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Understanding the Difference Between Primary and Secondary Clarifier Processes
Monday, February 15, 2021

Wastewater clarifiers are expensive to maintain, but using them incorrectly can cost you more than just a fine imposed by regulatory agencies.

Not following the proper standards and procedures can result in an entire community burdened by contaminated drinking water, which can transmit diseases such as cholera, diarrhea, typhoid, polio, and more.

This is why it's a must that wastewater operators know the difference between the primary and secondary clarifier. It might be easy to get them mixed up, but there is one clear physical property differentiating between the two.

The Density of the Sludge Handled

Each clarifier carries a distinct purpose in relation to density. The primary clarifier is designed to dispose of inorganic solids floating at the surface. It also tackles solids settling at the bottom. In this unit, the sludge is less dense.

In the secondary clarifier, 100 percent or nearly 100 percent of the sludge, is organic. In this clarifier, the sludge is compact-ready and significantly denser.

The Primary Clarifier Process

Also known as sedimentation, the primary clarification process can be described as the removal of inorganic solids before biological treatment. The step-by-step process is as follows:

  1. Water enters the clarifier tank.
  2. The inorganic solids (scum) floating on the surface are removed by skimmers.
  3. The sludge settling at the bottom is collected by a rake and then removed by the sludge removal system.
  4. Biologically treated effluent leaves the clarifier over a weir.

A good primary clarifier is expected to remove 90 to 95 percent sludge, 40 to 60 percent scum, and around 25 to 50 percent Biological Oxygen Demand (BOD).

What Influences Primary Clarifier Efficiency?

The objective of any primary clarification process is to remove as much waste as possible, but there are some factors that can prevent the best results.

  • Hydraulic overload and hydraulic underload: Overload decreases hydraulic detention time and underload prevents the equipment from working at full capacity, wasting its use and energy.
  • Sludge buildup: This decreases tank volume.
  • Highly concentrated waste streams: This includes a high solid count and high BOD levels. In this case, the clarifier will have trouble processing its sludge volume and won't operate properly.

Proper management of the primary clarifier process will ensure smooth transitioning to the secondary clarifier process.

The Secondary Clarifier Process

The secondary clarifier can be described as a circular basin where effluent from the activated sludge process is held. The process is as follows:

  1. The biomass of microorganisms settles to the bottom in the form of activated sludge.
  2. After settling over a period of time, this biomass of microorganisms is returned to the first aeration tank.
  3. This cycle repeats until only clean water is left.
  4. The clean water that remains flows to the top of the clarifier and is further sent for disinfection.

More often than not, there is excess biomass after each settling process is completed. It has to be thickened and pumped into a digestion tower alongside the primary sludge, without oxygen. This produces methane gases which quickens the sludge removal process.

There is Much More To Know About Waste Water Management

Understanding the difference between the primary and secondary clarifier is only one piece of the puzzle. There's a universe of information regarding wastewater management.

Here at Entech Design, we not only provide educative material regarding water purification treatment on our blog page, but we also provide products and services for all your wastewater management needs.