Depending on the operations and processes utilized, the sludge produced by wastewater treatment operations and processes often takes the form of a liquid or a semi-solid liquid, and typically comprises 0.25–12% solids by weight.
Due to the increased volume of sludge and the presence of organic content in it after biological treatment, its processing and disposal are arguably the most challenging issues. To supply a comparatively steady, homogeneous feed to sludge processing plants, various procedures are carried out on the sludge, and various products for sludge dewatering are thus utilized.
What are the various products used for sludge dewatering?
The following technologies can be used to treat the sludge:
· Filtration press
· A belt press
· A screw press
· Anaerobic digester
· Screw dehydrator
1: Filtration press or Filter press
· Sludge and bio-solids are dewatered using a filter press, which lowers their moisture content.
· The series of square plates with recessed edges make up the fixed volume.
· Recessed plate filter press are supported face to face in a vertical position, on a frame with a fixed and moveable head.
· Each plate has a filter cloth attached to it or hung over it. The plates are firmly fastened together in order to shut them off, from the pressure used during the filtration process.
· The process involves pumping chemically treated sludge into the gap between the plates, applying and maintaining pressure, and forcing the liquid through the filter cloth.
· The cake is then taken off the platters and separated.
· The moisture content of the sludge cake ranges from 48 to 70%, and its thickness ranges from 25 to 45 mm.
2: Belt Press
a. Continuous sludge dewatering is possible using belt filters between two filter belts. Sludge travels through many zones in it.
b. After the sludge and flocculant have been blended, the sludge is flocculated and then evenly spread across the filter belt.
c. When the inflow stream enters the clarifier, it becomes still. In order to catch the solid particles that are settling to the bottom of the clarifier, as the liquid is being cleared and leaving, hoppers are kept there.
d. The picket fences that are lying freely on the first belt aid with drainage, since the sludge drains there. The majority of the water is removed through flocculation, creating a water line in a zone of gravity drainage.
e. The majority of the water is removed through flocculation, creating a water line.
f. The sludge is compressed between two filter belts after the water liberated, by flocculation has been drained in azone of progressive compression.
A gradual pressurization begins with the upper belt's appearance:
· For low-pressure belt filters, up to 4 bars.
· For medium pressure belt filters, up to 5 bars.
· For high-pressure belt filters, up to 7 bars.
g. In a scraping area for cakes, when pushed, the sludge takes on a more solid appearance. It is referred to as a sludge cake. The two belts that divide at this level are then scraped clean of this cake.
h. In a high-pressure washing station, each belt is continuously cleaned by a bank of nozzles operating at 7 to 8 bars (100 to 120 PSI).
3: A screw press
· A ram is forced up and down by a screw in a machine known as a screw press.
· The downward force is produced by converting the rotation of the lever, into a coarse screw.
· Usually, balls act as flyweights in the lever. These weights aid in maintaining the tool's push and motion, making it simpler to use.
· The solid-liquid separation process is sped up by centrifugal force in a centrifuge, sometimes referred to as a centrifugal decanter.
· A centrifuge is a horizontally rotating conical cylinder decanter with an overflow of clarified water, and an Archimedean screw to extract the dewatered sludge.
· The solid particles experience a centrifugal force as a result of the rotation, which causes them to move much more swiftly.
· Through an injection hose, the flocculated sludge is placed inside the centrifuge bowl. The particles in the clarifying zone are flattened against the bowl's sides, while the bowl rotates quickly.
· Then, in the sludge spin-dry zone, these particles are driven by an Archimedean screw towards the bowl's cone's end.
· The distilled liquid known as concentrate, a cleared liquid, is drained from the other end of the bowl, via overflow.
5: Anaerobic digester
· The biological process of digestion results in the steady decomposition of organic materials. By breaking down sludge's volatile constituents, it lowers the overall mass of solids, and facilitates the dewatering or drying of sludge.
· Hydrocarbon gas, which can be used for any thermal application, is created from these volatile materials.
Anaerobic digestion characteristics
· The goal is to reduce the amount of SRT (Sludge Retention Time) needed to destroy, the most volatile suspended solids.
· Typically, the operating range for digestion is 30 to 45 degrees Celsius.
· In situations when the retention duration for concentrated sludge is between 10 and 20 days, single stage high rate digestion is suggested.
· To boost SRT, which in turn increases generation of digestive gas, sludge must be thickened.
· Due to the reduced volatile content and lower handling costs, the digested sludge is about half as concentrated, as the untreated sludge feed.
· An external recirculation system composed of draught tubes is used, to mix the sludge. This configuration is simpler to operate, has greater mixing control, and allows for the recirculation of scum layer and sludge deposits.
· Digested gas typically ranges from 0.75 to 1.12 m3/kg of volatile solids eliminated, and comprises 65 to 70% methane by volume.
6: Screw dehydrator
· As a new component of the screw press, it consists of a stationary ring, a moving ring, and a screw that presses the sludge, and thrusts the filter element.
· The inner pressure of the filter element rises as a result of the volume compression effect, which thickens and dewaters the sludge, and the gaps between the rings and the screw pitch are intended to gradually get smaller, towards the direction of the sludge cake outflow.
· These are designed for use in a variety of slurry applications in sewage and effluent treatment facilities.
Characteristics of a screw dehydrator
· Flexible in handling solids from 0.8% to 5% in slurry.
· It can lessen the requirement for sedimentation and tank thickening.
· It can take the place of decanters, filter presses, belt presses, and other dewatering equipment.
· Screw rotation occurs at a relatively low speed, which results in minimal electricity and water usage.
· Slurry storage, a flocculation tank, a dehydrator zone, and a dosing system are all included in a single skid.
· The system can be configured to run unattended and shut down without operator involvement, once it has been launched.
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