filter column was obtained, which was 0.153 L\(\cdot\)d\({}^{-1}\). In the experiment, however, a dosage of 0.100 L\(\cdot\)d\({}^{-1}\) was used. A brine concentration of 5.0 g\(\cdot\)L\({}^{-1}\) at a volume of 0.05 L (dosage calculated at a semi-technical scale) was applied to the second group of columns (2a, b, c) for regeneration, which took place every 10 days (10 d\(\cdot\)0.100 L\(\cdot\)d\({}^{-1}\)--i.e., every 1 L). Its dosage was estimated from a proportional calculation of the actual volume of water used between regenerations of the domestic water softener and from an approximate amount of water used to rinse the bed. Rinsing the ion exchange bed consumed approximately 3.9 m\({}^{3}\) of water, and the process itself consumed between 105 and 175 L of water.