Course: BFM 203 Water Treatment and Control

Lecture

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Sludge Removal

Typically sludge contains aluminum sulfate, with a concentration that varies from 0.25 to 10 percent in the sludge zone. Under normal conditions, 50 percent of the floc will settle out in the first third of the basin length. Sludge must be periodically removed to prevent interference with the settling process. Regular removal also will prevent the sludge from becoming septic or providing an environment for microorganism growth and to prevent excessive reduction in the cross-sectional area of the basin, which reduces detention time. While aluminium sulfate is the most common primary coagulant, other chemicals may be used. These other chemicals will cause varying metallic properties in deposited sludge. For example, ferric chloride sludge contains iron instead of aluminum, a criteria that must be considered when choosing coagulant chemicals.

Excess sludge buildup in the basin bottom can increase the chance of short circuiting and should be regularly monitored. Most removal methods can be controlled by adjusting the operating frequency and/or speed of the mechanism. Sludge may be removed using a variety of methods. Mechanical rakes push sludge toward the outlet. Drag-chain and flight systems are comprised of a continuous chain with wooden flights that move sludge into a sump. This type of removal results in higher operational and maintenance costs. A traveling bridge system consists of a bridge that holds traveling sweeps. These sweeps span the width of the basin and suction sludge from the bottom.

Operational Considerations For Sludge Removal

Many factors affect the decisions related to sludge removal from sedimentation units. How often and how much must be removed is based on the following criteria.

- Sludge is periodically removed for further processing and disposal.
- Sludge removal frequency depends on the rate of sludge buildup.
- Measuring sludge blanket depth is required to determine sludge removal frequency.
- Sounding the blanket is the most common method of measuring blanket depth.

Taking a core sample is a common method of sounding the sludge blanket as well as observing other sedimentation basin characteristics. A core sample is taken by inserting a clear plastic tube, with a check valve in the bottom, fully into the basin. The stratified contents of the tube are then observed for blanket depth, density, and supernatant water clarity.