Water treatment desanders - what are they and how do they work?
What is a desander?
Desanders are used to remove suspended solids from liquid process streams. One of the primary applications is to improve the quality of waste water.
Treatment of waste water normally involves multiple process steps to remove the impurities and ensure that the final effluent discharge is compliant to regulatory specifications. A simplified breakdown of the steps involved is as follows:
- Removal of solids
- Removal of hydrocarbons
- Chemical treatment
- Testing and release
Each step can be accomplished through a number of process units depending on the complexity of the operation and the potential contaminants present. A desander fits into the solids removal step of waste water treatment.
How does a desander work?
A desander has an entry port near the top on the side of the vessel, a hydrocyclone central component, and two discharge points (one on the top of the vessel and one on the bottom). The hydrocyclone is oriented so that the wide opening is at the top and the narrow opening at the bottom.
The product stream enters the vessel at an angle such that the flow path creates a circular pattern around the inside of the hydrocyclone. The narrowing shape of the cyclone causes the water to swirl around the cyclone at high speed. Centrifugal forces push the heavier solid particles against the sides of the cyclone, where they migrate downwards to be removed from the bottom of the vessel. The water and very light particles are pulled towards the central axis by the cyclonic action and rise to exit the discharge port on the top of the vessel.
Hydrocyclones are designed according to the efficiency of solids removal required and the flow rate of the product stream. A typical waste water desander will remove particles with a mean of up to 10 microns. Any flow rate can be accommodated by appropriate sizing of the hydrocyclone design.
Waste water screens are sometimes used upstream of desanders to remove large items such as rags and bolts, and protect the cyclones from damage. A desander can operate for up to 10 years if properly protected and maintained.
Environmental benefits of using desanders?
In terms of waste water treatment, the specific role of the desander is to remove solids which may contain some bacteria, thereby improving the clarity of the water.
Regulators use turbidity to set specifications for the safe discharge of waste water. Turbidity is a measure of the amount of light that passes through a water sample. The higher the amount of suspended solids, microbes, and other substances in the water, the less light will be transmitted and the higher the turbidity result. Water with high turbidity tends to be hotter because solids absorb heat and this causes a reduction in dissolved oxygen, which has a negative impact on plant and animal life.
Mechanical benefits of using desanders
Downstream filtration elements require cleaning, maintenance and replacement as part of the normal operating cycle of a waste water treatment process. The efficiency of the desander has a direct influence over the load placed on the filtration equipment and therefore its cost of operation and reliability. An excess of suspended solids that are not removed by a desander will enter the filter and build up quickly on the filtration element.
Pumps in the waste water processing system are also vulnerable to solid contaminants. Every pump operates with a seal that prevents product leakage and solid particles are very harmful to these seals. The scouring effect of pumping water with suspended solids damages the seal faces resulting in leaks, environmental concerns and equipment outages.
Where are desanders used?
Wastewater and other product streams in any industry where suspended solids may be present can benefit from the introduction of a desander in the treatment process.
- Mine sites use desanders for removal of suspended solids in the waste water treatment process.
- Waste water from wash down bays, construction sites, factory processes and other sources always contain solid particles, which must be removed for regulatory compliance or to prepare the water for recycling.
- Industrial process water is used within factories and is normally a closed loop system. Desanders are essential to keep suspended solids from causing damage to equipment.
Contact Cleanawater for more information about desanders and water treatment
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Cleanawater on 26 September 2023
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