Common Wastewater Treatment Techniques

Wastewater is the result of water being disposed in sewer systems from storms, industry, and your home. This water is often high in disease causing microorganisms and must be treated before it is returned to the external environment. Below are the most common techniques for treating wastewater.

Preliminary treatment

Preliminary treatment is the first step in the wastewater treatment process. Once wastewater arrives near the sewage treatment plant via a network of sewer pipes, the wastewater is sent through a bar screen. This screen removes large objects such as sticks and other large debris that could damage the pumping equipment at the treatment plant. Next the wastewater travels through the grit chamber. It is here where the water flow is slowed and heavy items such as rocks and sand are allowed to settle to the bottom. Those items that are captured during the preliminary treatment are usually disposed of in a landfill.

Primary treatment

The second step in the wastewater treatment process is primary treatment. It’s here that the wastewater is sent to a settling tank called a clarifier where the wastewater is allowed to sit undisturbed for several hours. As a result, solids sink to the bottom of the tank and greases float to the top on the tank. The solids are removed from the bottom while the grease is skimmed off the top. The wastewater is now clearer in appearance and moves to the next stage of the process.

Secondary treatment

Now that he majority of the solids have been removed from the wastewater, the organic matter needs to be removed as part of the secondary treatment. Here, the wastewater is pumped with air during the aeration process and microorganism cultures are added to the wastewater. The microorganisms, fueled by the oxygen that is in the air, consume much of the dissolved organic matter and grow in number rapidly. After several hours, the aeration is stopped and the wastewater is moved to another clarifier. The microorganisms settle to the bottom of the clarifier tank and become sludge. A small portion of the sludge is pumped back into the aerator to treat the next steam of wastewater. The remaining sludge that is not used for “seeding” is removed and sent for sludge processing.

Final treatment

The wastewater is now treated with a chlorine to kill off any harmful microorganisms that remain. Once the wastewater has been chlorinated and the microorganisms have been killed, the wastewater needs to be dechlorinated. Sodium Bisulfite is most commonly used to remove the chlorine. After dechlorination, the wastewater can be discharged back into an external body of water or stream.

Sludge processing

After the sludge has been removed from the primary settling tank during the primary treatment process, microorganisms are mixed in with the sludge to reduce the odor, kill some disease causing microbes and to dissolve some of the organic matter. The sludge is then pressed or spun in a centrifuge to reduce the water content of the sludge. Lastly, the sludge is heated to remove the excess water and to kill off the remaining disease causing microbes. The sludge can now be put in a landfill, burned or used for fertilizer.
The sludge for secondary treatment does not have to go through as many steps as the primary sludge process and can be spun or pressed immediately after it is removed from the secondary clarifier. After most of the water is removed, the sludge is heated to remove the excess water and put in a landfill, burned or used as fertilizer as well.