In the Caribbean harvesting of rainwater is a common way of obtaining water for a variety of uses because fresh water is a scarce resource.
In the continental US, the harvesting of rainwater is becoming more important in areas with drought conditions or because of an increasing interest in environmental damage that can be caused by storm water run off.
In cities, where open ground is limited, rainwater is simply flushed to the storm sewers where it enters rivers and lakes carrying the oils, debris and bacteria from the city.
Using rainwater means that the equivalent volume of water does not have to be take from surface or ground sources and treated.
This saves money and energy.
Green building standards encourage the use of rainwater harvesting and more buildings are trying to capture and use rainwater.
Typical uses of rainwater include toilet flushing and landscape irrigation.
The collected water is normally not used directly, but is treated to make it safer and aesthetically appealing to the building occupants.
Water collected from a roof, for example, can contain dirt, bird droppings and roofing materials.
This can contaminate the water with pathogens and organic compounds that might make the water unsafe, color the water or create unpleasant odors.
Even an application such as landscape irrigation can create an aerosol of water that could spread pathogens.
Legionnaires disease can be caused by water spray coming off building cooling towers.
So proper treatment of rainwater is a prudent approach to using this resource while keeping building occupants and neighbors safe and happy.
A rainwater treatment system should have at least two components: filtration and disinfection.
Filtration is the simplest and least expensive portion of the system.
The filtration system should contain a strainer to prevent large objects from the collection surface from entering the storage tank.
A finer filter can be added to remove particulates of dirt from the water.
Cartridge or bags filters would be good options for the fine filter application.
While there are a variety of disinfection options available, two green options are UV and ozone.
UV involves exposing the water to UV radiation from a submerged lamp.
It is regularly used to disinfect water for drinking and wastewater discharge.
It is effective against a wide range of pathogens and does not require the storage or addition of any chemicals.
One disadvantage of UV is that it cannot project its disinfecting power to the tank or the distribution system.
Only water that passes by the UV lamp is disinfected.
If microorganisms are living up or down stream of the lamp they will be unaffected.
The other green disinfection option is ozone.
Ozone also does not require the addition or storage of any chemicals.
The ozone is produced from the oxygen found in air.
Ozone is a strong disinfectant and works against a wide range of pathogens likely to be found in rain water.
Unlike UV, ozone maintains a residual in water.
So it not only disinfects the water that passes through the injection system, but it also maintains its disinfecting power in the storage tank.
This is beneficial since bacteria or other microorganisms might adhere to the tank walls providing a continuous source of pathogens in the water.
Besides being a safety threat, these organisms could create odor in the tank that might be unpleasant for the building occupants and neighbors.
Another advantage of ozone is that it can remove color and odor from the water.
In some applications like toilet flushing, building occupants may not appreciate the water having a tint or discoloration.
While the water might be safe, it may give the occupants the feeling the water is not clean.
Ozone would eliminate this problem.
It is possible to combine ozone and UV for rainwater treatment.
In this approach you use ozone to make sure nothing grows in the tank and to remove color and potential odor problems with the rainwater.
The UV treats the water prior to entering the distribution system or application.
Using both systems provides two levels of protection.
The Thursgood Marshall Federal Building in NYC is being built with an ozone/UV rainwater harvesting system to provide this higher level of protection.
Rain water harvesting is good for the environment and can also save money, but it needs to be done in a safe and effective manner.