Anaerobic Reactor

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Fundamentals

Anaerobic processes involve converting the organic matter in wastewaters to methane andcarbon dioxide through a series of reactions involving a consortium of facultative and obligateanaerobic microorganisms. Complex organic waste constituents - starches, fatty acids, proteins,

alcohols, complex organic chemicals, and the like - are converted first through enzymatichydrolysis to lower-molecular-weight soluble intermediates such as sugars, alcohol and aminoacids. These soluble substances are converted further through fermentation reactions to formorganic acids. Acetogenic microorganisms convert the higher-molecular-weight organic acids toacetic acid plus hydrogen in a free or bound form. Hydrogen and acetic acid are the primaryprecursors to the mineralization end products: methane and carbon dioxide. This methane gashas potential value for heating the digester for improved treatment efficiency or for production ofelectrical power for other in-plan processes. A principal advantage of anaerobic treatment is thesmall amount of excess biomass produced during biodegradation. Net yields as low as 2 kgVSS/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 inone case may not be the best in another. Wastewater characteristics affect the success or failureof specific processes. Fixed-media processes seem to be most suitable for treating low-yieldwaste constituents, while suspended-growth reactors are most suitable for treating high-yieldwastes that readily produce granules. However, with proper selection of organic loading rate andoperating conditions, each system can treat almost any type of wastewater but possibly will notperform 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 howthat biomass is retained in the system. Suspended-growth processes are systems where thebacteria grow and are suspended in the reactor liquid. Typically, suspended-growth systemshave sludge that is considered to be granular or flocculent in nature - oftentimes both granularand flocculent sludges coexist in a reactor. Attached-growth processes utilize either fixed filmor 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, combinesuspended- and attached-growth processes in a s ingle reactor to take advantage of both biomasstypes.

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 bulkvolume reactors and anaerobic contact reactors. Both these approaches are effective atretaining flocculent (i.e. nongranular) sludge due to lower organic and hydraulic loading ratesthan the high-rate systems. In particular they are very well suited for industrial applications thatdo 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 ofbiogas however is essential for environmental protection as well as potential alternative energysource. Gas composition is similar to biogas derived from engineered anaerobic systems and canbe used in just the same way.

The oldest and most simple designs were just uncovered, unmixed lagoon having retention timesmeasured in weeks or months. Suspended biomass concentration is very dilute. Consequently

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biological treatment capacity is limited. In order to make this option practical and/or competitivea number of items are incorporated:

mixing means to improve contact between constituents and biomass covers of some type, e.g. floating, natural, to help control odor, temperature and gas

collection inlet waste distribution grids recycle or sludge return piping

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 bulkvolume reactors. In the latter case heat is normally provided by in-plant boiler that would use thebiogas or a hot water boiler that would be installed at waste treatment and run on biogas. Eithershell and tube (dirty side) or spiral HX are used. The spiral is the best but its cost is quite a bitmore than shell and tube (~3x). Another method for heating that is used frequently is to injectlive steam into the equalization tank or into a mix tank ahead of the anaerobic reactor. Thisoption, if live steam is available, usually is more economical than heat exchangers and is easierto operate. Bulk volume reactors can take some low temperature days but one shouldn't allowthe temperature to drop any lower than 29 °C for periods up to perhaps 1-2 weeks if at allpossible. One would expect there to be a significant loss in performance for this period of time -ust a few percent COD.

One cannot store gas under the lagoon cover. It is essential that the cover always be on top of thewater. It is most common/required, that the system be operated with a slight vacuum to keep thecover down on the top of the water. If you want to store gas, you will need to add a storage tankwhich can store the gas under a known pressure and then pump from the lagoon to the tankcontinuously. 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 cattleabattoirs and dairies, it is not recommended the use of a low-rate covered reactor without DAF toremove grease first, even if the temperature is as high as 32C. The minimum temperature forgrease biodegradation is 32C. If at any time the temperature is lower, grease will accumulateunder the cover of a low rate digester and will not resuspend. That is, it will remain under thecover. Therefore, if the temperature in the winter time is less than 32C, recommend itsremoval. The system for a low-rate anaerobic option with DAF would also include a sludgedigester for the grease. This second digester would have to be heated to 35C to guaranteebiodegradation of the grease. A high-rate option would eliminate the need for DAF and solidsdigestion.

Aside from the already mentioned methane emission issue, large lagoons such as 80-d HRT willhave very little operational needs, but will have uncontrolled output.

Anaerobic Contact Process

Anaerobic contact reactors employ an external clarifier or vessel to settle solids and subsequentlyrecycle them back to the reactor tank. Typical configurations include large tanks due to the loworganic and hydraulic loading rates employed in their design. Anaerobic contact systems areparticularly effective when granulation is difficult or wastewater contains higher than desirableamounts of troublesome constituents, e.g. O&G, suspended solids. Anaerobic contactalternatives are effective at successfully retaining flocculent, i.e. nongranular sludge, thuspermiting maintaining appropriate anaerobic biomass inventory levels.

Granular Sludge Processes

Granular sludges exhibit high settling velocities and activity rates that reduce required reactorvolumes 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 complexand considerably researched both by investigators and vendors. These factors are varied butprincipally relate to wastewater characteristics, system configuration and loading condition.Typically, these systems retain granular sludge by employing specially designed oftenproprietary gas-liquid-solids (GLS for short) separation devices. Most representativeconfigurations include UASB (upflow anaerobic sludge blanket) and EGSB (expanded granularsludge bed) reactors.

Attached-growth

Attached-growth systems include fixed-film and fluidized bed reactors. In fluidized bedreactors, suspended carrier media such as sand are used to provide support surface for themicroorganisms. Fixed film processes have bacteria reside on some type of more static supportsurface such as rocks, plastic rings or media modules. Most representative configurationsinclude the classical AF (fully packed anaerobic filter) and of historical value, fluid bedvariants.

Hybrid Anaerobic Reactors

The hybrid reactor design combines an lower section functionally identical to an UASB and anupflow 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 forflocculant and/or granular sludge formation. The upper 50 to 70 percent of the reactor is filledwith crossflow plastic media and behaves as an anaerobic filter.

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Advantages of the hybrid anaerobic reactor design include:

granular sludge formation is not essential, i.e. flocculant sludge will perform satisfactorilyand attain good stability at relatively high loadings

biomass developed in the fixed media section add to the reactor's inventory

The hybrid reactor has been particularly suitable for treating wastewaters where granular sludgedevelopment is difficult such as some chemical industries wastes. The attached growth on themedia in the upper portion of the reactor together with the formation of a granular or flocculentsludge bed in the bottom section help add up significant biomass inventories leading to increasedprocess stability and higher removal. The crossflow media modules also act as effective gas-liquid-solids separator further enhancing biomass retention.

Two-Stage Reactors

Some installations use two reactors in series. Not only is the COD removal efficiency improvedover that of single-stage reactors operating at the same hydraulic retention time, but operation inthe cyclic mode - that is, by periodically reversing the lead and follow reactors - producessignificant reduction in excess solids production. A number of plants of this type have beenplaced in operation since 1987. One kind of two-stage option uses the first stage as a hydrolysisreactor, 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 highstrength 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 volumesoluble constituent wastewaters.

www.AerationFundamentals.com - www.ExtendedAeration.com - www.OxidationDitches.com - www.TricklingFilters.com

www.Biotowers.com - www.MembraneBioreactors.com - www.AnaerobicReactors.com - www.AnaerobicFilters.com

www.UASBs.com - www.EGSBs.com - www.CoolingTowerFundamentals.com - www.EvaporativeCondensers.com

www.DewateringFundamentals.com - www.BioremediationFundamentals.com - www.IncinerationFundamentals.com


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