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Follow the Purple Pipe

AMANDA RICHTER | Irrigation

Irrigation is the most common use of recycled water, although it may also be used for other nonpotable purposes such as feeding water features or fighting fires and even in carpet manufacturing facilities as part of the dyeing process. The one thing it’s not commonly used for is drinking, although some facilities do treat water to a level at which it can be consumed. In order to keep them distinct from potable water lines, reclaimed water pipelines are colored purple, the universal symbol of recycled water. Mark Tettemer, recycled water manager of the Irvine Ranch Water District (IRWD) in Irvine, California, explains the importance of using recycled water. IRWD was the first district in the state to receive approval for unrestricted use, meaning that its recycled water can be used for any purpose other than drinking. Tetttemer says, “Recycled water is one of the best potable water conservation tools available. It’s a drought-proof supply that is essentially unlimited, and there aren’t any pressures to conserve it. It’s also environmentally beneficial. Moreover, recycled water is a local supply and is not subject to some of the regional and state-wide pressures that occur with other suppliers of water.”

Tettemer continues, “Recycled water also lowers landscaping costs. IRWD’s landscape irrigation customers see a 10% savings relative to purchasing potable water. And lastly, it’s the right thing to do. Using potable water for some nonpotable uses is a waste of potable water.”

Although recycled water is a smart way to use one of our most important assets, it does require a shift in thinking. But if we follow the purple pipe, we will find a great “untapped” resource at our disposal.

11.jpgAcross the country

Southern California, with its large population and arid climate, has been one of the biggest proponents of recycled water. IRWD is testament to this, although other areas have caught on as well. In San Diego, city officials require new developments near reclaimed-water pipes to connect to the city’s system.

California isn’t the only state that’s recycling water, however. In Reno, Nevada, city officials are considering a proposal to use recycled water for irrigation of residents’ lawns and groundwater replenishment. Over in North Carolina, one of the places hit hardest by drought, officials have implemented tough water restrictions. Many green industry workers have complained that the industry is disproportionately affected by the restrictions in comparison to other industries. In response, the city of Durham has begun to allow commercial and industrial use of recycled water in lieu of potable water.

The North Durham treatment plant is giving away 80,000 gallons of recycled water per day. Only two stipulations apply: users have to take at least 250 gallons and must first attend a training session. Although small, the move could prove to be significant for contractors and city crews. “Some businesses were hurt by the restrictions, so we wanted to give them another option,” says John Dodson, plant superintendent of the North Durham Water Reclamation Facility. “So far we’ve had a good response, and I can only see the program growing.”

Further east, South Florida is turning to recycled water to alleviate some of its ongoing drought problems. Water reuse has gained momentum in the region since the mid-1990s. Currently, 28% of the water cycled back through public treatment systems is used for agriculture and other irrigation needs.

Last November, the city of West Palm Beach, Florida, began a reclamation project that combines advanced wastewater treatment with habitat restoration. It’s a system that could make a big difference in lessening the area’s dependence on Lake Okeechobee for water. The city empties up to 10 million gallons of reclaimed water onto the Grassy Waters Preserve. About two years are needed for the water to filter down through native plants before being pumped to the city’s reservoir, where it will be processed for drinking. The vegetation and soil help clean the water of nitrogen and phosphorus. Similarly, more than 40 states have adopted guidelines for recycled water use. For many of these areas, recycled water is essential to survival.

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How it works

Although recycled water is as clean as the stuff that runs from the taps, people tend to cringe at the thought of reusing the water from their sinks, washing machines, showers and hoses. For the most part, this is an unfounded fear. Wastewater doesn’t enter a water treatment facility in pristine condition. It is subject to a series of cleansing processes that send it out in much better shape than it comes in. In fact, some treatment facilities hold their recycled water to the same or even higher standards than drinking water. Knowing a little about the path that wastewater takes to become clean again may help clear up some of the misconceptions about recycled water.

Environmental Engineering Associate III John Mays of the Donald C. Tillman Water Reclamation Plant in Van Nuys, California, shared the following explanation of how the Tillman plant operates. Their cleansing process is similar to most other treatment plants; however, Tillman does not treat solids. All solids are processed at another location.

In the preliminary treatment stage, about half of the larger solids are removed from incoming (influent) water. After being piped to the plant through two main sewer lines, the influent is sent to channels that remove sand, rocks and grit. With some of the larger solids removed, the water is then lifted by large screw pumps to a screening facility, which removes the balance of the solids.

The water then flows into primary tanks, where most of the remaining solids settle to the bottom and are removed. Gas and oils, called scum, float to the top and are removed. Then the secondary treatment begins.

In 20' deep aeration tanks, the water encounters bacteria, fungi and protozoa. These particles are called activated sludge. The water then undergoes a nitrification-denitrification process. During nitrification, oxygen is added to help the particles consume the organic waste in the wastewater, converting it to carbon dioxide, water and new cells. The amount of nitrogen in the water is reduced. While this is happening, stainless steel discs that resemble large floor polishers act as “mixers” to prevent solids from settling. Following nitrification-denitrification, the wastewater becomes rich in activated sludge. It flows to the secondary clarifiers.

In the secondary clarifiers, the activated sludge settles to the bottom of the tank. Scum is skimmed off of the surface. A large portion of the sludge, along with a polymer, flows back to the aeration tanks to maintain biological equilibrium.

Next, the tertiary treatment begins. Wastewater travels through shallow-bed sand filters that whittle down the remaining solids. Residual polymers from the secondary clarifiers help coagulate the solids, while cloth filters also provide excess filtering.

13_1.jpgLastly, liquid chlorine is added to the wastewater to kill any pathogens or disease-carrying organisms. Then the water is dechlorinated and is ready for transport to off-site grounds for use as irrigated water. The whole process takes about 11.5 hours. By the Tillman plant’s estimate, more than 25 million gallons of water are recycled each day at the facility.

The reclaimed water is used to irrigate a nearby municipal golf course and onsite at a lush Japanese garden and lake. The lake is a clean playground for ducks and 12 types of fish. The black pines and other flora unfold into an odor-free, bright garden that serves as the background for many films.

Precautionary measures for using recycled water

When using recycled water, extra care must be taken to prevent cross connection with potable water supplies. Recycled water is carried through a separate set of irrigation lines that have a purple line to distinguish them. In addition, all tools and equipment such as flow control knobs, solenoids, valve boxes and sprinkler heads also must have purple markings and be approved for recycled water use. Prior to retrofitting existing irrigation systems, a cross-conduction test is usually required.

Cities, businesses and other groups that use recycled water for irrigation are often required to post signs warning people not to drink from the irrigation system or use it to wash food. Also, recycled water use may be subject to time-based watering restrictions. Other precautionary measures may include using quick coupling valves in public areas rather than hose bibs. The valves are regulated through a special coupler key so that the valve is opened and closed only when authorized.

A certified backflow tester should maintain and test backflow prevention devices on a yearly basis. Like pipes, these must be designated for reclaimed water status. Potential users should also check with their local regulatory agency for any additional provisions or permitting restrictions.

14.jpgWater that has undergone advanced stages of treatment can contain higher concentrations of dissolved salts than potable water. Although most plants are not affected by the high saline content, some plants may endure tip burning. To minimize this risk, drip irrigation is the recommended method of irrigation. Also, it may be necessary to over water plants by about 10% due to the potential for salt accumulation.

As we continue to seek out ways of supplementing our water supplies, recycled water will continue to gain importance and hopefully, lose the stigma associated with it. In time, it’s likely that we will become less resistant to using water that has been reclaimed. Purple pipelines may become as commonplace as recycling bins. If the drought and water shortages continue, we may not have a choice.

 
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