Scum removal equipment is desirable on secondary clarifiers. Skimmers are either of the type that rotates automatically or manually. The most important thing to consider is the sludge and scum collection mechanism. We will talk about “flights and chains”. They move the settled sludge to the hopper in the clarifier for return and they also remove the scum from the surface of the clarifier. The flights are usually wood or nonmetallic flights mounted on parallel chains. The motor shaft is connected through a gear reducer to a shaft which turns the drive chain. The drive chain turns the drive sprockets and the head shafts. The shafts can be located overhead or below.
Some clarifiers may not have scum removal equipment so the configuration of the shaft may very.
As the flights travel across the bottom of the clarifier, wearing shoes are used to protect the flights.
The shoes are usually metal and travel across a metal track.
To prevent damage do to overloads, a shear pin is used. The shear pin holds the gear solidly on the shaft so that no slippage occurs. Remember that the gear moves the drive chain. If a heavy load is put on the sludge collector system then the shear pin should break. This means that the gear would simply slide around the shaft and movement of the drive chain would stop.
Top picture, a clarifier’s raking mechanism. Bottom, scum armature equipment.
Let’s take a moment to review. Answer each of the questions below in the space provided.
1. What is the purpose of the flights and chains?
2. What is used to prevent wear of the flights at the bottom of the tank?
3. What is used to prevent damage to the unit during overloads? What could have caused the overload?
4. If you were creating a preventative maintenance program for this unit, in the space provided below what would be done during a plant shutdown?
Scum Removal Equipment
In some circular or square tanks rotating scrapers are used. The diagram below shows a typical Scum removal equipment.
The most common type has a center pier or column. The major mechanic parts of the clarifier are the drive unit; the sludge collector mechanism;
and the scum removal system. There is also some related equipment that we will consider briefly. Let’s look at the drive unit first.
There are three main parts to the drive unit; the motor (or gear motor); the gear reducer; and the turntable.
The motor is connected to a gear reduction unit which is commonly connected to additional gearing.
The drive cage is rotated around a center column by the motor and gear reduction unit. Although the drive motor runs about 1800 rpm’s, the gear reducer lowers the output speed so that the sludge collector mechanism goes through one revolution every 20 to 30 minutes. Usually the motors used on clarifiers mechanisms are totally enclosed, fan cooled motors, suitable for outside operation.
The horsepower of the motor is dependant on the size of the clarifier. The motor drives the chain and sprocket which drives the worm gear. The worm gear drives the gear that is mounted on a shaft that drives the turntable. The motor shaft speed is reduced by a series of gear reducers.
We looked at the main parts of the drive unit, now let’s take a look at the sludge collector and the scum removal system mechanism. The main parts of the unit are: the rake arm; the scraper blades; the adjustable squeegees; the surface skimmer; the scum baffles; and the scum box.
The surface skimmer rotates at the same speed as the collector mechanism and is usually supported by the collector rake arm. The scum baffle prevents scum from flowing over the effluent weir. The surface skimmer collects the scum and deposits it in the scum box. The stilling well or influent baffle projects above the liquid and directs the influent downwards to assist in the settling of suspected solids and reduce short circuiting. Another important part of the secondary clarifier is the effluent weir, launder and pipe. An effluent weir goes around the circumference of the tank and allows clarified liquid to flow evenly from the tank. The effluent launder collects the tank overflow and takes it a low point in the launder where a pipe is used to take the effluent to the chlorine contact basin or other means of treatment.
Some clarifiers may have a scum trough heater. The scum removal system rotates around the clarifier at a very slow rate. In subfreezing temperatures, the scum box and pipe could freeze. This problem can be overcome by using immersion heaters, or putting infrared lamps over the scum box.
Some clarifiers are covered.
Scrapers
As you have read depending on the design and operation of the process, activated sludge has several interrelated components:
1. Single aeration tank or multiple aeration tanks designed for completely mixed or plug flow.
2. An aeration source to provide adequate oxygen and mixing: sources can be compressed air, mechanical aeration, or pure oxygen.
3. A clarifier to separate the biological solids (activated sludge) from the treated wastewater.
4. A means of collecting the biological solids in the clarifier and recycling most of them (return activated sludge, RAS) to the aeration tank.
5. A means of removing or wasting excess biological solids (waste activated sludge, WAS) from the system.