Rain harvesting offers many important benefits—reducing flooding and erosion where it occurs; decreasing or eliminating combined sewer overflows (CSOs); reducing water pollution from storm water; and complying with Clean Water Act regulations and permits. An often underutilized benefit that is becoming more urgent is onsite and decentralized, or distributive, rainwater harvesting for non-potable direct beneficial use to replace potable water. Most parts of the country are under regulatory orders to reduce polluted storm water flows entering receiving waters or to reduce storm water flows to reduce CSOs, which result in raw sewage spills.
Generally, storm water management focuses on reducing storm water in the public right of way, alleys and streets by detaining, treating and releasing water after storm structural devices. These solutions do not perceive storm water as a local water supply that can be harvested for onsite use. Given the present critical water supply shortage in California and throughout the U.S. Southwest, and cycles of shortages in many other sections of the country, harvesting rain at its source offers all of the benefits described above and provides a supplemental (or possibly total) water supply in place of a municipal or private well supply. This strategy is also a low-impact development (LID) solution that minimizes negative impacts on the environment and maximizes benefits.
Sustainable Practice
This onsite rainwater harvesting and direct use strategy is one of many in the city of Santa Monica, Calif.’s Watershed Management Plan and 2020 Sustainable Water Master Plan. The strategy’s goal is to eliminate the city’s use of imported water, which currently makes up about 30% of its water supply, so that it can rely on local water only. A breakthrough in non-potable use occurred last year when a new city library that uses rainwater for indoor flushing opened. This end use is significant, because using rainwater for subsurface irrigation sparks no public health debate, but using it for flushing urinals and toilets, and spray irrigation does. This is because using rainwater for irrigation when it rains is unnecessary, but using it inside the building when it is not needed for irrigation opens a new opportunity to reduce drinking water use and become more sustainable and self-sufficient.
The Pico Branch Library in Virginia Avenue Park was completed in the spring of 2014, in time to harvest enough rain to fill the 12,000-gal cistern or storage tank, and provide enough water to offset potable water until mid-summer. Even during the summer, some brief, sporadic monsoonal storms added more water to the supply; and during the annual “June gloom” period—weeks of low clouds filled with moisture—the system collected some water.
The System
Rain that lands on the library’s 13,250-sq-ft roof flows to a pretreatment device—a screen, spinning (vortex) and separation product that removes floatables, debris, heavier sediment and leaves. A storm event of up to 1 in. results in 8,000 gal of harvested water. In the city’s average year of 13 in. of precipitation, the harvest would be 104,000 gal. This can replace about 60% of the potable water that would have been used for indoor flushing annually. A larger tank could harvest enough water for 100% replacement, but the additional expense was high and the tank size required was not available.
After pretreatment, the rainwater flows into the underground storage tank, which is under a fire lane. The tank has a recirculation system and spray nozzles that mix the water daily to prevent mosquito production, as well as oxygenate it to prevent it from going anaerobic. A number of utility covers provide access to different parts of the tank to allow servicing and to clean residues not removed by the pretreatment devices.
From the storage tank, rainwater flows to a multi-stage treatment system that includes a sand filter and microfiltration cartridges that clarify the water and remove fine materials, then undergoes ultraviolet disinfection. Effective treatment is required not only to protect the public from microorganisms, but also to remove fine material that could damage flushing equipment and invalidate warranties. A secondary, or residual, disinfection device injects copper molecules into the water as it flows through. These molecules act as an effective neutralizer of the bacteria and algae that may reside in the distribution pipe to the bathrooms.
Instead of treating the entire storage tank of water, a 305-gal day tank stores treated water for immediate, on-demand use. When a urinal or toilet is flushed, a pump on the treatment skid activates to draw treated water from the day tank to replace water used for the flush. The day tank has its own pump to recirculate water back to the treatment train for additional treatment twice a day to maintain high water quality; this also is a safeguard for longer periods (other than overnight) when the library is closed, when bacteria and algae could grow.
Should the storage tank have insufficient water, a potable water makeup line brings municipal water to a T-connection after the day tank. The potable backup water does not go into the storage tank, as it already has been treated. It is more efficient for the backup supply to enter the system post-treatment, where it can directly enter the building for use.
Summary
This project demonstrates how rainwater can be harvested onsite for beneficial non-potable uses in a building; if necessary, the rainwater also can be treated to drinking water quality with relative ease because it is fairly clean to begin with. This LID strategy is a sustainable solution for promoting the use of local water resources and self-sufficiency; for reducing dependence on imported potable water, which benefits distant watersheds by keeping more water there; and for reducing negative impacts from storm water, which can carry numerous pollutants, flowing in the public right-of-way and entering the ocean through Santa Monica Bay.
