INTERNAL CIRCULATION CIRCULATION REACTOR: PUSHING THE LIMITS OF ANAEROBIC INDUSTRIAL EFFLUENTS TREATMENT TREATMENT TECHNOLOGIES David Tshilumba Mutombo Biwater (Pty) Ltd,10 Vervoer Street, Kya Sand, Randburg, Johannesburg, PO Box: 2216, Honeydew, 2040. Tel: 0027 11 549 7600. Fax: 0027 11 462 6266. E-mail:
[email protected] . Website: www.biwater.co.za and www.biwater.com
ABSTRACT Biological anaerobic technologies are widely used for the treatment of high strength industrial effluents. Since it's development in the 1970s, the UASB reactor is considered as a breakthrough allowing high organic loading rates of up to 10 - 15 kg COD/m3.day. New generations of UASB reactors using expended beds have also been introduced with much higher Organic Loading Rates (OLR). Among them the Expended bed Granular Sludge Blanket (EGSB) reactor with OLR up to 20 kgCOD/m3.day and the Internal Circulation (IC) reactor with OLR up to 35 kg COD /m3.day in full scale plants are the most used anaerobic systems. The higher OLR of the IC reactor is principally due to its internal circulation that allows an improved contact between the biomass and the influent wastewater, resulting in higher biomass activity. Furthermore the IC reactor posses 2 sets of 3-phases separation modules, that separate the gas, the liquid and the biomass. This feature improves biomass retention and consequently allows higher biomass activity and improves the final effluent quality. The reactor has been applied in the following industries: Brewery and beverage industry pulp and paper industry, distillery and fermentation industry, chemical and petrochemical petrochemical industries. More than 161 IC reactors have been constructed worldwide for the past ten years, making the IC I C reactor one of the most demanded anaerobic technologies technologies in the t he world. Keywords: Keywords: Anaerobic treatment, EGSB, IC, OLR, and UASB
INTRODUCTION Since the 1972 world summit in Rio de Janeiro, environmental protection campaigns have raised the level of awareness on the importance of protecting the ecosystem from industrial pollution. Governments, public concerns, civil societies and environmental protection agencies have constantly put industries under pressure to reduce levels of pollution in their discharges (1). This has resulted in strict environmental regulations on the industrial emissions for the chemical and other related industries (2). Among the diversity of industries, brewery and beverage industry, industry, distillery and fermentation industry, food industry, pulp and paper industry, chemical and petrochemical industries are commonly characterized by high organic pollution in their effluents, with COD values of more than 2000 mg/l. For this level of organic pollution, it is an established fact that anaerobic treatment is more cost effective than aerobic treatment (3). The reasons include principally the higher energy input requirement and the excess bio mass production that would implicate further costs for treatment and disposal in aerobic processes. An added benefit of the anaerobic treatment is the generation of energy in the form of biogas and production of very little biomass. Different anaerobic technologies have been used for the treatment of industrial effluents. They range from anaerobic lagoons to the UASB reactor. Since its development in the 1970s by Prof. Lettinga, the Up flow Anaerobic Sludge Blanket reactor is by far the most widely studied reactor Proceedings of the 2004 Water Institute of Southern Africa (WISA) Biennial Conference ISBN: 1-920-01728-3 Produced by: Document Transformation Technologies 608
2 –6 May 2004 Cape Town, South Africa Organised by Event Dynamics
configuration for wastewater treatment and its primary use is for the treatment of higher strength industrial wastewater (4). To improve its performance and increase the OLR, different biotechnology houses have modified and patented changes made to the UASB. Developments of this reactor have focused principally on the three phases (Gas-liquid-solid) separation internal modules, the internal mixing of biomass and the incoming inffluent and a reduced foot print. Consequently, several UASB configurations are commercialized for high strength industrial effluents. Among them, the Internal Circulation reactor has so far achieved the highest organic loading rate (>30 COD/m3/day) with a regular operational out put of more than 80% COD reduction (5). This paper attempts to explain the major advantages of the Internal Circulation reactor compared to other anaerobic effluent treatment technologies.
ANAEROBIC TREATMENT OF WASTE WATER The concept of anaerobic treatment of industrial wastewater is based on a microbial consortium involving many classes of bacteria. The bacteria are active anaerobically to execute a complex process with several intermediate steps: The complex organics found in the substrate are first hydrolyzed into simpler organics after which they are fermented to volatile acids by the acidogens. Volatile acids longer than two carbons are then converted to acetate and H 2 gas by obligate hydrogen producing acetogens. Finally the acetate and H 2 gas are converted to CH 4 by methanogens (6). This relatively complex process is pictured in figure 1 below.
Figure 1. Schematic representation of anaerobic treatment processes.
To obtain high rates of the series of metabolisms pictured in figure 1; the microbial community must be conditioned to operate to its optimum activity. These conditions include the pH, the temperature, nutrients, presence of toxic substances, contact between bacteria and the substrate (mixing), and the retention time. Furthermore, after treatment an adequate separation between the treated effluent and the solid biomass is important in order to achieve a clear effluent and keep the biomass within the system. Optimum values for these environmental parameters can be obtained from literature. In this regard, the optimum pH for acidogens is 5.5 to 7 and 6.5 to 7.5 for methanogens. The optimum temperature range for methanogens is between 30 to 40 degrees Celsius. Although scientifically the basis of anaerobic biotechnology is common to all anaerobic processes, practical designs of reactors remain different. The differences are based on the phases' separation, internal mixing, biomass retention system, and the ratio height to width. It must also be mentioned 609
that the pretreatment of the wastewater prior to the anaerobic reactor plays a crucial role in the over all process performance. The pretreatment system must be designed to facilitate biological processes within the reactor. Effluents are generally pre-fermented to hydrolyze larger molecules and obtain short chain fatty acids that easily biodegradable in the anaerobic process (6).
ANAEROBIC VS AEROBIC EcKenfelder and Esley (7) reported the use of activated sludge process for the treatment of wastewater from pulp and paper industries, food and beverage industry etc. Comparing aerobic and anaerobic treatment is a complex topic in the sense that process, operation, and site constraints must all be combined in the final decision. For comparison purposes we refer to the Waste Activated Sludge (WAS) process as the aerobic treatment technology. Wastewater engineering books generally suggest the use of aerobic treatment for lower COD and BOD values. Grady and Lim (3) advised to use aerobic treatment for COD concentration of less than 1500 mg/l and anaerobic treatment for higher COD values up to 50 000 mg/l. There are different factors that one would consider in the process selection mechanism. We have only considered a few of them. The supply of oxygen in the aerobic process is achieved using mechanical equipment, principally mechanical aerators or diffused air equipment. To obtain Dissolved Oxygen (DO) level of 2 mg/l, mix and maintain the biomass floating, considerable energy input is required. Although mixing energy is required in Continuous Stirred Tanks Reactor (CSTR) type of anaerobic technologies, other anaerobic technologies would use the generated biogas to partially mix their contents. Furthermore aerobic biomass yield can be up to 20 times higher than the anaerobic biomass yield. This is illustrated in the table 1 bellow compiled by Field and Sierra (8). From an operational point of view this high biomass yield combined with a very short doubling time would imply more sludge production for the aerobic processes than for anaerobic treatment with the consequence of more sludge handling and disposal costs for aerobic treatment. Table 1. Aerobic vs. anaerobic kinetic parameters.
Doubling time (days) Aerobic bacteria
0.030
Cell Yield KgVSS/Kg COD 0.40
Cell activity Kg COD / Kg VSS/day 57.8
Ks mM 0.25
Fermentative bacteria Acetogenic bacteria
0.125
0.14
39.6
-
3.5
0.03
6.6
0.4
Methanogenetic autotrophs Methanosacharina
0.6
0.07
19.6
1.5
0.04
11.6
0.00 4 5
Methanosaete
7.0
0.02
5
0.3
Table 1 indicates also a higher cell activity for aerobic processes, which suggests a better-polished final effluent quality. It also indicates a low doubling time for aerobic microorganisms. In other words, in anaerobic systems, microorganisms reproduce less rapidly than in aerobic systems and a longer minimum sludge retention is required to accommodate the slow net growth rate (9). In this regard, biomass retention is one of the most important aspects of anaerobic technologies. The longer the biomass is retained, the higher the microbial activity and consequently a higher OLR is achieved compared to conventional activated sludge processes. Consequently aerobic treatment technologies require larger reactor volume and larger footprint than that of anaerobic 610
treatment technologies. Generally industrial locations are space limited and therefore require small foot print technologies.
ANAEROBIC TECHNOLOGIES FOR HIGH STRENGH INDUSTRIAL EFFLUENTS Anaerobic treatment technologies differ among themselves in the way they retain biomass and the internal mixing of biomass and the influent wastewater (10). These parameters would impact on the amount of biomass in the reactor and consequently the biomass activity. Tchobanoglous et al., (11) classify the anaerobic processes into two categories: The anaerobic suspended growth processes and the anaerobic attached growth processes.
Anaerobic Suspended Growth Treatment Processes Include :
The Simplest Anaerobic Systems Like Lagoons and CSTR Reactors These reactors have no special sludge retention system. To keep a growing microbial population in the reactor, CSTR and lagoons are designed with a dilution rate of less than the growth rate. In general, the sludge retention in these reactors equals the Hydraulic Retention Time (HRT) with the consequence of very diluted sludge. This means a limited biological treatment capacity (10) and therefore, these types of reactor are not suitable for high strength industrial effluents.
The Anaerobic Contact Process Reactors In this type of reactors, the content of the reactor is completely mixed and then separated into a clarifier, a vacuum flotation unit or a lamella clarifier, and the supernatant is discharged as effluent. Settled anaerobic sludge is then recycled to seed the incoming influent. The sludge produced in this type of reactor will have a flocculent like and dilute nature and consequently a limited organic loading rate.
The Upflow Anaerobic Sludge Blanket Reactors The concept of the upflow anaerobic sludge blanket was developed in the 1970s by professor Lettinga (12). The key feature of the system is the microbial aggregation into a symbiotic multilayer structure called granule (13). In this process the wastewater is introduced in the bottom of the reactor and flows upward through sludge blanket composed with biologically formed sludge granules. Treatment occurs as the wastewater comes in contact with granules. Literature has reported granules formation up to 4 mm, with very high settling characteristics (settling velocity of 30 to 80 m/hr) (15). In the UASB reactor, sludge granulation favors optimum retention because the problems of wash out and nutrient uptake of active biomass are minimized (14). UASB reactors are designed with a 3 phases separation system that will retain the biomass within the reactor. By retaining the maximum mass of sludge granules in the reactor, the activity of the anaerobic microbial consortia is increased within the system and therefore allows the reactor to accept a high OLR. In other words, the higher bomass concentration implies that contaminant transformation is rapid and highly concentrated or large volumes of organic waste can be treated in compact reactor (15). Apart from the wastewater pretreatment and its distribution within the reactor, the three-phase separation module is a key component to the success of the UASB and constitutes the main fundamental difference in UASB technology suppliers. Special attention must be given to the understanding of the separation modules and to comparing different existing modules prior to purchase a UASB technology.
Anaerobic Attached Growth Treatment Processes Include :
Anaerobic Filter Processes Anaerobic filter processes use column reactors filled with various types of solids media. The influent flows upward through the column, contacting the media on which anaerobic bacteria grow and are retained. These reactors offer a great potential of clogging due to solids in the influent. 611
This may cause short circuiting and dead reactor zones that are difficult to identify.
Expended Bed Processes These processes use appropriate medium (Sand, coal, expanded aggregate) on which a biological growth develops to treat the influent. The effluent is recycled to dilute the incoming wastewater and to provide an adequate flow to maintain the bed in an expanded condition. The reactors used in these processes have very small footprints compared to UASB reactors. Biomass concentration of up to 40 000 mg/L have been recorded in these reactors, allowing a relatively very high organic loading rate (11).
Anaerobic CSTR
Anaerobic Contact Process
Anaerobic Baffled Reactor
UASB
Anaerobic Filter
DPBR
UPBR
Expended bed reactor Fluidised bed reactor Figure 2. Schematic diagrams of anaerobic treatment processes.
NOVEL ANAEROBIC TREATMENT PROCESS TECHNOLOGIES: EXPENDED SLUDGE BED REACTORS (EGSB AND IC REACTORS) Frankin and Zoutberd (16) achieved high values of OLR in an expended bed reactor without using a solid support medium. Therefore it was possible to avoid costs and effects of carrier material of the expended bed processes while keeping it's high loading rate and its reduced footprint. With this 612
new concept, the EGSB (Expanded granular sludge Blanket) and IC (Internal Circulation) reactors were introduced into the anaerobic technologies. Figure 3 bellow illustrates two reactors.
Figure 3. Schematic representation of an EGSB and IC reactors.
EGSB reactors are defined as vertically stretched versions of UASB reactors (10). UASB reactors are commonly built of 4.5 to 6.5 m while EGSB are 12-16meters tall. Dinsdale et al., (17) define the EGSB reactor as a modification of the UASB reactor in which the granules are partially fluidized by effluent recycle at a liquid upflow velocity of 5 to 6 m. The EGSB reactor shows improved mass transfer characteristics over the UASB reactor. It is generally applied in situations where the volumetric gas production rate is low and mixing in a USAB reactor by up flow velocity alone (0.5 to 2 m/hr) is insufficient (17). Other researchers have compared the overall performance of an EGSB to the UASB reactor and the EGSB performed better than the UASB (18). To summarize them the EGSB reactor offers a smaller footprint, higher mixing due to the higher up flow velocity and consequently an improved mass transfer and biomass activity than the UASB reactor. As a derivative of the UASB, it responds to the need of small and medium sized industries (19). To reduce the energy costs of obtaining a high upflow velocity, the internal circulation reactor introduces the so-called internal circulation system induced by biogas collected within the system. As illustrated in figure 2, the IC reactor can be considered as two anaerobic treatment compartments (like UASB) on top of each other, one highly loaded and one low loaded. Its special feature is the separation of biogas in two stages within a tall reactor (up to 24m). The first reactor compartment contains an expanded granular sludge bed where the most organic pollution is converted into biogas. The biogas produced in this compartment is collected by the lower level 3-phases separator and is used to generate a gas lift by which water and sludge are carried upward via a riser pipe to the gas liquid separator on top of the reactor. In this separator the biogas is separated with the water/sludge mixture and leaves the system. The water/sludge mixture is directed downwards to the bottom of the reactor via a concentric downer pipe, resulting in internal circulation flow. The 613
effluent from first compartment is post treated in the second, low loaded compartment where remaining biodegradable COD is removed. The biogas produced in the upper compartment is collected in the 3-phase separator, while the anaerobic effluent leaves the reactor via overflow weirs. When comparing the IC to other expended bed reactors (EBR), the following parameter apply:
Sludge Retention The IC reactor uses two sets of separator modules while other EBR like the EGSB contain only one separator module. This explains why the IC retains more biomass and produces a clear effluent than the EGSB. This would improve the overall performance of the reactor in terms of COD removal. Furthermore, it will implicate relatively lower cost and high efficiency of the post anaerobic polishing process.
Sludge Bed Fluidization and Mixing in the System The IC reactor relies on the biogas produced to generate an internal circulation flow by creating a difference in gas hold up between the riser pipe and the downer pipe. This reduces the cost of extra pumps for mixing and the cost of mixing energy. While the other EBR use the hydraulic up flow and the biogass up flow for biomass wastewater contact, the IC reactor uses both forces plus an extra internal circulation, resulting in more intimate contact between biomass and wastewater and therefore a higher biological activity. Table 3 below provides values of organic loading rates as reported by different authors for full-scale anaerobic plants. Table 3. Typical reactors’ loading rates.
Reactor
UASB
EGSB IC IC
Organic Loading rates (Kg COD/m3/day) 4 to 12 5 to 15 5 to 10 5 to 20 5 to 25
Reference
20 to 30 15 to 30
(6) (21)
(11) ( 6) (10) (6) (8)
It can be seen from table 3, that Internal circulation reactor offers the highest possible organic loading rate. Combining the high organic loading rate to other advantages that the IC reactor offers, like the quality of the effluent and the internal circulation features, the IC reactor is by far the most sophisticated anaerobic process technology that exists in the market . Full-scale references of the IC reactor exist in variety of industris. These include the brewery industry, the pulp and paper industries, the distillery and fermentation industries, ago-industries, sugar Industry etc (20), (21). To illustrate the increased demand of the IC reactor, data from the paper and pulp industry and from the brewery industry have been plotted by Habets et al., (22). The IC reactor was introduced in the pulp and paper Industry in 1996. In 1999, all the effluent treatment plants constructed in the world for pulp and paper effluent were IC reactors. In 1997 only 32 IC installations were recorded in the world. Among them, 18 for breweries, 6 for potato processing, 2 for confectionery and 1 unit for the sugar, paper mill, starch mill, distillery, dairy plant, food industry each (20).
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By April 2003, 161 internal circulation reactors were constructed in the world; 89 of them in brewery and soft drink industry, 33 of in the pulp and pulp industry, 39 in the food industry, 9 in the distillery industry and 8 in varied chemical industries including tanneries and textile industry (22).
CONCLUSION The development of the upflow sludge blanket reactor by Prof. Lettinga, was considered as a major breakthrough in the anaerobic treatment of high strength industrial effluent treatment. The reactor uses granular biomass with very good settling characteristics. It also uses 3 phases’ separation modules to separate generated gas, the biomass and the final effluent. This results in improved biomass retention and consequently a higher biomass activity of the granular sludge. The major challenges of this reactor was the way of achieving the mixing within the reactor to optimize the biomass wastewater contact and the design of the separators modules that still differentiate UASB manufacturers world wide. A new generation of anaerobic reactors uses expanded beds to optimize the internal biomass wastewater contact. This generates higher organic loading rates compared to the UASB reactor. The ESGB reactor and the IC reactor can achieve respectively up to 25 and 35 kg COD/m3/day. However the IC presents other advantages over the ESGB reactor. It uses an internal circulation based on airlift principle, generated by the biogas that is produced inside the reactor. This internal mixing energy provides a more intimate contact biomass wastewater resulting in higher organic loading rate achievable. Furthermore the IC reactor uses two sets of the 3-phase separator modules, and therefore produces a higher quality effluent compared to other anaerobic processes reducing the cost of potential polishing step.
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