One of the less favorable jobs I’ve had at work over the years is fixing sewer backups. I want to apologize in advanced for anyone who is grossed out by this topic. My reason for writing about it anyway is that sewer systems are in fact essential infrastructure in our lives. They allow for convenience and improved sanitation. Before the advent of the modern sewer system, disease was spread through contaminated drinking and bathing water. Sadly, certain underdeveloped countries still lack adequate systems. I plan to discuss what makes up such a system,and how it works.

Not many people know what actually happens when you flush a toilet. Fortunately (or unfortunately) I have spent a little time working on the systems that manage our waste and want to share how it works. I don’t have experience with the urban setups like they have here at Penn State, so I will just talk about rural/suburban infrastructure.

When you flush a toilet or use a sink, or take a shower, the effluent flows down, through a “trap” and then through a network of downward sloping pipes. The trap is basically a u shaped piece of pipe that is designed to “trap” some of the water flowing through it. The point of this is to block the passage of sewer gasses from backtracking into your bathroom and making it smell bad. The pipes are sloped (as a rule of thumb 1/4 inch drop per foot) so that gravity does all of the work moving the effluent along. Old pipes were made of cast iron and new pipes are PVC. The drain pipes from each toilet shower bath and sink link together and flow downwards until they reach outside of the house. At this point, the main sewer line goes into what is called a septic tank. The septic tank is a large buried concrete box designed separate solids from liquids and allow for bacteria to consume the waste. Over time healthy bacteria will consume much of the solids and dissolve it into liquid form. On the opposite end of the sewer main entrance there is a outlet pipe. All of the liquid from the septic tank flows out of outlet pipe and into the distribution box. The so called “d-box” is pretty much just a fitting that allows for multiple drainpipes in the drain field. The liquid flows from the d box into perforated drain pipe ( pipe with holes). These drain pipes are under ground like the rest of the system and are run in long lengths away from the septic tank. The purpose of these is to allow for the liquid to drain into the ground. Although this may seem odd, bacteria and fungi in the soil work to digest the liquid and any chemicals in it and return it to pure water. One has to pay careful attention to the type of soil the drain field is made from because sandy ones drain too quickly and therefore do not give the bacteria time to clean up the water.