The advantages of membrane bioreactor systems
With Figure 5: Reverse Osmosis
With Figure 6: Aerobic MBR Applications
Membrane separation technology benefits:
Continuous process, resulting in automatic and uninterrupted operation. Low energy utilization involving neither phase nor temperature changes Modular design no significant size limitations Minimal moving parts with low maintenance requirements No effect on form or chemistry of contaminants Discreet membrane barrier to ensure physical separation of contaminants No chemical addition requirements By Peter Cartwright, PE
| Conventional vs. Crossflow Filtration |
Membrane separation technologies have been around for more than fifty years. These technologies utilize a membrane to separate contaminants from water streams, and are typically based on the engineering principle of “cross flow filtration.” Figure 1 compares the crossflow mechanism with conventional filtration. Membrane separation technologies can be subdivided into four categories, based on the class of contaminant that is removed.
| Microfiltration |
Figure 2 depicts the mechanism of microfiltration. Generally, microfiltration involves the removal of particulate suspended materials ranging in size from approximately 0.01 to 1 microns (100 to 10,000 angstroms).
| Ultrafiltration |
Figure 3 depicts ultrafiltration, which is used to separate materials typically smaller than 0.01 micron (100 angstroms). The removal characteristics of UF membranes can be described in terms of “molecular weight cutoff” (MWCO), the maximum molecular weight of compounds that will pass through the membrane pores. MWCO terminology is expressed in Daltons. Basically, ultrafiltration is used to remove dissolved nonionic contaminants, while suspended solids are removed by microfiltration.
| Nanofiltration |
This is an intermediate process between ultrafiltration and reverse osmosis. The molecular weight cutoff (MWCO) properties of nanofiltration membranes are in the range of 300 to 800 Daltons (4) are rejected as much as 99 percent, while monovalent salts such as sodium chloride (NaCl) may have rejections as low as 10 percent. Figure 4 illustrates the mechanism of nanofiltration.
Reverse Osmosis (RO)
| Reverse Osmosis |
This process will remove all dissolved organic (nonionic) solids with molecular weights above approximately 100 Daltons, as well as a high percentage of ionic materials. Because reverse osmosis membranes are not perfect (they will typically remove 95㭟 percent of the ionic contaminants), for high purity water production, they are generally used as pretreatment to a final “polishing” deionization unit. Figure 5 illustrates the reverse osmosis mechanism.
Membrane bioreactor technology
The utilization of bacteria to degrade biological contaminants in water is perhaps the oldest and most widely used wastewater treatment technology. For example, virtually all municipal wastewater treatment plants take advantage of naturally occurring bacteria that break down sewage as part of their normal metabolic activity. In actuality, there are two distinct biodegradation processes in use today, aerobic and anaerobic. Aerobic utilizes air to encourage the activity of aerobic bacteria, whereas the anaerobic process utilizes another kind of bacteria which thrive in the absence of air. In general, the contaminants that are broken down as a result of biodegradation are classified as BOD (Biochemical Oxygen Demand), organic compounds that constitute the food for the bacteria. The key to effective and efficient biodegradation processes is the creation and maintenance of a healthy population of thriving bacteria, know as the biomass. Typically, the biomass is either suspended in the wastewater in what is called a suspended-growth environment, or attached to a surface, known as fixedfilm. Within the suspended growth category there are a number of design options including: Defused aeration Jet aeration Surface aeration Under the fixed film category, design options include: Trickling filters Rotating biological contactors Submerged biological contactors
An interesting offshoot of the suspendedgrowth concept and the latest membranebased technology is membrane bioreactor systems. MBR involves utilizing an MF or UF membrane to filter the treated water to remove particles and microorganisms. The permeate is recovered, and, if necessary, further purified with RO or another polishing process. The concentrate stream is returned to the bioremediation tank. Compared to other biological processes, MBR offers the following advantages: Highquality effluent, almost free from solids The ability to disinfect without the need for chemicals Complete independent control of HRT (Hydraulic Retention Time) and SRT (Sludge Retention Time) Reduced sludge production Process intensification through high biomass concentrations with MLSS (Mixed Liquor Suspended Solids) ~ 8,000-15,000 mg/L Treatment of recalcitrant organic fractions and improved stability of processes such as nitrification Ability to treat high strength wastes The device configurations most commonly used today are capillary fiber and plate and frame, although tubular devices are becoming more widely used. The most common biological treatment is aerobic and, typically, air is bubbled into the treatment tank. A popular approach is to immerse the membrane element in the treatment tank and either allow the hydrostatic head of the solution to provide the driving force, or to use a pump to pull the permeate through the membrane (or both). In this case, air bubbles are also directed over the surface of the membrane (air scouring) in an effort to reduce fouling. Another design involves pumping water through the membrane system external to the treatment tank. Yet another uses a separate tank for membrane processing downstream of the biological treatment tank. Additional designs and configurations are sure to appear as MBR technology becomes more widely used. Figure 6 illustrates aerobic MBR applications for both “immersed” and “external” designs.
This interesting melding of old and new technologies underscores the technology advancements aiding both industry and municipalities in their quest for new sources of high quality water based on reuse and recovery. Peter Cartwright received his BChE degree from The University of Minnesota in 1961, and is a registered professional Engineer in the State of Minnesota. He entered the water treatment industry in 1974 and started his consulting engineering firm in 1980. He has authored almost 100 articles, written several book chapters, presented over 125 lectures in conferences around the world, has been awarded three patents, and is a 2001 recipient of the Award of Merit from the Water Quality Association. He is also a founding member of the Technical Review Committee for Water Conditioning &Purification magazine, on the Editorial Advisory Board of Filtration News magazine, and the Editorial Board of “Industrial Water World” magazine, and a peer reviewer for the “Journal of Membrane Science.” More information is available by calling (952)854-4911, emailing firstname.lastname@example.org, or visiting www.cartwrightconsulting.com.
How It Works
Photoacoustic infrared technology builds upon the success of basic infrared technology for ambient gas monitoring. This technology also exposes the gas sample to infrared light, but unlike absorptive infrared, the reading is based on what happens to the gas after
it absorbs the infrared light. With this method, a comparison to a reference sample is not required, so a direct gas reading is obtained. In a photoacoustic infrared instrument, a gas sample is introduced into the measurement chamber of the monitor, and the sample is exposed to a specific wavelength of infrared light. If the sample contains the gas of interest, it will absorb an amount of infrared light proportional to the concentration of gas present in the sample. However, photoacoustic infrared analysis extends beyond simply measuring how much infrared light is absorbed. Photoacoustic infrared technology observes what happens to the gas once it has absorbed the infrared light. The molecules of any gas are always in motion, and as they move around inside the measurement chamber, they generate pressure. When a gas absorbs infrared light, the molecules\rquote temperatures rise, and they begin to move more rapidly. As a result, the pressure inside the measurement chamber increases, creating an audible pulse that can be detected by a sensitive microphone located inside the photoacoustic infrared monitor. The gas is then irradiated with pulsed infrared energy, and as the molecules absorb this energy, they heat and cool. The pressure also changes as a result of the heating and cooling of the molecules measured by the detector. The gas is exhausted, a fresh sample enters the cell, and the sampling process is continuously repeated. Because the optical filter will only pass the particular wavelength of light for the gas in question, a pressure pulse indicates that the gas is present. If no pressure pulse occurs, then no gas is present. The magnitude of the pressure pulse indicates the concentration of the gas present and the stronger the pressure pulse, the more gas that is present. The microphone inside the monitor can detect the smallest of pressure pulses, enabling it to detect even the lowest levels of gas.
Benefits of Photoacoustic Infrared Monitors
The term Threshold Limit Value (TLV) refers to the concentration of airborne substances under which workers can be repeatedly exposed without adverse health effects. The purpose of gas detection instruments is to ensure that gases are identified at concentrations equal to or lower than the TLV to ensure a safe working environment. With certain gases, the TLV can be extremely low, requiring a detection method that can identify very low levels of that gas. The higher an instrument\rquote s sensitivity, the lower the levels of gases it can detect. Absorptive infrared monitors can easily detect gases in percent by volume or high partpermillion (ppm) levels; however, the detection limits for many absorptive infrared monitors can be well above the TLV of many gases. In order to achieve the required ppm level of detection, these instruments need to have longer sample chambers, increasing the overall size of the monitor, as well as the cost. Photoacoustic infrared monitors can detect gases at low ppm, and even partperbillion (ppb) levels due to the high sensitivity and stability of their microphone. This microphone can detect small pressure pulses, allowing photoacoustic infrared monitors to detect the presence of a toxic compound before the concentration reaches the TLV for many gases. Zero stability, or maintaining a stable baseline, is very important for low ppm detection. Instability can compromise lowlevel detection by causing inaccuracy, false alarms, and limited detection levels. A common problem with absorptive infrared monitors is the fact that the zero derived from the sample to reference ratio has a tendency to drift based on a number of factors. These factors include application, age, light source variability, and physical changes in the detector over time. Because the absorptive infrared monitor reading compares the readings of the sample gas to the reference gas, it\rquote s critical that the balance between cells is maintained. If not, the monitor must be calibrated or rezeroed to ensure a correct zero point. Otherwise, the monitor may present a false alarm, or become unable to detect low concentrations of the gas in question. Another downfall to this technology is that the rezeroing process takes time away from the sampling process. Photoacoustic infrared technology offers zero stability because it eliminates the need to adjust for zero drift. There is no zero point involvedproviding more accurate and reliable readings. Crosssensitivity, the ability to differentiate between various gases that may be present within a single sample, is a key factor to consider when choosing gas detection technology. When testing for a specific toxic or combustible gas, it is quite possible that another gas with similar absorption characteristics is present in the chamber. Even ambient air can cause crosssensitivity problems due to the variability of carbon dioxide or relative humidity in the atmosphere. For example, if a monitor is crosssensitive to CO2
, merely breathing on the monitor can cause a false reading. Photoacoustic infrared monitors, like other infrared monitors, are designed to minimize crosssensitivity through the use of specific optical filters. Given the stability of photoacoustic infrared technology and the use of optical filters, one can achieve the sensitivity and selectivity required for low ppm detection. For installations that require detection of a toxic or combustible gas at very low levels, particularly in an environment where crosssensitivity is an issue, photoacoustic infrared monitors are an excellent choice, as they provide precise, lowcost, highperformance monitoring for a variety of gases. These monitors can currently detect more than 60 common industrial gases including carbon monoxide, carbon dioxide, cleaning agents, heat transfer fluids, and a host of common industrial chemicals, with many other possible applications. Photoacoustic infrared monitoring systems can be expanded to observe up to eight separate locations. Additional sensors can be added within the same instrument enclosure to monitor noninfrared detectable gases such as catalytic bead sensors for combustible gas detection and electrochemical sensors for monitoring oxygen, carbon monoxide, and other toxic gases. Absorptive infrared monitors are often a suitable choice in gas detection, particularly when higher detection levels are acceptable. However, when the situation calls for an extremely lowlevel alarm in the presence of other gases, and when reliability is critical, photoacoustic infrared monitors offer the best package of performance and value. Allan Roczko is an MSA Product Line Manager and has worked with photoacoustic products for over thirteen years. Allan has BS and MS degrees in chemistry from the University of Pittsburgh and an MBA from Robert Morris University. MSA, P.O. Box 427, Pittsburgh, PA 15230, manufactures and sells personal safety equipment designed to protect the safety and health of people throughout the world. Product lines include respiratory protective equipment, thermal imaging cameras, gas detection instruments, head, eye, face and hearing protectors and fall protection equipment. For more information visit www.msagasdetection.com.