If you haven't already, read Part 1: The Ultimate DAF System Buyer's Guide
3. Materials of Construction
When selecting a DAF system, durability is paramount. The harsh environments that DAF systems are exposed to demand robust materials to ensure long-term performance and minimize replacement costs. Let’s explore the most common options for tank construction, along with their advantages and limitations:
Concrete
Steel-reinforced concrete basins are commonly used in large municipal wastewater treatment plants. These basins are robust and leak-resistant but come with high costs due to the extensive civil work required, including excavation, steel reinforcement, concrete forming, and coating. Additionally, because they must be built on-site, concrete DAF basins are not typically practical for industrial facilities.
Polypropylene
Polypropylene is favored by some manufacturers for its lower material cost, decent strength, and broad chemical resistance; however, it has drawbacks. Exposure to extreme temperatures or UV radiation can degrade the material, causing discoloration and cracks. At temperatures below 32°F, it becomes brittle and prone to cracking. Furthermore, polypropylene’s degradation over time makes refurbishment rarely viable. Most manufacturers offer a limited warranty of around 10 years for polypropylene tank structures.
Epoxy-Coated Carbon Steel
Epoxy-coated carbon steel combines the strength of steel with the corrosion resistance of an epoxy coating, making it suitable for applications with high Total Dissolved Solids (TDS). However, in industries like food processing, this material is less reliable. Free fatty acids present in floating sludge can erode epoxy coatings, exposing the steel to rust and compromising structural integrity. While initially perceived as a cost-effective alternative to stainless steel, achieving comparable strength and corrosion resistance often makes epoxy-coated carbon steel similarly expensive.
Stainless Steel
Stainless steel is widely used in DAF tank construction due to its durability and versatility. Its natural chromium oxide layer prevents rust, allowing it to withstand temperatures from -320°F to 1500°F. Stainless steel is well-suited for both indoor and outdoor applications, and modifications can be easily made without the need for recoating. Tanks made from stainless steel can remain structurally sound for decades, retaining a high resale value and often being refurbished for continued use. However, stainless steel does come at a higher initial cost and may not perform well in environments with high chloride concentrations, which can cause pitting or corrosion.
4. Sludge Thickening Mechanisms
The core function of a DAF system is to remove solids and oil contaminants from wastewater. Sludge disposal is one of the largest costs of operating a DAF system. Achieving dryer sludge reduces the sludge volume which increases efficiency and reduces disposal costs. Sludge consistency depends on several factors, with the largest being the chemical processes used to treat the wastewater. That said, a carefully engineered design will include features that can increase sludge dry solids performance.
Sludge Dewatering Grid
A dewatering grid is a stationary framework of angled steel plates installed at the water’s surface. This grid locks sludge in place as it rises, allowing it to dewater before skimmer blades push it toward the sludge ramp. Retention in the grid ensures higher dry solids content, resulting in less watery sludge.
Without a dewatering grid, sludge may accumulate near the ramp and get forced back into the water, undoing any prior dewatering.
Speed Adjustable and Time Adjustable Skimmer System
Varying the speed of the skimmer system on a DAF unit can influence the dry solids content of the sludge. If the skimmer runs too quickly it can create turbulence which can resuspend solids causing carry over in the effluent. A skimmer system that runs too slowly allows the sludge blanket to overthicken which can cause the same issue. An optimal speed should remove the last 10-15% of sludge from the surface of the DAF.
In applications where the sludge volume produced is low, it may not be necessary to run the skimmer continuously. Turning the skimmer off intermittently will allow the sludge blanket to thicken and dewater producing a dryer sludge. A system with the ability to run the skimmer at operator adjustable intervals can improve the dryness of the sludge without requiring operator intervention.
Easily Adjustable Effluent Weir
The water content of the DAF sludge is directly influenced by the level of the water inside the DAF vessel. In most DAFs this level is set with adjustable weirs. If the weir is too high, the skimmer system removes more water with the sludge. If the weir is too low the sludge can overthicken causing resuspension of solids and carryover in the effluent. A weir system that is quick and easy allows for optimization of the water level in the DAF which improves sludge dryness.
By incorporating these features, DAF systems produce thicker sludge, leading to significant cost savings. Thicker sludge requires less storage, smaller dewatering equipment, and reduced chemical usage for filtrate reprocessing.
When evaluating DAF systems, ask manufacturers how their designs optimize sludge consistency. Key questions include:
What mechanisms ensure drier, thicker sludge?
How can operators adjust sludge thickness to meet process requirements?
By prioritizing these features, you can select a DAF system that balances performance, reliability, and cost-efficiency.
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