How City Water Purification Works: Drinking and Wastewater

I'm Jake O'Neal, creator of Animagraffs. And  this is how city water purification works: from drinking water to wastewater  to nature and back again. Let's start with drinking water, which comes  from surface sources like rivers, lakes, and reservoirs, or from underground wells or springs.

Water treatment plants have near endless variations in design, so I've chosen major components and  processes to create a general working model. Cylindrical intake screens are  placed in a reservoir in such a way to best avoid ingesting silt from the reservoir  bottom, or floating material from the surface. A lift house with vertical pumps draws water  from a well and into the treatment plant.

The purification process starts  with coagulation and flocculation. Particles suspended in water, like clay,  sand, and some larger organic particles such as algae, have an inherent electric  charge that causes them to repel each other. Coagulants are substances  with opposite electric charge that neutralize these particles and encourage  them to stick together into clumps called "floc".

The coagulant and water is vigorously  mixed, and floc clumps start to form. To make floc particles heavier and therefore  even easier to eventually settle out, micro-sand is added as the water is  drawn through a special mixing tube, with a polymer to help the sand stick. The now flocculated water flows through a set of baffles to slow its  turbulence for the settling process.

As the water travels upwards, heavy floc particles  quickly settle to the bottom of the tank. Any remaining suspended particles  collide with an array of angled plates, and slide down to the settling zone. Slowly rotating scraper blades continuously  remove the combined sludge and sand layer from the bottom of the tank while clarified  water flows upwards into collection troughs.

At this stage the water already looks clear,  and a lot less cloudy. Now it's on to ozonation and filtration to treat microscopic taste  and odor causing agents. These can include inorganic elements such as iron or  sulfur, or harmful bacteria and viruses.

In the ozone tank, ozone gas bubbles are injected  into the flowing water through diffusers. The tank is divided, which slows the  flow and gives the water a longer path with more time for ozone to do its job. An ozone gas molecule is made up of 3 oxygen atoms.

The normal oxygen we breathe has just  2 oxygen atoms. When ozone is injected into the stream, the extra oxygen molecule can  bond with contaminants, or "oxidize" them, with various desirable results. For example,  oxidized metals have a weaker bond with water, and are therefore easier to extract.

Oxygen also bonds  with elements in virus or bacteria cell walls, disrupting their function, and altering their  surface charge for easier filtration downstream. When ozone gives up its extra oxygen  molecule it just becomes normal oxygen, leaving no byproducts in the treated water, unlike  other purification methods such as chlorination. The water flows to the filtration  stage to remove the oxidized particles, and any other remaining contaminants.

Water travels through a dense layered bed  of activated carbon granules and microsand. It's called "activated" carbon because  the resulting engineered granules, which are processed from  common materials like wood, coal, or even coconut husks, have a relatively  huge surface area with many features and pores, giving ample opportunity for contaminants to catch  or stick to the surface as water passes through. Generally speaking, the activated carbon layer  filters biological and chemical elements, while the sand layer filters inorganic  elements like unwanted metals.

Passing particles stick to a  carbon or sand granule's surface due to naturally occurring attractive  forces, called Van Der Waal's forces. The filtered water flows to final disinfection  in the UV tank. Water passes through banks of ultraviolet lights.

UV light at various  wavelengths can destroy or disrupt viral, bacterial, and other pathogen's DNA or cellular  structure, effectively destroying them. The water is now ready for public consumption. It flows into a what's called a clearwell for  storage, or into the municipal system for use.

Now let's look at what happens  to water after we use it! Wastewater flows from our homes through the  municipal collection system or city sewer pipes to a wastewater treatment plant, where it's processed  before being released into a natural water source. The headworks is a group of processes that removes large debris and heavier inorganic  particles from the incoming wastewater flow.

Water arrives at the plant mostly by gravity.  Depending on local geography, it may need to be pumped or lifted into the wastewater plant. Screw pumps are a rugged, mechanically simple design built to handle  this coarse incoming mixture.

Bar screens trap debris, including literal tons  of items that really shouldn't be flushed or sent down the drain, such as baby wipes, q-tips,  diapers, paper towels, medication, and so on. A moving platform called a “rake”  scrapes the bars, removing the debris for separate processing and disposal. The water then travels to the grit chamber to remove heavier, inorganic gritty particles  like sand, silt, clay, coffee grounds, eggshells, and so forth, while allowing lighter  organic material to pass through.

A spinning plate with fins, called an impeller, creates an axial vortex  which is a sort of vertical spiral force along its spin axis, at a specific speed so as  to catch particles in a defined weight range. These heavier grit particles are forced  down chamber walls and out at the bottom. The grit is collected to undergo its own  separate dewatering and washing process.

The wastewater leaves the headworks  towards primary clarification. Primary clarification separates  organic matter from the wastewater. Water flows in at the center of large circular  basins, called sedimentation tanks.

A baffle slows down the flow rate to aid the settling process. Floatable solids like grease and oil drift to the top of the tank. A rotating skimmer continually  pushes this material into a collection trough.

Settleable solids sink to the angled bottom  of the tank and form a sludge. Scraper arms push the sludge out of the tank into a sludge pit. This organic matter has its own purification process, and can eventually be  used, for example, as fertilizer.

A baffle at the edge keeps floating material  from mixing with outgoing processed water as it flows over a lip at the  edge of the tank called a weir. At this point, the water is starting  to look a lot cleaner and clearer. The water flows to secondary treatment,  which in our model, is an aeration tank.

In the aeration tank, helpful microorganisms  are mixed in with wastewater. These are special bacteria and protozoa that consume  biodegradable waste products, such as human waste, food waste, soaps and detergents. These helpful microorganisms also need oxygen to live, so air is vigorously pumped  through the mixture.

They either directly absorb soluble particles as food, or emit enzymes that  eventually allow solid particles to be digested. The microorganisms naturally stick together  over time, forming clumps called floc. The water and floc mixture  travels to final clarification.

Here, the floc sinks to the bottom of  the tank. Some of this settled floc is collected for re-use in the aeration process  as the helpful organisms are still active. Processed water flows over the edge of the tank en route to final disinfection  in a UV light exposure tank.

After final disinfection, the water is  ready to be released back into nature. A specially designed outfall  pipe with diffuser nozzles mixes treated water evenly while causing minimal  disruption to existing environmental conditions. Treated wastewater and untreated drinking water  almost never share the same immediate natural water system, though some rare places  in the world are challenging that model.