Rising Above the Rest: How DAGYEE's DAF Achieves Unmatched TSS and FOG Removal
DAGYEE Dissolved air flotation (DAF) Systems: Comprehensive FAQ
## Section 1: Fundamentals & Basic Concepts
### **Q1: What exactly is Dissolved Air Flotation (DAF), and what is its primary purpose?**
**Dissolved air flotation (DAF)** is a water treatment process that clarifies wastewaters (or other waters) by the removal of suspended matter such as oil or solids. The removal is achieved by dissolving air in the water or wastewater under pressure and then releasing the air at atmospheric pressure in a flotation tank basin.
The primary purpose of a DAF system is to reduce **Total Suspended Solids (TSS)**, **Biochemical Oxygen Demand (BOD)**, and **Fats, Oils, and Greases (FOG)** from a waste stream. Unlike sedimentation, which relies on gravity to make heavy solids sink, DAF relies on buoyancy to make light solids float.
### **Q2: How does the DAF process actually work?**
The process operates on the principle of **Henry’s Law**, which states that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid.
The operation follows these steps:
1. **Recycle Stream:** A portion of the clarified effluent water is recycled, pressurized via a pump, and mixed with compressed air in a saturation vessel.
2. **Saturation:** Under high pressure (typically 4–6 bar or 60–90 psi), the air dissolves into the water.
3. **Pressure Release:** This air-saturated water is released into the main flotation tank through pressure-reducing nozzles.
4. **Microbubble Formation:** The sudden drop in pressure causes the dissolved air to come out of the solution, forming millions of microscopic bubbles (typically 30–50 microns in diameter).
5. **Attachment:** These microbubbles attach themselves to suspended solid particles and oil droplets.
6. **Flotation:** The bubbles act like tiny life vests, reducing the density of the solids to less than that of water, causing them to rapidly float to the surface.
7. **Skimming:** The accumulated floating sludge (froth) is removed by a mechanical skimming device, while the clear water is drawn from the bottom.
### **Q3: What is the difference between a DAF system and a traditional Sedimentation Clarifier?**
While both systems separate solids from water, they use opposite forces:
* **Sedimentation Clarifiers:** Rely on **gravity**. They are effective for heavy solids (sand, grit, heavy sludge) that sink naturally. They usually have a large footprint and require long retention times.
* **DAF Systems:** Rely on **buoyancy**. They are effective for light, neutral-density particles (algae, oil, grease, fibrous solids) that do not settle easily. DAF systems generally have a much smaller footprint and faster processing times than clarifiers.
### **Q4: What specific pollutants does a DAF remove?**
DAF systems are highly efficient at removing:
* **TSS (Total Suspended Solids):** Removal rates typically exceed 90–95%.
* **FOG (Fats, Oils, and Grease):** Removal rates often exceed 95–99%.
* **Insoluble BOD/COD:** By removing the organic solids, the associated biological and chemical oxygen demand is also significantly reduced (often by 40–70%, depending on the soluble vs. insoluble ratio).
* **Algae:** Commonly used in municipal water treatment for algae bloom removal.
* **Heavy Metals:** If precipitated chemically beforehand, DAF can float the metal flocs.
## Section 2: Technical Specifications & Design
### **Q5: What is "White Water" in the context of DAF?**
"White water" refers to the recycle stream after it has been pressurized and released into the flotation tank. Because it is saturated with millions of microscopic air bubbles, the water appears milky or white. The quality and consistency of this white water are critical; without a uniform "cloud" of microbubbles, the flotation process will fail.
### **Q6: Why is bubble size important?**
Bubble size is a critical design parameter.
* **Too Large (>100 microns):** The bubbles rise too fast, causing turbulence that can break up fragile floc structures. They also have less surface area for solids to attach to.
* **Too Small (<10 microns):** They may not have enough buoyancy to lift the solids effectively.
* **Ideal Range (30–50 microns):** This size provides the optimal balance of rise velocity and surface area, ensuring gentle lifting of solids without shearing the floc.
### **Q7: What is the Air-to-Solids (A/S) ratio, and why does it matter?**
The Air-to-Solids ratio is a measure of the mass of air released relative to the mass of solids in the influent. It is defined as:
$$A/S = \frac{Mass_{air}}{Mass_{solids}}$$
A typical A/S ratio ranges from **0.02 to 0.06**.
* If the ratio is too low, there aren't enough bubbles to float all the solids.
* If the ratio is too high, it wastes energy and creates excessive turbulence.
Properly calculating the A/S ratio is essential for sizing the recycle pump and saturation system.
### **Q8: What is the Hydraulic Loading Rate (HLR)?**
The HLR represents the volume of water flowing through the DAF per unit of surface area per unit of time (e.g., $m^3/m^2/hr$ or $gpm/ft^2$).
* Standard DAF units typically operate between **5 to 15 m/hr**.
* High-rate DAF units (often using plate packs) can operate at **15 to 30 m/hr**.
Exceeding the design HLR results in "carryover," where solids are pushed out with the clean effluent because the downward velocity of the water exceeds the upward rise velocity of the bubbles.
### **Q9: Do I need Plate Packs (Lamellas) in my DAF?**
**Plate packs** are inclined plates located inside the DAF tank. They increase the effective separation area without increasing the physical footprint of the machine.
* **Pros:** They allow the unit to handle higher flow rates in a smaller space and encourage laminar flow.
* **Cons:** They can become clogged if the wastewater contains sticky, tenacious solids or high levels of calcium carbonate.
* **Verdict:** Use plate packs for clean, high-flow applications. Avoid them for extremely greasy or sticky industrial sludges unless you have a robust cleaning protocol.
## Section 3: Chemical Treatment (Coagulation & Flocculation)
### **Q10: Can DAF work without chemicals?**
Technically, yes, but only if the solids naturally float (like pure free oil). However, for **emulsified oils** or **colloidal suspended solids**, DAF alone is rarely sufficient. Chemical pre-treatment is almost always required to achieve environmental discharge limits.
### **Q11: What is the difference between Coagulation and Flocculation in a DAF process?**
These are two distinct steps usually performed in a pipe flocculator or mixing tank before the water enters the DAF:
1. **Coagulation:** This involves adding a chemical (like Ferric Chloride or PAC - Polyaluminum Chloride) to neutralize the negative electrical charge of the suspended particles. This allows the particles to come together rather than repelling each other.
2. **Flocculation:** This involves adding a polymer (polyelectrolyte) to bridge the coagulated micro-particles into larger, visible "flocs" (clumps). These large flocs entrap air bubbles easily and float rapidly.
### **Q12: What is a "Jar Test" and why do I need one?**
A **Jar Test** is a laboratory simulation of the coagulation/flocculation process. It is the only reliable way to determine:
* The correct type of chemical to use.
* The optimal dosage (ppm).
* The required mixing time.
* The anticipated clarity of the effluent.
Without a jar test, sizing a DAF and predicting its performance is effectively guessing.
### **Q13: How does pH affect DAF performance?**
pH is crucial for the chemical reaction. Most coagulants work within a specific pH range (e.g., Aluminum-based coagulants usually prefer pH 6.0–7.0). If the wastewater pH fluctuates (e.g., in a food processing plant with CIP cycles), you must install a **pH Neutralization** step upstream of the DAF. If the pH is outside the optimal range, the floc will not form, and the DAF will not separate the solids.
## Section 4: Applications & Industries
### **Q14: Which industries benefit most from DAF technology?**
DAF is versatile, but it is the "gold standard" for:
1. **Food & Beverage:** Dairy, slaughterhouses, meat processing, breweries, and snack food production. These waste streams are high in FOG and organic solids.
2. **Oil & Gas:** Produced water treatment and refinery wastewater.
3. **Pulp & Paper:** Fiber recovery and water recycling.
4. **Textiles:** Color removal and suspended solids removal.
5. **Municipal Desalination Pre-treatment:** Removing algae and suspended solids to protect Reverse Osmosis membranes.
### **Q15: Can DAF handle hot water?**
Yes, but temperature affects the solubility of air (air dissolves less effectively in hot water) and the viscosity of the water.
* **Above 60°C (140°F):** Special design considerations are needed. The saturator may need to operate at higher pressures, or the recycle rate may need to be increased to compensate for lower air solubility.
* **Materials:** Ensure the tank materials (polypropylene vs. stainless steel) can withstand the operating temperature.
### **Q16: Is DAF suitable for removing dissolved solids (TDS)?**
No. DAF is a physical separation process. It removes **suspended** solids. It does **not** remove dissolved salts, sugars, or minerals (TDS). To remove TDS, you would need biological treatment, evaporation, or membrane technologies (RO) *after* the DAF.
### **Q17: How does DAF compare to a Membrane Bioreactor (MBR)?**
* **DAF:** Primary treatment. Removes bulk solids and grease. Cheap to operate, robust.
* **MBR:** Secondary/Tertiary treatment. Uses biological degradation + physical barrier. Produces extremely high-quality water but is expensive, sensitive to oil/grease, and high maintenance.
* **Synergy:** DAF is often used *before* an MBR to remove the heavy oil load that would otherwise foul the membranes.
## Section 5: Operation, Maintenance, and Sludge Handling
### **Q18: How dry is the sludge produced by a DAF?**
One of the major advantages of DAF is sludge thickening. Because the skimmer scrapes the solids from the surface (where water drains away naturally), DAF sludge is significantly drier than sedimentation sludge.
* **DAF Sludge:** Typically 3% to 6% dry solids.
* **Sedimentation Sludge:** Typically 0.5% to 1% dry solids.
This higher solids concentration means significantly lower volumes of sludge to haul or dewater, reducing disposal costs.
### **Q19: What is the maintenance schedule for a DAF?**
Routine maintenance is relatively low but critical:
* **Daily:** Visual check of the "white water" and effluent clarity. Check chemical dosing pumps.
* **Weekly:** Grease skimmer bearings and chain drive. Drain the air compressor tank.
* **Monthly:** Clean the pressure release nozzles (if clogging occurs). Check oil levels in the recycle pump.
* **Annually:** Inspect the internal tank for sediment buildup (bottom sludge) and flush if necessary. Inspect saturator vessel seals.
### **Q20: Do I need a bottom auger/scraper?**
This depends on your waste stream.
* **Floating Solids Only:** If 99% of your solids float (e.g., pure oil), a bottom scraper may not be necessary; a simple V-bottom design with a periodic drain valve is sufficient.
* **Settleable Solids:** If your wastewater contains grit, sand, or heavy organic matter that might sink, a bottom auger is **mandatory** to prevent solids from accumulating and turning septic (rotting) at the bottom of the tank.
### **Q21: What materials of construction should I choose?**
* **Stainless Steel (SS304):** The standard for food and beverage. Corrosion-resistant and sanitary.
* **Stainless Steel (SS316):** Required for high-salinity water, acidic wastewater, or marine applications.
* **Carbon Steel (Epoxy Coated):** A cost-effective option for general industrial, non-corrosive applications (e.g., oilfields).
* **Polypropylene/Plastic:** Excellent for high-corrosion environments (strong acids/bases) or smaller flow rates.
## Section 6: Troubleshooting Common Issues
### **Q22: My effluent is cloudy/turbid. What is wrong?**
Cloudy effluent usually points to a chemical or physical failure:
1. **Chemical Imbalance:** Check your dosing pumps. Are you adding enough coagulant? Is the pH correct? Perform a new jar test.
2. **Shearing:** Are you mixing too aggressively upstream? You might be breaking the flocs before they enter the DAF.
3. **Hydraulic Overload:** Are you pushing more water through the system than it was designed for?
### **Q23: The solids are not floating; they are just suspended in the middle.**
This is likely an **Air-to-Solids Ratio** issue.
1. **Check the White Water:** Is the recycle pump running? Is the compressor running?
2. **Nozzles:** Are the injection nozzles clogged?
3. **Saturator:** Is the saturator waterlogged (full of water with no air head)? You may need to adjust the level control in the saturator tank.
### **Q24: I have excessive foam on top of the DAF.**
While some froth is normal, excessive billowing foam can be caused by:
1. **Surfactants:** High levels of soaps or detergents in the cleaning water.
2. **Air Pressure:** Excessive air release.
*Solution:* You may need a **defoaming agent** (chemical) or water spray nozzles to knock down the foam.
### **Q25: Why is sludge carrying over into the clean water weir?**
1. **Skimmer Speed:** The skimmer might be running too slowly, allowing the sludge blanket to get too deep and flow under the baffle. Increase skimmer speed.
2. **Recycle Rate:** If the recycle flow is too high, it creates turbulence that pulls sludge down.
3. **Solids Loading:** The influent solids load has spiked beyond the design capacity.
## Section 7: Commercial & Purchasing Considerations
### **Q26: How do I size a DAF system for my facility?**
Never size a DAF solely on flow rate (e.g., "I need a 10 m³/hr machine"). You must consider:
1. **Peak Flow Rate:** The maximum surge, not just the average.
2. **Solids Loading (TSS):** A machine handling 100 ppm TSS is very different from one handling 5,000 ppm TSS.
3. **Desired Outlet Quality:** Are you discharging to a sewer (loose limits) or a river (strict limits)?
*Consult with the manufacturer to calculate the surface area loading and solids loading rate.*
### **Q27: What is the typical ROI (Return on Investment) for a DAF?**
ROI comes from three sources:
1. **Surcharge Reduction:** Municipalities charge heavy fines for high TSS/BOD/FOG discharge. A DAF can eliminate these surcharges, often paying for itself in 6–18 months.
2. **Product Recovery:** In dairies or paper mills, the recovered solids (butterfat or pulp) can sometimes be re-sold or reused.
3. **Water Reuse:** Treating water allows for recycling it for wash-down or irrigation, lowering freshwater utility bills.
### **Q28: What is included in a standard DAF package?**
A complete DAF system usually includes:
* The main Flotation Tank (with skimmer and bottom auger).
* The Recycle Pump (high pressure).
* The Air Saturation Vessel (ASV).
* The Air Compressor.
* The Control Panel (PLC/HMI).
* *Optional but recommended:* Pipe Flocculator, Chemical Dosing Skids, and Feed Pumps.
### **Q29: How much energy does a DAF consume?**
DAF systems are generally energy-efficient. The main energy consumers are:
1. **Recycle Pump:** The largest consumer (typically runs continuously).
2. **Air Compressor:** runs intermittently.
3. **Skimmer Motor:** Very low consumption.
Compared to biological blowers in aerobic treatment, DAF energy consumption is low.
### **Q30: What information do I need to provide to get a quote?**
To get an accurate quote, provide:
1. **Water Analysis:** TSS, FOG, BOD, pH, Temperature.
2. **Flow Rate:** Max and Average ($m^3/hr$ or GPM).
3. **Discharge Requirements:** What limits must you meet?
4. **Space Constraints:** Available footprint (L x W x H).
5. **Power Supply:** Voltage and frequency available on site.
Contact DAGYEE:
- Company: Wuxi Dajiang Environmental Technology Co., Ltd. DAGYEE
- Phone/WhatsApp:+8613961861780
- Email: info@dagyee.com
- Website: www.dagyee.com
DAGYEE – Your Partner in Advanced Water Treatment Solutions.