June 2008, Vol. 20, No.6

Small Communities

Small MBRs: Not Your Father’s Package Plant

James C. Pyne

Your locality has a small but dense population center that needs wastewater service, and you’ve decided for various reasons that a small package plant is the best option, given the circumstances. One elected official you report to has just read an article about membrane bioreactors (MBRs), is excited about the technology, and wants you to consider them. So, where do you start?

Package treatment systems come in many sizes and complexities. Starting with small and relatively simple fixed-film units, the spectrum of package plants is populated with varying treatment capabilities and operational complexity systems. Traditionally, extended aeration units were employed, but their track records were generally unsatisfactory owing to inadequate operation and maintenance. In recent years, newer activated sludge designs such as sequencing bioreactors have been introduced, and most recently the MBRs. To some degree, as the treatment capability increases, so do the complexity and, typically, the unit operating cost. Included in the unit operating cost are such items as maintenance personnel, power, chemicals, parts, and something that often is overlooked, supervisory time.

Before selecting a package plant system, a utility must fully understand its needs, the available technologies, and all of the associated costs. The Hampton Roads Sanitation District (HRSD; Virginia Beach, Va.) recently undertook this process and selected an MBR as the most appropriate technology to install at the 378.5-m3/d (100,000-gal/d) King William treatment facility. We considered several factors before making this selection.

First, we learned that the solids-separation part of the MBR process, the membrane unit, is not a magic bullet. Rather, it is only one part of the process. The upstream biological process is the heart of the MBR system, and its design is critical to the system’s performance. Unlike other processes using gravity solids separation, a system utilizing membranes is effluent-flow-limited. That is, there is an absolute maximum on how much flow can pass through any membrane system under a specific set of operating conditions. Therefore, determining the allowable peak flow is a critical design parameter. In a system with a wide range of flow rates due to infiltration and inflow or other influences, an MBR could be costly.

Emergency power also is critical, because most MBRs require pumping to pressurize flow through the membranes. Even the coarser membranes that can operate with only gravity flow still require aeration to maintain the biological process and to scour the membranes while in operation, so these too will be able to operate for only a limited time without power.

Technology enhancements may reduce staff time in some areas but increase it in others. While we have found that our MBR requires somewhat less general operation and maintenance effort than we anticipated, most of the maintenance required is in the area of controls and instrumentation. HRSD is lucky in that being a large agency, we have these resources in-house; personnel are on call at all hours to respond to emergencies.

Operating underloaded facilities can be challenging. Most activated-sludge-based biological systems don’t perform well at loads significantly lower than the design point, and small package systems typically don’t have the capability to reduce tank volumes to match the influent conditions. MBRs seem to be a little more forgiving at low-load conditions, because settling of the solids is not a key issue. In HRSD’s case, because of the downturn in housing sales, we are currently operating at about 13% of the plant’s hydraulic design capacity and will be for some time. Fortunately, HRSD built the facility as two independent process trains, so we are able to operate only one-half of the plant at a time. At these low loadings, the facility is still able to function quite well in comparison to conventional facilities that often have difficulty with gravity solids separation at such low loadings and stringent nitrogen requirements.

Another consideration is manufacturer support. MBR facilities, if properly maintained, should remain serviceable for 30 years or more. Many of the parts and fixtures in package MBRs are proprietary, especially the control systems, membrane cassettes, and their mounting systems. The control system can be retrofitted if necessary, although with some difficulty. However, it would be extremely difficult, if not impossible in some cases, to retrofit the package plant to a different membrane-cassette system if the original manufacturer is unable to supply units that match the original equipment. Therefore, an investigation of the manufacturer’s long-term viability should be an essential part of the equipment-selection process.

A final consideration is how operation and maintenance of a new technology will integrate with the organization’s existing work processes. Does the organization need to be expanded to manage the new technology? How many of the existing staff must be retrained and reassigned to the new facility? Can the existing staff be retrained to a level adequate to operate and maintain a complex new technology properly, or is outside assistance required? Can the tasks be split among the existing staff and contractual specialists? These are tough questions, and the means of addressing them might cause some labor discontent in the organization.

In HRSD’s case, the MBR process was chosen for several reasons. First, it has been our experience that the majority of treatment problems we encounter with our older package plants are related to gravity separation. MBRs eliminate this problem. Second, since the service area is still in active development, we were looking for a process that could better tolerate changes in influent conditions and still meet rigorous final effluent requirements. So far, our MBR has shown that it can perform well under very low loading conditions and can tolerate occasional variations in loading from commercial establishments in the service area. Finally, HRSD wanted to investigate membrane technology for other possible applications at its larger plants or water reuse facilities, and using MBRs at this facility provided an opportunity to gain valuable operational experience.

While the above is not an exhaustive list, careful consideration of these issues and others that are site-specific will help ensure that your organization chooses an MBR or other package-plant technology based on all the factors that can affect its performance and your long-term satisfaction with that technology.

James C. Pyne is chief of the Small Communities Division, Hampton Roads Sanitation District (Virginia Beach, Va.).

James C. Pyne is chief of the Small Communities Division, Hampton Roads Sanitation District (Virginia Beach, Va.).