Economic Sustainability of Selected Individual On-Site Systems of Rural Sanitation Under Conditions in Poland

Abstract

The sustainability of rural areas depends on effective wastewater management to reduce human impact on the environment, including the risk of pollution to surface water, groundwater, and soil from human waste. However, organized sanitation systems, which include pipeline networks and wastewater treatment plants in rural communities with low population densities, often have very low profitability and cost-efficiency, which greatly reduces their acceptance and residents’ willingness to pay. This study examines the economic profitability and cost-efficiency of selected on-site household sewage collection and treatment systems operating under real economic conditions in Poland. An evaluation was conducted on seven contemporary models of individual bioreactors, as well as a standard anaerobic septic tank equipped with drainage filters. Additionally, all options were tested on permeable and poorly permeable soils. For each variant, investment costs, as well as operation and maintenance expenses, were calculated. Financial evaluation utilized indicators of economic profitability and cost-efficiency, including the Payback Period, Net Present Value, Benefits–Cost Ratio, and Dynamic Generation Costs. The potential financial benefits included savings from avoiding the use of holding septic tanks and sewage transport by slurry wagons. All the studied designs of on-site sanitary sewage management showed significant economic feasibility and cost-efficiency.

1. Introduction

The sustainable development of rural areas requires the availability of unpolluted surface water, groundwater resources, and clean arable soil [1,2,3,4,5,6]. Thus, limited freshwater resources, endangered by climate change and anthropogenic pressure, should be treated as one of the most important concerns for rural societies. Generally, the negative impacts of rural populations on the availability of clean water are mainly related to agricultural activity, as well as waste and excrement management [7,8,9,10,11,12], allowing possible point or area pollutant migration. The problem of sewage disposal and treatment in urban areas may be solved by collective sewage treatment plants. However, in the case of rural settlements, the construction of collective sewage collection and treatment systems poses a significant problem and challenge for municipalities [13,14,15]. Thus, a significant amount of the sewage generated in rural areas is illegally discharged into the environment without prior treatment, which negatively affects the quality of surface and underground waters. According to the current report of the Polish Geological National Research Institute [16], 28.2% of the tested rural groundwater monitoring points presented unsatisfactory or bad quality of water, class IV and V of water quality according to the legal regulations in Poland [17]. The disorganized water and sanitary sewage management, and the insufficient isolation of the groundwater table from the infiltrating surface water, were recognized as some of the main reasons for groundwater pollution in rural areas of Poland [18]. Similar observations have been reported in other countries around the world [19,20,21,22,23,24].

According to estimates, by 2030, over 22 million inhabitants of Central and Eastern Europe will not be connected to centralized sewage treatment plants and sewage systems [9]. Therefore, limiting the possibility of contamination of soils, surface waters, and groundwater by untreated sanitary sewage is crucial for the sustainability of rural areas [11,25,26,27,28,29,30]. Hence, it seems necessary to develop sustainable wastewater management systems, primarily small, decentralized wastewater treatment plants, ensuring as high a degree of purification as possible [31,32,33,34]. Moreover, in many regions of the world, the clear and visible disparities between access to water and sanitation in urban and rural areas have been observed [26,35,36,37,38,39,40].

Currently, centralized sanitation in rural areas of Poland, which cover over 50% of the country’s territory and are inhabited by approximately 15,300,000 people, is underdeveloped. Despite noticeable improvements in water supply and sewage systems described in [35,41,42,43,44,45,46], government data [47] show that 86.4% of rural residents in Poland are connected to centralized water supply networks, while only 45.4% are connected to sewage systems. The remaining rural population relies on periodically emptied holding septic tanks and household sewage treatment plants. In 2023, there were 3001 active collective sewage treatment plants with a capacity of 2,090,661 m³ annually, along with 335,765 individual household sewage treatment plants. For comparison, in 2023, 96.8% of urban residents were connected to water supply networks and 91.0% to sewage networks. In the Lublin Voivodeship, one of the less developed regions in Poland and the European Union [35], only 24.4% of the rural population was connected to organized sewage disposal systems in 2023 [47].

The construction of centralized sewage systems and sewage treatment plants in rural areas with low population density, large dispersion, small amounts of sewage produced, and difficult topography often poses major technical and economic problems [14,48,49,50,51,52,53,54,55]. The application of gravity sewage disposal systems in areas with dispersed development is very often associated with a significant pipe recess and the need for frequent pipeline flushing, which negatively affects the costs of network construction (due to the significant volume of earthworks required) as well as operation and maintenance. However, the use of pressurized systems requires an uninterrupted energy supply and regular maintenance. As frequently reported, the construction of collective wastewater transport and treatment systems is often unprofitable or even impossible for rural communities without external financing, e.g., in the form of EU subsidies [13,56,57,58,59]. A significant limitation in the construction of centralized sewage systems is also related to social acceptance, the involvement of residents in the construction, and the willingness to pay the regular sewage fees [60,61,62,63,64,65]. The social acceptance and willingness to sustain the costs of sewage disposal are influenced by the income of the population, awareness, fears, the value of water and sewage services payments, satisfaction with the current sanitary service, age, gender, and level of education [13,56,61,65,66,67,68].

For many rural communities in Poland and other EU countries, the only solution for sewage disposal is individual on-site household systems, including septic tanks and household sewage treatment plants [69,70]. The available sources, for example [10,62,71,72,73,74], show that septic tanks do not provide wastewater treatment, only retention. Therefore, sewage requires frequent transport to collective wastewater treatment plants, which significantly increases the operational costs of such facilities [75,76,77,78,79,80]. Moreover, due to frequent leakages, holding septic tanks are a common source of pollution for surface water, groundwater, and soil [49,81,82,83,84,85]. Despite the many disadvantages of this solution, septic tanks are still the main devices for sewage management in rural areas in Poland [11]. The construction of household sewage treatment plants enables the treatment of sanitary sewage, which is discharged into water or to the soil following the treatment [62]. The choice of the appropriate technological solution is determined by the amount and source of wastewater, local soil and water conditions, the available size of the plot, and the investment and operating costs [86,87,88,89]. Researchers from European countries have confirmed that the use of modern, decentralized wastewater treatment systems can be a more sustainable option from both environmental and economic perspectives [90,91,92]. In centralized wastewater management, approximately 80–90% of the total costs are related to wastewater transport, and only 10–20% are related to the treatment process. The results of the research conducted by Zhang et al. [80], using economic analysis and life cycle assessment of three selected decentralized wastewater treatment systems, confirmed that the main advantage of decentralized wastewater treatment plants is lower capital expenditure resulting from the lack of the need to build an extensive sewer network.

Several types of household sewage treatment plants are available in the market [93,94,95,96,97,98,99]. The simplest devices include sewage anaerobic tanks with infiltration drainage, a sand filter, and soil–plant treatment plants, in which the sewage is pre-treated in the settling tank and the main treatment processes occur during infiltration into the soil. In accordance with the binding Polish law [88], sewage can be discharged into the soil provided that the distance of 1.5 m from the groundwater table is maintained and the values of two basic indicators, i.e., biochemical oxygen demand (BOD₅) and total suspended solids, are reduced by 20% and 50%, respectively. Nowadays, the requirements for household sewage treatment plants in Poland are definitely lower than the requirements for sewage treatment plants serving a larger number of inhabitants [88]. For example, according to the Polish guidelines, for treatment plants with a population equivalent of less than 2000, the permissible values of pollutants are BOD₅—40 mg O₂/dm³, chemical oxygen demand (COD)—150 mg O₂/dm³, suspended solids 50 mg/dm³, total nitrogen—30 mg N/dm³, and total phosphorus—5 mg P/dm³ [88]. However, in the case of larger agglomerations, for wastewater treatment plants with an equivalent population of over 100,000, a higher degree of pollution reduction is required, at a minimum level of BOD₅—90%, COD—75%, total suspended solids—90%, total nitrogen—70-80%, and total phosphorus—80%.

Many authors [9,14,40,43,62,65] have indicated that sewage treatment plants, in which the main treatment processes take place during the infiltration of pre-treated sewage in the settling tank, pose a serious threat to the quality of surface and underground waters. Therefore, drainage systems should not be used as independent systems for wastewater treatment, but only for the discharge of biologically treated sewage. In addition, in such cases, it is hardly possible to monitor the processes taking place in the sewage discharged into the ground, so it is difficult to determine whether the treatment meets the applicable standards. The above may be extremely important in light of the currently implemented EU directive on wastewater treatment [100], according to which individual systems in settlements with a population equivalent above 1000, ensuring the same level of treatment as the secondary and tertiary treatments in collective systems, should be monitored and controlled at regular intervals.

More advanced devices for on-site sewage management include household treatment plants with activated sludge and trickling filters, using sequencing batch reactor (SBR) technology, in which the main treatment processes take place inside the tank volume and the drainage is only used to discharge the treated water into the ground. According to the available manufacturers’ data [93,94,95,96,97,98,99] and the literature, these devices ensure a high degree of pollution reduction, meeting the requirements set out in the regulation for treatment plants with a population equivalent of 2000 to 9999 [88]. Among these devices, objects of a very simple structure can be distinguished, in which all processes take place in single-chamber tanks, as well as more complex ones, in which separate chambers are used. The efficiency of wastewater treatment depends on the continuity of sewage inflow to the tank, systematic control of pumping systems, oxygenation, and continuity of electricity supply [9,32,33,62,101]. Research findings have indicated that modern decentralized wastewater treatment systems offer several advantages over their centralized counterparts. These solutions are characterized by flexibility, economic- and cost-effectiveness, and sustainable development [102,103]. Thus, it is visible that meeting the principles of sustainable development of rural settlements in regions of low population density, where collective organized sanitation is unavailable or would be unprofitable, and would not find social acceptance or willingness to pay, requires economically and environmentally efficient on-site sanitation. Previous studies have indicated the economic and cost-efficiency advantages of selected decentralized wastewater treatment systems. However, these studies are local in nature and usually cover a limited number of designs, providing possible applications without considering the diverse soil conditions. Thus, there is a need for a study covering the economic aspects of numerous available individual on-site sanitation systems that allow for a high degree of sanitary wastewater treatment under different local soil conditions and allow for a comparison of their economic feasibility and cost-efficiency to the most popular management manner, that is, septic tank and sewage transport by sanitary vehicles. Economic analysis of selected technological solutions can be a useful tool for supporting decision-making and the development of rural areas, thus increasing the sustainability and cost-effectiveness of wastewater infrastructure [74,80,92].

To fill this gap, we present a study aimed at determining the economic- and cost-effectiveness of several selected possible sustainable devices of on-site household individual sanitary sewage management for rural settlements under the actual economic conditions of Poland. Seven variants of up-to-date, technologically advanced domestic wastewater treatment devices allowing a high degree of pollutant reduction, based on reactors with aerated active sludge and trickling filters, were considered. The eighth variant covered the standard anaerobic septic tank equipped with drainage pipes and an infiltration layer, for which anaerobic treatment is being performed in the tank, and later the sewage is treated inside the infiltration layer and in the soil. For each variant, the investment installation costs as well as the operation and maintenance (O&M) costs were determined.

2. Materials and Methods

2.1. Object Description

A single-family house with four residents located in an area without connection to the organized municipal sanitary wastewater network was included in the conducted study. The mean daily water consumption and sewage discharge were assumed to be 90 dm³/(day∙resident) according to the binding legal standards in Poland [104]. Thus, the mean annual volume of generated sanitary sewage was assumed to be 116.80 m³. All studied individual on-site wastewater treatment plants were connected to the domestic sewage installation by a 160 mm PVC pipe of 10 m length.

2.2. Studied Variants

The assumed variants, together with the estimated investment and O&M costs, are presented in

Table 1. Studied on-site household sanitation devices.

Variant	Description	Investment Costs (Euro)	Annual Mean O&M Costs (Euro)
1	Single-chamber tank, aerated trickling filter, length 2.15 m, width 1.24 m, earthworks volume 19.68 m3 Energy consumption 0.72 kWh/day Sludge transport once per 24 months	a 5178.98 b 5644.09 c 4788.44 d 5329.14	a and c 119.26 b and d 209.49
2	Three-chamber tank, active sludge and aerated trickling filter, volume 2.3 m3, length 1.8 m, width 1.6 m, height 1.45 m, earthworks volume 20.75 m3 Energy consumption 0.69 kWh/day Sludge transport once per 6 months	a 4135.12 b 4704.88 c 3744.58 d 4389.93	a and c 213.91 b and d 304.13
3	Five-chamber tank, active sludge and aerated trickling filter, length 2.46 m, width 1.42 m, height 1.7 m, earthworks volume 22.63 m3 Energy consumption 0.9 kWh/day Sludge transport once per 12 months	a 4335.58 b 4905.35 c 3945.05 d 4590.40	a and c 131.07 b and d 221.30
4	Three-chamber bioreactor tank, anaerobic treatment, aerated active sludge, secondary chambers, length 2.3 m, width 1.7 m, height 1.1 m, earthworks volume 18.71 m3 Energy consumption 0.60 kWh/day Sludge transport once per 9 months	a 4511.79 b 5081.56 c 4121.26 d 4766.61	a and c 157.85 b and d 248.07
5	Six-chamber vertical bioreactor tank, anaerobic treatment, aerated active sludge, secondary sludge and clarification chambers, length 1.8 m, diameter 1.41 m3, volume 1.79 m3, earthworks volume 10.07 m3 Energy consumption 0.60 kWh/day Sludge transport once per year	a 4405.16 b 4974.93 c 4014.63 d 4659.98	a and c 173.11 b and d 263.34
6	Six-chamber cylindrical vertical bioreactor, low-loaded activated sludge, dual recirculation and aeration systems, diameter 1.4 m, height 1.8 m, earthworks volume 14.77 m3 Energy consumption 0.90 kWh/day Sludge transport once per year	a 4610.79 b 5180.56 c 4220.26 d 4865.61	a and c 173.11 b and d 263.34
7	Conical two-chamber tank, active sludge and aerated trickling filter, diameter 1.7 m, height 2.85 m, earthworks volume 14.53 m3 Energy consumption 1.42 kWh/day Sludge transport once per 18 months	a 4821.79 b 5180.56 c 4447.14 d 5092.49	a and c 298.34 b and d 388.57
8	Single-chamber tank, volume 2.3 m3, length 1.61 m, width 1.91 m, height 1.5 m, earthworks volume 19.99 m3 Drainage filter and pipelines flushing once per 12 months Sludge transport once per 24 months Filtration layer exchange after 20 years	a 5157,00 b 5416.07 c 2898.56 d 3401.47	a 202.29 b 345.32 c 154.78 d 297.80

and Figure 1. For each variant, the materials (tanks, pipes, aerating devices), workload, earthworks, energy consumption, and services were included in the cost estimations [13,22,105]. In the medium and long terms of the life cycle, costs such as hardware replacement, system modernization, and potential failure repairs were considered. Detailed data on O&M costs are presented in Table S1. All the developed variants were based on the actually market-available devices as well as their purchase and installation costs [93,94,95,96,97,98,99,106,107,108]. All the prices used in this paper were based directly on market offers in Polish zloty (PLN) and converted to Euro using the mean currency EUR 1 = PLN 4.3 (May 2025).

Each tested variant of on-site sewage treatment was combined with two possible drainage devices, drainage pipes or drainage packages, for two types of soils, permeable and poorly permeable, with the threshold value of coefficient of saturated hydraulic conductivity Ks = 1∙10⁻⁵–10⁻⁶ m/s. [109]. Based on the assumed soil type, the infiltration devices were designed to be installed in excavations (permeable soil) and embankments (poorly permeable). Thus, the following combinations were considered: a—drainage pipes in excavation, b—drainage pipes in embankment, c—drainage packages in excavation, and d—drainage packages in embankment. The proposed infiltration devices for both types of soils are presented in Figure 2.

The required design of a holding septic tank for further comparative analyses covered a 10 m³ volume concrete tank with the length, width, and height of 3.0 m, 2.4 m, and 1.8 m, respectively, with a 10 m of 160 mm PVC sewage connection. The required volume of earthworks was 35.72 m³, while sewage transport by slurry wagon to the wastewater treatment plant was designed as once a month. The assumed investment costs of a septic tank were EUR 1079.40, while the annual O&M, covering mainly sewage transport each month by at least 10 m³ slurry wagon, were assumed to be EUR 1172.09 [110,111].

Table 1 shows that the mean annual O&M costs for Variant No. 8 are different for each assumed infiltration device, which is related to the required exchange of infiltration drainage gravel/sand–gravel layer, working as a trickling filter, after approximately 15 years of operation [112].

Table 2. The declared reduction and the reduction reported in the literature of BOD₅, COD, and TSS for the studied devices of on-site household sanitation [93,94,95,96,97,98,99,106,107,108].

	Reduction (%)
Variant	BOD5	COD	TSS
Variant 1	99	97.4	94.6
Variant 2	93	92	96
Variant 3	97	86	92
Variant 4	92.9	88	92.4
Variant 5	98.2	94.4	97.2
Variant 6	97	92	95
Variant 7	92.4	84	91
Variant 8	90	83.5	70

presents the declared values of pollutant concentration reduction for all studied variants of on-site sewage management. According to the presented data, all the proposals met the legal requirements concerning the reduction in pollutant concentrations in water introduced to soil after the sewage treatment in Poland for agglomerations with an equivalent number of inhabitants below 2000 [88]. Variants 1–7 also met the pollution reduction requirements for large agglomerations, for which a higher degree of reduction is required. Only Variant 8, a septic tank with infiltration drainage, if installed in an agglomeration with an equivalent population of over 2000, would not ensure an adequate degree of total suspended solids reduction.

Drainage pipes for all the studied variants of on-site WWTP were designed as 20 m 110 mm PVC, with an inspection chamber and a ventilation pipe. The drainage layer consisted of 10 m³ of gravel, 25 m³ of sand–gravel aggregate, and 25 m² of geotextile. The excavation volume for permeable soils was assumed to be 57.3 m³. In the case of drainage pipe application on poorly permeable soils, the embankment earthworks were assumed to be 32.5 m³. Additionally, the wastewater pumping station with a Uniqua Cesspit J10P sewage pump with power consumption of 300 kWh/year was designed. Only in the case of the traditional septic tank, without aeration and a trickling filter, combined with the infiltration layer, the required length of 110 mm PVC drainage pipes was longer and was assumed as 45 m. In this case, the filtration layer consisted of 25.6 m³ of gravel, 64 m³ of sand–gravel aggregates, and 64 m² of geotextiles, with a drainage earthworks volume of 140.8 m³. The number of drainage packages for this variant was increased to 16, which increased the size of the drainage filter and the volume of earthworks.

Drainage packages were designed as 8 PVC modules with dimensions of 1.16 m (length) × 0.8 m (width) × 0.51 m (height) [96] with an inspection chamber and ventilation pipe. In this case, the drainage layer consisted of 5 m³ of gravel, 12.5 m³ of sand–gravel aggregate, and 12.5 m² of geotextile. The earthworks volume for excavation was assumed to be 33.12 m³. For the application of drainage packages in poorly permeable soils, a wastewater pumping station with a Uniqua Cesspit J10P sewage pump with a power consumption of 300 kWh/year was designed.

2.3. Economic Profitability and Cost-Efficiency Analysis

The assessment of economic feasibility and cost-efficiency of the tested on-site sanitation variants was based on four popular indicators. Economic profitability was determined using a single indicator, Payback Period, and two dynamic indicators: Net Present Value (NPV) and Benefits–Costs Ratio (BCR) [113,114,115,116,117,118]. The used indicators were calculated as follows:

$$\mathrm{P}\mathrm{P}={\displaystyle \frac{IC}{NCF}}$$

(1)

where PP—Payback Period, years, IC—initial investment costs (Euro), and NCF—net cash flow (Euro/year).

$$\mathrm{N}\mathrm{P}\mathrm{V}=\sum _{t=0}^N{\displaystyle \frac{{NCF}_t}{{(1+i)}^t}}$$

(2)

where NCF_t—net cash flow for t year of investment operation (Euro), N—total number of periods (years), and i—discount rate (%).

$$\mathrm{B}\mathrm{C}\mathrm{R}={\displaystyle \frac{{PV}_b}{{PV}_c}}$$

(3)

where PV_b—present value of investment benefits (Euro), and PV_c—present value of investment costs (Euro).

$${PV}_b=\sum _{t=0}^N{\displaystyle \frac{{CF}_{bt}}{{(1+i)}^t}}$$

(4)

$${PV}_c=\sum _{t=0}^N{\displaystyle \frac{{CT}_{ct}}{{(1+i)}^t}}$$

(5)

where CF_bt—benefits cash flow for a t period (Euro), and CF_ct—costs cash flow for a t period (Euro).

The Payback Period indicator determines the time necessary to recoup funds spent on the investment due to the income or savings possible during its operation. PP is a simple and easily understandable indicator (the shorter the PP, the more profitable the investment), showing the ratio of possible inflows to spent investments. However, it has one important disadvantage: it ignores the time-related value of money [115,119,120].

The Net Present Value indicator presents the total sum of discounted cash flows, covering incomes and expenditures, during the assumed period of assessment, including the investment capital costs [115,118,121,122]. The profitable investment is characterized by NPV > 0 (or eventually equal to zero for the neutral investment).

BCR, presenting the ratio of discounted incomes (or savings) to discounted costs, is a very sound and easy-to-understand indicator of investment profitability. In the case of the investment bringing no profits, the BCR value is lower than 1.0. While the investment with a determined BCR > 1.0 is profitable, it should be assessed positively.

The assessment of the cost-efficiency of studied variants of on-site sustainable sanitation for rural settlements was based on the popular Dynamic Generation Cost (DGC) indicator [123,124,125]:

$$\mathrm{D}\mathrm{G}\mathrm{C}=p_{EE}={\displaystyle \frac{\sum _0^{t=n}\frac{{IC}_t+{EC}_t}{{\left(1+i\right)}^t}}{\sum _0^{t=n}\frac{{EE}_t}{{\left(1+i\right)}^t}}}$$

(6)

where IC_t—annual investment costs in given year (Euro); EC_t—annual operation and maintenance costs in given year (Euro); t—year of investment time duration, from 0 to n, where n is the last assessed year of investment activity (year); i—discount rate (%), p_EE—price of the ecological unit effect of the investment (Euro/m⁻³); and EE_t—annual ecological unit in given year (m³).

The DGC indicator determines the cost of the ecological effect of the investment, taking into account investment and O&M costs. In this study, DGC describes the cost of the ecological unit effect, which was one cubic meter of treated domestic sewage. The application of DGC is relatively simple: the more financially effective the method is, the lower the value of the determined indicator.

The following input data were assumed for conducting economic feasibility and cost-efficiency calculations for the proposed manners of on-site sewage treatment for rural settlements: (i) time duration 30 years, according to [126,127]; (ii) discount rate of 5% (based on the similar projects adjusted to the actual economic situation in the region and object of analysis [13,60,128,129,130]); (iii) mean annual volume of sanitary sewage 116.80 m³/year; (iv) annual price of sewage transport by 10 m³ slurry wagon EUR 1172.09 based on available offers [110,111]. The annual benefits cash flow CF_bt, required for NPV and BCR calculations, was assumed as savings possible due to avoiding sewage transport payment from the septic tank to the wastewater treatment plant [22,124]. The value of possible savings was calculated as 10 m³ slurry wagon fee [110,111] multiplied by the number of transports per year, i.e., 12 times per year.

2.4. Sensitivity Analysis

To assess the possible changes in the results of the presented financial analysis affected by a variation in the assumed input data, sensitivity analysis was performed for the variable discount rate, commonly assumed as the most crucial variable highly influencing the results of the dynamic economic profitability and cost-efficiency indicators [131,132,133,134]. Thus, the determination of economic feasibility and cost-efficiency was repeated for changed values of the discount rate, with assumed values of i equal to 4% and 6%. Additionally, the following important factors affecting the financial aspects of on-site sanitary sewage management were also introduced to the performed sensitivity analysis: variable energy prices, time of operation, and mean unit cost of sewage and sludge transport to the municipal wastewater treatment plant [110,111,128,135]. The assumed set of input data for the sensitivity analysis, reflecting the historical variations in recent decades, is presented in

Table 3. Assumed input data for sensitivity analysis for economic profitability and cost-efficiency determination.

Input Data	Initial Value	Minimal Value	Maximum Value
Discount rate (%)	5	4	6
Energy price (EUR/kWh)	0.29	0.14	0.44
System lifetime (years)	30	20	40
Mean price of sewage and sludge transport (EUR/m3)	11.86	8.90	14.83

.

The performed sensitivity analysis was based on the sensitivity coefficient (SC), determined according to the following formula [136,137]:

$$\mathrm{S}\mathrm{C}={\displaystyle \frac{X}{Y}}·{\displaystyle \frac{Y_{max}-Y_{min}}{X_{max}-X_{min}}}$$

(7)

where X—initial value of parameter; Y—predicted output value for X input parameter; X_max—maximal value of input parameter; X_min—minimal value of input parameter; Y_max—predicted output value for X_max input; Y_min—predicted output value for X_min input.

The analysis of the sensitivity coefficient results is rather simple: the outcome value of SC equal to zero reflects no relation between changes in input parameters and changes in the output value. In contrast, SC values of −1.00 or 1.00 reflect the proportional linear relationship (decrease or increase) between changes in input parameters and changes in the outcome, that is, a 25% increase in input parameter results in a 25% decrease (for SC = −1.0) or increase (for SC = 1.00) in the determined output value during sensitivity analysis.

2.5. Weighted Sum Model

The weighted sum model (WSM) was selected to perform the final assessment of the proposed manners of on-site sewage management (Equation (8)). Two groups of criteria were selected: economic profitability (BCR) and cost-efficiency (DGC).

The assignment of weight factors for each studied criterion (see

Table 4. Assumed weight factors for assessed economic criteria.

Indicator	Weight Factor (%)
Profitability	60
Cost-efficiency	40

) was based on the authors’ previous experience, local conditions, and the literature studies [8,13,18,60,124,138,139,140,141,142]. In this study, greater emphasis was placed on economic feasibility issues as mainly affecting the applicability and social acceptance of on-site rural sanitation [7,22,121].

$${PC}_j=\sum _{i=1}^n{PI}_{ji}{w}_{ji}$$

(8)

where PC_j—performance value of j criterion; n—number of indicators included in the criterion; PI_ji—performance value of indicator in the criterion; w_ij—weight factor of the indicator in the criterion.

The performance values of the indicators in Equation (7) were based on the determined indicators of economic profitability and cost-efficiency. However, taking into account that an increase in profitability is related to an increase in BCR values, while an increase in cost-efficiency is reflected in a reduced DGC value, the inverse of the latter (i.e., 1/DGC) was calculated. Then, to directly calculate PC, the rescaling max normalization, for the sums of profitability and cost-efficiency indicators for each variant, was applied according to the following formula [143,144]:

$${PI}^{\prime }={\displaystyle \frac{PI}{{PI}_{max}}}$$

(9)

where PI’—the scaled normalized value; PI—the original value; PI_max—the maximum performance value.

3. Results

Figure 3 presents the results of the Payback Period determination for all studied variants of on-site sewage management under the conditions of rural settlements without organized collective municipal sanitation. The presented PP values show that all proposed sewage management devices are characterized by a relatively short time of investment return in relation to the traditional septic tank and sewage transport by slurry wagon. The minimum determined time of Payback Period was equal to 2.85 years for Variant 8 and drainage packages in excavation located in permeable soils. The longest return time, 6.90 years, was observed for Variant 7 and drainage pipes in embankment in poorly permeable soils.

Generally, the shortest determined PP values were noted for variants of sewage management using drainage packages in excavations in locations with permeable soils. In these cases, the required investment and O&M costs are lower because of the high hydraulic efficiency of drainage packages and lower energy consumption without the required application of sewage pumps. A relatively long Payback Period was also determined for a standard septic tank equipped with a drainage filter, located on both studied soils types, permeable and poorly permeable, which is 5.32 and 6.55, respectively.

The results of the dynamic economic profitability indicator Net Present Value are presented in Figure 4. It is clearly visible that all proposed variants present a satisfactory economic profitability with a positive value of NPV even exceeding the value of EUR 12,000–13,000 for a 30-year assessment duration.

The best economic performance, determined by the highest NPV, which is EUR 13,757.33, was shown by Variant 8, the septic tank equipped with drainage packages located in excavation in permeable soils. However, this assumed method of sanitary wastewater management was effective only in this specific case. In the case of drainage pipes in drainage filters located in permeable and poorly permeable soils, the determined NPVs were rather low, EUR 10,721.00 and 8120.30, respectively. As seen in Figure 4, only one variant of the domestic wastewater treatment plant with aerated active sludge and trickling filters showed lower values of determined NPV. It is worth underlining that Variant 3 for both applied types of infiltration devices and both studied types of locally available soils presented relatively high economic profitability, with NPVs in the range of EUR 10,661.46–13,099.02. However, the economic efficiencies of Variants 1, 4, 5, and 6 were only slightly lower. In contrast, the lowest efficiency was presented by Variant 7, also equipped with all tested infiltration devices determined by NPV in the range of approximately EUR 7420.79–9858.35. Taking into account the results of sensitivity analysis for NPV calculations assuming variable values of the discount rate, it can be seen that the positive assessment of all studied variants was not affected by the assumed changes in the discount rate value.

Similar observations were made for the BCR economic feasibility indicator; the values for each tested variant are presented in Figure 5. All developed variants of on-site individual sanitation are above the threshold of profitability determined by BCR ≥ 1.0, for which the possible benefits of the investment are greater than its costs, in relation to the traditional septic tanks and sewage transport. The determined values of BCR were in the range 1.63–3.53. Again, as described above, the highest economic efficiency, represented by BCR = 3.53, was achieved by Variant 8 equipped with drainage packages located in excavation in permeable soils. However, the use of drainage pipes instead of packages or the application of infiltration devices in embankment on poorly permeable soils significantly reduced this value to the level of BCR = 1.73–2.32. Relatively high values of the BCR indicator, i.e., BCR = 2.25–3.15, were calculated for Variant 3 equipped with all studied infiltration devices. As expected, the lowest profitability, but over the threshold value, was determined for Variant 7. The results of the sensitivity analysis performed for the BCR calculations in relation to the variable discount rate showed that an increase in its value to 6% does not affect the positive assessment of all studied variants of on-site sanitary management.

The determined values of the Dynamic Generation Cost indicator, which measures the cost-efficiency of all tested on-site sewage management methods, along with the results of the sensitivity analysis, are shown in Figure 6. It is clearly visible that the cost of ecological effect (in this case, one cubic meter of collected and treated sanitary sewage by the proposed designs) ranges from 2.84 to 6.15 EUR/m³ and is significantly lower than the cost calculated for a traditional holding septic tank (10.60 EUR/m³). Therefore, the largest reduction in the DGC value, approximately 73.2%, was achieved after introducing an on-site wastewater treatment plant, while the lowest reduction, approximately 41.9%, was also noted. Consequently, the proposed technologies should be viewed positively in terms of cost-efficiency. The most effective devices were those equipped with drainage packages installed in permeable soils, and the results of the sensitivity analysis clearly showed a lower DGC value when the discount rate was changed.

The lowest value of the DGC indicator, 2.84 EUR/m³, determining the highest cost-efficiency, was observed for Variant 8, the standard septic tank equipped with drainage packages located in excavation in permeable soils, presenting also the highest economic efficiency. However, the other studied infiltration devices combined with the septic tank resulted in clearly higher values of determined DGC, in the range 4.33–5.79 EUR/m³. Among the studied bioreactors, i.e., Variants 1–7, the lowest calculated DGC values reflecting the highest cost-efficiency were observed for Variant 3 equipped with both studied infiltration devices and located in permeable and poorly permeable soils. However, the highest value of ecological effect price was, as expected, determined for Variant 7. The DGC sensitivity analysis showed that an even increase in the discount rate to 6% results in higher cost-efficiency of the proposed on-site methods of sanitary sewage management in relation to the standard septic tank and sewage transport by sewage trucks. The highest observed values of DGC for on-site treatment and i = 6% were in the range 6.28–6.48 EUR/m³, while, as mentioned above, the determined DGC for the septic tank was 10.60 EUR/m³.

Figure 7a presents the observed relations between the economic profitability indicator (BCR) and the most important components of investment costs, i.e., underground bioreactor treatment plant tank price (including equipment), and required volume of earthworks. Additionally, the relationship between BCR and the main components of O&M costs, including the required energy consumption and required annual price of sludge transport for each variant, is presented in Figure 7b. As is expected, the profitability of design, understood as the highest possible value of BCR, increases in relation to a decrease in investment as well as operation and maintenance costs, which are related to the main components of these costs, i.e., tank price and energy consumption.

A similar situation is observed in Figure 8 for the cost-efficiency of the studied designs, for which the relationship between the DGC value and selected components of operation and maintenance costs was examined. The value of the ecological effect decreases with investment and O&M costs. Figure 7 and Figure 8 also clearly show that the selection of the infiltration method of treated sewage into soil (drainage packages and pipes) and the type of locally available soil (permeable and poorly permeable local soils) clearly affect the results of economic feasibility and cost-efficiency assessment.

Figure 9 presents the results of weighted sum model calculations performed for normalized profitability and cost-efficiency indicators for all studied variants of on-site sewage management. Among all tested designs, the highest performance value in WSM was determined for Variant 3, which covered a five-chamber tank with sewage treatment based on active sludge and an aerated trickling filter. This variant, as mentioned above, presented very high and even economic feasibility and cost-efficiency in combination with both tested drainage devices on permeable and poorly permeable soils. The relatively lower performance value of Variant 7, visible in Figure 9, is related to the results of its economic profitability and cost-efficiency assessment and may be relayed to the highest operating costs, which are primarily influenced by energy consumption, which is over 2.3 times higher than, for example, Variants 4 and 5, and the use of biopreparations that improve the functioning of the treatment plant. Additionally, this variant is characterized by very high investment costs (higher only for Variant 1), which are associated with the high costs of purchasing the treatment plant. However, the economic profitability indicators for this variant are clearly higher than the threshold value, and its determined cost-efficiency is definitely higher than that calculated for a standard septic tank and sewage transport by vehicles to the municipal wastewater treatment plant. On the other hand, the vertical arrangement of the tank in Variant 7 may increase the applicability of this solution, especially for plots of limited size, dense development, or unfavorable shape.

The results of the sensitivity analysis covering variable values of crucial import data to economic considerations, including discount rate, energy prices, system lifetime, and mean price of sewage and sludge transport, are presented in

Table 5. Sensitivity coefficient values calculated for variable discount rates and all tested variants of on-site wastewater management systems.

Variant	Sensitivity Coefficient for Variable Discount Rate
	Profitability				Cost-Efficiency
	a	b	c	d	a	b	c	d
Variant 1	−0.389	−0.333	−0.381	−0.326	0.388	0.333	0.380	0.325
Variant 2	−0.290	−0.260	−0.276	−0.251	0.289	0.260	0.276	0.250
Variant 3	−0.358	−0.308	−0.347	−0.299	0.358	0.307	0.346	0.299
Variant 4	−0.341	−0.297	−0.329	−0.289	0.340	0.297	0.329	0.289
Variant 5	−0.326	−0.287	−0.314	−0.278	0.325	0.286	0.313	0.278
Variant 6	−0.337	−0.296	−0.325	−0.287	0.336	0.295	0.325	0.287
Variant 7	−0.266	−0.246	−0.255	−0.238	0.266	0.246	0.255	0.238
Variant 8	−0.326	−0.262	−0.286	−0.220	0.326	0.262	0.285	0.220

,

Table 6. Sensitivity coefficient values calculated for variable energy prices and all tested variants of on-site wastewater management systems.

Variant	Sensitivity Coefficient for Variable Energy Prices
	Profitability				Cost-Efficiency
	a	b	c	d	a	b	c	d
Variant 1	−0.177	−0.302	−0.187	−0.314	0.175	0.295	0.186	0.306
Variant 2	−0.158	−0.277	−0.167	−0.286	0.157	0.271	0.165	0.319
Variant 3	−0.245	−0.363	−0.261	−0.378	0.241	0.351	0.257	0.364
Variant 4	−0.148	−0.276	−0.156	−0.286	0.145	0.270	0.153	0.280
Variant 5	−0.145	−0.271	−0.153	−0.281	0.144	0.266	0.152	0.275
Variant 6	−0.216	−0.328	−0.228	−0.340	0.213	0.319	0.225	0.330
Variant 7	−0.259	−0.341	−0.269	−0.351	0.254	0.331	0.264	0.340
Variant 8	0.000	−0.130	0.000	−0.174	0.000	0.129	0.000	0.173

,

Table 7. Sensitivity coefficient values calculated for variable systems lifetime and all tested variants of on-site wastewater management systems.

Variant	Sensitivity Coefficient for Variable System Lifetime
	Profitability				Cost-Efficiency
	a	b	c	d	a	b	c	d
Variant 1	0.319	0.276	0.312	0.270	−0.343	−0.293	−0.215	−0.184
Variant 2	0.242	0.218	0.231	0.210	−0.255	−0.229	−0.156	−0.142
Variant 3	0.295	0.256	0.286	0.249	−0.316	−0.271	−0.196	−0.169
Variant 4	0.281	0.248	0.272	0.241	−0.206	−0.262	−0.186	−0.163
Variant 5	0.174	0.156	0.169	0.151	−0.197	−0.253	−0.177	−0.157
Variant 6	0.278	0.246	0.270	0.240	−0.204	−0.261	−0.184	−0.162
Variant 7	0.223	0.207	0.214	0.200	−0.234	−0.217	−0.144	−0.134
Variant 8	0.270	0.219	0.238	0.186	−0.287	−0.231	−0.161	−0.124

and

Table 8. Sensitivity coefficient values calculated for variable sewage and sludge transport costs and all tested variants of on-site wastewater management systems.

Variant	Sensitivity Coefficient for Variable Sewage and Sludge Transport Costs
	Profitability				Cost-Efficiency
	a	b	c	d	a	b	c	d
Variant 1	0.920	0.937	0.916	0.935	0.111	0.141	0.105	0.136
Variant 2	0.705	0.767	0.689	0.759	0.475	0.603	0.451	0.583
Variant 3	0.941	0.955	0.938	0.954	0.067	0.088	0.063	0.085
Variant 4	0.788	0.835	0.775	0.829	0.295	0.379	0.278	0.366
Variant 5	0.844	0.878	0.835	0.874	0.225	0.289	0.213	0.279
Variant 6	0.846	0.879	0.837	0.875	0.228	0.292	0.216	0.282
Variant 7	0.825	0.856	0.818	0.852	0.453	0.550	0.436	0.535
Variant 8	0.866	0.897	0.792	0.863	0.264	0.345	0.169	0.258

.

The calculated values of sensitivity coefficients determined for variable values of discount rates, energy prices, and system lifetime (see Table 7) influencing economic profitability and cost-efficiency of studied on-site sanitation systems remain at the comparable level, from approx. —0.40 to 0.40. A different situation is observed in Table 8, which presents the results of the sensitivity analysis performed for variable sewage and sludge costs. The calculated values of the sensitivity coefficients for the DGC cost-efficiency indicator are clearly lower than the values of SC determined for the profitability indicator BCR, 0.07–0.60 and 0.70–0.94, respectively. This is related to the fact that the costs of sewage transport from the septic tank to the municipal wastewater treatment plant are excluded from the cost-efficiency determination of the tested variants of on-site sanitation. However, these costs are crucial in calculating the economic profitability indicators because they are the basis for determining the benefits cash flow (CF_bt in Equation (4)), which are the possible savings that can be achieved by the investor due to installation of on-site sanitation devices.

A negative SC value reflects a decrease in dynamic profitability or cost-efficiency indicator values (BCR and DGC) related to an increase in the assumed input parameter value. In contrast, positive SC values reflect an increase in the economic indicator value related to increased input parameters. However, it should be noted that, while an increase in the value of the economic profitability indicator, i.e., BCR, reflects the increase in the designs’ profitability, the situation with their cost-efficiency is different. Increased cost-efficiency is reflected by the reduced value of the DGC indicator. Thus, results presented in Table 5, Table 6, Table 7 and Table 8 show that variable values of discount rates, energy prices, and system lifetime affect, affect the results of economic profitability and cost-efficiency determination to some extent. The greatest impact on profitability was determined for the variable costs of sewage and sludge transport, which highly influenced operation costs and possible benefits related to the application of the tested on-site sanitation designs. However, it should be highlighted that the increased costs of sewage transport, by 10m³ slurry vehicle, from the septic tank to the municipal wastewater treatment plant clearly positively affected the determined economic profitability of on-site treatment systems, allowing for greater benefits resulting from money savings, for which such transport is not required.

4. Discussion

The presented above determined economic feasibility and cost-efficiency of the studied devices of on-site household sanitary sewage treatment shows the high applicability of this method of sanitation in low-density rural populations without access to organized sewage systems. The visible positive economic assessment of all tested on-site sanitation systems in relation to the popular standard of holding septic tanks and sewage transport by slurry wagons may result in wide public acceptance and an increase in willingness to pay.

The profitability of selected manners of on-site wastewater treatment based on active sludge and tricking filters determined in this study is in agreement with the results published by Karczmarczyk et al. [62] in a study concerning 10 years of costs and benefits analyses of 23 existing treatment plants of different size and daily sewage discharge per resident, 74.5–134.0 dm³/day, located in rural regions of Mazovia Voivodship, Poland. The economic feasibility analysis in this study was related to the type of bioreactor, i.e., activated sludge, sequencing batch reactor (SBR), and activated sludge combined with trickling filters. The tested activated sludge bioreactors were assumed to be the most efficient. The presented results of cost-efficiency calculations represented by the determined DGC indicators, showing a significant difference between the cost of ecological effects for eight studied variants of different manners of on-site sanitation under conditions of rural Poland, are in agreement with the previous studies [75,77], suggesting that the costs of sanitary sewage collection in septic tanks and its transport by slurry wagons are approximately twice the costs of sewage treatment in domestic, on-site WWTPs (due to the significant costs of sewage vehicle transport). Similar conclusions based on cost analysis have also been reported for regions with low-density settlements in different countries [23,50,70,145,146,147].

Considering the possible threat to groundwater and soil posed by the domestic wastewater treatment plants based on anaerobic tanks and filtering drainages [11,81], the significant economic profitability and cost-efficiency of proposed bioreactors utilizing aerated active sludge and trickling filters, allowing a higher degree of pollutant reduction [62], should be emphasized. Moreover, the financial sustainability of up-to-date devices for on-site wastewater treatment was possible in the case of two types of infiltration device applications (drainage pipes and drainage packages combined with sand–gravel filters) in different types of soil, permeable and poorly permeable. However, as expected, the localization of domestic on-site sanitation devices on poorly permeable soils requires higher investment and operation costs. However, the obtained results suggest that the use of domestic wastewater treatment plants on poorly permeable soils is feasible and cost-effective and should be recommended. Using advanced bioreactor-based wastewater treatment plants on poorly permeable soils ensures the required level of wastewater treatment. Therefore, they are ecologically efficient solutions. Economic analyses have shown that such solutions require a longer Payback Period for the investment costs incurred. The cost-efficiency of decentralized wastewater treatment plants, especially on poorly permeable soils, could be improved by incentives in the form of subsidies at the municipal or provincial level. Currently, the programs available in Poland under the National Fund for Environmental Protection and Water Management offer reimbursement of incurred costs ranging from EUR 697.67 to EUR 1860.46 (depending on the province). Municipal subsidies (available only in the selected municipalities) provide a refund of up to 80% of investment costs, and the average subsidy amount is between EUR 697.67 and 2790.70.

The simplest solution for on-site sewage management tested in this study, an anaerobic septic tank equipped with two types of drainage filters, presented satisfactory high economic profitability and cost-efficiency only under the conditions of drainage packages applied on permeable soils. However, its installation would be profitable compared to the standard septic tanks and sewage transport by slurry wagons. Previous research, also based on NPV, BCR, and DGC determination, demonstrated that this manner of sewage management would be more financially attractive for the rural population than organized sanitation in rural settlements with sparse spatial development and a limited amount of sewage discharged to the sewage network [51,124]. However, it should be emphasized that the proper and safe operation of the simplest anaerobic tank and drainage devices requires the awareness and responsibility of their users to avoid a decrease in environmental efficiency [11,81,124,148]. Therefore, the use of septic tanks equipped with infiltration drainage, although economically attractive, may pose a greater environmental risk than bioreactors, which is a key aspect of overall sustainability. Therefore, efforts should be made, for example, through financial incentives, to encourage potential investors to choose systems that offer higher levels of wastewater treatment.

The economic feasibility and cost-efficiency of the sustainable on-site domestic wastewater treatment methods studied in this paper agree with numerous reports assessing the treatment technology choice for different parts of the world [50,81,146,149,150,151,152], suggesting that decentralized solutions should be popularized owing to the high costs of centralized sewage systems. Efficient on-site sanitation for low-density settlements may reduce the financial costs of sewage management and promote better watershed management.

The application of bioreactors with a high degree of pollution reduction to domestic sewage may significantly improve the water balance of catchments distorted by the current climate change. The infiltration of efficiently treated sewage into soil improves its water retention [153]. On the other hand, the treated wastewater may also be directly used to water the garden or cultivated vegetation, reducing the tap water demand. However, in such cases, additional investment and maintenance costs are required.

In the authors’ opinion, the applied method of economic feasibility and cost-efficiency analyses of rural individual on-site devices of sanitary sewage collection and treatment is simple to understand and may be applied by the stakeholders on their own. Additionally, considering the profitability and cost-efficiency of the design, it includes not only the investment and (constant or time-related) maintenance costs, variable value of money, but also the possible financial benefits of the investment.

5. Conclusions

The economic profitability and cost-efficiency analysis of eight variants of on-site household sanitary wastewater collection and treatment, including seven variants assuming up-to-date bioreactors with a very high degree of pollutant removal, for regions without an organized municipal sewage network allowed the authors to draw the following conclusions:

• All the proposed devices for on-site household sewage management presented significant economic profitability and cost-efficiency in relation to the use of septic tanks and sewage transport by slurry vehicles.
• In most cases, the proposed bioreactors, which allowed for a very high pollutant reduction inside the tank, before introducing the treated sewage to the soil, were more economically profitable and cost-efficient than the standard septic tank equipped with drainage filters that introduced partly treated sewage to the soil.
• In all tested cases, the application of drainage packages instead of traditional drainage pipes allowed for higher economic- and cost-effectiveness.
• Drainage packages installed in embankments should be recommended for treated wastewater infiltration, especially in cases of locally available soils with poor permeability.
• The observed economic- and cost-effectiveness of the discussed on-site household wastewater treatment plants, in relation to the septic tank, could be sustained even on poorly permeable soils, but with higher investment and O&M costs.
• According to the weighted sum model assessment, most of the studied on-site devices presented relatively comparable economic feasibility and cost-efficiency.
• Even devices with lower determined performance values, according to the assessment, presented a high degree of economic profitability and cost-efficiency in relation to septic tanks and sewage transport by slurry wagons, presenting additional advantages, such as a reduced required area, allowing for application on small plots with dense development or unfavorable shape, in the case of treatment plants with vertical tanks.
• The sensitivity analysis showed that the profitability and cost-efficiency assessment results of the studied bioreactors are related to variable energy costs and the process of sewage and sludge transport by slurry vehicles.
• Taking into account the economic feasibility and cost-efficiency, the discussed on-site household bioreactor wastewater treatment plants should be encouraged, not only instead of traditional septic tanks but also instead of septic tanks equipped with drainage soil filters.
• To encourage the application of rural on-site sanitation systems, especially in locations with poorly permeable soils, an increase in social acceptance using municipal or governmental subsidies seems to be required.