Membrane bioreactors (MBRs) are coming of age. Once the technology was considered too high-maintenance and not cost-effective, but the drivers for selecting wastewater treatment technologies has changed. Now, as discharge criteria are becoming more stringent and reuse applications are increasing, membrane bioreactors are getting another look. Adaptable to both municipal and industrial wastewater treatment for aerobic and anaerobic applications, the field is expanding with more manufacturers, municipalities, and industrial users.
Peter Bouchard is global director marketing and communications with Koch Membrane Systems (KMS) Inc., a company that’s developed and manufactured membranes for over 25 years. Bouchard has seen more municipalities take an interest in the technology, as well as more manufacturers, and believes this scenario serves everyone better.
“The introduction of membranes to the municipal wastewater treatment market is relatively new, and, therefore, the technology continues to improve with more research,” says Bouchard. “Along with the growth of the market, we see a growing list of companies attempting to participate in providing membranes to the municipal wastewater treatment segment. The entry of new companies in this market further drives innovation, efficiency, and, ultimately, better technology at a lower price.”
Scott Christian is a process engineer with ADI Systems Inc., a company with a long history of developing and manufacturing anaerobic digesters and membrane technology. Christian agrees that the MBR market is dynamic, and that companies are constantly looking to improve performance while lowering costs.
“ADI Systems continuously studies the MBR processes and undertakes extensive research and development efforts to improve the aerobic MBR and anaerobic MBR [AnMBR] technologies,” says Christian.
“These activities are focused on developing an understanding on what new applications the technology is well suited for, as well as developing methods and modifications which allow the technology to be more cost-effective for all sizes of industry,” he continues. “For instance, through research and development, the ADI-AnMBR process is the most cost-effective choice for anaerobically digesting high-strength wastes with high COD [Chemical Oxygen Demand] and suspended solids, due the compact nature of the process, while operating at higher volumetric organic loading rates and producing high-quality effluent within one simple process.”
The basic operation of an MBR system is this: Where the standard activated sludge process includes a primary clarifier, biological reactor, and a secondary clarifier for solids and liquid separation, MBRs eliminate the need for a separate secondary clarifier. MBRs can be installed within the digester or as a separate unit. Either way, the filter acts as a physical barrier to solids. Because the difficulty of settling floc using gravity is no longer an issue, MBR systems have a higher number of microorganisms which increases the rate at which pollutants are removed; thereby, increasing the organic component of effluent quality. One of the major factors in MBR performance is aeration, which not only provides oxygen to the biomass—it scours the membrane surface and keeps solids in suspension.
What this technology always did, and continues to offer, is improved effluent quality with a smaller construction footprint.
“The selection of membranes for a wastewater treatment facility is typically driven by the need for high-quality effluent [typically for reuse], or when a wastewater treatment facility has a restricted footprint,” says Bouchard. “The capital cost of membrane facilities can be very competitive with conventional treatment facilities, but the O&M [operation and maintenance] costs of membrane facilities is typically greater than conventional treatment facilities, due to membrane replacement costs and increased power consumption.”
In certain parts of the country, as water supplies dwindle, the need to find alternative water sources increase. While alternative sources may not be potable themselves, they serves as a substitute for other demands such as irrigation.
“Many municipalities in water scarce areas—typically the Southeast and Southwest areas of the US—are using membranes to provide reuse of wastewater where appropriate,” says Bouchard. “Reusing wastewater places value on the water and recognizes it as a resource, while reducing demand on the potable water supply.”
Advanced Technology in Santa Paula
For the city of Santa Paula, CA, 65 miles north of Los Angeles, CA, membrane technology helped keep them from paying millions of dollars of fines.
“The city of Santa Paula was out of compliance and was facing $8 million of fines by the Los Angeles Regional Water Quality Control Board for discharge to the Santa Clara River,” explains Marian Clayton, marketing director for PERC Water, which partnered with Alinda Capital Partners to design, build, operate, and finance the $58-million facility. “The original plant was built in 1939; over time there had been major upgrades, but now it was time to start fresh.”
Starting fresh also meant starting in a hurry to design and build the 4.2-million-gallons-per-day (MGD) water recycling facility (WRF). The city had previously hired an engineering consultant to design the new wastewater treatment system. Unfortunately, it did not meet their budget requirements, and the city took a step back to look for alternatives. The clock was running out and fines loomed.
The city was introduced to California’s Code 5956, which encourages private investment to solve public infrastructure needs, and decided to switch to a Design-Build-Operate-Finance (DBOF) delivery method of procurement for the new WRF. They sought a single entity to finance, design, build, operate, and maintain the new WRF, and awarded the contract to Santa Paula Water LLC, a joint venture between Alinda Capital Partners and PERC Water Corporation. The DBOF approach allows the project to commence without requiring any capital costs from the city during the design and construction of the WRF.
“What’s interesting about this model is that we lease the land from the city,” says Clayton. “Santa Paula Water LLC owns the facility. Wastewater is conveyed to the facility, and recycled water is delivered back to the city for their future use. It’s like any other utility, power, et. cetera; except, the city of Santa Paula continues to set the rates and provide service to the residents of Santa Paula. Santa Paula Water charges the city a set monthly service fee, at which they rely on to set and maintain the rates.”
Another advantage to using this model was the ability to meet the compressed schedule. The city was required to have a new treatment system in place by December 2010. PERC Water received the notice to proceed with engineering on May 6, 2008, and broke ground only two months later. Operations of the new WRF started in April 2010, seven months ahead of schedule.
While the schedule was challenging, the expedited, fast-track nature of design and construction was relatively straight forward for PERC Water, because the company has extensive experience with the design, construction, and commissioning of very similar facilities. They have designed, built, and operated 22 other facilities with a similar design, all in the southwest US. This is their first venture working with an independent private equity firm.
The design is compact. The actual treatment area takes up less than 1 acre. The tank structures are built beneath the operations buildings, which reduces noise and odors for nearby neighbors. Because of the belowground tanks, all the process equipment, SCADA, and controls are strategically located directly above the associated process tanks, keeping piping and wiring, and associated costs, minimal. The operations buildings are also built with both the public and employees in mind.
“The office includes an educational area, and the main lobby has two flat screen televisions, where visitors can learn about the water recycling process,” says Clayton. “Operators enjoy working at our facilities because it has a professional environment.”
Another reason Clayton believes that the operators take pride in working at their facility is the advanced equipment. “Because we are responsible for a 30-year term of operation,” says Clayton, “ it is advantageous for us to invest in top-of-the-line equipment up front, knowing they will consume less energy and have a lower life cycle cost.”
The progressive equipment selected for the WRF includes Koch PURON Membrane. The hollow fibrous membranes, 0.05 µm in diameter, are similar to straws, fixed at the bottom and individually sealed at the top. Each group of membranes is called a bundle. Bundles are connected in rows, and these rows are lined up a stainless steel form.
A vacuum is applied at the base of each bundle, which draws water in through the sides of the membrane tubes and down through the base. Aeration is used to both supply the microorganisms with oxygen, and remove hair and other deleterious material. A single header in the center of the bundles forces air up through the membranes. This provides a scouring mechanism to remove material and allows the material to float away from the membranes. Pumping air through the middle also alleviates the potential for a dead zone to occur, where dead microorganism collect on the membranes, reducing efficiency.
KMS provided the MBR for the system and proved to be an advantage when it came to space. The total tank area is 32,000 square feet. This freed up space that had been originally set aside by the city for the new WRF, and will be used for a city corporate yard facility.
“KMS is the membrane supplier for the membrane bioreactor treatment facility,” says Bouchard. “In addition to supplying membranes, KMS also provided air control valves, instrumentation, permeate pumps, chemical cleaning equipment, and engineering support during the design phase of the facility, and startup support during the construction and commissioning phases of the project.”
For now, effluent from the recycling plant will be discharged to 13 acres of percolation ponds. But this is temporary. The city is working with the state to receive a grant to perform a feasibility study to determine the most effective reuse system. Once results shake out from the study, they will be one step closer to utilizing the MBR system to its fullest potential.
From Wastewater to Drinking Water
For now, MBR technology is primarily used to reach effluent goals needed for reuse and reclamation or improving pre-treatment operations. When used for reclamation, it’s replacing potable water—not in ice-filled glasses, but on lawns and landscapes, in toilets, for dust control, and even as industrial process water. Will Americans ever find wastewater effluent potable, or palatable?
Some parts of the US have already started down that path. Orange County’s (CA) Groundwater Replenishment Plant utilizes MBR technology for ultrafiltration of its wastewater. While it’s not pumped directly to citizen’s taps, it is pumped to the drinking water aquifer. Here it keeps the water table elevated to avoid saltwater intrusion,
but it will also serve to alleviate future drinking water shortages.
For now, the use of MBR systems continues to grow, mostly for the purpose of reuse. But as water sources begin to tap out, that clean, clear glass of wastewater effluent may start to look tasty.