Membrane Bioreactors 101
As a result of the U.S. Environmental Protection Agency’s Clean Water Act, many of the nation’s wastewater treatment facilities were upgraded in the late 1970s and early 1980s to meet secondary treatment standards. Today, in addition to replacing aging equipment, municipalities have to contend with population growth, stringent and changing regulations, tight capital improvement, operation and maintenance budgets and, finally, new technologies.
One relatively new and evolving technology is the use of membranes in wastewater treatment applications.
Research combining membranes and wastewater treatment started more than 35 years ago, with the first commercial applications appearing in the mid-1980s. In a conventional activated sludge process, the liquid-solid separation takes place in secondary sedimentation tanks, where the process depends on the settling characteristics of the activated sludge and sedimentation basin design. With membranes, the liquid-solid separation is a function of pore size. Currently, membranes can be categorized into four groups depending on pore size.
Combining microfiltration or ultrafiltration membranes with a biological process results in a membrane bioreactor (MBR). The other two types of membranes, nanofiltration and reverse osmosis, have been widely used in the water industry for many years and are now beginning to emerge as additional processes for removing dissolved salts from MBR effluents.
Information provided by four MBR membrane manufacturers shows a rapid increase in both the number of facilities using this technology and the size of facilities in which MBRs are being used. The capacity of older facilities is usually less than 2 million gal per day (mgd), increasing in capacity during the last five years to 6 mgd. This can be compared with other facilities either in construction or final design capable of treating up to 47 mgd.
There is no doubt that MBR technology is here to stay. The question is whether it is cost-effective for a particular facility.
The answer to that question should be based on economics and regulatory compliance. This is best illustrated by two HDR projects—one at an extended aeration plant and another at an activated sludge plant—where these factors were considered in developing the recommended plans.
The aeration plant
In 1999, HDR was retained by a southern California gaming facility to evaluate the operation of its extended aeration wastewater treatment plant, which was unable to meet strict effluent water quality standards mandated by the local regulatory agency. The facility, with a rated capacity of 150,000 gal per day (gpd) average flow, was unable to meet the required effluent quality when flows exceeded 70,000 gpd. The study concluded that the facility was unable to meet discharge requirements due to wide variations in flow rates and organic loading and the rapidly increasing flows generated by the gaming facility.
To improve treatment plant performance and capacity, HDR evaluated various treatment processes for the gaming facility’s ability to handle wide variations in flow and organic loading; maximize use of existing facilities; maximize use of available land; design and construct under an accelerated schedule; and facilitate implementation of a future water recycling project.
The various treatment alternatives included a parallel extended aeration train, a new conventional activated sludge plant, a sequential batch reactor using existing tanks and an MBR that would also use existing tanks.
The engineering evaluation concluded that the best treatment process for meeting the client’s objectives was the MBR process. However, additional work was required to assure the client and consulting engineers that this relatively new process was going to be a success. To this end, the team visited MBR facilities outside the U.S., reviewed operation and maintenance data and interviewed treatment plant operators. The consensus was to use the MBR process, provided that the project specifications included equipment performance bonds, extended performance guarantees and extended membrane guarantees.
The completed project included the following results: The membrane and associated equipment is modular, which facilitated subsequent treatment plant expansions. The facility is now rated for 300,000 gpd average flow. The rated treatment capacity was achieved using the same tanks as the original extended aeration plant.
The process is producing effluent with turbidities about one-tenth the limit set by California’s Administrative Code Title 22 standards for recycled water. This is accomplished without secondary sedimentation, flocculation, coagulation and filtration.
The process can handle large fluctuations in organic loads. Sludge production is lower than extended aeration activated sludge. The membranes remove a significant portion of total coliform bacteria, significantly reducing the chlorine demand in the disinfection process.
Activated sludge plant
Early this year, HDR was retained by a southern California agency responsible for wastewater treatment and disposal facilities that serve a community of 30,000 people. The existing facility, rated for an average flow of 4.5 mgd, is a conventional activated sludge plant using irrigation fields and percolation ponds for effluent disposal. Until recently, the solids generated by the treatment process were aerobically digested and land-applied to the irrigation fields. This practice contributed to high nitrogen concentrations found in the groundwater basin underlying the treatment plant site. Recently, the regulatory agency issued a cease-and-desist order prohibiting onsite solids disposal. The agency also anticipated that the revised waste discharge permit would limit total nitrogen effluent concentrations to 5 milligrams per liter (mg/L) or less.
Preliminary work indicated that the treatment capacities of the existing facilities would be lowered from 4.5 to 3.4 mgd once the activated sludge process was modified to operate in the nitrification-denitrification mode. It also suggested that additional facilities would be required to bring the treatment plant capacity back to 4.5 mgd and reduce the total nitrogen concentration to 5 mg/L or less. MBR is one of the technologies being evaluated to identify the recommended project. Conventional treatment process modifications required to maintain treatment capacity and meet the anticipated total nitrogen limit may require additional primary aeration and secondary sedimentation tanks, plus filters to remove organic nitrogen.
However, the MBR alternative being evaluated would not require additional tanks and would allow the existing shallow rectangular secondary sedimentation tanks to be converted into a chlorine contact tank. The basis of the recommended project would be the results of an economic analysis comparing MBR and nitrification-denitrification followed by filtration. Other factors of the evaluation include anticipated wastewater treatment plant expansions to meet projected population growth and the desire to recycle plant effluent for landscape irrigation.
For the most part, municipalities that need to increase treatment capacity, improve effluent quality or consider a water recycling program will be confronted with challenges similar to those illustrated by the above examples. The following facts about MBR can help municipalities make an informed decision.
MBR technology continues to evolve at a rapid pace. Now that MBR operating and maintenance data are available, municipalities and the engineering community are no longer concerned about being the first ones to try something new.
MBR can be economical when high quality effluent is required.
The process achieves a superior effluent quality with fewer treatment processes. In water reclamation, secondary sedimentation tanks are eliminated, and there is no need for coagulation, flocculation and filtration.
The cost for disinfection is reduced because membranes remove pathogens at higher levels than other treatment technologies.
MBR eliminates the need for several treatment processes, thereby requiring less space than other treatment technologies to produce superior effluent quality.
Adding membrane modules can easily expand a well-planned facility that uses MBR. MBR generates fewer solids than conventional processes to produce a higher quality effluent.
It is also forgiving and thus requires less operator interaction. The MBR process is well suited for retrofits where existing tanks can be modified to fit the process.
The seven commercially available MBR systems use different membrane pore sizes, methods to clean the membrane, methods to withdraw permeate, hydraulic loading per square ft of membrane surface area and membrane configuration.
This represents an additional challenge with the equipment procurement process and the detailed design. The most frequent approach is to preselect a membrane equipment supplier before embarking on the detail design.