DAF Surface Skimmer Operation and Maintenance

The Surface Skimmer on a Dissolved Air Flotation Unit is a key piece for DAF Performance.  

The Surface Skimmer typically consists of a series of flight scrapers (skimmer blades) and chain mechanism.  The gearbox mechanism and motor allows for periodic skimming. A Variable Frequency Drive (VFD) is included to adjust the speed of the skimmer motor as well as a timer. 

The Surface Skimmer will remove the top sludge blanket in the DAF. The chain and flight system has a timer that allows the operator to adjust the skimmer motor to rotate between one and ten minute intervals and turn off for zero to ten minute intervals. 

The Surface Skimmer Speed can affect the DAF sludge blanket thickness and effluent quality.  The skimmer speed should be set so that a thin three to six inch sludge blanket be maintained on the surface, with a six inch depth near the beach. A The sludge blanket should always present on the surface of the DAF. 

The sludge blanket helps in the removal of solids as the wastewater flow enters the DAF.  The surface skimmers  should also be adjusted so that they allow for maximum thickening of the sludge blanket. The slower the skimmer speed and frequency, the thicker the sludge blanket.  The faster the skimmer speed and frequency the thinner the sludge blanket.  

The skimmer speed should also be optimized to minimize any turbulence that could be caused while the skimmer is operating. If the skimmer speed is too fast it may generate turbulence that can result in higher than usual effluent suspended solids carryover. If the skimmer is operating too slowly and the  sludge blanket is allowed to get too thick, it will most likely result in higher than normal suspended solids carryover in the effluent as the solids start to break away from the surface sludge blanket and get carried away with the effluent flow. 

The skimmer time interval should operate in the ON mode for one minute and OFF Mode for five minutes.  The speed of the skimmer should be able to keep the last 25% to 33% of the DAF surface clean when skimming.

Periodic checks should be made by the operator on the flotation chamber to observe that a sludge blanket is being formed and that it is being skimmed off on to the beach and into the sludge trough. The effluent weir can be adjusted to help raise and lower the sludge blanket depth.  

The Ellis Surface Skimmer design is one of the most robust skimmer systems to remove DAF surface sludge.  It is designed to maintain long service life compared to others.  The chain is constructed of stainless steel instead of plastic.  Skimmer Flights are made to rotate down like a ferris wheel instead of staying upside down as they rotate over the top of the DAF surface.  This minimizes wear and tear on the pivot points. The chain is supported on a track to minimize weak strain on pivot points of skimmer flights.

Ellis Skimmer
Ellis Skimmer Flights Flip Down Like a Ferris Wheel Reducing Strain on Pivot Points and Chain. Chain rides along on support track to reduce strain
Ellis DAF Surface Skimmer
Robust steel chains are used on the Ellis DAF Surface Skimmer Assembly. The steel minimizes down time and repair compared to assembles that do not use steel and assembly supports
Skimmer Flights Pivot Points
These Skimmer Flights do not pivot putting strain on the chain and pivot points leading to premature wear and tear.
Skimmer Assembly Uses a Plastic Chain
This skimmer assembly uses a plastic chain which stretches over time and adds wear causing links to break. The skimmer flights are not allowed to pivot putting additional strain on the chain further causing stretching
Problem Possible Cause Action
Skimmer Blades do not reach the sludge blanket. The level in the flotation chamber is too low. The effluent weir is set too low. Adjust the effluent weir higher to raise the water level in the flotation chamber higher.
Wastewater Feed Pump is not Operating. No increase in the flotation tank water level. Set the Effluent Weir Higher.
Switch the wastewater feed pump on.
There is a leak on the flotation chamber drain valve to remove bottom solids. Check the seal on the flotation chamber drain valve for a tight seal.
There is no sludge blanket on top of the flotation chamber

No incoming air. No microbubble formation to float solids to the flotation chamber surface. Check the microbubble aeration pump to ensure air bubbles are being generated.
Skimmer Motor Not Operating

VFD Not Functioning Properly Check VFD Operation
Output Speed Set Tool Low Increase Hz of Output Speed
Timers Not Set Properly Check Timer Settings
No Power to Motor Check Control Panel to see if Motor Switch is ON
Sludge Skimmers Stalling Out

Sludge Blanket Too Thick Increase “ON” Time for Skimmer
Reduce Flocculant Dosage
Skimmer Stuck on Beach Check Chain Tension. Inspect Chain Links. Replace Broken or Weak Links
Sludge Blanket Too Thin Decrease Skimmer Run Time, Maintain 3” to 6” sludge blanket depth.
Sludge Blanket Too Thick Increase Skimmer Run Time, Maintain 3” to 6” sludge blanket depth.
Solids Overload Condition Compare Actual Solids Loading to Design Loading

Maintenance Task Recommended Maintenance Schedule
Check Skimmer Sprockets and Chain for sludge solids build up.
Wash Chain and Inspect Chain Links.
Re-lubricate chain links with liberal amounts of wheel bearing grease.

Clean Skimmer Flights and the pin insertion points supports and chain support angles.
Lubricate Skimmer Sprocket and Gear Box Bearings
Unlike many DAF manufacturers that use plastic chain for the surface skimmers, Ellis Skimmer Chain is constructed of steel.
Steel chain does not stretch like plastic chain so frequent tightening of the skimmer chain is not required.
Inspect the chain and replace weak links as necessary.