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ANAEROBIC REACTORS  (www.engetec.info)

By Shannon R. Grant, MScE, PE, Shashi Gorur, ME, PE, James C. Young, PhD, PE, Robert Landine, PhD, PE, Albert C. Cocci, PhD, PE, and Calvert Churn III, PhD, PE

A comparison of anaerobic treatment technologies for industrial wastewater

Anaerobic treatment of industrial wastewater has come a long way in the past 10 to 20 years. Prior to 1980, the process was deemed unreliable for its lack of robustness and overall instability, primarily due to misunderstandings of the biochemical pathways involved and the factors governing sludge characteristics. Systems also suffered from inferior design and improper materials selection. Today, anaerobic treatment is being used successfully throughout the world to treat a wide range of industrial waste streams. Several different anaerobic technologies are available in the marketplace. The best technology in one case may not be best in another.

Benefits and Applications
Anaerobic treatment is a biological process that utilizes a mixed culture of bacteria in the absence of free oxygen to remove organic matter that is present in the wastewater. The overall process yields a useful byproduct in the form of biogas, primarily methane (CH4) and carbon dioxide (CO2). This unique feature means that much of the available energy in the wastewater is converted to a gaseous form, resulting in very little energy left for new cell growth. In a nutshell, three significant benefits are associated with this process, namely the production of biogas energy, much less biosolids waste and a low energy requirement for the treatment process, in addition to these benefits:
Less nutrients required;
System can be shut down for extended periods without serious deterioration; and
Can handle organic shock loads effectively.

As with any process, however, anaerobic treatment does have certain drawbacks, including the following:
Anaerobic treatment cannot achieve surface water discharge quality without post-treatment;
Reduced sulfur compounds are produced, which need to be properly addressed in terms of corrosion, odor and safety; and
Longer start-up period.

A straight comparison between anaerobic and aerobic treatment clearly illustrates the operating benefits that could be realized with anaerobic treatment. This is shown in Table 1, for a given biodegradable chemical oxygen demand (CODB) waste load.

Table 1: Anaerobic vs Aerobic Treatment for 1000 kg CODB/dParameter Anaerobic Aerobic
Power consumption (kW) 1.5 65
Net biosolids prod. (kg TS/d) 15-100 200-600
Energy produced (kW) 140 Nil

In many cases, the optimum wastewater treatment configuration is an anaerobic process followed by aerobic polishing (for final biochemical oxygen demand (BOD) reduction and/or sulfide oxidation). This configuration typically guarantees that the benefits of anaerobic and aerobic treatment are realized while minimizing their respective limitations.

Wastewaters that are commonly treated using anaerobic processes are listed in Table 2 together with typical COD and BOD removal performances. Achievable removals are very much dependent on the type of wastewater being treated. Reactor configuration also affects removals, with the lower rate systems typically achieving somewhat better COD and BOD removals than high-rate systems.

Table 2: Typical Wastewaters and Anaerobic System PerformancesIndustry COD Removal (%) BOD Removal (%)
Brewery 70-90 > 90
Distillery 70-90 > 90
Recycle Paper 65-80 80-90
NSSC Pulp 50-60 80-90
TMP/CTMP Pulp 45-55 55-70
Kraft Pulp 75-85 >90
Potato Processing 80-90 80-95
Cheese/Whey/Dairy 80-90 >90
Starch Production 70-85 80-95
Chemical 60-90 >90
Pharmaceutical 50-80 >90
Yeast 55-75 >90
Leachate 70-90 >90
Sugar Beet 80-90 > 90

In many ways, anaerobic treatment can be considered to have matured significantly over the past thirty years. Many full-scale applications have been recorded and can be drawn from to anticipate potential problems or future difficulties. As a result, piloting for many applications may not be essential. Today, this is normally the case for brewery applications, potato plants, starch production, recycle paper mills, sugar beet plants and dairies.

Of course, no two industrial plants are alike. This is particularly true within some sectors more so than others (pulp mills, yeast plants, chemical plants and pharmaceutical applications). Within these particular areas, as well as in unusual applications, piloting is still highly desirable.

Commercial Installations and Technologies
Figure 1. Anaerobic system installations.

Figure 1 demonstrates the growth of commercial anaerobic installations in the world over the last 30 years (Totzke, 2000). As the figure shows, there are currently more than 1500 commercial anaerobic installations. This does not include hundreds of non-commercial (i.e., consultant or owner-designed) installations, such as anaerobic lagoons.

Several anaerobic technology configurations have been developed over the last 30 years or so. Some process configurations have been researched and described in the literature, but have not had significant commercial application; some of these include newer systems that have not yet achieved commercial success. In other cases, older technologies, which were commercially successful in the past, now have more limited use as they are replaced by more evolved and advanced technologies.

The focus in research and development of anaerobic processes has been on maximizing biomass retention and substrate-to-biomass contact — two objectives that have been challenging to combine. For example, improving biomass-to-substrate contact typically means more mixing and biogas production, which can lead to washing out the biomass unless special design considerations are used to counteract this washout.

Anaerobic systems can be categorized according to the type of biomass they depend on and how that biomass is retained in the system. Suspended-growth processes are systems where the bacteria grow and are suspended in the reactor liquid. Typically, suspended-growth systems have sludge that is considered to be ‘granular’ or ‘flocculent’ in nature (oftentimes both granular and flocculent sludges coexist in a reactor). Attached-growth processes utilize either fixed film or carrier media (which is suspended in the liquid) for the bacteria to grow on and attach to.

Granular sludge-based systems include the upflow anaerobic sludge blanket (UASB) reactor, the BIOPAQ® Internal Circulation (IC) Reactor and the Biobed® Expanded Granular Sludge Bed (EGSB) reactor. Granular sludges exhibit high settling velocities and activity rates that reduce the required reactor volume and increase the organic loading rate. Thus, these processes are considered to be high-rate systems. The factors that create the formation of a good granular sludge are complex and have been studied for the last two decades or more by several academic researchers and anaerobic system vendors. These factors are varied but principally relate to wastewater characteristics, system configuration and loading condition. Typically, the granular sludge is retained in the system by specially designed gas-liquid-solids separation devices, which are often proprietary equipment.

Low-rate suspended-growth anaerobic systems, such as the ADI-BVF® reactor and anaerobic contact process are effective at retaining flocculent (non-granular) sludge due to lower organic and hydraulic loading rates than the high-rate systems mentioned above. These low-rate systems are particularly effective when treating wastewaters that do not granulate well or have substances that effect the retention of granules at high loading rates (i.e., high concentrations of fat, oil or grease (FOG), total suspended solids (TSS), COD, salts, total dissolved solids, calcium, etc., in the wastewater).

Attached-growth processes include expanded/fluidized bed reactors and fixed-film processes. In an expanded/fluidized bed reactor, suspended carrier media (such as sand or porous inorganic particles) are used to develop an attached film. Fixed film processes, as the name would suggest, rely on the bacteria attaching to a fixed media, like rocks, plastic rings, modular cross-flow media, etc. Some systems, such as the anaerobic hybrid process, combine suspended- and attached-growth processes in a single reactor to utilize the advantages of both types of biomass.

In some cases, a particular type of wastewater has been treated successfully by several different types of anaerobic technologies. In other cases, experience has shown that a particular technology is more appropriate (i.e., more cost-effective, stable and/or efficient) for certain wastewaters than others.

The Upflow Anaerobic Sludge Blanket (UASB) Reactor
The UASB reactor is the world’s most widely applied anaerobic technology after lagoons, with hundreds of systems having been installed over the last 20 years. The UASB process is a high-rate granular sludge-based system with typical loadings in the range of five to 15 kilograms of chemical oxygen demand per cubic meter per day (kg COD/m3/d). There are several vendors throughout the world that commercially market and sell UASB technology.

Influent flow is typically equalized, neutralized and partially acidified in a separate tank ahead of the reactor. The influent flow is often mixed with effluent recycle and then distributed into the lower part of the reactor below the sludge bed. The upper portion of the reactor typically has a gas-liquid-solids separator (GLSS) that removes biogas and clarifies the effluent.

UASB reactors typically require low influent TSS concentrations (< 15 percent of the influent COD concentration) and FOG concentrations (< 100 milligram per liter (mg/l)).

The Biobed® EGSB Reactor
The Biobed® EGSB reactor is another more recently developed granular sludge-based system. The EGSB utilizes the same operating principles as the UASB but differs in terms of geometry, process parameters and usually, construction materials. The EGSB has a substantially smaller footprint, and tanks are usually 12 to 18 meters in height, typically consisting of fibreglass-reinforced plastic (FRP) or stainless steel (Zoutberg and Flick, 1999). Loading rates are typically in the range of 10 to 25 kg COD/m3/d.

The BIOPAQ® IC Reactor
The BIOPAQ® IC reactor is another example of a commercial high-rate granular sludge-based system. Wastewater with low TSS and FOG can be processed through the reactor in as little as a few hours, depending on the strength of the waste. Locations where space is at a premium can be particularly suitable for high-rate technology, with its small footprint and silo-like design. Typical volumetric organic loadings range from 15 to 35 kg COD/m3/d. The IC reactor has been particularly effective for treating wastewaters from the beverage, brewery and paper industries.

A schematic of the IC reactor is shown in Figure 2, and a photograph is shown in Figure 3.
Figure 2. Schematic of BIOPAQ IC reactor.

Figure 3. Photo of a BIOPAQ IC reactor at a Kraft pulp mill in the USA.

The influent is pumped into the reactor via a distribution system, where influent and recycled sludge/effluent are well mixed. The first reactor compartment contains an expanded granular sludge bed, where most of the COD is converted to biogas. The biogas produced in this compartment is collected by the lower level separator and is used to generate a gas lift by which water and sludge are carried upward via the “riser” pipe to the gas/liquid separator located on top of the reactor. Here the biogas is separated from the water/sludge mixture and leaves the system. The water/sludge mixture is directed downwards to the bottom of the reactor via the concentric “downer” pipe, resulting in the internal circulation flow. The effluent from the first compartment is post-treated in the second, low-loaded compartment, where any remaining biodegradable COD is removed. The biogas produced in the upper compartment is collected in the top 3-phase separator, while the final effluent leaves the reactor via overflow weirs.

Anaerobic Fluidized Bed Reactor
The anaerobic fluidized bed reactor uses an inorganic carrier, such as sand or other type of particle, that provides sufficient area for bacterial growth, as well as weight to help hold the bacteria in the reactor. The reactor tank is tall and cylindrical, and recirculation rates are relatively high to fluidize the sludge and provide good biomass-to-substrate contact. Fluidized bed reactor technology is similar to the granular sludge-based systems mentioned above in that it is best applied to high-strength wastewaters low in TSS and FOG concentration.

One commercially successful fluidized bed reactor is the Anaflux process. This reactor uses a natural porous inorganic particle that has a very high surface area for bacterial growth. A triple-phase separator at the top of the reactor is used to separate liquid, biogas and solids. The solids are transferred from the separator back into the reaction chamber of the tank by gravity or pump. The Anaflux reactor can be applied at loading rates of 60 kg COD/m3/d or more if a separate acidogenic stage is also included ahead of the reactor.

Anaerobic Filter Reactor
The anaerobic filter is a fixed-film technology. Instead of depending on the growth of granular sludge with good settling characteristics or bacterial attachment to a suspended carrier, biomass becomes attached to fixed media in the reactor. Several different reactor configurations (upflow, downflow and hybrid) and types of media (random pack, cross-flow, pall rings, etc.) have been employed with the process.

The volumetric organic loading to the anaerobic filter is typically in the range of five to 15 kg COD/m3/d. Similar to previously mentioned processes, the anaerobic filter is primarily used for removal of soluble organics and has similar loading limits in terms of FOG (< 100 mg/l) and TSS (< 15 percent of COD) concentrations.

Anaerobic Hybrid Reactor
Figure 4. Schematic diagram of ADI-Hybrid reactor.

The hybrid reactor is a combination of suspended- and fixed-film growth processes. Typically, the upper 50 to 70 percent of the reactor is filled with cross-flow plastic media that serves as the fixed-film zone (or anaerobic filter section). The lower 30 to 50 percent is the suspended-growth zone (or UASB section). A schematic diagram of the ADI-Hybrid reactor is provided as an example of such technology that is available commercially (see Figure 4).

Organic loading rates are typically in the range of five to 15 kg COD/m3d. Similar to the other high-rate systems described previously, this process is used primarily for soluble organics removal and has similar constraints in terms of influent FOG and TSS concentrations.

The hybrid reactor has been particularly suitable for wastewaters where the development of granular sludge has proven to be difficult, such as in some chemical industries. The attached growth on the media in the upper portion of the reactor together with the formation of a granular or flocculent sludge bed in the lower section helps concentrate biomass in the system, thus promoting better process stability and higher performance. The cross-flow media also serves as an effective gas-liquid-solids separator, further enhancing the biomass retention abilities of the process.

Low-Rate Anaerobic Reactor
An example of a low-rate anaerobic technology that is available commercially is the ADI-BVF® reactor. This process consists of a suspended-growth reactor, with typical loadings in the range of 0.5 to 3 kg COD/m3/d. The lower volumetric loading rate allows the reactor to retain non-granular flocculent biomass and to treat wastewaters that have higher COD, TSS and FOG than can be handled by high-rate processes. As such, the process is particularly effective for treating wastewaters, such as potato processing, dairy and cheese, yeast and distillery.

The BVF® reactor can be constructed of in-ground, lined concrete and/or earthen basins or in above-ground concrete or steel tanks. The system utilizes a flexible insulated geomembrane cover.

The larger volume of the system means that it occupies more land area; however, the larger volume retains a large amount of biomass, which gives the process more stability and robustness than higher rate systems. Furthermore, the system can operate at lower temperatures than other processes and generates less waste sludge on a dry weight basis.

In many cases, up-front clarification is not necessary, thereby eliminating the added capital and operating costs associated with primary solids handling. Activated sludge from downstream aerobic polishing processes can be digested in the BVF reactor, eliminating sludge dewatering equipment, lowering waste sludge production and operating costs and simplifying the overall sludge handling/disposal process.

Conclusions
Anaerobic treatment has become an accepted and standard avenue for treatment of high-strength industrial wastewaters. Over the last 20 years there has been significant advancement in the understanding of the microbiology involved and in applying the fundamental principles to the design and development of anaerobic treatment technologies. There are now over 1500 full-scale commercial anaerobic installations throughout the world treating a wide variety of wastewaters. Depending on the type of wastewater and other factors regarding each application, one anaerobic technology may be more appropriate and cost-effective than another. Energy and sludge considerations push anaerobic processes to the forefront when large organic loads are involved.

This article originally appeared in the November/December 2002 issue of Water & Wastewater Products, Volume 2, Number 6, page 18.

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