Secondary Clarifier | Activated Sludge Process | Wastewater Treatment Plant

Secondary Clarifier

Sludge Blanket Level

• The formation of a sludge blanket is caused by the high volume of flocculant solids in the secondary clarifier. The sludge blanket's thickness varies and can become so thick that biomass is washed out of the clarifier and into the final effluent. Experience must be used to determine the optimal sludge blanket level, which must provide adequate settling depth and sludge storage.
• Secondary clarifiers typically have a thickening depth of 2-3 feet, a buffer zone of 3 feet between the thickened sludge and the clarification zone, and a clarification depth of 8 feet.
• To avoid biomass loss from the clarifier, the operator should keep accurate records of the sludge blanket levels. The sludge blanket level can be manually or automatically monitored.

 

• Manual – Sludge Judge

Manually measuring the sludge blanket level with a clear plastic tube with a check valve on the bottom (e.g., Sludge Judge) is possible. This device is simply lowered slowly until it reaches the bottom of the clarifier. As the tube is raised out of the tank, a check valve on the bottom of the tube traps the solids and water inside. The tube has markings one foot apart to assist in determining the location of the sludge blanket's top.

• Automatic 

An automatic sludge blanket monitor is a more sophisticated (and more expensive) method of monitoring sludge blanket level. These devices come in a variety of sizes and configurations, and they detect the interface between clear water and sludge blanket using a variety of technologies. In real time, these devices can measure the depth of the sludge blanket as well as the concentrations of solids in the clarifier.

Sludge Return Rate 

• The sludge return rate, also referred to as the return activated sludge (RAS) rate, is critical for achieving the desired level of treatment by controlling and maintaining an adequate MLSS concentration in the aeration tank.
• RAS pumping rates for large plants are typically 50 to 100 percent of the wastewater flow rate, while RAS pumping rates for small plants can be up to 150 percent of the wastewater flow rate.
• Inadequate RAS pumping rates can cause a sludge blanket to form in the secondary clarifier.
• The flow rate of return sludge should be adjusted to keep the sludge blanket as low as possible.

Floating Solids on Clarifier Surface

• Floating solids on the clarifier surface indicate a problem known as "rising sludge."
• This problem occurs when the DO content in the secondary clarifier drops, causing anoxic (lack of oxygen). Nitrifying bacteria convert nitrate to nitrogen gas under anoxic conditions. The floc particles float to the surface as a result of the nitrogen gas bubbles clinging to them.