Rainwater Harvesting Systems, Inc.

What is the difference between rainwater and greywater?

It is important to distinguish between rainwater and greywater for the simple reason that these two types of water have very different qualities.

Rainwater is any water that has been collected from rainfall and has not yet touched the ground. It is possible to know what materials the rainwater has come into contact with on its way to the holding tank and therefore, one is able to confidently determine whether or not the water is safe to drink. As soon as rainwater hits the ground, pavement or other unknown surfaces, it becomes surface water. Generally, surface water is not safe to drink, even after filtration due to the possibility of chemical contamination.

Greywater is water that was once purified to a high quality and has been used once or perhaps twice for non-contaminating applications. Examples of greywater are laundry water, shower water and some sink water.

Used toilet water and kitchen sink water are considered to be blackwater because this water has come into contact with potentially dangerous bacteria. It is not recommended that individuals recycle blackwater on site due to health concerns. The exception is an aerobic septic system, but even in this case, the water should only be applied to the landscape, never directly recycled into potable water.

Rainwater is not greywater, it is considered freshwater.

Greywater can be recycled on site, usually for landscape irrigation, but the possibility exists to recycle this water to drinking water quality.

Blackwater is water that is most certainly contaminated and should be sent through a water treatment plant or septic system.