Dewatering is a filtration process designed to separate water from solids using force exerted by vacuum and centrifugal dewatering equipment. 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.
In construction, dewatering is used to remove groundwater or surface water from a construction site via pumping or evaporation. It is usually done before excavation for footings and will help to lower the water table that might cause problems during excavations. Dewatering can also refer to the process of removing water from the soil by wet classification. Wet classification is a construction process that looks at the size of particles that make up the underlying soil in a project site and the flow of fluids through those particles.
Construction dewatering is used on most construction sites due to accumulated water in trenches and excavations or in places with an inadequate slope or high water table. In construction projects, this water should be removed to keep working as scheduled or to provide a safe workplace. Normally, builders tend to use water pumps to dewater these areas, but if they are not paying attention to the place where water is discharged, erosion and other problems may occur. It is important to follow best management practices when water is being pumped to lakes, wetlands, or directly to storm sewer inlets.
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.
There are numerous ways to dewater if construction sites with open excavation or trenches. The simplest of these involves using a gravity drain i.e. sloped drainage channels carrying away water from the area to be worked to the discharge point. Other dewatering methods include pumping, siphoning, or using large construction machinery buckets to scoop and dump water from the selected area.
In other applications, dewatering can also be done using centrifuges, filter presses, drying beds, sludge lagoons, and gravity and low pressure devices. Each method is described in detail below:
Centrifuges: 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.
Vacuum filters: 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. 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.
Filter presses: 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. 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.
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. 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: This method consists of using 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. 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: 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. 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.
When selection the most effective dewatering equipment for an application, consider the site's drying requirements, the cost constraints, the sludge characteristics, and the available area. 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. Also 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.
Interested in learning more? Give one of these local dewatering service providers a call today and ask for their expert advice!