Water Recycling Method

Water Recycling Method

Source: https://www.edwardsaquifer.net/waterrecycling.html

[Reproducing it here for study purpose]

Step 1: Physical Systems

Physical processes are the first step in the water recycling process.  These include screening, grit removal, and clarification. Raw sewage passes through bar screens which are simply metal rods immersed in the influent flow to separate large objects such as sticks and rags from the water.  They are used to protect pumps and other rotating mechanisms further in the treatment process.  After the water passes through bar screens, it enters a grit removal system.  Here the influent flow is slowed and a mechanical separation of sand and gravel is achieved by systems such as addition of air or inducement of a vortex. Primary clarifiers allow further slowing of the wastewater so that settleable organics precipitate to the bottom while fats, oils, and greases float to the top.  These physical processes remove approximately 30 to 40 percent of the contaminants in wastewater.

Raw sewage
	Raw sewage is 99.9% water.  Large objects such as sticks and rags are removed from raw sewage as it passes through bar screens.

Vortex grit removal system
	A newly installed vortex grit removal system at San Antonio's Dos Rios Water Recycling Center in spring of 2014. It replaced a system that used injection of air to promote separation of grit and sand.

Primary clarifier

Spet 2: Biological Systems and Final Clarification

Biological processes remove most of the rest of the contaminants.  Microorganisms consume the organic material as food, and when it becomes part of their cell mass, it is now converted into a form that will settle out in a final clarifier.

The microorganisms do their work in a chamber called an aeration basin, where air is mixed with the water so the bugs have oxygen to survive.   Many operators of water recycling plants consider themselves "bug farmers", since they are in the business of growing and harvesting a healthy population of microorganisms.  Because the process is biological, any chemical or substance harmful to life can interfere with the operation of the plant.  This is why cities prohibit discharges of untreated industrial wastes to sewers and promote education among citizens regarding the harmful effects that dumping household chemicals can have on the treatment process.  When the water recycling plant cannot operate properly because chemicals are killing the microorganisms, water reuse programs are jeopardized and the quality of water discharged to receiving streams is lowered. 

Aeration basin
	Air is mixed with the partially treated wastewater so that microorganisms can survive to consume organic material in the water. The vast amounts of air being pumped into the chamber give it the apearance of boiling.

Final clarifier
	After water spends 4-6 hours in the aeration basin, it enters a final clarifier where material is once again allowed to settle out, just as in the primary clarifier.

Return activated sludge

Here's where the Return Activated Sludge process gets its name. As sludge settles to the bottom of the final clarifier, it forms a blanket in which the material at the bottom is several days old. This sludge is rich in microorganisms - is is biologically activated. But they haven't had much to eat in several days, and the stress from lack of food causes them to reproduce. The large screw pump returns some of this activated sludge back to the aeration basins. This jump-start the process with hungry bugs that quickly cause a population explosion.

Step 3: Chemical Systems

After the bugs do their work and the water is clarified for a second time, chemical systems such as chlorine contact chambers are used to kill the remaining microorganisms not captured in final clarifiers.  It is not desireable to have residual chlorine in the rivers and lakes, however, so chlorine is then removed using sulfur dioxide or vigorous aeration.  This protects the aquatic life in the receiving stream.  Using and storing highly toxic chlorine gas poses risks, so many facilities are beginning to use ultraviolet radiation instead of chlorine to provide final disinfection of water.  The point where treated water is discharged into a stream or body of water is called the outfall.