Commercial rainwater harvesting: Going beyond the barrel

by BRIAN SODERHOLM,

contributing writer

Like much talked about solar and geothermal energy technologies, the harvesting of rainwater harvesting, or “rainwater reclamation,” has become a become a much talked about, and in-demand, aspect of “green” mechanical system design. Once collected, rainwater can be reused for non-potable applications such as toilet/urinal flushing, cooling tower make-up water, lawn or nursery irrigation, vehicle washes, industrial applications, and more.

While the interest in reclaiming stormwater from roofs and drain tiles is great, knowledge about system requirements and potential pitfalls is very limited within the construction community. Nowhere is this scarcity more apparent than in the commercial construction arena, where making use of reclaimed rainwater can significantly contribute towards six or more LEED® points under the current New Construction document, and 11 or more points under the proposed LEED 2009.

One of the great advantages of reclaiming rainwater in commercial applications lies in the potential for water production capacity. With roof sizes in the tens of thousands of square feet, significant amounts of water can be collected and stored for later use. With this quantity of stored water, delivery systems in the hundreds of gpm flow rates can be incorporated for uses like toilet flushing and irrigation. The main downside to these systems, however, lies in the potential liability associated with delivering water that has been collected and stored in an environment with the potential for contamination. Sanitation, and the accompanying controls, become critical in order to assure human safety.

Although codes authorities are beginning to develop standards and guidelines for reclaimed rainwater, they have not kept up with the rapidly increasing demand for this technology. The few states and municipalities that have relevant codes in place are the exception and not the rule. With the significant liability to engineers, installers, and owners of these systems, it is critical to design these systems with adequate sanitation and failsafe mechanisms.

Currently, the Uniform and International Plumbing Code contain language regulating the use of wastewater which has been reclaimed on-site or treated on a municipal basis. This language, although somewhat helpful, is not well-tailored to rainwater reclaim applications. There is currently activity within IAPMO to make the UPC’s language more all-encompassing (and better suited to rainwater harvesting, as well as other non-gray water applications).

Until such time as the codes provide more specific guidance, a useful design resource is the USEPA’s 2004 publication titled, “Water Reuse Guidelines.” Chapters 3 and 4 of this document do a good job of describing ideal product water quality levels, disinfection methods, fail-safe mechanisms, and monitoring procedures.

When designing commercial rainwater reclamation systems, four variables must be taken into consideration: Collection, storage, disinfection and delivery. Some tips regarding each of these variables are as follows:

Collection: Carefully consider just how much water you can -- and need to -- collect. Large systems can be costly, so there’s no use designing for more water than your fixtures/ equipment will use -- or more than your roof will be able to provide. Good engineering practices would say that you can harvest approximately 0.62 gallons of water per inch of rainfall for every square foot of flat roof surface you have available. Consider mesh screens or ballast guards on your roof drains, as well as vortex downspout separators or gutter filters, in order to limit the amount of debris, sediment, insects, rodents, and bird droppings that are ultimately able to settle in the storage tank.

Storage: Smaller stormwater storage tanks (also known as cisterns) will typically be constructed of polyethylene, while larger units will likely be built from fiberglass or concrete. Verify that the tank material is compatible with the disinfection method being used. Cisterns should incorporate manhole access covers, adequate inlet/outlet tappings, and an overflow which can be directed to grade or a storm sewer system. Cisterns may be located indoors, outdoors, or below grade, depending on the application and the local climate.

Disinfection: Cistern tanks are highly susceptible to algae, mold, bacteria, cysts, rodents, amphibians and a host of other infestations which can create public health issues, as well as unpleasant colors and odors. No engineer, contractor or owner should risk diffusing reclaimed rainwater from irrigation spray heads or cooling towers -- or delivering it to public use fixtures like toilets -- without adequate disinfection systems in place. Ultraviolet light treatment, or ozone injection, when combined with particulate filtration, can both provide effective means of disinfection when properly controlled. The best systems will monitor ozone concentrations or UV light penetration (which can be inhibited by water discoloration) and provide for failsafe mechanisms should disinfection levels show to be inadequate. Chlorination is also an option, though it is usually not preferred in “green” building applications because of the harsh chemicals involved, and associated problems with irrigation uses.

Delivery: Fixtures and equipment supplied with reclaimed stormwater require separate piping systems to prevent any cross connection with potable water supplies. Most states will require this piping to be color coded and labeled “non-potable.”

Additional requirements for labeling fixtures supplied with non-potable water may also exist (see your local codes for details). Each system should include a potable water bypass mechanism, in the event the cistern runs dry. Any such bypass must incorporate proper backflow protection (typically an RPZ assembly). If connections to a potable water supply are prohibited, a mechanism for supplying potable water to the cistern tank, via an air gap, may be required.

If this all seems like a lot to swallow, consider eliminating the headaches and worries by using a turnkey system that ties together, and controls, different systems for collection, storage, disinfection, and delivery. Pre-fabricated, skid-mounted systems are available that can perform this complex task -- even when large roof areas and high delivery flow rates are involved. Typically, these systems can guarantee water quality levels which meet or exceed current USEPA recommendations, and will likely satisfy local codes authorities, even when no standard exists.

Demand for commercial grade rainwater harvesting systems is sure to grow. With proper design and installation, your reputation -- and your bottom line -- can grow together with it.

Brian Soderholm is product manager for Rainwater Reclamation Systems at Water Control Corporation in Ramsey, Minnesota. Since 1973, Water Control has been manufacturing commercial water conditioning and filtration systems. Their current product array includes turnkey, fully controlled, commercial rainwater reclamation systems, which incorporate UV and ozone-based disinfection. Brian can be reached at 866/405-1268 or www.watercontrolinc.com.