## 3 Results and Discussion
### Technological Reliability of the Studied Systems
1; (**B**)—facility no. 2). Notation: dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent. In facility no. 2, the reliability of TSS removal was 95% (Figure 3B), indicating faulty operation of the WWTP for 19 days per year. Taking into account the previously mentioned guidelines [58], it may be concluded that the level of TSS negatively influenced the facility assessment for 10 days per year. Lower reliability levels for the analyzed indicator (about 65%) were found in some studies concerning household WWTP with activated sludge conducted by [PERSON] et al. [59] and [PERSON] [32]. However, 100% technological reliability of TSS removal in a small sequencing batch biofilm reactor (SBBR) with activated sludge was obtained by [PERSON] et al. [35]. Research on two other household hybrid CW WWTPs of \(\mathrm{VF/HF}\) type (with vertical and horizontal flow) operating in the RNP indicated that they also provide quite high (92-100%) reliability of TSS removal [36]. Additionally, [PERSON] et al. [33] obtained 100% reliability of TSS removal in a household CW. On the basis of the data obtained and the literature review, it appears that activated sludge systems have lower reliability of TSS removal than CW WWTPs. **BOD5. ** The technological reliability of BOD5 removal in facility no. 1 was 95% and that in facility no. 2 was 76% (Figure 4). In facility no. 1, the level of this indicator was higher than the permissible value for 10 days a year. Similarly, in facility no. 2, the level of BOD5 was elevated for 78 days a year. In comparison, in a WWTP of identical design but without a preliminary settling tank, the reliability of BOD5 reduction was 70% [32]. On the other hand, [PERSON] et al. [59] in a household system Biocompact BCT S-12 determined the reliability of reducing this  
Figure 6: Weibull cumulative distribution functions and the technological reliabilities determined for TN ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation:dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent. Figure 7: Weibull cumulative distribution functions and the technological reliabilities determined for TP ((**A**)—facility no. 1; (**B**)—facility no. 2). Notation: dashed green line—reliability function, continuous green line—confidence intervals, red arrows—probability of achieving the indicators limit in the effluent. indicator at the level of 88%. The BOD5 removal reliability in a small SBBR was 77% [35]. On the other hand, maximum (100%) reliability of BOD5 removal was reported in two other household hybrid CW WWTPs in the RNP [36]. Moreover, [PERSON] et al. [33] achieved 100% reliability of BOD5 removal in a household hybrid CW. **COD. ** The reliability of COD removal was 96% in facility no. 1 and 87% in facility no. 2 (Figure 5). The obtained reliability levels are lower than the minimum required values given by [PERSON] and [PERSON] [58]. In relation to the annual reference period, WWTP no. 1 provided the required level of organic pollutant removal expressed by COD for 359 days, while this value for WWTP no. 2 was 326 days. The reliability indicators obtained in the analyzed activated sludge facilities in the RPN for COD are comparable to those obtained by [PERSON] et al. [59] in a household WWTP working in activated sludge technology. At the same time, in the facilities in the RPN, the reliability of COD removal was higher than that found in a treatment plant based on an identical tank design but without a preliminary settling tank (60%) [32]. The reliability of COD removal in a small SBBR was 97.8% [35]. However, similarly to the case of BOD5, maximum (100%) COD removal reliability was reported in two other household hybrid CWs WWTPs operating in the RNP [36]. Moreover, [PERSON] et al. [33] achieved 100% COD removal reliability in a household hybrid CW.