A Primer on Anaerobic Reactors | Fundamentals & Applications
Anaerobic processes involve converting the organic matter in wastewaters to methane and carbon dioxide through a series of reactions involving a consortium of facultative and obligate anaerobic microorganisms. Complex organic waste constituents - starches, fatty acids, proteins, alcohols, complex organic chemicals, and the like - are converted first through enzymatic hydrolysis to lower-molecular-weight soluble intermediates such as sugars, alcohol and amino acids. These soluble substances are converted further through fermentation reactions to form organic acids. Acetogenic microorganisms convert the higher-molecular-weight organic acids to acetic acid plus hydrogen in a free or bound form. Hydrogen and acetic acid are the primary precursors to the mineralization end products: methane and carbon dioxide. This methane gas has potential value for heating the digester for improved treatment efficiency or for production of electrical power for other in-plan processes. A principal advantage of anaerobic treatment is the small amount of excess biomass produced during biodegradation. Net yields as low as 2 kg VSS/kg COD removal are not uncommon.
Anaerobic Reactors: A Brief Overview of Available Systems
Several different anaerobic technologies are available in the marketplace. The best technology in one case may not be the best in another. Wastewater characteristics affect the success or failure of specific processes. Fixed-media processes seem to be most suitable for treating low-yield waste constituents, while suspended-growth reactors are most suitable for treating high-yield wastes that readily produce granules. However, with proper selection of organic loading rate and operating conditions, each system can treat almost any type of wastewater but possibly will not perform optimally. Unless previous experience is available with treating a specific wastewater, treatability tests are recommended highly.
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 carried media (the latter suspended in the liquid) for the bacteria to grow and attach to. Finally relatively recent developments, e.g. hybrid anaerobic filters or HAFs for short, combine suspended- and attached-growth processes in a single reactor to take advantage of both biomass types.
Systems can be further qualified based on process rate, e.g. low-rate systems and high rate systems. Thus one can have a low-rate, suspended-growth anaerobic system which include bulk volume reactors and anaerobic contact reactors. Both these approaches are effective at retaining flocculent (i.e. nongranular) sludge due to lower organic and hydraulic loading rates than the high-rate systems. In particular they are very well suited for industrial applications that do not granulate well or have higher than desirable amounts of troublesome constituents, e.g. high levels of O&G, high levels of SS.
Anaerobic Lagoons and Bulk Volume Reactors
Anaerobic lagoons are essentially large unsophisticated, low-rate anaerobic reactors. Capture of biogas however is essential for environmental protection as well as potential alternative energy source. Gas composition is similar to biogas derived from engineered anaerobic systems and can be used in just the same way.
The oldest and most simple designs were just uncovered, unmixed lagoon having retention times measured in weeks or months. Suspended biomass concentration is very dilute. Consequently biological treatment capacity is limited. In order to make this option practical and/or competitive a number of items are incorporated:
Contemporary low-rate designs include simple floating cover, lined
concrete and/or earthen basin systems all the way to aboveground tanks and
highly engineered, high performing bulk volume reactors. In the latter
case heat is normally provided by in-plant boiler that would use the biogas or a hot water boiler that would be installed at waste treatment and run on biogas. Either shell and tube (dirty side) or spiral HX are used. The spiral is the best but its cost is quite a bit more than shell and tube (~3x).
Another method for heating that is used frequently is to inject live steam into the equalization tank or into a mix tank ahead of the anaerobic reactor. This option, if live steam is available, usually is more economical than heat
exchangers and is easier to operate. Bulk volume reactors can take some low temperature days but one shouldn't allow the temperature to drop any lower than 29 °C for periods up to perhaps 1-2 weeks if at all possible. One would expect there to be a significant loss in performance for this period of time - just a few percent COD.
One cannot store gas under the lagoon cover. It is essential that the cover always be on top of the water. It is most common/required, that the system be operated with a slight vacuum to keep the cover down on the top of the water. If you want to store gas, you will need to add a storage tank which can store the gas under a known pressure and then pump from the lagoon to the tank continuously. The lagoon will produce gas at almost a constant rate during any day because of averaging the load over the the long hydraulic retention time.
In general, one would like to see a depth of 8 or 9 meters. Also
for applications such as cattle abattoirs and dairies, it is not recommended
the use of a low-rate covered reactor without DAF to remove grease first,
even if the temperature is as high as 32C. The minimum temperature for
grease biodegradation is 32C. If at any time the
temperature is lower, grease will accumulate under the cover of a low rate
digester and will not resuspend. That is, it will remain under
the cover. Therefore, if the temperature in the winter time is
less than 32C, recommend its removal. The
system for a low-rate anaerobic option with DAF would also include a
sludge digester for the grease. This second digester would have to be
heated to 35C to guarantee biodegradation of the grease. A high-rate
option would eliminate the need for DAF and solids digestion.
Anaerobic Contact Process
Anaerobic contact reactors employ an external clarifier or vessel to settle solids and subsequently recycle them back to the reactor tank. Typical configurations include large tanks due to the low organic and hydraulic loading rates employed in their design. Anaerobic contact systems are particularly effective when granulation is difficult or wastewater contains higher than desirable amounts of troublesome constituents, e.g. O&G, suspended solids. Anaerobic contact alternatives are effective at successfully retaining flocculent, i.e. nongranular sludge, thus permiting maintaining appropriate anaerobic biomass inventory levels.
Granular Sludge Processes
Granular sludges exhibit high settling velocities and activity rates that reduce required reactor volumes and increase allowable organic loading rates. Thus, these processes are considered to be high-rate systems. The factors that create the formation of good granular sludge are complex and considerably researched both by investigators and vendors. These factors are varied but principally relate to wastewater characteristics, system configuration and loading condition. Typically, these systems retain granular sludge by employing specially designed often proprietary gas-liquid-solids (GLS for short) separation devices. Most representative configurations include UASB (upflow anaerobic sludge blanket) and EGSB (expanded granular sludge bed) reactors.
Attached-growth systems include fixed-film and fluidized bed reactors. In fluidized bed reactors, suspended carrier media such as sand are used to provide support surface for the microorganisms. Fixed film processes have bacteria reside on some type of more static support surface such as rocks, plastic rings or media modules. Most representative configurations include the classical AF (fully packed anaerobic filter) and of historical value, fluid bed variants.
Hybrid Anaerobic Reactors
The hybrid reactor design combines an lower section functionally identical to an UASB and an upflow AF on top, the idea being to combine the strengths of each approach in a single tank. Thus, the lowermost 30 to 50 percent UASB-like portion of the reactor volume is responsible for flocculant and/or granular sludge formation. The upper 50 to 70 percent of the reactor is filled with crossflow plastic media and behaves as an anaerobic filter.
Advantages of the hybrid anaerobic reactor design include:
The hybrid reactor has been particularly suitable for treating wastewaters where granular sludge development is difficult such as some chemical industries wastes. 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 bottom section help add up significant biomass inventories leading to increased process stability and higher removal. The crossflow media modules also act as effective gas-liquid-solids separator further enhancing biomass retention.
Some installations use two reactors in series. Not only is the COD removal efficiency improved over that of single-stage reactors operating at the same hydraulic retention time, but operation in the cyclic mode - that is, by periodically reversing the lead and follow reactors - produces significant reduction in excess solids production. A number of plants of this type have been placed in operation since 1987. One kind of two-stage option uses the first stage as a hydrolysis reactor, while the second is designed to optimize methane production.
Sludge or Solids Digesters
Early reactor designs, mainly batch and continuous stirred tank digesters, dealt with high strength particulate wastes such as sludge or low strength, low volume liquid wastewaters, e.g. onsite systems. Septic tanks and complete mix anaerobic reactors are good representatives. Anaerobic reactors have come a long way and routinely address high strength, high volume soluble constituent wastewaters.
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