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Sludge scraper mechanisms – Removal of settled sludge

Clarifier sludge scraper mechanism - sludge removal from clarifiers

About the blog

Karl-Uwe Schmitz
Karl Schmitz is a consultant and sales agent for FBBR, diffused air and lamella system solutions. We are working together with the lead manufacturers for drinking water and waste water components in the United States.
Analytical Technology (ATi)

Sludge scraper mechanisms – Removal of settled sludge

One of the most important treatment steps in wastewater and drinking water plants is sludge removal by sedimentation. Sludge in clarifiers can either be settled down to the bottom of the basin or float on the water surface (scum). In most aeration basins the sludge is “activated” meaning it contains live bacteria which are required for biological processes within the treatment plant. Therefore, the settled sludge, as well as scum, need to be collected but without interruption in operation and without basin dewatering.

For removal of settled sludge in clarifiers many different technologies are available but the most common are:

  1. Sludge hoppers
  2. Chain scraper/ rotary scraper mechanism
  3. Vacuum suction systems

Sludge hoppers are usually used for smaller round or square basins. Especially free-standing clarifier tanks made of steel use this type of sludge removal technology. The hopper walls should have a minimum gradient of 30 degree to ensure a good sludge slide down. At the very bottom of the hopper is the sludge outlet which in most cases is connected to a sludge removal pipe. When enough sludge has accumulated in the hopper a sludge pump is turned on and sludge is sucked out of the basin.

The advantage of sludge hoppers is their simplicity. Sludge hoppers don’t require any moving parts so maintenance and operational costs are low. Also, the degree of sludge thickening can be adjusted by changing the sludge removal cycles.

Chain scraper and rotary scraper mechanisms are mechanical sludge removal systems. The scraper arms drive along the basin floor and continuously carry the sludge with them. In rectangular basins, the sludge will be carried to a ditch and the removed with sludge pumps. In circular basins, the sludge will be carried to the center of the basin where it is removed through a pipe outlet. Both mechanisms need a motor for sludge movement. Scraper arms in rectangular basins are connected via chain links to the motor whereas scraper arms in round tanks are attached to a center located shaft which is connected to a motor on a service bridge.

Scraper mechanisms are often the best solution for larger tanks. They work for different types of sludge (light or heavy) and are easy to operate. Motors are accessible for maintenance without basin dewatering. Their operation and maintenance costs are overall higher compared to sludge hoppers.

Vacuum suction systems work in a similar way compared to scraper mechanisms. However, instead of moving scraper arms, this system uses moving pipes with suction orifices. A compressor on the top of the basin creates negative pressure so that sludge and water are sucked into the system.

Vacuum systems require little space and are therefore used for basins with limited accessibility for example underneath tube settlers. However, compared to scraper mechanisms vacuum systems tend to clog easier and they achieve less sludge thickening which could result in more costs for downstream sludge handling equipment.

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