Screenings cake exiting the wedge wire (dewatering press) Click here to enlarge imageAs wastewater passes through the moving mesh belt, solids are deposited and a mat of solids forms. Solids are removed by the mesh screen. Liquid that passes through the filter is collected after the screen cloth and discharged into the outlet pipe. As the mat builds, the effective opening becomes an order of magnitude finer than the openings in the mesh, allowing very high solids removal rates. At the same time, as effective head loss across the belt increases, the system is monitored by the PLC, where an internal algorithm speeds up or slows down the speed of the belt to maximize mat thickness while exposing clean belt to minimize head loss. Once the water is filtered, it is baffled around the return belt to maximize solids removal.
After the matted belt is rotated out of the wastewater, solids deposited on the mesh screen are then transported upward to the sludge compartment, where the belt is cleaned and the solids are thickened and dewatered. The belt cleaning system uses an air knife, blowing air in the opposite direction as the mat was formed, to produce a 4% to 8% solid that is collected in a trough. A take-away screw in the bottom of the trough then takes those solids to a wedge wire extruder screw assembly producing a final solids cake of 24% - 40%.
Five units are modularly sized for a peak flow rates of 0.2, 0.5, 1, 2, or 3.8 million gallons per day (MGD). Typical total suspended solids removal is between 40% and 80%. Due to the ultra fine effective opening in the belt mesh mat, the filter effectively captures hair as part of that TSS removal.
Additional Benefits
Additionally, the Salsnes filter effectively acts as a primary clarifier with a fraction of the footprint and capital cost. The unit will function as a primary clarifier by inherently removing up to 30% of the particulate biochemical oxygen demand (BOD). Soluble BOD is returned to the plant in the filtrate from the wedge wire extruder screw assembly. Removing BOD enables the plant to operate less aeration, use less basins, decrease biosolids digestion, and decrease biosolids disposal, thus saving significantly on capital and operating expenses. Independent engineering reports concluded that a 6' x 8' Salsnes footprint could take the place of a 60' primary clarifier.
Case Study
A demonstration study was performed at a Southern Alabama municipal wastewater treatment plant. The Salsnes unit was operated with a 350 micron and a 250 micron belt.