Montana Earth Science Picture of the Week

When you flush, where does it go?

There are three ways that we deal with sewage and other wastewater from modern homes. For those who live in one of Montana's larger cities, a wastewater treatment plant (a.k.a. sewage treatment plant) is used. In smaller communities, wastewater most likely ends up in a "lagoon" or series of lagoons. For homes that are not connected to community systems, septic systems are used to treat the material that goes down drains and toilets.

The dirty work . . .
This photo shows the "septic tank" portion of a septic system for a house that was being built near Montana City. Eventually the tank was covered with dirt. Once completed, gravity will pull wastewater from the house (through a pipe) into the concrete box where solids will settle to the bottom. Helpful bacteria will beak down this solid waste into a material referred to as "sludge". Lighter solids such as hair and grease will float to the top. Periodically both of these layers will need to be removed by a septic pumping business. NOTE: A baseball cap has been placed on the septic tank to give you a sense of how big it is.

Lawns like it . . .
The liquid portion of the wastewater, called "effluent" flows out of the tank through a pipe that takes it to a drain field. The drain field (a.k.a. leach field) consists of a series of underground pipes that are surrounded by gravel so that the effluent can easily trickle out through holes in the pipe(s) and soak into the soil. The soil acts as a natural filter, removing disease-causing bacteria, nitrates, and phosphates. The effluent provides moisture and nutrients, so grass and other vegetation tend to grow well above the drain field.

Groundwater quality . . .
Modern septic systems are effective in most situations with proper maintenance. As long as there are several feet of the right kind of soil between the drain field and the water table, groundwater contamination should not be a problem. However, in some situations where the water table is high, nitrate contamination from human waste may be a concern. Water tables tend to be close to the surface near lakes and streams, and near the lower parts of valleys. Nitrates that seep into lakes or rivers from septic systems can contribute to algae blooms, leading to lower levels of dissolved oxygen in the water. High levels of nitrates in drinking water is very unhealthy for babies, and may lead to a disease called "blue baby syndrome". Furthermore, the presence of nitrates in well water is an indicator that effluent from the septic system is affecting the quality of the groundwater.

Inspection programs in the future? . . .
Recent testing of water from some wells in the Helena Valley Aquifer found all sorts contaminants, including trace amounts of medications and sunscreens. Medications enter the septic system through the urine of those taking the medication (anti-depressants, hormones, etc.), while sunscreen and other personal care products enter as showers drain into the septic tank. Concern about the quality of water in the Helena Valley Aquifer has prompted the county to initiate a program that requires septic system owners to have their systems tested on a regular basis.

Courtesy of Thurston County (Washington State) Public Health & Social Services Department

Term: blue baby syndrome


*How septic systems work
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