Introduction to wastewater treatment

Two fundamental terms are used to evaluate the quality of wastewater treatment systems:

influent: wastewater flowing into a treatment plant

effluent: treated wastewater flowing out of any treatment unit (the effluent should be better than the influent as a result of treatment).

Two more terms are fundamental to following the progress through a wastewater treatment facility:

sedimentation: the process used in both primary and secondary wastewater treatment, that takes place when gravity pulls particles to the bottom of a tank (also called settling).

sludge: any solid, semisolid, or liquid waste that settles to the bottom of sedimentation tanks (in wastewater treatment plants or drinking water treatment plants) or septic tanks. 

With these terms defined, we can now talk about the first major phase of wastewater treatment, referred to as preliminary treatment.

Preliminary treatment is used to condition the wastewater. Very little reduction in wastes goes on. Instead, it aids the processing that occurs downline. Examples of preliminary treatment include the following.

1. screening- to remove large objects, such as stones or sticks, that could plug the flow through treatment units.

2. grit chamber: a chamber or tank that slows down the flow of wastewater. By doing so, heavy, large solids (grit) settle out and are removed.

Primary treatment is the second major phase of wastewater treatment. It uses physical treatment methods (later on, we will see that secondary methods use biological methods). It removes settleable or floating solids only. When completed, it generally removes less than half of the suspended solids and BOD in the wastewater. By far, the most common method for primary treatment is the primary sedimantation tank (also called the primary settling basin) as shown below. It is a vessel in which solids settle out of water by gravity. The settleable solids are pumped away (as sludge), while oils float to the top and are skimmed off. Sedimentation tanks can also be adapted for secondary and tertiary processes, and can also be used to treat drinking water.


Notice several things about the above graphic. First, the gravitational settling of sludge is sent off to another area for further treatment and disposal. Second, the treated effluent is sent off for further secondary (biological) treatment. And third, this particular design skims off the grease and other floating scum for further treatment and disposal.

The diagram below shows another design. Notice that the "scrapers" move very slowly to help collect the sludge that settles to the bottom.


Primary sedimentation tank

The final diagram given below is the Imhoff tank (not to be confused with the Imhoff cone mentioned above, which is simply glassware used in the lab). Pay special attention to the small opening at the bottom of the smaller chamber. This Imhoff tank is used in smaller municipalities.

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