Wastewater contains numerous pollutants like wood, plastic, oil, inorganic & organic substance, chemicals, sand, microorganisms, etc. Therefore, such wastewater should be treated before releasing into the natural water stream. 

Many equipment and mechanisms are used in wastewater treatment plants to remove contaminants from water and produce clean water for reuse. One of the widely used and integral parts of wastewater treatment is clarifiers. 

What are clarifiers?

Clarifiers are settling tanks and have mechanical means to remove deposited solids continuously. In clarifiers, solids will settle down at the bottom of the tank, and they will be collected by scrapping. Moreover, coagulants are added that boost floc formation and enhance the performance of clarifiers. 

Generally, wastewater treatment plants have primary and secondary clarification processes.

Primary clarifiers are located downstream of the plant and are used to remove solids pollutants from incoming raw wastewater. On the other hand, secondary clarifiers are located near biological treatment facilities before aeration basins or filters. The primary task of secondary clarifiers is the clarification of wastewater and the thickening of sludge using chemicals.

Types of clarifiers

• Radial sludge scraper

Radial sludge scrapers are built to remove sludge from the bottom of the primary clarifier and are used as radial primary clarifiers in wastewater treatment plants. Major components of radial scrapers are a bridge, platform, flights with scrapers, and driving trolley. 

It operates in continuous mode, and a driven trolley activates the scraper flights. Finally, the scraper moves the sludge to the clarifier’s center, where it is directed for further dewatering. 

• Radial suction scraper

Radial suction scrapers are intended to eliminate sludge from primary clarifiers, and it contains components like a bridge, sludge receiving collector, sludge intake devices, and a driven trolley. The bridge allows access to the center of the sludge sucker, and the driven trolley comes with variable speed. 

Design of clarifiers

When designing clarifiers, water and wastewater treatment plant manufacturers consider factors like percent removal, hydraulic detention time, surface loading rate, weir overflow rate, biosolids pumping, BOD & SS removal, overflow rate, and settling velocity. 

However, one of the significant factors neglected by many treatment plant manufacturers is side water depth and its effect on the clarifier’s performance. Historically, detention time was considered in selecting side water depth, but currently, side water depth is determined independent of detention time. 

What is side water depth?

Side water depth is the measurement of the depth of water held in the basin, and the depth measurement will be taken near the wall of a unit. Hence, it is known as ‘side water depth.’ 

Importance of side water depth in clarifiers

Side water depth is determined by the distance needed between effluent weirs and sludge blanket, and adequate side water depths are required to prevent sludge blanket solids from entering the effluent lauder. 

In secondary clarifiers, deeper side water depth is likely to improve effluent quality, ranging from 14 to 16 feet. Moreover, the typical side water depth for primary clarifiers ranges from 12 to 14 feet.

It is essential to maintain enough depth to maintain safe operations and avoid the ‘gross carryover’ of solids. Moreover, insufficient side water depth can hinder the proper operating procedure. 

Designing of side water depth

• • If you employ industrial storage tanks with grit collectors, you can keep less side water depth because the material acts according to Stokes law. 
  • In wastewater treatment primary units, side water is determined by holding solids inventory.
  • In chemical clarifiers, side water depth is determined based on solids inventory and detention time for flocculation and separation. 
  • Often, side water depth is determined based on the sludge detention time and the need to handle additional mixed liquor suspended solids  (MLSS).
  • When determining side water depth, a vital factor is examining different settling zones created by activated mixed liquor. 

Those zones are: Clarified water zone, individual particle zone, hindered zone, transition zone, compression zone, and alternate concentration zone. 

 

However, the top and bottom zones are critical for final clarifiers because, 

• The bottom concentration zone is affected by mass loading and removal devices.
• The top zone also called the clarified zone, is affected by the overflow rate and the type of inlet. 

Clarifiers are an integral part of the wastewater treatment system because they remove the majority of large solids from water and prevent downstream equipment like pumps, motors, and pipes from extensive damage and clogging. In addition, when used before filtration, it averts the blockage of membranes and reduces the maintenance costs of filters.