RAINWATER HARVESTING VERSION 1.2

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1 DRAFT VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 6 RAINWATER HARVESTING VERSION 1.2 Note to Reviewers of the Stormwater Design Specifications The Virginia Department of Conservation and Recreation (DCR) has developed an updated set of nonproprietary BMP standards and specifications for use in complying with the Virginia Stormwater Management Law and Regulations. These updated standards and specifications were developed with significant assistance from Tom Schueler of the Chesapeake Stormwater Network (CSN), staff of the Center for Watershed Protection (CWP), the Northern Virginia Regional Commission (NVRC), and the Engineers and Surveyors Institute (ESI) of Northern Virginia. These standards are based on both the more traditional BMPs and the newer LID practices. The advancements in these standards and specifications are a result of extensive reviews of BMP research studies incorporated into the CWP's National Pollution Removal Performance Database (NPRPD). In addition, we have borrowed from BMP standards and specifications from other states and research universities in the region. Table 1 describes the overall organization and status of the proposed design specifications under development by DCR. # Practice Notes 1 Rooftop (Impervious Area) Disconnection Includes front-yard bioretention 2 Sheetflow to Conserv. Area or Veg. Filter Strip 3 Grass Channels 4 Soil Compost Amendments 5 Vegetated Roofs 6 Rainwater Harvesting Includes cisterns 7 Permeable Pavement 8 Infiltration Includes micro- small scale and conventional infiltration techniques 9 Bioretention Includes urban bioretention 10 Dry Swales 11 Wet Swales 12 Filtering Practices 13 Constructed Wetlands 14 Wet Ponds 15 Extended Detention Ponds Reviewers should be aware that these draft standards and specifications are just the beginning of the process. Over the coming months, they will be extensively peer-reviewed to develop standards and specifications that can boost performance, increase longevity, reduce the maintenance burden, create attractive amenities, and drive down the unit cost of the treatment provided. Timeline for review and adoption of specifications and Role of the Virginia s Stormwater BMP Clearinghouse Committee: The DCR will be soliciting input and comment on each standard and specification until August 21, 2009, the conclusion of DCR s Public Comment Process for proposed revisions to the Virginia SWM Rainwater Harvesting 1 of

2 Regulations. Significant peer review has already been conducted on earlier drafts of these specifications, with comments received from the research, design and plan review community. The additional feedback will ensure that these design standards strike the right balance between prescription and flexibility, and that they work effectively in each physiographic region. The collective feedback will be presented to the BMP Clearinghouse Committee to help complement their review efforts. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR. The revisions to the Virginia Stormwater Management Regulations are not expected to become finalized until late 2009 and to take effect until July 1, The DCR intends that these stormwater BMP standards and specifications will be finalized by the time the regulations take effect. The Virginia Stormwater BMP Clearinghouse Committee will consider the feedback and recommend final versions of these BMP standards and specifications for approval by DCR, which is vested by the Code of Virginia with the authority to determine what practices are acceptable for use in Virginia to manage stormwater runoff. As with any draft, there are several key caveats, as outlined below: Many of the proposed design standards and specifications lack sufficient graphics. Graphics will be produced in the coming months, including CADD files and photo tutorials. The design graphics shown in this current version are meant to be illustrative. Where there are differences between the schematic and the text, the text should be considered the recommended approach. There are some inconsistencies in the material specifications for stone, pea gravel and filter fabric between ASTM, VDOT and the DCR 1999 SWM Handbook. These inconsistencies will be rectified in subsequent versions. While the DCR 1999 SWM Handbook was used as the initial foundation for these draft standards and specifications, additional side-by-side comparison will be conducted to ensure continuity and consistency among the specifications regarding their organization and formatting. Other inconsistencies may exist regarding the specified setbacks from buildings, roads, septic systems, water supply wells and public infrastructure. These setbacks can be extremely important, and local plan reviewers should provide input to ensure that they strike the appropriate balance between risk aversion and practice feasibility. DCR requests that you provide an copy of your comments, etc., to Scott Crafton at The final version will provide appropriate credit and attribution on the sources from which photos, schematics, figures, and text were derived. Thank you for your help in producing the best stormwater design specifications for the Commonwealth. Rainwater Harvesting 2 of

3 DRAFT VIRGINIA DCR STORMWATER DESIGN SPECIFICATION No. 6 RAINWATER HARVESTING VERSION 1.2 SECTION 1: DESCRIPTION Rainwater harvesting systems intercept, divert, store and release rainfall for future use. The term rainwater harvesting is used in this specification, but it is also known as a cistern or rainwater harvesting system. Rainwater that falls on a rooftop is collected and conveyed into an above or below ground storage tank where it can be used for non-potable water uses and on-site stormwater disposal/infiltration. Non-potable uses may include, flushing of toilets and urinals inside buildings, landscape irrigation, exterior washing (e.g. car washes, building facades, sidewalks, street sweepers, fire trucks, etc.), supply for chilled water cooling towers, replenishing and operation of water features and water fountains, and laundry, if approved by the local authority. Replenishing of pools may be acceptable if special measures are taken, discussed in section 6, and approved by the appropriate regulatory authority. While non-potable uses of captured rainwater are most common, in some limited cases where potable water supply is insufficient, rainwater can be treated to potable standards, so long as treatment methods and end use quality all meet drinking water standards and regulations and is accepted by the local governing authority. Treating to potable standards may drive up installation and maintenance costs significantly. Rainwater Harvesting 3 of

4 Typically, pre-fabricated rainwater harvesting cisterns range in size from 250 to over 30,000 gallons. The capture and re-use of rainwater can significantly reduce stormwater runoff volumes and pollutant loads. By providing a reliable and renewable source of water to end users, rainwater harvesting systems can also have environmental and economic benefits beyond stormwater management (increased water conservation, water supply during drought and mandatory municipal water supply restrictions, decreased demand on municipal or groundwater supply, decreased water costs for the end-user, potential for increased groundwater recharge, etc). Rainwater harvesting systems can be combined with other rooftop disconnection on-site practices, such as micro-infiltration practices (Spec No. 8), rain gardens and foundation planters (Spec No. 9), to enhance their runoff reduction and nutrient removal capability. 1.1 Tank Configurations There are three basic tank design configurations to meet the various system layouts that are described in the Design System Configurations -- Section 1.2. Figure 1.1. Tank Configuration 1, Storage Associated with Treatment Volume only The first tank (Figure 1.1) maximizes the available storage volume associated with the Treatment Volume to meet the desired level of Treatment Credit. This layout also maximizes the storage that can be used to meet a demand. An emergency overflow exists near the top of the tank as the only gravity release outlet device (not including the pump, manway or inlets). It should be noted that it is possible to address Channel Protection and Flood volumes with this tank configuration, but the primary purpose is to address the water quality Treatment Volume (Tv). Rainwater Harvesting 4 of

5 Figure 1.2. Tank Configuration 2, Storage Associated with Treatment, Channel Protection and Flood Volume The second tank configuration (Figure 1.2) utilizes tank storage to meet Treatment Volume (T v ) objectives as well as utilizing an additional detention volume above the treatment volume space to also meet some or all of the Channel Protection and Flood Volume requirements. An orifice outlet is provided at the top of the design storage for T v and an emergency overflow at the top of the detention volume. This specification only addresses the storage for T v, however the Runoff Reduction spreadsheet in combination with other approved hydrologic routing programs may be used to model and size the Channel Protection and Flood (detention) volumes. Figure 1.3. Tank Configuration 3, Constant drawdown, Storage Associated with Treatment, Channel Protection and Flood Volume Rainwater Harvesting 5 of

6 The third tank (Figure 1.3) creates a constant drawdown within the system. The small orifice at the bottom of the tank needs to be routed to an appropriately designed on-site secondary practice (e.g., rain garden, micro-scale infiltration, urban bioretention) that will allow the rainwater to be treated and allow for groundwater recharge over time. The release should not be discharged to a receiving channel or storm drain without treatment, and maximum specified drawdown rates from this constant drawdown should be adhered to, as the primary function of the system is not for it to act as a detention facility. For the purposes of this tank configuration, the secondary practice shall be considered a component of the rainwater harvesting system with regard to the runoff reduction percentage received in the Runoff Reduction Spreadsheet. In other words, the runoff reduction associated with the secondary practice shall not be added (or double-counted) to the rainwater harvesting percentage. The reason for this is that the secondary practice is an integral part of a rainwater harvesting system with a constant drawdown. The exception to this would be if the secondary practice were also sized to capture and treat impervious and/or turf area beyond the area treated by rainwater harvesting (for instance, the adjacent yard or a driveway). In this case, only these additional areas should be added into the Runoff Reduction Spreadsheet to receive credit for the secondary practice. While a small orifice is shown at the bottom of the tank in Figure 1.3, the orifice could be replaced with a pump that would serve the same purpose of conveying a limited amount of water to a secondary practice on a routine basis. 1.2 Design System Configurations Many system variations can be conceived of to meet user demand and stormwater objectives. This specification focuses on providing a design framework to address the water quality treatment volume (T v ) credit objectives for assessing stormwater compliance criteria. From a stormwater treatment volume standpoint, there are six basic rainwater harvesting system configurations that are presented in this specification. Other configurations may be conceived of and may be acceptable, if approved by the local regulatory authority. However, several key considerations and concepts should be adhered to in order to properly meet the design intent and purpose: Dedicated Year-round drawdown for credit. While seasonal practices (such as irrigation) may be incorporated into the site design, they shall not contribute to the treatment volume credit for stormwater purposes, unless a drawdown at an equal or greater rate is also realized during non-seasonal periods (e.g. during non-irrigation months). Utilization of rainwater as a resource to re-use for on-site demand or design for infiltration to promote groundwater recharge. Nutrient and pollutant reduction from site is realized through reduction of runoff volume leaving the site. Rainwater Harvesting 6 of

7 Reduction in peak flows is realized due to reduced volume of runoff. The design system configurations include rainwater harvesting systems that provide: 1. Interior non-potable water. 2. Interior non-potable water and seasonal outdoor irrigation, but lack On-site Infiltration. 3. Interior non-potable water to meet a constant internal water demand throughout the year, provide seasonal outdoor water to an automated irrigation system, and provide on-site stormwater infiltration/disposal during times that irrigation is not utilized in an approved practice. 4. Interior non-potable water supplemented by approved year-round on-site stormwater infiltration/disposal to augment water level drawdown within tank for greater treatment credit. 5. Rainwater harvesting systems with year-round or seasonal use with constant year-round use of a secondary treatment practice (tank configuration 3). 6. Approved year-round on-site infiltration and/or treatment in a secondary practice, but no reuse of water. This practice would be used to recharge groundwater supplies to the extent possible and to realize treatment credit (tank configuration 3). 1.3 On-Site Infiltration/Filtration in a Secondary Practice (Tank Configuration 3) Recent rainwater harvesting system design materials do not include guidance for on-site stormwater infiltration or disposal. The basic approach is to provide a dedicated secondary runoff reduction practice or infiltration area on-site that will ensure water within the tank will gradually drawdown at a specified design rate between storm events. Secondary runoff reduction practices may include: Rooftop Disconnection: Design Specification No. 1; excluding rain tanks and cisterns. This may include release to a compost-amended filter path Front yard bioretention area: Design Specification No.1 Grass filter strip: Design Specification No. 2 Micro-bioretention (rain garden): Design Specification No. 9 Micro-infiltration: Design Specification No. 8 Grass channel: Design Specification No. 3 Dry swale: Design Specification No. 10 Storage and release in foundation planter (Spec No. 9, Appendix A) Underground infiltration soakaway pit. The secondary practice approach is useful when demand is not enough to sufficiently draw water levels in the tank down between storm events to achieve the desired treatment credit. Of course, Rainwater Harvesting 7 of

8 if demand for the harvested rainwater is relatively high, then a secondary practice may not be needed or desired. While design specifications are available for most of the secondary practices proposed, an Underground infiltration soakaway pit may prove useful in some situations and may be used in conditions where soil has moderate to high infiltration rates. The soakaway pit must be properly designed to adequately infiltrate the controlled design release rate. Design approval is subject to the reviewing authority. Use of a secondary practice may be particularly useful to employ in sites that utilize captured rainwater for irrigation during part of the year, but have no other use for the water during nonirrigation season months. During non-irrigation months, credit cannot be realized unless on-site infiltration/treatment or another drawdown mechanism creates a year-round drawdown, as no stormwater benefit would be realized during non-seasonal periods. Soil types, ground surface areas, release rates, methods of conveyance (gravity fed or pumped), time periods of operation and inverts should all be accounted and designed for to determine the disposal rate and sizing of the receiving mechanism (both storage volume and surface area). On-Site Infiltration Using Below Ground Cistern Because the practice will often involve winter operation, the effect of operation during freezing periods must be considered and designed for. On-Site Infiltration Using Above Ground Cistern The practice may be used for an above ground cistern as well, however special caution and design considerations must be designed for. These include insulating exterior pipes used to convey water, ensuring minimum volume levels are maintained to prevent freezing of water within the tank, and other factors. 1.4 Summary of Treatment Volume Credit (T v ) The overall stormwater functions of the rainwater harvesting systems are described in Table 1. Illustrations of the six rainwater harvesting system configurations at a typical residential site are shown in Figures 1.4 through 1.9. Table 1: Stormwater Functions of Rainwater harvesting systems Annual Runoff Reduction Rate Variable 2 Total Phosphorus Removal 1 0% Total Nitrogen Removal 1 0% Channel Protection Partial. Reduced Curve Numbers and Increased Time of Concentration Rainwater Harvesting 8 of

9 Flood Mitigation Partial. Reduced Curve Numbers and Increased Time of Concentration 1. Nutrient mass removal is equal to runoff reduction rate. Zero additional removal rate for rainwater harvesting system only. Higher removal rates may apply to SRPs 2. Credit is variable and determined using Cistern Design Spreadsheet. Credit up to 90% is possible if all water from storms of 1 or less is used through demand and tank is sized such that no overflow occurs. Total credit not to exceed 90%. 1.5 Configurations Layout The six Design System Configurations described in section 1.2 are displayed graphically in the figures below: The first configuration supplies indoor re-use only on a year round basis. Figure 1.4. Configuration #1 - Interior Non-potable Demand year-round The second configuration utilizes both year round indoor use as well as seasonal irrigation. Because no on-site infiltration is incorporated into the design for non-irrigation months, the system must be modeled, for stormwater credit purposes, without seasonal drawdown. Rainwater Harvesting 9 of

10 Figure 1.5. Configuration #2 - Interior Non-potable Demand year-round, seasonal Irrigation, without On-site Infiltration during non-irrigation months The third configuration is the same as the second, however it utilizes an on-site secondary practice during non-irrigation months in order to yield a greater stormwater credit. In this case, the drawdown due to seasonal irrigation must be compared to the drawdown specified during non-irrigation months and the minimum modeled for stormwater credit purposes. Figure 1.6. Configuration #3 - Interior Non-potable Demand year-round, Seasonal Outdoor Demand, On-site Stormwater Disposal during non-irrigation months The fourth configuration uses indoor demand, and no irrigation. In this case, on-site infiltration is utilized in order to garner greater credit for stormwater management purposes or in order to Rainwater Harvesting 10 of

11 meet groundwater recharge goals. Because both practices are year round, they are both modeled to determine treatment credit. Figure 1.7. Configuration #4 - Interior Non-potable Demand year-round, On-site Stormwater Disposal Year-round The primary year round drawdown mechanism in the fifth configuration is the on-site stormwater infiltration. For stormwater credit purposes, the system must be modeled considering only on-site infiltration since it is the only year-round practice. When on-site infiltration is the primary drawdown practice, the storage associated with the treatment credit volume in gallons must be a minimum of 50% of the roof area in SF (e.g. a 5000 SF roof must dedicate a minimum of 2500 gallons to storage associated with Treatment Volume). Figure 1.8. Configuration #5 Year round or Seasonal Demand with Dedicated Constant Year-Round Drawdown with On-site Infiltration Rainwater Harvesting 11 of

12 The sixth configuration utilizes only groundwater recharge and does not re-use the rainwater to meet any on-site demand. The primary objective with this layout is to either meet the stormwater treatment credit requirement or to meet groundwater recharge objectives. When onsite infiltration is the primary drawdown practice, the storage associated with the treatment credit volume in gallons must be a minimum of 50% of the roof area in SF (e.g. a 5000 SF roof must dedicate a minimum of 2500 gallons to storage associated with Treatment Volume). Figure 1.9. Configuration #6 On-site Infiltration for Groundwater Recharge, No Re-use of Stored Water SECTION 2: PHYSICAL FEASIBILITY A number of site-specific features influence how rainwater harvesting systems are designed. These should not be considered comprehensive and conclusive considerations, but rather some recommendations that should be considered during the planning phases of incorporating rainwater harvesting systems into site design. Some of the key considerations include: Available Space: Adequate space is needed to house the tank and any overflow. Space limitations are rarely a concern with rainwater harvesting systems if they are considered during the initial building design and site layout of a residential or commercial development. Storage tanks can be placed underground, indoors, on rooftops in buildings that are structurally designed to hold the added weight, and adjacent to buildings. Designers can work with architects to creatively site the tanks. Underground utilities or other obstructions should always be identified prior to definitively identifying the location. Site Topography: Site topography and tank location should be considered as they relate to all of the inlet and outlet invert elevations in the rainwater harvesting system. The total elevation drop will be realized beginning from the downspout leaders to the final mechanism receiving gravity fed discharge and/or overflow from the cistern. Rainwater Harvesting 12 of

13 These elevation drops will occur along the sloping lengths of the underground roof drains from roof drain leader downspouts at the building all the way to the cistern. A vertical drop occurs within the filter before the cistern. The cistern itself must be located sufficiently below grade and below the frost line forcing an additional elevation drop. When the cistern is used for additional volume detention for channel protection and flood mitigation, an orifice may be included with a low invert specified by the design user. An overflow will always be present within the system with an associated invert. Both the orifice (if specified) and the overflow will drain the tank during large storms routing this water through an outlet pipe with varying lengths and slopes. All of these components of the rainwater harvesting system have an elevation drop associated with them. The final invert of the outlet pipe must match the invert of the receiving mechanism (natural channel, storm drain system, etc.) that receives this overflow. These elevation drops and associated inverts should be considered early in order to ensure that rainwater harvesting systems are feasible for a particular site. Site topography and tank location will also affect the amount of pumping needed. Locating storage tanks in low areas will make it easier to route roof drains from buildings to cisterns, however it will increase the amount of pumping needed to distribute the harvested rainwater back into the building or to irrigated areas situated on higher ground. Conversely, placing storage tanks at higher elevations may require larger diameter roof drains with smaller slopes, however it will also reduce the amount of pumping needed for distribution. In general, it is often best to locate the cistern close to the building, ensuring minimum roof drain slopes and cover over top of roof drain pipes is sufficient. Available Hydraulic Head: The required head depends on the intended use of the water. For residential landscaping uses, the cistern should be sited up-gradient of the landscaping areas or on a raised stand. Pumps are commonly used to convey stored rainwater to the end use in order to provide the required head. When the water is being routed from the cistern to the inside of a building for non-potable use, often a pump is used to feed a much smaller pressure tank inside the building which then serves the internal demands through gravity-fed head. Cisterns can also use gravity-fed operations for indoor residential uses, such as laundry, that do not require high water pressure. In cases where cisterns are located on top of building roofs in order to operate under gravity-fed conditions, the structure must be designed with the added weight of the rainwater harvesting system and water into consideration. Water Table: Belowground storage tanks are most appropriate in areas where the tank can be buried above the water table. They should be located in a manner that will not subject them to flooding. In areas where the tank is to be buried below the water table, special design considerations must be employed, such as incorporating appropriate provisions to sufficiently secure the tank, conducting buoyancy calculations when the tank is empty, and other measures. The tank must also be installed according to tank manufacturer s specifications. Rainwater Harvesting 13 of

14 Soils: Storage tanks should only be placed on native soils or on fill in accordance with the manufacturer's guidelines. Belowground tanks must be buried and weighted or secured appropriately to avoid uplift bouyancy. The ph of the soil should also be considered in relation to it s interaction with the cistern material. Proximity of Underground Utilities: All underground utilities must be taken into consideration during the design phase of underground rainwater harvesting systems, treating all of the rainwater harvesting system components and storm drains as typical stormwater facilities and pipes. The underground utilities must be marked and avoided during the installation of underground tanks and piping associated with the system. Appropriate minimum setbacks from septic drainfields, as specified by Virginia code should be observed. Contributing Drainage Area: The contributing drainage area to the cistern is the impervious area draining to the tank. In general, only rooftop surfaces should be used. Parking lots and other paved areas can be used in rare circumstances with appropriate treatment (oil/water separators) and approval of the locality. Areas of any size, including portions of roofs, can be used based on the sizing guidelines in this document. Contributing drainage areas should be routed directly from rooftops to rainwater harvesting systems in closed roof drain systems or storm drain pipes avoiding surface drainage, which could allow for a further potential contamination of the water. Rooftop Material: The quality of the harvested water will vary according to the roof material it serves. Water harvested from certain types of rooftops, such as asphalt sealcoats, tar and gravel, painted roofs or galvanized metal roofs, may leach trace metals and other toxic compounds and should, in general, be avoided unless new information determines that these materials are sufficient for the intended use and is allowable by Virginia code. Designers should also note that the ph of rainfall in Virginia tends to be acidic ranging from 4.5 to 5.0 which may leach metals from the roof surface or tank lining. Once rainfall leaves rooftop surfaces, ph levels tend to be slightly higher, ranging between 5.5 to 6.0. The 2009 Virginia Rainwater Harvesting Manual and other references listed at the end of this specification describe the advantages and disadvantages of different roofing materials. Hotspot Land Uses: Rainwater harvesting systems can be an effective method to avoid contamination of rooftop runoff with ground-level runoff from a stormwater hotspot operation. In some cases, however, industrial roof surfaces may also be designated as a stormwater hotspot. Setbacks from Buildings: Cistern overflow devices should be designed to avoid causing ponding or soil saturation within ten feet of building foundations. Storage tanks should be designed to be watertight to prevent water damage when placed near building foundations. In general, it is recommended that underground tanks be set 10 ft from the building foundation. Rainwater Harvesting 14 of

15 Vehicle Loading: Below-ground rainwater harvesting systems should be placed in areas without vehicle traffic, whenever possible or be designed for the live loads from heavy trucks, a requirement that may significantly increase construction costs. SECTION 3: RAINWATER HARVESTING SYSTEM DESIGN FOR POTABLE WATER APPLICATIONS In situations with insufficient potable water supply, rainwater can be treated and used for potable water supply subject to state and local health requirements (The Virginia Department of Health maintains regulations pertaining to reuse of water for potable uses). This rainwater harvesting system use is not covered in this specification, although there is growing interest in using harvested rainwater for potable drinking water. If this use is permitted by the appropriate public health authority, and the rainwater harvesting system is equipped with proper filtering equipment, the increased water re-use rate would reduce the demand on municipal water systems sharply. It would also enable more standard plumbing system, since potable and non-potable water would no longer need to be separated. SECTION 4: COMMUNITY AND ENVIRONMENTAL CONCERNS Although rainwater harvesting is an ancient practice, it is enjoying a revival due to the inherent quality of rainwater and the many beneficial uses that it can provide (TWDB, 2005). Some common concerns associated with rainwater harvesting that must be addressed during design include: Winter Operation: Rainwater harvesting systems can be used throughout the year if they are located underground or indoors to prevent problems associated with freezing, ice formation and subsequent system damage. Alternately, an outdoor system can be used seasonally, or year round if special measures and design considerations are incorporated. See Section 11.4 for further guidance on winter operation of rainwater harvesting systems. Local Plumbing Codes: Designer and plan reviewers should consult local building codes to determine if they explicitly allow the use of harvested rainwater for toilet and urinal flushing. Rainwater harvesting systems should be required to have backflow preventers or air gaps to keep harvested water separate from the main water supply, in the cases where a municipal backup supply is used. Pipes and spigots using rainwater must be clearly labeled as non-potable. Mosquitoes: In some situations, poorly designed rainwater harvesting systems can create habitat suitable for mosquito breeding and reproduction. Designers should provide screens on above- and below-ground tanks to prevent mosquitoes and other insects from entering the tanks. If screening is not sufficient in deterring mosquitoes, dunks or pellets containing larvicide can be added to cisterns when water is intended for landscaping use. Rainwater Harvesting 15 of

16 Child Safety: Above grade home rainwater harvesting systems cannot have unsecured openings large enough for children to enter the tank. For underground cisterns, manhole access should be secured to prevent unwanted access. SECTION 5: TYPICAL RAINWATER HARVESTING SYSTEM DETAILS There are six primary components of a stormwater rainwater harvesting system: 1. Rooftop Surface 2. Collection and Conveyance System (e.g. gutter and downspouts) 3. Pre-screening and First Flush Diverter 4. Storage Tank 5. Distribution System 6. Overflow, Filter Path or Secondary Runoff Reduction Practice 5.1 Rooftop Surface: The rooftop should be made of smooth, non-porous material with efficient drainage either from a sloped roof or an efficient roof drain system. Slow drainage of the roof leads to poor rinsing and a prolonged first flush and can decrease water quality. If the harvested rainwater will be used for potable uses, or uses with significant human exposure (e.g. pool filling, watering vegetable gardens), care should be taken in the choice of roof materials. Some materials may leach toxic chemicals making the water unsafe for humans. Rainwater can also be harvested from other impervious surfaces, such as parking lots and driveways, however this practice is much less common and will require more extensive pretreatment or treatment, as the quality of water is typically much lower. Rainwater Harvesting 16 of

17 Figure 5.1 Sample Rainwater harvesting system System Detail 5.2 Collection and Conveyance System The collection and conveyance system consists of the gutters, downspouts and pipes that channel stormwater runoff into storage tanks. Gutters and downspouts should be designed as they would for a building without a rainwater harvesting system. Aluminum, round bottom gutters and round downspouts are generally recommended for rainwater harvesting. Minimum slopes of gutters should be specified. Gutters should, at a minimum, be sized with slopes specified to contain the 1 storm for treatment credit volume. They should be designed to convey the 10 year storm, specifying size and minimum slope, if volume credit will be sought for channel protection. In all cases, gutters should be hung at a minimum of 0.5% for 2/3 rd of the length and at 1% for the remaining 1/3 rd of the length. Pipes (connecting downspouts to cistern tank) should be a minimum of 1.5% and sized/designed to convey the intended design storm. In some cases, a steeper slope and larger sizes may be recommended and/or necessary to convey the required runoff. Gutters and downspouts should be kept clean and free of debris and rust. Rainwater Harvesting 17 of

18 5.3 Pre-Treatment: Screening, First Flush Diverters and Filter Efficiencies Pre-filtration is required to keep sediment, leaves, contaminants and other debris from the system. Leaf screens and gutter guards meet the minimal requirement for pre-filtration of small systems, although direct water filtration is preferred. All pre-filtration devices should be low maintenance or maintenance-free. The purpose of pre-filtration is to significantly cut down on maintenance by preventing organic buildup in the tank, thereby decreasing microbial food sources. For larger tank systems, before rainwater enters the storage tank, the initial first flush diversion, must be diverted from the system. Designers should note that the term first flush in rainwater harvesting design is not the same term that had been historically used in the design of stormwater treatment practices. In this specification, the term first flush diversion is used to distinguish it from the traditional stormwater management first flush term, and can range anywhere from the first 0.02 to 0.06 inches of rooftop runoff. A variety of first flush diverters are described below. In addition to the initial first flush diversion, filters have an associated efficiency curve that estimates the percentage of rooftop runoff that will be conveyed through the filter to the storage tank. If filters are not sized properly, a large portion of the rooftop runoff may be diverted and not conveyed to the tank at all. For the 1 storm treatment volume, a minimum of 95% filter efficiency is required. This efficiency includes the first flush diversion. The Cistern Design Spreadsheet, discussed more in Section 6, assumes a filter efficiency rate of 95% for the 1 storm. The diverted flows (first flush diversion and overflow from filter) must be directed to an acceptable pervious flow path, that will not cause erosion during a 2 year storm, or to an appropriate BMP on the property, for infiltration. Preferably the diversion will be conveyed to the same secondary BMP in series practice that may be used for tank overflows. First Flush Diverters: First flush diverters direct the initial pulse of stormwater runoff away from the storage tank. While leaf screens effectively remove larger debris such as leaves, twigs and blooms from harvested rainwater, first flush diverters can be used to remove smaller contaminants such as dust, pollen and bird and rodent feces (Figure 5.2). Simple first flush diverters require active management, by draining the first flush water volume to a pervious area following each rainstorm. However, if water is to be used for indoor uses, this may be a preferred pre-treatment technique. Vortex filters may serve as an effective pre-tank filtration and first flush diverter. Leaf Screens: Leaf screens are mesh screens installed either in the gutter or downspout and are used to remove leaves and other large debris from rooftop runoff. Leaf screens must be regularly cleaned to be effective; if not maintained, they can become clogged and prevent rainwater from flowing into the storage tanks. Built-up debris can also harbor bacterial growth within gutters or downspouts (TWDB, 2005). Roof Washers: Roof washers are placed just ahead of storage tanks and are used to filter small debris from harvested rainwater (Figure 5.3). Roof washers consist of a tank, usually between 25 and 50 gallons in size, with leaf strainers and a filter with openings as Rainwater Harvesting 18 of

19 small as 30-microns (TWDB, 2005). The filter functions to remove very small particulate matter from harvested rainwater. All roof washers must be cleaned on a regular basis. Figure 5.2 First Flush Diverter Figure 5.3 Roof Washer Vortex Filters: For large scale applications, vortex filters can provide filtering of rooftop rainwater from larger rooftop areas. The images below displays two examples of the filter. The first image in Figure 5.4 provides a plan view photograph showing the interior of the filter with the top off. The second image in Figure 5.5 displays the filter just installed in the field prior to the backfill. Rainwater Harvesting 19 of

20 Figure 5.4 Interior of Vortex Filter 5.4 Storage Tanks Figure 5.5 Installation of Vortex Filter prior to backfill The storage tank is the most important and typically the most expensive component of a rainwater harvesting system. Cistern capacities range from 250 to over 30,000 gallons. Multiple tanks can be placed adjacent to each other to increase overall storage on-site as needed and connected with pipes to balance water levels. Typical rainwater harvesting systems for Rainwater Harvesting 20 of

21 residential use are 1500 to 5000 gallons. Storage tank volumes are calculated to meet the water demand and stormwater treatment volume credit objectives, as described in Section 6 of this specification. The tanks can be made of many materials and configured in various shapes, depending on the type used. While many graphical representations of cisterns in this specification utilize that of a cylindrical shape, for the purposes of communicating visual concepts, cisterns can in fact be configured in many different shapes. Many configurations are rectangular, L shaped, or step vertically to match the topography of a site. Other factors that should be considered when designing a rainwater harvesting system and storage tank are: Aboveground storage tanks should be UV and impact resistant. Belowground storage tanks must be designed to support the overlying sediment and any other anticipated loads (e.g., vehicles, pedestrian traffic). Belowground rainwater harvesting systems should have a standard size manhole opening or equivalent to allow access for maintenance purposes. This access point should be secured/locked to prevent unwanted access. All rainwater harvesting systems should be sealed using a water safe, non-toxic substance. Rainwater harvesting systems may be ordered from a manufacturer or can be constructed on site from a variety of materials. Table 2 compares the advantages and disadvantages of different storage tank materials. Storage tank should be opaque or otherwise protected from direct sunlight to inhibit algae growth and screened to discourage mosquito breeding and reproduction. Table 2 below discusses the advantages and disadvantages of various cistern materials. Table 2 Advantages & Disadvantages of Various Cistern Materials (Cabell Brand, 2007, 2009) Tank Material Advantages Disadvantages Fiberglass Polyethylene Modular Storage Plastic Barrels Galvanized Steel Commercially available, alterable and moveable, durable with little maintenance, light weight, integral fittings (no leaks), broad application Commercially available, alterable, moveable, affordable, available in wide range of sizes, can install above or below ground little maintenance, broad application Can modify to topography, can alter footprint and create various shapes to fit site, relatively inexpensive Commercially available Inexpensive Commercially available, alterable and moveable, available in a range of sizes, film develops inside to prevent corrosion Must be installed on smooth, solid, level footing. pressure proof for below ground installation..expensive in smaller sizes Can be UV- degradable. Must be painted or tinted for above ground installations. Pressure proof for below ground installation Longevity may be less than other materials, higher risk of puncturing of water tight membrane during construction Low storage capacity (20 to 50 gallons) Limited application Possible external corrosion and rust, must be lined for potable use Can only install above ground soil ph may limit underground applications Rainwater Harvesting 21 of

22 Steel Drums FerroConcrete Cast in Place Concrete Stone or concrete Block Commercially available, alterable and moveable Durable and immoveable, suitable for above or below ground installations. Neutralizes acid rain Durable, immoveable, versatile, suitable for above or below ground installations. Neutralizes acid rain Durable and immoveable, keeps water cool in summer months Small storage capacity, Prone to corrosion and rust can lead to leaching of metals, verify prior to reuse for toxics, water ph and soil ph may also limit applications Potential to crack and leak Expensive Potential to crack and leak. Permanent Will need to provide adequate platform and design for placement in clay soils. Difficult to maintain Expensive to build The images below in Figures 4.2 to 4.4 display three examples of various materials and shapes of cisterns, discussed in the categories above. Figure 4.2 Example of Multiple Fiberglass Cisterns in Series Rainwater Harvesting 22 of

23 Figure 4.3 Example of two Polyethylene Cisterns Figure 4.4 Example of Modular Units Tanks should be accessible for cleaning, inspection, and maintenance. Dead storage below the outlet to the distribution system and an air gap at the top of the tank should be added to the total volume. For gravity-fed systems a minimum of 6 inches of dead storage should be provided. For systems using a pump, the dead storage depth will be based on the pump specifications. Rainwater Harvesting 23 of

24 Any hookup to a municipal backup water supply should have a backflow prevention device to keep municipal water separate from stored rainwater, and/or as specified by code; this may include incorporating an air gap to separate the two supplies. 5.5 Distribution Systems Most distribution systems require a pump to convey harvested rainwater from the storage tank to its final destination, whether inside the building, the automated irrigation system or gradually discharge to an onsite runoff reduction practice following a storm event. The rainwater harvesting system should be equipped with an appropriately sized pump that produces sufficient pressure for all end-uses. Separate plumbing, labeled as non-potable, is sometimes required by the municipality. The typical pump and pressure tank arrangement consists of a multistage centrifugal pump, which draws water out of the storage tank and sends it into the pressure tank, where it is stored for distribution. When water is drawn out of the pressure tank, the pump kicks on and supplies additional water to the distribution system. The backflow preventer is required to separate harvested rainwater from mains. Distribution lines from the rainwater harvesting system should be buried beneath the frost line. Lines from the rainwater harvesting system to the building should have shut-off values that are accessible when snow cover is present. A drain plug or cleanout sump, also draining to a pervious area, should be installed to allow the system to be completely emptied, if required for any reason. Above ground outdoor pipes should be insulated or heat wrapped to ensure operation during winter. 5.6 Overflow, Filter Path and Secondary Runoff Reduction Practice An overflow system should be included in the rainwater harvesting system design in order to handle an individual storm event that exceeds the capacity of the tank or in the case that multiple storms occur in succession to fill the rainwater harvesting system storage capacity. Overflow pipes should have a capacity equal to or greater than the inflow pipe(s) and have a diameter and slope sufficient to drain the cistern maintaining an adequate freeboard height. The overflow pipe should be screened to prevent access to the tank by rodents and birds. The filter path is a pervious or grass corridor that extends from the overflow to the next runoff reduction practice, the street, adequate existing or proposed channel or the storm drain system. The filter path shall be graded to have a slope such that sheet flow conditions are maintained. If compacted or impermeable soils are present along the filter path, compost amendments may be needed (see Stormwater Design Specification No. 4). It is also recommended that the filter path be used for first flush diversions. In many cases, rainwater harvesting system overflows are directed to a secondary runoff reduction practice to boost overall runoff reduction rates. These options are addressed in Section 1.3. Rainwater Harvesting 24 of

25 SECTION 6: SIZING OF RAINWATER HARVESTING SYSTEMS The rainwater harvesting cistern sizing criteria presented in this section was developed using best estimates of indoor and outdoor water demand, long term rainfall data and rooftop capture area using a spreadsheet model developed by Sarah Lawson, PhD of RMS and Alex Forasté, PE of Mckee Carson, The Cistern Design Spreadsheet is primarily intended to provide guidance in sizing cisterns and to quantify the runoff reduction volume and credit for input into the Runoff Reduction Spreadsheet for stormwater management purposes. A secondary objective of the spreadsheet is increasing beneficial uses of the stored water, utilizing it as a valuable natural resource, which few other stormwater BMPs can realize. 6.1 Incremental Design Volumes within Cistern Rainwater tank sizing is determined by accounting for varying precipitation levels, captured rooftop runoff, first flush diversion (through filters) and filter efficiency, low water cut-off volume, dynamic water levels at the beginning of various storms, storage needed for treatment volume (permanent storage), storage needed for channel protection and flood volume (detention storage), seasonal and year-round demand use and objectives, overflow volume, and freeboard volumes above high water levels during very large storms. See Figure 6.1 for a graphical representation of these various incremental design volumes. Figure 6.1. Incremental Design Volumes associated with tank sizing This specification does not provide design guidance for sizing the Channel and Flood Protection volume, but rather provides guidance on sizing for the 1 target storm Treatment Volume (T v ) Credit. See Chapter 11 of the Handbook: Uniform Stormwater BMP Sizing Criteria for more information on design volumes and sizing criteria associated with various target storm events. Rainwater Harvesting 25 of

26 Note that the Treatment Volume is different from the Storage Associated with the Treatment Volume. The Treatment Volume, as defined by DCR in Table 11.2, Chapter 11, is calculated by multiplying the water quality target rainfall depth (one inch) with a composite of three site cover runoff coefficients (forest, disturbed soils and impervious cover). In the case of rainwater harvesting, because only rooftop surfaces are captured, only one runoff coefficient is applicable, impervious cover. Therefore, the only variable for Treatment Volume is surface area captured. The Storage Associated with the Treatment Volume is the storage within the tank that is modeled and available for re-use. While the Treatment Volume will remain the same for a specific rooftop capture area, the Storage Associated with the Treatment Volume may vary depending on demand and runoff reduction credit objectives. It includes the variable water level at the beginning of a storm and the low water curt off volume that is necessary to maintain for pumping requirements. 6.2 Cistern Design Spreadsheet (CDS) This specification is intimately linked with the Cistern Design Spreadsheet (CDS). The spreadsheet utilizes daily rainfall data from September 1 st, 1977 to September 30 th, 2007 to model performance parameters of the cistern under varying rooftop capture areas, demands on the system and tank size. The precipitation data is the same that was utilized by the Center for Watershed Protection (CWP) to determine the 90 th percentile 1 target storm event, as presented and explained in Figure 11.1 of the Handbook, Chapter 11. Precipitation data for four different regions throughout Virginia can be selected for use within the model. 1. Richmond International Airport 2. Reagan Airport 3. Lynchburg Regional Airport 4. Millgap 2NNW near Harrisonburg A runoff coefficient of 0.95 for rooftop surfaces and a filter efficiency rate of 95% for the 1 storm are assumed. It is assumed that filters are to be installed on all systems and that the first flush diversion is incorporated into the filter efficiency. The remaining precipitation is then added to the water level that existed in the cistern the previous day, with all of the total demands subtracted on a daily basis. If any overflow is realized, the volume is quantified and recorded. If the tank runs dry (reaches the cut off volume level), then the volume in the tank is fixed at the low level and a dry frequency day recorded. The full or partial demand met in both cases is quantified and recorded. A summary of the water balance for the system is provided below. Water Balance: [NOTE: Insert text in this subsection.] Water Contribution: A. Precipitation to rooftop Volume of water contributing to rainwater harvesting system is a function of rainfall and rooftop area captured, as defined by designer. Rainwater Harvesting 26 of

27 B. Municipal Backup (optional) In some cases, the designer may choose to install a municipal backup water supply to supplement tank levels. Note that municipal backups may also be connected post-tank (i.e. connection is made to the non-potable water line that is used for pumping water from the tank for re-use), thereby not contributing any additional volume to the tank. Water Losses: A. Rooftop Runoff Coefficient The rooftop is estimated to convey 95% of the rainfall that lands on it s surface, R v = 0.95 B. First Flush Diversion The first inches of rainfall that is directed to filters is diverted from the system in order to not clog it with debris. This value is assumed to be contained within the filter efficiency rate. C. Filter Efficiency Each filter has an efficiency curve associated with the rate of runoff and size of storm that it will receive from a rooftop. It is assumed that, after the first flush diversion and loss of water due to filter inefficiencies, the remainder of the 1 storm will be successfully captured. This means that, a minimum of 95% of the runoff from a 1 storm should be conveyed. The filter efficiency value is adjustable and can be modified as an input value in the CDS, but should remain a minimum of 95%. Some localities may require that a minimum filter efficiency for a larger storm event is met (e.g. minimum 90% filter efficiency for 10 year storm), depending on design objectives and local reviewing agency policy. D. Drawdown (Runoff Reduction Volume) This is the stored water within the cistern that is reused or directed to a secondary runoff reduction practice. It is the volume of runoff that is reduced from the rooftop drainage area. This is the water loss that grants rainwater harvesting the Runoff Reduction Volume Credit. E. Overflow For the purposes of addressing treatment volume (not addressing channel or flood protections volumes), both orifice outlets for detention and emergency overflows are treated the same. This is the volume of water that may be lost during large storm events or successive precipitation events. See Appendix A.1 for a detailed description of Spreadsheet Inputs 6.3 Results for all Precipitation Events The performance results of the rainwater harvesting system for all days during the entire period modeled, including the full spectrum of precipitation events, is included in the Results tab. This tab is not provided in order to address Runoff Reduction Volume Credit, but rather may be a useful tool in assisting the user to realize the performance of the various rainwater harvesting system sizes with the design parameters and demands specified. Rainwater Harvesting 27 of

28 Demand Met The demand met for various size cisterns and rooftop area/demand scenarios is reported. A graph displaying the percentage of demand met for various cistern sizes is provided in this tab. Normally this graph assists the user in understanding the relationship between cistern sizes and optimal/diminishing returns. An example is provided below in Figure Percent rainwater can meet demand (% ,000 20,000 30,000 40,000 50,000 60,000 70,000 80,000 90, ,000 Cistern Storage Associated with Treatment Volume Credit (gallons) Figure 6.2. Percent Demand Met Vs. Storage for Re-use (Example) At some point, larger cisterns no longer provide significant increases in percentages of demand met. Conversely, the curve informs the user when a small increase in cistern size can yield a significant increase in the percentage of time demand is met. Dry Frequency Another useful measure is the dry frequency. If the cistern is dry a substantial portion of the time, this can inform that user that they may want to decrease the size of the cistern, decrease the demand on the system or they may want to explore capturing more rooftop area to provide a larger supply, if feasible. It can also provide useful insight for the designer to determine whether they should incorporate a municipal backup supply if they need to ensure water supply through the system at all times. Overflow Frequency A metric of both overflow frequency and average volume per year for the full spectrum of rainfall events is provided. This will inform the user on the design parameters and magnitude of demand and associated performance of the system. If the system overflows at a high frequency, then the designer may want to increase the size of the cistern, decrease the rooftop area captured, Rainwater Harvesting 28 of

29 or consider other mechanisms that could increase drawdown (e.g. increase area to be irrigated, incorporate or increase on-site infiltration, etc.). Inter-relationships and Curves of Diminishing Returns Plotting various performance metrics against one another can be very informative and reveal relationships that were previously not evident. One such inter-relationship is the percentage of demand met versus tank size compared to percentage of overflow frequency versus tank size on the same graph. There is a range of cistern sizes that tends to emerge informing the Designer where a small increase or decrease in tank size can have a significant impact on dry and overflow frequency. Conversely, outside this range, changes in cistern sizes would yield small changes to dry and overflow frequencies, yet yield a large trade-off compared to cost of rainwater harvesting systems. 6.4 Results for Precipitation Events of 1 or less The amount of rooftop runoff volume that the tank can capture and use and/or drawdown for all precipitation events of 1 or less is also quantified and recorded. These results are presented on the Results Treatment Volume tab. It is with this information that the Treatment Credit is calculated. The primary result from this tab is the Treatment Volume Credit that can be transferred and used as input to the Runoff Reduction spreadsheet. Treatment Volume Credit A series of Treatment Volume Credit values are calculated for multiple sizes of cisterns. A trade-off curve plots these results which allows for a comparison of the credit earned versus cistern size. While smaller tanks may yield less credit than larger tanks, they are more cost effective. Conversely, while larger tanks yield more credit, they are more costly. The curve assists the user to choose the appropriate size based on their objectives and site needs, as well as realizing the rate of diminishing returns. The Runoff Reduction and Treatment Volumes are also quantified, however these results will automatically be calculated in a similar manner on the Runoff Reduction spreadsheet with the use of the credit earned. Therefore, only the credit needs to be transferred, not the volumetric results. Overflow Volume from 1 storm The frequency that the cistern overflows and the average volume that it overflows per year as a result of precipitation events of 1 or less are also reported in this tab. A chart of the Treatment Volume Credit and Overflow Frequency for the 1 storm versus storage size is provided, such as in the example shown below in Figure 6.3. Rainwater Harvesting 29 of

30 Figure 6.3. Percent Runoff Reduction Credit Vs. Storage for Re-use (Example). These plotted results enable an establishment of a trade-off relationship between these two performance metrics. For instance, in this case, a 20,000 gallon cistern optimizes the runoff reduction credit and the overflow frequency (near the inflection point of both curves). 6.5 Results from Cistern Design Spreadsheet to be transferred to Runoff Reduction Spreadsheet There are two results from this Cistern Design spreadsheet that are to be transferred to the Runoff Reduction Spreadsheet. They are: 1. First Value to Transfer: Once the cistern storage volume associated with the Runoff Reduction Credit has been selected, simply transfer that credit amount into the Runoff Reduction Spreadsheet column called Credit in the 2.f. To Rain Barrel, Rainwater harvesting system, Cistern row in the blue cell (cell F28). 2. Second Value to Transfer: Then enter the rooftop area that was used in the Cistern Design Spreadsheet in the same row into the Credit Area (acres) column in the blue cell. See Appendix A.2 for STEP BY STEP INSTRUCTIONS for using the Cistern Design Spreadsheet. Rainwater Harvesting 30 of