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What is a Rainwater Harvesting system?

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asked Oct 14, 2015 by Devan Talebi
edited Oct 14, 2015 by Devan Talebi

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Example for 2,500 square feet roof:

Rooftop rainwater and cooling condensate are captured, filtered, sanitized and then pressurized to support approximately landscaping. Municipal water is added to the system as a back-up if the rainwater tank is empty. The system should meet about 37% of total demand, saving an average of 11,000 gallons of municipal water each year.

1.   Pre-Treatment.  Rainwater will be collected from 2,500 square feet of and pre-filtered through a vortex filter that will screen out all particulates larger than 280 microns. Pre-treatment is essential to removing trash, sediment and organic materials that can quickly foul rainwater stored for any length of time making it unsuitable for harvesting. Cooling condensate can be sent directly to the tank.

2.   Storage. Screened rainwater will then enter a 3,400 gallon tank system. Water will be transferred from the base of the tank and through a floating filter to the processing system cabinet via a submersible pump.

3.   Processing. A NEMA 3R enclosure "wet box" mounted on a wall or post will contain components for final filtration, sterilization and electrical connections.

a.   Conveyance to Processing System. Water from the tank is pumped to the processing skid via a submersible 15 GPM pump located in the base of the tank. The water is drawn through a floating filter to minimize any sediment entering the filtration system.

b.   Filtration. Water passes through a one-step bag filtration process.  The bag removes particulates down to 50 microns in size which is sufficient for 50 UVT UV exposure.

c.   Sanitation. Sanitizing the water before leaving the system ensures that it is safe for use in public areas.

Water is sanitized by an ultraviolet system rated to handle 15 GPM and providing a minimum UV exposure of 40 mJ/cm2 which exceeds NSF/ANSI 55 Class A requirements for potable water. 

d.   Pressurization. When there is demand for non-potable water, a drop in the system pressure signals the cistern pressurization pump to begin sending pressurized water through the treatment system and out to the irrigation area.

e.   Municipal Make-Up. Should the system demand additional water from the tank when the tank is empty.   

4.   Controls. The control system monitors water levels in the tank and tracks the amount of harvest water used.

       5.   Water Quality Management.  The following steps are included in the design to manage water quality:

a.   Proper Pre-Treatment.  Proper pre-treatment of stormwater prior to entry into the cistern shall be achieved using a pre-filter that will remove organic material and sediment. 

b.   Rotation of Water.  The design of the system shall bring storm water into the bottom of the tank through a smoothing inlet. Harvested water shall leave the tank from through a floating filter that pulls water from below the surface and minimizes disruption of fine sediment in the tank. 

c.   Processing Treatment. The processing system is designed to treat the harvested water to a standard suitable for non-potable use through comprehensive filtration and sanitation steps defined by the EPA.

6.   Maintenance and Operation Requirements – This system has been design to require minimum maintenance and operation efforts and a maintenance schedule and training are included with the system. Periodic maintenance will include:

a.   Primary Filter. The vortex filter is self-flushing, but the fine screen will need to be removed and cleaned with a stiff brush 2-3 times per year to maximize its efficiency.

b.   Bag Filter. Bag filters are disposable and should be changed when full in order to maintain efficiency (approximately every 4-8 weeks).  A manual inspection of the differential pressure gauges and the bags should be done on a regular basis.

c.   UV Bulbs.  UV bulbs come with a wiper that should be used to clean the bulbs periodically in order to ensure optimum sanitation performance.  Replacement of the UV bulb is necessary only every 10,000 hours (approximately one full year of continuous operation). Bulb replacement is a simple task that can usually be completed by building maintenance staff.

7.   Electrical Requirements.  This project requires single-phase power, 220v. We will assume that 220v is available unless otherwise notified.




answered Oct 14, 2015 by Devan Talebi
edited Oct 14, 2015 by Devan Talebi

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