## 1 Introduction
The application of such a device aims to reduce the hardness of the water, among other things, in order to reduce the adverse effects of limescale and sediment deposits on the water supply, heating systems, and the devices that use the water. Water softening in individual installations takes place on the basis of ion exchange [18,19]. As a result of the water-softening process, calcium and magnesium cations, responsible for the hardness of the water, are exchanged for sodium cations. Once the ion-exchange capacity of the bed has been exhausted to a certain extent, a regeneration process takes place from time to time in the SW by automatically flushing the bed with a solution of water and salt, called brine. The solution contains a high concentration of sodium ions with which the bed is re-supplied with sodium cations [20, 21, 22]. During the regeneration process, grey water is produced, which is usually discharged into the sewage system [20]. It is possible that there may be a negative impact of water softener regeneration grey waters on the operation of DWTPs, both in relation to biological wastewater treatment and the leaching system [23]. In the context of biological methods [24], the influent brine may interfere with the wastewater treatment bacterial microflora (present in the form of free-floating activated sludge or immobilised on a biomass carrier), reducing the efficiency of the domestic treatment plant. An important factor of this impact is, among other things, the size of the tank (e.g., septic tank) where the wastewater pre-treatment is performed. In wastewater infiltration systems [25], in turn, crystallising salt can have a negative effect. In the case of a soakaway pit, due to the point of introduction of wastewater, the crystallising salt deposits can, after a few or several years, reach a salt layer thick enough to obstruct the free flow of wastewater into the native soil. With infiltration drainage, on the other hand, the negative impact may be less significant compared to the previously discussed systems, due to the distribution of treated wastewater over larger areas. As demonstrated in a laboratory study by [PERSON] and [PERSON] (2007), the brine from a softener rinse contributed to the stratification of the zones in the tank and the increase in the sludge layer [26]. Based on the observed phenomenon, it was concluded that the salty water had accumulated at the bottom of the tank and lifted the sludge, which may have adversely affected the proper operation of the domestic wastewater treatment plant. [PERSON] and [PERSON] (1997a) showed that high concentrations of sodium can lead to defloculation in activated sludge systems [27]. [PERSON] et al. (1997a) also demonstrated that when the monovalent-to-divalent cation ratio exceeded two, effluent characteristics in activated sludge systems deteriorated. Promoting the concept of \"reduce, reuse, and recycle\" in DWTPs [21] and following the principles of a circular economy, brine used in various industries needs to be recovered and/or managed in an environmentally friendly manner [28]. In the softener regeneration process discussed in this paper, the brine is traditionally directed to a cesspool, discharged underground into a dry well or routed through building pipes to grade level. Excessive amounts of salt in the soil may result in, among other things, physiological drought for plants, which is related to the loss of their ability to extract mineral nutrients from the soil solution [25]. Sodium also influences the swelling of some soil particles, making it much more difficult for water to infiltrate the soil. The rationale and method for controlling the infiltration of grey waters into the soil, along with other negative effects, are presented later in this paper. The primary objective of this study was to carry out six-month field tests at a DWTP facility equipped with a septic tank and measure the concentration of sodium chloride at the outflow of a septic tank into which the grey waters from the regeneration of a water softener (WS) flowed.