How Wastewater Treatment Works
Wastewater treatment is a relatively new development, though humanity has use plumbing to direct wastewater for thousands of years. As recently as 1940, major cities in the US like Boston, St. Louis, Pittsburgh and Kansas City did not treat their wastewater. Everything that went down the drain or was flushed down the toilet went straight into a nearby river, harbor, or lake.
In 1940, New York City treated about 1/4 of its sewage. It didn't treat all of its wastewater until 1986. Milwaukee has been treating their wastewater and selling Milorganite fertilizer since 1925. Milorganite is short for MILwaukee ORGAnic NITrogEn, and they ship out tons of it every year. There are benefits of wastewater treatment other than just clean water.
Wastewater comes from many uses. Besides what gets flushed down the toilet, there's water disposed of after washing dishes and clothes, bathing ourselves, our pets, and our cars. It is estimated that the average person in the US uses 100 gallons of water a day, and the large majority of that either waters our lawns or becomes wastewater, eventually.
How Does Wastewater Get Treated?
Disposal or Reuse
This decision determines what treatment wastewater calls for. Wastewater that is going to be disposed of needs to be treated, but not to the standards that are called for if the water is to be recycled for reuse. Acceptable levels of impurities in wastewater that will be disposed of vary, depending on where it will be disposed.
Wastewater disposed of in the ocean is subject to standards set by international treaties. Wastewater fed into rivers that cross international borders must fit standards set by a treaty between the affected countries. Wastewater in rivers that are entirely within a country's borders must satisfy standards set by states and cities downstream.
Reclaiming wastewater for reuse must live up to even higher standards. In the US, tap water quality is regulated by the EPA, and their standards are often more strict than standards for bottled water. Yet parts of the US have been doing this since the 1970s.
The drinking water supplied by Fairfax Water in northern Virginia has contained reclaimed water since the 1970s. About 5% of the area's drinking water is from reclaimed wastewater.
Orange County in California also began purifying wastewater in the 1970s, and they have increased the capacity of their purification plant so much that it serves 20% of the area's water demands. Singapore now uses reclaimed wastewater to supply almost 1/3 of their freshwater needs.
A typical water treatment plant uses large concrete pools or tanks to clean its wastewater, in several different stages.
Sedimentation means letting solids settle to the bottom of the tank or pool, preventing anything that isn't liquid from traveling further into the process. Removal of greases and oils is also done in this stage, with the oils reclaimed and either used as fuel or turned into soap. Debris that floats is also removed and disposed of.
After the sediment has settled out, the next step is the aeration tank, where oxygen is added to the wastewater in order to maintain the microbes that eat the organic material that is dissolved or suspended in the water. These microbes perform the same job in rivers and lakes, as well. When the water tests low enough in organics, it is moved to the next step.
In this tank, the microbes that are running low on food become dormant, clump together, and settle to the bottom of the tank. They are then recovered and returned to the aeration tank to clean the next batch of water. The remaining water from this tank is then pumped to the next step in the process.
The wastewater is cleaned further by passing it through a filtering substrate. The filter is made of layers of gravel, sand, and coal, often anthracite coal. Inorganic particles become trapped in these filter layers, and the coal, just like charcoal, is an effective purification agent. After this step, the water is clean enough to dispose of safely or be processed further for reuse.
The next step in purifying the ex-wastewater is a much finer filtration process. A membrane with a pore size of 0.1 – 10 µm allows water to pass through, trapping impurities and bacteria. Viruses are smaller than bacteria, and many of them get through this stage.
Reverse Osmosis Filtering
Reverse Osmosis uses a semipermeable membrane that blocks salts, the smallest of viruses and pharmaceuticals to purify water.
The last of the trace biological contaminants are killed. Intense ultraviolet light or hydrogen peroxide can be used to perform this task.
A Macro View of Wastewater Treatment
The water that passes through the entire process is just as clean and pure as any other tapwater. This is a known, reliable technique that still meets public opposition, mostly because of the 'icky' factor, but that opposition has no real reason behind it.