ID#1602 made on
WEc1.1 - Water efficient landscaping - reduce by 50%
LEED BD+C: New Construction, LEED BD+C: Core and Shell, LEED BD+C: Schools
Background: Our project is a 40,000 sf, four-story office building on a tight urban site. Our goal is to capture and reuse as much rainwater as possible, for reuse as both irrigation water and sewage...
Background: Our project is a 40,000 sf, four-story office building on a tight urban site. Our goal is to capture and reuse as much rainwater as possible, for reuse as both irrigation water and sewage conveyance, and to apply this system to credits WE 1.2 and WE 2. Proposed Rainwater Use Strategy: We are proposing an annual water balance approach, similar in concept to the monthly water balance approach outlined in the CIR ruling for WE 1.2 and WE 2 dated 3/6/06. Per this ruling, it appears acceptable that water used for irrigation not be drop-for-drop from rainwater when served by the rainwater cistern, thereby allowing for greater rainwater usage year round. In our case, the local climate is such that there is very little rainfall during the summer months, while peak rainfall occurs during the winter months. Demand for sewage conveyance will be daily and relatively constant throughout the year, while demand for irrigation water will peak over the summer months and diminish during the winter months. The continuous draw from the rainwater cistern for sewage conveyance will result in optimal rainwater reuse, but rainwater will not actually be 'stored' for dry season irrigation. We would like to request that a water balance over the course of a year, rather than monthly per the 3/6/06 CIR, be considered when determining the amount of rainwater saved and reused onsite. This will allow for greater water conservation for this project overall, while requiring a smaller, simpler rainwater collection system which will also save resources. Proposed System: Our rainwater system collects and reuses 39% of the annual rainfall that collects on the building roof. A cistern with a capacity of 5,600 gallons will collect enough rainwater over the course of a year to offset potable water use for 100% of irrigation needs and contribute to 71% of sewage conveyance needs. This equates to approximately 95,899 gallons per year of reused rainwater. This is much more than the amounts required by the credits to achieve 100% irrigation (8,080 gallons) and the amount of rainwater needed to achieve 50% sewage conveyance (20,000 gallons). In effect, with our proposed strategy we are saving and reusing an additional 67,819 gallons of water per year. A monthly water balance demonstrates that the cistern will provide enough rainwater for irrigation and toilet flushing for most months, but that there will be a few where it would fall short, due to the dry summer season. However this shortfall would be more than made up in the months on either side of the dry period. If we sized the cistern to store enough rainwater to irrigate through the dry season, the cistern would need to nearly double in size, and toilet flushing would need to switch over to city domestic supply during part of this period to reserve the cistern water for irrigation needs. Not only does this require more controls and piping, but it also requires a much larger tank. We believe that the building is best served by the smaller, simpler system that saves much more water per year, and conserves materials and resources. Request: Please confirm that an annual water balance on the collected rainwater and potable water savings is an acceptable approach for achieving credits WE1.2 and WE2. We have used an annual balance sheet, with daily rainfall inputs and daily irrigation and toilet use extractions, similar to that appearing in tables 4-2 and 4-3 of the Texas Guide to Rainwater Harvesting, to demonstrate that cistern sizing is appropriate to meet the overall rainwater savings per these credits.
The project is requesting approval for achieving WEc1.2 by demonstrating that the total annual on-site rainwater collection exceeds the total annual irrigation consumption. The project rainwater collection and storage system serves both toilet flushing and irrigation needs and is sized to meet a consistent baseline use throughout the year. The system does not, however, store enough water to meet peak summer irrigation demand. The proposed system design is not an acceptable alternative to achieving zero use of potable water for irrigation. The project design is making a trade-off between reserving captured water for irrigation and using it for the building plumbing. You can receive credit for the water reused for toilet flushing in WEc3. The monthly water balance referenced in the 3/6/06 CIR is the appropriate means for calculating water storage needs to achieve this credit. Applicable Internationally.
Related Addenda (Corrections & Interpretations)