Inclined-plate Sedimentation


Following flocculation, the particles are settled out using the inclined settling plates.  Flow enters the plates through specially sized feed openings located in the lower sides of the plates. The solids then settle on the surface of the plates while

effluent is discharged from the top of the plates. The clarified effluent then flows to a collection through and over v-notch weirs to ensure even flow distribution. The heavier particles, which have settled on the inclined plates, slide down the plates

while the lighter material accumulates on the plates. The lighter material continues to accumulate on the plates until a critical mass is reached at which point gravity causes the solids to slide down into the sludge storage/thickening area below.


Unhindered settling tank size is governed by the time to be allowed for particles to settle through the depth of water. The simple theory shows that efficiency of removal of particles is governed by the area available for settlement. An approach to providing a large area with a small footprint is to use inclined plates. These are usually constructed with lightweight material in modular form which can be easily positioned in a concrete or steel tank. Flow between such inclined plates can be co-current, counter-current or cross-flow. In the co-current arrangement, the water flows downward between the plates in the direction of particle settlement. In the counter-current arrangement, the water flows upward between the plates against the direction of particle settlement. In cross-flow the water flows across the plates, i.e. horizontal, at right angles to the direction of particle settlement. A design challenge for inclined settling is to maximize distribution of flow of water within and between plates and thereby maximize efficiency of particle removal.







Principle of the Inclined Plate


This devise is designed to increase the settling capability of a basin. The design principle utilizes the fact that the depth of a gravity settler has very little bearing on its’ settling capacity. Of much more importance are the available settling areas of 140 square feet in each inclined plate clarifier plate pack.


In a conventional clarifier or sedimentation basin, the available settling area is limited to the area of the bottom. The inclined plate clarifier principle utilizes a series of inclined plates mounted together in a pack. There is only a short distance in the plate openings of 1 ¼” in the inclined plate clarifier design. With this principle, the available settling area becomes the total area of the plates projected on a horizontal surface. It should be remembered that in a conventional settling basin, the sludge particle often has a considerable distance to traverse before reaching the bottom. In an inclined plate clarifier this settling distance is something less than 1 ¼”.


Since the inclined plate clarifier plates are mounted at a 60-degree angle to the horizontal, the settling sludge can easily slide down the plates for collection.

With the inclined plate clarifier the water entering the unit is introduced slightly below the clarifier separator, allowing the majority of the solids to be discharged over, and settle out over essentially the entire surface of the sludge collection basin, the lower part of which is designed as a hydraulically static thickener. This design allows for 100% use of the 140 square feet of the settling area in the pack to be utilized for separation of a reduced solids concentration up-flow. With the efficiency of horizontal plate separators being fixed for each type of waters and solids particles, the net effect of the design is lower supernatant suspended solids concentrations.





Operation of the Inclined-Plate Sedimentation

The standard inclined plate clarifier consists of a flash mix/flocculation chamber, the specially designed plate pack and the coned thickener underneath. For many applications, polymer addition is necessary to increase particle size. Where required, polymer is applied into the first chamber. An air mixer provides the rapid mixing necessary for uniform application and dispersion. The waters then are transferred through a series of overflow/underflow baffles, where a gentle mixing accomplishes floc growth. Retention time required with this inclined plate clarifier design is 1 to 2 minutes.


The flocculated water enters the inclined plate clarifier in an influent launderer and flow distributor. Uniform distribution of flow accomplished by the launderers allows for even distribution of solids onto the sludge bed and hydraulic loadings upward through the plate pack. The inclined plate clarifier water rises up the plates and is discharged near the top over weirs into effluent troughs mounted above the plate packs. The settling sludge slides down the plate into the thickening compartment below the plate pack and is applied to the surface of the sludge bed.


The dense sludge collected in the cones is removed on a timed basis for additional dewatering. Sludge removal is made by a timer actuated solenoid valve controlling an automated butterfly valve on the cone. As air pressure is applied to the valve, the valve opens allowing the dense sludge to be driven by the hydrostatic head pressure of the unit, through a three-inch sludge-piping network into a storage tank. A two-foot differential head pressure between the incline plate clarifier surface and the sludge storage tank is recommended. Applications requiring a taller sludge storage tank in excess of the two-foot differential will require a lift station and a suitable diaphragm pump. Pumping of sludge directly from the inclined plate clarifier cone is not recommended as “rat-holing” can occur.


The inclined plate clarifier method of removing only the densest sludge through the full three-inch pipe eliminates the requirement for supplemental expensive thickening equipment, and prevents “rat-holing” in the sludge bed. For each application, the timed sludge withdrawal is field set. Sampling ports provided on the inclined plate clarifier’s cones allows for a convenient method for determining proper settling.




Main advantages of the Inclined Plate Sedimentation


Conservative sizing provides for plate pack hydraulic loading rates of 0.2 GPM per square foot.


Cone volume and static thickening, coupled with timed sludge blow down, eliminates the need for further thickening.


Maximum unit height of 12 feet allows for installation in most any plant without building modifications.


Use of a launderer system for flow distribution rather than submerged orifices insures uniform flow because of possible plugging of orifices.


Surface area of sedimentation basis reduced up to 80% against conventional and up to 50% versus tube settler systems.


Ability to produce easily dewatered sludge underflows, typically four to six times conventional sedimentation or tube settle systems.


Elimination of need for second stage thickening.


Up to 50% reduction in cost of sedimentation basins.


Performance relatively unaffected by changes in the solids loading.


Immediate start-up of system under full load.


Effluent quality unaffected by hydraulic shock loads.