Gravity Thickeners | Activated Sludge Process | Wastewater Treatment Plant

 Gravity Thickeners 

Gravity thickening of wastewater sludges separates solids from the sludges that are being treated.

Solids which are heavier than water are compacted to the ground of the thickener by the weight of the solids which are still settling.

A gravity thickener is designed in the same way as a primary or secondary clarifier, and it primarily consists of:

• The sludge is introduced into the thickener via the inlet and distribution assembly.
• Sludge rake for transporting sludge to a sludge hopper – rotates slowly to transport settled solids to the tank's center.
• Pickets or vertical steel members – gently stir or flocculate the settled sludge and release trapped gas to prevent rising sludge.
• Weir for effluent or overflow – collects and removes effluent or thickener overflow.
• Equipment for removing scum – collect and remove floating debris

Gravity thickeners operate based on the following factors:

• The type of sludge
• The feed sludge's age
• Temperature of sludge
• Depth of sludge blanket
• Detention times and loadings for solids and hydraulics

Because secondary sludges contain more bound water than primary sludges, they are less dense and thus less suitable for gravity thickeners.

Bound water is a water which is present inside the sludge cell mass or is firmly held on colloidal particle surfaces and is one of the causes of sludge bulking.

The performance of a gravity thickener is influenced by the following factors:

• The sludge collection mechanism's speed
• Changing the rate of sludge withdrawal
• Controlling the depth of the sludge blanket

The flow through a thickener is typically continuous and should be as consistent as possible.

Surface water and effluent should be clear and free of solids and gas bubbles under normal operating conditions. The effluent is returned to the plant's headwork's for further treatment.

The depth of the sludge cover is typically between 5 and 7 feet deep and the speed of the sludge collectors should be sufficiently rapid to enable the settled solids to move towards the sludge collection swamp.

The typical loadings and output concentrations vary depending on the sludge type and thickener operation.

Troubleshooting gravity thickeners, like any other wastewater treatment process, begins with visual inspections and an understanding of the expected results by comparing design values with operating criteria. Typically, performance can be determined by observing items such as liquid surface and effluent quality, as well as being aware of unusual odors.