De-watering Equipment
In wastewater treatment, any dissolved organic or inorganic chemical compounds are converted to floc that is collected as sludge. All the suspended solids are settled and collected as concentrated or thickened sludge.
However, the sludge is still in liquid form, and is difficult to handle. Moreover, the cost of disposal is more if the charges are based on weight. To remove most of the water from the sludge, several methods can be used.
Filter Press
Liquid sludge is pumped to a filter press assembly that contains filter cloths, compression plates, chamber plates. The compression plates and chamber plates are pressed together to hold the cloth in between them. During the filtration stage, as the liquid sludge is pumped through the cloth, water passes through the cloth, while the filter cloth traps the solids. The filtration process through the filter cloth is carried out for certain duration of time.
When sufficient time has elapsed, compressed air is led into the diaphragm in the chamber plates to compress the solids sticking to the cloth. The wet sludge is thus squeezed by the compressed air. As a result of the squeezing, the sludge becomes dried to the consistency of modeling clay.
The compression and chamber plates are then separated. The clay-like sludge separates from the cloth and drops down to a collection pan at the bottom of the filter press. This can be collected and stored in bags for disposal.
Some filter presses are designed for automatic operation. The sequences of filtration, squeezing, cake discharge, cloth washing, collection of dried sludge, and transportation of the sludge to collection bags is all programmed into the system.
Belt Press
This is another method of squeezing the wet sludge. In this method, two moving cloth belts are used. The sludge is continuously carried up on top of one of the moving cloth belt. Once the wet sludge is evenly distributed on top of the belt, the water drops down by gravity, leaving the solids trapped on the cloth.
By a system of rollers, the two belts are brought one on top the other, with the sludge sandwiched in between them. The belts in contact are passed through a succession of rollers arranged from big sized to small size. By going through this array of rollers, the pressure exerted against each other is gradually increased. The sludge gets squeezed more and more.
The result of the squeezing is a dry cake sticking on the surface of the belts. The cake is scrapped off by spring-loaded scrappers and collected. Conveyors may be installed to transport the dehydrated sludge to collection bags.
Because the belts move continuously, there is no distinct timing, and stages like in the filter press. Sludge is continuously squeezed and collected. Since the belts are in constant motion, there is a need to prevent them from straying from the course.
Built-in controls for detecting and correcting drifting movement has to be incorporated in the design. Controls for the belt speed can be used to match the rate of generated sludge to the system. Belt washing is also necessary for optimum efficiency in separation.
Centrifugal Separator
When liquid containing suspended solids is led into a rapidly spinning bowl, the heavier solids are flung out to the perimeter of the bowl while the liquid remain. It is like speeding up the process of settling solids in a settling tank. Centrifugal force is used to make the separation process faster.
Centrifugal separator can work well only if the specific gravity of the solids is different from that of the liquid.
As the solids are flung to the outer perimeter of the bowl, it will remain there unless they are removed constantly. In the centrifugal separator, a screw conveyor is installed inside the rotating bowl. As the solids collect in the perimeter of the bowl, the screw conveyor scrapes them off and transports them to an opening. The solids are discharged and collected in bags to be disposed.
A typical centrifugal separator bowl is rotated at speeds up to 3250 rpm. At this speed, the centrifugal force exerted on the solid particles can be up to 2100 times the force of gravity. Because of the high speeds, the bowl must be dynamically balanced for best performance and minimal vibration. The screw conveyor is rotated at a much slower speed.
Sludge Dryer
The purpose of the sludge dryer is to remove moisture from the de-watered sludge obtained by the above methods. Usually heat is used to remove the moisture. The sludge that is produced feels dry to the touch.
When de-watered sludge is transported to the hopper of the sludge dryer, it tends to stick together like mud. The hopper of a sludge dryer usually contains mechanism for breaking the sludge cake, and an extruder for forcing the sludge through small holes in a screen. After being extruded, the sludge comes out in the form of short cylinders, very much like toothpaste coming out from the tube.
Dropping on to a slow moving chain conveyor, the droppings are carried over a heated area. This is where most of the moisture in the sludge is removed. The pellets that emerge after heating has become dry, and can be collected in bags for disposal.
The heating can be done by burning of propane gas or natural gas, or by electrical heating elements. The moisture from the hot gas is exhausted to the atmosphere after passing through a fume scrubber.
A typical sludge dryer has exhaust temperatures of between 160 to 200 degrees Fahrenheit. It may also have a reversing action, speed control, sludge level control, temperature control and other features.
|
