UASB Reactor for Wastewater Treatment

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AKSES TANGGAL 29 JUNI 2005, OLEH al Akh abu Muhammad MAHMUD Hasan TIP’01 UGM
Web Site : http://www.uasb.org/discover/anaerobic_biotechnologies.htm
Author : Jim Field, jimfield@email.arizona.edu
Date Created : April 25th, 2002 Last Updated: April 17th, 2003



Anaerobic Biotechnologies
Anaerobic Wastewater Treatment: Anaerobic wastewater treatment is the biological treatment of wastewater without the use of air or elemental oxygen. Many applications are directed towards the removal of organic pollution in wastewater, slurries and sludges. The organic pollutants are converted by anaerobic microorganisms to a gas containing methane and carbon dioxide, known as "biogas" (see Figure 1 below).
Figure 1. Conversion of Organic Pollutants to Biogas by Anaerobic Microorganisms
COD Balance: In the wastewater engineering field organic pollution is measured by the weight of oxygen it takes to oxidize it chemically. This weight of oxygen is referred to as the "chemical oxygen demand" (COD). COD is basically a measure of organic matter content or concentration. The best way to appreciate anaerobic wastewater treatment is to compare its COD balance with that of aerobic wastewater treatment, as shown in Figure 2 below.
Figure 2. Comparison of the COD balance during anaerobic and aerobic treatment of wastewater containing organic pollution
Anaerobic Treatment: The COD in wastewater is highly converted to methane, which is a valuable fuel. Very little COD is converted to sludge. No major inputs are required to operate the system.
Aerobic Treatment: The COD in wastewater is highly converted sludge, a bulky waste product, which costs lots of money to get rid of. An aerobic wastewater treatment facility is in essence a "waste sludge factory". Elemental oxygen has to be continuously supplied by aerating the wastewater at a great expense in kilowatt hours to operate the aerators.
Spectrum of Applications: Most environmental engineers are aware that anaerobic processes are used to stabilize sludge such as a sludge digester at a municipal treatment plant. Less fully appreciated is the fact that "high rate" anaerobic wastewater treatment technologies can also be utilized to treat dilute to concentrated liquid organic wastewaters (distillery, brewery, paper manufacturing, petrochemical, etc). Even municipal wastewater (sewage) can be treated in tropical countries with "high rate" anaerobic technologies. "High rate" anaerobic treatment is a mature technology. At least 1200 full-scale plants have been documented world-wide for the treatment of industrial effluents (the actual number is estimated at 2500).
"High Rate"Anaerobic Treatment: High rate anaerobic treatment systems refer to bioreactors in which the sludge retention time (time for sludge biomass solids to pass through system) is separated from the hydraulic retention time (time for liquid to pass through system). The net effect is that slow growing anaerobes can be maintained in the reactor at high concentrations, enabling high volumetric conversion rates, while the wastewater rapidly passes through the reactor. The main mechanism of retaining sludge in the reactor is immobilization onto support material (microorganisms sticking to surfaces, eg. filter material in the "anaerobic filter") or self-aggregation into pellets (microorganisms sticking to each other, eg. sludge granules).
Other Applications High rate " anaerobic wastewater treatment is not limited to removal of bulk organic pollution in wastewater. There are a number of established and emerging technologies with various applications such as:
• sulfate reduction for the removal and recovery of heavy metals and sulfur
• denitrification for the removal of nitrates to
• bioremediation for the breakdown of toxic priority pollutants to harmless products
Sulfate Reduction: Sulfate reducing bacteria can be utilized to convert sulfate (SO42-) or sulfite (SO32-) to sulfide (S2-) as shown in Figure 3. The bacteria utilize electron-donating substrates present in wastewater (organic pollution) or added substrates for the reduction of sulfate. The substrates are either partially oxidized (eg. to acetate) or fully oxidized to carbon dioxide. Sulfate behaves as an alternative electron acceptor to support anaerobic respiration. The formation of biogenic sulfide is the first step in biotechnological processes directed at the removal and recovery of sulfur or heavy metals.
Figure 3. Sulfate reduction process, resulting in the formation of biogenic sulfide.
Heavy metal removal and recovery: Biogenic sulfides form highly insoluble precipitates with heavy metals (such as copper or zinc). Thus the sulfides can precipitate soluble heavy metals in wastewater streams or polluted groundwater as shown in Figure 4. The resulting metal sulfides precipitates can be removed. Since the metals ions are highly concentrated in the precipitate, they can be recycled back into industry for reuse.
Figure 4. Precipitation of heavy metals by biogenic sulfides.
Sulfur removal and recovery: Biogenic sulfides can be partially reoxidized under microaerophilic conditions (low oxygen concentrations) by chemotrophic bacteria to form insoluble elemental sulfur (S0) as shown in Figure 5. The elemental sulfur sedimented from the wastewater and can be collected for reuse in industry. A microaerophilic sulfoxidation reactor is typically placed as a post-treatment to a sulfate reducing bioreactor in order to remove and recover sulfur. Sulfoxidation reactors can also be used to clean gas streams which contain hydrogen sulfide (H2S).
Piles of elemental sulfur are shown in Figure 6.
Figure 5. Oxidation of sulfide under microaerophilic conditions by chemotrophic bacteria to elemental sulfur
Figure 6. Elemental sulfur
Denitrification: Denitrification is an anoxic process in which either an organic or inorganic electron-donating substrates are oxidized at the expense of reducing nitrate (NO3-) or nitrite (NO2-) to dinitrogen gas (N2) as shown in Figure 7. Dinitrogen is an inert gas which accounts for 70% of our atmosphere; thus the denitrification process converts nitrate- or nitrite- pollutants into a environmentally benign products. Denitrification processes are becoming popular as a post treatment method in order to remove nitrogen nutrients before treated effluents are discharged into environment. The removal of nitrogen nutrients is important to prevent eutrophication in receiving waters.
Effluents from anaerobic treatment will typically contain nitrogen in the form of ammonium (NH4+). The ammonium must first be oxidized by chemotrophic bacteria to nitrate with oxygen (known as nitrification), prior to applying the denitrification process.
Figure 7. Denitrification process with organic substrates as electron donor, resulting in the formation of inert dinitrogen gas.
Bioremediation. Anaerobic technologies are not only suitable for the removal of bulk COD they can also be utilized for the biodegradation or biotransformation of toxic priority pollutants. Microbial communities in anaerobic environments can either cause the oxidation of the pollutants resulting in its mineralization to benign products (e.g. CO2) or they can cause the reductive biotransformation of pollutants to less toxic substances (e.g. dechlorination of polychlorinated hydrocarbons). Anaerobic bioremediation can take place in bioreactors, such as the case in the treatment of industrial effluents containing toxic pollutants. Or anaerobic bioremediation can take place in situ in groundwater or sediments at contaminated sites (Figure 8).
Figure 8. Example of a hazardous waste contaminated site.
Among the most successful applications of anaerobic treatment for the oxidation of toxic pollutants is the case of the treatment of effluent in the plastic industry containing high concentrations of terephthalate. These effluents are generally high in COD and aerobic treatment would result in excessive sludge production. A complex microbial community of anaerobes is feasible to maintain in bioreactors permitting the total conversion of terephthalate to carbon dioxide and methane in high rate anaerobic bioreactors (Figure 9). Anaerobic technology has now been fully accepted as the main treatment technology for effluents of the polyethylene terephthalate (PET) industry.
Figure 9. Anaerobic biodegradation of terephthalate to carbon dioxide and methane by a complex microbial community
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Web Site: http://www.uasb.org/discover/agsb.htm
Author: Jim Field, jimfield@email.arizona.edu
Date Created: September 15th, 2002
Last Updated: April 27th, 2003

anaerobic granular sludge bed reactor technology
What is a UASB? Anaerobic granular sludge bed technology refers to a special kind of reactor concept for the "high rate" anaerobic treatment of wastewater. The concept was initiated with upward-flow anaerobic sludge blanket (UASB) reactor. A scheme of a UASB is shown in Figure 1 below. From a hardware perspective, a UASB reactor is at first appearance nothing more than an empty tank (thus an extremely simple and inexpensive design). Wastewater is distributed into the tank at appropriately spaced inlets. The wastewater passes upwards through an anaerobic sludge bed where the microorganisms in the sludge come into contact with wastewater-substrates. The sludge bed is composed of microorganisms that naturally form granules (pellets) of 0.5 to 2 mm diameter that have a high sedimentation velocity and thus resist wash-out from the system even at high hydraulic loads. The resulting anaerobic degradation process typically is responsible for the production of gas (e.g. biogas containing CH4 and CO2). The upward motion of released gas bubbles causes hydraulic turbulence that provides reactor mixing without any mechanical parts. At the top of the reactor, the water phase is separated from sludge solids and gas in a three-phase separator (also known the gas-liquid-solids separator). The three-phase-separator is commonly a gas cap with a settler situated above it. Below the opening of the gas cap, baffles are used to deflect gas to the gas-cap opening.
Figure 1. The upward-flow anaerobic sludge bed (UASB) reactor concept.
Brief History UASB. The UASB process was developed by Dr. Gatze Lettinga (Figure 2) and colleagues in the late 1970's at the Wageningen University (The Netherlands). Inspired by publications of Dr, Perry McCarty (from Stanford, USA), Lettinga's team was experimenting with an anaerobic filter concept. The anaerobic filter (AF) is a high rate anaerobic reactor in which biomass is immobilized on an inert porous support material. During experiments with the AF, Lettinga had observed that in addition to biomass attached on the support material, a large proportion of the biomass developed into free granular aggregates. The UASB concept crystallized during a trip Gatze Lettinga made to South Africa, where he observed at an anaerobic plant treating wine vinasse, that sludge was developing into compact granules. The reactor design of the plant he was visiting was a "clarigestor", which can be viewed as an ancestor to the UASB. The upper part of the "clarigestor" reactor design has a clarifier but no gas cap.
Birth of UASB. The UASB concept was born out of the recognition that inert support material for biomass attachment was not necessary to retain high levels of active sludge in the reactor. Instead the UASB concept relies on high levels of biomass retention through the formation of sludge granules. When the UASB concept was developed, Lettinga took into account the need to encourage the accumulation of granular sludge and discourage the accumulation of disperse sludge in the reactor. The main features for achieving granular sludge development are firstly to maintain an upward-flow regime in the reactor selecting for microorganisms that aggregate and secondly to provide for adequate separation of solids, liquid and gas, preventing washout of sludge granules.
First UASB. The UASB reactor concept was rapidly developed into technology, the first pilot plant was installed at a beet sugar refinery in The Netherlands (CSM suiker). Thereafter a large number of full-scale plants were installed throughout the Netherlands at sugar refineries, potato starch processing plants, and other food industries as well as recycle paper plants. The first publications on the UASB design concept appeared in Dutch language technical journals in the late 1970's and the first international publication appeared in 1980 (Lettinga et al. 1980).
Figure 2. Photograph of Dr. Gatze Lettinga made for the cover the proceedings of his farewell symposium at the time of his retirement in 2001.
EGSB. An expanded granular sludge bed (EGSB) reactor is a variant of the UASB concept (Kato et al. 1994). The distinguishing feature is that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed. The increased flux permits partial expansion (fluidization) of the granular sludge bed, improving wastewater-sludge contact as well as enhancing segregation of small inactive suspended particle from the sludge bed. The increased flow velocity is either accomplished by utilizing tall reactors, or by incorporating an effluent recycle (or both). A scheme depicting the EGSB design concept is shown in Figure 3. The EGSB design is appropriate for low strength soluble wastewaters (less than 1 to 2 g soluble COD/l) or for wastewaters that contain inert or poorly biodegradable suspended particles which should not be allowed to accumulate in the sludge bed.
Figure 3. The expanded granular sludge bed (EGSB) reactor concept.
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Overview Reactor Performance. In a recent survey (Frankin, 2001), 1215 full-scale high rate anaerobic reactors have been carefully documented, which have been built for the treatment of industrial effluents since the 1970's throughout the world. An overwhelming majority (72% of all plants) of the existing full-scale plants are based on the UASB or EGSB design concept developed by Lettinga in The Netherlands. This statistic emphasizes that the anaerobic granular sludge bed design concept has been the most successful for scale-up and implementation. The average full-scale design loading of the UASB of 682 full-scale plants surveyed was 10 kg COD/m3.d. Note: COD stands for chemical oxygen demand and refers to the organic matter in the wastewater expressed as the weight of oxygen to combust it completely. The average full-scale design loading of the EGSB of 198 full-scale plants surveyed was 20 kg COD/m3.d. COD removal efficiencies depend largely on wastewater type; however the removal efficiency with respect to the biodegradable COD is generally in excess of 85 or even 90%. The biodegradable COD is sometimes reflected in the parameter biological oxygen demand (BOD).
The four top applications of high rate anaerobic reactor systems are for:
1. Breweries and beverage industry
2. Distilleries and fermentation industry
3. Food Industry
4. Pulp and paper.
Together, these four industrial sectors account for 87% of the applications. However, the applications of the technology are rapidly expanding, including treatment of chemical and petrochemical industry effluents, textile industry wastewater, landfill leachates as well as applications directed at conversions in the sulfur cycle and removal of metals (see Other Applications). Furthermore in warm climates the UASB concept is also suitable for treatment of domestic wastewater.
Literature cited
Franklin, R. J. 2001. Full scale experience with anaerobic treatment of industrial wastewater. Wat. Sci. Technol. 44(8):1-6.
Kato, M., J. A. Field, P. Versteeg and G. Lettinga. 1994. Feasibility of the expanded granular sludge bed (EGSB) reactors for the anaerobic treatment of low strength soluble wastewaters. Biotechnol. Bioengineer. 44:469-479.
Lettinga, G., A. F. M. van Velsen, S. W. Hobma, W. De Zeeuw, A. Klapwijk 1980. Use of upflow sludge blanket reactor concept for biological waste water treatment, especially for anaerobic treatment. Biotechnol. Bioengineer. 22: 699-734.
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WWW.waterandwastewater.com AKSES TANGGAL29 JUNI 2005
Methods for UASB Reactor Design
Guest article by Nguyen Tuan Anh
Introduction
Anaerobic treatment is now becoming a popular treatment method for industrial wastewater, because of its effectiveness in treating high strength wastewater and because of its economic advantages.
Developed in the Netherlands in the late seventies (1976-1980) by Prof. Gatze Lettinga - Wageningen University, UASB (Upflow Anaerobic Sludge Bed) reactor was originally used for treating wastewater from sugar refining, breweries and beverage industry, distilleries and fermentation industry, food industry, pulp and paper industry.
Figure 1. Essential Components of an UASB Reactor (courtesy: http://www.uasb.org/discover/agsb.htm)
In recent times the applications for this technology are expanding to include treatment of chemical and petrochemical industry effluents, textile industry wastewater, landfill leachates, as well as applications directed at conversions in the sulfur cycle and removal of metals. Furthermore, in warm climates the UASB concept is also suitable for treatment of domestic wastewater.
In recent years, the number of anaerobic reactors in the world is increasing rapidly and about 72% consist of reactors based on the UASB and EGSB technologies.
Anaerobic Processes in the UASB Reactor
There are 4 phases of anaerobic digestion in an UASB reactor
• Hydrolysis, where enzymes excreted by fermentative bacteria convert complex, heavy, un-dissolved materials (proteins, carbohydrates, fats) into less complex, lighter, materials (amino acids, sugars, alcohols...).
• Acidogenesis, where dissolved compounds are converted into simple compounds, (volatile fatty-acids, alcohols, lactic acid, CO2, H2, NH3, H2S ) and new cell-matter.
• Acetogenesis, where digestion products are converted into acetate, H2, CO2 and new cell-matter.
• Methanogenesis, where acetate, hydrogen plus carbonate, formate or methanol are converted into CH4, CO2 and new cell-matter.
Specifics of the UASB Reactor
When comparing with other anaerobic reactors, we conclude that the differences as well as the specifics of an UASB are existence of granules sludge and internal three-phase GSL device (gas/sludge/liquid separator system)
Granules sludge: In an UASB reactor, anaerobic sludge has or acquires good sedimentation properties, and is mechanically mixed by the up-flow forces of the incoming wastewater and the gas bubbles being generated in the reactor. For that reason mechanical mixing can be omitted from an UASB reactor thus reducing capital and maintenance costs. This mixing process also encourages the formation of sludge granules.
Figure 2. Shape and size of granules sludge
The sludge granules have many advantages over conventional sludge flocs:
• Dense compact bio-film
• High settle-ability (30-80 m/h)
• High mechanical strength
• Balanced microbial community
• Syntrophic partners closely associated
• High methanogenic activity (0.5 to 2.0 g COD/g VSS.d)
• Resistance to toxic shock
Internal three-phase GSL device: Installed at the top of the tank, the GSL device constitutes an essential part of an UASB reactor with following functions:
• To collect, separate and discharge the biogas formed.
• To reduce liquid turbulences, resulting from the gas production, in the settling compartment.
• To allow sludge particles to separate by sedimentation, flocculation or entrapment in the sludge blanket.
• To limit expansion of the sludge bed in the digester compartment.
• To reduce or prevent the carry-over of sludge particles from the system.
UASB Design
In general, there are two ways to design an UASB reactor
1. If input COD: 5,000 - 15,000 mg/l or more, the design method should be used based on Organic Loading rate, (OLR)
2. If input COD < 5000 mg/l, the design method should be calculated based on velocity. Calculation UASB Tank Base on OLR If input COD: 5,000 - 15,000 mg/l with Organic loading rate ORL: 4 - 12 kg COD/m3.d and Hydraulic retention time HRT: 4 - 12 h COD treatment efficiency: E = (CODinput – CODoutput)/CODinput In Calculation, Percent of COD removal is 75 - 85 % Organic loading rate ORL = Q (CODinput – CODoutput) * 103 Volume of tank W = C * Q / OLR = (kg COD/m3 * m3/h) / (kg COD/m3.h) C: concentration of COD in wastewater Q: flow rate of wastewater H (m) the height of tank can be calculated by: H = HS + HSe The height of sludge layer Hs is: Hs = V * HRT Where Hs: the height of sludge layer area (main reactor) and Hse: the height of sedimentation area Where V = Velocity of flow 0.6 to 0.9 m/h HRT = Hydraulic retention time (h) In general, the height of sludge layer will be chosen in Table 1: In general, the height of sludge layer will be chosen in Table 1: Table 1. Sludge Layer Height Selection COD input Sludge layer height < 3000 mg/l 3 – 5 m > 3000 mg/l 5 – 7 m
Note: Sludge layer is longer than sludge bed layer
The height of setting area HSe ≥ 1.2 m and
The area surface of an UASB tank (m2): A = HRT * Q / H
Figure 3. A typical model of an UASB design
Calculating an UASB Tank Based on Velocity
When input COD < 5,000 mg/l, using the method base on ORL is not effective in operation process because the granular sludge will be hardly formed. Therefore, the design criteria must be: Up-flow velocity V £ 0,5 m/h. • Hydraulic retention time HRT ³ 4 h • Chosen in table 1, the height of sludge is Hs = 3 – 5 m • The height of setting area HSe ³ 1.2 m The volume of the UASB reactor: W = Q x HRT The area of the UASB reactor: A = V / Q GSL Separator Design Slope of the separator bottom from 45 – 60o Free surface in the aperture between the gas collectors: 15 – 20% of reactor area. Height of separator from 1.5 – 2 m The baffles to be installed beneath the gas domes should overlap the edge of the domes over a distance from 10 – 20 cm Construct material: In the anaerobic conditions of an UASB reactor, there is a risk of corrosion in two main situations: • Some H2S gas can pass the GSL separator and accumulate above the water level in the top of the reactor. This will be oxidized to sulphate by oxygen in the air to form Sulphuric Acid that will in turn cause corrosion of both concrete and steel. • Below the water level: Calcium Oxide, (CaO), in concrete can be dissolve with by Carbon Dioxide, (CO2), in the liquid in low pH conditions. To avoid these problems, the material used to construct the UASB reactor should be corrosion resistant, such as stainless steel or plastics, or be provided with proper surface coatings, (e.g. coated concrete rather than coated steel, plastic covered with impregnated hardwood for the settler, plastic fortified plywood, etc). Operation Operation criteria: The optimum pH range is from 6.6 to 7.6 The wastewater temperatures should not be < 5 °C because low temperatures can impede the hydrolysis rate of phase 1 and the activity of methanogenic bacteria. Therefore in winter season, methane gas may be needed to heat the wastewater to be treated in the reactor. Always maintain the ratio of COD : N : P = 350 : 5 : 1 If there is a deficiency of some of these nutrients in the wastewater nutrient addition must be made to sustain the micro-organisms. Chemicals that are frequently used to add nutrients (N, P) are NH4H2PO4, KH2PO4, (NH4)2CO3... Suspended solid (SS) can affect the anaerobic process in many ways: • Formation of scum layers and foaming due to the presence of insoluble components with floating properties, like fats and lipids. • Retarding or even completely obstructing the formation of sludge granules. • Entrapment of granular sludge in a layer of adsorbed insoluble matter and sometimes also falling apart (disintegration) of granular sludge. • A sudden and almost complete wash-out of the sludge present in reactor • Decline of the overall methanogenic activity of the sludge due to accumulation of SS Therefore, the SS concentration in the feed to the reactor should not exceed 500 mg/l In phase 2 and 3 the pH will be reduced and the buffer capacity of wastewater may have to be increased to provide alkalinity of 1000 – 5000 mg/l CaCO3 Start-up: An UASB reactor requires a long time for start-up, e.g. from 2 – 3 weeks in good conditions (t > 20oC) and sometimes the start-up can take up to 3 – 4 months. In start-up process, hydraulic loading must be £ 50% of the design hydraulic loading.
The start-up of the UASB reactor can be considered to be complete once a satisfactory performance of the system has been reached at its design load.________________________________________
Web Site: http://www.uasb.org/discover/granules.htm
Author: Jim Field, jimfield@email.arizona.edu
Date Created: September 20th, 2002
Last Updated: April 17th, 2003
Granulation
What are sludge granules? Sludge granules are at the core of UASB and EGSB technology. A sludge granule is an aggregate of microorganisms forming during wastewater treatment in an environment with a constant upflow hydraulic regime. In the absence of any support matrix, the flow conditions creates a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Eventually the aggregates form into dense compact biofilms referred to as "granules" (see Figure 1 below). Due to their large particle size (generally ranging from 0.5 to 2 mm in diameter) , the granules resist washout from the reactor, permitting high hydraulic loads. Additionally, the biofilms are compact allowing for high concentrations of active microorganisms and thus high organic space loadings in UASB and EGSB reactors. One gram of granular sludge organic matter (dry weight) can catalyze the conversion of 0.5 to 1 g of COD per day to methane. In layman terms that means on a daily basis granular sludge can process its own body weight of wastewater substrate.
Figure 1. Anaerobic sludge granules from a UASB reactor treating effluent from a recycle paper mill (Roermond, The Netherlands). The background is millimeter paper indicating the size of the granules. Red arrows point to gas vents in the granules, where biogas is released.
Granulation Process: The process of granular sludge formation is one of the most interesting and enigmatic questions when attempting to understand the fundamentals of anaerobic granular sludge technology. This topic has fueled many PhD research projects. There are many theories, ranging from extracellular polysaccharide slime to calcium as key players in the initial aggregation process. However, the most promising theory is the "spaghetti" theory (proposed by Dr. W. Wiegant) in which filamentous microorganisms become entangled in one another analogous to the formation of fungal pellets as shown in Figure 2 below. In support of theory is the fact that the methanogens known as Methanosaete, which are better adapted for low substrate concentrations (a condition desired for wastewater treatment), happen to be filamentous microorganisms. The initial pellets ("spaghetti balls") of Methanosaete can serve as a surface of attachment or support matrix for other microorganisms involved in the anaerobic degradation process. For the attachment of diverse microorganisms to the pellet, perhaps slime layers and calcium may play an important role.
Figure 2. The spaghetti theory of granulation. I) disperse methanogens (filamentous Methanosaeta); II) floccule formation via entanglement; III) pellet formation ("spaghetti balls"); and IV) mature granules, with attachment of other anaerobic microorganisms onto the pellet.
Look Inside a Granule: Each granule is an enormous "metropolis of microbes" containing billions of individual cells and perhaps thousands to millions of different species. Follow the link in Figure 3 below to a slide show which takes you inside a granule to take a closer look at the microorganisms inside.
Figure 3. Take a look inside a granule [look inside slide show].
Settling Properties: According to Stoke's law, sedimentation rates are a function of particles size squared. Due to their large particle sizes, anaerobic sludge granules have exceptional settling properties. The rapid settling velocities permits the application of high hydraulic loads to UASB and EGSB reactors without having to be concerned about wash-out of biologically active sludge particles (responsible for the bioconvresions). Because high hydraulic loads are tolerated, UASB and EGSB systems can handle wastewater streams with relatively low concentrations of substrate, even as low as a few hundred milligrams COD per liter (previously considered impossible for anaerobic treatment). As is illustrated in Figure 4, granular sludge settles extremely rapidly and is completely clarified within a few minutes. By comparison dispersed sludge (like that from an anaerobic digester at a municipal treatment plant) has not even begun to clarify in the same time scale. Flocculent sludge, also clarifies rapidly but not as fast and as granular sludge.
Figure 4. Comparison of the settling properties of granular, flocculent and disperse sludge after 5 minutes of settling time .
Stoke's Law
v = 2r2g(d- Stoke's Law D)/9N
v = velocity of sinking
r = radius sludge particle
g = gravity
d = density of sludge particle
D = density of water
N = viscosity
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