Advantages

Disadvantages

1. Multiple tube arrangement may remove sludge where it settles.  

a. Creates some stirring motion, which may keep the sludge blanket in motion.

b. RAS tubes interfere with the flow from the centerfeed inlet ports.  

2. Should minimize denitrification and P-release by removing sludge rapidly.

3. Creates a stirring motion, which may enhance flocculation

a. Creates a stirring motion, which may prevent the blanket from compacting and reduce RAS concentration.

4. Creates a stirring motion, which may enhance the release of nitrogen bubbles, thereby reducing rising sludge due to denitrification.

5. Provide for sludge removal control for each "ring" area of the clarifier floor.

a. Practically speaking, many operators do not attempt individual control of RAS tubes.

6. Individual terminal control valves allow optimizing control of each RAS tube flow.

a. Practically speaking, control of tube flow rate is often determined by the setting that minimizes plugging.

b. Some types of control valves are not operator-friendly; not useful for varying flows.

7. Selective sizing of tubes provides for sludge removal in proportion to the area of floor covered.

a. Use of large diameter tubes at the outside may lead to plugging due to low velocities.

b. Use of small diameter tube near center almost always leads to plugging due to small diameter.

8. Surface sight well provides for observation of RAS.

a. RAS well box has seal that may leak, causing dilution of the RAS.

b. RAS well depth lowers inlet feed location, may require deeper centerwell.

9. Minimum floor slope required.

Advantages

Disadvantages

1. Removes sludge where it settles without channeling it to a single draw-off tube; allows total floor area to be used.

a. Improperly designed orifices can defeat suction advantages.

2. "Sucks" its way through the blanket, creating little disturbance.

a. This motion would not enhance flocculation, but the truss on the opposite side does.

3. Suction action while rotating, plus small front section, reduces drag (torque).

4. The lower "profile" creates less disturbance moving through the sludge blanket.

5. Less disturbance of the blanket leads to a more concentrated RAS

6. A more concentrated RAS reduces the power cost for RAS pumping.

7. An increased RAS concentration permits higher MLSS concentration.

8. Direct connection to RAS pumps simplifies RAS control.

9. A properly designed tapered header maintains scouring velocity in header; minimizes clogging.

a. An improperly designed header will contribute to clogging.

10. No floor slope required for RAS concentration; reduces construction costs.

a. Some minimum slope is required for dewatering.

11 "Squeegee" design maintains a clean floor.

12. Suction plus squeegee action can maintain a lower blanket and rapid total removal, reducing opportunities for P-release and denitrification.

a. Suction header seals need maintenance or replacement in order to maintain good suction.

Advantages

Disadvantages

1. Conventional, best known technology

a. The perception that this is "old technology"; has fallen into disfavor.

2. Squeegee blades system permits clean scraping of the floor.

a. Sludge blanket is probably re-suspended as the blades pass thru it, esp. near the perimeter due to normal 10-12 fpm tip speed.

3. Echelon collector moves sludge continuously toward the hopper

(…. . however, the sludge should move to the hopper as a flowing fluid. )

a. It’s claimed that this plowing action takes several revolutions of the mechanism to move solids to the center sludge hopper.

b. …. However, at 10-15 fpm tip speed, the blades should plow thru the sludge.

c. Scraper collection should require steeper floor slopes; deeper center depths; increased cost.

d. Sludge removal should deteriorate with higher SVI's.

4. Plow collectors are claimed to eliminate the jetting action of the RAS drawn along the collector arm by RSR suction technologies, leading to a "trailing wave of solids" loss at the weirs.

a. Note:  We have not been able to locate signs of any "jetting" along the collector arm. In fact, we have shown by field experiment that this phenomenon is not present.

5. Use of a sludge hopper permits thickening of the RAS prior to pumping.

a. Standard hopper design may also lead to fluctuating RAS concentrations as collectors pass over and beyond the hopper.

b. Central sludge hopper is in the zone where scouring of the blanket may take place.

6. Proponents claim that the use of a "sludge ring" leads to a more uniform RAS draw-off.

a. Use of a "sludge ring" should cause drawing a more dilute sludge from the ports opposed to and behind the scraper blades …. unless there's sufficient sludge blanket over it.

b. Carrying a "sufficient" sludge blanket reduces the effective clarifier volume and may increase solids carry-over.

Advantages

Disadvantages

1. Perceived as the "latest" collector technology

a. It's been around Europe/UK since the early 1900's, fell into disfavor; has now been “rediscovered”.

2. Can be used with a simple center sludge hopper.

a. Standard hopper design leads to fluctuating RAS concentrations as collectors pass over and beyond the hopper.

b. Central sludge hopper location can be subject to scouring action.

3. Claimed that "…. deep scraper blade effectively moves solids to the hopper in one revolution or less. "

a. The sludge doesn’t have to be scraped to the hopper; it flows like a thick "soup".

b. A deep scraper blade is required to prevent overloading the outside with solids.

c. A deep scraper blade should promote fluffing of the sludge blanket, esp. at 10 to 15 fpm tip speed.

d. Multiple blades required for larger clarifiers should exacerbate fluffing of the blanket.

e. The deep blade profile should increase torque.

f. Scraper collection should require steep floor slopes; deeper center depths; increased cost.

g. Sludge removal should deteriorate with higher SVI's.

4. Claimed that "…. . coupled with a sludge ring, provides for effective removal of the sludge. "

a. Use of certain sludge rings should cause drawing a more dilute sludge from the ports opposed to and behind the scraper blades …. unless there's sufficient sludge blanket over it.

b. Carrying a "sufficient" sludge blanket reduces the effective clarifier volume and may increase solids carryover.

c. Carrying a deep sludge blanket increases the likelihood of denitrification and P-release.