Pretreating Stormwater
Source Control:
Selecting the appropriate catchment area for a stormwater harvest system is critical to success. If you’re attempting to use heavily polluted water, the level of treatment necessary to render it safe for use is likely to be prohibitive from a cost/benefit consideration. Such heavily contaminated stormwater should be collected and transported, instead, to a well-equipped wastewater facility.
Debris Removal:
This is typically the very first step in managing stormwater runoff and may be as simple as grates or grids that limit entry of relatively large objects such as tree branches or other trash into a collection area.
Settling and Filtration
One of the major polluting components of stormwater is suspended sediment. Sediment is damaging in a stormwater harvesting system not only because it can foul and damage system components as it accumulates, but because sediment particles themselves attract and carry contaminants such as nutrients, heavy metals, organic chemicals, bacteria and other pathogens. This means that the more sediment your harvested water contains, the more polluted it is.
Sediment reduction and management is a major focus throughout the development of a stormwater system, and the two major approaches are settling and filtration. Detention basins, hydrodynamic separators, wet vaults, or other components that slow or stop the movement of water, allow sediment to settle and be retained in the bottom of the basin. This approach requires little energy, but it can require both time and space to work. In a harvesting system, a faster approach such as filtration may be preferred.
In filtration mechanisms, water is passed through one or more layers of granular media such as gravel, sand, charcoal, and peat. As it passes, the media strains the water, retaining suspended solids and any pollutants they carry, within the filter medium. Fine filters retain more sediment but clog faster and require more attention.
Treating Stored Stormwater
Precipitating Contaminants:
Certain types of pollution cannot be adequately filtered out using a simple physical system. Dissolved minerals, phosphates and salt are important examples, particularly because of the threat each pollution type presents to human, surface, and aquatic ecosystems.
Phosphates, salts, and dissolved metals are difficult to separate from stormwater, and each requires its own specific process. For dissolved metals, the first step is to induce precipitation or adsorption of the molecules into particulates. The degree of success in this step depends on the concentration of suspended solids within the stormwater, where a higher concentration of solids generally equates to higher levels of particulate metals. Once in particle form, the metal contaminants can be filtered out using standard filtration methods. Depending on the concentration of metals in your site, however, be aware that it may be necessary to use hazardous waste protocols in sediment disposal.
Disinfection:
Not all beneficial uses for harvested stormwater require disinfection. If you’re going to use your stormwater to wash your car, flush your toilet, or water non-edible landscaping during the spring and summer, there’s no reason to treat it at this level. Removal of phosphates and salts using a precipitation method will keep your plants happy but disinfecting, even with inexpensive chlorine, is an unnecessary step.
If you’re using your harvested water to cook, brush your teeth, or rinse and water plants that you or your pets are going to eat (e.g., tomatoes, lettuce), you’ll need to go through a full disinfection process or simply switch to municipal water for those uses. Filtering sediment and dissolved solids does not eliminate bacteria and viruses, which are much too small to be blocked by anything but specialized membranes.
If disinfection is in your future, there are three effective options for disinfecting harvested water.
Chlorination
Chlorination is the most popular disinfection technology and is widely used by municipal water systems across the country. It’s relatively simple and certainly is the lowest cost option. It effectively treats bacteria, viruses, and other organic pathogens.
This practice uses chlorine in dry, liquid, or gas forms to kill harmful microorganisms. One advantage to chlorine disinfection is its residual effect. As chlorinated water travels through your distribution system, it continues to disinfect, and can be relied upon to remain free of harmful critters for a limited time.
Remember, though, that chlorine can be dangerous when handled improperly, so follow instructions carefully, and be sure to only use chlorine that’s certified for potable water use (ANSI/NSF 60).
Ultraviolet Light
UV light is an increasingly popular disinfection treatment, especially where the odor or taste of chlorine is considered offensive, but it does come with some important caveats. UV disinfection works by passing water through a tube that contains a UV bulb. As the water is exposed to the light, the DNA of microorganisms are disrupted, meaning they can’t reproduce. However, only Class A UV lights are rated to destroy pathogens. Class B lights can remove some residual impurities from water, but they are ineffective against unsafe microorganisms. UV is sometimes preferred because it produces no byproducts and requires very little energy.
It’s important to remember that UV disinfection relies exclusively on the ability of the light to permeate every molecule of water. If the water is colored, cloudy, or contains even small amounts of suspended solids, the UV light will be blocked, and disinfection will be incomplete. For this reason, high level filtration cartridges must be employed before the UV step.
Ozonation
Ozone is a colorless gas that is introduced to water through an aeration mechanism. It’s a powerful oxidizing agent that produces a broad-spectrum biocide that destroys all bacteria, viruses and cysts. Not only is ozone the most effective of the three disinfection methods at eliminating pathogens, but it also deodorizes and decolorizes treated water, which can make it more appealing.
Ozonation does have some significant disadvantages. It’s relatively expensive, both in capital and operating expenses, particularly in high energy requirements. Ozone gas is irritating (and possibly toxic) so off-gases must be destroyed to prevent exposure to operators. Ozone is also corrosive, so elements of your storage, and distribution systems should be selected accordingly.
