From the headworks the water flows to primary clarifiers--tanks through which the wastewater flows extremely slowly, not unlike the settlement basins at the water plant, and with the same goal: allowing solids to settle to the bottom, creating a "primary sludge blanket," though here grease and oil also float to the top. Scrapers make a circuit along the surface, scooping grease into a small flume, then cycling to the bottom where they shepherd the sludge to its own pumps. Water cleanliness is measured by biochemical oxygen demand (BOD)--the amount of oxygen the bacteria in the water use to remove its organic impurities. The lower the BOD, the smaller the next basins can be and the lower the plant's operating costs. This is a crucial point, given how much energy the next step takes. That step is what Lynch calls "the absolute heart and soul" of his plant: the activated sludge process.
So after a few hours in the clarification tank, the water flows into aeration basins, six concrete pools of several million gallons apiece, the bottoms of which are crisscrossed by air nozzles. These enormous tanks of what looks like boiling brown sewage are just what you imagine when you think "sewage treatment." But it's not boiling: What's happening is aeration, which provides oxygen, in the presence of which bacteria love to eat poop. "We are bug farmers," Lynch says. Bacteria in the basins multiply rapidly, like the starter for sourdough bread. The wastewater provides the food, the nozzles provide the oxygen, and the bacteria feel like they're on a cruise: nothing but breathing, eating, and reproducing, with free food all day long.
This process removes all the harmful chemicals from the water--except nitrates, which feed algae in rivers. These algae propagate wildly and then die. "And the decomposition sucks up all the oxygen in the river, killing the fish. So now we're going into the same tanks and we put walls in there and we have different zones where we stimulate the bacteria to do different things" that remove nitrogen. After about a day of that, the water spends 2 days flowing very slowly through secondary clarification basins: a dozen large circular tanks, each the size of an aboveground swimming pool. "It's very quiet," Lynch says. "The bacteria settle down and create what we call the sludge blanket," which sinks to the bottom and is pumped out. The water trickles out through V-notch weirs around the top of the tank, by which point it has a barely yellowish tinge and no odor whatsoever.
We'll get to the next stage of water filtration, but what's more interesting at this point is what happens to the sludge. Most of it becomes RAS--return activated sludge--and rejoins new wastewater in the aeration basins, providing the starter for a whole new treatment reaction. Bacteria can go around the cycle half a dozen times, Lynch says, but eventually they become WAS--waste activated sludge. In the WAS stream, the bacteria go to four big covered tanks called aerobic sludge digesters, in which instead of wastewater nutrients they eat each other. We climbed up to the top of one of the basins and looked through a porthole inside: You could almost hear the screaming. Okay, not really; it's just a tank of sludge. "If it's good sludge, it's got that nice brown color--it's oxygenated, and it don't smell bad up here," Lynch says. If it's black, something's out of whack--it probably needs more oxygen.
Aerobic digestion, Lynch says, is actually inefficient--it costs money, since the plant has to pump in oxygen; anaerobic digestion, to which the plant hopes to switch, is the opposite: It generates methane, which the plant can use to generate electricity to operate its pumps and blowers; the waste heat from the generators can even help provide the heat the anaerobic digesters need to keep the bugs at optimum eating-each-other temperature. "There are plants now that generate their entire power needs from that methane," he says. "They pull no energy from the grid."
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16 Comments
Add CommentExcellent article, kudos to SA for reprinting it. I've been studying water and wastewater treatment with an eye toward employment. The more I learn, the more impressed and fascinated I am by the ingenuity that had made WWTPs amazingly efficient and virtually smell-free, producing very clean water. It's unfortunate that most people can't or won't ever visit one of these high tech plants to see how much they aren't like you'd expect, and how sensible it is to re-use tertiary treated water.
Reply | Report Abuse | Link to thisRecycled water is the only thing that makes sense in a space colony. Of course, you could reduce all the water in sewage to hydrogen and oxygen, then burn it, if you have enough energy. I'm always looking for alternative concepts. Got ideas?
Reply | Report Abuse | Link to thisExcellent article, with information that should be, but is generally not, understood by citizens. Many still protest the fees they must pay for wastewater treatment as part of their utility bill. I hope this lengthy article can be condensed into newspaper features and municipal handouts to better educate citizens about a vital service. The city of Arcata, CA, has a very interesting wastewater treatment system that may warrant the attention of the author and readers. Link: http://www.cityofarcata.org/departments/environmental-services/water-wastewater/wildlife-sanctuary
Reply | Report Abuse | Link to thisone of the best articles i've read in along time....contained both historic and current detail...thanks Scott for your effort...this should be one of the main articles in sciam...not just a link to the side...then again 11 pages might be to much for most;-)
Reply | Report Abuse | Link to thisThe four letter word for this stuff is actually an old acronym from the days of sail. When guano (bird droppings) was discovered in huge quantities on pacific islands and transported back in bales in the cargo holds of wooden sailing ships, any stored in the very bottom would get wet, give off methane gas which would be ignited by any sailor with a candle doing inspections. Several ships were lost before survivors told of huge explosions and fires that happened during inspections and the problem and mystery was resolved. Thereafter, maritime laws required all guano bales be marked "Store High In Transit".
Reply | Report Abuse | Link to thisQuestion; Does it make sense to pump city water from wells (from the aquifer) use it, treat it, and dump it in the river-shed?
Reply | Report Abuse | Link to thisA rural home pumps from a residential well, uses it, treats it and returns it to the ground. Not a river headed for the Gulf of Oil.
I agree wholeheartedly with several others here that this was a fascinating and well-written article. Who knew sewage treatment could be so interesting?
Reply | Report Abuse | Link to thisA well written story on the history of used water distribution and treatment around the world. Ending off with a a fine and entertaining write up on the process of water reclamation. I love the point which says that what the used water treatment plant is doing is in fact mimicking what the river is doing, just quicker!
Reply | Report Abuse | Link to thisA well written story on the history of used water distribution and treatment around the world. Ending off with a a fine and entertaining write up on the process of water reclamation. I love the point which says that what the used water treatment plant is doing is in fact mimicking what the river is doing, just quicker!
Reply | Report Abuse | Link to thisThis is hogwash, or perhaps a good example of the word it purports to define. See the Etymology section of http://en.wikipedia.org/wiki/Shit, which shows the origins going back to Roman times.
Reply | Report Abuse | Link to thisThis text was meant to reply to the comment on the origin of the s-word, not a comment on the article or reply to any of the other comments:
Reply | Report Abuse | Link to thisThis is hogwash, or perhaps a good example of the word it purports to define. See the Etymology section of http://en.wikipedia.org/wiki/Shit, which shows the origins going back to Roman times.
The article itself was wonderful, a fascinating read, with a mind-boggling amount of information I could hardly wrap my mind around (though Mr. Huler's style certainly was as clear as . . . triple-treated waste water!).
I live in flat Phoenix and I'm really wanting now to know how our system works. And all the other infrastructure we ignore but wouldn't want to be without these days.
Thanks for the kind words; mm, you beat me to the punch on the etymology. And in answer to Quinn, no, I don't think that makes sense; surface water -- like water from a reservoir -- basically just makes its way to the ocean through the city and its residents. That's where it was going anyhow, so wastewater generated by a community with surface water as its source can reasonably be released back into the stormwater system -- the riverbasin from which it emerged.
Reply | Report Abuse | Link to thisAquifer water is on the same journey, but it may take much longer, and continued pumping from an aquifer can deplete it; using septic systems to replace wastewater from such systems into the aquifer is probably a wiser move. Just the same, in my experience most houses on pumps -- using aquifer water -- are also using septic systems too, so the wastewater just trickles back into the ground.
There's a long chapter in "On the Grid" about stormwater -- much current stormwater treatment is about keeping it where it lands and helping it soak into the earth, instead of using curb and gutter to sluice it into creeks. It has a job to do as part of the hydrological cycle, and we do ourselves and the planet no favors by getting rid of it so quickly.
Many sewerage treatment stations only work at around 15% efficiency, because the operators cut the electricity supply to aerators and turbines to reduce expenses. The liquids that are then put into rivers are therefore a serious cause of pollution.
Reply | Report Abuse | Link to thisPyrolysing sewerage is a new, simpler and cheaper technique that should totally replace traditional plant. See www.eprida.com for more details.
How sewage treatment works in this country is so complicated! Why all the bother?
Reply | Report Abuse | Link to thisThey've got it figured out in Kibera; who needs sewers? Just poo in a bag and throw it in the street!
Thank you for this article; it has been a wonder to me that we honor or police and firefighters (rightfully so) but never give a second thought about those water and wastewater professionals who also keep us safe and healthy day in and day out. This too is a dangerious profession, there are more people who die in water/wastewater industry than in the police/fire profession. Again, kudos, keep up the good work.
Reply | Report Abuse | Link to this@cwclark: I did not know that -- do you have a source for that statistic? It's important and I will share it. Thanks for your kind words.
Reply | Report Abuse | Link to thisScott