Harvest Greywater or Rainwater?

By John Bauer, Wahaso President

We find that there is a lot of confusion out there about greywater, (also called gray water and graywater), and rainwater usage. Which system is the best for a particular use? The usual assumption is that a greywater system, which must filter and sanitize water from showers and sinks, will be more expensive than a rainwater system. This may or may not be true, depending on your situation.

Generally, we find that a rainwater system makes the most sense when the following conditions apply for a property:

  • The building is already existing, so it is economically unfeasible to separate the existing waste streams from the toilet, showers and sinks to capture greywater.
  • The water use requires a very clean source with little dissolved solids – for cooling tower make-up, washer rinse or vehicle washing.
  • There is an abundant source of rainwater in the area so that the storage size can be minimized.
  • There is an abundant source of rainwater in the area so that the storage size can be minimized.

Greywater systems are often a better choice when these conditions exist:

  • The building houses residents, so there is ample supply of greywater from showers and sinks.
  • The primary uses of the harvested water will be toilet flushing and irrigation. Greywater is an excellent source for these uses.
  • The building plumbing has not yet been completed so there is time to change the layout to capture shower and sink water separately from the toilet black water.
  • The property is located in an arid part of the country that gets little or very seasonal rainfall.

If greywater is an option for your property – that’s good news. Unlike rainwater supply that is highly dependent on local rain events, greywater is a very reliable source, tied directly to building usage. Residents flushing toilets are also showering and washing their hands.

And while a greywater processing skid is somewhat more expensive than a rainwater system, the reliable source of greywater means that we usually only need to store a few hundred gallons of processed water at a time. That saves a tremendous amount of the cost and space requirements associated with a large rainwater system. A typical commercial rainwater system can require tens of thousands of gallons of cistern storage.

The net result is that a greywater system can predictably save more municipal water for a lower total system cost than a rainwater system. The one caveat here is the added cost of running a separate waste line in a building to capture the greywater for harvesting. Depending on the plumbing layout for a building, this cost can be relatively insignificant or a sizable investment.

And we should note that a greywater system will need a bit more maintenance than a simple rainwater system. Greywater sanitation requires the addition of chlorine, and there is additional filter maintenance. But these maintenance needs are typical of the many systems in a commercial building and require no special skills or training.

Our best advice about whether rainwater or greywater harvesting is right for you is to have you talk with us at Wahaso. We can help you evaluate the feasibility and cost considerations for both options as they relate to your unique building. Please contact Wahaso. And you can learn more about greywater harvesting and rainwater harvesting by visiting our website.