Archive for November, 2009

Waste Water Treatment Plants: One of The Greatest Public Health Inventions, Ever.

November 4, 2009

So my environmental microbiology class recently had a field trip to the waste water treatment plant here in Tucson.  What an experience!  Nearly all of the sewage that is produced in Tucson flows downhill (in sewer pipes, of course) towards the Pima County Waste Water Treatment Center on Roger Rd. sitting on the banks of the Santa Cruz River.

Many of us don’t even think about what happens when we flush the toilet or turn on the garbage disposal and wash our dinner scraps down the sink, but the invention of waste water treatment has made our lives healthier by removing unhealthy microbes from where we live.  Just imagine how hard life would be if we were always concerned about cholera, dysentery, typhoid fever, or giardiasis.  These are real problems in much of the developing world, and many places only wish they had waste water treatment facilities like we have in every city in the US.  Interestingly, modern waste water treatment technology is not that old and in Tucson, our modern facility was only installed in the late 70’s.  Before 1900, Tucson didn’t have a sewer system.  From 1900 to the early 1950’s raw sewage was shipped away from the city and used for irrigation.  A modern waste water treatment center was built in 1951 and it has expanded ever since.  The modern facility at Roger Rd. can handle 41 million gallons per day of raw sewage; 41 million gallons per day!!  That’s a lot of waste.

So what is modern waste water treatment?  Essentially, there are three stages to treatment:

wastewater treatment process

Courtesy of

Primary Treatment: All the “chunks” are physically removed from the sewer water, collected in a truck, and sent to the landfill.  This is the really smelly part of the process and Tucson’s facility has a tent that covers this part of the process to cut down on the smell. The rest of the “waste stream” is sent to a settling tank – called a primary clarifier so that the waste is concentrated into a “sludge.”

Secondary Treatment: The sludge from the primary treatment is sent to a system of honeycomb looking things called “Bio-Towers.” Here the waste stream “trickles” down the honeycomb to “filter out” and convert all of the biosolids (organic wastes) from the sludge to food for microbes that cover the honeycomb.  This is called a “trickling filter” for obvious reasons.  This is also where most of the waste water treatment occurs. The waste stream is sent to another tank that is pumped full of oxygen and where microbes break down the rest of the organics.


Trickling filter. (Courtesy of

The last stage of waste water treatment is to add chlorine to the water to kill any residual microbes and then it’s released to the Santa Cruz River.  Although this water has been treated and is released to the environment, it’s not a good idea to swim in or drink this water.

That’s it.  Without microbes using our wastes as food, we’d be swimming, literally, in our own sewage.  The science of microbiology has made our lives much healthier.  You might be wondering what happens to all of the solid sludge that is a product of treatment.  Well, it’s applied to cotton fields as fertilizer; don’t worry, it’s not used on any crops we eat.

Recently, I’ve been asked by several people if we’re going to be drinking treated waste water in the future in Tucson (often called toilet-to-tap).  I’ve heard many things about this and it’s not conclusive either way.  What is certain is that we currently use treated waste water for irrigation at 90% of our golf courses here in Tucson.  This is a great use for this water, because drinking water is expensive and there’s no benefit to spraying it out on our golf courses to grow grass.  In the desert, water is a very valuable resource and it’s always a good idea to conserve it where we can.  One solution that the city of Marana is exploring is to use a “duel delivery system.”  This is where treated waste water is sent to houses, separate from drinking water, to flush toilets, water lawns, etc. – uses that don’t need to be up to drinking water standards.  This will hopefully cut costs to consumers and establish a tradition of reuse and recycling our water in Southern Arizona.