Membrane bioreactor technologies successfully treat municipal, commerical, and industrial wastewater—resulting in high-quality treated water—for discharge and reuse applications.
Failed wastewater treatment systems are on the rise. Until recently, solutions were limited at best: Force people from their homes or extend existing sewer lines at astronomical costs. With the utilization of membrane bioreactor (MBR) technology, deteriorating septic systems can be upgraded and suitable alternatives for onsite wastewater treatment can be achieved. An MBR is a solid-liquid separation device that can be used to filter high-quality water from activated sludge.
Since 1980, MBR systems have been successfully used to treat municipal, commercial, and industrial wastewater for discharge and reuse applications. The result is better-quality treated water than most city plants are able to provide.
Submerged in each MBR are membranes that physically reject pathogens and other suspended solids. However, it is the biological process that removes contaminants such as biochemical oxygen demand (BOD) and nitrogen. If necessary, phosphorus removal can be achieved with simple chemical addition. As such, MBRs are generally only one part of a system that is designed to biologically treat wastewater.
Pilot Project in Washington State
In 2002, the Woodinville Water District completed a remodel of the headquarters facilities, which included the addition of two new buildings—requiring a second mound-type septic system. One year after the remodel was completed, the older mound system failed.
Research of the MBR began in July 2003. Staff was given tours of the pilot project at the King County West Point Treatment Facility as well as the two facilities at the Tulalip Tribe. After evaluating costs to replace the failed mound, the Board of Commissioners decided to purchase an MBR system from Enviroquip for their pilot project. The next few months were spent determining design and layout of the system.
| Picture 1 |
 |
| The only signs of an existing MBR system here are hatches and a wall-mounted control panel. |
| Picture 2 |
 |
| The Kubota FS25 SMU |
In 2004, the Woodinville Water District installed an Enviroquip flat plate MBR to replace the failing mound system at the district offices. The new system is housed completely underground in the back parking lot of the equipment yard at the district headquarters. Easy-access hatches and a control panel mounted to an existing property-boundary wall are the only visible signs of the system (Picture 1).
The Enviroquip MBR incorporates Kubota flat plate membrane technology. The Kubota FS25 Submerged Membrane Unit (SMU) contains 25 panels and is designed to process 3,000 gallons per day (gpd) of effluent and up to 6,000 gpd in a 24-hour period. A coarse bubble diffuser is housed in the bottom portion of the SMU, which adds oxygen to the mixed-liquor and provides scrubbing action over the face of the flat plate membranes as it travels up between panels. Each panel is approximately 1 meter long by 0.5 meter wide. The effective opening of the flat plate membrane is <0.1 micron—preventing the passage of suspended solids while allowing free passage of filtered water (Picture 2).
The MBR is divided into three chambers. As soon as wastewater enters the equalization chamber, it is pumped to the anoxic chamber and then to the membrane chamber for further biological treatment and filtration through submerged membranes. Currently the permeate water is being sent to the district’s mound system, with no disinfection provided prior to discharge (Diagram).
An equipment vault is used to house the two permeate pumps, small blowers for the Geyser pumps, and the blower for the course bubble diffuser. A NEMA 4 stainless steel control panel is mounted on the block wall next to the MBR that houses the electrical controls for the pumps and blowers.
The force mains from the existing septic system were intercepted between the septic tanks and the mounds, with the flow directed into the first chamber of the MBR unit. Flows out of the system were then tied back into the force mains and conveyed to the mounds. The equipment was initially tested with the three chambers of the MBR filled with clean water and operated for approximately 11 weeks while the district waited for its permit from the county. The permeate drawn through the membrane panels was returned to the first chamber and continuously recycled through the system.
Testing of the MBR was performed during the pilot study as mandated by the local health department. Influent/effluent sample analyses are run three times per week for BOD, total suspended solids (TSS), pH, ammonia, and fecal coliform. Once a week samples are taken for TSS of the mixed liquor in the membrane chamber. Finally, samples are taken for dissolved oxygen and fats, oils, and grease once a month.
Following the six-month pilot project, the MBR was found to be very effective in removing suspended solids, BOD, fecal coliform, and ammonia from the effluent out of the district’s septic tanks. Once the system passed a normal break-in period for treatment plants, the unit consistently produced Class A reuse water. Minimal time is spent operating the system and less time expended for maintenance.
The first membrane cleaning occurred after 275 days of operation and required approximately two hours of downtime. After a chlorine solution is fed into the membranes, it must soak for approximately one and a half hours before the permeate line can be reconnected and the MBR restarted. The first sludge wasting came after 311 days of operation and took about 30 minutes. The MBR was cleaned and sludge wasted for the second time following another 252 days of operation. The membranes were actually cleaned less frequently than the manufacturer’s estimate of every six months.
The unit has been somewhat of an attraction. As this technology appears to be the next generation of sewage treatment, plant operators, engineers, utility district commissioners, county and state regulators, and state representatives have shown great interest in the large municipal and smaller satellite station applications.
Operating costs of this unit have been minimal. There was no noticeable change in power usage for the district campus during the pilot project and the year prior to MBR installation. During the six-month pilot project, more time was spent taking/evaluating samples and giving tours than maintaining and operating the unit.
King County has issued a permit for continued operation of the MBR at the Woodinville Water District, spelling out the following testing requirements:
- The district will continue to pull samples once a month for BOD, total suspended solids, pH, FC, ammonia, and NOx for influent/effluent from the system.
- A quarterly report with test results and information detailing functional/operational status of the MBR system is required.
- The local health department must be notified of any changes/alterations of the MBR system and/or repair to the attached mound systems, prior to implementation.
- The permits must be renewed annually.
- Operation of the MBR is under direct supervision of the engineering services
supervisor.
The conditions of the permit also indicate that if the project is expanded to other operational situations, alternative requirements may be developed.
Upon completion of the pilot project, there were issues at the district, county, and state levels. Ken McDowell, Woodinville Water District engineering services supervisor, addresses the issues.
 |
| MBR technology is the next generation of sewage treatment. |
Where Will the District Go With MBR Technology?
From the onset, the engineering department has been responsible for the maintenance and operation (M&O) of the MBR. With the permit in place and testing parameters identified, M&O efforts will be turned over to the maintenance department.
Currently, a third party analyzes test samples taken from the MBR. The district board will decide whether to continue this practice or assemble an onsite lab.
In the event additional units are installed in the district, the board will address maintenance and operation, staffing, and installation design standards.
Operational Issues
Influent Transport—The equalization tank has to be large enough to store the influent directed to it and then meter it to the MBR. If the tank is not large enough, flows will need to be coordinated so that all buildings connected to the MBR do not discharge to the MBR at the same time, causing the equalization tank to overflow.
Disinfection Requirements—If the permeate is to come in contact with humans, UV lighting or chlorination must be utilized.
Equipment Life Cycle—As the Woodinville MBR was a pilot project, there is no data.
Requirements
Redundancy—Larger treatment plants (>14,500 gpd) are regulated by Washington State Department of Ecology and require redundancy be included in the design. If the system needs to be shut down for any reason, a backup system is available to keep the plant in operation. The system installed at the district uses the existing mound system for redundancy. If the MBR is taken offline, the flow is bypassed directly to the permitted mound system.
Testing—The King County Health Department has established parameters for testing the district’s MBR.
Who will regulate MBRs within the district?
- King County Health Department <3,500 gpd
- Washington State Department of Health 3,500 to 14,500 gpd
- Washington State Department of Ecology
- Septic systems >14,500 gpd
- Mechanical systems (such as the MBR) >3,500 gpd
Credits for Use of This Technology
Although the treated water being directed to a drainfield/mound system from the MBR meets reuse standards, the permeate is considered graywater by the regulating agency.
Due to the quality of the effluent from an MBR, consideration should be given to providing some form of permit modification/allowances or credits for use of this technology.
onsideration could include:
- Reduced size of drainfield
- Less vertical separation to groundwater or impervious layer
- Reduced set-back requirement to surface water
- Permeate discharge into storm system or use of groundwater injection wells
- Existing systems-to-MBR retrofit issues
The addition of the MBR identified large amounts of inflow and infiltration into existing septic tanks that required repair.
Existing operational drainfields can be used for permeate discharge.
A collection system needs to be installed in order to bring the influent to the MBR.
 |
| MBR systems are available in sizes to meet most demands. |
Synopsis
“Enviroquip’s small flow membrane bioreactor technology provides a cost-effective solution for decentralized wastewater management. The quality of the treated water is suitable for beneficial reuse, therefore providing a valuable tool as we look for sustainable practices to offset the ever-increasing demands on water supply,” says Jim Gleason, Pacific Northwest representative for Enviroquip, a division of Eimco Water Technologies.
The MBR system has performed very well to date and all effluent parameters have been well within state requirements. “The district has found the MBR to be a viable alternative to onsite septic systems. Once regulators become more familiar with the maintenance and operation of these units, they will hopefully see the benefits and create new regulations that can take advantage of their potential,” says Ken McDowell. He goes on to say, “The one item to keep in mind with an MBR is that it is more work to maintain and operate than a conventional septic system. You don’t just put it in the ground, walk away, and forget about it. You need to make sure all the equipment is operating and that the microorganisms are alive and in check.”
MBR technology has proven to be a cost-effective and reliable solution to onsite wastewater treatment in the region, and another tool in the district’s ongoing mission to protect and improve water quality. The small unit at the water district can handle 3,000 gallons a day—much more than is generated by a single-family residence.
The MBR is now connected to the SCADA system, which monitors high/low level alarms and run times for the two permeate pumps. The district is scheduled to install an in-line pH meter, as the pH of the permeate is a quick indicator of the condition of the biological process.
This system is clearly a more effective way to treat wastewater-—producing clean water, taking up less ground than traditional systems—and can be used where groundwater levels have previously restricted the use of traditional systems.
MBR systems come in many sizes and can be scaled to meet nearly any demand. With larger systems it would be possible to have many homes on a single system, thereby reducing the cost to any one home for installation and maintenance. Technologies like the MBR system are changing how we think about our future.
Robyn Dack is a Forester Communications staff writer.
OW - March/April 2007 |
