## 1 Introduction  
In non-urbanised areas where, for technical or economic reasons, it is not possible to connect to the sewage network, drainless reservoirs or domestic wastewater treatment plants (DWTPs) are used. The latter are becoming increasingly popular due to their lower odour nuisance and lower maintenance costs in comparison to the costs incurred for the disposal of wastewater from drainless reservoirs [1; 2]. The simplest DWTPs are those consisting of a septic tank (ST) and a wastewater leaching system (e.g., a subsurface leaching system) [3; 4]. The ST is usually the first unit in a domestic wastewater treatment plant and is used for the primary treatment of wastewater. More extensive domestic wastewater treatment plants utilise aerobic biological processes (flow-through activated sludge, sequencing batch reactor (SBR), trickling filter) after the preliminary treatment of wastewater, e.g., in a septic tank [5; 6]. Effective sorption materials of various origins are also increasingly used in DWTPs. Sorption is a frequently used method of water purification and uses, e.g., polymer composites, membranes, graphene, carbon nanomaterials, and natural adsorbents such as aerogels of biological and natural origin adsorbents. The latter can be further divided into organic and inorganic materials [7]. Currently, adsorbents used to remove various types of pollutants from water, including oils, antibiotics, and heavy metals, can be divided into natural--of organic and inorganic origin--and synthetic materials [8,9]. In a septic tank, the wastewater treatment process mainly takes place through the sedimentation of solids with a higher density than the liquid in the tank. The solids settling to the bottom form sludge. Meanwhile, for the suspended solids lighter than the liquid in the tank, a flotation process occurs, resulting in the formation of scum in the vicinity of the wastewater table. Anaerobic biological processes have little influence on the treatment of wastewater discharged from septic tanks [10, 11, 12]. Psychophilic fermentation takes place in the accumulated sludge at the bottom of the tank due to the low temperature inside the septic tank [9]. A study conducted in Poland observed a range of liquid temperatures in the septic tank from 3 to 19 \({}^{\circ}\)C, with an annual air temperature of \(-\)13 to 30 \({}^{\circ}\)C [13]. The density of wastewater flowing into a septic tank is influenced by the concentration of suspended solids and their temperature [14,15]. Wastewater with a high content of suspended solids acts as if it has a higher density than the wastewater in the septic tank and settles to the bottom. As a result, the slab velocity is much higher than the theoretical one, and the septic tank has worse efficiency. When density differences are influenced by the temperature, there are two possibilities. The first is that when the wastewater entering the septic tank is cooler than the wastewater in the tank, a bottom slab is formed, which lifts the sludge accumulated at the bottom. This phenomenon is referred to as spring turnover or boil of the septic tank and may result in a deterioration of the quality of the treated wastewater. This situation is particularly prevalent during the summer when cold water is used more frequently. Instances where warmer wastewater causes a surface slab are particularly noticeable during winter. In this situation, very little treated wastewater enters the septic tank outlet [2]. In order to prevent the negative effects of the temperature difference between the inlet wastewater and the interior of the septic tank, it is recommended to use special inlet and outlet design solutions in septic tanks [13,16] or dedicated connections between chambers for multi-chamber tanks [17]. In the last few years, water softeners (WS) have been installed in detached houses to improve water quality.