## 1 Introduction  
In national parks and protected areas, there are usually museums, froester's lodges, hostels, or tourist trails with resting places for visitors, which should be equipped with sanitary infrastructure that ensures their proper functioning. According to the Law on Nature Protection [1] in Poland and the Council Directive 92/43/EEC [2], in the area of national parks and nature reserves, it is forbidden to build or reconstruct any buildings or technical facilities with the exception of facilities and devices that serve to achieve the goals of the given national park or nature reserve. For this reason, it is essential in protected areas to use water supply systems and wastewater treatment plants (WWTPs) that do not interfere with the environment [3, 4] and meet the criteria of sustainable development and nature protection [5, 6, 7]. Domestic wastewater generated by various tourist facilities in national parks or protected areas, just as in rural areas, is most often discharged to non-return tanks (septic tanks) and is then taken to collective WWTPs or disposed of in individual wastewater treatment systems, i.e., in so-called small household WWTPs [8, 9]. A similar way of dealing with domestic sewage in protected and rural areas, where sewerage systems and collective treatment plants are lacking, is also in force in other countries around the world [10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. In recent years, household WWTPs have increasingly become one of the basic elements of technical infrastructure in protected areas where, due to natural and landscape values and large dispersion of tourist buildings, the installation of a sewage network and a collective WWTPs is not justified [9; 13; 14; 17; 19]. In accordance with Polish Standard PN-EN 12,566 [20], household WWTPs are defined as facilities for 50 inhabitants. However, on the basis to the Water Law [21], the maximum capacity of such systems in Poland should not exceed 5 m\({}^{3}\)/day. Various technological solutions are currently used to treat small amounts of wastewater, such as systems with drainage pipes, systems with a sand filter, WWTPs with activated sludge, systems with a biological bed, hybrid systems (activated sludge + biological bed), and constructed wetland systems (CWs) [7; 22]. A review of the literature on the evolution of wastewater management and its development over the centuries was presented by [PERSON] and [PERSON] [23]. Currently, the activated sludge method is the most commonly used for wastewater purification in the world. Activated sludge is most often used for urban wastewater treatment [24; 25; 26; 27], as well as for industrial wastewater [28; 29]. The popularity of the activated sludge method used in large WWTPs around the world has led to the development of \"miniature\" facilities of this type, whereby for over 30 years, attempts have been made to replicate the technological processes [24; 30; 31; 32; 33; 34; 35]. However, it is important for household WWTPs with activated sludge, which are used to treat small sewage amounts, to meet the appropriate criteria. The most important aspects that should be considered during the selection of a technological solution involving small WWTPs are the efficiency of pollutant removal and the reliability of operation [4]. These criteria should be taken into account especially for WWTPs installed in protected areas [36]. In the case of WWTPs, efficiency refers to the degree of removal of particular types of pollutants, which is determined by the amount of pollutants retained in the system in relation to the amount of pollutants entering the system. Reliability, on the other hand, is defined as the ability to treat wastewater to the degree required by the wastewater receiver over the assumed operating time, including changes in the quantity and the composition of the inflow [32; 33].