Lesson 1: On-site testing will reduce risks
Not all facilities have the same waste stream characteristics. Some of the differences include:
- Grinder pumps. Some systems will include grinder pumps, which will reduce the size of the debris before it enters the lift pumps.
- Industrial contribution. This typically includes smaller debris.
- Flushable wipes. These can quickly blind grids.
- Septage has different characteristics than a typical waste stream.
- Gravity vs. pumped force main. A gravity-flow system to have larger solids, whereas a pumped system will bring in smaller solids.
- Volume and size of grit in the system.
- Pre-screening. Your pre-screening process should be effective before the flow moves on to the ultrafine screen.
- FOG. These includes fats, oils, and greases.
On-site testing can help define the grid size and type to maximize capture of solids in the wastewater stream. The screening grid is the surface used to capture undesirable solids and keep them from fouling downstream processes. Many grid types and sizes are available. MBR applications typically require two stage screening, where the design objective is to have each screen remove about 50% of the debris load. On site testing can be used to determine the grid size and type of the first – coarse screening stage.
Lesson 2: Two Stage Screening
Two screening stages are typically used to protect a membrane bioreactor system. A coarse screening stage and an ultra-fine screening stage are used. Membrane manufacturer warranties are typically based on keeping all solids greater than 2mm in diameter out of the MBR System Tank. This is achieved by using screen grids composed of perforated holes 2mm or less in diameter. On-site testing can be used to simulate both coarse and fine screening grids to optimize solids capture. It is important to balance loading between the coarse and fine screens such that one stage is not blinded off by excess captured material.
Variable frequency drives (VFDs) can increase the flexibility of the system by increasing or decreasing the grid speed of each screen. During peak flow events, increasing the speed will quickly remove solids and maintain the optimal headloss across the screen. The stages should be operating about the same time intervals each day such that both screens are working about the same to remove solids at all flow conditions.
Lesson 3: Redundancy is key to operational success
A lot of screening systems are built with redundancy, but remember that you need redundancy in all components, such as:
- Solids handling systems
- Control panels
- Grit removal
If one fails, the redundant component can pick up and do its part for the operations. If you don’t have redundancy, having extra storage capacity can work for peak flow events. Typically the term N+1 is applied to describe the required redundancy. This means if 2 screens are needed for treatment (operating in parallel) than one more is typically added assuming it can operate if needed for a total of 3 screens in this example.
Lesson 4: Be aware of common pitfalls with creating a screening solution
All waste streams are not the same. Once you recognize this, you can avoid some common errors that undermine performance:
- Undersizing the system due to the use of a low-blinding factor. Screen grid blinding factors need to be appropriate for match the influent waste stream. For post-primary clarifiers, we typically apply a blinding factor of 30 percent. For dual-stage screening, we typically want 50 percent blinding for the coarse screen and the fine screen. If there is no coarse screen, then we will use a high 85-90 percent which results in a very large ultrafine screen.
- Coarse screen grid openings that do not consider waste stream characteristics.
- Insufficient pre-screening. Not all 6mm screens are alike. On average, multi-rake screens and step screens remove about 35 percent of debris. In contrast, center-flow band screens with 6mm perforated plate openings have over 80 percent capture. This is why it’s important to understand the technology being used, because the performance of the coarse screen will determine how much solids loading demand will be placed on the fine screen.
- Insufficient freeboard to handle high loading surges. Transient operating events can lead to a higher blinding rate, one that’s higher than the one calculated for the system design. This leads to higher headloss, which increases the level upstream of that screen. Having sufficient freeboard will help prevent operational issues.
- Screens aren’t designed to handle abnormal situations where they shut down unexpectedly. A screen that is shutdown with forward flow still occurring will become 100% blinded. The level upstream of the screen will then rise until it overflows an upstream weir. Meanwhile, the downstream level can drop to zero. Screens need to be able to handle the difference between those levels structurally. They also may have to be designed to startup and operate under a high differential.
- Insufficient screen handling system design. Well-designed systems use coarse and fine screens because that’s how they handle the high volume of solids and washwater. An undersized solids handling system will not be able to process the high solids and washwater load. The discharge location is also critical to this operation, because adding a sluice will add more water to the process.
- Unloading/clearing. When screen grids become loaded with debris, you need an effective way of offloading them. At Hydro-Dyne, we do not use rotating brushes, since they don’t sufficiently nor consistently clean the screening girds. Instead, we use two washwater spray bars. Key to the success of these spray bars is their operating pressure and washwater volume. Maintaining pressure is especially important when you have more than one screen operating at a time.
Hydro-Dyne’s best practices deliver outstanding headworks performance
Hydro-Dyne mitigates the impact on screen performance by following best practices, including:
- Avoid grinder pumps
- Effective prescreening (50 percent of total load)
- Hooks and trays to lift small objects
- Exceptional grid-to-grid and grid-to-frame sealing
- Limit grid velocities
- Remove grit prior to fine screening
- Heated spray wash
Speak with a Hydro-Dyne expert about your facility and which screening system is right for you. Call us at 813-818-0777 or contact us online.