Pretreatment for membrane bioreactors is imperative for performance

By Kevin Bates

Toward the end of the 1980s, Professor Kazuo Yamamoto from Tokyo University gave a lecture to a skeptical crowd on immersed hollow fibre membrane bioreactors (MBRs). Since then, much work has been done to make them a common step in the wastewater treatment process.

To maximize the success of MBRs, pretreatment should be a top priority. This is because, even if the smallest solid or debris slips through, the process can be compromised.

Operators can attest to the incredible challenges of operating an MBR when debris accumulates between the membrane sheets or fibres. Nothing better illustrates the need for pretreatment than the numbers on how quickly one piece of hair can disrupt a system. Studies have shown that 1 ppm of debris can quickly add up to 3.8 kg per 3.8 MLD per day.

The benefits of MBRs are only possible when proper pretreatment is put in place. Unprotected units will often require fouling control and membrane cleaning and replacement, all of which increase operating costs. Installing a screening system as pretreatment will ensure performance is maintained.

Screening

Placing a screen upstream 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 fibre that slip through, or debilitating damage inflicted by sharp particles.

A number of screen designs can be used to protect MBRs, including rotary drum and band screens. The key is to ensure the screen is designed for the types of debris it will encounter and that it is sized correctly for the influent.

Evolution of pretreatment in membrane bioreactors

In the early days of pretreatment for MBRs, a 3 – 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 fibre contents 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 shows 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 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 – 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 less cleanings, a better capture rate and improved operations. Fine screens of 2 mm or less are recommended for pretreatment of MBRs, but ultra-fine screens are preferable to ensure a higher solids capture rate. Ultra-fine screens of 1 mm are better suited to truly protect MBR systems from even the smallest piece of fibre or hair.

Protecting all flows

It is not just the main process flows that must be considered when planning a pretreatment strategy. Plant designers and operators have also learned that they must consider fugitive fibre and debris sources.

Wash down and filter back flushing are some of the more common sources of fugitive debris that, if left unchecked, can foul membrane bioreactors. Designers must also look at the potential for less frequent incidents like system repair work or overflow events that can create problems. All such fugitive debris sources must be reintroduced into the process, prior to the pretreatment screening.

Kevin Bates is with JWC Environmental. This article appears in ES&E Magazine’s February 2018 issue.