How to Select Dewatering Equipment
What is Dewatering Equipment?
Dewatering equipment is designed to separate water from solids using force, including vacuum and centrifugal motion. Used widely in waste management, dewatering equipment can save money by reducing solids handling or disposal expenses that are charged on a unit weight basis. The weight percentage of water in landfill-bound solids and raw wastewater sludge can effectively double or triple the cost of removal or treatment services.
Depending on the type of solid and the size of the operation, dewatering can be more cost effective than heat drying systems for water removal because the energy cost to power an oven or microwave can be much higher than to power a motor or pump. It may also be easier to streamline dewatering methods into a process line. In applications demanding very high levels of water removal, dewatering is used generally as the preliminary method, and is followed by stages of heat drying or digestion.
Selecting a Dewatering Method
Characteristics of dewatering equipment vary by method. However, the same performance specifications and design considerations are applicable to all dewatering technologies. Specifications that describe dewatering performance include cake dryness, solids recovery and drying time. Considerations for design and operation of this equipment include operating costs, chemical usage, and required maintenance.
|Type||Cake Dryness||Solids Recovery||Process / Drying Time||Operating Cost||Capital Cost||Chemical / Flocculent|
|% solids||% solids capture||time (measurement)||Usage|
|Vacuum filters||16 – 45*||85 - 95||Fast (minutes or hours)||High||Moderate||Moderate|
|Filter presses||40 – 60||80 - 95||Very fast (minutes)||High||High||High|
|Centrifuges||20 – 35**||85 - 90||Fast (minutes or hours)||Moderate||Low||High|
|Drying beds||25 – 60||90 - 100||Slow (weeks or months)||Very low||Low||Low|
|Sludge lagoons||20 – 40||90 - 100||Very slow (months or years)||None||Very low||None|
|Gravity/Low Pressure||10 – 50||90 - 96||Moderate (days or weeks)||Low||Moderate||High|
There are many different dewatering methods. These include centrifuges, filter presses, drying beds, sludge lagoons, and gravity and low pressure devices. Each method is described in detail below.
Centrifuges separate solids from liquids through sedimentation and centrifugal force. Solids are fed through a stationary feed tube, accelerated through ports in the conveyor shaft, and distributed to the periphery of the bowl. The bowl, spinning at high speeds, separates the water from the solids which are compacted against the bowl walls. Solids can then be conveyed to additional centrifuge drying stages while the separated liquid is discharged continuously over adjustable weirs at the other side of the bowl.
Figure 1 – Diagram of a typical dewatering centrifuge. Image Credit: Global Environment Centre
|Parameter and Performance Correlations for Centrifuges|
|Performance & Parameters||Pool Volume||Bowl Speed||Conveyor Speed||Feed Rate||Feed Consistency||Temperature||Flocculent Usage|
Vacuum filters involve creating a vacuum to draw out water from solids. The filter consists of a drum over which the filtering medium is laid. The drum is set in a tank with one quarter submerged in the cake or sludge. Valves and pipes are arranged so that a vacuum is applied to the inner side of the filter medium as the drum rotates slowly, causing water to be drawn from the sludge. Once the drum has lifted the sludge into the atmosphere, a water layer has been drawn to the top of the cake and the cake is subsequently removed by a stationary knife blade.
Figure 3 – Diagram depicting rotary drum vacuum filter operation and process flow. Image Credit: Komline-Sanderson.
Operating costs, including sludge conditions for vacuum filtration, are usually higher than other processes. However, vacuum filters are independent of seasonal conditions, require a small area, and run as a continuous process. They can dry solids enough to eliminate the need for subsequent steps such as digestion or heat treatment before disposal, incineration, or usage. As a measure of performance, vacuum filters can be rated in pounds per hour dry solids filtered per area of filter surface.
Figure 2 – Photo of a rotary drum vacuum filter. Image Credit: Komline-Sanderson.
Filter presses use a porous media and mechanical pressure to separate solids from liquids. The solids are directed between two or more porous plates or into porous cavities until full. Solids are captured in these pores and built up on the surface of the plates, reinforcing the solid-liquid separation action. Water is forced through the pores either from plate pressure by pushing the plates together or from a buildup of solids pressure by continuously pushing solids into the cavities. Plates are generally pressed via a hydraulic cylinder or other type of powered piston and sludge pumps provide the energy to push solids into the cavities and force the water through the media. While some filter presses are designed for capturing and retrieving the extracted liquid, those designed for dewatering applications generally focus on retention of the solids.
Figure 4 – Diagram depicting the layout of a filter press, including sludge flow and components. Image Credit: Lenntech BV
Filter presses generally operate as semi-continuous or batch drying processes. They have very high drying efficiency while requiring a smaller design area than most other dewatering machinery. Costs tend to be very high for filter presses based on their power requirements, maintenance needs, and use of pretreatment chemicals.
Video depicting an automatic filter press in operation. Video Credit: Micronics Filtration
Gravity and Low Pressure Devices
Gravity and low pressure devices are a subset of filter presses that utilize a combination of gravity drainage (used in some drying beds) and low pressure pressing devices. Some employ rotating cylindrical beds to continually expose different areas of the sludge cake to proper drainage. Low pressure belt presses can be utilized along the gravity beds to allow for increased solid-water separation.
Figure 5 – Photo of a gravity belt thickener. Image Credit: Siemens, Scheel & Company, Inc.
This equipment tends to offer simplicity, low cost, low energy and maintenance costs, small space requirements, and little noise. All of these factors tend to be convenient for smaller treatment and operating plants. However, the solids being treated with these devices require large doses of chemical conditioners prior to dewatering in order for effective handling and draining.
Drying beds consist of perforated or open joint drainage pipes laid within a gravel base covered with a layer of sand. Sludge is placed on top of this sand layer and allowed to dry. Water is removed through natural evaporation and by gravity draining from the sludge mass through the supporting sand to the drainage piping. Cracks develop as the sludge dries, allowing evaporation to occur from the lower layers which accelerates the drying process.
Figure 6 – Diagram depicting how a drying bed operates. Image Credit: VCCS.edu.
Drying bed sludge needs to be well digested prior to drying. Design parameters include depth of sludge, moisture content of sludge, and available sand bed area. Drying beds are able to achieve very dry cakes for low cost if given enough time; however, the time required for extensive drying can be months and varies based on the weather (if outside). If the feed is not well digested, odors can also be a problem during drying.
Sludge lagoons are excavated areas in which digested sludge can be deposited and dried for several months to a year or more. Depths may range from two to six feet. After the solid dries, it can be removed for lagoon reuse or leveled to be developed into lawn or soil.
Figure 7 – Diagram depicting how a lagoon operates. Image Credit: VCCS.edu
Lagoon utilization requires only the needed land area and equipment required for their excavation; thus, they essentially cost nothing to operate. They have very little versatility however. They are limited to applications with no drying time constraints and solids that contain no hazardous materials that could contaminate groundwater.
Selecting the most effective dewatering equipment for an application can be summed into four major considerations. These are the drying requirements, the cost constraints, the sludge characteristics, and the available area.
The drying requirements and cost constraints will come first in the selection process. Most importantly, the selected method must be able to dewater the sludge to the desired level within a certain period of time. The costs associated with chemical usage, capital (including the machine and all auxiliary equipment and fixtures), and operation may also render some methods impractical.
In addition, an engineer or industrial buyer must keep in mind that equipment compatibility can be affected by the corrosion potential of the sludge, hazardous contaminants, moisture content, and its level of digestion. It may also require equipment to be constructed with durable, possibly expensive materials. Though most machines come in various sizes, the area available may also need to be compared to the size requirements of different methods.