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Ask an Engineer Why and How, Installment 3: What Happens After I Flush? Number 2, Water Reclamation Plants

Friday, March 1, 2019

Centralized Wastewater Treatment

In most municipalities there is a sewerage system (sewers, manholes, pump stations, forcemains, service lines – collectively “sewerage”) that collects the sewage (the wastewater in the system, no “r”) from the homes and businesses and transports it to the wastewater treatment plant (water reclamation facility, sewage treatment plant, sh## plant – they all mean the same thing) where the wastewater is treated to make it acceptable for discharge to the environment.  A very simplified and generalized flow chart of what happens at a treatment plant follows:

Flow Streams:

  1. Primary Influent (Raw Sewage)- from the sewage collection system
  2. Primary Effluent- some solids, floatables and rags reduced
  3. Secondary Effluent (or Mixed Liquor for some processes, this Flow Stream has different characteristics dependent upon what happens in the previous treatment step)- organic components and nutrients reduced
  4. Settled Effluent- floatables (scum) and solids reduced from the secondary effluent
  5. Final Effluent- the liquid stream meets the regulatory requirements for surface discharge
  6. Primary Sludge- floatables (scum) and solids from Primary Treatment
  7. Secondary Sludge- floatables (scum) and solids from Secondary Treatment
  8. Stabilized Sludge- used on farmland as fertilizer, or landfilled


Treatment Steps:

  1. Primary Treatment, the step (or steps) to reduce inorganic components (what the bacteria in the next steps can’t remove) through physical processes such as settling, straining, or skimming
  2. Secondary Treatment, the step (or steps) to reduce organic components and nutrients usually through biological processes.  With some industrial wastestreams we can (must) perform this step with chemical processes, but for most plants this involves aerobic biological processes.
  3. Secondary Settling, the physical process step to reduce floatables (scum) and solids in the treatment stream
  4. Disinfection the final step in treatment to reduce any bacteria or viruses not removed in the prior steps.  It also further oxidizes remaining organic components and can stabilize some nutrients
  5. Sludge Stabilization (digestion), reduces the volatile organics (making it less “stinky”) in the sludge.  This is either and aerobic or aerobic biological process

The most treatment is happening in Step B.  Bugs are eating the poop from the wastewater thereby “cleaning” the water.  I can list maybe a dozen different technologies that we use to accomplish this that involve concentrating microorganisms (bacteria and/or algae) and giving them the ideal amount of food (wastewater), the right amount of oxygen (a lot = aerobic, a little = anoxic, almost none = anaerobic; the bacteria that thrive in each different environment are specialists at ingesting specific nutrients and/or organic matter), and the right environment (temperature, pH…) to allow them to reproduce and form dense colonies that we can separate from the treatment stream in Step C.

The secret to this is that we treat the wastewater only to the point where it will cause no harm to the environment. Further dilution and treatment occurs in nature after we have done our job at the treatment plant. The old adage, “Dilution is the solution to pollution” governs all treatment plans.  Any stories that you’ve heard about drinking the treated effluent from a wastewater treatment plant are really just stories about crazy people.  That said, this newspaper article hangs on the side of the bookshelf behind my desk as a reminder of the importance of my profession:

June 27, 1912:

The village board are contemplating putting up a sign at the outlet of the sewer into the DuPage River, below Electric Park, so the camp dwellers can distinguish it from the spring nearby.
A few days ago some of the campers were remarking on what delicious drinking water they had found running from a tile at the river bank. Inquiry and investigation revealed the fact that it was the village sewer.

Excerpts from June, 1912 "The (Plainfield) Enterprise”


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Drink responsibly!