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Article

Discussion on the Need for Harvested Rainwater Quality Standards Tailored to the Reuse Purpose

by
Emil Tsanov
,
Dobril Valchev
,
Irina Ribarova
* and
Galina Dimova
Water Supply, Sewerage, Water and Wastewater Treatment Department, Faculty of Hydraulic Engineering, University of Architecture, Civil Engineering and Geodesy, 1046 Sofia, Bulgaria
*
Author to whom correspondence should be addressed.
Processes 2023, 11(3), 665; https://doi.org/10.3390/pr11030665
Submission received: 14 January 2023 / Revised: 19 February 2023 / Accepted: 20 February 2023 / Published: 22 February 2023
Browse Figure
Review Reports Versions Notes

Abstract

:
Rainwater harvesting attracts rising interest in solving the new challenges associated with climate change and socio-economic development. Rainwater harvesting is addressed in various policies, but standards related to the harvested rainwater quality (HRWQ) are currently set mostly for reuse for agricultural purposes. This paper discusses the necessity for the introduction of specific legislative requirements for the HRWQ tailored to the reuse purpose, which would reduce the health and environmental risks. Based on a literature review of research outcomes regarding HRWQ parameters and existing legislation, the paper discusses the complexity of the factors influencing HRWQ and provides some thoughts for possible actions forward that could be undertaken toward the development of specific legislation. The actions include the application of a risk-based approach, the development of a database and guidance with technological solutions.

1. Introduction

The rainwater harvesting (RWH) is an approach, applicable since ancient times, but nowadays, it attracts continuously rising interest for solving the new challenges associated with climate change (floods, droughts, heat waves, etc.) and the socio-economic development (expanding urbanization, polluted air, emerging pollutants, etc.). In the summer of 2022, some regions of Europe suffered from one of the most severe droughts of the last century, but others were hit by storms and floods [1]. Both the climate change threat and the expanding city growth intensify the need for searching for more resilient engineering solutions, including more adaptive ways for rainwater collection and discharge as well as reuse of the harvested rainwater.
RWH and its subsequent use have been addressed in one way or another in a number of EU policies and legislative documents.
The EU Water Framework Directive promotes the development of “efficiency and reuse measures” and acknowledges the potential human impact on the groundwater through “alterations to the recharge characteristics such as rainwater and run-off diversion through land sealing” [2]. A communication from the European Commission (EC) from 2012 identifies the need to establish water reuse standards to mitigate water scarcity and the vulnerability of water supply systems [3]. The EU Circular Economy Action Plan [4] states the need to keep resources in the economy as long as possible. This also corresponds to the established hierarchy of waste, including the consistent application of measures for the prevention of waste formation, preparation for reuse, recycling, other recovery, and disposal.
The recent proposal by the European Commission for revision of the urban wastewater directive (Directive 91/271) acknowledges the role of urban runoff in the formation of urban pollution and sets mid-term deadlines for its consideration in the integrated urban wastewater management plans for all the agglomerations above 10,000 population equivalents [5]. This includes specific requirements for the evaluation of the runoff quantity and the respective pollution load. Broader application of RWH, green water zones, and permeable surfaces in urbanized areas are suggested as preventive measures for decreasing the contribution of urban run-off in the urban wastewater cycle. At the same time, the new proposal acknowledges the present lack of sufficient data on urban runoff quantity and quality and sets a close deadline for establishing monitoring in place (i.e., by 2025).
The EU water-related policies recognize the need for harvested water use; however, specific legislative requirements concerning harvested rainwater quality (HRWQ) in dependence on the rainwater utilization purpose have not yet been developed.
This paper discusses the necessity for the introduction of specific legislative requirements for the quality of harvested rainwater tailored to the reuse purpose. Based on a literature review of research outcomes regarding the HRWQ parameters and existing legislation, the paper acknowledges the complexity of the factors influencing HRWQ and provides some thoughts for possible forward actions that could be undertaken toward the development of specific legislation.

2. The Approach

The study is structured into two main parts indicated in Figure 1. It begins with an analysis of the state-of-the-art, which includes two aspects—research outcomes, showing the wide variations of the HRWQ, and current legislation (left block in Figure 1).
Based on the review in part 1, in part 2—the complexity of the factors, influencing HRWQ, is discussed (right block in Figure 1). Some forward actions are suggested as well.

3. State Of-The-Art

3.1. Research Outcomes Regarding Variations of the Quality of the Harvested Rainwater

The review on the HRWQ shows high variability. Based on a review and analysis of relevant recent research papers, some HRWQ parameters appear to be essential and worth being considered in the legislation. They are presented below.

3.1.1. Physical Parameters of Harvested Rainwater

The minimum and maximum values of the physical parameters found in the scientific literature are presented in Table 1.
Conductivity is a relatively easy-to-measure indicator, which indirectly gives information about the salinity of harvested rain water (HRW). The data in the literature from studies conducted in different countries and at different latitudes (Spain, France, Greece, Ethiopia, China, South Korea, Pakistan, Nigeria, South Africa, Poland, Malaysia, etc.) suggest a wide variation of this parameter with values ranging from about 8.3 to about 615 µS/cm, as shown in Table 1.
Usually, the turbidity and suspended solids of the collected rainwater are highest at the initial runoff. This is due to the presence of dust and larger particles (pebbles, sand, etc.), accumulated on the surface of the roof, which falls into the water mainly in a suspended state. When the water is retained in the reservoir under steady-state conditions (i.e., no hydraulic disturbance from incoming water), most of the suspended particles settle down, and the turbidity of the water decreases significantly [8,17]. In Texas USA, Mendez and co-authors reported values of turbidity and SS concentration after the first-flush removal in the range of 5–35 NTU and 10–50 mg/L for metal-coated roofs and 6–23 NTU and 20–150 mg/L for shingles roofs [7]. In Poland, Zdeb and co-authors 2020 reported the highest values of turbidity of 14 NTU for an epoxide-coated roof and the lowest for a zinc-coated metal sheet [14].
Farreny and co-authors in their review paper considered monitored parameters from different studies in real conditions with flat roofs, with values ranging from 13 to 120 mg/L suspended solids, but in their own study in Spain, they measured significantly lower SS concentrations that were in the order of 0–38 mg/L [6]. The concentration of SS is directly dependent on the ambient environment—see item 3.1 [8,17,18]. With the correct selection of roof coverings and location, it is possible to achieve low concentrations of SS in the generated rainwater runoff (<50 mg/L) without the need for additional treatment [8,17].

3.1.2. Chemical Indicators

The minimum and maximum values of the chemical parameters found in the scientific literature are presented in Table 2.
  • pH
Rainwater pH values can vary widely from the acidic to the alkaline spectrum (from about 3.60 to about 11.40). The most common values are in the slightly acidic to neutral spectrum (pH = 6.5–7.5) due to the content of carbon dioxide in the atmosphere, which is retained in the raindrops [8,9,20,21]. Full-scale studies report mean pH values close to neutral in Greek suburban areas, some areas in France, the Ontario region, Canada (around pH = 7.3 in all studies), Austin, USA (pH = 7.8), Sao Jose, Brazil (pH 6.23–6.44), Seoul, South Korea (pH = 6.5–8.5), Barcelona, Spain (pH = 7.6); and Rzeswoz, Poland (pH = 7) [7,20,23,27,28]. Despins and co-authors reported a relationship between the type of storage tank and the pH values of the collected rainwater [20]. When storing water in concrete or reinforced concrete tanks, it is possible to increase the pH to an average value of 7.7 and a maximum measured of 10.3, which has a favorable bactericidal effect and reduces the risk of the development of unwanted microorganisms in the water medium [20].
  • Nitrogen
Nitrogen is among the most important biogenic elements, which is necessary for the construction of numerous cellular structures such as proteins, nucleic acids, etc. From this point of view, it is necessary for soil fertility, and its presence in the water used for irrigation would be advantageous. On the other hand, the presence of high concentrations of nitrate anions in the water could lead to gradual salinization of the humus layers and deterioration of the conditions for the growth of plants or direct accumulation of large amounts of nitrates in them as well as the contamination of groundwater with such ions [29,30]. In addition, an excessive concentration of ammonium nitrogen in the soil causes toxicity and directly suppresses plant species [31]. Data from northeastern Greece, China, North Korea, various parts of Europe, and Africa report concentrations of nitrate nitrogen in the storage tank, after first–flush removal, from 0.01 to 12 mg/L (at a norm < 20 mg/L) [14,26,32] and ammonium nitrogen from 1.53 to 4 mg/L (at a norm < 5 mg/L) [14,32], which testifies for the safe use of the collected rainwater for irrigation needs [6,8,17,21].
  • Phosphorus
Another essential biogenic element for plant growth is phosphorus, which is key to energy exchange in cells. Low concentrations of phosphorus in irrigation water are beneficial to plant growth, but its increased content in the soil can cause problems with the assimilation of some microelements such as iron, zinc, etc., leading to rapid wilting of plants. A review of the world practice shows that orthophosphate ion concentrations in rainwater runoff after the first-flush removal are mostly below 1 mg/L, in the order of 0.1 to 0.4 mg/L [6,8,14,33]. A study from South Africa shows lower than 0.5 mg/L concentration in all of the samples from three roof types (tile, thatch, metallic) without the removal of the first flush [12]. Single measurements in Spain showed concentrations of up to 6.60 mg/L [6]. With the proper maintenance of the roofs and the storage tanks, as well as their appropriate location in the urban or non-urban environment, the accumulation of phosphorus in the collected water would be insignificant and should not lead to problems with the soils when it is regularly used for irrigation.
  • Chemical oxygen demand (COD)
The COD indicator gives indirect information about the content of organic compounds in water. Carbon is a basic building block of cells and is needed in soils, but higher COD values, corresponding to higher organic loading, can cause rainwater in the storage tank to rot regularly, giving off unpleasant odors, attracting insects, and developing pathogenic microorganisms. In the literature review, no values greater than 31 mg/L were found when using ordinary flat roofs and separating the first-flush [27,33,34,35]. The COD minimum and maximum values are presented in Table 3.
  • Heavy metals;
  • The minimum and maximum values of the chemical parameters found in the scientific literature are presented in Table 3.
Monitoring and control of heavy metals in irrigation water are of paramount importance to prevent their accumulation in soils and plants. A study from Lee and co-authors shows extremely low concentrations of a number of commonly monitored heavy metals—cadmium, chromium, selenium, aluminum, arsenic, copper, iron, lead, and zinc, even in a very poorly maintained rainwater harvesting system and roof [36]. The lack of proper maintenance (regular cleaning of the roof, gutters and first flush diverters) and associated long-term accumulation of heavy metals do not cause a significant increase in metal concentrations, which continue to be in the order of micro or even nano concentrations [36]. This is especially valid when the separation of the first flush is in place. The low levels of Al, F, and Zn content (<100 µg/L) are noticed also in other studies [14,15,16]. Furthermore, Lai and co-authors do not observe significant differences between rain water and runoff in a study, which was carried out for nine roofs (three clay tiles, two metal, two polycarbonate, and two reinforced concrete) with different slopes [15]. Similar conclusions for the close values of Cu, Ni, Cr, Pb, As in rain water and runoff are reported in the study of Zdeb and co-authors, but they observed higher values for Zn and Mn from galvanized steel roof runoff in comparison with the ambient rain [14].
Unusual high values (above 1000 µg/L) for some heavy metals (Al, As, B, Cd, Cr, Fe, Zn) are reported in a study carried out in Bergville, South Africa, in 34 households with three roof types (metallic, thatch and tile) [12]. Higher are only the values for the rainwater collected from the metallic roofs. However, it should be underlined that these values do not refer to the runoff but are based on samples taken from plastic rainwater collecting tanks.

3.1.3. Microbiological Indicators

The minimum and maximum values of the reported biological indicators in the scientific literature are presented in Table 4.
  • Total coliforms
The presence of coliform bacteria, including fecal coliforms, has been observed in most of the studies and regardless of the roof type, as shown in Table 4. When water samples are taken from the water tank, the maximum values are higher [40,41,42,43,44]. One of the main reasons is seen to be the improper manner of storage [45]. Bae and co-authors found coliform bacteria in 92% of the samples, with fecal coliforms in 89%, but the data included rainwater and rainwater runoff and did not mention the separation of the first flush [37]. The study detected concentrations of total coliforms up to 1000 CFU/100 mL and fecal coliforms up to 100 CFU/100 mL from flat roofs. Gikas and Tsihrintzis reported values in rainwater storage tanks of 400 CFU/100 mL [8]. Lee and co-authors measured values in the range of 70 to 197 CFU/100 mL without first-flush removal on roofs with different coatings, which, however, decreased to 1 ÷ 12 CFU/100 mL when removing the first, most polluted, runoff [38]. Morgado and co-authors report that after the removal of the first flush, no Salmonella spp. or L. monocytogenes isolates were detected in any of their analyzed water samples [46]. Furthermore, the average E. coli levels from all harvested rainwater samples were in the low range of 1.2 to 24.4 CFU/100 mL. The timely separation of the first flush and the prevention of HRW retention in the storage tank for long periods of time could significantly reduce the risk of intense coliform bacteria development and their subsequent wide spreading [47]. In a study of ten rainwater harvesting systems (RWHS) without first-flush diverters (part of a pilot study in Kleinmond, South Africa), higher values of TCF (above 10,000 CFU/100 mL) and FCF (above 50,000 CFU/100 mL) were observed [48].
In a study by Mendez and co-authors, despite the better performance on average in terms of bacterial contamination of green roofs, high concentrations of bacteria were detected in single events, indicating that the contamination of catchment surfaces is likely at any time [7].

3.2. Legislative Framework and Guidance Documents

  • Legislation at EU level
HRW has been widely used for different purposes such as irrigation, toilet flush, cooling, etc. However, the standards regulating the quality of the harvested rainwater, tailored to its “final destination”, have not been set up at the EU level yet. The literature review showed that the EU countries regulate at national level some quality parameters but mostly when collected rainwater is used for irrigation purposes.
The standard EN16941-1:2018 “On-site non-potable water systems” [49] defines the minimum requirements for rainwater collection and use of rain water on site as non-potable water. This excludes the use for drinking water and for food preparation; the use for personal hygiene; and infiltration. However, the standard does not provide answers to a number of emerging issues. For example, the standard does not indicate all the risks associated with the collection and use of rainwater. A lot of the design decisions are left to the designers. First-flush separation is not considered even though it has a massive effect on the quality. Bacteriological contamination is another issue that is not well represented in relation to the use. The standard provides a very general section concerning water quality. It only shows the corresponding link between roof surface and the potential effect like Green roof—Coloration; Bitumen containing material—Coloration; Cement with fibers—Emission of fibers in the long term; Copper, lead or zinc roofs—Increased concentrations of heavy metals; Weathered rough surfaces—Wash out of solids. However, numeric values for the qualitative parameters are not provided [49].
  • Legislation in the USA and Australia
In the USA and Australia, there are no state regulations on harvested rainwater reuse [50,51]. Different states/municipalities apply different approaches and in some RWH is even prohibited [50]. However, similar to the EU, there are a number of guidance documents that presents practices on non-potable water reuse, including rainwater reuse [50].
  • United Nations (Food and Agriculture Organization) (FAO)
The FAO published a guideline for irrigated agriculture that considers three degrees of restrictions for water used for irrigation—None, Slight to Moderate, and Severe [52]. The categories are based on long-term application and are aimed to be used as general evaluation. None restriction means that no soil or cropping problems are expected. The list of parameters includes EC, TDS, Sodium, Chloride, Boron Nitrogen Bicarbonate, and pH. Furthermore the specific values of some of the parameters are linked to the type of irrigation method—sprinkler or surface and are available in Water quality for agriculture [52].
  • Country-specific international examples
Standards for water quality used for drinking or irrigation needs are set in most countries. These standards usually do not distinguish between the water sources (natural water, harvested rainwater, reclaimed wastewater, etc.).
The national standards for water used for drinking purposes are similar, and they comply with the guidelines of the WHO and the EU Directive 2020/2184 (i.e., the Drinking directive) for the EU countries [53,54]. Because of this similarity, the values of these standards are not included in the paper.
The national standards for water used for irrigation purposes have some specificities. Examples from four countries are included in this paper (Table 5).
In the USA, the EPA (2004) sets different standards for water quality for irrigation depending on the type of application: for long-term and short-term applications. Values for long-term use are conservatively determined concerning the sensitivity of different soils and crops. These maxima are below concentrations that result in toxicity, but the long-term application may lead to accumulation and, as a result, cause phytotoxicity. For example, some of the indicators are tailored to sandy soils, which have a small capacity to retain the specified elements. A value of 0.2 mg/L has been set for nickel, but for many crops, the toxicity level is in the range of 0.5 to 1 mg/L. Other indicators are consistent with the maximum yield, and for others, the toxicity value also depends on the pH level. For these reasons, the proposed values rather define universally good water for application in the long-term irrigation of crops. Short-term use (up to 20 years) defines irrigation water with a wider range of quality indicators with threshold values corresponding to finely textured neutral and alkaline soils with a high capacity to remove various pollutants [55].
In Bulgaria, Ordinance No. 18 sets two lists of control indicators—a full list and a short list [32]. The short list is applied in cases where, for the previous year, there was no exceedance of the maximum permissible values according to the full list, and no reason appeared to lead to their exceedance. The frequency of sampling is not less than once per irrigation season. The list of the Bulgarian water quality indicators is the longest one among the presented four countries in Table 5.
The Hungarian values presented in Table 5 refer to the reclaimed wastewater used for irrigation purposes. It can be seen that these values do not differ considerably from the values in the other countries, which refer to any water source.
In Romania, there are two types of restriction, one for high irrigation rates usable in arid areas and fine soil texture, and the second one for reduced irrigation norms usable in wet areas and fine soil texture.
It might be concluded that regardless of the national specificities, the values of the indicators are in general close to each other.

4. Discussion

4.1. Complexity of the Factors Influencing HRWQ

Many factors of different natures can play a significant role in the quality of the HRW [56]. Below, conclusions of different research concerning the influence of climate, and rainwater runoff characteristics (e.g., intensity, frequency, and the number of consecutive dry days), the level of urbanization, the roof material, and overhanging vegetation are discussed. Each of these factors has a different effect, which has to be taken into consideration before using the HRW. Furthermore, these factors can even play a role in the selection of the elements of the rainwater harvesting system.
  • Frequency and intensity of rainfall
Weather conditions affect the quality of rainwater and runoff from roofs. One of the most important climatic factors is the frequency of rainfall. On days without precipitation, pollutants accumulate on roof surfaces, which are washed away with the first rainfall. Lee and co-authors reported a high correlation between the number of dry days and the content of suspended solids [38], while Zhang and co-authors observed a significant pollutant concentration increase in the first 10 dry consecutive days, and then, the effect of longer dry spells did not lead to a significant increase [18].
  • Air pollution
Zhang and co-authors report that air pollution has a direct effect on rain runoff. Higher concentrations of dust pollution also lead to higher values of turbidity, ammonium nitrogen (NH4+-N), total nitrogen, total phosphorus, SS, and COD [19]. However, the amount of rainfall also plays a role. Intense rainfall events result in lower runoff concentrations and cleaner air. A similar effect regarding the dissolved organic matter was also reported by Li and co-authors [57].
  • Roof type and slope
The roof type has a significant impact on the quality of rain runoff. A study by Lee co-authors comparing four types of roof materials (galvanized steel, wooden shingle tiles, concrete tiles, and clay tiles) indicated that galvanized steel metal roofs were the most suitable for collecting and using rainwater [38]. No coliforms were found with them. One of the main reasons for the good results is the ultraviolet sun rays and the high temperature of the roof, which has a bactericidal effect. In comparing five types of roofing (asphalt fiberglass shingle, Galvalume metal, concrete tile, cool and green), Mendez and co-authors obtained similar results, i.e., lowest bacterial contamination of the runoff from metal roofs [7]. The same was reported by Zdeb and co-authors (2020) comparing galvanized steel, concrete tile, ceramic tile, and epoxy resin floors [14,25]. Bae and co-authors obtained no statistical differences when comparing five roof types (concrete tile, cool, green, Galvalume metal, and asphalt fiberglass shingle), but they also obtained the lowest values for metal roofs [37]. The effect of solar radiation in the latter two studies was reduced because the metal roof was north facing, but the lower concentrations were also reported as a consequence of the higher roof temperature.
Despite the good bacteriological effect, metal roofs can lead to an increased release of metals into rain runoff depending on the type of roof and its coating [9,14,17]. However, on the other hand, they are very suitable for collecting rainwater due to the low concentrations of bacterial contamination.
Concrete roofs, tiles, asbestos cement, and stone do not stand out in the studies reviewed. Since all pavements are porous, this creates conditions for bacteriological contamination of the runoff. Concrete pavements have a beneficial effect on rain runoff as they slightly increase pH [7,15,38].
In the case of green roofs, conflicting data are obtained in various studies. While in some, they help reduce pollutant concentrations, in other studies, they lead to increased concentrations in the runoff. For example, Todorov and co-authors reported no change in rain runoff parameters over a 4-year period from a green roof situated in Syracuse, NY [58], while Harper and co-authors found initial high concentrations of nitrogen and phosphorus, which significantly decreased at the end of the nine-month study of a green roof in Missouri. Undoubtedly, part of the reason lies in the use of a different base (substrate) for the plants, its different composition and thickness, as well as the plants themselves [59].
Green roofs show an increase in the electrical conductivity of runoff relative to rain water [7,34]. An increase in pH was also observed in many studies [34,59,60,61,62]. However, when rain water is in the alkaline spectrum, the green roof might lower the pH [34].
In some studies, green roofs have lower levels of fecal coliform contamination than concrete, cool roof, galvanized, and asphalt shingles [7].
Roof type also responds differently to the number of dry days. For example, on a sloping roof (30°) with ceramic tiles, the number of dry days led to an increase in the concentrations of NH4+-N, Ca2+, Mg2+, and total phosphorus, while on a flat concrete roof (1% slope), they decreased as a result of the easier removal of pollutants by wind [8].
Roof slope may also play a role, as reported by Gikas and Tsihrintzis [8] and Farreny and co-authors, but in both studies, in addition to the roof slope, the roof materials are different [6].
In the study of Lai and co-authors (2018) based on a comparison of four roof materials (metal, plastic, clay tile, and reinforced concrete) and four slopes (0°, 15°, 30°, and 45°), the best runoff quality was noticed for clay tile roof (45°) [15].
The roof age is another factor influencing the rainwater quality. Gikas and Tsihrintzis reported an increase in pH from a concrete roof as a result of roof age [8]. However, Mendez and co-authors also observed higher values for concrete roofs compared to the other roof material studied, even with new roofs [7].
  • First-flush removal
Separation of the first runoff looks like a mandatory condition for any modern rainwater collection and use system. Precipitation strips pollutants from the air and washes those onto the roof, and in the first 10–20 min after runoff formation, a rapid drop in pollutant concentrations is observed [18].
While in rainwater storage tanks from two types of roofs (ceramic tiles and concrete) no significant difference was observed in turbidity, electrical conductivity, alkalinity, NH4+-N, total nitrogen, SO42−, Mg2+, Ca2+, Na+, and K+, in the collected first runoff, the concentrations were significantly higher [8]. A similar effect has been observed for other indicators—total coliforms, dissolved substances, nitrites, and nitrates [7]. Proximity to urbanized areas, overhanging vegetation, and roof age did not result in a difference in quality indicators after the removal of the first runoff [7].
In a two-month study of runoff from seven concrete and sheet metal roofs conducted in Dhaka, one of the most polluted cities in the world, after the removal of the first runoff (15 min after its formation), a number of parameters met local drinking water standards [10]. Fully compliant (total seven covers/sites) are turbidity, conductivity, solutes, nitrites, sulfates (SO42−), and chlorides. Observed deviations from the requirements for some sampling points were observed for pH (4 with a min value of 6.37), nitrates (NO3−) (2 with a max value of 14.42 mg/L), and fluoride (3 with a max value of 2.27 mg/L).
All studies show that the separation of the first runoff has a major influence on the composition of rain runoff and should not be underestimated.
  • Other factors
As pointed out above, there are many factors affecting rainwater quality, and some of them act in parallel. It is worth mentioning that HRWQ depends also on the storing facility (material and retention time) as well as ambient storing temperature.
Zhang and co-authors evaluated the impact of the ambient temperature (4 °C, 20 °C, and 30 °C) and hydraulic retention time in the storage facility (up to 60 days) [47]. They conclude that high ambient temperature might be the reason for regrowth in total bacteria. Acanthamoeba spp. was found in all samples when the retention time was above 50 days regardless of the temperature. This study indicates that prolonged storage can pose additional risks. Zdeb and co-authors evaluated the temperature effect (20 °C and 37 °C) in different seasons and obtained opposite results. The total number of bacteria was reduced with prolonged storage time. They concluded that in order to improve microbiological parameters, at least six weeks of storage with no fresh water added is necessary. They also point out that different systems can behave differently [25].

4.2. Forward Actions

The screening and the analyses presented in this paper show that the quality of the HRW from building roofs varies in a wide range and depends on a number of factors that interact in a complex way and in most cases their effect could not be easily quantified.
The role of urban wastewater runoff, however, as a key component of the integrated urban wastewater management plans is emphasized in the new proposal for the revision of the EU urban wastewater treatments directive [5]. It is expected that in the near future, the urban runoff, expressed as quantity and pollution load, will be subject to in-depth analyses, as the decrease in its share in the overall wastewater balance at the settlement level will be a challenging task. The RWH is specifically indicated as a preventive measure aiming at avoiding the entry of rain water into the sewer collecting systems.
The data collected from the different studies on the quality of rain runoff vary greatly, which is a result of the many factors affecting the quality of harvested rainwater. This indicates a need for an introduction of a necessary execution of specific studies for each specific case into the legislation requirements.
Based on the considerations above, it appears necessary that HRWQ legislative requirements, tailored to the reused purpose, should be established to prevent possible health and safety risks.
Some possible actions and considerations toward the establishment of the needed standards could be the following.

4.2.1. Application of a Risk-Based Approach

The HRWQ standards should be linked to the risks that arise with the final purpose of the application. Rainwater reuse for non-potable needs (e.g., irrigation, toilet flushing) seems to be a predominant option.
  • Considerations for irrigation
When irrigation of large farmlands is the ultimate goal, the legislative associated risks are mostly related to soil health and plant health. Thus maximum permissible concentrations have been determined for some pollutants in some national requirements. The national requirements could be a good basis for developing EU regulations or at least recommendable values and approaches, especially if water is used for crop production.
However, the values could differ when the focus is irrigation on relatively small yards in settlements. The way of irrigation and the potential risks for the people nearby also need to be taken into account, because in collected rainwater, the presence of pathogenic microorganisms, and various viruses, including Legionella, is possible [37,39,47,48,51]. Their dispersion in the air, e.g., through sprinklers may pose a risk of infection.
  • Considerations for potable water use
When the ultimate goal is drinking use, then the water should meet the requirements of drinking water standards. Some of the studies indicated that HRW met some drinking water standards [38]. However, in most of the studies, it is reported that additional treatment (such as disinfection) is required [7,10,15,39,45,47].
Another challenge is the lack of reliable database in regard to the diseases due to untreated rain water. Since the number of affected people is usually small, the current epidemiological methods are not sufficient [51,63].
  • Considerations for non-potable domestic water use
When the ultimate purpose of rainwater use is non-drinking domestic water use, to avoid risk to human health, there are currently no requirements related to specific water quality parameters and specific (limiting) concentrations. However, some guidelines or good practices are developed [50,51]. This is also a prompt for applying a common EU approach and setting standards.

4.2.2. Development of a Targeted Database

Many research studies have been carried out to answer the need for a better understanding of all aspects of harvested water reuse. The extensive data set, however, is not collected and is not easily assessable. Collecting the existing knowledge in one platform, with well-structured data organization and wide access would probably facilitate to a great extent different types of analyses, which would lead to the setting of quality standards in dependence of the final reuse destination.
The requirements of the proposed revision of the urban wastewater treatment directive, related to the monitoring of storm water discharges, will contribute to a great extent to the collection of a large database in this regard.

4.2.3. Development of Guidance with Technological Solutions

The standard EN16941-1:2018 “On-site non-potable water systems” lays down a good basis for the design of rainwater harvesting systems. However, as discussed above, a number of issues should be more thoroughly considered. For example, the diversion of the first runoff from the collecting reservoir might be considered a mandatory element of any rainwater collection and use system, as it mitigates the effect of factors such as air pollution, overhanging vegetation, age of the roof, etc. The absence of this element can make the collected water unsuitable for direct reuse. Proper storage of water and maintenance of the storage facilities as well as if necessary, regular testing on some main water quality parameters should be addressed. Good practices for the safe application of HRW such as drip irrigation can also be included.
The development of guidance with technological solutions appears to be a helpful tool to ensure safe systems for HRW (collection, treatment, and supply) reuse.

5. Conclusions

RWH is consistent with the EU policies for reducing pressure on water bodies, mitigating water scarcity, and reducing flood risk.
Harvested rainwater quality appears generally appropriate for non-potable use, especially if removal of the first flush is applied. The first-flush removal can neutralize factors such as proximity to urbanized areas, overhanging vegetation, and roof age. In some cases, this single step is even sufficient to reduce specific quality parameters to meet the local drinking water standard values. However, removal of the first flush is not well defined, and different approaches are reported, such as time from the start or mm of rain. Not only the first-flush removal but also the rainwater harvesting systems in general may significantly affect the quality of collected water in a positive or negative aspect. Low temperatures of storage reduce the bacteriological regrowth, but high temperatures might result in increased bacteriological regrowth. In addition, short time storage and quick application (i.e., small to medium size tanks) might turn out to be a preferred option over large tanks due to the threat of bacteriological regrowth.
However, HRWQ varies greatly, and many factors such as rain intensity, roof material, and climate can have a significant effect on it. The reported wide range of values for the major qualitative parameters from different sites around the world shows the need of setting legislative requirements to reduce the specific risks associated with different reuse purposes of the harvested rainwater. Further actions such as the application of a risk-based approach, the development of a world database, and guidance with technological solutions appear helpful in this process.

Author Contributions

Conceptualization, E.T. and I.R.; methodology, E.T. and I.R.; validation, E.T., D.V. and G.D.; investigation, E.T., D.V. and G.D.; data curation, E.T., D.V. and G.D.; writing—original draft preparation, E.T., D.V. and G.D.; writing—review and editing, E.T., D.V., I.R. and G.D.; supervision, I.R.; project administration, E.T.; funding acquisition, E.T. All authors have read and agreed to the published version of the manuscript.

Funding

The current research development under contract БH-252/21 is financially supported by the Center for Scientific Research and Design at the UASG.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 11 00665 g001 550
Figure 1. The research approach.
Figure 1. The research approach.
Processes 11 00665 g001
Table
Table 1. Physical indicators.
Table 1. Physical indicators.
ParameterUnitMin ValueMax ValueReferences
ConductivityµS/cm8.3615[6,7,8,9,10,11,12,13,14,15,16]
TurbidityNTU040[6,11,12,13,14]
NTU-110[7], first flush
Suspended solidsmg/L 730[7], pilot scale
mg/L1050[7], full scale
Table
Table 2. Chemical indicators.
Table 2. Chemical indicators.
ParameterUnitMin ValueMax ValueReferences
pH-3.6011.40[6,11,14,15,16,17,19,20,21,22,23,24,25,26]
Nitrate Nitrogen (NO3−-N)mgN/L0.019.34[6,15,25]
Ammonium Nitrogen (NH4+-N)mgN/L0.044.00[6,17]
Orthophosphate (PO43−)mg PO43−/L06.60[6,8,25]
CODmgO/L135[27]
Table
Table 3. Metal content in rainwater runoff after removal of the first flush.
Table 3. Metal content in rainwater runoff after removal of the first flush.
ElementUnitMin ValueMax ValueReferences
Aluminum (Al)µg/L63490[15,36]
Zinc (Zn)µg/L10>3000[12,14,15]
Copper (Cu)µg/L40120.7[14,36]
Lead (Pb)µg/L0.22534.5[14,36,37]
Chromium (Cr)µg/L0.0219.2[14,36,37]
Arsenic (As)µg/L0.0083[14,36,37]
Cadmium (Cd)µg/L0.0193[36,37]
Manganese (Mn)µg/L15>3000[14,36]
Mercury (Hg)µg/L0.040.916[37]
Nickel (Ni)µg/L0.1660.827[14,37]
Selenium (Se)µg/L0.00440.32[37]
Table
Table 4. Biological indicators.
Table 4. Biological indicators.
ParameterUnitMin ValueMax ValueReferences
Total Coli FormsCFU/100 mL01000[7,14,19,21,24,25,37,38,39]
Fecal Coli FormsCFU/100 mL-100[14,24,25,37]
Table
Table 5. Quality indicators of water for irrigation of crops.
Table 5. Quality indicators of water for irrigation of crops.
IndicatorsUnitUSA (EPA)BulgariaHungaryRomania
- Long-term useShort-term use High levels of irrigationLow levels of irrigation
A. Salinization
Conductivity (Ec) *µS/cm--2000
B. Permeability
Sodium (Na) *mg/L--300
Calcium (Ca) *mg/L--400
Magnesium (Mg) *mg/L--300
Potassium (K) *mg/L--350
C. Toxicity
Boron (B)mg/L0.7521.00.70.752
Chlorine ions (Cl) *mg/L--300150
Manganese (Mn) *mg/L0.2100.250.23
Iron (Fe) *mg/L5205.02015
Copper (Cu)mg/L0.250.2 0.25
Cobalt (Co)mg/L0.0550.050,050.055
Zinc (Zn)mg/L2102.05210
Molybdenum (Mo)mg/L0.010.050.010.020.010.05
Lead (Pb) *mg/L5100.05125
Mercury (Hg) *mg/L--0.0010.010.020.05
Aluminum (Al)mg/L5205.010520
Beryllium (Be)mg/L0.10.50.01 0.10.5
Nickel (Ni)mg/L0.220.210.22
Vanadium (V)mg/L0.110.1 0.11
Cadmium (Cd) *mg/L0.010.050.010.02
Selenium (Se)mg/L0.020.020.01 0.020.05
Arsenic (As)mg/L0.120.10.20.12
Chromium-hexavalent (Cr(+6))mg/L--0.050.50.11
Chromium-triad (Cr(+3)) --0.5
Fluoride (F)mg/L1151.0 15
Lithium (Li)mg/L2.52.52.5 2.52.5
D. Sanitary and hygienic indicators
Total coliformscm3--<0.1
E. colicm3--<1.0
Fecal coliformscm3--Not allowed
E. Other indicators
Ammonium nitrogen (N-NH4+) *mg/L--5
Nitrogen nitrate (N-NO3−) *mg/L--20
Carbonates (CO32−) *mg/L--200
Hydrocarbons (HCO3−) *mg/L--300
Sulfates (SO42−) *mg/L--300
Phosphates (PO43−) *mg/L--3
pH *-recommended value 66–9
Phenols (volatile)mg/L--0.05
Cyanides (CN)—totalmg/L--0.5 0.20.2
Petroleum productsmg/L--0.3
Detergentsmg/L--1.0
CODmg/L--100
BOD5mg/L--25
Extractables with tetrachloromethanemg/L--5.0
Temperature *°C--28
Dissolved oxygenmg/L-->2.0
Total hardness *mgeqv/L--14
Undissolved substances *mg/L--50
F. Radioactivity
Radium 226 (Ra 226)mBq/L--150
Total beta radioactivitymBq/L--750
TDSmg/L500–2000 -
Bariummg/L---4
Chromiummg/L0.11-2.5
Free Chlorine Residualmg/L<1 -
* Indicator from the full and shortlist.
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Tsanov, E.; Valchev, D.; Ribarova, I.; Dimova, G. Discussion on the Need for Harvested Rainwater Quality Standards Tailored to the Reuse Purpose. Processes 2023, 11, 665. https://doi.org/10.3390/pr11030665

AMA Style

Tsanov E, Valchev D, Ribarova I, Dimova G. Discussion on the Need for Harvested Rainwater Quality Standards Tailored to the Reuse Purpose. Processes. 2023; 11(3):665. https://doi.org/10.3390/pr11030665

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Tsanov, Emil, Dobril Valchev, Irina Ribarova, and Galina Dimova. 2023. "Discussion on the Need for Harvested Rainwater Quality Standards Tailored to the Reuse Purpose" Processes 11, no. 3: 665. https://doi.org/10.3390/pr11030665

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