Financial Sustainability of Selected Rain Water Harvesting Systems for Single-Family House under Conditions of Eastern Poland

Recent climate changes limiting the available water resources require careful sustainable water management in the cities, the locations of highest drinking water consumption and sanitary sewage and stormwater generation. Over 50% of water demand in the residential areas of cities cover activities in which non-potable water could be used, e.g., toilets and laundry facilities, cleaning, garden irrigation and washing vehicles. Thus, rain water harvesting (RWH) systems are the sustainable alternative water supply, lowering drinking water consumption, by the usage of non-potable harvested water, and limiting the anthropopressure on natural water reservoirs. However, in many cases the social acceptance of RWH and willingness to pay may be affected by financial sustainability, including the affordability and profitability of the investment. This paper presents a case study concerning the financial sustainability of thirteen designs of RWH systems for a single-family house under the climatic and economic conditions of Eastern Poland, one of the poorest regions of the European Union. The financial sustainability of the tested RWH designs were based on indictors of cost-efficiency: dynamic generation cost, payback period, net present value and benefit–cost ratio. The performed analyses showed the limited profitability of the studied RWH designs and the insufficient governmental financial support which may significantly affect the social sustainability of the designs under the local conditions.


Introduction
The sustainable development of society, in general, is understood as a growth at which the needs of the current and the future generations are satisfied, also by the proper usage of natural resources, including drinking water, the reserves of which are threatened by urbanization, population growth and climate change [1][2][3][4][5][6][7][8][9]. Around one third of the EU territory is exposed to permanent or temporary water shortages [10,11]. The volume of available drinking water is related to the natural factors including climatic conditions, droughts, limited precipitation, infiltration, surface and underground flows, and may be also affected by anthropopressure caused by water usage by populations, services and industry, the improper handling of sanitary sewages and municipal solid wastes, increased urbanization triggering elevated runoff, etc., so the protection of water resources is crucial to the sustainable development of urbanized regions [4,9,[12][13][14][15][16]. Thus, the sustainable management of water resources can prevent water shortage [15,17,18].
Water demand in the residential areas of developed countries, including single-family housings, covers not only the actual drinking, food preparation and personal hygiene but also water operation and maintenance as well as the volume of collected and reused rain water. Thus, a financially sustainable RWH design should be affordable and cost-effective during the extended time duration of its operation and the choice of technology should be based on the extensive decision-making process.
In our opinion, RWH systems economics analyses, as aimed at local residents and designers, should be based on clear, easy-to-understand and sound cost-effective analysis, based on several simple and dynamic financial indicators, allowing the easy assessment of the RWH system profitability already at the stage of conceptual design and decision making [48][49][50][51][52].
This paper presents a case study concerning the financial sustainability of several designed rain water harvesting systems for a single-family house model occupied by four residents under the climatic and economic conditions of Eastern Poland. The financial sustainability of thirteen RWH designs of various complexity, and the rain water reuse ratio were based on simple and dynamic indicators of cost-efficiency: dynamic generation cost, payback period, net present value and benefit-cost ratio. The required costs and cash inflows or possible savings, necessary to determine the aforementioned financial sustainability indicators, were based on a determined preliminary investment costs estimation, the calculation of the operation and maintenance costs, the actual mean prices of tap water and sanitary sewerage discharge as well as the stormwater fee.

Materials and Methods
The performed studies of the financial sustainability of various on-site rain water harvesting systems application were conducted for two cases, without and with the obligatory payment of the stormwater fee by the owners of the house (this obligation is not uniform in Poland, its application nowadays depends on the decision of the local water supply and wastewater removal companies). The presented economic analyses were based on several simple and dynamic indicators of investment cost efficiency.
The assessment of the financial sustainability of several studied rain water harvesting systems was performed for a typical single-family house located in the eastern part of Poland, Lubelskie and Podlaskie Voivodeship, regions of relatively low level of economic development, generally one of the less developed regions of Poland and the European Union [53].
The single-family house of a roof area of 230 m 2 occupied by 4 residents owning a car and cultivating a 400 m 2 garden, located in the area of a mean annual rainfall height H = 600 mm [54] was used for the presented analyses. The annual rain water yield was calculated according to the following formula: where: Q-annual rain water yield, L/year, A-roof area, m 2 , ϕ-dimensionless outflow coefficient, assumed ϕ = 0.8, according to [55], q-mean annual rainfall, L/m 2 , according to [54]. The assumed household is connected to a municipal water supply system, a sanitary sewerage and a stormwater system. The mean drinking water demand for one resident (125 L/(d·resident), including 38 L/(d·resident) for toilet flushing, 14 L/(d·resident) for laundry and 5 L/(d·resident) for cleaning the house), washing vehicles (1.9 L/(d·resident)) and watering the garden (60 L/(year·m 2 ) i.e., 0.16 L/(d·m 2 )) were assumed after actual Polish standards [22]. The determined mean annual water demand for the building occupied by four residents was 182.5 m 3 per year, while the water demand extended by vehicle washing and watering the garden reached the level of 209.3 m 3 . The domestic water supply system was designed as layered cross-linked polyethylene pipelines PEX-Al-PEX PN10, of the following diameters 32 × 3, 26 × 3, 20 × 2 and 16 × 2 mm, equipped with the standard water meter DN 20 and non-return valve type EA DN 20 [56,57].
To perform the economical assessment of several possible, up-to-date RWH systems for a single-family house, the meeting requirements of actually biding Polish and European law as well as utilizing modern technologies, thirteen variants of storm water collection, storage and utilization were designed. All the developed variants are presented in Figure 1, while the general information regarding the components of each assumed variant and the planned usage of rain water are presented in Table 1. As it is visible in Table 1, the designed variants allow the reuse of rain water in volumes from 26.6 m 3 (watering the garden and washing the vehicle) to 110 m 3 (as above plus domestic use including toilet flushing, cleaning and laundry), which constitutes from 12.8% to 52.6% of the total household water demand. It is visible, that most of the designed RWH variants were aimed to maximize the reuse of rain water and saving the tap water. Only one variant was designed as allowing to deliver all the stored rain water to the soil through drainage boxes, with zero water reuse (in this case, the only economic advantage may be the savings possible due to avoiding the stormwater charge). In all the designed cases, the overflow 110 mm PVC (polyvinyl chloride) pipe was designed to remove the exceeding volume of water from the RWH system to the municipal stormwater system or road-side drainage ditch. In all the studied cases, the volume of harvested rain water tank was calculated according to the formula [58]: where: V-tank volume, L, Q-annual rain water yield, L/year, P-annual water demand on given purpose, according to [22], L/year, n-duration of dry period, assumed n = 21, according to [58].
Rain water installation was in all the tested variants, designed as 110 mm PVC pipes to collect and direct the stormwater [55]. The designed variants consisted of several technical units allowing to collect, store, treat and reuse harvested rain water, presented in Table 1. The underground stormwater tank, selected as the main unit storing rain water in Variants No. I, II, II, IV and XII was designed as 7 m 3 high-density polyethylene (HDPE) reservoir. The volume of HDPE over ground rain water storage tank was different in several variants, starting from 4 m 3 for variants IX, X and XII, through 6 m 3 for Variant No. XIII to 7 m 3 for Variants V-VIII [58]. A multistage automatically operated submersible pump with an output up to 120 dm 3 /min (7.2 m 3 /h) and head up to 42 m, was used to allow water flow from the tank to the installation was used in Variants I, II, V, VI, IX, X and XIII. The collected rain water in all tested variants underwent partial basic mechanical treatment underground, integrated with the tank, (Variants I, III, V, VII and XIII) or downpipe (Variants II, IV, VI, VIII-XII) filters [59]. Both types of filters applied in these designs had the same purpose: the removal of solid particles flushed from the roof to the rain water gutter piping. The main difference, besides the size and costs of both filters, was their location, underground inside or outside the water tank, and the second directly on downpipe, respectively. Four designs (III, IV, VII and VIIII) were equipped with a rain water central consisting of drinking water reservoir, a submersible water pump with an electromagnetic valve of maximum performance 4.2 m 3 /h and a pressure head of 48 m. The designed central allowed the automatic use of drinking water, instead of rain water, in cases of long lasting dry period and limited access to water from precipitation. Variant XI, in which no reuse of rain water was designed, assumed a storage of 110 m 3 /year of intercepted stormwater and infiltration to soil by a set of polypropylene drainage boxes of a total volume 2.35 m 3 .
stormwater system or road-side drainage ditch. In all the studied cases, the volume of harvested rain water tank was calculated according to the formula [58]: where: V-tank volume, L, Q-annual rain water yield, L/year, P-annual water demand on given purpose, according to [22], L/year, n-duration of dry period, assumed n = 21, according to [58]. In order to allow the financial analyses, the preliminary investment cost estimation and the assessment of operation and maintenance (O&M) costs under the commercial conditions of Eastern Poland were performed. The preliminary investment cost calculations included materials, assembly and earthworks, while the estimation of the O&M costs covered energy prices (assumed 0.1 Euro for 1 kWh), required fittings and pipeline services or exchange, as well as filters and reservoir cleaning. It was assumed that simple conservation works may be performed by the users of the RWH systems. The determined investment and O&M costs of all the designed variants, together with a sole domestic water supply installation, are presented in Table 2. The financial cost efficiency as well as the investment profitability, affecting the sustainability of the designed RWH systems for single-family housing, were assessed based on four indicators, one simple payback period (PP) and three dynamic: dynamic generation cost (DGC), net present value (NPV) and benefit-cost ratio (BCR) [50,60,61].
The payback period presents the time required to recoup funds spent on the investment due to the incomes, or savings, possible during its operation. The PP is a simple an easily understandable indicator, showing the ratio of inflows to investments, but it has one important disadvantage: it ignores the time value of money [62,63]. The PP was calculated according to the formula [63]: where: PP-payback period, years, IC-initial investment costs, Euro, NCF-net cash flow, Euro/year. Dynamic generation cost (DGC) is a rather popular and easy-to-understand cost-efficiency indicator expressing the cash value required to obtain the discounted revenues equal to the discounted costs, so it shows the price of the ecological effect, in this case 1 m 3 of collected, stored and reused rain water during the whole assumed duration of the investment, taking into account investment as well as O&M costs. The application of DGC to decision making is very simple: the lower the value of DGC, the more economically acceptable the investment is. The DGC may be calculated using the formula [64][65][66]: where: DGC-price of the ecological unit effect of the investment, Euro/m 3 , IC t -annual investment costs, Euro, EC t -annual operation and maintenance costs, Euro, t-year of investment time duration, from 0 to n, where n is the last assessed year of investment activity, years, i-discount rate, %, EE t -ecological effect unit in given year, m 3 .
The next applied dynamic, introducing the variable value of money, the indicator net present value presents the total sum of discounted cash flows, inflows and outflows, during the assumed time duration, reduced by the investment capital costs [67]. Thus, the profitable investment should obtain a Sustainability 2020, 12, 4853 7 of 16 positive, greater than zero, or eventually equal to zero, value of the NPV indicator, in monetary units. The NPV may be calculated as follows [60][61][62][68][69][70][71]: where: NCF t -net cash flow for a i year of investment operation, Euro. The last applied indicator, benefit-cost ratio (BCR), presents the dimensionless relation of discounted investment benefits (cash inflows or savings) to its discounted costs (initial investment plus O&M). The value of the BCR indicator for the profitable investment should be BCR ≥ 1. The BCR may be calculated using the following formula [60][61][62]72,73]: where: PV b -discounted value of benefits, Euro, PV c -discounted value of costs, Euro, IB t -investment benefits (cash inflows) in given year, Euro.
The assumption, as well as the input data required for the determination of the aforementioned financial sustainability indicators of thirteen designs of RWH systems for single-family housing covered: (i) the time duration of the analysis: 30 years; (ii) the actual discount rate: 6%; (iii) the price of 1 m 3 drinking water delivered by a municipal water supply network and 1 m 3 of sanitary sewerage collected by sanitary sewage network (determined as the mean price for 12 main cities of discussed regions, the payment for water delivery and sanitary sewage management is in Poland generally based on water meter readings): 2.06 Euro/m 3 ; (iv) the price of 1 m 3 stormwater delivered to a municipal stormwater system (determined as the mean price for 11 cities in Poland in which this fee is obligatory): 0.94 Euro/m 3 .
To allow the direct comparison all the proposed variants of RWH designs from the point of view of economical sustainability, based on all the determined financial indicators, the weighed sum model (WSM) was applied, due to its clarity and simplicity [12,48,49,51,52,74,75]: 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, points values, from 13 for the best design to 1 for the weakest, were assigned, w ji -weight factor of the indicator in the criterion, for each criterion weight factor was assumed 25%. Additionally, the minimal value of co-founding allowing to reduce the investment costs and to raise the public acceptance of RWH systems was determined. For each variant, the BCR calculations were repeated for the variable value of the outside costs refund until the resultant indicator value reached the threshold of profitability BCR ≥ 1. Figure 2 presents the obtained values of the DCG financial indicator for all the tested variants of the RWH systems for single-family housing, supported by DGC, determined for the drinking water supplied from the municipal drinking water distribution system. The calculated costs of ecological effect for all the tested variants varied between 1.43 Euro/m 3 and 8 Euro/m 3 , of collected, stored and reused or infiltrated rain water. Obviously, the value of the determined DGC indicator was affected by investment and O&M costs but also by the amount of retained and reused stormwater (see formula (4)). It is also visible, that in most cases, i.e., excluding Variants V and XI, the DGCs determined for 1 m 3 of harvested and reused rain water are on a similar level (e.g., Variants I, II, VI, VIII and XIII), or clearly higher (Variants III, IV, VII, IX, X and XII) than the total cost of 1 m 3 of tap water delivered by the municipal water supply system, even taking into account the rather high costs of water supply and wastewater removal.  The determined payback period values, the simple indicator of investment profitability for all the tested RWH variants, calculated in two scenarios, without and with the obligatory payment of rain water discharge to the municipal stormwater system, is presented in Figure 4. It is visible that in some cases, i.e., IX, X and XII, the determined PP without possible savings resulting from avoiding the stormwater charge (in these variants, incomes represent the savings possible with the use of rain water instead of tap water) is longer than the assumed time duration of the investment: 42.95, 61.07 and 37.99 years, respectively. In the case of Variant XI, in which all the collected rain water is stored and infiltrated into the soil, the calculated PP reaches infinity, as the variant generates no possible savings. The application of obligatory stormwater payment, allowing to obtain the significant The dependence between the complexity of the studied RWH system, understood as a number and degree of the technical sophistication of the applied components, affecting the cost of the ecological effect, is presented in Figure 3 for the Variants I-VIII, allowing to harvest and reuse the maximal required volume of rain water, i.e., 110 m 3 /year, for the assumed single-family housing.  The determined payback period values, the simple indicator of investment profitability for all the tested RWH variants, calculated in two scenarios, without and with the obligatory payment of rain water discharge to the municipal stormwater system, is presented in Figure 4. It is visible that in some cases, i.e., IX, X and XII, the determined PP without possible savings resulting from avoiding Comparison presented in Figure 3 shows that the increase in complexity and technical sophistication of tested RWH causes an increase in the DGC value caused by the increase in total investment as well as the operation and maintenance costs.

Results and Discussion
The determined payback period values, the simple indicator of investment profitability for all the tested RWH variants, calculated in two scenarios, without and with the obligatory payment of rain water discharge to the municipal stormwater system, is presented in Figure 4. It is visible that in some cases, i.e., IX, X and XII, the determined PP without possible savings resulting from avoiding the stormwater charge (in these variants, incomes represent the savings possible with the use of rain water instead of tap water) is longer than the assumed time duration of the investment: 42.95, 61.07 and 37.99 years, respectively. In the case of Variant XI, in which all the collected rain water is stored and infiltrated into the soil, the calculated PP reaches infinity, as the variant generates no possible savings. The application of obligatory stormwater payment, allowing to obtain the significant savings, clearly changes the determined values of PP. In most tested cases, the duration of payback period decreased below the assumed time duration of the investment. The only exception is Variant X, which was rather costly, but allowed the limited saving due to the reuse of only 26.8 m 3 of rain water per year. Generally, the shortest time of payback period was obtained for variants in which the reuse of rainwater was maximized, in these cases up to over 50% of the total water demand.
Sustainability 2020, 12, x FOR PEER REVIEW 10 of 17 period decreased below the assumed time duration of the investment. The only exception is Variant X, which was rather costly, but allowed the limited saving due to the reuse of only 26.8 m 3 of rain water per year. Generally, the shortest time of payback period was obtained for variants in which the reuse of rainwater was maximized, in these cases up to over 50% of the total water demand. . Determined values for payback period for the tested variants, with and without obligatory stormwater charge included; possible savings due to avoiding stormwater fee reduced the duration of the payback period, however in several cases, the PP is longer than the assumed duration of the analyses. Figure 5 presents the determined values of the discounted costs-benefits indicator net present value for all the tested RWH variants, with and without the obligatory payment of the stormwater fee. It is visible that in most tested cases, without and with possible savings due to avoiding stormwater fee payment, the profitability of the proposed designs is rather questionable because they generate costs higher than financial incomes (savings). In the first scenario, only one variant, No. V, reached the level of NPV > 0, however, rather unsignificantly, with the determined NPV = 39.08 Euro for the 30 years of investment duration. Under the conditions of the second scenario, with the savings consequent from the avoided stormwater fee, the situation was better. Six design variants, i.e., Variants I, II, V, VI, VIII and XIII, showed NPV values greater than zero, some of them even allowed NPV from approx. 559 to 1569 Euro for the 30 years of operation. Thus, it is visible that . Determined values for payback period for the tested variants, with and without obligatory stormwater charge included; possible savings due to avoiding stormwater fee reduced the duration of the payback period, however in several cases, the PP is longer than the assumed duration of the analyses. Figure 5 presents the determined values of the discounted costs-benefits indicator net present value for all the tested RWH variants, with and without the obligatory payment of the stormwater fee. It is visible that in most tested cases, without and with possible savings due to avoiding stormwater fee payment, the profitability of the proposed designs is rather questionable because they generate costs higher than financial incomes (savings). In the first scenario, only one variant, No. V, reached the level of NPV > 0, however, rather unsignificantly, with the determined NPV = 39.08 Euro for the 30 years of investment duration. Under the conditions of the second scenario, with the savings consequent from the avoided stormwater fee, the situation was better. Six design variants, i.e., Variants I, II, V, VI, VIII and XIII, showed NPV values greater than zero, some of them even allowed NPV from approx. 559 to 1569 Euro for the 30 years of operation. Thus, it is visible that in order to increase the profitability of investment, the possible savings should be maximized and the investment and O&M costs should be minimized. Otherwise, the social acceptance of the proposed variants of RWH by owners of households may be limited.
Sustainability 2020, 12, x FOR PEER REVIEW 11 of 17 Figure 5. Determined values of the net present values for the tested variants, with and without the obligatory stormwater charge included; the limited number of tested RWH systems exceeded the profitability threshold, NPV > 0, even after assuming the obligatory stormwater fee payment. Figure 6. Calculated values of the dimensionless benefit-cost ratio for the tested variants, with and without the obligatory stormwater charge included; again, the profitability threshold benefit-cost ratio (BCR) ≥ 1.0 was achieved by certain RWH variants, mostly in cases with savings possible due to avoided obligatory stormwater fee payment. The determined values of the last applied indicator of financial sustainability of the proposed RWH designs, the discounted benefit-cost ratio, were calculated for both tested scenarios and are presented in Figure 6.
Similarly, like in the case of the calculated NPV, the determined BCR values show a low profitability of the studied RWH designs, under the conditions of both applied scenarios. Without obligatory stormwater charge, and possible savings, the value of BCR ≥ 1.0 was achieved by only one variant, again Variant No. V. Increased possible savings, due to stormwater management and reuse instead of direct discharge to the municipal stormwater system, allowed the increase in the profitability of the proposed designs. Variants I, II, V, VI, VIII and XIII showed a BCR greater than 1.0. The highest determined values of BCR 1.20, 1.31, 1.49 were obtained for the Variants I, VI and V, respectively. Thus, in the cases of systems generating cash incomes, due to money savings, greater than the total investment costs, the level of social acceptance related to the affordability and economic profitability of the RWH designs may be clearly higher. Figure 7 presents the determined performance points (PC), by weighted sum model, for all the tested variants of the RWH systems under the conditions of two analyzed scenarios. In both cases, the relative, in comparison to the remaining designs, the best financial performance was observed for Variant V, which showed a very low DGC, the shortest payback period, the highest positive NPV and the highest BCR, at both scenarios greater than 1.0. It was also observed that there were some variants, i.e., VI, I and II, which showed insufficient profitability under the conditions of the first scenario, but after increasing the possible savings due to the obligatory payment of stormwater fee, under conditions of the second scenario they became profitable. Figure 5. Determined values of the net present values for the tested variants, with and without the obligatory stormwater charge included; the limited number of tested RWH systems exceeded the profitability threshold, NPV > 0, even after assuming the obligatory stormwater fee payment. Figure 6. Calculated values of the dimensionless benefit-cost ratio for the tested variants, with and without the obligatory stormwater charge included; again, the profitability threshold benefit-cost ratio (BCR) ≥ 1.0 was achieved by certain RWH variants, mostly in cases with savings possible due to avoided obligatory stormwater fee payment.
Similarly, like in the case of the calculated NPV, the determined BCR values show a low profitability of the studied RWH designs, under the conditions of both applied scenarios. Without obligatory stormwater charge, and possible savings, the value of BCR ≥ 1.0 was achieved by only one variant, again Variant No. V. Increased possible savings, due to stormwater management and reuse Figure 6. Calculated values of the dimensionless benefit-cost ratio for the tested variants, with and without the obligatory stormwater charge included; again, the profitability threshold benefit-cost ratio (BCR) ≥ 1.0 was achieved by certain RWH variants, mostly in cases with savings possible due to avoided obligatory stormwater fee payment. instead of direct discharge to the municipal stormwater system, allowed the increase in the profitability of the proposed designs. Variants I, II, V, VI, VIII and XIII showed a BCR greater than 1.0. The highest determined values of BCR 1.20, 1.31, 1.49 were obtained for the Variants I, VI and V, respectively. Thus, in the cases of systems generating cash incomes, due to money savings, greater than the total investment costs, the level of social acceptance related to the affordability and economic profitability of the RWH designs may be clearly higher. Figure 7 presents the determined performance points (PC), by weighted sum model, for all the tested variants of the RWH systems under the conditions of two analyzed scenarios. In both cases, the relative, in comparison to the remaining designs, the best financial performance was observed for Variant V, which showed a very low DGC, the shortest payback period, the highest positive NPV and the highest BCR, at both scenarios greater than 1.0. It was also observed that there were some variants, i.e., VI, I and II, which showed insufficient profitability under the conditions of the first scenario, but after increasing the possible savings due to the obligatory payment of stormwater fee, under conditions of the second scenario they became profitable. Taking into account the presented above results of financial sustainability analyses of thirteen on-site rain water harvesting designs for single-family houses, showing limited or even no profitability of the studied systems, limiting the social acceptance and discouraging the possible investors of such investments, the outside, governmental or European Community, the co-founding of part of the investment costs seems to be required. The reduction of investments, in relation to the Taking into account the presented above results of financial sustainability analyses of thirteen on-site rain water harvesting designs for single-family houses, showing limited or even no profitability of the studied systems, limiting the social acceptance and discouraging the possible investors of such investments, the outside, governmental or European Community, the co-founding of part of the investment costs seems to be required. The reduction of investments, in relation to the same operation and maintenance costs, as well as the savings possible due to the volume of retained and reused water, allows the increase in the benefit-cost ratio indicators (NPV and BCR) and the decrease in DGC and the duration of payback period. Table 3 shows the determined values of the percentage of the refund rate and the required amount of cash to obtain the minimal profitability of the proposed design, under the scenario of lowered savings due to the lacking stormwater fee. Currently, there are two available programs of RWH refunding in the assumed region of investment: (A) "Catch rain water" in Lublin, Lubelskie Voivodeship, allowing to obtain a 70% refund for the purchase and installation or renovation of RWH units, but not more than 1110 Euro; (B) "Stormwater management" in Białystok, Podlaskie Voivodeship, allowing even a 100% refund (but no more than approx. 1330 Euro) for RWH for households connected to the municipal stormwater system, and a 50% refund (no more than approx. 666 Euro) for buildings not connected to the municipal stormwater system [76,77]. Table 3. Determined refund ratio and value required to obtain the minimal profitability of the RWH investment, i.e., BCR ≥ 1.0; A-"Catch rain water" project, Lublin, B-"Stormwater management", project Białystok. *-design profitable without refund; **-no reuse of rainwater assumed, infiltration only, no savings possible; ***-only for households already connected to the municipal stormwater system. Table 3 shows that in many cases, the local refund programs for RWH systems, especially based on sophisticated technical devices, are unavailable or are unsatisfactory. Even obtaining the maximal possible refund in the cases of Variants II, IV, VII, VIII is insufficient to ensure even the minimal profitability of the investment allowing to gain the interest and acceptance of the potential investors.

Conclusions
The performed analyses of the financial sustainability of thirteen designs of on-site rain water harvesting installations allowed the following conclusions:

•
The performed financial assessment of the proposed RWH designs showed that their economical sustainability under the local conditions is in most of the cases questionable; local investors would not obtain the cash incomes or savings expected as result of their investment.

•
The determined cost of ecological effect, i.e., the total discounted cost of 1 m 3 of harvested and stored or reused water for domestic demands rain, was only in two cases lower than the similar cost of tap water supplied by municipal water distribution system. • The profitability of the proposed RWH designs under the economic conditions of Eastern Poland is very low, as in most cases both the applied benefits-costs indicators, NPV and BCR, were below the profitability threshold, and only the application of additional savings resulting from avoiding the stormwater fee in 50% of the studied designs allowed to exceed the threshold.

•
The application of variable, modern, up-to-date technologies and equipment of RWH frequently may cause a decrease in the design profitability, due to the high investment as well as O&M costs, thus, such designs may not be selected by local investors.

•
The increase in profitability of RWH design is highly affected by cash savings, so the replacement of the highest volume possible of tap water by rain water in a wide variety of domestic applications is strongly advised.

•
The outside partial refund of investment costs, without the assigned monetary value limit, should be available to increase the profitability of the RWH designs, thus, to increase their financial attractiveness, required for the local population's acceptance.