The University of Washington Botany Greenhouse is a prime candidate for rainwater reclamation. Capturing rainwater that flows through the existing gutter system and re-routing it to storage tanks located within the confines of the greenhouse would save from purchasing city water, increase the quality of water used for irrigation, save energy currently used to warm irrigation water, and dramatically increase the thermal mass of the enclosed greenhouse space which would save on space heating energy demands.
The greenhouse demands approximately 350 gallons of water per day (corresponding to 128,000 gallons of water annually). In order to adequately meet this demand there must be a balance between the quantity of rain falling on the roof and the physical storing capacity of the greenhouse. It turns out that the limiting factor is the storing capacity of the greenhouse. To date there is available space underneath six of the tables to store six, 3ft diameter by 32ft long water tanks, allowing for a maximum storing capacity of 7050 gallons at any given time. These tanks would not impose a spatial burden on the greenhouse and could be connected to existing plumbing systems with relative ease. A volume calculation that took into consideration the average monthly rainfall rates in Seattle (in ft) and the available roof area that projects onto the floor of the greenhouse (in ft^2) gave the volume of rainwater supplied equal to 92,800 gallons/year. This accounts for 73% of the total water demanded, or in other words, 265 days of irrigation. As stated above, physical restrictions within the greenhouse are limiting the number of tanks to six. However, a year or two from now if it is decided that the rainwater reclamation system is effective enough to warrant an increase in storage capacity, it would be very possible to relocate materials currently located underneath the tables to a new location, freeing space for additional tanks. This proposal focuses on six tanks because it utilizes the space that is currently available, and also because inherent in any rainwater collection system is uncertainty in rainfall rates and overall system functionality. At this point in time 73% is a happy medium between available space, adequate irrigation supply, and reducing overflow potential on a high volume rain day.
Environmental impact: The first benefit of this system is the reduction in city water usage. The proposed system is expected to save the university 92,800 gallons of water/year, with the potential for even more savings further down the road. Rainwater is healthier for the plants than city water because of its lower pH and reduced TDS (Total Dissolved Solids count). A second advantage is the added thermal mass that the six tanks would bring to the greenhouse. Greenhouses are notorious for heating up quickly when the sun is out and losing heat when the sun is gone. This results in the venting of warm air during the day, and the consumption of energy (steam) at night. Right now there is a deficiency of mass capable of storing heat in the greenhouse. By dramatically increasing the thermal mass of the structure, we can reduce the venting of energy by day and reduce the addition of energy at night. Additionally, since the water from the roof will acclimate to room temperature, it will no longer be necessary to add warm water prior to irrigating. One last, but far from trivial benefit would be the reduction of run-off going into the city sewer system. Seattle faces significant challenges to water-treatment facilities by the fact that our storm water is combined with our sewerage.
Student Leadership and Involvement: This project is simple enough to be handled and managed almost entirely by dedicated students who are willing to put the time and effort into seeing each step through to its entirety. It doesn’t require vast technical knowledge on any one subject and it also provides students with real life project management experience under the supervision of professionals. Doug Ewing, the greenhouse manager, is highly knowledgeable with everything relating to the greenhouse and what it needs to run properly. Depending on how much freedom we are granted by facility services, there should be ample opportunity for a number of interested students to help with the installation process as well as routine maintenance checks once the system is in place. I can personally list 10 underclassmen (mainly pre-engineers) that have expressed interest in helping with the project.
Education, Outreach, and Behavior Change: One of the main goals of this project is to spread the concept, methods, and benefits of rainwater harvesting. At this writing, similar systems exist in Seattle but are more focused on turning rainwater into a potable source of drinking water. Very few systems, if any, use rainwater for the irrigation of greenhouse plants. The results of this project will serve as a guideline for systems of a similar nature located in a similar climate. Once the system is in place a continuous stream of data may be collected and interpreted as the seasons progress. Gallons of city water saved, heating energy saved, plant health before/after implementation, consecutive days with/without rainfall, etc. This data could then be used as a compelling argument to spread the use of similar systems in various regions around the Northwest.
Feasibility, Accountability, and Sustainability: The greenhouse manager Doug Ewing has been in direct contact with a few representatives from facility services in the capital projects department. The senior project estimator John Barker is currently on vacation but has agreed to bid the project. He gets back March 11th. We have already received an estimate on material costs that includes the tanks, pump, pressure tank, and piping. This figure came out to be $24,500. What remains to be priced is the cost of labor, which will be forthcoming upon the arrival of Mr. Barker.