The dewatering packages are as the name clearly implies, equipment to remove water from sludge. Sludge dewatering is typically the final step for industrial wastewater treatment processes and plants. After wastewater treatment the sludge remaining is very high in water content (>95% as minimum) that can be reclaimed through sludge dewatering.
Apart from water reclamation, most wastewater treatment plants pay for sludge disposal by weight, and water is heavy. So if one removes as much water as possible, dewatered sludge is lighter and thus costs less to dispose of sludge and increases the number of recycling options and reduces risk of outlet availability. The bottom line is that effective Sludge dewatering saves money.
The most common way to dewater sludge is to physically squeeze the water out of the sludge. As with all sludge dewatering technologies one can approach dewatering in various ways depending on requirements and eventual disposition of sludge.
Normally following methods are applied:
- Pressure filtration dewatering,
- Belt press dewatering filtration,
- Air sludge drying processes,
- Vacuum filtration,
- Sludge dewatering by centrifugation.
In addition a sludge drier can be utilised at the end of the process. Driers are oven like equipment that actually bakes out the water.
A brief synopsis of each method is given below.
Pressure Filtration Dewatering: Pressure filtration is a process similar to vacuum filtration where sludge solids are separated from the liquid. Leaf filters probably are the most common type of unit.
Like vacuum filtration, a porous media is used in leaf filters to separate solids from the liquid. The solids are captured in the media pores; they build up on the media surface; and they reinforce the media in its solid-liquid separation action. Sludge pumps provide the energy to force the water through the media.
Lime, Poly Aluminium chloride (PAC), and ferric salts have been commonly used to condition sludge prior to pressing. The successful use of ash pre coating has also been reported. Minimum chemical costs are supposed to be the major advantage of press filters over vacuum filters.
Leaf filters represent an attempt to dewater sludge in a small space quickly. But, when compared to other dewatering methods, they have major disadvantages: (1) batch operation, and (2) high operation and maintenance costs.
Some other types of pressure filters include hydraulic and screw presses, which while effective in dewatering sludges, have a major disadvantage of usually requiring a thickened sludge feed.
Sludge cakes as high as 75% solids using pressure filtration have been reportedly accomplished.
Belt Press Filtration: The belt filter (sometimes called a belt filter press) is used for solid/liquid separation processes, particularly the dewatering of sludges in the chemical industry, mining and water treatment. The process of filtration is primarily obtained by passing a pair of filtering cloths and belts through a system of rollers. The feed sludge to be dewatered is introduced from a hopper between two filter cloths (supported by perforated belts) which pass through a convoluted arrangement of rollers. As the belts are fed through the rollers, water is squeezed out of the sludge. When the belts pass through the final pair of rollers in the process, the filter cloths are separated and the filter cake is scraped off into a suitable container. The sludge can be combined with a filter aid or flocculant the help the filtration process and reduce blinding of the filter cloth.
Filter cloths can be cleaned throughout the operation of the process by means of water sprays positioned on the return section of the belt.
Air sludge drying processes: Drying beds are generally used for dewatering of well digested sludges. Attempts to air dry raw sludge usually result in odor problems. Sludge drying beds consist of perforated or open joint drainage pipe laid within a gravel base. The gravel is covered with a layer of sand. Partitions around and between the drying beds are generally open to the weather but may be covered with ventilated green-house type enclosures where it is necessary to dewater sludge in wet climates.
The drying of sludge on sand beds is accomplished by allowing water to drain from the sludge mass through the supporting sand to the drainage piping and natural evaporation to the air. As the sludge dries, cracks develop in the surface allowing evaporation to occur from the lower layers which accelerates the drying process.
The only side stream is the drainage water. This water is normally returned to the raw sewage flow to the plant or to the plant head works. The drainage water is not normally treated prior to return to the plant.
Large plants will normally utilize mechanical equipment for handling the dried sludge. Some communities have encouraged public usage of the dried sludge. In some cases users are allowed to remove the sludge from the beds, but this may not be satisfactory in many cases.
Vacuum Filtration: The vacuum filter for dewatering sludge is a drum over which is laid the filtering medium consisting of a cloth of cotton, wool, nylon, fiber glass or plastic, or a stainless steel mesh, or a double layer of stainless steel coil springs. The drum with horizontal axis is set in a tank with about one quarter of the drum submerged in conditioned sludge. Valves and piping are so arranged that, as a portion of the drum rotates slowly in the sludge, a vacuum is applied on the inner side of the filter medium, drawing out water from the sludge and holding the sludge against it. The application of the vacuum is continued as the drum rotates out of the sludge and into the atmosphere. This pulls water away from the sludge, leaving a moist mat or cake on the outer surface. This mat is scraped, blown or lifted away from the drum just before it enters the sludge tank again.
The moisture content in the sludge cake also varies with the type of sludge from 80 to 84 percent, for raw activated sludge to 60 to 68 percent for well digested primary sludge.
While operating costs, including conditioning of sludge for vacuum filtration, are usually higher than with sludge beds, filtration has the advantage of requiring much less area, is independent of seasons and weather conditions, and can eliminate the necessity for digestion since raw sludge can be dewatered sufficiently to be incinerated.
Centrifuge: Basically, centrifuges separate solids from the liquid through sedimentation and centrifugal force. In a typical unit sludge is fed through a stationary feed tube along the center line of the bowl through a hub of the screw conveyor. The screw conveyor is mounted inside the rotating conical bowl. It rotates at a slightly lower speed than the bowl. Sludge leaves the end of the feed tube, is accelerated, passes through the ports in the conveyor shaft, and is distributed to the periphery of the bowl. Solids settle through the liquid pool, are compacted by centrifugal force against the walls of the bowl, and are conveyed by the screw conveyor to the drying or beach area of the bowl. The beach area is an inclined section of the bowl where further dewatering occurs before the solids are discharged. Separated liquid is discharged continuously over adjustable weirs at the opposite end of the bowl.
Two factors usually determine the success of failure of centrifugation — cake dryness and solids recovery.
Centrifugation has some inherent advantages over vacuum filtration and other processes used to dewater sludge. It is simple, compact, totally enclosed, flexible, can be used without chemical aids, and the costs are moderate. Industry particularly has accepted centrifuges in part due to their low capital cost, simplicity of operation, and effectiveness with difficult-to-dewater sludges.