Pretreatment is imperative to the MBR process and designers must consider a multitude of operational situations when building their pretreatment strategy
Toward the end of the 1980s, Professor Kazuo Yamamoto from Tokyo University gave a lecture to a skeptical crowd. His topic? Immersed hollow fiber membrane bioreactors. Since the day that first audience listened to the possibilities of MBRs, much work has been done to make membrane bioreactors a common step in the wastewater treatment process.
While the benefits of MBR are now clear to many — high-quality effluent, a smaller footprint than traditional activated sludge processes, the potential for better biotreatment — the pretreatment steps are not always as clear. To maximize the success of MBR treatment, however, pretreatment should be a top priority.
A case for pretreatment
When even the smallest solid or bit of debris slips through, the MBR process can be compromised. Operators can attest to the incredible challenges of operating an MBR when debris accumulates between the membrane sheets or fibers. Nothing better illustrates the need for pretreatment than the numbers on how quickly one piece of hair can disrupt your system. Studies have shown that 1 ppm of debris can quickly become 8.35 pounds per million gallons, which adds up to 249 pounds a month.
The benefits of MBRs are only possible when the proper pretreatment is put in place. Unprotected MBRs will often require fouling control and membrane cleaning and replacement, all of which result in increased operating costs. Without pretreatment, the benefits of MBRs are quickly overshadowed. Installing a screening system as pretreatment will ensure that the benefits of your membrane bioreactor shine through.
Placing a screen upstream of a membrane bioreactor is as significant to the treatment process as the MBR itself. Screening protects the membrane from all manner of damage, such as clogs formed from loose hair and fiber that slip through, or debilitating damage inflicted by sharp particles. It serves as a crucial component of the MBR system.
A number of screen designs can be used to protect membrane bioreactors, including rotary drum screens and band screens. The key is to ensure the screen is designed for the types of debris it will encounter and is sized correctly for the influent.
Evolution of MBR pretreatment
In the early days of pretreatment for MBRs, a 3 to 6 mm opening size on the screening media was deemed to be an adequate level of protection. Since then, there have been shifts in both the understanding of MBR pretreatment needs, as well as changes in the influent in waste streams. In recent years many facilities have seen fiber content increase with the popularity of disposable personal hygiene products like wipes. This change in the makeup of typical sewage, along with the experience that the industry has gained over years of operating the technology, is showing that finer screening – in the range of 2 mm or less – is a must.
Reaching the necessary level of fine screening is not typically achieved in just one step. It is common to have multi-step screening to accomplish the task. For combined sewer applications some form of coarse screening (greater than 6 mm) is frequently utilized to remove large solids. Even dedicated sanitary sewer flows employ 3 to 6 mm fine screens as the first line of defense to prevent over-loading, or blinding, a much finer screen.
A properly sized and designed pretreatment strategy will contribute to fewer cleanings, a better capture rate and improved operations. Fine screens of 2 mm or less are recommended for pretreatment of MBRs, but ultrafine screens are preferable to ensure a higher solids capture rate. Ultrafine screens of 1 mm are better suited to truly protect your MBR system from even the smallest piece of fiber or hair.
Plant designers and operators have also learned that they must consider fugitive fiber and debris sources when building out their pretreatment strategy. Some of the fugitive sources that must be contained include washdowns, filter back flushing, repairs and overflows. If not controlled these situations can lead to membrane fouling.
Protecting all flows
The small town of Field, British Columbia, which had once been a stop for trains on their way over the continental divide, now welcomes tourists in the thousands as part of Canada’s Yoho National Park. And with those tourists comes an increase in waste.
When the town of 300 needed to build a new facility to handle the waste, Parks Canada reviewed the latest in advanced treatment technology and opted for a membrane bioreactor system. The newly installed MBR plant quickly became a prized facility for the town. The plant’s operators recognized that to keep their new state-of-the-art equipment running smoothly, they’d need to protect the MBR with fine screens.
Field’s effluent can be best characterized as variable. In the summer months, tourists use Field as a pit stop, putting a large demand on the wastewater treatment plant, while during the subzero, mid-winter period, usage falls dramatically due to the small number of permanent residents. With low temperatures and short sewer lines, there is little natural breakdown of organic solids before entry to the treatment plant. Also, the low flow and long residence time in lift station sumps means the solids settle, creating a significant variation in solids concentration from the standard 300 ppm.
To handle the highly variable influent, the facility looked to JWC Environmental to provide protection as advanced as their MBR equipment. They settled on an IPEC rotary screen and an IPEC screenings washer/compactor.
The rotary screen was fitted with 800-micron (opening size) stainless steel mesh for the primary separation. The high porosity allows for a high hydraulic process rate in a compact physical size. A special in-feed header on the screen comfortably handles both the high flow and the low flow/high solids concentration conditions efficiently, while maintaining the required separation. The rotary screen also processes the highly concentrated compactor pressate that is reintroduced by a separate pump system. The screen is self-cleaning, allowing for hands-free operation.
The solids from the rotary screen, which include all the typical nonbiodegradable matter mixed with a much higher than normal biodegradable faction (fecal matter, tissue) are accepted by the PLT press. The screened organic components are effectively broken up and returned to the plant for further biological processing, while the remaining inorganic solids are discharged, in compacted form, to a bagging device. The bagging device includes an 80-meter-long plastic tube that allows for practical isolation of the rejected solids being disposed of from the plant operator.
The IPEC screen and press combination is designed to continuously protect the membranes. In yearly inspections of the bioreactor in Field, no contaminates have been detected.
The advanced technology of membrane bioreactors also calls for progressive protection of these systems. Pretreatment is imperative to the MBR process and designers must consider a multitude of operational situations when building their pretreatment strategy. Just as we’ve advanced and improved our knowledge of membrane bioreactor technology since its early days on the market, we’ve come to recognize the true power of protection in the process.
For more information on MBRs and other processes, visit www.jwce.com.