What’s the Difference Between Rainwater and Stormwater?
Rainwater is collected exclusively from roof surfaces and may be directed into simple rain barrels or cisterns for reuse. Because the water is collected from a single surface, it’s generally considered less contaminated than water that has run over multiple ground surfaces. It’s important to remember, though, that some types of roofs can produce dangerous pollutants of their own. Metal roofs that contain copper or lead, roofs covered with asbestos shingles, or even some wood shake shingles with chemical treatments will produce rainwater that needs treatment to be considered safe even for your lawn and plants.
Stormwater is any rain that has traveled over the ground, including streets, sidewalks, structures, open lots, etc. Even rain that falls on a roof is considered stormwater if it flows over other surfaces before it’s collected. Stormwater is assumed to be more polluted because hard surfaces in developed areas harbor many different types of toxins and pollutants, including oil and gas products, chemicals, heavy metals, dangerous microorganisms, and more. As stormwater passes over these surfaces, it inevitably picks up pollutants and carries them along.
What’s the Difference Between Wastewater and Stormwater?
Stormwater is produced by precipitation in the form of rain, snow, ice, etc. in developed areas and is often gathered, treated, or disposed of through municipal stormwater systems. Stormwater and sewage wastewater are sometimes intermingled in combined sewage systems, which present significant environmental and public health risks.
Wastewater, on the other hand, is water that has been used and then discarded by the user. In developed areas, consumer wastewater is collected through sewer systems and includes toilet flushes, dishwasher and laundry effluent, shower runoff, etc. Because the different sources all flow together, municipal wastewater is considered highly polluted and must undergo several treatment processes to remove solids, digest organics, filter, and disinfect the remaining water. Wastewater that goes through these processes and passes quality tests at the end is considered potable and can be returned to the municipal drinking water supplies.
In Comparison to Treated Municipal Water and Well Water, Where Does the Quality of Harvested Rainwater Stand?
It’s popular to announce that rainwater is the cleanest of all available water on the planet, and that’s probably true, for all intents and purposes. The key, however, is what happens to that rain once it falls to Earth. Once raindrops hit the earth, they’re exposed to a potentially wide range of pollutants and contaminants. The practice of rainwater harvesting is typically restricted to collection from roofs, and so it’s considered less contaminated than stormwater which has, by definition, traveled along the ground where it’s had the opportunity to pick up pollutants from a wide variety of sources.
Even so, roofs are not pristine, sterile surfaces. They may have bird droppings, insect carcasses, debris carried by the wind, including particles from smoke and car exhaust, and some roofing materials themselves, such as asphalt shingles and wood shakes, may be treated with chemicals to fight mold or make them fire retardant. Even clean rainwater, collected from one of these surfaces, is not considered safe to drink. Before harvested rainwater is safe for human consumption, potential contaminants like toxic metals and exhaust particulates need to be filtered out, then the water must be disinfected to remove any potentially harmful microorganisms.
Of course, if you’re planning on using harvested rainwater to keep your garden green, none of that is likely to be a concern. Still, for drinking water purposes, your source with the highest initial quality, (barring any specific known issues,) is municipal water, followed by well water, followed by harvested rainwater.
How Much Stormwater Can Be Harvested from Discrete Surfaces Like Roofs?
The amount of rainfall available to harvest depends on where you live in the US. East of the Mississippi River, the states typically get plentiful rain and it’s spread out fairly evenly throughout the year. This is an ideal situation for supporting your home with captured rainwater.
From the central states stretching westward, rain is much less plentiful, but that doesn’t mean it’s not worth the trouble. You may choose to dedicate your harvested water to typical graywater uses, saving yourself the expense of setting up a disinfection system, while still saving plenty of money with reduced water bills. Alternatively, you can run entirely on harvested water but have a backup source (even if that’s municipal water) if your harvested supply runs out.
After rainfall, the next limit for how much you can collect is the size of your collection surface. If you’re setting up and sizing your system, a good rule of thumb for estimating collection amounts is 550 gallons per inch of rain on a collection surface of 1000 square feet. Keep in mind, as you’re performing these calculations, that the square footage of your home is NOT the square footage of your roof. You’ll need to consider the pitch and complexity of your roof when you determine how much roof you have and how much of it can be used as a collection surface.
Are Standalone Rainwater Harvesting Systems Expensive?
Rainwater harvesting systems can range from remarkably cheap to remarkably expensive. Some of that will depend on whether you’re purchasing a custom or pre-fab system or building your own from scratch, while the rest of it will depend on how much treatment capibility you’ll need, based on your ultimate use. A fully engineered and professionally installed system that produces both a graywater stream and a drinking water stream for your home will cost many thousands of dollars more than a couple of simple rain barrels used to supply water for your houseplants.
When you begin to consider rainwater harvesting, think about the scope of your ideal system. Do you plan just to provide water for outside activities like lawn irrigation and car washing? A system like that, that needs nothing beyond the most basic filtration, is pretty affordable, even if you purchase a ready-made system. The more involved you go, however, the higher the price. If you’re in a very cold climate and you’ll be using the system year-round, be sure to include the cost of burying your system below the frost line. Chlorine disinfection systems are relatively cheap and effective, but UV and ozone systems are sometimes preferred because they don’t leave that chlorine taste behind. Both those alternatives, of course, require some specialized equipment and are more expensive to set up and to operate.
What About Contaminants on the Roof?
Contaminants on a roof are a legitimate concern for those who harvest rainwater, but the degree of concern depends on your ultimate beneficial use. In situations where grey water is suitable to fill the need, most harvested rainwater can also be used with little concern. On the other hand, if you intend to use harvested rainwater indoors for cooking, washing dishes, or drinking, the contaminants become a significant concern and that rainwater will need to undergo filtration and disinfection before use.
If you have specific knowledge about contaminants that are likely to be carried off your roof, that information can be helpful in deciding where the safest beneficial use will be, or in designing a treatment protocol to raise the quality to desired levels. For a metal roof that’s known to contain toxic chemicals like lead or copper, you may choose to install a high-quality filtration system in your harvesting setup. If your roof is a frequent host to a variety of migrating birds, however, you’ll probably want to consider adding both filtration and disinfection even if you’re only planning on using it as a dust suppressor in your poultry lot. Avian flu is no joke.
