Municipal sewer systems are examples of combined sewers, whereby waste from each building in the system flows into a central pipe and from there to a community treatment facility. The waste isn't treated until it gets to the treatment facility, so the pipes must maintain a sufficient downward flow to keep it moving. When the terrain doesn't allow for this downward slope, the system must include a pumping station. Manholes installed at each point in the system at which the pipe changes direction or slope allow access for clearing blockages and general maintenance.
Primary and Secondary Treatment
Since the passage of the Clean Water Act in 1972, municipal sewage systems must include both primary and secondary treatment facilities. In primary treatment, large solids are screened out, and grits and other particulate matter allowed to settle into a sedimentation tank. Some treatment facilities rely on gravity, while others use mechanical means to hasten the separation. In secondary treatment, wastewater facilities use biological processes to remove the bulk of the organic matter from the water. Treatment methods include attached growth processes and suspended growth processes.
Secondary Treatment Methods
In plants that employ attached growth processes, water passes over a medium, such as rocks or plastic, on which microbes are present. The medium can be as simple as a bed of rocks or slag, but modern plants are more likely to have trickling filters made of plastic. Using oxygen in the air, microbes on the medium consume the organic matter in the water. In suspended growth process plants, the microbes are suspended in aerated water. Sewage lagoons are examples of this method of waste treatment; the water is circulated by a pump much like a fountain and is in direct sunlight, allowing the organic processes to occur quickly and efficiently.
Private Sewage Systems
Buildings that aren't connected to a municipal sewer must rely on a private septic system that typically consists of a holding tank and a drainage field. The septic tank is the primary treatment facility, functioning like the sedimentation tank at a municipal treatment plant. In most septic systems, water overflow from the tank flows directly to a drainage field, where it is distributed by perforated pipes to leach into the soil. If the soil is too dense or clay-like for efficient dispersal, the system may employ an aeration tank or bio filters to pre-filter the water, or the water may flow through a system of drip lines and emitters, much like those used for irrigation.