Membrane bioreactors are a newer type of aerobic treatment unit recently found more and more often in the market for small-scale effluent treatment systems.
First developed in the 1960s, these units have seen significant modifications since the late 1990s resulting in a more robust and practical pretreatment unit for applications in onsite systems.
ATUs pretreat effluent by adding oxygen to break down organic matter, reduce pathogens and transform nutrients in what is known as the activated sludge treatment process. Naturally occurring microorganisms consume organic material in sewage. Commonly, bacteria and other microorganisms are considered to be undesirable components of effluent, yet only a small fraction of the microbes found in effluent are truly pathogenic.
Supporting bacteria
Aerobic effluent treatment encourages the growth of naturally occurring aerobic microorganisms as a means of treating effluent. These microbes are the engines of effluent treatment.
When dissolved oxygen is available, microorganisms in decomposing organic matter consume it. The oxygen available in the MBR allows nitrifying bacteria to effectively transform the ammonia to nitrate. Under anoxic conditions, the nitrate is denitrified to nitrogen gas. Some MBRs are designed to also provide denitrification as part of their operation.
Design modifications include intermittently supplying air and recirculating the nitrified effluent into the anoxic regions within the treatment unit. The advantages of MBR systems over conventional ATUs include better effluent quality, smaller space requirements and ease of automation. Specifically, MBRs operate at higher volumetric loading rates, which result in lower hydraulic retention times. The low retention times mean that less space is required compared to a conventional system.
What’s happening inside
MBRs combine two basic processes — biological degradation through the AS treatment process and membrane separation — into a single process. This allows suspended solids and microorganisms responsible for degradation to be separated from treated water by membrane filtration units, which pull water through a membrane with very small pores.
There are several types of MBRs used: flat sheet, hollow fiber, ceramic and tubular membranes. Typically, they are immersed in a tank, and a slight suction is applied to pull the treated effluent through the membrane.
While there are many designs of MBRs, the systems typically share characteristics. For instance, most MBRs use ultrafiltration membranes with a 0.02 to 0.05 micron pore size, which traps solids on the outside. The membrane is often made of polypropylene, cellulose acetate, aromatic polyamides or thin-film composite. With time, a thin biofilm forms on the membrane, which in turn reduces the pore size and further limits the diameter of the organism, which can pass through the barrier.
MBR systems retain the biomass, which results in a high level of mixed liquor suspended solids (MLSS – the volume of suspended solids in the mixed liquor of an aeration tank) benefiting the bacteria with low growth rates.
The rate of effluent passing through a unit area of the membrane per unit time is defined as the flux rate and is an important design variable. The membranes are kept clean by various strategies including low flux operation, air scouring by bubbling, intermittent operation and backwashing.
Crucial characteristics
There are two very important design considerations when you are dealing with MBRs:
- Accurate flow determination and appropriate application. The system has to be able to handle peak flows while the units are also sized to handle a specified flow.
- MBRs also need to have an appropriate screening process. Things that could either clog, scrape or cut the unit need to be excluded. Most of the time, the septic tank and an effluent screen provide enough filtering.
The activated sludge process combined with membrane separation is able to achieve very high removal efficiencies of organic material, with >95% common for BOD and TSS. In addition to removing biodegradable organics and suspended solids, MBRs remove a very high percentage of pathogenic organisms, providing disinfection of the effluent with 99.9% removal of fecal coliform with two- to five-log virus removal and more than five-log removal of protozoa.
MBRs with advanced removal process design have been found to eliminate 60-90% of total nitrogen and phosphorus. In order to achieve these rates of nutrient reductions, special design modifications are required, including varying aeration schemes and recirculating effluent to an anaerobic mixing tank.
Modern solutions
In recent years, pharmaceutical and personal care products in wastewater have become a concern. It is known that conventional activated sludge processes are not able to eliminate these compounds completely. Membrane bioreactors are widely believed to achieve further removal of micropollutants. The Water Technology Center in Karlsruhe, Germany analyzed samples from a MBR pilot plant for 12 common pharmaceuticals and endocrine disruptors such as ibuprofen, naproxen and nonylphenol. Removal efficiencies by the bioreactor process were between 20% and 100% with half of the substance having removal efficiencies above 80%. Other research studies have shown similar results.
Residential water reuse is a topic of increasing interest. With growing water shortages in the Southwestern United States and other areas, the ability to reuse water onsite as opposed to sending it all to a wastewater treatment facility becomes more and more appealing. Additionally, reuse of water is generally considered to be a part of sustainable resource management practice, so it is increasingly being utilized or considered in areas that do not necessarily suffer from water shortages.
MBRs are a technology that produces effluent that can potentially be reused. NSF/ANSI 350 is a testing and certification process for reuse systems. This product testing is for residential and commercial facilities and covers four different types of influent water — combined blackwater and graywater, graywater only, bathing water only and laundry water only with intended uses for nonpotable applications, such as surface and subsurface irrigation and toilet and urinal flushing. MBRs are one of many types of technologies that could meet reuse quality effluent standards.
The primary disadvantage of MBR systems is the typically higher capital and operating costs than conventional ATU systems for the same throughput. O&M costs include membrane cleaning and fouling control, and eventual membrane replacement. Energy costs are also higher because of the need for air scouring to control bacterial growth on the membranes. They are a solution to consider with stringent effluent requirements when small footprints are needed.
