Published on March 17th, 2008 | by Michelle Bennett9
In-Depth Look at Clean Sewage Tech
March 17th, 2008 by Michelle Bennett
There’s been a lot of focus on renewable energy on Cleantechnia recently but energy is not the only “clean” tech out there. What about recycling wastes? What about recycling… human wastes? What’s the difference between traditional wastewater treatment and sewage purification (also known as ‘indirect potable water reuse”)? Is it safe and cost-effective? And does it smell like… you know…? Turns out, it’s a lot cleaner than you think.
Let’s get the “yuck” factor out of the way. If you’ve ever passed a waste water treatment plant on a hot day, you probably got a whiff of what we’re talking about: the result of all our glorious indoor plumbing. Traditional waste-water treatment has one main goal: “reduce pollutants in wastewater to a level nature can handle.” When they’re done filtering out most of the undesirables, they release it into a waterway – in fact most major waterways. These traditional plants handle everything that goes down the drain: sewage, runoff, litter, chemicals, and even medication. This cloudy mix includes dreaded disease-causing bacteria but also nutrients, minerals and metals. Here are the basic steps for normal waste water treatment:
1) Screen and Grind: Everything that washes into our sewers is supposed to end up at the plant. This includes sticks, trash, and plenty of “unmentionable” solid particles. The first step to treatment is to grind everything down to a manageable size. Now the pipes won’t get backed up (that’s one hell of a clog) and the next steps can work their magic.
2) Separation: The next step is to separate the grease and solid particles from the more manageable waste water.Everything sits quietly in a tank (or perhaps moves slowly through pipes) until the most difficult ingredients, called “sludge” can be skimmed off the top and bottom. These require special treatment. Anaerobic bacteria and sometimes caustic chemicals are used to decompose sludge. The bulk of wastewater, now deemed “clear,” (but not clean) moves on to the next major step.
3) Micro Dine-In: The “clear” wastewater is fed to bacteria that happily eat organic materials. There are many different ways to feed these helpful bacteria, but the result is always the same: the organic material is digested away, leaving disease-causing bacteria, nutrients, and other chemicals behind. Now the waste water moves on to the final stage before being released into a river or ocean.
4) Final Treatment: Plants must disinfect the wastewater before releasing it back into a waterway. Chlorine or ultraviolet light work best, but neither are cheap. Too much chlorine can damage natural habitat, and ultraviolet light requires a lot of energy. In most wastewater treatment plants this is the last stage, meaning many nutrients, metals, and medication have not been scrubbed from the wastewater. Some plants also employ “Advanced Treatment” that removes excess nutrients, like nitrogen or phosphorous, from the water. This step protects natural habitat because excess nutrients cause “dead zones.” (For more information on these wastewater processes, click here.)
But what about “sewage purification”? On January 10, 2008 Orange County, California began full-scape operation of an “‘indirect potable water reuse” plant which removes *ahem* impurities from sewage to extract potable water. They then pump that purified water back into aquifers to replenish the drinking-water supply . Some people are a little wary. Are they drinking their own “impurities”? How safe is it? And what does it cost? A quick look at the technology should put some of these fears to rest:
1) Micro Filter: All water is gently pumped through special filters – only objects smaller than about two-tenths of a micron may pass. That’s smaller than many bacteria (including the disease-causing kinds), and obviously the larger solids we usually associate with sewage. But have no fear, the water is not yet “clean”. The filters are cleared every twenty minutes and all pipes are cleaned weekly to ensure purity. The solids (sludge) from this process are piped to an existing traditional waste treatment plant for decomposition.
2) Reverse Osmosis: High-pressured pumps force the filtered effluent through several layers of thin plastic. This scrubs “viruses, salts, pesticides and most organic chemicals” out of the water. Now all that’s left are a few organic chemicals and pharmaceuticals. Not to worry, the next step is equal to the task of removing even these hearty offenders.
3) UV Bath: Powerful UV lamps react with hydrogen peroxide (added in the water) to fry the rest of the impurities. Now we’re getting some clean water. Plant officials claim that the result is cleaner than bottled water, and they’re right. The purified water is then pumped back into the aquifer, ready for use. Designers claim they have planned and installed numerous fail-safe designs into the system to prevent anything unwanted from passing through their system.
But what about the cost? This plant, the largest of its kind in the world, cost $480 million. But pricing a traditional waste treatment plant is tricky because it depends on the size, location, and methods used. The Northend Wastewater Treatment Plant cites $38 million for a “conventional treatment system” but fails to mention details. Nevertheless, given the gap between these estimates, I think it’s safe to assume that sewage purification is pricey. The cost of this kind of plant may only be rivaled by the cost of piping water across long distances or difficult terrain.
The benefit of recycling water is that you reduce your dependence on local water supplies, which are not always reliable. The drought in the southeast United States has spurred Georgia to reconsider the state line. My hometown of Chattanooga, TN replied by declaring “Give our Georgia Friends a Drink Day”, and delivered bottled water to Atlanta in a truck dutifully escorted by a Davy Crockett.
All joking aside, water is a serious issue around the world. Orange County itself is rightfully worried about its water supply. Ron Wildermuth of the Orange County Water District told Living on Earth, “Because of growing population, we knew that we needed to put more water in the groundwater basin.” Their new facility pumps 700 million gallons of clean water back into the aquifer (not directly to homes) each day; that’s enough water for the daily needs of about 500,000 people. “It will give us a supply unaffected by drought,” Mehul Patel, Orange County Water District’s principal process engineer, told C&EN.
Though successful, the idea hasn’t caught on. Proposals in other states have met with resistance from residents and politicians. The cost and “yuck factor” are the leading complaints against indirect potable water reuse. To put the whole issue into perspective, traditional waste water plants release non-potable water into rivers everyday. Cities and towns downstream are already cleaning that water to make it safe for drinking. Sewage purification, though expensive, cuts out the middle man.
(Image of pipes courtesy of the New York Times)
(Image of “three steps” courtesy of Wired.com)
(Image of Davy Crockett courtesy of WJBF News Channel 6)
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