Disadvantages of the Gravity
Sludge Drying Bed

Disadvantages of Conventional
Sand Beds or Lagoons

• Requires 3-4 days between operating cycles
• Requires larger area (move space) to process solids load
• Typically only produces 8-10% solids
• TLC required by loader operator not to damage plates
• Periodic removal of plates necessary to clean solids buildup

• Climate dependent
• Large land area required
• Cake removal is labor intensive
• Loss of sand during cake removal
• Sand becomes compacted and plugged
• Undesirable public appearance and odor
• Only cost effective for very small plants

Advantages of the Vacuum
Dewatering Bed System (VDBS)

Sludge Dewatering Techniques

• Capable of dewatering solids to at least the paint filter test requirement
• Complete cycle typically takes less than 24 hours
• More efficient, uses land less space
• Few moving parts, low maintenance
• Overall lower operating costs

• Centrifuge (greater than 5,000mgd)
• Rotary Drum Vacuum Filter
• Belt Filter Press (greater than 5,000mgd)
• Gravity sludge bed
• Sand bed
• Vacuum Sludge Dewatering Bed

How the Vacuum Sludge Dewatering Bed (VSDB) Works

Gravity dewatering begins as the bed is filling.  The clear liquid, separated from the flocculated solids flows down through the porous media, through the support plenum and out of the bed structure.

After the bed is filled to maximum liquid level, the conditioned sludge feed is shut off.

The vacuum pump is started creating a vacuum in the plenum and media causing a uniform pressure on top of the cake.

Motorized filtrate drain valves automatically controlled by level sensors discharge the filtrate from the plenum of the VSDB.   Typically, filtrate suspended solids are less than 30 mg/l, indicating very high solids capture in the dewatered sludge.

As the sludge continues to consolidate and shrink, the resulting cake will start to crack.  This will continue until the bed is uniformly cracked and the vacuum gradually lost.  As the plenum area loses vacuum the vacuum pumps shut down.

The stop-gates can now be removed to allow a front-end loader access to the bed to remove the dewatered sludge.  The surface is then washed down with high pressure, low volume water.  This washdown water is discharged to the sewer or directed back to the head of the plant.

Following cleaning, the stop-gates are placed back into position and the facility is ready for another dewatering cycle.  A complete dewatering cycle is typically 24 hours.