Plate settlers are the most reliable, low maintenance and long lasting method of settling enhancement available.
Plate Settlers have been in existence since the turn of the century. Only recently has the technology advanced to the point that plate settlers are cost effective. As more engineers and plant personnel realize the cost savings and efficiency associated with plate settlers they are becoming extremely popular. Plate settlers can decrease the footprint of the settling basin by as much as 90% over an open basin or as much as 50% over a basin with tube settlers. This results in savings in real estate and concrete costs. Since there are no moving parts, maintenance is virtually non-existent.
Plate Settlers are the pre-treatment of choice ahead of membrane filters.
The quality of feed water that plate settlers can provide will result in longer membrane life and lower operating costs. Using plate settlers ahead of membranes also results in lower rates of membrane fouling, higher recovery rates, lower operating pressure, reduced cleaning requirements, extended membrane life and better water quality.
Membranes are highly vulnerable to turbidity spikes, and other types of raw water inconsistencies. One of the biggest benefits of plate settlers is that variations in raw water are buffered in the plate settlers and not passed through to the membranes.
Disadvantages of other methods of pre-treatment for membranes are:
- Ballasted settling processes require the use of special polymers. Polymer residual will increase membrane fouling.
- Ballasted processes can pass on ballast particles that can damage the membranes.
- Operating costs and procedures when dealing with the pumps, ballast separation and replacement ballast and ballast storage.
FRP Plate Settlers
- Fiberglass plates can shed “shards” that cause damage to membranes.
- Fiberglass plate settlers are known to “warp” or “distort” over time which unbalances the flow causing short circuiting.
- The overhead troughs completely block the tops of the plate settlers for access and inspection. The flexible trough joints are known to fail causing short circuiting.
- Tube settlers will shed sharp chips as they age and degrade which can damage membranes by cutting the thin membrane fibers.
- Tube settlers will last 5 – 10 years for the vacuum formed type, and 10-15 years for the folded type (MRI) depending on service conditions.
- Tube settler’s effluent quality will be less than MRI plate settlers
Advantages of MRI plate settlers over other methods of pre-treatment:
- In cases tested, the MRI plate settler effluent to the membranes can be maintained to < 2 NTU without the need for polymers which do not work well in conjunction with membranes.
- The all stainless steel design will not shed any materials that may enter the membranes causing possible damage.
- The stainless steel plate settlers have no moving parts and require no maintenance except occasional cleaning.
Advantages of MRI plate setters over other plate settlers:
Settling enhancement has been the expertise of MRI for over 25 years. MRI designs, patents, and manufactures its own plate settlers of stainless steel and aluminum, not FRP. FRP plate settlers can degrade and distort due to water absorption and in the presence of UV light. Stainless steel and aluminum plate settlers will never degrade or distort.
- MRI Plate Settlers are the only Plate Settlers available that can be walked on-
MRI plate settlers have the effluent trough to the side of the plate packs which provides an unobstructed view of the top of the plate settlers. MRI’s patented “Top Deck” allows contractors and operators to walk out on the plates for installation and maintenance (local OSHA regulations should be followed). The robust Top Deck also allows high pressure washing without damage to the plate settlers.
- Influent Distribution – MRI’s Inlet Infuser lowers the inlet velocity and blends the influent water with the basin water and eliminates diving currents, short circuiting and other differential density anomalies.
GEWE Design – MRI manufactures the only true GEWE style plate settler made in the United States. We also manufacture plate packs.
- Smooth Plate Surface - The surfaces of the MRI Plate Settlers are completely smooth. There is nothing to interfere with the distribution across the plate or the movement of the solids traveling down the plate, i.e. textures, ribs, or stampings either vertical or horizontal.
- Effluent Distribution - MRI has incorporated two very important features to insure an absolutely even distribution throughout the basin. The first is the patented “Top Tube”. The Top Tube contains 4 to 6 orifices (depending on flow rate) spaced evenly across the width of the Top Tube. The orifices, usually .5” to .75” in diameter, act as a weir across the Top Tube. This creates an even draw across the width of the Top Tube resulting in even distribution over the width of the plate settler. The two figures below show a comparison between the overhead trough type and MRI’s top deck type using CFD analysis of the two styles.
CFD Analysis of MRI Plate Settler
Older designs are not as efficient as the MRI Plate Settler and, therefore, do not produce as good an effluent. Probably the most common design is the “Overhead Trough” design where the effluent trough is located over the plate settlers and the flow travels through orifices located in the bottom of the trough or over a weir on the top of the trough. This style of plate settler concentrates the flow into two exit points which creates uneven velocities over the plate surface as seen in the CFD analysis below. Not only is the velocity higher in the center of the plate settler, but there is a region down each side where the flow re-circulates and creates higher velocities. This results in poor efficiency and is why manufacturers of this design specify 80% efficiency.
Another problem with the overhead trough is that as the velocity in the trough increases toward the discharge end, it creates a difference in pressure drop as opposed to the “blind” end of the effluent trough. This results in higher flows through the orifices (plates) closest to the trough discharge than at the beginning of the trough. Another draw back to this design is that the trough covers the plate settlers and makes them inaccessible.
CFD Analysis of Trough-Type Plate Settler
The second feature that insures even distribution throughout the basin is the flat weir on the effluent trough. This weir sets the downstream water elevation and insures that all top tubes discharge evenly throughout the basin, as shown in the illustration below. The combination of these two elements insure even distribution through each plate and over the entire basin.