When you flush your toilet, you expect it to work. Most people don’t spend much time thinking about how it all works – where that water comes from, how it got there or where it goes when it leaves your sight. There are many things that happen on a daily basis to make this a smooth process. (Learn how to keep your sewer system at home trouble free.)
Wastewater (or sewage) comes from domestic residences, commercial properties, and industries, and contains a wide range of contaminants. It’s made up of 99% water (added during flushing to help things move down the pipes) and only a small amount of solids. If wastewater is left untreated, it can pose public health and environmental risks. Hence, wastewater treatment is “pollution control.”
Thankfully, wastewater treatment systems exist to collect sewage and deliver it to treatment plants where wastewater is treated to meet required effluent standards. The treated wastewater is then returned to the environment as effluent (e.g. discharged into streams or other waterways, or put to beneficial reuse, such as irrigation). The solid waste byproduct of the treatment process is disposed of in accordance with state and federal standards.
Waste from homes and businesses is collected through a series of pipes that lead to a sewer main, which runs underground, often down the middle of the street, for example. Manhole covers allow access to the main for maintenance purposes. Sewer mains connect to progressively larger pipes until wastewater reaches the treatment plant. To conserve energy used for pumping, natural gravity helps most of the collection go along, but when gravity is not enough, the system also relies on grinder pumps or lift stations to help “lift” sewer over a hill.
The treatment of wastewater depends on the sophistication of the treatment plant where it ends up. There are three basic treatment processes. Primary treatment removes settleable or floating solids through physical operations, such as screening, skimming or sedimentation. The system allows the heavy stuff to sink in the water while the lighter scum rises to be skimmed from the surface. The resulting raw primary biosolids are collected and dumped in a landfill, incinerated or further treated for use as a fertilizer.
Secondary treatment allows for higher treatment quality, removing a majority of the organic matter. This is done with the help of bacteria. Sewage is mixed in large tanks where the microorganisms consume everything they can. The main technique used is the activated biosolids process. In this process, bacteria eat up most of the organic matter. This biological breakdown is helped by air (i.e. oxygen). To finish off the secondary treatment, effluent (treated water coming out of a sewage plant) is usually disinfected with chlorine to kill the harmful bacteria, readying the water for release back into the environment.
Tertiary treatments (also known as advanced treatment techniques) include physical-chemical separation techniques (filtration, carbon adsorption, distillation and reverse osmosis.) These processes can achieve a high degree of pollution control and wastewater treated in such manners can be used for industrial (i.e. equipment cooling), agricultural (i.e. reclaimed water irrigation) or recreational purposes.