MBR configuration assessed

As membrane bioreactor technology comes of age for large-scale municipal wastewater treatment applications, Frank Rogalla and Cindy Wallis-Lage look at the various systems on offer

The largest membrane bioreactor (MBR) plant in the world is set to be constructed between 2006 and 2010 in King County, Washington State. The facility will treat municipal sewage at an average day flow of approximately 144Ml/d, with peak flows of up to 204Ml/d.

The projected flow brings the MBR application up to the same scale as large drinking water applications of microfiltration membranes, such as Singapore (270Ml/d) and Minneapolis (295Ml/d). This is a change of magnitude compared to the largest operating MBR plants to date, such as the Traverse City in Michigan, with a peak monthly flow of 32Ml/d and a peak factor of two, which started up in mid-2004.
In Europe, the last few years have seen the start-up of two large MBR systems: Nordkanal, Germany, peak flow of 45Ml/d, start-up in December 2003 and Brescia, Italy, 42Ml/d, start-up in November 2002. In the UK, the 20 operating MBR plants are of more modest size, being used mostly for smaller coastal discharges to avoid chemical disinfection and/or long sea outfalls. One of the longest-serving plants is at Swanage, at 13Ml/d, which was started up in 2000. The largest to date is in Buxton, with a peak flow of 16.7Ml/d, started up in August 2004. Other examples are: Campbeltown, Scotland, peak flow 6.5Ml/d, start-up in November 2001; Daldowie, Scotland, peak flow of 10Ml/d, start-up in December 2001; Lowestoft, England, peak flow of 14 Ml/d, start-up in 2002. The are other plants in design, such as Dunbar, Scotland.

Most MBR installations are less than ten years old, so the design criteria for the technology is still evolving. Until recently, less than a handful of manufacturers offered this technology. While the number of vendors is increasing, comparison of the MBR systems available shows significantly different configurations and design approaches.

MBR manufacturers

The number of MBR suppliers is increasing, however, the manufacturers with most municipal experience are Zenon, USFilter, Mitsubishi and Kubota (represented in the UK by Copa-MBR, and in the US by Enviroquip). Zenon has supplied the majority of large municipal MBR applications worldwide, with close to 200 plants and most of the sites listed previously. The system is composed of 0.04µm hollow fibre tubular membranes assembled in cassettes and submerged in compartmentalised cells within the activated sludge basin.

This configuration enables operators to clean the membranes in each cell in-situ, keeping the remaining membranes in service. The membrane cassette configuration has evolved over time. The modules, first built with unitary surface of 46.4m2 have been modified to hold a membrane surface area of 32m2. The modules can be arranged together in groups of 32-48 modules to form cassettes. These units allow permeate to be withdrawn from both the top and bottom.

USFilter’s MBR system combines the Jet Tech pumping and aeration system with Memcor’s 0.08µm submerged hollow fibre membranes. Around ten plants are currently in operation, the largest of which is 7,560m3/d. This system utilises multiple racks – each rack consists of as many as 40 modules, sized at 9.3m2/module and oriented in ten groups (clover leafs) of four modules.

The membranes are installed in separate basins from the activated sludge basin to facilitate in-basin cleaning of a portion of the membranes while keeping the remaining membranes in service. The Kubota system utilises a 0.4µm membrane plate in lieu of a hollow fibre tube. Each plate has a surface area of 0.8m2. The membrane panels are lined up adjacent to each other, 8mm apart to form cassettes.
A cassette can contain up to 200 panels in a single stack or 400 panels if double-stacked. The cassettes are installed single file, perpendicular to the flow, in multiple aeration basins. Kubota has a reference list of more than 400 plants, mostly small applications in Japan. The largest operating system in the US is 7.6Ml/d, and of the around 20 plants in the UK, Swanage is the largest.

The Mitsubishi unit, Sterapore, uses a 0.04µm hollow fibre membrane tube. Unlike most hollow fibre configurations, the membranes are oriented in horizontally, allowing stacking of the units in up to three layers.

Each module has a filter area of 210m2. Mitsubishi has more than 1,000 MBR facilities, predominately small domestic units in Japan and Korea, with only a couple of facilities in the US. The company’s largest US facility is 0.4Ml/d.

Design considerations

There are several design and operating issues common to all MBR applications, however, each manufacturer has customised its design to assemble a successful MBR facility:

  • Pre-treatment requirements – all MBRs require pretreatment, for example, screening and grit removal, to protect the membranes. Screening has historically been limited to 3mm, however, hair and fibre can still pass through this size of screen and become embedded or wrapped around the hollow fibers. Zenon, USFilter and Mitsubishi have standardised their screen selections to a 2mm traveling band, punched screen. Conversely, the flat surface of the Kubota plate membrane has experienced less problems with hair and fibre, and standardised to a 3mm screen.
  • Flux management – with high mixed-liquor suspended solids (MLSS) concentrations surrounding the membranes, an MBR system must be designed to limit solids caking onto the membranes in order to maintain design flux rates. Some means of aeration is located directly under the membranes to scour solids from the membrane surface. Details on quantity of air and airflow frequency are shown in Table 1. In addition to this air scour, to prevent build-up in the membrane area, solids are recycled to the head of the aeration basin at a rate of four times the influent.
  • Flux rate – specific flux rates vary with temperature, solids concentration and solids retention time (SRT), however, most MBRs operate at an average flux rate between
    12.5-25 l/m2/h, with Mitsubishi’s unit operating in the lower range. The key flux rates that determine the number of membranes required are associated with the peak flow rates, as listed in Table 1.
    For plants with peaking factors of less than two, an MBR can handle the plant flow variation without significantly impacting the average design flux rate. Otherwise, equalisation needs to be provided with either a separate tank at the head of the facility or within the aeration basin, allowing sidewater depth variations during peak flow. Recent pilot testing with membrane performance-enhancing (MPE) polymers indicate their potential to improve the magnitude and duration of peak flux rates, as well as to reduce scour-air requirements.
  • Cleaning procedures – although membrane cleaning varies significantly, most manufacturers incorporate a frequent and automated maintenance cleaning programne to minimise permeability decline. This, in turn, prolongs the time between recovery cleaning requirements. In addition to air scouring to maintain flux rates, the two maintenance cleaning methods utilised are membrane backpulse and membrane relax.
    The backpulse method reverses the flow through the membrane using permeate or chlorinated permeate at a designated interval ranging from 10-15min. To relax the membranes, they are taken out of service for
    1-2min at 10-12min intervals. The recommended interval for a recovery cleaning ranges from two-six months and is heavily dependent on the biological environment.
    Longer SRT systems and moderate MLSS concentrations will have a longer cleaning interval. Recovery cleaning intervals for some recent installations have approached ten months, but USF recommends a more frequent interval of every two-three months to minimise the actual time of the recovery clean (for example, to reduce soak time to 2h) as well as to minimise the permeability decline at any time.
  • Biological parameters – biological considerations play a key role in effective operation of MBR systems. Most manufacturers have now reduced their recommended MLSS concentrations to between 8,000-12,000mg/l to reduce aeration energy, increase flux rates, and limit cleaning frequency. In addition, SRTs have been reduced to 10-20 days depending on design conditions.
    For example, biological nutrient removal (BNR) can be implemented with an MBR process but requires shorter SRTs to optimise biological phosphorus removal. Oxygen transfer is also a limitation in the sizing of the basin, because inappropriate combinations of SRT and MLSS concentrations can result in high oxygen demands that cannot be met.
  • Nutrient removal – because of the long SRT required to minimise fouling of MBR membranes, nitrification is a certainty assuming that sufficient oxygen is available and adequate pH levels are maintained. Denitrification requires an anoxic zone and a nitrified MLSS recycle line. The solids recycle line, which returns solids from the membranes to the biological basins, creates a potential for returning excessive oxygen to the anoxic zone.
    This dilemma can be avoided either by providing two separate recycle lines, leading to better operating flexibility, but resulting in an increase in the number of pumps and energy costs. Alternatively, an additional volume is required to deoxygenate the recycle line, significantly impacting capital costs. Incorporating simultaneous nitrification and denitrification (SND) into the aeration basin can provide for additional denitrification buffer. Traditionally, very low phosphorus concentrations can be achieved by chemical addition into MBRs.
    Following successful biological phosphorus-removal pilots, BNR can be integrated into an MBR facility with proper SRT and a true anaerobic zone. Currently there are two full-scale BNR MBR plants in operation in the US.
  • Wasting of solids and foam – solids can be wasted from the aeration or membrane basin. With the elevated solids concentrations, downstream solids processing can be reduced via direct feed to dewatering systems. Wasting can be intermittent without affecting the stability of the biological system. While scum and foam in an MBR basin do not have any adverse impact on effluent quality, plants should be designed with the capability to remove scum/foam to avoid unsightly nuisances.
  • Conclusion

    With increased demand for high-quality
    effluent for reuse and small footprints to accommodate expansion and upgrade of site-constrained WwTWs, MBR technology has advanced rapidly as a viable solution. A review of the systems available yields numerous differences in both design and operation, and helps determine which system is most suitable, based on configuration and/or operating requirements

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