1 Best Practices for BNR Mixing (Biological Nutrient Removal)Welcome to this webinar on Best Practices for BNR (Biological Nutrient Removal) Mixing. Thank you for joining us. Rana Elbittibssi, P.Eng.
2 Goals of BNR Mixing Prevent settlingPrevent short-circuiting of inflows Force strong contact between microbes and wastewater. Minimize energy consumption Maximize process flexibility
3 Mixing basics Mixers are like hammersMixers are like hammers – strong tools that do great work when you use the right size and when you aim them the right way …but with the wrong size or bad aim, you can waste energy, miss the mark, and even cause damage. Before we can talk about best practices for BNR mixing, it helps to understand the physics of submerged jet mixing. How does a submerged jet create mixing? As with a hammer, size and aim are very important. How does a submerged jet create mixing?
4 Submerged Jet Mixing Basics Creating Mixing and Bulk FlowMany flows, one source Inflow Primary flow Entrained flow Bulk Flow Jet Entrained flow Inflow 3 Good mixer positioning hinges on knowing how mixing with a submerged jet works. So before we get into the details of best practices with mixer positioning, let’s talk about how mixing is done with a submerged jet. It’s the jet that creates mixing, but how does it work? It helps to break down the jet into its component parts. In a well-mixed tank there are many flows with one source, the mixer. This slide shows three of these flows. The fourth one, bulk flow, we will talk about in the next slide. For now take a good look at this image and let’s talk about the three flows shown. The inflow is the flow that enters the mixer propeller. It’s important to know that most of the inflow enters radially. In other words, from all directions around the circumference of the propeller % of the flow enters from this direction versus only % which enters axially or from the back of the mixer. That’s why it’s important to have good clearance between mixers and side walls or floors -- to allow for unhindered approach flow. It also means it is OK to locate mixers close to a rear wall since little flow enters from the back of the propeller. This often means mixers are located in corners, which gives the benefit of longer jet length. The second type of flow is the primary flow. This is the flow that actually passes through the mixer propeller. The next type of flow is entrained flow. The primary flow and the entrained flow together make up the jet. The interesting thing that may not be obvious: most mixing takes place along the jet border. Entrained flow is indicated in the image by the small curved arrows along the borders of the jet. This zone is a very key part of mixing because it is actually here where the mixing takes place. This is very important because it means the longer the mixer jet before it is deflected by something like a wall, the more mixing happens. So to get the most mixing out of a mixer jet, the jet needs enough open space to fully develop. Then it can create a tremendous amount of mixing. Primary Flow Inflow Entrained flow 4
5 Mixer Positioning Creating Mixing and Bulk FlowIntensive mixing zone Bulk Flow 4 Now let’s talk about bulk flow. In addition to having room for good jet development, a well-positioned mixer also gets the entire tank contents moving in one large loop. This large, loop-shaped flow is what we call bulk flow, and you can think of it like a racetrack. So a well-designed mixing system includes two main ingredients; first, an intensive mixing zone directly in front of the mixer along the entire length of the jet; and secondly, a bulk flow loop. Once the tank comes up to speed, every drop of liquid in the tank, carried by the bulk flow, (think of the liquid as cars on the racetrack) – every drop eventually passes through the intensive mixing zone. And that, my friends, is the “magic” of submerged jet mixing. This image shows a well-placed mixer in the upper image and a not so well placed mixer in the lower image. The upper image shows a single, strong bulk flow loop while the lower image shows the jet split into two smaller, weaker bulk flow loops. This leads to an undesirable calm zone indicated here by the small circle with waves inside.
6 Mixer Positioning Mixer JetJet drives both primary flow and bulk flow Jet brings the surrounding liquid into motion The surrounding low-velocity liquid is entrained Majority of the mixing is not in the prop-area Intensive mixing happens along the jet border 5 In summary, the jet drives both primary flow and bulk flow. The jet brings the surrounding liquid into motion. The surrounding low velocity liquid is entrained into the jet, and that’s where the actual mixing takes place. Contrary to what you might think, the majority of the mixing does not happen in the propeller. Nearly all of the intensive mixing happens along the jet border. 6
7 Mixer Positioning for a bulk flow loopDetermine an efficient and strong bulk flow loop Locate the mixer along the streamlines of the loop Position for a long jet path Smooth Jet deflection Steer clear of obstacles 6. Now let’s talk about mixer positioning and how to make the most of the intensive mixing zone and the bulk flow loop. First, determine an efficient bulk flow loop. An efficient bulk flow loop has smooth jet deflection for low hydraulic losses. And because mixing happens along the jet border the longer the jet path, the more mixing you get. This often means the mixers are located in corners. Second, after you determine the bulk flow loop, locate the mixer or mixers so that are directed along the stream lines of the loop. Third, aim the jet for smooth jet deflection and to steer clear of obstacles.
8 Submersible Mixer Positioning 1Submersible Mixer Positioning 1. Determine the most natural, efficient bulk flow loop 7 Let’s look at each step with a visual to help picture it. First, determine an efficient bulk flow loop with a minimum of hydraulic losses. This image shows a typical tank with its length longer than its width. In this case always direct the loop along the length of the tank in an oval or race track shape.
9 Submersible Mixer Positioning 2Submersible Mixer Positioning 2. Locate the mixer along the streamline of the loop 8 Locate the mixer along the streamline of the loop.
10 Submersible Mixer Positioning 3. Position for a long jet pathLong jet paths entrain more flow and develop stronger bulk flow 9 Always position the mixer so that you have as long a jet path as possible. This yields more mixing because mixing takes place along the jet borders, illustrated here by the green arrows. This provides both the largest possible fluid entrainment and the largest possible bulk flow. This often means that the mixer is placed in a corner.
11 Submersible Mixer Positioning 4. Smooth jet deflectionSmooth jet deflection: Yields low hydraulic losses 10 Always position the mixer for smooth jet deflection. This yields low hydraulic losses and conserves the energy that creates bulk flow.
12 Submersible Mixer Positioning Long jet path & smooth deflectionPosition mixer cross diameter or center of the tank with longest jet path, Lead to worst possible jet deflection 30o 15~30o Aim for both long jet path and smooth jet deflection 11 Creating both long jet path and smooth jet deflection sometimes requires optimizing or compromising. In round tanks for example don’t aim the mixer directly towards the center or across the diameter as shown in the image on the left with the red X. Even though that would be the longest jet path, you would get the worst possible jet deflection. Instead, aim the mixer about 30 degrees away from the center of the tank. In square or rectangular tanks there’s a similar situation. The longest jet path would be from one corner to the other. However, this of course would also be the worst position for smooth jet deflection. A good compromise is about a 15 degree angle off the side wall.
13 Submersible Mixer Positioning 5. Steer clear of obstaclesPipes, Pillars ... Bends, Aerators ... 12 Always aim the mixer away from obstacles such as pipes or pillars. Don’t aim the mixer into a wall. When positioning in front of bends, turning vanes and aerators, leave sufficient clear distance downstream of the mixer. 8
14 Best Practice: Minimize Mixing EnergyOversizing mixers should be avoided Wastes energy Too much turbulence can reduce equipment life. Especially if not properly positioned, the jet from an over-sized mixer can damage nearby mixers.
15 Madison, WI Nine Springs WWTPAnaerobic Selector Basin Dimensions 33’ Long 30’ wide 17’ deep
16 Less Mixing Yields Big Savings
17 Phosphorous removal the same Before & After7.5 HP 2.5 HP
18 Calculated costs
19 Electrical usage Before After
20 Best Practice: When heavier solids are removed from wastewater by screening and grit removal, mixing energy can be safely reduced.
21 Weyauwega, Wisconsin Strategy for long narrow tank with one mixer
22 Best Practice Case Study: Hilbert, WI Take control of your oxidation ditch Decouple Mixing from Aeration Enables BNR Saves energy Saves chemicals 2 Disc Aerators Removed 25 HP Replaced with 1 mixer, 3.5 HP Annual energy savings: $15,000
23 Hilbert, WI Biological Phosphorous Removal
24 Decoupling Aeration and Mixing: Annual Energy savings: $15,000 Payback: 2 yearsHilbert, Wisconsin WWTP Oxidation Ditch Additional Savings: Chemicals reduced maintenance
25 Riverside, California WQCP Anoxic ZonesTwo co-rotating loops provide for virtual wall between two sub-zones
26 Example: Aeration Zones 59’ x 59’ x 15’Hyperbolic Top Entry: Consumes 4.9 kW Submerged Horizontal Jet Would consume 1.0 kW
27 Milwaukee MSD BioP Removal High efficiency mixing systemMixer Design: 8.2 ft Dia Adjustable speed 0 – 32 RPM High Efficiency Basin Size: 47 feet long 30 feet wide 15 feet deep Mixing Power 0.42 kW 0.02 kW/1000 cubic feet
28 Milwaukee MSD BioP Removal High efficiency mixing systemPositioned in The corner of the mixing zone for an Efficient bulk flow loop
29 Optimizing Mixing Efficiency in Swing Zones (anoxic/aerobic)Swing Zones - operate as both fully Aerobic Zones or as Anoxic Zones with Aeration Mixing. Swing Zone Mixers – designed to provide a High Efficiency Mixing system to keep solids in suspension (MLSS or RAS), keep varying inflow streams and tank volumes completely mixed while preventing short circuiting of flow from one zone to another. Mixer System Design – is carefully evaluated to ensure the produced Thrust will not cause damage to the diffuser system.
30 Optimizing Mixing Efficiency in Swing ZonesMixer Efficiency – (Thrust / Consumed Electrical Power) measured in (N/kW) is factory tested by the mixer manufacturer in accordance to ISO21630:2007 Mixed Liquor Recirculation (MLR) Pumps - recirculate a portion of the mixed liquor from the Swing Zone to the Anoxic Zone for Denitrification and to produce high quality effluent. Typical design criteria for a BNR facility is four times the influent flow rate. Varying the MLR flow rate effects the Nitrogen Removal %
31 Measuring Mixer PerformanceISO Standard, 21630:2007
32 Impact on Required Mixer Thrust from Varying MLR Flow Rates(MAXIMUM) Mixed Liquor Recirculation Flow Rate Required Mixer Thrust (INCREASES) Zone Retention Time (DECREASES)
33 Impact on Required Mixer Thrust from Varying MLR Flow Rates(MINIMUM) Mixed Liquor Recirculation Flow Rate Required Mixer Thrust (DECREASES) Zone Retention Time (INCREASES)
34 Optimizing Produced Mixer Thrust to Save EnergyUtilize a High Efficiency Variable Speed Mixer Adjust the Mixer Output Speed (RPM) Optimize the Produced Mixer Thrust to closely match the Required Mixer Thrust FIXED SPEED MIXER VARIABLE SPEED MIXER MLR Flow Rate (gpm) Required Thrust (N) Produced Thrust (N) Produced Thrust (N) Retention Time (min) 45,966 6,504 7,818 6,632 5 22,983 1,631 1,811 10
35 Optimizing Mixing Efficiency in Swing ZonesSWING ZONE MIXERS FIXED SPEED VARIABLE SPEED FIXED SPEED Consumed Electrical Power(kW) 4.26 3.22 0.59 Speed (rpm) 43 38 18 Produced Thrust(N) 7,818 6,632 1,811 Required Thrust(N) 6,504 1,631 ISO Mixing Efficiency (N/kW) 918 1030 1535 Recirculation Rate(gpm) 45,966 22,983 Electricity Cost ($/kWhr) $ $ $ Operating Hours(hr) 8760 Energy Cost ($/yr) $ ,478 $ ,385 $ ,478 $ Total Energy Cost ($/yr) – (2) mixers per Tank $ ,956 $ ,770 $ ,956 $ ,240 TOTAL SAVINGS with VARIABLE SPEED ($/yr) $ ,186 $ ,716
36 LATEST IN MIXING INNOVATIONFIXED SPEED OR ADJUSTABLE SPEED High Efficiency Submersible Mixer with Integrated VFD
37 Eliminate the Tank Side External VFDSimple Integrated Design Save Time & Money Higher Reliability Clear Responsibility Better Drive Environment Factory Tested as a Complete Unit Reduce Inventory Management this is extremely useful for WWTP's that have several different sizess of mixers
38 MBBR Process (Moving Bed Biofilm Reactor TechnologyA process that uses fixed film carriers to remove biological contaminants from wastewater or to reduce or increase nitrogen content and reduce biochemical and chemical oxygen demand Two basic types (used interchangeably): MBBR (Moving Bed Biofilm Reactor) IFAS (Integrated Fixed Film Activated Sludge)
39 MBBR Process Tanks are filled (30-70% by volume) with small plastic carriers that promote bacterial growth on their interior surfaces to clean the wastewater These carriers need to be circulated throughout the tank to contact the entire tank volume Air diffusers (in oxic zones) and mixers (anoxic or anaerobic zones) are used to keep the carriers moving
40 Mixing Requirements Anoxic and Anaerobic TanksNew carriers have a density slightly less than water (i.e. float) so mixers are used to circulate the carriers throughout the entire tank. After a while the carriers will have a density slightly higher than water and will tend to sink so the mixers must also keep the media from settling. Speed must be kept low to prevent the destruction of the carriers and to promote bacteriological growth on the carrier surfaces
41 Mixing Requirements The speed of the mixer has to be low to keep from damaging the carriers. (MBBR kit available) Too much agitation will cause the growth on the carriers to “fall off” and may prevent new growth from developing Not enough agitation will leave portions of the tank with no carrier contact and may allow carriers to settle. High efficiency is desired since these applications run continuously.
42 Questions?