UNDERSTANDING ACTIVE APPROACHES FOR RAINWATER HARVESTING
UNDERSTANDING ACTIVE APPROACHES FOR RAINWATER HARVESTING
There is a fast-growing interest in capturing and reusing rainwater in commercial buildings to reduce costs for municipal water and also to reduce the impact that a building has on the surrounding environment and on the municipal sewer and stormwater systems. Rainwater from rooftops can be captured or "harvested" and used to replace municipal drinking water for many non-potable applications. Rainwater harvesting can be categorized as either "active" or "passive". This article will help the reader understand the major components of active commercial rainwater harvesting systems.
By "actively" harvesting rainwater, we are referring to formal systems for collecting, filtering, storing and recycling water that would otherwise be sent to municipal sewer or stormwater systems. These are differentiated from passive harvesting systems that usually detain rainwater in vegetated swales so that it can infiltrate naturally back into the soil.
While active systems are more complex and more expensive than passive systems, they have the ability to significantly reduce the costs for municipal water and water treatment- especially in commercial buildings with large rooftops and concentrated numbers of occupants. A commercial building that captures all of its rooftop rainwater for harvesting can often save 1-2 million gallons of municipal drinking water per year.
These systems are most efficient when designed into the mechanicals of a new building so that capture, treatment and application of the harvested water are integrated into the rest of the building particularly when the harvested rainwater is used to flush toilets in the building. To flush toilets with harvested rainwater, separate non-potable water lines must be run to each toilet in the building; this can be prohibitively expensive in a large existing building. Systems can be economically retrofitted into existing buildings, however especially if the harvested rainwater is used for irrigation of landscaping or for cooling tower.
Systems in commercial buildings that process millions of gallons of harvested rainwater per year require proper engineering design and commercial-grade components to meet higher standards of treated water safety and reliability. Active systems generally include the following major components:
Collection: Rainwater is typically collected from building rooftops. Rainwater collected from parking lots and other impermeable surfaces is termed "stormwater" and is treated differently than rainwater because it typically has more contaminants than water collected from relatively clean rooftop surfaces. "Gently used" water collected from showers, sinks and washers is called "greywater" and requires more complex treatment. Water from toilets and kitchen sinks is termed "blackwater" and normally not included in water harvesting systems.
Pre-Filtering: Rooftop rainwater is relatively clean, but still needs to be screened of leaves, gravel and other debris. This can be done with a "first flush" system that sends the first few minutes of rain to flush the debris to the stormwater system. More efficient systems screen the water mechanically or use a vortex action to separate out larger debris particles in the 200+ micron range.
Storage: There are many options for storing harvested water, and the best option depends on many factors - like the amount of supply and demand, the frequency of rainfall in the area and the availability of space to house or bury the tanks.
Tank sizes can range from a few hundred gallons to a half a million gallons or more and be made of polyethylene, fibreglass, concrete or steel, depending on their size and location in the building.
For more information on storage tanks, see Storage Options for Harvested Water
Sanitation: Most commercial rainwater harvesting systems include one or more processes that sanitize the water to kill bacteria, viruses and other pathogens. This stabilizes water being stored for long periods of time, and ensures public safety of water the will be brought into the building or sprayed on landscaping around the building.
There are many approaches to sanitation, which can include the addition of chlorine, chlorine dioxide, ozone, or by circulating the water past ultraviolet (U.V.) sterilizers. Ultraviolet sterilization is ideal for water being applied for irrigation because it is chemical-free and very low maintenance. When the water is used inside the building usually to flush toilets most municipalities require adding chlorine as the disinfectant so that there is a measurable "residual" level of pathogen-killing capability.
Final Filtration ("Polishing"): A final filtration step is usually included in commercial systems to remove remaining particulates in the water before it is applied; this step improves the water's purity and clarity, "polishing" it for final use. This can be achieved with a mechanical filter using a fine micron screen, sand filters or bag filters. When higher levels of purity are required -especially when harvesting greywater, ultra-filtration, reverse osmosis and carbon filters are also used. These can take the water quality to potable or near-potable purity.
Controls: Water harvesting systems in commercial and institutional buildings are held to a higher standard than residential buildings. These systems generally require a sophisticated control system that manages all aspects of the process.
These smart systems can determine when to add municipal water or divert excess water and waste to municipal sewer systems, the levels and purity of stored water, the status of filtration devices, etc. Sophisticated systems can interface with building automation systems for alarms and optimization.
The cost for commercial rainwater harvesting systems is directly related to the amount of water captured and stored and applied. Areas with regular monthly rainwater events require less expensive systems due to reduced storage capacity requirements. System costs generally range from $25 100K including engineering design and installation support. System payout is directly related to the costs for municipal water replaced by the system and can range from 6-12 years. Wahaso Water Harvesting Solutions, Inc. provides full design-build support; initial system scoping is provided at no cost by the company.
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