The Poop Question: What Happens After You Flush?

It might be something you've never considered, or it might be a question you've never had the guts to ask.  Either way, it's one of life's little curiosities: what happens to our waste after we flush the toilet?  Where does it go?  Many of us may be aware that cities and municipalities in the United States run wastewater treatment plants where sewage is processed, but few of us have any real understanding of what these facilities do.  In this article, we are going to shed some light on the mystery of where all our poop goes after the toilet is flushed.

The goal of any wastewater treatment plant is the same: to change contaminated water into water that is safe to be discharged back into the environment.  Wastewater entering a sewage treatment plant may be contaminated with physical debris such as cans or bottles, chemical pollutants like household cleaners, and biological contaminants, including the pathogens from our waste.  All these contaminants must be removed before the water is considered clean and non-toxic.  To this end, wastewater will go through various processes usually referred to as primary, secondary, and tertiary treatment.

Primary treatment can be thought of as the "settling" stage.  Wastewater is placed in very large sedimentation tanks where sludge settles to the bottom and grease and oils rise up to the top.  The sludge is removed so that it can be separately processed, and the grease and oil are skimmed away.  The end result is a homogenous liquid that moves into the secondary stage of treatment.

Secondary treatment removes the biological contaminants that are polluting the water, or at least reduces them to an acceptable level.  Quite simply, this is done by exposing the water to various types of bacteria that literally eat the pathogens out of the water.  There are different types of processes to accomplish this, but the majority of wastewater treatment plants use aerobic processing.  This means that the bacteria need oxygen in order to break down the pathogens in the water.  In aerobic processing, the water will need to be aerated so as to provide sufficient oxygen to the bacteria.

Tertiary treatment can mean a number of different things, and so any process that takes place after secondary treatment is generally considered tertiary treatment.  Some examples of tertiary treatments might include filtering the water to further remove any suspended matter.  Additional reduction of biological or chemical contaminants would also be a type of tertiary process.  If wastewater has a high level of nutrients such as nitrogen or phosphorus, then it may need to undergo tertiary treatment to remove the excess nutrients so as to prevent algal blooms once the water is released back into the environment.  Excessive algae growth on water is a problem because it deoxygenizes the water to the point that fish and other wildlife can't survive.

You may be wondering what happens to the sludge that was removed in the first step of the process.  Sludge is decomposed by bacteria, similar to the bacteria that remove pathogens from wastewater in secondary treatment.  This decomposition can be either aerobic or anaerobic, but the purpose is always the same: to reduce the volume of matter and to reduce the number of pathogens in the material that could cause disease.  Disposal of the finished sludge varies.  Traditionally, most wastewater treatment plants would truck the material to a landfill and dispose of it.  However, today some plants are beginning to sell the material to companies that turn it into fertilizers that can be applied to farmland.

So that's the answer to the poop question!  Wastewater treatment isn't something that many of us care to consider, but it is a very necessary process.  In developing countries where wastewater treatment isn't regulated, there are devastating effects to the environment and public health caused by exposure to contaminated water.  In short, those of us living in developed countries can be thankful for the wastewater treatment plants that process our waste on a daily basis.